Wednesday, November 1, 2017

Plasma: The Fourth State Of Matter

Understanding what plasma is can be confusing, even though plasmas are part of our everyday lives. The Northern Lights, lightning, flames, fluorescent bulbs, neon signs and even the Sun are all examples of matter in this particular physical state. Although most of us are well acquainted with at least three states of matter - solid, liquid and gas - getting to know plasma, and how substances become plasma, can be daunting, but also fun and very useful.

Plasma Is Everywhere

It might surprise us to learn that plasma is by far the most abundant physical state of matter in the universe, much more so than our familiar three states of solid, liquid and gas. Perhaps even more surprising is the fact that plasma is just one of over 30 different states of matter currently listed on Wikipedia. More yet will undoubtedly be discovered in laboratories. Most of these exotic states are unknown beyond research circles because they tend not to be observed except under extraordinary conditions. Plasma, in contrast, is the most common state of ordinary matter under natural conditions in our universe.

Most of our visible universe consists of plasma, and it is a fortunate thing because that is the key reason why we can observe it with our telescopes. Stars are entirely made up of it. Most interstellar gas is plasma. Some sources estimate as much as 99% of visible matter in the universe is in a plasma state. By the end of this article we will understand how plasma can emit light. Even in our own solar system, over 99% of its mass is in a plasma state, thanks to the overwhelming mass contribution of our Sun. In our everyday life we regularly, but temporarily, witness plasma as lightning, fires, and the auroras, when energy is applied to matter, usually in its gaseous state. I did not explore plasma in high school and I had few occasions to explore it in my first years as an undergrad. One reason why plasma is not more thoroughly studied at these basic levels is that it can be difficult to jump from the fairly straightforward molecular theory needed to understand solids, liquids and gases into atomic and sub-atomic particle theories required to get a handle on the plasma state. Yet it's not only do-able; it's fascinating.

What Is a Phase Change?

A common theoretical thread to solids, liquids and gases is temperature. We can extend this temperature spectrum at the cold end by exploring the exotic-sounding Bose Einstein Condensate (BEC). In this state, mysterious quantum behaviours of matter are on a scale that we can observe. I wrote an article a few years ago that explores what happens when matter gets VERY cold and transforms into a BEC. Because stars are very hot, it is tempting to add plasma to the upper temperature end of this physical state spectrum. That, however, would be misleading and incorrect. Gases, when extremely hot, can break down into plasma but we do not need to heat a gas to transform it into plasma. Applying an electrical charge alone to a gas can do this. Think of a glowing neon tube light, for example. It is filled with gas in a plasma state but you can comfortably touch it with your hands.

There is an additional reason why temperature alone does not necessarily dictate the phase of a substance. We often first learn about phase changes by exploring water as a gas, liquid and solid, but when we talk about solids, liquids and gases, we really need to consider pressure in addition to temperature, which together determine what state matter is in. Under constant atmospheric pressure, temperature alone determines which state water will be in. This is how we encounter water in everyday life. We can also keep water at a constant temperature and change the pressure. Under enough pressure water will condense into liquid and even into exotic forms of ice. In a near-vacuum, water ice can sublimate directly into vapour.

Temperature, pressure and electrical potential are the three changing factors behind the phase changes we will focus on here. A phase change of any type relies on a boost or withdrawal of energy into/from matter. These aren't the only kinds of phase change that exist. The application of a magnetic field can change the physical state of magnetic material, for example. For all phase transitions (which is also called a change in physical state), the physical arrangement or ordering of atoms changes. In this article I'd like to start by exploring transitions between solids, liquids and gases in depth. Then I will compare those changes to transitions into a plasma state and explore what plasma is as a physical phase of matter.

Exploring Solids, Liquids and Gases Using Water as an Example

Matter changes from one physical state to another through a process called a phase transition. Every chemical compound and element undergoes a phase transition under a specific combination of temperature and pressure. The periodic stable below lists all the known elements. At 0°C and 1 atmospheric pressure, all elements in red type are gases, elements in green type are liquids, elements in black type are solids, and it is unknown what physical state those in grey type are. These "mystery" elements start at atomic number 100, fermium. These elements, plus several other smaller atomic number elements are synthetic or man-made. This is the highest atomic number element that can be created in a macroscopic amount, although a pure sample of this metal has not been created yet.


We can compare the melting and boiling points of most of the elements above but we are all most familiar with water. Not only is water familiar but it is also exhibits some unique and fascinating properties unlike most other substances, and this will allow us to explore change of state in greater depth. It is a chemical compound of hydrogen and oxygen (H2O). We drink it in a liquid state. At 0°C, water freezes into ice, a solid state, and at 100°C it evaporates into a gas, water vapour. These transitions assume a pressure of one atmosphere (atm; the average air pressure at sea level)). To understand how pressure plays a role in the physical transition of a substance, imagine water vapour in a container fitted with a gas-tight plunger. The temperature is carefully maintained throughout the experiment at 130°C. The starting pressure is 1 atm. As the plunger lowers down into the container, the volume of gas decreases and the gas pressure increases. Soon the water vapour will be compressed into liquid water. 130°C is the boiling point at around 5 atm. Above 5 atm, water at 130°C will liquid. It will have to be compressed much further, to about 100,000 atm (because liquid water strongly resists compression) to solidify into scalding hot 130°C ice in the container.

Researchers at Sandia National Laboratories performed a similar experiment several years ago. They subjected liquid water (starting at 1 atm) to extremely rapid compression (to 70,000 atm). The water shrank abruptly into a dense phase of solid ice. When the pressure was relieved, it melted (and expanded) back into liquid water. The ice formed under these conditions is not the everyday ice we make in our freezers. Ordinary freezer ice, as most of us know, is less dense than liquid water. At pressures above 100,000 atm, however, water only exists as different kinds of very dense ice, even at temperatures of hundreds of degrees centigrade. Scientists think such hot ice exists in the deep interior of exoplanet GJ 436 b, a Neptune-size ice giant that orbits very close to its parent red dwarf star, GJ 436, which is 33 light-years away from Earth.

A Phase Change is a Molecular Rearrangement

Chemically, water in any physical state remains water. That is, it retains its chemical properties. Any matter undergoing a phase change from solid to liquid to gas and vice versa retains its chemical properties. Each water molecule consists of two hydrogen atoms covalently bonded to an oxygen atom. What changes during a phase transition is the molecular arrangement of these molecules (see below). At any particular temperature and pressure, water molecules will adopt the most thermodynamically stable arrangement possible for that environment. In order to transition into a new arrangement, the molecule-molecule interactions must change. Molecule-molecule, or in the case of elements, atom-atom interactions can shift and reorganize a substance, changing its physical properties in the process. The interactions between molecules in a substance tend to be attractive. In solids, these attractive forces are strong enough to be called chemical bonds, but these bonds are still much weaker than the chemical bonds that bind each molecule itself together (such as the two covalent hydrogen bonds in water). The simple images below highlight the general differences in atomic/molecular arrangement between solids, liquids and gases.

Materialscientist;Wikipedia
This is an actual atomic resolution image of the lattice-like arrangement of molecules in solid strontium titanate. Chemical bonds between strontium titanate molecules hold them close together in a tight regular arrangement. Above about 2000°C, strontium titanate crystals will melt, breaking these relatively weak intermolecular chemical bonds.

Kaneiderdaniel;German-language Wikipedia
This highly simplified two-dimensional representation (left) shows how atoms or molecules might be arranged in a typical liquid in a beaker. They have contact with their closest neighbours, where they experience very weak attractive inter-molecular forces, but there is no overall order to their arrangement, and the atoms/molecules can slide freely past one another.



As the diagram right simplistically shows, the spaces between molecules/atoms in a gas are vast compared to the size of the atoms or molecules themselves, many magnitudes greater than this simple diagram suggests. This is true even for a highly pressurized gas. The atoms/molecules can move freely in all directions. They experience no appreciable attractive forces between them so they move independently of one another. The ideal gas law assumes negligible molecular volume relative to gas volume and it assumes the absence of any molecular attraction. It quite accurately predicts how volume, temperature and pressure relate to one another in a gas. This means that most gases in reality act much like a theoretical ideal gas.

Water molecules bond with each other in a solid phase according to the Bernal-Fowler ice rules. This is where water gets quite interesting. Every oxygen atom can bond to 4 hydrogen atoms. Two bonds are strong. These are the covalent chemical bonds that hold each water molecule together, and they don't change during a phase transition. However, oxygen has 6 valence (or outer, chemically available) electrons. In water vapour, four electrons remain unbound as two lone pairs. As water vapour condenses into liquid water, some of the four unbound electrons take part in very weak molecule-molecule bonds with hydrogen atoms in adjacent molecules. These are called hydrogen bonds. Unlike the strong covalent hydrogen-oxygen bonds holding each molecule together, these bonds are weak and in the liquid state, they are transitory. The (proton) positive charges of hydrogen atoms are attracted to the negative charge zones of nearby oxygen lone electron pairs.

The attractive force behind hydrogen bonding is about 90% due to the attraction between opposite charges. This is an electrostatic phenomenon. 10% of the bonding force is due to electron sharing. This is a quantum mechanical phenomenon and it is also responsible for (much stronger) covalent bonding. In liquid water, the weak hydrogen attractions form and break very easily, allowing water molecules to slip past one another. The much stronger covalent intra-molecular bonds holding each water molecule together only break when water undergoes a chemical decomposition reaction called electrolysis, producing hydrogen and oxygen gases as products.

The diagram below models the hydrogen bonds between water molecules. Two lone electron pairs force every water molecule into a triangle shape, which makes it polar - each molecule has a positively charged region and a negatively charged region. The positive charge of hydrogen atoms is attracted to the negative charge zone of oxygen atoms. Hydrogen bonds are indicated by dotted lines.

User Qwerter at Czech Wikipedia    
The length and strength of hydrogen bonds is strongly dependent on temperature. As liquid water freezes, additional hydrogen bonds form between molecules. Every hydrogen atom is, in effect, bonded to two oxygen atoms - one bond is strong and one bond is weak. Under sufficient pressure and/or cold, water molecules come close enough together to bond into regular and stable lattice-like arrangements. As ordinary ice, the molecules form bonds that result in a hexagonal crystalline lattice arrangement. Under more extreme pressure/temperature regimes, water ice can transition into a variety of different lattice structures such as cubes, rhomboids and tetragons, which allow for denser molecular arrangements. Each transition into a denser crystalline lattice is a phase change. There are at least 17 known physical states of water ice alone. Changes in the number, length and strength of hydrogen bonds underlie each of these additional solid phase changes.

Most liquid substances solidify into denser molecular arrangements but there are a few exceptions, and they are always due to unusual bonding between the molecules. Water is an example. Ordinary water ice is actually less dense than liquid water. That is why it floats. This phase is called hexagonal ice, or ice lh. It is the only solid water phase encountered on Earth. Due to the unusual chemical bonds of water (responsible for its molecular shape and polarity), the hexagonal lattice arrangement of lh ice keeps molecules a bit further apart from one another than they are in the liquid state (yet the bonds themselves are stronger and more permanent). Ih ice has a density of 0.9167 g/cm3 compared to liquid water with a density of 1.18 g/cm3. The diagram below shows what the hexagonal lattice of Ih ice looks like. Gray dashed lines indicate hydrogen bonds.

NIMSoffice;Wikipedia
In addition to a number of solid crystalline lattice states, water can also exist as a solid lacking any crystalline structure, called amorphous ice. On Saturn's icy moon, Enceladus, water ice strewn onto the surface from its many cryovolcanoes is amorphous. Liquid water flash-freezes in the vacuum of space (zero pressure) before it can organize into any structure. In contrast, along Enceladus's unique "tiger stripes," the ice is crystalline because here it is kept warm enough long enough from the heat of geothermal activity to arrange into a more thermodynamically favourable crystalline structure.

Below is Cassini's view of Enceladus's south pole, showing 4-5 "tiger stripes," which are tectonic fractures.

