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Your lukewarm cup of coffee won’t suddenly heat itself up, no matter how long you put off the trek to the microwave. But the same rule doesn’t necessarily apply to quantum systems. Like chilly air warming a mug, heat can spontaneously flow from a cold quantum particle to a hotter one under certain conditions, researchers report November 10 at arXiv.org. This phenomenon seems to reverse the “arrow of time,” the idea that natural processes run forward but not in reverse (SN: 7/25/15, p. 15).

The existence of an arrow follows from the second law of thermodynamics. The law states that entropy, or disorder, tends to increase over time. That rule explains why it’s easy to shatter a glass but hard to put it back together, and why heat spontaneously flows from hot to cold but not the opposite direction.

The new result, however, “shows that the arrow of time is not an absolute concept, but a relative concept,” says study coauthor Eric Lutz, a theoretical physicist at the University of Erlangen-Nürnberg in Germany. Different systems can have arrows of time that point in different directions, Lutz says. While the arrow was apparently reversed for the two quantum particles the researchers studied, for example, the arrow pointed in its typical direction in the rest of the laboratory.

Reversing the arrow of time was possible for the quantum particles because they were correlated — their properties were linked in a way that isn’t possible for larger objects, a relationship akin to quantum entanglement but not as strong. This correlation means that the particles share some information. In thermodynamics, information has physical significance (SN: 5/28/16, p. 10). “There’s order in the form of correlations,” says physicist David Jennings of the University of Oxford, who was not involved with the research. “This order is like fuel” that can be consumed to drive heat to flow in reverse.

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Category: Science