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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

#61

Post by kirykl »

BurnGpuBurn wrote: post #46
Animats wrote: post #16

Nice. A related result from the neutron star collision is that the speed of light and the speed of gravity are the same, to within something like 1 part in 10^15. Both light and gravity waves from an event 130 million light years away came in less than 2 seconds apart.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

I think this helps answer your question https://physics.stackexchange.com/questions/252015/gravity-v...

Re: Gravitational waves provide dose of reality about extra dimensions

#62

Post by foxyv »

car wrote: post #25
Animats wrote: post #16

Nice. A related result from the neutron star collision is that the speed of light and the speed of gravity are the same, to within something like 1 part in 10^15. Both light and gravity waves from an event 130 million light years away came in less than 2 seconds apart.

How do we know that the 2 types of waves came 2 seconds apart? Didn't we have to hunt for the light source after the gravitational wave detection, which must have taken some time? Or were there cameras already trained on this part of the sky? Edit: Ok, read the linked article, and the explanation is the detection of gamma radiation, which is part of the electromagnetic spectrum 1.7 seconds after the gravitational wav…

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 different radiation when they hit a target. (Compton Scattering I think...) The downside of this method is the angular resolution is super low.

Some Gamma ray telescopes are directional like Swift, but yeah, these Gamma Ray Bursts can be detected by non-directional observatories.

Re: Gravitational waves provide dose of reality about extra dimensions

#63

Post by car »

foxyv wrote: post #62
car wrote: post #25

Earlier quoted context omitted.

How do we know that the 2 types of waves came 2 seconds apart? Didn't we have to hunt for the light source after the gravitational wave detection, which must have taken some time? Or were there cameras already trained on this part of the sky? Edit: Ok, read the linked article, and the explanation is the detection of gamma radiation, which is part of the electromagnetic spectrum 1.7 seconds after the gravitational wav…

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 detection gave the direction? You said cube, so I assume it's the latter, but wanted to be sure.

Re: Gravitational waves provide dose of reality about extra dimensions

#64

Post by BurnGpuBurn »

andrewla wrote: post #52
BurnGpuBurn wrote: post #46

Earlier quoted context omitted.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

Looking at just the Earth-Sun interaction, Earth is moving in a gravitational field that is unchanging. It's not that Earth is being pulled towards where the sun was eight minutes ago, but rather that it continues to move in free-fall in a region of space-time that is curved in a time-invariant manner. If the sun were to vanish completely, then an observer above the elliptic of the solar system would see Earth transi…

Thank you for your answer. I've heard the "it's an unchanging gravitational field" argument before, but it doesn't really explain the question for me. If the suns gravity is a field, then why is the movement of that field (which is locked to the suns mass I presume) instantaneously "known" to the earth, so the earth "free falls" in the curved space time in the right direction holding its orbit. Why doesn't the earth, at 1 au, free fall into the space time curvature that was created by the sun 8 minutes ago? It certainly appears the earth "knows" where the sun is now, which would imply instant information travel.

I also think you contradict yourself in your last statement, in which you say that the gravitational field of the sun does take 8 minutes to reach earth.

Re: Gravitational waves provide dose of reality about extra dimensions

#65

Post by BurnGpuBurn »

Sharlin wrote: post #60
BurnGpuBurn wrote: post #46

Earlier quoted context omitted.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

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.

Re: Gravitational waves provide dose of reality about extra dimensions

#66

Post by [deleted] »

kirykl wrote: post #61
BurnGpuBurn wrote: post #46

Earlier quoted context omitted.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

I think this helps answer your question https://physics.stackexchange.com/questions/252015/gravity-v...

[deleted]

Re: Gravitational waves provide dose of reality about extra dimensions

#67

Post by BurnGpuBurn »

sxv wrote: post #54
BurnGpuBurn wrote: post #46

Earlier quoted context omitted.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

I'll let someone else answer your question, but if you enjoy thinking about these things, you might enjoy this vsauce video about the "speed of dark": https://www.youtube.com/watch?v=JTvcpdfGUtQ

Thank you, I will watch it when off of work.

Re: Gravitational waves provide dose of reality about extra dimensions

#68

Post by BurnGpuBurn »

kirykl wrote: post #61
BurnGpuBurn wrote: post #46

Earlier quoted context omitted.

I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.

I think this helps answer your question https://physics.stackexchange.com/questions/252015/gravity-v...

Thanks! That's a long read. I've started at the original question, but it'll take a while to process.

Re: Gravitational waves provide dose of reality about extra dimensions

#69

Post by BurnGpuBurn »

Sharlin wrote: post #59
SketchySeaBeast wrote: post #57

Earlier quoted context omitted.

That's true - you'd have a possible gravitational blue/red shift at different points.

Gravitational waves aren't how gravity propagates, however. They are really only relevant in exotic situations like mergers of massive compact objects, where spacetime is warped enough that orbits aren't stable and the bodies spiral inwards, radiating their excess gravitational potential energy in the form of gravitational waves.

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

Re: Gravitational waves provide dose of reality about extra dimensions

#70

Post by Sharlin »

BurnGpuBurn wrote: post #69
Sharlin wrote: post #59

Earlier quoted context omitted.

Gravitational waves aren't how gravity propagates, however. They are really only relevant in exotic situations like mergers of massive compact objects, where spacetime is warped enough that orbits aren't stable and the bodies spiral inwards, radiating their excess gravitational potential energy in the form of gravitational waves.

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 hypothetical graviton. But as of now we don’t have a good idea of what such a theory might look like; unifying QM and GR is famously one of the great unsolved problems in physics.

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