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Does Gravity Travel at the Speed of Light? (1998)

math.ucr.edu

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Re: Does Gravity Travel at the Speed of Light? (1998)

#121

Post by InclinedPlane »

prawn wrote: post #55
diminoten wrote: post #45

Earlier quoted context omitted.

The linked article directly disagrees with this post (except the last paragraph, which itself disagrees with the rest of the post), however: > The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement con…

I think she pulled up stumps and left at some point over issues with how some of the community was responding to her? Something like that. Real shame because she inspired strong interest in so many who read her explanations.

She probably just got tired of being treated the way highly visible women often are on the internet. This is not always visible to everyone else since there are private messages on reddit. It's possible things are better now, it's hard to say. I tried to perform an experiment on reddit a while ago by posting under a handle that was obviously feminine but I had to cancel the experiment because it didn't really work due to my posting style being too abnormal.

Re: Does Gravity Travel at the Speed of Light? (1998)

#122

Post by Filligree »

davedx wrote: post #120
aidenn0 wrote: post #103

Earlier quoted context omitted.

That can't happen without them annihilating very quickly. Otherwise it would be violating conservation of mass.

What about Hawking radiation? [1] Then one of the pair escapes and the other falls into a black hole. [1] http://en.wikipedia.org/wiki/Hawking_radiation

That reduces the mass of the black hole. Effectively, you have a particle tunneling out of the hole, mass is still not appearing from nowhere.

Re: Does Gravity Travel at the Speed of Light? (1998)

#123

Post by Filligree »

tocomment wrote: post #85
AlwaysBCoding wrote: post #28

This is the winning comment from the reddit discussion from /u/RobotRollCall * This is a far more interesting question than it might seem at first glance, and it deserves some attention because it tells us something fundamental and wonderful and just bloody awesome about the universe. But I don't know how to tell the story succinctly. So I'm going to do that thing I do. I am very, very sorry. Please feel free to move…

So if we ever invent artificial gravity that we can turn on and off with a switch all that stuff he said breaks down? And we get some really strange behavior?

Yep. Which is good evidence that such a thing is impossible.

Re: Does Gravity Travel at the Speed of Light? (1998)

#124

Post by jheriko »

radarsat1 wrote: post #115
tedsanders wrote: post #87

Earlier quoted context omitted.

No, I think you're right. RobotRollCall sets up a straw man with the Wile E Coyote example. Electric attraction travels at the speed of light, yet you would feel it right away after walking off a cliff.

In fact you "feel" gravity all the time, even when you're not falling... in the mechanoreceptors in your feet.

don't forget the 'sixth sense' - there are three little accelerometers in each of your ears, and cool little feedback control system that keeps your eyes constantly cancelling out the tiny variations in position that your head constantly undergoes.

http://en.wikipedia.org/wiki/Vestibular_system

afaik its the only sense which doesn't primarily detect electromagnetic effects.

Re: Does Gravity Travel at the Speed of Light? (1998)

#125

Post by jheriko »

c is a conversion factor between space and time coordinates when we project them into a space that we can intuitively grasp.

the obsession with the 'speed of light' always seems a bit senseless in this context - its taking a concept which is known to break down in some limit and then applying it right at that limit. this is bound to be confusing.

really the speed of light is like infinity in the geometry of our universe... it just happens unfortunately that the geometry is not as simple as an infinite euclidean space with an independent time coordinate so we end up attributing a bad interpretation to the 'speed' of light.

Re: Does Gravity Travel at the Speed of Light? (1998)

#126

Post by jheriko »

aidenn0 wrote: post #103
mistercow wrote: post #97

Earlier quoted context omitted.

>Like you said, what if "suddenly a black hole appeared?" > >Well, the answer of course is that that never happens, ever. Can't it, though? It would be extremely unlikely, but pairs of particles and antiparticles pop into existence constantly. Again, it would be so unlikely as to be of only academic concern, but as I understand, it is technically possible that ~10^57 of antiparticles could pop up all over the univers…

That can't happen without them annihilating very quickly. Otherwise it would be violating conservation of mass.

doesn't the 'big bang' violate this completely?

i feel any fundamental theory must violate the conservation of mass as we perceive it in order to explain existance (not to say that it isn't a very good approximation with a true underlying conservation law)

Re: Does Gravity Travel at the Speed of Light? (1998)

#127

Post by hmsimha »

acchow wrote: post #109
stcredzero wrote: post #79

Earlier quoted context omitted.

No. The change in gravitational fields propagates at the speed of light. The gravitational force vector for a star in the sky N light years away points to where the star was N years ago.

> The gravitational force vector for a star in the sky N light years away points to where the star would be if kept moving at constant speed for N years. FTFY.

Interesting. I don't know enough about physics to have a clue one way or the other, but even if it was the prior case (we base our model on where the star is for gravitational computations based on the light we are receiving), that's just what it is -- a model. So if there is a lapse in our understanding that doesn't take into account that the star's gravitational effect on us actually depends on its current position then that may need to be corrected.

Re: Does Gravity Travel at the Speed of Light? (1998)

#128

Post by radarsat1 »

jheriko wrote: post #124
radarsat1 wrote: post #115

Earlier quoted context omitted.

In fact you "feel" gravity all the time, even when you're not falling... in the mechanoreceptors in your feet.

don't forget the 'sixth sense' - there are three little accelerometers in each of your ears, and cool little feedback control system that keeps your eyes constantly cancelling out the tiny variations in position that your head constantly undergoes. http://en.wikipedia.org/wiki/Vestibular_system afaik its the only sense which doesn't primarily detect electromagnetic effects.

not to mention that the force that your muscles are exerting to stay standing up continuously keep your brain informed about proprioception.

Re: Does Gravity Travel at the Speed of Light? (1998)

#129

Post by fubari13 »

acchow wrote: post #109
stcredzero wrote: post #79

Earlier quoted context omitted.

No. The change in gravitational fields propagates at the speed of light. The gravitational force vector for a star in the sky N light years away points to where the star was N years ago.

> The gravitational force vector for a star in the sky N light years away points to where the star would be if kept moving at constant speed for N years. FTFY.

Constant speed is irrelevant in space-time calculations.

Re: Does Gravity Travel at the Speed of Light? (1998)

#130

Post by tocomment »

Filligree wrote: post #123
tocomment wrote: post #85

Earlier quoted context omitted.

So if we ever invent artificial gravity that we can turn on and off with a switch all that stuff he said breaks down? And we get some really strange behavior?

Yep. Which is good evidence that such a thing is impossible.

But what about the albicuierre drive? (sp)
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