we can't reach absolute zero -- 7/18/18
Today's selection -- from Nothing: Surprising Insights Everywhere from Zero to Oblivion edited by Jeremy Webb. We can't reach absolute zero:
"The quest for lower temperatures in large pieces of material has stalled on the fact that the thermal conductivity and heat capacity of all materials plummet as temperature falls. This means it takes longer and longer to remove even tiny amounts of heat from a substance. Also, any experimental technique you use to study the properties of a substance will warm it up. If a butterfly happened to find itself in a refrigerator containing a cubic centimeter of copper at 0.001 K, the very act of the butterfly falling 10 centimeters would raise the copper's temperature 100-fold.
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| Robert Boyle pioneered the idea of an absolute zero. |
"For smaller amounts of material -- up to just a million atoms or so -- we can cool them atom by atom using laser light. This has slowed atoms from moving at around 1 meter per second at 1 millikelvin to roughly 1 millimeter per second at 1 nanokelvin. Although applications of such technology seem unlikely at the moment, given the last century of progress we would be unwise to bet against future widespread application.
"This state-of-the-art technique can get us very close to absolute zero, and I don't doubt that we will eventually get colder still. Which raises the most common question asked of cryogenic scientists: why can't we reach absolute zero? The impossibility of cooling an object to absolute zero is the essence of the Third Law of Thermodynamics, and there is no way around this.
"Here's one way to understand why: conventional fridges work by placing a target to be cooled in 'thermal contact' with a cooler substance, typically a recirculating fluid. We know that the fluid must be colder than the target so that heat can flow from the target. By the same principle, to get heat flowing out of a target that you want to reach absolute zero, the fluid coolant would have to be colder than 0 K to begin with! Being below absolute zero is -- of course -- nonsense: it is clearly impossible to make molecules move slower than not moving at all.
"Techniques such as laser cooling seem to overcome the limitation of conventional cooling by simply damping the motion of atoms, but in fact all that has changed is the level of sophistication of the coolant. Even at 1 nanokelvin, atoms are moving at about 1 millimeter per second slow, but still a long way from stationary.
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| Velocity-distribution data of a gas of rubidium atoms at a temperature within a few billionths of a degree above absolute zero. Left: just before the appearance of a Bose_Einstein condensate. Center: just after the appearance of the condensate. Right: after further evaporation, leaving a sample of nearly pure condensate. |
"It might seem odd that a century after Kamerlingh Onnes took us to 4.2 K, we are still investigating what happens in those few degrees above absolute zero. But this is perhaps because slowing down the vibration of atoms creates the equivalent of a quiet room in which one can hear tiny noises, and the logarithmic scale of the decibel -- which we use to measure sound levels -- could also describe the realm of cryogenic investigation. We shouldn't think about the single degree between 1 K and absolute zero, but about the factor 1,000 difference in temperature between 1 K and 1 milliKelvin. Cooling through this range, one encounters as many changes in properties as in the change from 1 K to 1,000 K.
"For each factor of 10 we cool a substance, we probe atomic interactions at a new level of subtlety. So even at 1 nanokelvin, there is plenty of room for further cooling -- to picokelvin, femtokelvin and beyond. And we really have no idea what we will find when we get there!"
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