NASA/JPL/Space Science Institute
Critical Point and Triple Point

The temperature and pressure at which a phase change occurs is called a critical point. The temperature of the critical point depends on the pressure of the system and vice versa, so on a graph you have a line rather than a single point (see the phase graph below for water). However, there is a single point, a specific temperature and pressure, at which gas, liquid and solid states of water all have an identical free energy (or internal molecular energy). All three phases coexist at this point, which is called the triple point. A critical point is a line and a triple point is a point. In the graph below, two black lines represent the solid/liquid critical point and the liquid/gas critical point of water. In the lower center of the graph you can see the single triple for water. It is a single point on the graph found at 0.01°C (273.6°K) and 0.006 atm (611.657 pascals).


mglee;Wikipedia
A note about pressure units on the above graph: 1 atm pressure is roughly equal to 1 bar pressure. 1 bar is equal to 100 kilopascals (kPa). The pascal is the internationally recognized SI unit for pressure. The bar and the atmosphere are older non-Si units. I am in the old habit of using atm units, where 1 atm is standard air pressure at sea level.

This 6-minute video from Bergen University in Norway demonstrates how water behaves as it approaches its triple point.



At the triple point, all of the water in a system can be changed into vapour, liquid or solid just by tweaking the pressure or temperature a tiny bit. The triple point is also the lowest pressure at which liquid water can exist. At lower pressures, as on the surface of an icy moon (at near vacuum), water ice, perhaps warmed when facing its star, will sublimate directly into vapour, bypassing the liquid state altogether. For most substances, the triple point is also the lowest temperature at which the liquid state can exist but water, due to its unusual hydrogen bonds, is an exception. Notice the odd little horn on the green section in the graph above. This is an anomaly of water. If the temperature is just below the triple point, a steadily increasing pressure will transform ordinary solid water ice (Ih) into denser liquid water (at around 1000 atm) and then back into an even denser solid (now as ice VI) at around 10,000 atm.

A Brief Look at The Thermodynamics of a Phase Change

For the phase changes I've described so far, the transition process is abrupt, or more scientifically put, discontinuous. At a phase transition point (a critical point), such as the boiling point for water for example, two phases - liquid water and water vapour - co-exist at one specific temperature. In thermodynamic terms, they have the same average Gibbs free energy. In any system some molecules will randomly have slightly more energy and some will have slightly less. However, at critical point the substance cannot be distinguished as either gas or liquid.

All thermodynamic systems tend to assume the lowest possible Gibbs free energy. Systems also tend toward increasing entropy. Gibbs free energy can be looked at as a measure of potential chemical energy in a system that is available to do work in the system, while entropy can be thought of as a measure of the orderliness of a system. Highly ordered crystalline water ice has very low entropy compared to the high entropy of water vapour. Both states, however, are stable within their own temperature/pressure regime because each state has maximized its entropy according to the Gibbs free energy available to it.

Below the boiling point, the liquid phase is more thermodynamically stable (which means it has reached maximum possible entropy at a lower Gibbs free energy). As water vapour condenses into liquid water, it enters a more highly ordered state (lower entropy).  As this phase transition occurs, an energy exchange takes place. As the water enters a state of lower Gibbs free energy, it releases the extra free energy as latent heat. Conversely, if we wanted to melt ice into water we would have to apply heat to it. In other words we would have to add latent heat to the system so that we can increase the entropy of the water molecules.

So far we've focused on water as our example, but every chemical substance and element can exist as a solid, liquid and gas, depending on its temperature and pressure. Each substance has its own unique critical points and triple point, and it may have additional solid lattice states as well. These critical points are unique to each substance and depend on the substance's unique molecular bonding.

The Relationship Between Pressure and Phase Change

We've explored how substances and elements transition from solids to liquids to gases and vice versa. Increasing the pressure on a substance or increasing its temperature increases the average kinetic energy of that substance. Pressure and temperature determine which physical state a particular substance is most thermodynamically stable. The relationship between temperature and physical state is fairly straightforward as we've seen. However, the relationship between pressure and physical state depends on the starting physical state of the substance. A gas responds to pressure by shrinking in volume and increasing kinetic energy. A gas resists compression through thermal pressure: Even though a container full of gas appears static, the molecules in it are in constant motion. Collisions between those molecules and the sides of the container are detected as a force per unit area, or pressure. If a gas is compressed, it will heat up because the total thermal energy of that gas, which doesn't change, is now concentrated into a smaller volume. If heat is continually removed from the system as it's compressed, the gas will remain at the original temperature it was even though it will eventually condense into a liquid and then further solidify into a solid.

Liquids, unlike gases, tend to be fairly incompressible. This means that liquids don't experience much increase in kinetic energy when they are compressed, but they will compress under extreme pressure. The pressure that resists compression is not thermal pressure, as with gases,  but another kind of outward pressure (which gases also express but is masked by thermal pressure). This pressure is the result of the Pauli exclusion principle. This quantum mechanical principle means that when electron orbitals in adjacent atoms are forced into very close proximity, they will powerfully resist overlapping one another into the same atomic orbital. To note, this is not the same thing as orbital sharing, in which an electron will occupy an empty orbital in an adjacent atom, and through such sharing create a chemical bond. I will explain this more accurately in a moment.

Under increasing pressure, the liquid will likely first become denser and more viscous and then it will transition into a solid, in which molecules will pack more closely together but in a very ordered arrangement that respects the exclusion principle. If you recall my earlier example of liquid water being subjected to sudden extreme pressure and condensing into a dense form of ice, in both cases it eventually becomes more thermodynamically favourable for the water molecules to transition into a high-density highly ordered form of ice than to remain as a highly pressurized liquid. That "decision" made by a substance is abrupt, which means it happens all at once.

Water Under Very Intense Pressure

What happens if you continue to increase the pressure on a solid such as water ice? Solids, like liquids, resist compression. If we take a look at the water phase graph above once again, we see that above 100,000 atm (or bars), water exists only as a solid no matter what the temperature is, at least up to 350°C. Cubic ice VII will transform into an even denser cubic lattice called ice X as the pressure is increased. Its crystalline lattice will transition into increasingly denser molecular arrangements, each transition being a phase change. At the highest pressure on the graph (around 6-10 million bars) we see a phase of ice called high-pressure ice XI. This extremely dense XI hexagonal ice is hypothetical at this time and should not be confused with ice XI orthorhombic, which likely forms at temperatures below -200°C at zero pressure, such as on the surface of Pluto's moon, Hydra, for example.

A 2010 article by Burkhard Militzer and Hugh Wilson explains what researchers think happens when the pressure on ice increases even further than our graph goes. In this theoretical case, water bypasses hypothetical high-pressure ice XI. They suggest instead that above about 3 million bar, Ice X could transition into a different more complex lattice that contains 12 hydrogen atoms per unit. If the temperature is also increased to about 2700°C, they think the lattice might transition into yet another new arrangement. The hydrogen atoms may become mobile within a stable lattice consisting only of oxygen atoms. This would create a super-ionic phase. If the temperature is increased even further under this intense pressure regime, the oxygen atoms themselves may also become mobile and the lattice itself might melt into a new extremely dense unstructured phase of matter that no longer consists of water as we know it. The atoms are no longer chemically bound to each other. This is an exception to the simpler rule we started with - that the chemistry of substances does not change during a phase transition. As one goes deeper into almost any theory, rules based on simpler understandings tend to hit the roadside.

At temperatures below 2700°C and pressures above 48 million bar, a transition to a metallic ice phase is possible. Such intense pressure is expected to exist deep within icy Uranus or Neptune. In this phase, the structure of the ice would resemble stacked corrugated sheets of oxygen and hydrogen atoms, which would be electrically conductive.

Even more extreme pressures can be forced on matter, for example, inside the core of a rapidly collapsing star. What would happen to water under such conditions? As pressure is increased within a solid, individual atoms resist being pressed closer together. To explain this, we need to revisit the electron orbital. Hydrogen is a simple example to illustrate this. This atom (at ground state) has one electron in its 1s electron orbital. Any orbital can hold a maximum of two electrons, so 1s could also accommodate a second electron if there was one. An electron in another atom's orbital can temporarily occupy that role. If it is another hydrogen atom, then a covalent electron-sharing chemical bond is created, turning atomic hydrogen into hydrogen gas, H2.

An atom is a system, and like all physical systems, it tends toward the lowest free energy state possible. There are many forces going on in a hydrogen molecule. The two electrons repulse each other. The two protons repulse each other. Each electron is attracted to both proton nuclei and vice versa. There is an optimum distance between electrons and protons where all of these electrostatic forces add up to a lowest possible energy state. This is the most stable state. When atoms are squeezed closer together (or pulled further part), they resist and a force must be applied. Although perhaps easier to visualize, the electrostatic interactions that I've just described here are insignificant compared to much more powerful quantum interactions that respect the exclusion principle. Two or more electrons cannot share identical quantum numbers. If two electrons share an orbital they must be of opposite spin (spin is a quantum number). This exclusion principle translates into a repulsive quantum force that very powerfully resists additional pressure put onto an already extremely dense solid phase.

As pressure increases, the atomic orbital structure itself is forced to shift. Normally, electrons fill only a few energy levels in an atom. Many energy levels are unoccupied. Under extreme pressure, all the electrons are forced into the lowest energy level orbitals, as close to the nucleus as possible. This is ultra-dense electron-degenerate matter that is expected to exist in the core of a white dwarf stellar remnant. The exclusion principle means that two same-spin electrons won't share an orbital no matter how strongly they are forced together. Electrons, experiencing such intense repulsive quantum forces, respond by moving faster and faster in these lowest orbitals. Under enough pressure, they approach the speed of light, which is the limit of this arrangement. Protons begin to absorb electrons, creating a degenerate phase of matter, called neutron matter which is the densest state of matter possible. It is thought to exist in super nova stellar core fragments called neutron stars. Such transitions into extremely dense matter are technically phase changes as well, as is the final transition of matter into a black hole, in which matter as we know it collapses entirely.

Where Plasma Fits In As A Phase Transition

Where does plasma fit into this progression of phase transitions? The temperature of a substance or element is the measure of the average vibrational energy of its constituent molecules or atoms. Gas molecules generally have a great deal of vibrational energy as well as kinetic energy. They move around in all directions at great velocities, widely separated from each other. They occasionally bump into one another, and the force of these collisions translates into gas pressure. If a gas is cooled, it will eventually condense into a liquid at its critical point. Pressurizing a gas adjusts that critical point to a higher temperature. If you take another look at the phase diagram of water, you notice that water will boil into water vapour at just 50°C, rather than at 100°C if the pressure is 1/10th that of atmospheric pressure. At the other extreme, the temperature of water streaming from a deep sea hydrothermal vent can reach over 400°C but it does not boil because the pressure is over 300 times atmospheric pressure at depths over 3000 m, where most hydrothermal vents are located.

One way to make plasma is to heat gas. What happens if we heat a container of gas rather than compress it? We increase the total kinetic energy of the molecules. If we heat a sample of hydrogen gas enough, we will eventually supply enough energy to break the chemical bonds and dissociate the gas into hydrogen atoms. The bond dissociation energy of H2 is about 50,000°C but this temperature depends on the density of the gas. As density increases, the bond dissociation energy increases (due to two particles taking up more space than one). Now a mono-atomic gas, the hydrogen atoms can be further energized into ions. The ionization energy of hydrogen is about 150,000°C at 1 atm. This is also pressure-dependent. As density increases, the ionization energy decreases. As atoms are forced together, the gaps between electrons energy levels get shorter, which means less energy is required to ionize an atom.

Ionization means that the atom loses one or more electrons to create a gas that contains positive and negative ions. If we ionize hydrogen gas, we create a cloud of electrons and protons, called plasma. A very dense liquid-like plasma state in which protons are surrounded by a sea of mobile electrons, called metallic hydrogen, might exist deep inside Jupiter and Saturn. Because electrons are not confined to orbitals, this is also called a degenerate state of matter.

The process of ionization from a neutral gas into charged plasma is considered by many researchers to be the fourth physical phase after gases, liquids and solids. This phase transition, however, is a different and gradual, rather than abrupt, process. Atoms with more than one electron tend to lose electrons gradually as increasing energy is applied to a gas. The other three transitions - solid to liquid to gas - are each marked by an abrupt change in the arrangement of the molecules in a substance. In a transition from a gas into plasma, unlike the other three phase transitions, the substance also chemically changes because at least some molecular bonds are broken. For example, water vapour can ionize into hydrogen/oxygen plasma within a lightning bolt. The process of ionization into plasma is reversible, a characteristic in common with the three other changes of state. If energy is removed from the plasma, the ions will recombine into neutral atoms and molecules. However, in a complex mixture of gases, some new molecules might form as different ions react with one another.

