Gravity's speed of propagation

[ramparts]

Gravity propagates at c, but unlike light, once gravity is there, it stays. That's the difference. So whatever is causing this gravitational pull (if that's in fact what this is - this result has a few question marks) was at one point visible to us (pretty much everything was at one point), but no longer is. Still, since gravity doesn't disappear instantaneously, the gravitational field remains.

 

[ianke]

Hi again rampart,
So if I get this straight, even thought the claim is ceretainly in need of further study at best, It doesn't break any accepted GR or SR principles as it is hypothesized?

This sounds to me like an interesting lead to follow because, as far as I know, it is the first possible evidence of the universe actually being larger than the observed size.

I realize the size of the observed universe now is several times the size we see as we are looking at what was the edge about 13.7 Billion years ago (once you factor for spacetime expansion). As far as I know though, no extra matter has ever been proved to be out past the limitations of the known stuff.

 

[ramparts]

Hi again rampart,
So if I get this straight, even thought the claim is ceretainly in need of further study at best, It doesn't break any accepted GR or SR principles as it is hypothesized?

Yep - nothing wrong with that No obvious violations of well-established physics.

This sounds to me like an interesting lead to follow because, as far as I know, it is the first possible evidence of the universe actually being larger than the observed size.

I realize the size of the observed universe now is several times the size we see as we are looking at what was the edge about 13.7 Billion years ago (once you factor for spacetime expansion). As far as I know though, no extra matter has ever been proved to be out past the limitations of the known stuff.

Well, it's hard to "prove" the existence of matter outside the visible universe, but we can make a pretty good guess that there's stuff we can't see. Think about it - we see the same amount of matter in every direction on the sky, and we don't see an end to any of it. So if the universe isn't bigger than what we can observe, then it has to be exactly the size of the observable universe, and we have to be directly in the center of it. That's a huge philosophical jump, and extraordinarily unlikely.

 

[ianke]

Thank you ramparts and the rest for giving me some more food for thought. You guys are always a great pleasure to converse with.

I had forgotten about that improbable center of the galaxy part of the whole thought experiment.

It seems like any time you try to break down the whole into smaller bite sized pieces, the whole elephant gets pissed and steps on your foot. Elephants are odd that way.

 

[ramparts]

Yes. Exactly. Elephants.

 

[SpaceTas]

Warning signs of a pissed-off elephant: staring at you flapping ears, stomping, ---> mock charges.


Yes there is/was a big debate about the speed measurement for gravity using radio waves deflected by Jupiter's gravity. This is very close to using gravity to measure the how constancy of the sped of light. There is some very deep thinking needed which hasn't been resolved. Good luck to all those who attempt it.

The "simple" experiment would be to measure the time delay between a gravity wave signal measured at two observatories.

Signs of a pissed (drunk) elephant; the mini parked in front of the pub, and the empty kegs

 

[xmo1]

The general consensus is the the propagation speed of gravity is the speed of light, and I would agree here, but disagree that gravity is a curvature of space. I prefer to consider it to be wave originating from matter.

I like the instantaneous idea; gravity as a geometric condition, since no particle has been found. The mathematical model is easy, as calculating instantaneous conditions is a done deal. Space-time would be observed if an object were to be removed (or destroyed) within it. There shouldn't be any question at this point in time whether gravity propagates or is an instantaneous (geometric) condition.

General consensus? Please. Haha.

 

[ramparts]

General consensus? Please. Haha.

I know. It's only based on silly things like theoretical consistency and rigorous experimental evidence. Pshaw.

 

[R1]

Is it possible that dark energy and hyperinflation are somehow closely related?

 

[ramparts]

It's possible - indeed, that would be an elegant solution. However, I'm unaware of any model which successfully does this (I'm sure someone has contrived one somewhere, but there's no reason to buy it), and there's certainly no observational indications of it.

 

[R1]

Gravity propagates at c, but unlike light, once gravity is there, it stays. That's the difference. So whatever is causing this gravitational pull (if that's in fact what this is - this result has a few question marks) was at one point visible to us (pretty much everything was at one point), but no longer is. Still, since gravity doesn't disappear instantaneously, the gravitational field remains.

I'm not sure I understand you completely.

...whatever is causing which gravitaional pull?...

 

[ramparts]

I was referring to an article posted somewhere back in this thread. I believe it was one talking about an observational result claiming to see distant galaxies moving in response to a gravitational source outside our observable universe.

 

[R1]

Oh..ok.

 

[R1]

Which came first, inflation?
Or such thing as 'mass' ?

 

[origin]

Which came first, inflation?
Or such thing as 'mass' ?

Inflation. Quark formation came at the end of the inflationary period.

 

[R1]

And is it safe to think that gravity did not exist until the instant that 'mass' existed?

If gravitinos (or whatever they're called) do seem to exist, could they have existed prior
to the moment that mass first appeared?

 

[xmo1]

General consensus? Please. Haha.

I know. It's only based on silly things like theoretical consistency and rigorous experimental evidence. Pshaw.

Theoretical - Experimental? What about a statement like: Astronomy has not observed to date the gravitational effects of objects large enough or far enough apart to conclude that gravity is either a force or a geometric condition.

Is it that our collection of devices fails us, or that no one is looking?

 

[ramparts]

Whether gravity is a "force" or actually a condition of geometry might be an unanswerable question - the geometric picture accords extremely well with the rather elegant math, which makes little sense otherwise, but then again it's possible that gravity as geometry is just a convenient picture.

Either way of looking at gravity, though, predicts that gravity propagates at the speed of light, and that if it doesn't, it has to propagate more slowly. In the quote you just used, I was referring to your claim (one post up) which seemed to say that if gravity is geometric, then it would propagate instantaneously. Nonsense. Theoretical consistency with special relativity - which is a very, very well-tested theory - demands that it be unable to propagate faster than light, lest we get into hairy issues of causality. Theoretical consistency with quantum field theory - another very well-tested theory - demands that no force travel faster than light. And on top of being theoretically consistent with well-established theories (which is very important), the propagation speed has been measured to be within a small percentage of the speed of light by direct experiment.

 

[xmo1]

Whether gravity is a "force" or actually a condition of geometry.

Even if it were a geometric condition rather than a force, the structure (space-time) would have light speed constraints. I got it. Whew, thanks.

 

[ramparts]

Not a problem c is fundamentally engrained in spacetime (rather than just being a consequence of electromagnetism), so it makes sense that gravity - an effect of spacetime - would propagate at that speed.