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Gravitational waves provide dose of reality about extra dimensions

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Re: Gravitational waves provide dose of reality about extra dimensions

#71

Post by Sharlin »

BurnGpuBurn wrote: post #65
Sharlin wrote: post #60

Earlier quoted context omitted.

Remember that Newtonian concepts of gravity such as the equation g = Gm1m2/r^2 are only strictly valid within the Newtonian framework which, among other things, does assume that the effect of gravity is instantaneous. To derive accurate equations of motion under finite propagation speed of gravity, you need the machinery of general relativity. Of course, in mundane situations like the Earth-Sun system the two models…

Thank you for your answer. True, Newton basically uses "instant gravity", and his equations really do work. I don't know enough about general relativity I must say, I've just never been able to really get it.

His equations work in the limit of slow speeds and low masses; GR keeps giving valid predictions in cases Newtonian mechanics breaks down. Famously, Newton cannot explain the slight observed precession of Mercury’s orbit which is something GR predicts precisely. But the effect is tiny and in almost all cases you don’t need a full relativistic treatment to predict the motions of Solar System bodies sufficiently accurately.

One quite mundane situation where one does need GR is compensating for the clock drift of GPS satellites caused by their height and speed relative to a ground-based observer (time is relative, not absolute as Newton thought!) Because positioning critically relies on very precise clocks, a non-adjusted satellite positioning system would very quickly start giving completely inaccurate results.

Re: Gravitational waves provide dose of reality about extra dimensions

#72

Post by foxyv »

car wrote: post #63
foxyv wrote: post #62

Earlier quoted context omitted.

I had a physics professor that ran a Gamma Ray observatory in a high altitude balloon. (He accidentally spotted a few nuclear powered satellites in the process!) The way it worked is that instead of having a lens/mirror and an aperture pointed at a specific part of the sky it had a cube of sensors. Based on which sensors are triggered you can determine direction because high energy Gamma rays cause a cascade of diffe…

Thank you, very interesting. Were those nuclear satellites known ones, as in this article: https://www.businessinsider.com/nuclear-powered-satellites-s... ? And were the Gamma ray sensors behind a collimator (basically lead plates with through-holes that only allow a very narrow angle of rays to be detected, e.g. used in Gamma cameras for nuclear medicine). Or were they in a 3D arrangement, and the timing of detectio…

He didn't mention which satellites they were, I think he was under the impression they were Russian military satellites. Unfortunately this was all during my lower division courses so I didn't get a chance to work with him. When I was doing my senior project with a different professor we used different types of detectors for gamma ray spectroscopy. The two we had were these little sodium iodide detectors and a larger liquid nitrogen cooled gallium arsenide based detector. Pretty much just chunks of metal or salt that caused Compton scattering and were detected by photon multipliers or single photon counting modules. They often had multiple channels and were not very sensitive.

It's been six years so my memory is kinda fuzzy. >_<

Re: Gravitational waves provide dose of reality about extra dimensions

#73

Post by BurnGpuBurn »

Sharlin wrote: post #70
BurnGpuBurn wrote: post #69

Earlier quoted context omitted.

If gravitational waves are not how gravity propagates, how does gravity propagate?

In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c , the speed of causality. Gravitational waves are a special case of those changes predicted by GR, inspiraling massive objects making the spacetime around them ”ring” like a bell. An eventual theory of quantum gravity may entail another viewpoint of gravity as a quantum field and mediated by a hypothetic…

"In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c, the speed of causality."

I really hope you meant speed of light there.

Regardless, this thing is still a mystery to me. Why, if gravity propagates at c (or, better perhaps: Changes in the space-time curvature, which is caused by mass-energy, propagate at c), why doesn't the earth spiral into the sun or away from it? If the earth is attracted to the point where the sun was eight minutes ago and not to where it is right now, it's orbit should not be stable. It's a very big difference compared to the "gravity is instant" way of Newton.

Re: Gravitational waves provide dose of reality about extra dimensions

#74

Post by candiodari »

JeromeBonnet wrote: post #26
mirimir wrote: post #15

> The gravitational waves from the collision reverberated in LIGO the morning of Aug. 17, 2017, followed by detections of gamma-rays, X-rays, radio waves, and optical and infrared light. If gravity were leaking into other dimensions along the way, then the signal they measured in the gravitational wave detectors would have been weaker than expected. But it wasn't. Do they really mean to say "followed by detections"?…

My understanding is that the detection order follows the underlying phenomena: first the neutron stars fell towards each other, producing gravitational waves, and only then did the stars collide, producing electromagnetic waves.