The process of ionization takes place over a series of steps. We will go back to hydrogen as our example. As energy is pumped into atomic hydrogen gas, the kinetic energy of the atoms increases and some atoms become excited. An atom is excited through two possible processes: collisions with other atoms and the absorption of electromagnetic energy (light).

The electron in a hydrogen atom at ground state is located near the nucleus in a lowest possible energy orbital, which is actually a cloud of possible locations rather than a defined circular orbit. The electron in each hydrogen atom can absorb either kinetic or photon energy and move to a higher energy orbital. Electrons can even move into higher energy orbitals that are not ordinarily occupied by electrons. The atom is now in an excited state. If energy is removed from this system, the electron will return to its lowest energy (ground) state by releasing exactly the same energy as the energy difference between the orbitals. The energy is quantized. It is released as photons of specific wavelengths. For hydrogen, some of these photons are in the visible range. An excited hydrogen atom commonly emits red or aqua blue photons depending on the orbital drop, but it can also emit higher energy ultraviolet photons if it absorbs and releases more energy. As energy is added to the system, the electron continues to move up into higher energy orbitals until it eventually breaks free from the atom altogether. The proton nucleus can no longer hold onto the electron so it is now a completely ionized nucleus without any electrons. If an atom with more than one electron loses some of its electrons, but not all,  it is partially ionized. Our hydrogen atom is now a dissociated electron and proton. A gas of these ions is called completely ionized plasma, shown in the simple diagram below.

Spirit469;Wikpedia
In this simple diagram (left) of completely ionized plasma, all electrons have been stripped from their nuclei, creating an electrically conductive "electron sea."

Under real conditions, low-energy plasmas often contain a mixture of excited, partially ionized and neutral atoms, which can be used to emit a beautiful glow in a gas-discharge lamp, for example. The tube below, filled with diffuse low-energy hydrogen plasma, glows pink - a mixture of red and aqua blue photon emissions.

www.pse-mendelejew.de; Wikipedia
The plasma in the tube above contains mostly neutral and excited atoms and only a few ionized atoms. The degree of ionization in plasma, sometimes called plasma density or electron density, is the number of free electrons in a volume of plasma. Even a partially ionized gas which contains only 1% ionized atoms, can be considered plasma if it exhibits plasma behaviours such as responding to magnetic fields and conducting electricity.

The plasma in the tube above was not created by heating hydrogen gas until it ionized; this tube is at room temperature. Extreme heat is one way to create plasma. Hydrogen in the interior of the Sun and other stars is extremely high-energy completely ionized plasma because it is an incredibly hot environment. Any substance, if hot enough, will transition into plasma. Even the exotic solid states of water, which remain solid even up to 400°C, would eventually break down into plasma as temperature is increased. I should note here that this process is not chemical ionization of water, in which water (H20) self-ionizes into hydroxide (0H-) and hydronium (H30+) ions. That is a chemical equilibrium reaction, and no change of phase occurs.

Rather than through heat, the hydrogen plasma in the tube above was created by applying an electric potential to hydrogen gas. This is another way that energy is applied to a gas in order to ionize it. Instead of electrons being "shaken off" of an atom with high kinetic energy, electrons (which, remember, are charge-carrying particles) are drawn off the atom by a powerful electrical force. It is maybe analogous to ducklings being swept away from their mom down a stream with a powerful current. It can take tremendous energy to completely ionize a gas. It all depends on which molecules and elements the gas consists of and what kind of energy is applied. When we focus in on what happens to an ionizing atom, it is more accurate to talk about energy in term s of electron volts (eV) than in temperature, which is an average of particle energies. The most tightly bound (and stable) atom is helium. It has two electrons, and therefore two ionization energies - it takes 25 eV to remove just one electron (this partially ionizes the atom). It takes much more energy (about 54 eV) to strip both electrons off the helium nucleus (now the atom is completely ionized). If a collision with another atom strikes with over 54 eV of energy, the target atom can be completely ionized.

It takes far less energy to excite atoms rather than ionize them. Even a relatively small 120V household circuit can light up a small neon lamp, which is a vacuum tube filled with mostly neon gas and a little argon gas.

In gases, ionization is reversible. If the energy is removed from the system, such as turning off the applied voltage gradient (unplugging the hydrogen tube above), the protons recombine with free electrons, the atoms return to ground state, and molecules recombine into neutral gas (H2). In such a closed system of only a single atomic gas, the gas can be ionized and recombined, transitioned into liquid and solid states and back again, over and over, demonstrating an entirely reversible process, but under real conditions, the extreme energy of some plasmas can trigger various chemical reactions as well, particularly combustion reactions. These reactions are non-reversible and add a non-reversible component to the phase change.

In solids, in particular, the process is not easy to describe in terms of a phase change. An example here might be a fulgurite. Lightning strikes sand and leaves behind a hollow tube of glass buried in the ground. Lightning is an extremely powerful electric potential, of up to 500,000 volts. The melting point of silicon dioxide (pure sand) is 1710°C and the boiling point is 2230°C. The interior of a lightning bolt can reach a temperature of 28,000°C, far higher than the boiling point of sand, enough to chemically break it apart and at least partially ionize its silicon and oxygen atoms. When lightning strikes sand, some sand is explosively vapourized into gas and plasma, leaving a hollow tube, where the bolt struck, surrounded by a layer of molten sand that quickly solidifies into glass. If there were impurities in the sand, such as soil and plant debris, these components would have combusted and reacted with each other, resulting in new chemical compounds in the fulgurite's glass. Air molecules in the vicinity are also broken down into plasma temporarily. In this case, a series of very rapid phase changes has occurred from solid to liquid to gas to plasma, but there was also opportunity for chemical reactions to take place, which are irreversible. Even the air itself, which is a mixture of gases, does not transition to a plasma state and then back into neutral gases without some chemical changes taking place. Some highly reactive oxygen ions and excited oxygen molecules will recombine into ozone molecules, for example, creating the fresh-air smell after a thunderstorm.

Why isn't a neon sign or lamp hot? Neon lights might get warm to the touch but they never get hot. Yet we can think of the ionized atoms in plasma as hot. The atoms have at least enough energy to become excited and lose some outermost electrons. The plasma itself contains some free fast moving electrons with significant kinetic energy, but in the case of the low-density plasma in a neon lamp, it also contains mostly neutral atoms with far less kinetic and thermal energy. This means that its average temperature will be simply warm to the touch. A neon light contains very diffuse plasma and most of the neon gas remains in a neutral gas state that absorbs the excess energy of the electrons that collide with them. It doesn't take much energy to create glowing plasma in which only a few outermost electrons of atoms are excited and fewer still are stripped off, creating a small electrical current in the tube that sustains the excited-atom glow. Ordinary air is an exception. It is actually a powerful electrical insulator and would make a lousy plasma light. It will ionize and glow (this is a lightning bolt) only when it is subjected to a very powerful electrical potential of more than approximately 100,000V. The dielectric strength of air, its ability to withstand a potential gradient before breaking down or ionizing, is 3.0 MV/m (million volts/metre), which is much higher than neon's, which is 0.02 MV/m. Dielectric strength is an intrinsic property of a material.

Understanding Plasma is Essential To Understanding Our Universe

Plasmas, no matter how they are created, have unique physical properties. They act quite differently from neutral gases. Like gases, plasmas do not have a definite shape or volume. They can be compressed fairly easily. Unlike gases however, which are electrically neutral, plasmas respond to electric and magnetic fields. Even though the charges are usually balanced overall in plasma, they are separate and free to move. This means they can produce electric currents and magnetic fields and they respond to them as well. Electromagnetic forces exerted on plasmas act on them across very long distances. This gives plasma special coherent behaviours that gases never display. The Sun and other stars are examples of plasma created under extreme heat. Their interiors consist of extremely dense, energetic, and completely ionized plasma. Powerful plasma currents exchange heat released from ongoing nuclear fusion in the stellar core. These physical currents are also powerful electrical currents because they are moving charges. The moving charges set up incredibly intense magnetic fields that can interfere with one another and snap. These are the mechanisms behind the violent solar weather that can damage communications, satellites and electrical systems 150 million miles away on Earth. While stars are made of dense plasma, most of the visible matter in the universe consists of very diffuse highly ionized plasma. Both kinds of plasma are the key reason why we can see distant stars and various gas clouds. They glow because they are plasmas that contain atoms that are continually being excited and returning to ground state, emitting photons of light in the process. Intergalactic space, interstellar space and interplanetary space are also all (extremely diffuse) plasma. Solar wind is plasma and Earth's ionosphere in the upper atmosphere consists of diffuse atmospheric gases ionized by solar radiation, or plasma in other words. At one point, prior to recombination, the entire universe consisted of completely ionized plasma. It was too energetic to support intact atoms. Expansion (and therefore cooling) allowed that primordial plasma to settle into the first and most abundant simple atoms such as hydrogen and helium. For these reasons, some theorists think that plasma theory should play a more dominant role in understanding the cosmology of the universe, alongside general relativity, high-energy astronomy, mechanics and dynamics.

Plasma behaviour can be extremely complex. Understanding the complicated dynamics of stars, for example, requires computer modeling based on plasma theory, much of that theory belonging to the field of magnetohydrodynamics, a field of study that only got under way in last few decades. Understanding the electrodynamics of plasma (the behaviour of an electrical fluid) on the largest scales might also help to explain the evolution of galaxies as well as their birth from the collapse of interstellar clouds into stars, which is not yet completely understood. Plasma theory might also shine some light on the mysterious nature of dark matter, required to explain the rotation curves of galaxies. Black holes, quasars and active galactic nuclei must all incorporate plasma dynamics in order for us to fully understand how they work. Extreme plasma dynamics could even represent missing pieces in our understanding of cosmic inflation and the accelerating expansion of the universe, the latter of which is now called dark energy. An intuitive understanding of plasma could be a useful key to understanding the way the universe works.


Thursday, August 17, 2017

The Perils of Cosmic Radiation: Are We Made for Deep Space Travel?

We know we are intrepid explorers by nature. Our pop culture reflects that fact. We believe that someday we will overcome all technological odds to travel to, and set foot on, distant but promising exoplanets, perhaps somewhere where an alien ecosystem has taken root. Maybe that life is biochemically similar to us. It's only a matter of time.

There is a problem, however, one that has not been given much play in our sci-fi movies and books. We are not made for outer space. We are extremely delicate organisms that have evolved within a pocket shielded from deadly solar wind and even more violent cosmic radiation. We live inside a thick envelope of gas surrounded by a powerful planetary magnetosphere, which in turn is enveloped in an even more powerful and far-reaching stellar magnetosphere. These powerful magnetic envelopes deflect most harmful radiation away and our atmosphere does a thorough job of absorbing what still makes it through. As a result, our bodies are exposed over our lifetimes to just a miniscule fraction of the fast atomic fragments that bombard every square meter of deep space.

Space is not the empty, cold, benign backdrop portrayed in most movies. It is a nuclear blast zone. Stars like our Sun are ongoing fusion reactions sloughing off electromagnetic radiation, protons, electrons, neutrinos, and small atomic nuclei in every direction. Almost infinitely more powerful stellar explosions and collisions, happening all the time across the universe, blast matter away at almost the speed of light. Nothing slows these deadly particles down as they fly across the light-years. An astronaut's suit or spaceship hull stands little chance against this constant cosmic onslaught. No currently available material is strong or dense enough to absorb or deflect cosmic radiation while also being lightweight enough to be launch-able. Any ship-wide magnetic deflection shield powerful enough to work will require an enormous supply of energy during the decades it will take to get to even a relatively nearby exoplanet is also a distant, if not impossible, technological dream.

Despite the inhospitable nature of space, we have inhabited it, at least close to home where we have some protection from cosmic radiation. The International Space Station (ISS) is an artificial biosphere that takes care of the physical needs of a few humans at least on the scale of many months. On its predecessor, Mir, Valery Polyakov, a Russian astronaut, spent 437.7 consecutive days in space during 1994/1995. What we might forget is that this is time spent in space orbiting close to Earth. Mir, for example, maintained a near-spherical orbit of between 352 km and 374 km above the surface of Earth, within the second most outer layer of Earth's atmosphere called the thermosphere, which is just above the ionosphere, which forms the inner edge of Earth's magnetosphere. At this altitude the density of air is extremely low so there is no atmospheric protection from solar or cosmic radiation. Air here is so diffuse that a single molecule of oxygen would have to travel on average one kilometer before it collided with another molecule. However, Mir was well within Earth's thick protective magnetosphere. Even at its narrowest region, where it is compressed by incoming solar wind on the side of Earth facing the Sun, the magnetosphere is about 60,000 km thick.

