Quantum Mechanics of Gravitational Waves | Hacker News Reader

Quantum Mechanics of Gravitational Waves

journals.aps.org

117 pointsshusaku15 comments

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

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.

No, it's just an unexplained but useful pattern in the math:

https://www.quantamagazine.org/how-gravity-is-a-double-copy-...

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.

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.

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?

"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?

I’m not sure I see any similarities. Could you elaborate on why you think this is similar to the aether theory?

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

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.

To put it another way, with aether, the zeroth derivative of position may be relative ("above me" may be "below you"), but the first derivative (in principle) had a universally agreeable absolute value as the velocity of something against the universal aether.

In current physics, it is only the second derivative that is absolute. We can universally agree how much acceleration something is undergoing, but neither position nor velocity have a method for absolutely measuring them.

This can be a difficult distinction to express in English but with this math terminology it should be clear how very significant the difference is.

>We can universally agree how much acceleration something is undergoing,

You can't tell the difference between acceleration and gravitation. That's the principle of equivalence from general relativity. [0]

[0] https://www.physicsoftheuniverse.com/topics_relativity_gravi...

Pretty sure you know this, but for the benefit of other readers, extracting from the SEP we might more properly put this as, "no-one can tell the difference between uniform acceleration and being at rest while immersed in a uniform gravitational field".

A uniform gravitational field is not a feature of our universe, and especially not around our planet. More concretely, a necessary condition for a spacetime equipped with a uniform gravitational field is a constant https://en.wikipedia.org/wiki/Scalar_curvature .

In sufficiently small (compared to galaxy clusters) patches of our universe we can get an excellent (corrections in less than parts per billion) local approximation of the SEP, however, as tested by human-built space probes like MESSENGER ( https://pgda.gsfc.nasa.gov/products/66 ) and observations of natural systems like Archibald et al.'s work on the PSR J0337+1715 triple ( https://astrobites.org/2019/03/25/testing-einsteins-equivale... and to save clicks, here are the linked-to the pre-referreed version https://arxiv.org/abs/1807.02059 and the pretty animation at https://vimeo.com/83397123 ).

Additionally, in our universe one can only accelerate uniformly for a finite time, whereas in a universe equipped with a uniform gravitational field, one can be at rest eternally.

The SEP (strong equivalence principle) imposes deep requirements on the mathematical structure of any general (as in insensitive to initial conditions) theory of gravitation that is compatible with it to such high precision and on the mechanisms that generate stress-energy (that is, the non-gravitational behaviour of matter).

Wilczek himself has been writing / speaking about ether. A couple things I could dig up on short notice: - https://www.quantamagazine.org/why-feynman-diagrams-are-so-i... - https://news.asu.edu/20170208-finding-nothing-conversation-f... I'm sure there's more, including in Wilczek's book.

Addendum: I was looking for this short article from Physics Today (https://physicstoday.scitation.org/doi/10.1063/1.882562) but page 2 is behind a paywall. Then there's the book (https://www.amazon.com/Lightness-Being-Ether-Unification-For...) which appears to expand on the theme (I've not read it).

Reply on news.ycombinator.com