Monday, April 13, 2009

Cold and Special States of Matter

Although the journey to extreme heat spans many trillion trillion trillions of Kelvin, the journey to extreme cold only goes down a few hundred Kelvin to 0 K, or absolute zero, about -273.15 Celsius and -459.67 Fahrenheit. I will use the Kelvin scale, as it is the most convenient for representing low temperatures (to convert Celsius to Kelvin, simply add 273.15). As we go down in temperature, the movement of particles slows, eventually freezing gases, and creating substances that defy gravity and nearly stop light beams.

The calculation of absolute zero came about in a fairly simple way. Two temperatures were found, and the movement of particles was measured for each temperature. This was done using the boiling and freezing points of water. By plotting these two points on a graph, and then continuing the line until it reached the temperature where there was no movement at all, a very accurate approximation of absolute zero could be found. This, of course, is assuming the function of temperature to particle movement was linear, or a straight line. If it wasn't, more points would be calculated before the zero point could be found.

We will start at room temperature, which is about 293 Kelvin, traveling down on the temperature scale. On this journey, we soon encounter the melting point of the element Mercury, which is a liquid at room temperature. Mercury freezes into a solid at 234 K. As we get lower, we encounter the well known dry ice. Dry ice is frozen carbon dioxide and does not have a "melting point" because on contact with warm air, carbon dioxide sublimes (going directly from solid to gas, without any liquid middle stage). The gas resulting from subliming carbon dioxide is fog. In fact, the liquid form of carbon dioxide cannot occur unless under pressure. Therefore, the "subliming point" of carbon dioxide is 194.65 K.

As the temperatures continue to drop, the gases begin to liquefy. One example is oxygen, which at 90.20 K, becomes a blue liquid. One property of this liquid is its ability to make objects dipped in it very brittle. The classic example of this is dipping a bouncy ball into liquid oxygen and dropping it. The brittle properties of the ball causes it to shatter. Oxygen is supposedly one of the permanent gases, (the term was coined by Michael Faraday) or a gas that cannot be liquefied by pressure alone. These gases are oxygen, nitrogen, and Hydrogen (Helium would be a permanent gas but it wasn't discovered until later).

The second of the so called permanent gases to liquefy is nitrogen at 77 K. Nitrogen then solidifies at 63 K. The next gas is Hydrogen, which liquefies at a very low 20.28 Kelvin. This was the coldest liquefaction point of any gas before Helium was discovered. Hydrogen also becomes a solid at 14.2 K. Finally, the last gas to liquefy is Helium, at an astounding 4.22 Kelvin. Helium solidifies at an even lower temperature, and the solid from of Helium usually requires pressure to keep it stable.

Now, that all the gases are liquefied, various strange phenomena occur, the first of which is superconductivity. Many metals conduct electricity, but it has been discovered that at very low temperatures, metals suddenly have zero resistance to electrical current. Therefore, magnets have the effect of floating on the metal's magnetic field. Until 1986, all metals known had a superconductivity point of less than 30 K. Over recent years, however, metals have been discovered with higher superconductivity points. The first metal to have a point higher than 30 K was LaBaCuO (La=Lanthanum, Ba=Barium, Cu=Copper, O=Oxygen) at 35 K. The next discovery was the metal whose acronym is YBCO, which had a point of 90 K, discovered in 1987. Progress continued over the years, until today, when the highest temperature superconductor is thallium barium calcium copper oxide (Hg (12 atoms) Tl (3 atoms) Ba (30 atoms) Ca (30 atoms) Cu (45 atoms) O (125 atoms)). This remarkable substance has a superconductivity point of 138 K, and possibly up to 164 K under more extreme pressures. If more high temperature superconductors could be discovered, there would be a definite commercial use for superconductivity and electric wires could conduct electricity without any resistance, increasing the efficiency of transporting electricity over long distances. Currently, it is not known why metals reach this curious state at low temperatures or why the temperatures would vary from metal to metal.

A magnet levitating on the magnetic field produced by a superconductor. The superconductor itself is not visible but the wisps of gas are a result of the liquid nitrogen (the coolant to the superconductor) evaporating.

