A BALL spinning in a vacuum should never slow down, since no outside forces are acting on it. At least that’s what Newton would have said. But what if the vacuum itself creates a type of friction that puts the brakes on spinning objects? The effect, which might soon be detectable, could act on interstellar dust grains.
In quantum mechanics, the uncertainty principle says we can never be sure that an apparent vacuum is truly empty. Instead, space is fizzing with photons that are constantly popping into and out of existence before they can be measured directly. Even though they appear only fleetingly, these “virtual” photons exert the same electromagnetic forces on the objects they encounter as normal photons do.
Now, Alejandro Manjavacas and F. Javier García de Abajo of the Institute of Optics at the Spanish National Research Council in Madrid say these forces should slow down spinning objects. Just as a head-on collision packs a bigger punch than a tap between two cars one behind the other, a virtual photon hitting an object in the direction opposite to its spin collides with greater force than if it hits in the same direction.
So over time, a spinning object will gradually slow down, even if equal numbers of virtual photons bombard it from all sides. The rotational energy it loses is then emitted as real, detectable photons (Physical Review A, DOI: 10.1103/PhysRevA.82.063827).
The strength of the effect depends on the object’s make-up and size. Objects whose electronic properties prevent them from easily absorbing electromagnetic waves, such as gold, may decelerate little or not at all. But small, low-density particles, which have less rotational momentum, slow down dramatically.
The rate of deceleration also depends on temperature, since the hotter it is the more virtual photons pop in and out of existence, producing the friction. At room temperature, a 100-nanometre-wide grain of graphite, the kind that is abundant in interstellar dust, would take about 10 years to slow to about one-third of its initial speed. At 700 °C, an average temperature for hot areas of the universe, that same speed decrease would take only 90 days. In the cold of interstellar space, it would take 2.7 million years.
Could this effect be tested in the lab? Manjavacas says the experiment would require an ultra-high vacuum and high-precision lasers to trap the nanoparticles, conditions that are “demanding but reachable in the foreseeable future”.
John Pendry of Imperial College in London calls the analysis a “fine piece of work” and says it could provide insights into whether quantum information is ever destroyed, for example, when it falls into a black hole. He says the real photons emitted during the deceleration process should contain information about the quantum state of the spinning particle, much as the photons thought to escape from black holes as Hawking radiation are thought to encode information about the holes.
“This is one of the few elementary processes that converts what appears to be purely classical mechanical energy into a highly correlated quantum state,” Pendry says.
How to float above a vacuum
Houdini would be proud. It seems there is a way to levitate an object in a vacuum just by channelling the quantum fluctuations.
The trick involves the Casimir effect, in which objects very close to one another are pulled together thanks to quantum fluctuations in the vacuum between and around them. When two plates are brought ever closer together, for example, fewer fluctuations can occur in the gap between them. Fluctuations on their outer sides, however, continue as normal. This pressure difference on either side of the plates forces them to stick together.
In recent years, physicists have been trying to develop ways to reverse the Casimir effect and repel nearby objects, causing them to levitate. Previous suggestions have included inserting various materials between the objects to be repelled – such as exotic metamaterials, which bend electromagnetic waves in the opposite way to that expected, reversing the Casimir effect.
Now, Stanislav Maslovski and Mário Silveirinha of the University of Coimbra in Portugal outline a way to repel objects with no filler material. Their setup, described in a paper to appear in Physical Review A, uses 40-nanometre-wide silver rods stuck in a substrate like candles on a cake.
The metallic “candles” would channel the fluctuations between them, pushing anything placed there away. So if a perforated metal bar was lowered over the candles, with a candle poking through each hole, the bar should float, repelled in all directions by the candles between and around each hole.
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