How does atmospheric cosmic and solar radiation protection work? Our atmosphere is transparent to low frequency electromagnetic (EM) radiation emitted from the Sun. Sunlight, for example travels through air to bathe us on the surface. High-frequency EM radiation from the Sun, such as X-rays and gamma rays, is absorbed by the plasma (consisting of electrons and electrically charged atoms and molecules) in Earth's ionosphere. In addition to EM radiation, the Sun also emits particles in all directions, most of which are protons, and they are traveling very fast, about 400 km/s. Earth's rotating metallic core generates a powerful magnetic field that deflects most of these charged particles away from the surface. Our atmosphere is also opaque to this radiation. It absorbs what isn't deflected. Consider Mars for a moment. It possesses neither a deflecting planetary magnetosphere nor a thick highly absorptive atmosphere. Its thin carbon dioxide rich atmosphere provides some radiation protection, but not much. It is roughly 2.5times less protective than Earth's very thin thermosphere through which ISS orbits. Although its surface at the equator approaches a livable temperature,  the continuous bombardment of radiation makes it more inhospitable than you might think.

While astronauts on Mir and now the ISS have very little atmospheric radiation protection, they are protected from solar wind by Earth's magnetosphere. Thanks to the Sun's magnetosphere they are also mostly protected from far more powerful radiation - gamma rays and particles with far higher kinetic energy than any solar wind particle. Yet even here close to Earth, astronauts can only stay on ISS for a limited time. A very small amount of cosmic radiation makes it through our solar magnetosphere, meaning that the astronauts do receive a low but cumulative dose of cosmic radiation. A  2014 study by radiation expert Francis Cucinotta indicates that ISS astronauts exceed their lifetime safe limit due to cosmic radiation in just 18 months for women and two years for men. 

Protons (hydrogen nuclei) and, to a lesser extent, larger atomic nuclei (such as alpha particles and helium nuclei), traveling near the speed of light which is 300,000 km/s, pervade every square centimeter of deep space. The ISS would never be feasible in this constant blast zone. However, protected within the magnetospheres of the Sun and the Earth, an aluminum hull just a few millimetres thick shields about 95% of the radiation that strikes it. Even thick plastic stops this radiation, which consists mostly of EM radiation along with relatively low-energy solar protons (averaging 400 km/s) that manage to pass through Earth's magnetosphere. We have to say average velocity here because the Sun isn't static. Magnetic storms sometimes rage across its surface, accelerating particles up to 3200 km/s, in the case of exceptionally violent storms called coronal mass ejections. A coronal mass ejection is a mass of highly magnetized plasma, a chunk of the Sun, hurled off into space when a magnetic field snaps. Velocities up to 3200 km/s have been recorded by the LASCO onboard the SOHO satellite orbiting between the Sun and Earth. Even this accelerated plasma, which would devastate Earth's electrical and communications systems, has nowhere the particle-to-particle punch of cosmic radiation, with velocities close to 300,000 km/s.

As mentioned, materials are still in the process of being developed that are both light and effective at shielding protons traveling close to light speed. Most cosmic radiation comes from supernovae. It consists of stellar particles violently spewed in every direction when a high-mass star reaches the end of its life. The shock wave of the explosion is powerful enough to accelerate material to near light speed. Only in extremely powerful accelerators can particles on Earth approach such velocities.

Radiation itself can be a confusing subject but at its core the concept of radiation is simple: it is an emission or transmission of energy. It comes in four basic kinds: electromagnetic (EM) radiation (radio waves, visible light, gamma rays etc.), acoustic radiation (sound waves, seismic waves), gravitational radiation (gravitational waves) and particle radiation (on Earth we typically deal with alpha radiation – alpha particles, beta radiation – electrons, and neutron radiation – neutrons). In terms of cosmic radiation, we are especially interested in protons, alpha particles and, to a much lesser extent, larger atomic nuclei.

Radiation is either ionizing or non-ionizing. With the exception of microwaves, sonic devices and intense or prolonged light exposure that can cause photochemical burns, non-ionizing radiation tends to present a minimal hazard to human health. This radiation simply doesn't have enough energy to ionize atoms in living tissue. I will explain what "ionize" means in a moment. Generally, a particle or wave must carry more than 10 eV (electron volts) of energy to ionize atoms and therefore damage biochemical bonds in molecules, but the line is blurry because some atoms ionize more easily than others do and some chemical bonds break more easily than others do. Consider the energy of a visible green light photon of about 2 eV. 2 eV is harmless (unless those 2 eV photons are concentrated into a laser. Then they can burn your retinas and blind you). Radiation of 10 eV or more, however, has enough energy to strip electrons off of (ionize) atoms and molecules, breaking chemical bonds between them in the process. This radiation can be harmful and even lethal to humans. Short wavelength (very energetic) EM radiation such as X-rays and gamma rays can break chemical bonds in DNA for example, creating genetic damage that can eventually lead to cancer. It can also damage biological proteins and enzymes, impairing cell function. All particle radiation from radioactive materials or in the form of solar radiation and cosmic radiation is ionizing. It causes biological damage at the microscopic level in our bodies. That said, The National Cancer Institute in the United States explains why radiation can sometimes be good thing. Targeted at cancer cells, which are dividing uncontrollably, the ionizing radiation damages their cellular DNA so severely that the cells program themselves to die, shrinking the tumour.

Most cosmic radiation consists of ultra-fast protons, and most of them are blasted out of exploding stars. A proton, a particle normally confined inside an atomic nucleus, is an exceptionally tiny object, weighing less than 2 x 10-27 kg. Despite its miniscule mass, a proton blasted from its atom and accelerated to near light-speed (now we call it a relativistic proton) packs a catastrophic punch, around 1 GeV (G means giga or billion electron volts). Compare this to the low end of ionizing radiation at 10 eV. This force is roughly equivalent to a baseball being thrown at you hard but with all of that impact condensed into an area about one millionth of a nanometre wide (a proton is about 10-15 m in diameter).

You might expect a single proton at such high velocity to fly right through your body, causing minimal damage along its sub-microscopic course. After all, a general rule in physics is that the higher the kinetic energy of a particle, the smaller the fraction of its kinetic energy tends to get deposited in the material. The problem is that the proton will, more than likely, glance off atoms in your body along the way. A homemade analogy here might be that while a massless "slender"  gamma photon elegantly dances through the atoms, a massive proton acts like a bull in a china shop. Those atoms in the proton's path will be ionized, and they will ionize other atoms and so on, depositing energy along an ever-widening path of damage in the body. This process is technically called linear energy transfer (LET), which I will explain in a moment. An astronaut onboard the ISS might by hit by only a few cosmic protons during a months-long mission but new research indicates that even low-dose infrequent cosmic radiation exposures can cause significant long-lasting damage, particularly to the brain. An astronaut on route to Mars or on the surface of Mars, with only the Sun's magnetosphere as protection (Mars doesn't have one), will receive a much higher exposure to cosmic radiation than an ISS astronaut. Of most concern is the cumulative damage to our fragile and very slow to heal brain.

Cosmic radiation is not a new discovery. Scientists have known about it for many decades. In fact, in 1909, Theodor Wulf discovered that the rate of ion production inside a sealed container of air was higher at the top of the Eiffel Tower than at its base, refuting the then-current theory that radiation originated from radioactive elements in the ground. A few years later Victor Hess launched ionization-measuring electrometers on a balloon during a near-total solar eclipse. His results ruled out the Sun as the source of this radiation and confirmed Wulf's data.

Primary cosmic radiation, originates outside the solar system and some of it comes from outside the Milky Way galaxy. About ¾ of cosmic radiation consists of protons (hydrogen nuclei). About a quarter consists of heavier alpha particles (helium nuclei) and about 1% consists of still heavier nuclei, mostly lithium, beryllium and boron nuclei. These heavier so-called HZE (high atomic number and energy) ions, though far less abundant, are especially damaging. Though very sparse even in deep space where there is no magnetic shielding, these infrequent impacts would contribute significantly to an astronaut's overall radiation dose. 

A radiation dose is measured as an absorbed dose. The SI unit is the gray (Gy). It measures the radiation's energy deposited in the body. Radiation is also measured by the action upon the matter of the body, by its linear energy transfer (LET). This means it measures the amount of energy lost per distance traveled. Two equivalent absorbed doses can do vastly different amounts of damage in the body depending on their LET values. A high LET means that the radiation leaves behind more energy and therefore causes more atoms to ionize in its wake. A higher mass particle, such as an alpha particle, will leave a track of higher ionization density than a proton, if both are going the same velocity when they strike the body. A gamma photon will have a far lower LET value yet. The chemical make-up of our cells also plays a significant role determining ionization damage. Low LET radiation, such as X-rays or gamma rays, ionizes water molecules inside cells. It breaks them up into H+ and OH- ions (also called radicals). It does this damage over a long track through the tissue so it tends to leave one event per cell. The often single H+ and OH- ion pair simply recombines to form water once again, releasing some energy in the process. When ionization occurs over a shorter wider track, many H+ and OH- ions can form within each cell. A pair of OH- ions near each other can recombine into H2O2 (peroxide) instead. This molecule causes additional oxidative damage to proteins, lipids and to DNA in the cell on top of the ionization damage to various biological molecules. Dense clusters of high LET ionization damage in the body means that cosmic particle radiation is extremely dangerous to astronauts, especially to their brains as we will discuss below in more detail.

Radiation Units: A Frustrating Labyrinth

Articles about radiation can be very difficult to grasp because so many unfamiliar units are thrown around seemingly at random and sometimes interchangeably. A large part of this confusion stems from the use of both "old" American units and SI (standard or metric international) units. Here in North America the switch over to SI has been particularly slow in the field of nuclear science in part because the stakes are so high. Even a small conversion error can lead to dangerous radiation exposures. In addition, a number of different units must be used to accurately describe radiation as it travels from its source, through the atmosphere or through space and then into our bodies. The rate of emission of radiation from its source is measured in curies (old) or becquerels (SI). Sometimes radiation emission is measured in terms of emission energy instead of rate. In this case it is measured in electrovolts (eV) or joules (J, an SI-derived unit). I used eV earlier to compare ionizing to non-ionizing radiation energy. Once the radiation is emitted and is now ambient, its ambient concentration is measured in roentgens (old) or coulombs/kg (SI). Once ambient radiation strikes a living body or other object, the raw amount that object absorbs is measured in eitherrads (or just rads; old) or grays (SI). I gray is equivalent to 100 rads. Rems (old) and Sieverts further complicate the picture. These units measure the effective dose, or dose equivalent, in other words the biological harm caused by radiation in living tissue. Rather than describing a radiation disaster in terms of rads, for example, seiverts or rems can offer a better description of how the radiation exposure will affect human health. This measurement reflects the different LET values of different kinds of radiation as well as the kind of tissue receiving the dose. Some body tissues are more sensitive to radiation than others. This measurement of dose equivalent is still an inexact science, however, as there are so many not entirely known biological factors to consider. Randomized experiments that test radiation damage to living human tissues are, of course unethical. Animal study results are sometimes difficult to extrapolate to humans because their bodies, tissues, and physiologies are different. We also still don't know much about how our human tissues react to various kinds of radiation exposures, especially from cosmic radiation.

The Edge of a Mystery: The Brain and It's response To Radiation

Scientists once assumed that brain tissue is less sensitive to radiation damage than other tissues because brain cells tend to multiply at a much slower rate than other cells in the body, such as gut and skin cells do, for example. Cells that divide less often spend less of their time in the process of division. The DNA in a quiescent cell is tightly coiled into a dense structure called chromatin. This structure is very stable and resistant to radiation damage and it offers a very small target as well. When a cell starts the process of dividing, its DNA must unravel so the machinery of DNA synthesis can replicate the entire genome. The DNA in this state is highly susceptible to damage. Luckily, cells have mechanisms that detect and repair DNA damage when it occurs, but if the radiation dose is high enough, the dividing cell can't fix all the damage before it completes replication. It will then either program itself to die off or it will pass on the DNA damage as mutations.

