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Comment by _Schizotypy | Hacker News Reader

I don't think this breaks the second law, it sounds like they put some energy into the system to reverse the process

This experiment doesn't break the second law, because they have perfect knowledge of the intermediate states, so they can apply a transformation to invert the evolution.

In a big macroscopic system this is imposible in practice, because you only get a parcial knowledge of a few important properties, not of every detail.

Moreover, in most systems it is imposible to know everything about the system, but in some specially build systems like a quantum computer you can know the state of all the qbits.

That was also my understanding. This looks less like a reversal than a simulation of a reversal. They are using perfect knowledge to engineer a system that operated in reverse to the norm. Actual reversal would reverse everything, including the unknown/unknowable.

If they can read a bit now that had itself set even one microsecond in the future, flash traders will arrive with truckloads of money.

Of course the entropy also only rises necessarily in the statistical average - which is quite strong for any system with a large amount of object.

I don't think this should be getting downvoted, in my undergrad we treated entropy as a measure of the number of possible "equivalent" states, multiplicity or probability equivalently. So if you had a small enough system you might see entropy go down, by chance.

If someone downvotes this can they reply to explain why?

Late answer, I didn't downvote this though.

Anyhow, both formulations of entropy - the thermodynamical one with the integral and the statistical physics one with multiplicity - are equivalent. At least the latter already contains statistics because of the combinatorial part.

And then there is one formulation of the 2nd law which states that on statistical average the entropy rises. So it doesn't break the law if it falls down. Given that this quantum computer doesn't involve many objects, the number of combinations isn't that high. But it's nice to observe Entropy falling! (And there are other situations in which this can happen anyway.)

The reason I made the statement was because the article said

'"This is one in a series of papers on the possibility of violating the second law of thermodynamics. That law is closely related to the notion of the arrow of time that posits the one-way direction of time from the past to the future," said the study's lead author Gordey Lesovik, who heads the Laboratory of the Physics of Quantum Information Technology at MIPT.'

When I don't see how violating the second law could even be a possibility.

I think we both agree that this is not violating the second law of thermodynamics.

We disagree in that this time reversal operation need energy. It's possible to do this experiment with a frequency doubler [1] crystal in reverse to split a photon in two photons with less energy, then use mirrors to reverse the photons, and then you will be able to "see" that the photons return to the crystal and produce the original photon. [Good luck aligning all the optical equipment perfectly. This is theoretically possible, but it would be very difficult to make the experiment. Perhaps it's easier with other particles.] Anyway, the time reversal operation in this experiment only use a few perfectly aligned mirrors, so it doesn't need additional energy.

In the experiment in the article, they use a setup that is like a quantum computer. It use energy to keep everything working, but the additional energy is not necessary for the main part of the experiment. (The energy is important to make the experiment possible, i.e. transform "perfect alignment" into "we can build this".)

[1] https://en.wikipedia.org/wiki/Optical_frequency_multiplier

I didn't read it as breaking the second law, but rather a quantum theory of the second law.

It violates the law locally.

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