Tests of General Relativity | Hacker News Reader

Tests of General Relativity

arxiv.org

42 pointsskolos19 comments

Should be renamed to 'Tests of General Relativity with GW170817', it sounds like a survey article.

Yes, please. The current title is quite misleading: I thought it was a “methods and history of GR tests” survey article when I clicked on it.

The test confirmed GR. In particular the test results contradict theories with massive graviton and theories with "gravitational leakage" into the large extra dimensions of higher-dimensional theories of gravity.

You mean "the test did for disprove" right?

Annoyingly yes. We know the theory can't be exactly true, just like we know quantum mechanics can't be, but this always happens when we test them.

As much as it'd be great to make a theory of everything, we need something to work off. Right now the physicists are honestly just spinning their wheels.

> just like we know quantum mechanics can't be

Wait, how do we know this?

The standard model doesn't include gravity, for a start.

Because we can't describe gravity with it?

General Relativity and Quantum Mechanics are incompatible with each other. You can't get GR out of QM or vice versa. Yet, annoyingly, the predictions of both continue to be shown to be correct. The grandparent is hoping for a discrepancy somewhere to provide a hook to base further theoretical developments on.

It's not that quantum mechanics isn't true, it's that the standard model must be incomplete (the standard model is a field theory that's built on top of the framework of quantum mechanics). In fact, one can derive Einstein's field equations from quantum mechanical arguments as a low-energy effective theory.

Because we exist.

Quantum field theory (QFT) does a great job describing the electromagnetic, strong and weak nuclear force, and has done a spectacular job describing reality in ways too numerous to count. But it has two principal failings.

The first is that every time someone tries to incorporate a quantum field theory of gravity, the inevitable conclusion is "the first time a graviton interacts with a particle, the entire thing collapses into a black hole". Because we haven't collapsed into a black hole, we know our theory is incomplete.

The second is the vacuum catastrophe. QFT predicts a value of the zero point vacuum energy (very closely related to dark energy, aka the cosmological constant) of roughly 10^112 ergs per cubic centimeter. With a bit of a hack, we can get this to cancel out to precisely 0. However, the observed value of the zero point vacuum energy is roughly 10^-8 ergs per cubic centimeter. Which isn't even close to 10^112, but more importantly, it is also not zero, so our hack to cancel it out doesn't work.

The pdf is 15 pages, but really only 6 pages of it is actual content. Is it common for there to be 3 full pages of authors on a paper like this? Seems a little bizarre to my untrained eye.

Yes, it's common for large scale collaboration experiment like this. Papers from ATLAS/CMS have about 15 pages for authors (for example https://arxiv.org/pdf/1806.00425.pdf https://arxiv.org/pdf/1804.02610.pdf).

As other comment say, yes, esp. for these honking-great detector devices like LIGO, LHC etc. Also from the article:

    * Deceased, February 2018.
    † Deceased, November 2017.

So sadly A. Giazotto and V. Brisson did not live to see this published.

> Is it common for there to be 3 full pages of authors on a paper like this?

Some subfields in physics have a specific culture around authorship. For example: In experimental particle physics, the authorship list can be similarly long. Despite the long list, there is an implicit assumption of a working particle accelerator, which is run by another large number of people who publish separately in their own journals.

So GR is right again, what a surprise...

It is, but it's really annoying.

What could be better than finding a situation where GR doesn't apply?

Oh yeah, it would be great, like the news about the particle whose speed exceeded c. Unfortunately I don't think it is going to happen after all these years and all these advanced experiments that only confirm it over and over.

Newton's theory of Gravitation stood for almost 230 years before being superseded by GR. During this time it was proven right over and over. And in fact in many ways what happens with great theories is not that they are proven wrong, but rather that they are extreme conditions where there predictions deviate slightly from reality. In the case of gravity I believe one of the very few deviations from Newton's gravity known was the precession of the perihelion of Mercury - there was not that much known that was wrong about it.

I think it is fair to at least hope to find deviations from GR which would have to be at much more extreme conditions given the greater precision of today's measurements.

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