At loggerheads: John Pendry (left) and Ulf Leonhardt (right)
By James Dacey
Everybody loves a good, strong disagreement between two academics at the top of their game, especially when their positions are polar opposites. Two recent papers, by John Pendry of Imperial College, London, and Ulf Leonhardt of St Andrews in Scotland, draw our attention to one such fracas that is really starting to heat up.
The issue in question is quantum friction – does it exist? In a nutshell: Pendry says, absolutely yes; while Leonhardt says, not a chance.
If such a force does prove to exist, as well as crowning a winner in this debate, it could be of great interest to engineers trying to improve the performance of ultra-small mechanical devices.
Let me give you a brief history of the issue…
Over the past few years, Pendry and a number of others have advocated the existence of quantum friction, by building on the pioneering work of Dutch physicist, Hendrick Casimir.
In the mid 20th century, Casimir worked out that two flat surfaces placed in a vacuum should be attracted to one another. This force arises from the fact that, according to quantum mechanics, the energy of an electromagnetic field in a vacuum is not zero but continuously fluctuates around a certain mean value, known as the “zero-point energy”. Casimir showed that the resulting radiation pressure outside the plates will tend to be slightly greater than that between the plates and therefore the plates will be forced together.
Pendry and others became interested in the situation where the first surface is moving relative to the second one, claiming that friction should exist between the two. Pendry, who is chair in theoretical solid-state physics at Imperial College, develops the idea in a new paper in the New Journal of Physics. He argues that fluctuations in the first surface appear to be moving with a Doppler-shifted velocity, relative to the first surface. This Doppler shift destroys the balance between fluctuations as there are now more of them travelling against the relative surface motion than there are heading in the opposite direction. This, he believes, leads to a net frictional force.
This argument, however, is strongly rejected by Leonhardt, who is chair in theoretical physics at the University of St Andrews. In a comment paper submitted to the arXiv preprint server, Leonhardt claims that Pendry has described quantum friction “qualitatively”, but not quantitatively. Leonhardt argues that, “there is no experimental evidence for or against this effect, no facts”. He criticizes Pendry’s idea of quantum friction claiming that “one could apply the same effect to extract an unlimited amount of useful energy from the quantum vacuum”.
Leonhardt contrasts Pendry’s academic efforts with his own approach to this topic, referring to a paper he co-authored last year. In this paper – developing the earlier work of Soviet physicist, Evgeny Lifshitz – Leonhardt carries out an “exact calculation” for a particular configuration of plates, which shows quantum friction to equal precisely zero.
This calculation of Leonhardt is scrutinized in Pendry’s latest paper, and the Imperial researcher is less than impressed by it. Pendry says that Leonhardt has essentially shifted the goalposts on the problem. “[Leonhardt's team] claim that a moving surface can be replaced by a stationary one that is bianisotropic,” he told me. “Of course, this leads to zero net friction in their theory.”
So, as you can see, the issues that still need to be resolved include: what exactly constitutes a moving surface; and the conditions that could trigger what Pendry refers to as a “Doppler-induced imbalance”.
For now though, the argument rages on.