Heat can quantum leap across a totally empty vacuum

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In a strange sort of quantum leap, heat can move across an area of complete vacuum without being carried by any particles at all. Instead, minuscule quantum fluctuations in the vacuum allow heat to leap between two objects that aren’t touching each other or emitting any form of light.

A layer of vacuum is normally a very good insulator, as anyone with a drinks flask knows, but at the quantum level, even a total vacuum isn’t completely empty: it is roiling with quantum fluctuations of energy.

In 1948, Dutch physicist Hendrik Casimir predicted that those fluctuations could create a force that pulls two objects in a vacuum towards one another in what is now called the Casimir effect. It was theorised that this effect could also cause heat to jump across a vacuum, but that was expected to take place at such small scales that it has never been measured before.

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Hao-Kun Li at the University of California, Berkeley, and his colleagues used the Casimir effect to demonstrate heat leaping between two tiny drum-like membranes. The two membranes were placed, in a vacuum, just about 300 nanometres apart separated by less than 5 per cent the width of the average red blood cell and each was attached to a reservoir of a different temperature.

The heat from the reservoirs caused the two drums to vibrate, the hotter one faster than the colder one. Because of the Casimir effect, those two sets of vibrations became coupled together at a quantum level, so the hotter membrane transferred heat to the cooler one until both were vibrating at a similar rate. This meant they had a similar temperature.

According to classical physics, there is no way to transfer vibrations from one membrane to the other without some sort of particle, like a photon, but with quantum fluctuations and the Casimir effect it is possible, says Li. So the vibration of one object can affect the vibration of another object.

This could be a useful tool in computing, says John Pendry at Imperial College London. Heat is a big issue in nanotechnology, he says, because it limits the number of computations a circuit can do, and how fast.

Now that this effect has been demonstrated, it could be used to cool circuit boards thousands of times faster than simply waiting for them to radiate the heat away, says Pendry, which could allow us to build faster computers.

Journal reference: Nature, DOI: 10.1038/s41586-019-1800-4