As I understand it gravitational waves aren't slowed down by anything whereas electromagnetic waves interact with matter along the way, which takes some amount of time.

There isn't much matter to interact with, but not quite zero either. Something like 1e-23 of the distance had some amount of matter in it (mostly very diffuse gas clouds, nothing more).

Re: Gravitational waves provide dose of reality about extra dimensions

#75

Post by Jernik »

BurnGpuBurn wrote: post #73
Sharlin wrote: post #70

Earlier quoted context omitted.

In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c , the speed of causality. Gravitational waves are a special case of those changes predicted by GR, inspiraling massive objects making the spacetime around them ”ring” like a bell. An eventual theory of quantum gravity may entail another viewpoint of gravity as a quantum field and mediated by a hypothetic…

"In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c, the speed of causality." I really hope you meant speed of light there. Regardless, this thing is still a mystery to me. Why, if gravity propagates at c (or, better perhaps: Changes in the space-time curvature, which is caused by mass-energy, propagate at c), why doesn't the earth spiral into the sun or…

The speed of light in a vacuum is c, which is the speed of information, or the speed of causality

Re: Gravitational waves provide dose of reality about extra dimensions

#76

Post by Sharlin »

BurnGpuBurn wrote: post #73
Sharlin wrote: post #70

Earlier quoted context omitted.

In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c , the speed of causality. Gravitational waves are a special case of those changes predicted by GR, inspiraling massive objects making the spacetime around them ”ring” like a bell. An eventual theory of quantum gravity may entail another viewpoint of gravity as a quantum field and mediated by a hypothetic…

"In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c, the speed of causality." I really hope you meant speed of light there. Regardless, this thing is still a mystery to me. Why, if gravity propagates at c (or, better perhaps: Changes in the space-time curvature, which is caused by mass-energy, propagate at c), why doesn't the earth spiral into the sun or…

I used the term "speed of causality" on purpose; that's what c is really about. The fact that the speed of light in vacuum happens to be c is just a side effect of photons being massless.

Re: Gravitational waves provide dose of reality about extra dimensions

#77

Post by BurnGpuBurn »

Sharlin wrote: post #76
BurnGpuBurn wrote: post #73

Earlier quoted context omitted.

"In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c, the speed of causality." I really hope you meant speed of light there. Regardless, this thing is still a mystery to me. Why, if gravity propagates at c (or, better perhaps: Changes in the space-time curvature, which is caused by mass-energy, propagate at c), why doesn't the earth spiral into the sun or…

I used the term "speed of causality" on purpose; that's what c is really about. The fact that the speed of light in vacuum happens to be c is just a side effect of photons being massless.

Thank you again for your of answering my questions. So.. well.. What is this speed of causality then? Can we put a number on it?

Besides this all, I still can't get anyone to give me a clear cut and dried answer on my original question. If gravity is instant in the Newtonian model, and that gives us stable orbits and all that, how does that translate to the Einsteinian model, where there's an upper speed limit of c? So, why would that even work, if the gravity of the sun takes 1au/c to get here and be felt by the earth? Or, is gravity somehow "above" this speed limit?

Re: Gravitational waves provide dose of reality about extra dimensions

#78

Post by kawakiole »

kakarot wrote: post #31
kawakiole wrote: post #27

Earlier quoted context omitted.

Anything is possible, but how would you falsify that hypothesis?

Good question and I don't know the answer, but this unknown still helps to define the limits of what we can definitely say is and isn't taking place. We can't say "we're definitely not experiencing displacement in X dimensions" if we don't definitively know the rules of displacement are homogeneous in every dimension. Since we don't definitively know the number of dimensions, we also can't exhaustively test these rul…

We don't ever definitely know anything. That's why science and bayesian reasoning in general is so useful to begin with ;)

We can be more or less confident about how stuff works though. For instance, thermodynamics is something we are very confident of having gotten right, because there are plenty of ways to test if doesn't work, and those test fail all the time (so far, at least!).

So this article basically says: would it be easier to explain what we see if there were other dimensions? and the answer is it wouldn't, because we'd have to assume that things that we are very confident about (thermodynamics) are not correct, while simultaneously not providing any way to gain confidence in this new hypothesis (that there are new dimensions with different thermodynamic laws).

This might change some day: before spectrography was a thing, all the theories about star composition were non-falsiable either!!

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