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Q & A: who needs gravitons?

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Most recent answer: 05/27/2013
Q:
Hi, I may be missing something here - but .... If gravity is not actually a force and is in fact just the effect of the warping of spacetime caused by mass, i.e. bodies are "falling" in a straight line in 4 dimensional space when we see them orbiting a larger mass, why is there any requirement for a graviton? Why search for one ? Regards Andy
- Andy Kelsall (age 46)
Melbourne, Australia
A:

That's a very deep question. The answer is more subtle than most of the ones we post here.

Although general relativity (GR) treats gravity as a geometrical effect of space time, it's still a sort of field. there can be waves in it, moving around very much like any classical wave. In fact, strong indirect evidence of these waves exists in the slowing of binary pulsars. So far, I still haven't given any need for gravitons.

The structure of quantum mechanics leads logically to certain uncertainty relations, by which no quantum state can have sharply defined values of each of some pair of variables, such as (position/momentum), (event time/ energy change), (x-spin/y-spin, for a spinor), etc. Now if there were any classical-like objects, whether particle-like or wave-like, lacking these uncertainties, then one could in principle make some sort of "microscope" to view the quantum state with those classical objects, unraveling the uncertainty relations, and revealing the true values of the apparently fuzzy variables. However, a series of experiments showing that certain relations, the Bell Inequalities (search this site and elsewhere), that must be obeyed if those true values exist are not in fact obeyed by our world. So we'd be left with a logical contradiction. There' seems to be no way out except for spacetime geometry to also have quantum properties, such as shot-noise in gravitational waves, like the shot-noise in light waves, arising from randomness in the graviton number analogous to randomness in the photon number. So far the quantum theory of gravity is a work in progress.*

Mike W. (posted without checking until Lee returns)

*The BICEP2 collaboration claimed to have seen what looks like the leftover effects of the quantum zero-point spread of gravitational waves, left as tiny "B-mode" ripples in the polarization of the cosmic microwave background radiation. It now looks like they really saw somethig more local and less interesting. If one of the more sensitive versions of this experiments works, then it will be fair to say that a real quantum gravity effect will have been seen. /Mike W.


(published on 05/26/2013)

Follow-Up #1: quantum gravity waves

Q:
Hi Mike, A little confused with this answer. If I'm reading it correctly the argument for Gravitons goes:- (1) Rotating Pulsars suggest that gravity waves exists in some form - even though 4 dimensional space-time treats gravity as a geometrical effect of mass (2) As Gravity waves may exists Quantum Theory uncertainty means wave particle duality exists for gravity as well hence the existence of gravitons. Is my interpretation correct ? If this is the case then is GR correct in suggesting that gravity is not a force and is a geometric effect - or is GR just using this as an analogy as it doesn't have an explanation ? I'm guessing that the identification of a Graviton would allow gravity to fit into QM - but does it have to ? I'm confused as to why it has to. trying to think in 4 dimension is impossible - but the three dimension approximation of the ball on the elastic mesh works well without any connection between the big ball and the small one - if this is extrapolated to 4 dimensions cannot things work without the force carrying particle or any interconnection between the two objects ? If one removes the big ball the mesh returns to its undeformed shape and this takes time (speed of light) meaning that the smaller ball moves off tangentially after the space-time has gone back to "flat". I just can't see the need for gravitons or gravity as a force if GR is right. I'm not a physicist - just read a lot of strange books (according to my wife) !! Regards Andy
- Andy Kelsall (age 46)
Melbourne, Australia
A:

Andy- you're on the right track, but my previous answer was too compressed.

1. The GR equations predict the necessary existence of gravity waves. Those pulsars are just the experimental case in which there's good evidence (slowing of the rotation rate) confirming that particular prediction. A wide variety of other observations confirm the accuracy of GR, which means that the waves have to exist,

2. I think you're getting at the right idea, but I never use the phrase "wave-particle duality", It's widely-used, but a relic of the early days of quantum mechanics. In most modern interpretations, nothing exists but the wave-like quantum state. Under some circumstances it has a fairly localized distribution and in other cases it's more spread out. The quantum roughness of the wave, as if particles were being counted, has a more technical description*, but no stepping outside the ordinary wave-like state. 

It's extremely awkward to try to cram GR effects into anything like Newton's spacetime. GR really is geometrical. Whether that picture will hold up well on the tiny scale on which quantum gravity becomes important, I don't know.

The existence of gravitons by itself would not suffice to fit GR comfortably with quantum mechanics. The people who work on this say that the problem is that just trying to do a routine quantization like that leads to infinities for all sorts of calculated quantities. Some sort of deeper fix seems to be needed. They say that one possibility that avoids the infinities is to extend the number of spatial dimensions to 9 (for string theory) or 10 (for M-theory).

Your argument about how the structure of GR doesn't require quantization looks ok to me. The requirement for quantization comes from all the rest of physics, which loses its self-consistency if mixed with any non-quantum ingredients.

Mike W. (posted without checking until Lee gets back)

* There exists a Hermitian operator with integer eigenvalues, which plays the role of a particle count.

 


(published on 05/26/2013)

Follow-Up #2: GR and QM

Q:
HI Mike, Thanks for your explanation. I guess that any answer is going to be abstract (to say the least ) given the topic !! It now makes more sense to me - GR doesn't need QM at the macro scale - and the geometric nature of GR explains the observable results. I wasn't aware that GR shows the need for "waves" before your explanation - so thanks for that. I guess the basic answer is that EVERYTHING else fits into QM - and therefore given the prediction that Gravity waves exist, gravity should also fit in there somehow...otherwise inconsistencies start to creep in. Not having gone through the learning process as you guys have (my degree is pure maths and is 20 years dormant) I'm jumping in at the deep end without having enough understanding of QM (or GR for that matter). All very interesting. Paul Davies' book started me on this stuff - don't get me started on entanglement or the more detailed double split experiments with secondary sensors !!! Just can't get my head around it. Ended up the only way I could understand it was to believe that everything is everywhere until you measure it - then it decides where it is....That sort of helps me sleep....Once again thank for your responses... Regards Andy
- Andy Kelsall (age 46)
Melbourne (Australia)
A:

Thanks Andy, You've got what I was saying exactly right. Sorry I couldn't make it simpler- that's probably an indication that I don't understand it as deeply as the real experts.

Mike W.


(published on 05/27/2013)

Follow-up on this answer.