Based on instances of acute radiation exposure studied after radiation accidents and war, tissues in which cells multiply most rapidly such as those in the gastrointestinal tract, the spleen and bone marrow, tend to present symptoms of radiation damage (radiation sickness) first. These assumptions are based on whole-body one-time exposures to an absorbed dose of between 6 and 30 Gy. Neurological symptoms (including cognitive defects) typically only manifest after a dose higher than 30 Gy occurs. This is a catastrophic dose; the victim will die within two days. There is a fascinating, if sobering, chart of whole-body dose effects here. Scientists are discovering that the effects of acute exposure cannot be extrapolated to the effects of sustained low-dose exposure, especially long-term effects. The results don't take into account cellular damage that takes weeks, months or years to manifest. More importantly, different tissues in the body deal uniquely to long-term radiation exposures with different LET values in complex ways that are still poorly known. The study we are most interested in here (explored in detail below) considers both Gy and LET values as indicators of damage, in this case, to brain cells. The results are based on both behavioural changes and structural tissue changes in mice exposed to radiation that mimics cosmic radiation.

Until a few years ago, NASA and other space agencies only suspected that long-term cosmic radiation caused cognitive impairment in astronauts. This suspicion was largely based on clinical data from cranial radiotherapy and radiation treatment for brain cancer and then comparing that data to before/after cognitive test results of astronauts. The extrapolation from clinical data to cosmic radiation exposure, as they knew, was problematic, much like comparing apples to oranges. A typical daily dose during cranial radiotherapy is about 2 Gy. Compare this to a far lower roughly 1/5000th daily dose of radiation of around 0.48 mGy expected for an astronaut during a round-trip and stay on Mars. Adding that exposure up for a 300-day trip, for example, still adds up to just 1.44 Gy, less than the dose of a single cranial radiotherapy treatment. Travel to Mars shouldn't be a problem right? The trouble with comparing the two doses is that they have vastly different LET values. In the clinic, X-rays and gamma rays are most often used. These energetic EM photons are not nearly as densely ionizing as cosmic particle radiation because a massless photon particle has far less momentum than a particle of mass traveling at nearly the same velocity.

To really understand how long-term cosmic radiation will affect astronauts during trips to Mars or on possible future deep space missions, you need direct experimentation, using relativistic particles. How would you design an experiment to do this? Charles Limoli, a neuroscientist and radiation biologist at the University of California School of Medicine, has taken some of the first steps in answering the question. His research findings to date can be found in the February 2017 issue of Scientific American. It's an excellent read that inspired me to write this article. He explains not only the radiation problems that current astronauts face based on current research findings, but he also outlines the challenges of getting the data scientists will need to plan future deep space travel. A reference paper for this article called What Happens to Your Brain on the Way to Mars by Vipan Parihar et al. provides a good background for this research. Here I try to provide a glimpse into his work, and offer some insight into the challenges and excitement of designing new experiments, into the process itself. The findings are preliminary so far but they are quite stunning. Undoubtedly a lot of future work will build upon it.

In this case, mouse brains are used as living models for astronaut brains in deep space. There are three significant challenges to this approach. First we have to ask if a mouse brain is similar enough to a human brain in terms of structure and function to make a valid model. We also have to ask if a mouse brain reacts to radiation the same way a human brain does. Rats are used as extensively as mice in neurological studies. There is a mountain of neurological data using either species, suggesting that they are reliable and useful models. I wondered, though, if rats would make a better model in this case. That turned out to be an interesting question. A 2016 research paper by Bart Ellenbroek and Jiun Youn explores the differences between rats and mice and how reliable they are as models for human brain behaviour. The species differ in their behaviour, as anyone who has worked with both species knows. For example, rats tend to be more comfortable with lots of human physical contact while mice can be stressed by similar attention. Unexpected stress on an animal could affect the results of behavioural tests. The researchers also point out that, while both rodent brains are very similar to human brains, there are fundamental differences between them that could affect the reliability of test results. Limoli's research presents a unique physical limitation on what kind of animal model you can use – it has to fit into the accelerator target area.

An additional thing to keep in mind is that the brain is still not fully understood. Neurology, neuroscience and psychiatry are very active fields of research. Still, at least one basic fact about the brain seemed to be firmly established: the brain contains two basic types of cells with neutrons performing the twin star roles of function and structure and with glial cells playing a supporting role. This is called the neuron doctrine. Research over the last few years calls these assumptions into question. For example, a  2013 Scientific American blog post by Douglas Fields points out an unexpected but key difference between mouse brains and human brains that suggests that glial cells are far more involved in function than researchers realized.  It was assumed that glial cells can't do any electrical signalling. Until now they've been thought of simply as physical and physiological support cells for neurons. The researchers transplanted human glial cells into mouse brains and discovered that these mice soon significantly surpassed their untreated siblings in both memory and learning. Somehow human glial cells imparted an improved, perhaps more humanlike, cognitive ability into a mouse mind. A specific type of cultured human glial cells called astrocytes are in fact much larger and have a more variable morphology than mouse astrocytes. It is a clue that these cells might be involved in the evolution of human intellect. Researchers now know that glial cells not only propagate calcium signals over long distances but they also form electrically coupled synchronized units through gap junctions (similar to how heart cells are synchronized to contract during a heartbeat).

The second question is how do we reliably test cognitive function in mice before and after radiation exposure? Fortunately mice have been bred and used extensively for various kinds of cognition and memory research for decades. There is lots of data to draw from as well as a large collection of reliable cognitive and memory test protocols available to use. This 2015 compilation paper by SM Holter et al. provides an overview of those tests. Third, how do we expose our test animals to specific doses of relativistic particles? A tremendous amount of energy must be used to accelerate particles to nearly the speed of light. Few natural mechanisms outside of stellar explosions can do the job. The only way to do this in a lab is to expose the mice to radiation inside a particle accelerator. Fortunately, NASA Space Radiation Laboratory, commissioned in 2003, is designed for exactly these kinds of experiments. Radiation consisting of a variety of particles with a range of very high energies is specifically designed to resemble cosmic radiation.

Out of practical time constraint limitations, the mice received a single dose of radiation equivalent to many months to years of actual cosmic radiation exposure according to Limoli's article. The article states a dosage of either 30 cGys or 0.3 Gy was used, which is very low, approximately 50 times lower than an expected round trip to Mars radiation dose. Would an astronaut's brain, exposed to the same amount of radiation but spread out over many months, have enough time to repair the damage between intermittent particle exposures? Researchers are not sure but the results of this experiment are not promising as we will see in a moment. This research is an essential first step to find out just how big a problem long-term cosmic radiation exposure could be for future astronauts.

A healthy brain neuron consists of a soma (cell body) which contains the nucleus and other organelles, dendrites (branched projections) and an axon (a long slender electrically conductive projection). See the diagram below. It connects to other neurons through specialized connections called synapses. 


(Wikipedia public domain)
One neuron can contact another neuron's dendrite, soma or, less commonly, its axon via a synapse. At the synapse an electrical signal is converted into a chemical signal by the release of neurotransmitter molecules into the gap between two cells. The neurotransmitter initiates an action potential in the connecting neuron. See the enlarged box in the diagram above. One neuron can connect to many other neurons to form complex neural networks in the brain.

To start, a healthy mouse explores toys in a box. Over a period of hours and days, its brain physically changes as it learns and forms memories. The neurons in its brain form new dendritic branches and trees and create new synaptic connections. Dendritic branching can be very extensive and complex. A single neuron can receive as many as 10,000 dendritic inputs from other neurons.

The toys in the box form part of a task that evaluates a mouse's cognition and memory abilities. In this task, called the novel object recognition task, a mouse is placed in a box containing toys to explore. After exploration for a fixed amount of time, the mouse was then removed from the box and the locations of the toys are changed and some are replaced with new toys. The mouse is returned minutes, hours or days later to the box to explore the novel landscape. A healthy mouse is curious - it will quickly notice changes and spend extra time exploring those changes. The time spent on checking out new things compared to overall time present in the box is that mouse's discrimination index, a reliable measure of its memory and learning ability. Memory and learning takes place primarily within the brain's prefrontal cortex and hippocampus. Limoli and his group discovered that a single dose of low-level cosmic-like radiation exposure greatly reduced the mice's discrimination index values. The deficit stood out in stark relief when irradiated mice were given the same follow-up tasks as healthy mice. Their curiosity was greatly diminished. They didn't seem to recognize changes made to their environment.

Six weeks after a single exposure of either 5 or 30 cGy of radiation, both very low doses representing sparse particle impacts, the performance of the mice dropped on average by a whopping 90%, regardless of dose. Furthermore, they also found that these impairments lasted 12, 24 and even 52 weeks after the exposure, suggesting that the damage to the mouse's brains didn't heal, at least within a year after being damaged.

The researchers confirmed the physical damage to the brain by imaging sections of the medial prefrontal cortex of healthy non-radiated mouse brains and of irradiated mouse brains. The imaging data revealed significant reduction in dendritic branching as well as a significant loss of dendritic spines. A dendritic spine is a tiny protrusion from the main shaft of the dendrite that contains the synapse that allows the dendrite to receive signals. If dendrites are the branches on the brain "tree," then spines are the tree's leaves, as Limoli describes it in his article. These structures are very plastic. They undergo constant turnover in a healthy brain. The growth of dendritic spines reinforces new neural pathways as an anatomical analogue of learning. They also maintain memories. Environmental enrichment (providing lots of learning opportunities) leads to increased dendritic branching, increased spine density and an increase in the number of synapses in the brain. Cosmic radiation exposure not only undid the changes associated with learning and new memory formation. It severely impaired the normal (baseline) cognitive function of the brain as well.

Only the medial prefrontal cortex, a region known to be associated with learning and memory, was imaged but it seems reasonable to assume that the radiation attacks the physical and functional integrity of synaptic connections across the entire brain.

Conclusion

This is disconcerting news to those of us who dream of mankind eventually traversing the universe to explore other planets and moons in person. Imagine the spectre of our best and most brilliant men and women gradually losing their cognitive abilities, losing their memories and themselves as they travel through deep space. Cosmic radiation would impair astronauts in the most critical way. The skills and mental acuity required to deal with maintenance issues and sudden problems as they arise during a long-term space flight are what set astronauts apart from the rest of us.  Stasis would be no solution, unless it is inside some as of yet undiscovered material that can block out cosmic radiation. The fact is, we evolved inside a layered magnetic cocoon where the threat of cosmic radiation doesn't exist. We'll have to rely on another facet of our evolution, our ingeniousness, to get past this hurtle.

Wednesday, February 15, 2017

BOSE EINSTEIN CONDENSATE

This amazing state of matter was mentioned years ago in my Very Hot, Very Cold article (2011). At first a curiosity, now scientists are starting to see that these condensates may not only pry open the inaccessible quantum world for us but they might offer us a wide variety of new breakthrough technologies as well. Meanwhile the techniques used to create and maintain matter in this ultra-cold condensed quantum state have steadily improved. Researchers at the University of Alberta (my old "home") just created the coldest Bose Einstein Condensate (BEC) ever, at just 40 billionths of a degree above absolute zero! What happens to atoms when their motion literally begins to freeze? This great little 1-minute video offers a glimpse into what a BEC is:



WHAT IS A BOSE EINSTEIN CONDENSATE (BEC)?

Some Background First

Just like a solid, liquid, gas or plasma, a BEC is a state of matter. We come across three states of matter - solids, liquids and gases - every day. Life on Earth relies on the fact that one substance in particular, water, exists in these three states within a relatively narrow temperature range. What determines the state of matter is the average kinetic energy of the atoms that make it up. Generally speaking, atoms are most tightly packed within solids, less so in liquids and farther apart still in gases, but when collections of atoms are subjected to pressure this trend can reverse. As you might know, increasing the pressure of a gas, liquid or solid will also increase its temperature. Solids, depending on the kinds of atoms making them up, are not very compressible but they will eventually compress under sufficient pressure into a dense liquid state when the internal energy increases enough to break apart the inter-molecular bonds that make the solid stiff. The resulting very dense liquid will compress into a very dense gas and the gas, a much more compressible state, will compress further into an extremely dense example of a fourth state called plasma. Not all plasmas are hot and dense like this, but the Sun's hydrogen/helium plasma is a good example. Plasmas can also be diffuse cold ionized gases such the those that populate interstellar nebulae. Excited atoms in plasmas emit electromagnetic radiation. You can witness the light emitted by plasmas when you see sunlight or a lit neon sign. In the case of sunlight, pressure and heat are both at work. In the case of the neon sign, atoms in the plasma are excited by an electrical current. Atoms in a plasma state have so much energy they can no longer hold onto their outermost electrons. They are in an excited state, creating a separation of electrical charge.

