Thursday, February 24, 2011

Friction in a Vacuum

Vacuum has friction after all 
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)...
 I'm kind of surprised that Stephen Hawking didn't pick up on this first. It's related to the way he predicted the evaporation of black holes, the creations of virtual particles near the object, and then an unequal distribution of the results. We'll, I guess he's got a lot on his plate.

So why doesn't it slow forward progress of an object through space? That's relatively easy.  By the principle of relativity, no object can be said to be absolutely moving in space, only with respect to other objects.  Therefore the virtual particle act evenly on the particle.  Spin, however, is absolute, and can be detected without reference to outside frames of reference.

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