5 ms·

Quantum Mechanics of Gravitational Waves

117 points by shusaku 5y ago

a9h74j 5y ago
Not the same approach as OP, but searching "double copy gravity" should yield interesting material. Evidently if you write formulas for scattering amplitudes, there is a remarkable symmetry or similarity between gravitation and electromagnetism. I have only watched this: https://www.youtube.com/watch?v=aJkknmiCrR4 https://www.youtube.com/watch?v=aJkknmiCrR4
- betwixthewires 5y ago
  I thought double copy was based on a hypothesis that gravity is a remnant of the strong force? Perhaps in thinking of a different hypothesis.
  - T-A 5y ago
    No, it's just an unexplained but useful pattern in the math: https://www.quantamagazine.org/how-gravity-is-a-double-copy-of-other-forces-20210504/ https://www.quantamagazine.org/how-gravity-is-a-double-copy-...
deleted 5y ago
[deleted]
cortexio 5y ago
I dont think gravity has anything to do with quantum mechanics and certainly do not agree with the statement of "the fundamental laws of physics are quantum mechanical"... that makes me facepalm.
imglorp 5y ago
It bears mentioning that one of the authors is Frank Wilczek, who named the axion and anyon, won a nobel in 2004 for work in the strong force, and proposed the idea of time crystals. Not a lightweight.
- a9h74j 5y ago
  Indeed. On reading through, in a bandwidth characteristic of existing gravitational wave detectors they predict distance fluctuations of order one Planck length, so yet unobservable. However, the fourier transform of the autocorrelation function, which gives a frequency spectrum, suggests a potentially detectable signature, perhaps in some important edge cases.
Qw3r7 5y ago
"We find that falling bodies in gravity are subject to random fluctuations (“noise”) whose characteristics depend on the quantum state of the gravitational field. " So I read this, and I have not touched up on physics in awhile, but can someone explain to me the difference between their postulozation and the lumeniferous aether?
- bradrn 5y ago
  I’m not sure I see any similarities. Could you elaborate on why you think this is similar to the aether theory?
- OGMcOW2 5y ago
  My very superficial understanding is that the aether was a fictional - fluid and solid - physical medium that was supposed to model behavior but created more problems. Quantum fields are mathematical representations that model physical behavior (including the probabilities inherent in quantum mechanics). Aether, in my interpretation, was a "thing", and quantum fields are just values that describe properties of things. Depending on how crazy you wanna go, that does of course also make them things, and you could also look at the aether as a mathematical abstraction, but afaik, quantum fields work ridiculously well and are a solid theory, while aether - despite maybe being cutting edge at its time - is more one of the homeopathies of physics. That said, I'm sure if looked over by someone who actually studied QT/QM/QFT/GR/SR, they'd find what I wrote to be comically primitive and inaccurate :) edit: punctuation
- whatshisface 5y ago
  Although aether and fields share a couple similarities, that they're both conceptualized as filling space, and both are wave media, they have a big difference, which is that fields behave the same no matter how fast you are going, whereas aether is like air or water in that you move "though" it. Removing the concept of through-moving from space was one of the big changes that relativity made to scientific thinking. The big consequence of through-moving was that if you send out a wave, and then move through its medium in the direction of the emission, you will "chase after" the wave front and it will escape from you more slowly than if you sat still after sending it. It turns out that this does not happen in real life when the waves involved are light or gravity. Instead, they escape you just as quickly no matter how fast you chase after them. This was discovered in the Michelson-Morley experiment where an attempt was made to detect the motion of the earth through the universe but instead no evidence of moving through the medium that bore light was found. Edit: By the way, I think the downvotes the parent comment received are completely unfair, they are asking for an explanation, not claiming there are no differences.
whatshisface 5y ago
This comment comes a second after reading the relevant paragraph, but if they're going up to second order and ignoring graviton-graviton loops, how do they know the noise isn't cancelled back out as all orders are taken in to account?

117 points by shusaku 5y ago

Not the same approach as OP, but searching "double copy gravity" should yield interesting material. Evidently if you write formulas for scattering amplitudes, there is a remarkable symmetry or similarity between gravitation and electromagnetism. I have only watched this: https://www.youtube.com/watch?v=aJkknmiCrR4 https://www.youtube.com/watch?v=aJkknmiCrR4

betwixthewires 5y ago
I thought double copy was based on a hypothesis that gravity is a remnant of the strong force? Perhaps in thinking of a different hypothesis.
- T-A 5y ago
  No, it's just an unexplained but useful pattern in the math: https://www.quantamagazine.org/how-gravity-is-a-double-copy-of-other-forces-20210504/ https://www.quantamagazine.org/how-gravity-is-a-double-copy-...

deleted 5y ago

[deleted]

cortexio 5y ago

I dont think gravity has anything to do with quantum mechanics and certainly do not agree with the statement of "the fundamental laws of physics are quantum mechanical"... that makes me facepalm.

imglorp 5y ago

It bears mentioning that one of the authors is Frank Wilczek, who named the axion and anyon, won a nobel in 2004 for work in the strong force, and proposed the idea of time crystals. Not a lightweight.

a9h74j 5y ago
Indeed. On reading through, in a bandwidth characteristic of existing gravitational wave detectors they predict distance fluctuations of order one Planck length, so yet unobservable. However, the fourier transform of the autocorrelation function, which gives a frequency spectrum, suggests a potentially detectable signature, perhaps in some important edge cases.

Qw3r7 5y ago

"We find that falling bodies in gravity are subject to random fluctuations (“noise”) whose characteristics depend on the quantum state of the gravitational field. " So I read this, and I have not touched up on physics in awhile, but can someone explain to me the difference between their postulozation and the lumeniferous aether?

bradrn 5y ago
I’m not sure I see any similarities. Could you elaborate on why you think this is similar to the aether theory?
OGMcOW2 5y ago
My very superficial understanding is that the aether was a fictional - fluid and solid - physical medium that was supposed to model behavior but created more problems. Quantum fields are mathematical representations that model physical behavior (including the probabilities inherent in quantum mechanics). Aether, in my interpretation, was a "thing", and quantum fields are just values that describe properties of things. Depending on how crazy you wanna go, that does of course also make them things, and you could also look at the aether as a mathematical abstraction, but afaik, quantum fields work ridiculously well and are a solid theory, while aether - despite maybe being cutting edge at its time - is more one of the homeopathies of physics. That said, I'm sure if looked over by someone who actually studied QT/QM/QFT/GR/SR, they'd find what I wrote to be comically primitive and inaccurate :) edit: punctuation
whatshisface 5y ago
Although aether and fields share a couple similarities, that they're both conceptualized as filling space, and both are wave media, they have a big difference, which is that fields behave the same no matter how fast you are going, whereas aether is like air or water in that you move "though" it. Removing the concept of through-moving from space was one of the big changes that relativity made to scientific thinking. The big consequence of through-moving was that if you send out a wave, and then move through its medium in the direction of the emission, you will "chase after" the wave front and it will escape from you more slowly than if you sat still after sending it. It turns out that this does not happen in real life when the waves involved are light or gravity. Instead, they escape you just as quickly no matter how fast you chase after them. This was discovered in the Michelson-Morley experiment where an attempt was made to detect the motion of the earth through the universe but instead no evidence of moving through the medium that bore light was found. Edit: By the way, I think the downvotes the parent comment received are completely unfair, they are asking for an explanation, not claiming there are no differences.

whatshisface 5y ago

This comment comes a second after reading the relevant paragraph, but if they're going up to second order and ignoring graviton-graviton loops, how do they know the noise isn't cancelled back out as all orders are taken in to account?