Another strange property of matter at extremely cold temperatures comes about when we reach 2.1768 K. Helium* (see footnote directly below this paragraph) at this point is a liquid, and it is a normal colorless liquid from 4.2 K down to 2.1768 K. Then, a strange thing happens. The liquid switches phases and turns blue. Also, its viscosity becomes zero. The viscosity of a liquid is, in common terms, the "thickness" of the liquid. For example, maple syrup, as you know, takes awhile to flow and clearly has high viscosity compared to water, which is "thin" and flows easily and quickly over a surface. However, even water encounters resistance and barriers, such as rocks or dams, can (temporarily) stop it. However, when a liquid has exactly zero viscosity, it is called a superfluid. The normal Helium 4 atom (which has two electrons, two protons, and two neutrons) becomes a superfluid at its "lambda point" or 2.1768 K, as mentioned above. The amazing properties of superfluid Helium allow it to, without friction, travel up surfaces and defy gravity. For example, if a empty container, devoid of superfluid Helium, was submerged into an area filled with the fluid, a thin film of Helium would travel up the walls of the container and fill it until the level equalizes. In fact, unless sealed, superfluid Helium would flow everywhere until it was heated above its Lambda point or until there was a film of superfluid Helium around the entire Earth! Also, below Helium's freezing point (not exactly calculated, but is probably 1.5 K for pressurized Helium and 0.95 K for regular Helium) Helium is conjectured to become a supersolid. A supersolid is identical to a superfluid, with the exception that a supersolid has solid-like properties that result in an orderly spacing of molecules. Therefore, the solid would be "flowing". Since superfluids move without friction, a superfluid fountain is a perpetual motion device. The fountain continues without any energy at all! The only problem is that superfluids exist at such low temperatures that there is no commercial use.

*The only substance that is capable of being a superfluid is Helium. This is because Helium is the only substance that is a liquid at this extremely low temperature. (Hydrogen freezes at 14.2 K)

A picture showing how superfluids can travel, as a thin film, up the walls of a container. Eventually, the levels will equalize. Also, notice that a thin film circumnavigates the entire structure. If the top was not sealed, the superfluid would creep out and escape.

In 1924, Satyendra Bose sent a paper to Albert Einstein on theories of matter at extremely low temperatures. Einstein applied his own calculations and together they discovered a peculiar property of matter at very low temperature called the Bose-Einstein condensate. The quantum properties of this state of matter are very technical, but it seems that the atoms themselves adopt wave-like properties and grow larger. As the temperature continues to drop, the waves become larger and larger, until they intersect with each other and become one single unit, moving (although very little because the temperature is so low) uniformly. The quantum physics of atoms and particles applies to the larger "atom" and allows events to be seen visibly that usually only occur on very small scales. However, nothing was physically learned about this state of matter until over seventy years later, in 1995, because the temperature needed to attain it was very low (below 0.000001 Kelvin). Before its discovery, it was thought that light atoms would be more useful in producing Bose-Einstein condensates, but the first sample synthesized was of a small sample of Rubidium at 170 nanokelvin (0.000000170 K). Later, another Bose-Einstein condensate was produced, this time with Sodium atoms. This condensate had about one hundred times more atoms, or about two hundred thousand atoms, and the results were very beneficial for seeing how Bose-Einstein condensates interact with each other. Also, the Bose-Einstein condensate has the interesting property of being able to slow down light to observable speeds. The Bose-Einstein condesate is also very fragile, and interaction with even one regular atom could turn the substance back into normal form.

A map of atomic velocities during the production of a Bose-Einstein Condensate. The colors represent how many atoms are moving at a certain velocity. For example, the color red represents that very few atoms are moving at the same velocity while the color white represented thousands of atoms moving at the same rate. The image on the left is just before formation of the condensate, and the atoms are moving to different directions at different speeds. The center and left images are progressions in the life of the Bose-Einstein condensate where the atoms are moving in unison, represented by the white peak.

The temperature at which a Bose-Einstein Condensation is achieved is still above the lowest attained temperature of 0.0000000001 Kelvin, and this is still above absolute zero. The colder you get, the harder the last bit of heat clings to the matter. What happens at absolute zero, and whether it is even attainable, is unknown and may never be known.