Wednesday, August 15, 2012

Superluminal Neutrinos and Observable Quantum Effects

Koukouji asks,

"Can apparent superluminal neutrino speeds be explained as a quantum weak measurement?"
This is, in fact, the title of a recent article by Berry, Brunner, Popescu, and Shukla and has the world's greatest abstract, two words -- Probably not.

Here's why it seemed like it might be:
"The idea, following analogous theory and experiment involving light in a birefringent optical fibre, is based on the fact that the vacuum is birefringent for neutrinos. We consider the initial choice of neutrino flavour as a preselected polarization state, together with a spatially localized initial wavepacket. Since a given flavour is a superposition of mass eigenstates, which travel at different speeds, the polarization state will change during propagation, evolving into a superposition of flavours. The detection procedure postselects a polarization state, and this distorts the wavepacket and can shift its centre of mass from that expected from the mean of the neutrino velocities corresponding to the different masses. This shift can be large enough to correspond to an apparent superluminal velocity (though not one that violates relativistic causality: it cannot be employed to send signals)."
The idea is that when you measure something, you are selecting a particular property state to measure, but if the system itself is in a superposed state which evolves into that particular property state, you are going to induce an error with the assumption that it maintained the property state throughout the time period. A promising explanatory candidate. But, they argue, that when you run the numbers, it doesn't work out.

Gwydion asks,
"Quantum entanglement in bird navigation: amazing, huh? Any other examples of macro-level effects of quantum mechanics?"
Well, radioactivity, I suppose, would be number one. Lasers in everything from our dvd players to supermarkets also rely on quantized energy for their extremely coherent light.