Under even more extreme pressure and heat, additional exotic physical states are possible, such as electron degenerate matter inside white dwarf star remnants (our Sun is destined to become one eventually). Atoms in this exotic state are crushed by pressure. Nuclei have lost all of their electrons and a high-energy dense sea of negative charge surrounds them. As pressure is increased further, neutron degenerate matter forms. In this case atoms are crushed into an ultra-dense neutron sea, the strange stuff of neutron stars, pulsars and magnetars. Electrons are so energetic that they combine with free protons to create additional neutrons. Increasing pressure further theoretically produces the densest state of matter possible - quark matter. In this state, neutrons are crushed into their normally confined component particles - quarks. Above this pressure, matter is completely crushed in the infinite gravity well of a black hole - a state, in which matter at the atomic scale can no longer be described using our current theory of quantum mechanics.

Just as there is a maximum threshold of energy above which atomic matter as we know it can't exist, there is a minimum threshold of energy below which atoms no longer behave in the ways we expect. Atoms become excited when their energy increases. In an analogous way, atoms become "de-excited" when their energy decreases. As atoms approach absolute zero, they become sluggish and ultimately condense into an additional physical state called a Bose Einstein Condensate (BEC). Consider a balloon filled with steam, the gaseous state of water. Cool it down to room temperature and it will contain a small puddle of water inside it. Put it in a freezer and that water will solidify into ice. It's easy to visualize the atoms in steam moving around fast and bumping into each other, or atoms sliding around one another in liquid water, but there is no visible motion within a block of ice. Yet, undetectable to our eyes, there is. As the water freezes into ice, the atoms get close enough and slow down enough to form attractive chemical bonds with each other. In the case of water ice, they create a three-dimensional lattice. The type of bond arrangement depends on the kinds of atoms involved. No matter what the solid material is, the bonds hold the atoms more or less in place but they don't stop the atoms from jiggling about, like a runner jogging in place. This jiggling or oscillating motion, averaged over the material, is what we perceive as its heat or temperature. Lets say we cool our balloon down much further inside a special box that removes energy. The oscillations, on average, will slow down. Eventually, the ice will theoretically get so cold that the atoms no longer oscillate at all. At this point it has reached absolute zero, a temperature measured as - 273.15°C or - 459°F or 0K. I say theoretically because it is not possible in practice to remove all the energy from a system (we are treating our balloon as a physical system). Scientists are finding ways to get very close to absolute zero, and as they do, matter begins to act very strangely. In theory, the ice in our balloon could transform from a solid into a BEC, and when it does it will exhibit some very interesting properties.

Now To the BEC Itself

Exotic states such as degenerate matter, which is thought to exist inside stellar remnants, represent the highest energy extreme of atomic matter, while BEC's represent matter's lowest energy extreme. Degenerate matter cannot be directly studied in the lab. Its creation would require an enormous input of energy that would be impossible to achieve and maintain. The gravitational pressure of an entire star is required. Unlike degenerate matter, which cannot be lab-created, no BEC should exist naturally at all. This state of matter is made in-lab only. Even the coldest atomic gas clouds in deep space are a million times too warm, at about 3K, to harbour any BEC matter. A temperature above even just a few microkelvins will disrupt the quantum mechanical quiescence of a BEC and transform it back into its original gas state. But why would such a cold state be a gas and not a solid? More on this to come.

Each particle in matter is described mathematically as a quantum wave function. Click on this HyperPhysics link to get an idea of what these kinds of equations looks like. Any matter particle exhibits wave/particle duality. It can act like a particle and like a wave. In a BEC, particles act completely like waves. These matter waves, as they are called, stretch out as the atoms slow down. They start to overlap one another, and eventually they completely overlap into one large matter wave. At this point the gas is condensed into a BEC state.

Though predicted decades earlier by Albert Einstein and Satyendra Nath Bose, the first BEC wasn't created until Eric Cornell and Carl Wieman succeeded in doing so in 1995. Hydrogen atoms were the logical first choice but providing the right conditions for BEC formation proved too difficult. Helium atoms were turned to next but they presented too many challenges as well. Instead, rubidium was the first successful BEC. Two techniques, lasers and evaporative cooling, which will be described in detail later on, were used to cool a diffuse cloud of rubidium atoms into a BEC state. The two men received the 2001 Nobel Prize in physics for their success in creating this new state of matter.

I mentioned that the universe is far too warm for natural BEC formation. It is possible, however, that all of the matter in the universe could very slowly transform into a BEC state as the universe continues to expand, and therefore, cool. Right now the universe is still "warmed" by radiation from the Big Bang. What were once the highest energy gamma rays possible are now stretched across the expanding universe into low energy microwaves, and they will continue to stretch into extremely long and imperceptibly weak radio waves. At this point, interstellar atomic gas clouds in the universe might be so cold that those atoms will begin to condense into BEC's.

We live within a very narrow range of temperatures, from about -50°C (for a few minutes) to about +50°C (for a few hours) with about 20°C being most comfortable. In this range, hydrogen is always a gas; iron is always a solid and so on, but every element can exist as a solid, liquid, gas and plasma, with each state having its own unique physical and chemical properties. As atoms are energized further, they lose their chemical and physical identities altogether when the nuclei themselves are torn apart. Although practical research is still at a young stage, BEC's also lose some of their original classical chemical and physical properties. In this case, a group of atoms takes on the properties of one single atom, and their normally hidden quantum nature reveals itself.

So, what does this look like? We can visualize what's going on by looking at how the atoms fall into a single lowest-possible energy state. The graphs below plot the energy distribution of a gas of rubidium atoms. At room temperature, the energy levels of the atoms (measured as their velocities) would be spread across a wide range. They would be evenly distributed across a grid like the ones shown below and it would be entirely red. Energy density increases from red, yellow, green, blue to white, the highest density. The three graphs below illustrate, left to right, an already very cold gas cooling into a BEC state. The change in energy distribution below is calculated using Bose Einstein statistics, which we will investigate further later on. The middle graph shows the energy distribution just before the appearance of a BEC and the graph on the right is of a nearly pure concentrate (notice the reduction in yellow around the peak). At this point, nearly all the atoms have condensed into identical lowest accessible (ground) quantum energy states, contributing to the peak density at the centre. Some researchers call this state a super atom, not to be confused with "superatom" which not a BEC but a cluster of distinct atoms.

Image created by NIST/JILA/CU-Boulder
A VARIETY OF BEC's HAVE BEEN MADE

The BEC state has been observed in very cold gases, in very cold liquids such as helium, and even within solid materials, in a special form. In addition to rubidium, lithium and other elements have been used, as well as a variety of molecules. Non-matter particles that have unique properties (these are boson particles such as quasiparticlescan also condense into BEC's. These particles do not form atoms. They can be thought of as localized excitations in an energy field. They include polaritons. A type of quasiparticle, polaritions are half matter/half light particles that will condense into a BEC state inside a solid semiconductor under the right conditions. Quasiparticle BEC's, like this example, can form at a much higher temperature than other BEC's do, at about 19 degrees above absolute zero. This makes them an interesting research focus because their unique BEC properties might have practical applications at temperatures that aren't too hard to achieve. Polaritons can also be created in gas BEC's, and as we will see later on, they play a key role in how light interacts with atoms in the BEC state. In 2010, researchers reported in Nature that they were even able to confine and condense photons (particles of light) into a BEC. Trapped in a "white box," blackbody radiation photons begin to act like a two-dimensional gas of massive bosons in a BEC state (photons are massless force-carrier bosons). As we will see, atoms also act like massive bosons in a BEC (in a three dimensional gas). It is fascinating that massless force-carrying bosons (photons) and matter particles (atoms) can be nudged into an identical quantum state. It is yet another hint at how closely light and matter interact with one another.

BEC's ARE RULE-BREAKERS

Normally, atomic matter doesn't act like boson particles of force. The particles follow very different rules. Atoms won't overlap in one spot. That's why even matter crushed down to a neutron star still takes up space. But photons, for example, can and do (think of constructive and destructive interference of light). Only when atomic matter is very cold will it fall into a single ground energy state (into a single matter wave), and when it does, it breaks a fundamental rule of matter called the Pauli Exclusion Principle (PEP). Matter particles are called fermions. Two or more identical fermions (electrons, protons, neutrons or composite particles such as atoms) cannot share the same quantum state at the same time. This energy distribution rule of atoms is laid out by Fermi-Dirac statistics from which the PEP is derived. It means that at most just one particle can occupy one quantum state in a system. Notice that this is NOT what you see happening in the graphs above. The atoms as matter waves are falling into the same energy state at the same place at the same time. The PEP is being broken.

This rule holds until the atoms approach BEC critical temperature. At this point, the atoms are quiet enough to fall into a single lowest possible energy state, breaking the PEP and now obeying Bose-Einstein statistics instead of Fermi-Dirac statistics. Bose-Einstein statistics describe the energy distribution of bosons (particles of force such as photons, and W and Z bosons of the weak force). Under these statistical rules, particles are described mathematically as symmetric wave functions (rather than fermionic asymmetric wave functions) and that means they can overlap. In a BEC, matter particles act like bosons.

They become one big single wave function. It is still atomic matter, but it is overlapped in one location. A BEC trapped in a magnetic bowl looks like a tiny spherical cloud with a dark dot in its centre. A gas cloud of atoms surrounds the actual BEC (the dark dot, which corresponds to the white peak in the graphs above). BEC's created from normally bosonic particles like photons and polaritons don't break the PEP because they don't fall under Fermi-Dirac statistics.

As an interesting aside, the Pauli Exclusion Principle (PEP) plays an essential role in atomic matter at the highest energy scales too. In these cases, rather than the principle being broken, it takes on a hero-like role that proves just how strong quantum forces within matter are. It explains degeneracy pressure. Ordinary atoms take up space because electron degeneracy pressure keeps electrons with the same quantum spin apart. Electrons also repel one another through electrostatic (same-charge) repulsion. That's a different force that's also at play here. Because of charge repulsion, electrons in atoms tend to spread out and partially occupy several orbitals, like passengers on a jet tend to do when it's half full. When outward nuclear fusion pressure no longer counteracts inward gravitational pressure, atoms are squeezed together and the electrons are forced to fill up the lower energy quantum states. (There's a bunch of cargo in the back of the jet so everyone has to fill up the first few rows). Electrons are forced to get close despite electrostatic repulsion, a classically described force. They refuse, however, to overlap into identical quantum states (two people can't sit in the same spot).

This much more powerful quantum, rather than classical, resistance is called quantum degeneracy pressure. Atomic matter in this state, with electrons forced into all the lowest but not identical orbitals, is found in the extremely dense electron degenerate matter of white dwarfs. In a more massive and even denser stellar remnant such as a neutron star, where gravitational pressure is much higher, the electrons, forced against the electrostatically repulsive (positively charged) nucleus are so energized they approach the speed of light. At this point it is more energetically favourable for them to undergo electron capture through inverse beta decay than to remain traveling near light speed because, at this velocity, their relative masses are approaching infinity, a prediction made by special relativity. Electrons combine with protons in the nucleus and transform them into neutrons (therefore the name "neutron" star). Unlike the white dwarf, just one kind of pressure prevents the neutron star from collapsing. This is the quantum degeneracy pressure of neutrons. These particles don't repel one another electrostatically because they are electrically neutral. In fact, they bind strongly to one another through the strong force, which operates at very short (intra-atomic) distances. As outlined by the PEP, neutrons, like electrons and protons, are matter particles whose wave functions cannot overlap. If the stellar remnant is more massive than a neutron star, matter collapses altogether into a black hole (of which a quantum mechanical description is not yet available). Are the neutron wave functions forced to overlap inside a black hole? Is it a BEC? There is currently no way to know. Isn't it fascinating? Even under the pressure of a massive collapsing star matter waves refuse to overlap one another. Yet, when energy approaches zero, matter waves expand and smear across one another of their own accord.

