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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)

#131

Post by prawn »

InclinedPlane wrote: post #121
prawn wrote: post #55

Earlier quoted context omitted.

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 du…

I never noticed gender come into it on the public side, at least. A good number of people wouldn't have even been aware she was female. (Think of the Groklaw thread the other day and many assuming PJ is male.)

There was an abrasive way RRC responded to some that fired up discussions and I got the impression that this ultimately frustrated her into ending her participation. I've seen that abrasive style/response feedback loop develop with males just the same.

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

#132

Post by robotresearcher »

tokipin wrote: post #49
aphyr wrote: post #30

Earlier quoted context omitted.

Cellular automata violate continuity and isotropy, which means they're inconsistent with the continuous spatial formalism used in special relativity, general relativity, classical electrodynamics, and as far as I know, quantum electrodynamics. We have no experimental evidence inconsistent with continuous models of space. We also have evidence that Lorentz symmetry holds over scales smaller than the Planck length, whi…

I want to point out that Wolfram doesn't think the universe is a giant cellular automaton. He gives CA as examples of how easy it is to produce impermeable complexity. His more serious proposal is a graph-based system, though I haven't perused enough to know if that's also for the sake of example or if it's intended to be taken as a serious physical theory. Most likely it's an earnest attempt at a heuristic towards t…

A CA is a graph-based system.

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

#133

Post by mistercow »

jheriko wrote: post #126
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.

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)

As I understand, the main hypothesis for how the big bang does not violate energy conservation is the zero-energy universe hypothesis: http://en.wikipedia.org/wiki/Zero-energy_universe

The general idea is that negative energy due to gravity (this is hard to wrap one's head around) perfectly cancels out all of the energy in the universe (including energy in the form of mass).

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

#134

Post by jessriedel »

vehementi wrote: post #58
jessriedel wrote: post #53

Earlier quoted context omitted.

Yes, I know, but that's not what the quoted reddit comment claims: > To the absolute limit of our ability to measure it ... when an object is dropped, it begins falling instantaneously. ...This is fairly earthshaking, really. Because it implies that somehow a "signal" of some kind is getting from the ground to Wile E. faster than the speed of light. Which is supposed to be impossible.

There is no contradiction. The thing you quoted is what happens in normal circumstances (i.e. linear motion) - you instantly start falling with no delay. Which is correct. It's the change in the linear motion that gets propagated at speed of light. But in any case you still react properly because things cancel. Woo!

I am not claiming a contradiction in experimental outcomes, I am claiming that the quote draws an incorrect inference.

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

#135

Post by stcredzero »

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.

> FTFY

You learn something new everyday!

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

#136

Post by stcredzero »

gliese1337 wrote: post #111
stcredzero wrote: post #78

Earlier quoted context omitted.

You could only modulate gravity as fast as you could move energy close to you or far away from you. An implausibly powerful gamma ray flashlight flicking on and off would let you do this. Get the beam powerful enough, and it will have a gravitational field.

Get the beam powerful enough and it will have meaurable gravitational field. All beams of any intensity technically do contribute some, albeit infinitesimal, component to the gravitational field. It's a fun exercise to calculate just how strong a laser beam you'd need to have to make a black hole entirely out of photons.

A black hole can actually cause an additional singularity due to the blueshifting of infalling light. IIRC, one researcher studied this phenomenon and called it a "blue sheet."

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

#137

Post by ajuc »

vehementi wrote: post #62
spenrose wrote: post #57

So I think this article says: - the earth revolves around where the sun is almost precisely and rules out: - the earth revolves around where the sun was ~8 minutes ago (where 8 minutes is about the travel time at c). Can anyone confirm?

Yeah that's what it's saying. But it's not saying we get updates as to where the sun is instantly due to FTL gravity. It's saying objects rotate around a projection. If the linear path of the sun were to change, those changes would take a while to get to us to update our path, but also due to how that can't "just happen", things don't end up getting messed up.

But isn't sun rotating around galaxy center? So the velocity of sun isn't line, but an orbit, so it isn't constant (it's always changing direction), and we (Earth) do orbit the place that sun would be if not for the acceleration of sun caused by combined gravity of galaxy?

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

#138

Post by Filligree »

tocomment wrote: post #130
Filligree wrote: post #123

Earlier quoted context omitted.

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

But what about the albicuierre drive? (sp)

The Alcubierre drive has other problems (Krasnikov tubes look more likely, for some values of 'likely'), but not this one. There's no "instantly" in Alcubierre drives.

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

#139

Post by aidenn0 »

acchow wrote: post #108
aidenn0 wrote: post #104

Earlier quoted context omitted.

Gravity travels at the speed of light, but it pulls you towards where the object is going to be when it left the object, not where it was when it left the object.

Thanks for this. Tweaking it slightly: > Gravity travels at the speed of light, but it pulls you towards where the object would be if the object kept moving at constant speed, not where it was when it left the object

It's been 10 years since I took a physics class, but isn't it constant acceleration, not constant speed?
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