Sunday, May 28, 2006

Gravity waves and spin entropy

New Scientist has promoted the following preprint

that proposes a method to observe gravitational waves via spin entropy and spin entanglement. Several people say that it is "quite an achievement" and there are only "some technical issues" that should be resolved. Frankly, I have no idea what they are talking about.

The basic mechanism underlying these 9 pages is the following simple fact.

Consider a particle with a well-defined momentum (or a wavefunction in the momentum space) and a well-defined spin state. Two-component spinors are always eigenstates of the angular momentum with respect to a particular axis in three dimensions. The particle discussed in this paragraph is described by a wavefunction that is a tensor product of the momentum state and the spin state.

Now you may want to boost the particle and switch to a difference reference frame (but please, don't switch to another blog - you won't find another Reference Frame anyway). The wavefunction in the new frame will no longer be a tensor product. You can easily see why: the wavefunction is not an eigenstate of any angular momentum in the new frame because the angular momentum in the new frame is the old angular momentum mixed with the boost generators in the old frame - and the state was not an eigenstate of the boost generator if the momentum wavefunction was general enough.




This means that by going into a different frame, you induce entanglement between the spin and the momentum. It may be the first time you hear about this trivial insight but it has been known for decades.

Ye Yeo et al. now realize that a gravitational wave is able to boost the spinning particle. Consequently, the gravitational wave is able to induce the entanglement between the spin and the momentum.

Well, a more correct statement is that the gravitational wave, much like any other influence or force in the Universe, is able to cause this effect. I have no idea why they have singled out the gravitational waves. Also, I am pretty sure that neither of the main methods and additional tricks is going to be able to improve the sensitivity so that we would need no LISAs, LIGOs, or VIRGOs. If there is no resonance in the picture - and it seems that there is none - there is no chance how this method could make the measurement easier than the classical measurements.

The amount of the "spin entropy" is proportional to the change of the momentum. If properly defined, they are of the same order. Moreover, don't forget that if a wavefunction has a component of order "epsilon", it only implies tiny new probabilities of order "epsilon^2". If the measurements are going to have a different sensitivity, they will be less sensitive, not more sensitive, than the traditional machines to detect gravity waves.

To summarize, the combination of ideas in that preprint looks illogical and random to me.

Incidentally, Cosmic Variance asks:

  • [W]hy do people so often use those words ["spooky" and "weird"] when talking about quantum mechanics? why? why? why? why?!

Let me answer this question. The answer will apparently be shocking news for Cosmic Variance.

People use the words "spooky" and "weird" because the laws of quantum mechanics are not only true and extremely accurate, but they are also the spookiest and weirdest aspects of reality that the humans have revealed so far.

I hope that the PC police will kindly allow me to use the term "laws of quantum mechanics". ;-) Thank you.

A celebrated Harvard physicist has said that if thousands of philosophers were trying to invent the weirdest thing possible, they would have never invented something as weird as quantum mechanics.

Niels Bohr, a father of quantum mechanics, said that anyone who has never been shocked by quantum mechanics has not understood it.

Einstein used the term "spooky action at a distance" for EPR correlations that are indeed predicted by quantum mechanics and experimentally verified. And finally, Richard Feynman paraphrased a comment that "only 12 people understood general relativity" by saying that "I think it is safe to say that no one understands quantum mechanics".

Although your humble correspondent thinks that it has been quite some time since the birth of quantum mechanics and this framework should sound much more familiar to us today, it is still the spookiest and weirdest feature of the Cosmos that we know so far. Does it make sense, Clifford?


This has nothing to do with idea by Erik Verlinde that gravity doesn't exist.

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