HOW A BEC IS MADE

The first rubidium BEC was made by trapping a tiny ball of a few rubidium atoms using lasers and magnetic fields. This process is tricky. If the atoms get too close to each other they will form Rb2 molecules. A gas at an ultra-cold temperature will condense into a liquid and then into a solid if the atoms are allowed to interact with each other. To cool the gas, infrared lasers bombard the atoms from every direction. One would think this should have the opposite effect of cooling. The intense photon energy should excite the atoms and add momentum to them, heating them up.

One trick to laser cooling is to understand temperature as the average random kinetic energy of a group of atoms. By making the atomic motions less random, lasers narrow the energy distribution of the group of atoms (and create the single sharp peak in the series of graphs above.). When a photon strikes an outer electron in an atom, it can be absorbed, exciting the atom, and then be re-emitted or it can be reflected. An electron will only absorb a photon that matches its orbital energy (called transition energy). This is where the word quantum derives its meaning; energy can only transition in discrete packets. In this case, a laser with a frequency just below that of the transition energy is used. A stationary atom in the cloud won't even "see" the photons. It won't absorb them because they aren't the right energy. An atom moving away from the laser also won't absorb a photon. It will "see" it as red-shifted, having even lower energy in other words. An atom moving toward the laser, however, will "see" the photon as blue-shifted, and therefore at just the right energy to absorb. The atom is excited and then re-emits the photon, in a random direction. Now statistics comes into play: the photon hits the atom coming toward it. The photon slows it. That's a pure loss of momentum. But the atom then re-emits a photon (with the same energy) in a random direction. This time the direction is random so the change in momentum is not a pure gain. When absorption and emission are repeated many times over a group of atoms, the average random kinetic energy of the group decreases (pure losses and not-pure gains) and that means the temperature decreases. In other words, the lasers eventually align all the velocity vectors of the atoms, and when that happens they are close to transforming into a BEC. Just like the laser light (which is made of aligned in-phase photons) used to make a BEC, the BEC itself is made of aligned particles. They act like waves in phase with one another rather than as discrete particles of matter.

Atoms are also tiny magnets because their spinning electrons create magnetic fields. By applying a carefully aligned magnetic field to the group of atoms, they can be held in one place once they are cooled. The lasers can be turned off at that point. The lasers aren't perfect; some atoms will still have higher kinetic energy than others. They are still "hot." These atoms simply jump out of the magnetic trap, and are eliminated, leaving only the coldest atoms, the ones that can't jump out, inside. This is evaporative cooling. Heisenberg's uncertainty principle ensures that even in a quiet state such as this, there are tiny random movements in the system that eventually destroy the perfectly aligned quantum state. The original group of about 2000 rubidium atoms, that formed a BEC in 1995, lasted for about 20 seconds before it lost its coherence and dissipated back into ordinary gas.

This 6-minute video describes how a Bose Einstein Condensate is made:



As mentioned earlier, the technology has steadily improved since 1995, creating new kinds of BEC's that last longer, as well as solid-state quasiparticle BECs that can exist at higher temperatures.

WHY RUBIDIUM?

Any element's physical and chemical properties change as they transform from one physical state to another. For example, rubidium, the first element made into a BEC, is a very soft silvery white metal at room temperature. Rubidium gets its name from the dark purplish red colour of its flame. With a melting point of just 39.3°C, it is partly melted in the vacuum sample tube shown below.

Dnn87;Wikipedia
A very reactive alkali metal, rubidium can explosively react with just the moisture in the air. As oxidation starts, it warms the solid into a liquid state, which is even more reactive. If this vacuum sample tube were broken, the rubidium would quickly explode. Rubidium readily vapourizes under ordinary pressure. Even an ordinary heating element (at around 700°C) will boil rubidium into a fairly colourless gas, which is one reason why it made the prefect first candidate for a BEC. How do we keep the rubidium gas in a gas state when we expect it to condense and then to freeze into a solid, and an exceptionally hard one at close to absolute zero? The answer is to start with a very hot (hundreds of degrees Celsius), therefore very diffuse gas. As it is cooled, conditions are carefully monitored to keep its density very low - about a million times less dense than air at Earth's surface. This prevents the atoms from condensing as they normally would.

The most important reason why rubidium works as a BEC is that these atoms are bosonic atoms. They are not actually bosons as we've previously learned, but under the right conditions they can display a bosonic nature. The general rule for bosonic atoms is simple (but the actual calculations are usually very complex). If an atom contains an even number of subatomic particles, it's a "boson." All neutral atoms have an equal number of electrons and protons so it's the neutron number that matters. Rubidium-87 is an isotope of rubidium with 50 neutrons, an even number, so it is a boson. From a quantum mechanical point of view it means that all the subatomic particles in rubidium can be spin-matched or polarized. This doesn't mean that all bosonic atoms make good BEC's (again, it's complicated). It also doesn't mean that a fermionic atom can't make a BEC. Helium-3, for example, is a fermionic atom - it has an uneven number of neutrons (one). It will, however, under extreme cooling, form a pair with another helium-3 atom (a Cooper pair like those in superconductors), which in effect creates a bosonic composite particle, which will condense into a BEC. Because it is fermionic, helium-3 must be much colder than helium-4, a bosonic atom, to condense.

In addition to being plentiful, fairly easy to vapourize, and bosonic, rubidium-87 atoms have one lone electron outside a completely filled electron shell and this lone (interactive) electron makes it ideal for magnetic trapping.

HOW DOES A BEC BEHAVE?

BEC's act a lot like superfluids. Superfluid helium, for example, is created when liquid helium is cooled to almost absolute zero. Helium happens to be the only element that will remain liquid under normal pressure right down to absolute zero. Interactions between helium atoms are so weak that its ground state energy stays too high to allow it to condense into a solid, unless additional pressure forces the atoms closer together. Helium will, however, transition into a superfluid state. In this state, these atoms, like those in a gaseous BEC, no longer vibrate much at all with heat energy. Instead, they enter a calm state where many atoms begin to vibrate in unison like a single particle. This sounds like a BEC but there are key differences. Still, the two states are strongly linked and it is easy to see why many of the strange behaviours of superfluids also apply to BEC's.

Comparison to a Superfluid

A condensed gaseous atomic BEC is not a liquid. Nor does it follow the laws of ordinary gas behaviour. But is it an example of a superfluid? The little bead of rubidium BEC in the magnetic trap is a very rare case of a macroscopic fully coherent quantum object. Loss of phase coherence is the hallmark of the transition from quantum BEC into classical gas. The group of rubidium atoms will grow out of phase with each other in a matter of seconds and return to a classical object, an ordinary gas. Superfluids such as ultra-cold helium exhibit many of the same strange behaviours we explore here with BEC's and they are of quantum origin as well, but these behaviours are hidden in the dense liquid - you can't directly track the gradual loss of interference and other behaviours, as you can with a gaseous BEC. Furthermore, with a BEC, you can fine-tune experimental parameters such as the kinetic energy of the atoms, or its density (more about this in a bit), things you cannot readily do with a superfluid.

Even though the interactions between helium atoms are weak compared to other elements in a liquid state, helium atoms in a superfliud state interact quite strongly with each other compared to those in a diffuse gas BEC. These atom-atom interactions complicate the behaviour of a superfluid, complications generally not encountered in BEC's.

Both superfluids and BEC's rely on bosonic atoms. Cold liquid helium-4 transitions into a superfluid at about 2.17K. Below 1K (well below superfluid transition temperature), it exhibits zero viscosity even though only about 7% of the atoms are at ground state. Compare this to a BEC, where all the atoms are at ground state. Helium-3 will not transition into a superfluid until temperatures dip to 2.5 mK (milli-Kelvins). At this point, helium-3 atoms pair up into bosonic Cooper pairs. A BEC must be much colder than a superfluid. A gas will not transition into a BEC until the temperature dips to just a few uK (micro-Kelvins). There is some disagreement among researchers whether a BEC is type of superfluid but most researchers agree that a superfluid is an example of partial Bose Einstein condensation.

Macroscopic Quantum Behaviour

You might be wondering at this point how you can observe a physical wave when what you seem to have is a coherent singular quantum wave, or mathematically put, a wave function. At this point quantum mechanics students get riled because they know a quantum wave function is a purely mathematical construct made up of a real part and an imaginary (not physically possible) part. You can't see one: it doesn't exist in the real world. However, there is a way around this conundrum. The probability density of the wave, which is the absolute value of the wave function squared, always gives you a real and positive value, a value, which amounts to the real standing wave that you can observe. In other words the probability density is the mathematical description of the physical phenomenon. Can we ever "see" the quantum wave function in action? Yes, we can. We can generate and observe a BEC interference pattern. Rather than mixing together like two gases would, two BEC's in different phases will interfere when they combine, setting up positive and negative lines of quantum interference, a quantum effect, which can be observed. In the negative lines there is only vacuum. Here, the matter waves of atoms interfere resulting in a space with no atoms. Although the total number of atoms in the mixture is conserved, the atoms simply disappear along lines of negative interference, a purely quantum effect that you can observe.

Quantum Vortices

Perhaps the most intriguing behaviour of BEC's is the quantum vortex, a behaviour that is already well studied in superfluids such as superfluid helium. The classical analogue is stirring a cup of coffee. A little liquid tornado forms with a depression or hole in its centre. Internal friction eventually slows down the rotational motion and this classical vortex dissipates. Both superfluids and BEC's are frictionless. They exhibit zero viscosity and this means the fluid flows without any loss of kinetic energy. If you could stir a superfluid it would just flow around the spoon with no resistance. You can't stir a BEC with a spoon because all current BEC's are too small. The "dot" mentioned earlier tends to be a spherical or pancake shape that is around a millimetre across. But you can whirl it around by rotating the magnetic trap that contains it and, when you do, you create multiple tiny string-like whirlpools in it. Unlike any classical system, all the rotational motion of the BEC is sustained only by these quantized vortices because all the atoms in a BEC are in one quantum wave function. Because these vortices are quantum, their angular momenta must be quantized. The angular momentum can only be expressed in whole integer packets. When a wave rotates (any wave, even a quantum wave), it forms a closed curve. In this case, those closed curves are confined to de Broglie wavelengths. Like any standing wave, the wavelengths must be whole (an integer value). In a classical fluid like coffee the velocity of rotation increases smoothly from the spoon in the centre toward the walls of the cup. In a quantum fluid (superfluid or BEC), the velocity can only increase in packets like 0, 1, 2, . . .  It takes less energy for the system to form lines rather than sheets, so you get a series of string-like vortices. In the BEC, the vortices tend to be multiple and they have very tiny holes, the width of which can vary depending on the atoms used. These holes, called filaments in a quantum system, don't decay by diffusion as they would in a classical system.

Put mathematically, a BEC quantum vortex is a direct result of the macroscopic wave function of the system. A BEC rotates by puncturing the condensate with filaments along which the quantum wave function vanishes to zero. These filaments are singularities in the wave function. A number in the wave equation, called the winding number, must be a whole integer (0,1,2, . . .). Mathematically this ensures that the wave function doesn't change value after each rotation. In physical terms this means that the velocity of the circulation has to be quantized. A quantum vortex can only spin at a discrete set of speeds and it can never die down smoothly like a classical vortex does. However, the motion around each vortex, called superflow velocity, acts classically like it does in the cup of coffee, and it can be described using ideal fluid dynamics.

The study of quantum turbulence (in which vortices are an example) began in the 1950's using superfluid helium. The availability of cold gas BEC's now offers a great advantage in this study because the turbulence can now be directly visualized in a BEC, rather than being hidden inside a dense liquid, which, depending on the temperature, can contain a complicating mixture of superfluid and classical viscous fluid. Even with this advantage, a lot of questions remain. It is not yet possible to predict where and how many vortices form and what their shapes will be. This problem isn't limited to the quantum vortex. Turbulence itself is a very complicated phenomenon. It is strongly nonlinear and this means you can't input data into an equation and get a predictably straightforward answer. This is why weather forecasts are notoriously unpredictable.

The study of quantum vortices might make the study of turbulence easier. A classical vortex tends to be messy: it's unstable, it appears and disappears randomly and its circulation is not conserved. Quantum turbulence is a simpler system. It is composed of a tangle of vortices that all have the same conserved circulation in the BEC. Even so, quantum turbulence is complicated; it is still a system with many (albeit fewer) degrees of freedom.

