Recently, an international crew of scientists gathered together and conducted a nifty quantum experiment that disturbed the law of thermodynamics: they managed to create a spontaneous heat flow from a cold system to a hot one, and they did it without breaking any physical law! In order to make this work, they connected quantum thermodynamics, quantum mechanics and time itself.
Everyone knows that heat usually flows from a hot system to a cold one. For example, if you put some ice cubes in your drink, it won’t get just suddenly get hotter. So, in fact, when this heat flow happens, actually the entropy of the system increases.
In other words, by looking at the entropy of the system, it’s possible to work out if we are looking at the system going forward or backward in time. The entropy increase is defining a thermodynamic arrow of time and the macroscopic world experiences this in the same way that we do.
Maybe this sounds pretty good, but we can’t forget about one major assumption: in order to flow heat from hot to cold, the systems must be uncorrelated and there shouldn’t be any link that connects the two before you put them in contact. In other words, the ice cubes don’t really have some kind of special relationship with the molecules that your soda.
But in the quantum world this is not so obvious. You can create quantum states that are linked together and that can make the direction of the arrow of time to be reversed. Theoretically, scientists have spoken about this in the past, but this is the very first study that actually proved it with an experiment.
That’s how they set up two correlated thermal systems and actually saw how the heat flow goes from the cold to the hot system.
The team of international scientists published a paper that Is available on the arXiv and they say that what happens is really a trade-off that is happening between the entropy of the system and the quantum correlation. This trade-off is able to bring the reversal of heat flow. The thermodynamic arrow of time depends on the initial condition of a system and that’s how this result happened.
The researchers in this study actually suggest that this is not possible only in extremely microscopic systems and in future they would investigate some kind of bigger setups. They don’t know if they will make it, but it is worth the shot since entropy plays such a huge role in the scientific definition of time.