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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.
…Back again, It’s 6:30 am here in jolly old Northern Wisconsin, so we all here need as much caffein as we can get at this ungodly time of day.
The Casimir effect arises from quantum fluctuations in the very spacetime continuum that Einstein spoke of. Quantum friction (QF) must arise between the moving plate (relative to the other plate or relative to the absolute continuum?) and the continuum itself. A zero point energy implies that the vacuum exists within a manifold of energy states. Yet, the effect of relative motion is still evident and QF means that there should be a sort of redshift or blueshift in the virtual radiation that the vacuum engenders. This all sounds very relativistic to me, yet it is actually a quantum phenomenon. Does not this suggest that QF may be a crack in the door of the vault sealing off relativity from quantum mechanics?
Maybe quantum mechanics and relativity are not so incompatible after all. And QF might be interpreted relativistically as well as quantum mechanically, thereby providing a sort of Rosetta Stone to translate one to the other.
Pendry’s friction was derived by several means, and other authors, and ultimately rests on the seemingly-solid idea that lag in the images of a surface will translate into drag. I can’t pierce through the math (at least not without great effort, that I’m not sure I’ll take), but it just seems *plausible*. It is also analogous to other cases of environmentally-induced drag, such as Brownian motion, that is due to thermal (rather than quantum) fluctuations. The symmetry breaking due to the Doppler effect, for example, is essentially analogous to the symmetry breaking noted by L. Diosi in developing a theory of quantum Brownian motion (Europhysics Lett, 1995). This is all circumstantial, none of these derivations (or any others that I know of) is a truly first-principles exact quantum derivation. We’ll have to wait for this argument to be sorted through thoroughly, but my money is on Pendry. Perhaps we’ll need to wait for an experiment, as Leonhardt implies.
The argument, however, is only about one sort of quantum friction – friction caused by quantum fluctuations that act as a dissipative environment. There are other sorts friction. Stephen M Barnett even claims that quantum Brownian friction is due to the affects of quantum measurements, so it too is actually quantum (I disagree). Your truly argued for the existence internal quantum friction, ultimately due to the non-commutative nature of time-dependent (quantum) Hamiltonian dynamics. There are lots of people talking about quantum friction, and they mean different things.
Yair Rezek
If the quantum friction is real, then the principle of relativity is false because vacuum would be the ultimate referential. If the vacuum is the same in all referentials then his fluctuations will be the same for all too. The only possible source of friction would come from the reflected photons between the plates. These photons can be reflected only if the Heisenberg’s uncertainty principle about energy and time is respected. The energy of the photon determines the time it will exist and if the distance between the plates is too long then they won’t be able to exchange photons emitted by each other.