Saturday, April 1, 2017

Quantum Thermodynamics

Interesting story at Nature on the physicists who study the messy intersection of classical thermodynamics with quantum mechanics:
Physicists have argued over the meaning of the three laws of thermodynamics since they were written in the nineteenth and early twentieth centuries. The laws say that energy cannot be created or destroyed; that the amount of disorder, or entropy, in an isolated system can never decrease; and that it is impossible to cool an object to absolute zero. But thermo-dynamics is paradoxical. The second law, which also puts limits on how efficiently heat can be converted into work — as happens in a steam engine — is particularly controversial.
Physicists do not agree on what the three laws of thermodynamics mean or where they come from. Are they purely the statistical product of trillions of particles interacting, or are they a fundamental part of all physics at any scale? To answer that question you need to understand how they relate to quantum mechanics, which describes the operation of single particles. Consider:
The law says that the production of disorder is irreversible. But some physicists argue that at the microscopic level, this seems to conflict with the laws of mechanics — be they those of Newton or of quantum physics. Mechanical laws, say these researchers, prescribe that all processes can be reversed.

Researchers have come up with different approaches to solving this conundrum, but none has satisfied everyone. “This has always been a bit of a dirty business,” says Christian Gogolin, a physicist at the Institute of Photonic Sciences in Castelldefels, Spain.
I bring this up because it shows again the limits of our physics. It is not just gravity that can't be reconciled with quantum mechanics, a problem that only matters when your are trying to understand black holes or other extreme situations. We cannot even reconcile ordinary thermodynamics, the kind we use to calculate the efficiency of engines, with quantum mechanics. In recent years there has been a great deal of work in this direction, and there have been reports of experiments that confirm our thermodynamic laws operate at the quantum scale. If we can figure this out, it may have all sort of useful applications:
Whatever the outcome of these debates, they may have implications for future technologies. Physicists have made ‘quantum heat engines’ — that can turn heat into work at the quantum level. Applications such as quantum computing are moving from the theoretical to the real world, so understanding thermodynamics on a tiny scale could be crucial. “You need to design algorithms that are not just faster,” says Renner, “but also thermodynamically optimized.”

1 comment:

G. Verloren said...

The laws of thermodynamics are only "laws" because they've always been observed to be true. If you can verifiably demonstrate a situation in which they do not apply, then there you have it - they're only mostly true, not always true.

Part of the problem might just be scale, and the way our limits on our ability to observe things shapes our perceptions of something "always" being true.

For example, Newton's Law of Universal Gravitation was (and still is) treated as a scientific "law", because under ordinary conditions, the calculations "always" hold true. But it turns out that at extreme scales, the theory begins to show small margins of error. When operating at cosmic scales or at quantum scales, the formula begins to produce ever so slightly wrong results. This led to developments such as Einstein's theory of General Relativity, among others, working to explain the discrepancies.

So what's stopping the three "laws" of Thermodynamics from being similarly flawed?

What if, under certain extreme conditions, it actually is possible for energy to be created or destroyed? We only believe this to be impossible because we've "always" observed it to be so. But what if 99.999999999999999999999999999+% of the time, it's impossible... but in incredibly rare situations, it actually can happen - even if just at incredibly small scales or rates?

Of course, it's could just as easily turn out to be that, if we do find evidence of something happening that we would lead us to think energy has been "destroyed", in fact the energy simply changed state in a way that we cannot yet detect. But how would we know the difference?

What does it even mean for something to be "destroyed"? You could argue that nothing ever truly gets destroyed, because the component parts always remain, just in some other arrangement. But isn't a big part of our very conception of "destruction" the notion that a desired arrangement has been changed or lost?

If you split an atom, do you actually destroy it? It's component parts still exist - but can the atom itself be said to exist? We used to believe the atom was indivisible - because we had "always" observed this to be so. But then we found the extreme circumstances where it isn't.

Compare to energy. We believe a quantum to be "indivisibile", because we've always observed it to be so.But what if you actually can "destroy" quanta, breaking them into yet unknown component parts? And what if those parts can't yet be detected? We wouldn't be able to determine whether we had violated the conservation of energy or not. We would lack the observational data to know one way or the other whether the "law" is wrong, or if it is right and we just aren't able to detect the results.