Check the NIST website [[1]] to be sure, but I believe the reason it is considered a gas and not a solid is that the sample consists of a very small number of atoms (on the order of ten or less) under high vacuum. In order for condensation to occur the atoms must collide and at these conditions collisions are fairly rare. In other words condensation is kinetically limited. Under some conditions, the scientists were able to observe formation of a Bose-Einstein condensate, so some form of condensation does occur. -- Matt Stoker |
Check the NIST website [1] to be sure, but I believe the reason it is considered a gas and not a solid is that the sample consists of a very small number of atoms (on the order of ten or less) under high vacuum. In order for condensation to occur the atoms must collide and stick together. Under the experimental conditions collisions are fairly rare, so condensation takes a long time. In other words condensation is kinetically limited. Under some conditions, the scientists were able to observe formation of a Bose-Einstein condensate, so some form of condensation does occur. -- Matt Stoker |
efficiency = 1 - TH/TC
it is impossible to obtain a temperature below 0 K. If such a temperature were possible, it would be possible to develop a thermodynamic cycle that exhausted heat at that temperature. The result would be an engine with efficiency >100%, which violates the first law of thermodynamics. (MS)
It is also possible to build a two level laser system, with the ground state being the low energy state in population inversion, but for various practical reasons these are a lot less fun.
The definition of temperature usually used for such systems is, derivative of entropy with respect to energy (perhaps multiplied by a constant for the system of units you are using). Adding energy to a system with population inversion actually decreases the entropy, and subtracting energy increases the entropy, hence negative temperature.
BTW for such systems, there are two good approximations to absolute zero. One is with all atoms in the low energy state. The other is with all in the high energy state (perfect population inversion).(GJ)
I did a little reading from: <a href="http://www.google.com/search?q=cache:FJjlpW9h4es:math.ucr.edu/home/baez/physics/neg_temperature.html+absolute+zero&hl=en">Physics FAQ</a>
One important distinction is that the negative temperature, so defined, is for only one mode (eg. the nuclear spin mode). Thus, the entire system (i.e. all modes) still has a positive temperature. This is why such a negative temperature is possible without violating the 1st & 2nd laws of thermodynamics. It would not be possible to exhaust heat to only the negative temperature mode (other modes would still play a role), so TC would still be >0 and efficiency < 100%. (MS)
The application of this particular low temperature work is for atomic clocks, the accuracy of which are improved by using very slow moving atoms. Check the NIST website for details. --Matt Stoker
How can they have a gas at a few billionths of kelivns? Doesn't helium, the hardest element to condense, liquify at 4K? I could understand if they could stretch the condensation point down to 1 K or .1 K by playing with the volume, but even just down to a μK is a difference of 6 orders of magnitude. Especially with a gravitational field pulling the atoms toward the bottom of the tank, I couldn't see them getting anywhere near absolute zero without letting the gas condense.
Also interesting to note is that there are rotational modes available to single atoms. The problem is that the amount of energy necessary to access them is enormous compared to the energy involved in temperature. because of the quantization of angular momentum.--BlackGriffen
Check the NIST website [1] to be sure, but I believe the reason it is considered a gas and not a solid is that the sample consists of a very small number of atoms (on the order of ten or less) under high vacuum. In order for condensation to occur the atoms must collide and stick together. Under the experimental conditions collisions are fairly rare, so condensation takes a long time. In other words condensation is kinetically limited. Under some conditions, the scientists were able to observe formation of a Bose-Einstein condensate, so some form of condensation does occur. -- Matt Stoker