I What's Known about the Wave Function and Gravity

[jon4444]

I'm trying to get a sense of the current state of knowledge regarding the relationship between gravity and quantum phenomena. For example, if you had a super-sensitive gravity detector, would that count as a "measurement" in the double-slit experiment in the same way that a particle detector does? (I.e., forcing the particle to definitely have gone through one slit or the other).
(And, tbh, someone here once firmly asserted the answer was yes, but I'm skeptical of firm assertions coming from the internet without citation.)

 

[vanhees71]

The current status is that we have a well-established quantum theory of elementary particles, the Standard Model, which is formulated in flat Minkowski space. At the same time we have a well-established classical theory of gravitation, which is General Relativity.

What we also have to some extent is quantum field theory in some not too complicated "curved" spacetimes. The crux here is to define proper asymptotic free states.

The big question is, whether there is a consistent quantum theory of gravity itself. It's hard to find it, because it's hard to find quantum effects of gravitation, because where the gravitational is detectable, it's from large macroscopic bodies, usually astronomical ones. Note that we just have the phantastic results from LIGO/VIRGO concerning (classical) gravitational waves from black-hole mergers and (much more interesting for my taste) the first from a neutron-star merger, which was also observed over a large range of wavelength in the electromagnetic spectrum. I'm also quite sceptical whether there's a chance to observe quantum effects of gravity in the near future, but you never know. With the gravitational waves, and I'm pretty sure with the recent success the measurements will get more and more refined pretty quickly, and this literally opens a entirely new window into the cosmos, and it might well be that multi-messenger astronomy (including electromagnetic and gravitational waves as well as neutrinos, where ICECUBE has made a breakthrough recently too) may give a hint at which direction the theory of quantum gravity has to take.

I do not understand which experiment you propose concerning the double-slit experiment.

 

[bhobba]

Just to add to Vanhees excellent answer - which is of course true - we do not have a theory uniting QM and gravity the same way as the theories of the standard model do. However these days we also have a slightly different take based on the re-normalization work of Ken Wilson - the effective field theory approach:
https://arxiv.org/abs/1209.3511

I think Ken viewed all our current quantum theories as just effective field theories. I could be wrong on that though - it is something I have been meaning to investigate in more detail along with the myriad of others stuff I would like to delve deeper into.

Thanks
Bill

 

[vanhees71]

It's the usual modern interpretation that all QFTs are effective theories. As usual Weinberg brings it to the point most clearly:

https://arxiv.org/abs/hep-th/9702027v1

 

[edmund cavendish]

In terms of the "effect" of the observer in terms of collapse of the wave function I think roger penrose's ideas are worth some consideration here. He posits in his talk to the consciousness symposium in san diego that gravitational facors can cause the collapse independently of any obsevation. This links I believe to his ideas about the emergence of consciousness within brain tubules and his use of gauss to assert the limits of computational intelligence. Youtube covers the consciousness conferences very well.Gravity may well therefore be crucial.

 

[jon4444]

At a crude, intuitive level, I think Penrose must be on the right track. Just because the nature of consciousness and wave function collapse are two phenomena in nature we don't really have any solid framework to understand based on our everyday world (well, maybe that's more true of consciousness than collapse). Intuitively, you might expect the two greatest unknowns in our knowledge to be linked to the same as-yet-not-understood area of Physics.

 

[jon4444]

"I do not understand which experiment you propose concerning the double-slit experiment."
I'm thinking of double slit experiments where the interference pattern can be eliminated by putting a photo detector behind one of the slits. Would the interference pattern also be eliminated if you were using electrons and detected the mass of the electron behind one of the slits?

 

[Nugatory]

Would the interference pattern also be eliminated if you were using electrons and detected the mass of the electron behind one of the slits?

Yes, assuming that an electron going through the other slit would not have its mass measured by the same measuring device.

 

[bhobba]

In terms of the "effect" of the observer in terms of collapse of the wave function I think roger penrose's ideas are worth some consideration here.

While in no way endorsing, or criticizing his views IMHO Roger Penrose is one of the better popular writers. Horror of horrors he actually uses math.

So while it's not peer reviewed literature, or a standard textbook, if it raises a query ask away. Note however he does touch a bit on philosophy which is off topic here - if in doubt write to a mentor before posting.

But just on the above the effect of the observer, even what is an observer, is a bit philosophical, I would suggest first becoming a bit more acquainted with QM proper before posing anything about it. Susskinds books are good for that at the beginner level:
https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

Also it must be said his views are not one of the major interpretations. It probably is a good idea to look at a more mainstream one first:
http://quantum.phys.cmu.edu/CQT/index.html

Thanks
Bill

 

[edmund cavendish]

Bill
Thank you for the feedback and reading suggestions.
Edmund