In a perfectly condensed BEC, well below its critical temperature, you might expect vortices to be indefinitely stable because there is no friction to diffuse them, but they do decay over time. Quantum vortices spontaneously and randomly reconnect, much like water spouts over an ocean do. This behaviour is analogous to eddies that form in a turbulent classical fluid. A quantum vortex can lose energy, dissipate over time in other words, by emitting sound. Sound appears to come from two processes. First, during the process of reconnection (depending on the angle of reconnection), vortex line length is destroyed. When this happens a rarefaction pulse is emitted, a sound in other words. A second source of sound emission may come from a cascade of Kelvin waves, which are excitations in the BEC that result from vortex reconnections. These particular waves, which usually have very long wavelengths in nature, may be short enough to cause sound radiation in this case. By these mechanisms of energy dissipation (loses through sound energy), all quantum vortices eventually decay.

The Bosenova

Under the right conditions, a BEC made of rubidium-85 will explode in a manner that resembles a tiny supernova, a bosenova (also less cutely called a BEC loss). Rubidium-85 is one of two naturally occurring isotopes of rubidium, the other being rubidium-87, the isotope which created the first BEC. Like rubidium-87, rubidium-85 is a bosonic atom, this time with 48 neutrons. One of the key differences between the two isotopes is that rubidium-87 has a positive s-wave scattering length. This means that the atoms naturally repel each other at low temperatures so it is easy to evaporatively cool the gas. Rubidium-85, in contrast, has a negative scattering length. This means that a condensate made of this isotope will tend to collapse in on itself, especially in a zero magnetic field. Because the atoms attract each other, it's also more difficult to cool it into a stable (non-interacting) BEC. It has to be just 3 billionths of a degree K above zero. Even with these challenges, rubidium-85 offers a unique bonus. In 2001, physicist Carl Wieman (half of the team that created the first BEC in 1995) adjusted the fine-tuning of a BEC droplet of rubidium-85 by changing the magnetic field in which the atoms are trapped. Doing this amounted to adjusting the self-interactions of the wave function (remember a BEC is a superimposed macroscopic wave function). In effect, he could dial between repulsion and attraction.

Dialed to repulsion, all the parts of the wave function push each other apart. The BEC droplet swells accordingly. Dialed to attraction, they pull together, and this is when unexpected dramatics begin. It starts to shrink gradually as expected, but then it shrinks suddenly, triggering an outward explosion that is tiny by everyday standards but with significant energy (about 100 nano Kelvins, nK) considering only a few thousand very low energy atoms are involved. After a few microseconds a much smaller remnant of BEC is left behind, surrounded by an expanding gas cloud of rubidium-85 atoms. This sudden collapse followed by an explosion that leaves behind a remnant and a gas cloud reminds one of a supernova, although the actual mechanisms involved are very different.

About half the original atoms vanish during the explosion. Researchers first thought that they might have either formed Rb2 molecules in the explosion or that some atoms flew out of detector range before they were measured. Or (sharp breath in), they really disappeared. Perhaps an even deeper mystery presented here is how very cold BEC atoms with minimal energy available to them could explode in the first place. The thermal energy released is greater than the free energy of the original BEC. And, just to season the broth, why does some of the BEC survive?

In 2003, Masahito Ueda and Hiroki Saito explained theoretically how a BEC collapses and explodes, thus resolving some of the mystery. The idea is that even though the BEC state is a single matter wave, it still consists of a diffuse gas of atoms and they will interact when they are nudged closer together. When the magnetic field is tuned to just barely favour attractive atomic interactions, the number of inelastic (interactive) collisions between atoms remains negligible at first. But as the atomic density gradually increases, the density of the concentrate increases toward it centre. After a short period of time, the collision rate suddenly jumps and becomes significant, but just within a tiny localized central portion of the BEC (about a millionth of the volume). This triggers instability in the BEC. Not one but several intermittent explosions/implosions occur in rapid-fire, and each time several tens of atoms are lost from the condensate. These researchers determined that atoms are removed due to three-body loss. Three-body loss, or three-body decay, occurs when three rubidium atoms get very close to each other, as they do in a shrinking condensate. Two of the atoms form a molecule (Rb2) in an excited state while the third atom carries away the energy released by the formation of the chemical bond. Each of these energies is much higher than the depth (energy confinement) of the magnetic trap, so three atoms (and their energies) fly off out of the system.

Although atoms are not lost irretrievably, the implosion viewed in terms of a closed system acts like a tiny atom drain, a tiny black hole. After the atoms are lost, outward kinetic pressure from the atoms minus the combined mass of the lost three atoms just surpasses the attractive force. This slight surplus in kinetic energy is enough to trigger a subsequent burst or explosion. Afterward, attractive energy then dominates and the BEC shrinks again. The cycle repeats until the number of atoms remaining is too small for attraction to overcome outward kinetic pressure. Because the collision-heavy region is confined to such a small portion of the BEC, some it survives the ensuing explosion.

BEC's Can Slow Down and Stop Light

The bosenova is a very interesting phenomenon but it seems to have few practical applications. The interaction between light and a BEC, on the other hand, is not only fascinating, it hints at new possibilities for storing and communicating information in the future.

The speed of light in a vacuum is an exact value, approximately 300,000,000 m/s (denoted c). In practice, however, a pulse of light contains many particles of light, or photons. Their average velocity is c. A pulse of light slows down (it refracts) when it travels from one medium such as air into another medium of different density such as water. If it strikes the new medium at an angle, the pulse of light appears to bend as its speed differs on different points along its wave front. The individual photons in that pulse do not slow down, however. They remain at vacuum light speed. What slows down is the pulse, because multiple interactions between the photons and the atoms in the medium take tiny amounts of time. As density increases, so does the number of photon-electron interactions, because there are more atoms for the photons to interact with. Photons may reflect off atoms and resume travel or they may be absorbed by an atom's outermost electron and then be re-emitted. These interactions mean it takes longer for the group of photons as a whole to get from point A to point B through the medium. The light pulse slows.

How about the photons themselves? Can anything slow a photon down? It turns out that a BEC can. In fact, it can even stop a photon (in effect) for a few seconds, then reconstitute it (with all the quantum information it contains still intact) and allow it to resume its course.

In 1999, Lene Hau and her associates slowed a light pulse down to just 17 m/s (that's 20 million times slower than light traveling in a vacuum, c) in a BEC. A BEC, as we know now, is a condensate of atoms. The atoms are still a diffuse gas. As matter waves, however, they spread or smear out as their atomic vibrations slow down. Compared to an ordinary gas at room temperature, the matter waves (the de Broglie wavelengths) are about 10,000 times shorter than the average distance between the atoms. These waves are longer than the distances between the atoms, so the matter waves overlap. It might be tempting to imagine a BEC as a clump of super-dense matter where the atoms themselves have all collapsed into one spot, but it is not. Only the matter waves superimpose.

Refraction, even at its extreme, slows light only by a small fraction (about a third). For example, gallium phosphide has one of the highest known refractive indexes of any material, and even through this, light slows only to about 86,000,000 m/s. A diffuse gas generally has a refractive index of just one, which means it does not slow light at all. To achieve slowing on the extreme scale here, something other than refraction must be involved. There are currently three theories used to explain how extreme slowing occurs. They are briefly described on the Wikipedia entry here. I will focus on the polariton (more precisely called microcavity exciton-polariton) approach. It's the theory that best explains stopped light (which we will talk about next). A polariton is a not a physical particle in the same sense that a particle of matter is a physical particle. It is a quantum-only entity that is a hybrid light/matter quasiparticle. It is an emergent phenomenon called a collective electromagnetic excitation. It can take on particle characteristics, which, as a result, can have physical (measurable) effects on a system. These particular particles act like a gas, they can be trapped, and they can move just like real particles do.

Normally you can't shine light through a BEC gas condensate. As the gas cools, it changes from transparent to opaque. However, Lene Hau and her associates were able to electromagnetically induce transparency in a BEC within a very narrow spectral range. In this case, a cloud of sodium atoms is treated the same way as the rubidium BEC explored earlier. It is cooled by lasers and by evaporative cooling into a BEC state that is confined within a magnetic trap. The BEC is then made transparent by exposing it to a specific arrangement of laser beams. The lasers also allow photons traveling through the BEC to combine with atoms to create polariton quasiparticles. More technically put, the laser treatment induces strong light-matter coupling into a structure that combines quantum wells (think of an atom stuck in one spot as a standing wave) and photon cavities (imagine a photon trapped between two very close tiny mirrors). This coupling is equivalent to a boson particle that is composed of a quantum well exciton and an optical cavity photon. Polaritons get mass from the atoms, so they travel slower than c. Remember that a BEC is, in effect, one big super-atom. It has the mass of all the atoms condensed into a single quantum state. It's mass is therefore the sum of the masses of all the atoms, so polaritons formed with incoming photons are likewise very massive and this means they must travel much slower than light speed. It is a quantum effect that significantly slows the photons traveling through the BEC because they enter a different particle state. This slowing effect is an entirely different mechanism from refraction, described earlier.

This 3-minute video describes how a BEC is used to slow down light.



In 2001, Ron Walsworth and his associates went one big step further. They stopped the propagation of light through a BEC altogether (and then restarted it). This time, using rubidium atoms once again, they gradually turned down the lasers once they had made the BEC transparent. The behaviour of the polaritons in the BEC shifted accordingly from photon toward atom. Eventually the polariton nature turned entirely into atom nature, and at this point the photons were effectively stopped in their tracks inside the BEC. Light stored in the polaritons as quantum information was now hidden in their atom-nature. Perhaps a better way of looking at it is to consider all the quantum information encoded in the light now trapped within the BEC gas, not in the atoms themselves but as quantum excitations within the gas. An exciton-polariton is a localized quantum excitation. Light in this way can be stored for up to seconds inside a BEC (and perhaps longer as the technique improves).

When the lasers were turned back up, the photon component of the polaritons increased and the light resumed its travel.

The ability to stop and restart photons could be very useful in a number of technologies. Imagine that instead of solid-state electronic qubits delivering information in a computer, photons could be used instead. They carry information faster, they don't heat up sensitive components and as we just saw their information storage can now be precisely controlled. One problem with using photons in communications has been how to stop them and decode their information. After flying down an optical cable they have to be stopped at your computer somehow. Nowadays, the information is transferred into an electronic system. The only way to stop a photon is to interact with it. A photon is absorbed by an ordinary atom. As it is absorbed it loses its quantum information, for example its polarization state, and an entirely new photon of equivalent energy is re-emitted in a random direction. In a laser-irradiated BEC, the laser is tuned in a way to prevent photons from being absorbed by the electrons in the gas atoms. The atoms instead form a cluster around each photon, creating a polariton quasiparticle. A BEC can stop and capture each photon, store its information intact (again, polarization could be used instead of 0's and 1's) for a specific period of tine and then send it on its way as a signal.

MANY POTENTIAL USES FOR BEC's

There is a potential goldmine of possibilities for BEC's both as research tools and in practical applications. This research is still in its early stages; it is less than two decades old, but as BEC-making technology improves and the list of BEC's with various properties grows, there is no doubt that some of their exciting quantum features will be exploited in new technologies. Because BEC's greatly magnify phenomena confined to the formerly inaccessible quantum world, BEC's might be manipulated as tools to directly observe and verify currently theoretical quantum behaviours such as the mass acquisition of a normally massless particle called a quasi-Nambu-Goldstone boson, which is thought to be the result of tiny quantum fluctuations. Until now, almost all investigations into the quantum world must be carried out in incredibly large and expensive particle accelerators. As well as expense, this limits the kinds of questions that can be asked. BEC's could be used as customizable quantum laboratories. They might also provide a direct window into the mysterious phenomenon of quantum entanglement. BEC's don't normally contain entangled atoms but researchers have very recently discovered it might be possible to fine-tune the magnetic field around the condensate in such a way to entangle all the atoms in it.

BEC's are already being exploited to learn more about solid-state physics. You can create an optical lattice in a BEC by using several lasers to make an interference pattern that looks (and acts) much like the crystal lattice patterns of atoms found in many solid materials. The big advantage here is that the same optical lattice can be repeatedly tuned and manipulated in different ways to see what happens. When using a solid, you have to regrow your sample every single time (and I assume you've got to be very consistent). The fine-tune-ability of a BEC also means it could be potentially used as a variety of high-precision measuring instruments. There is no obscuring noise to tune out in a purely quantum system. In quick succession science and technology have evolved through the golden age, the industrial age and the information age, Now, it seems that BEC's will help us usher in a quantum age.