| Author |
Message |
    
Miisa
flint knapper
Username: Miisa
Post Number: 755
Registered: 5-2003
| | Posted on Thursday, March 23, 2006 - 7:44 am: |     |
Something has been on my mind for a while, and it is "What is the speed of gravity?".
Suppose the sun just suddenly winked out of existence (bear with me). I know it would take us 8 minutes to notice the lack of light, and it would take about an hour before Jupitarians could see it, but what about gravity? At what point would the planets start spiralling out of orbit? All of them at the same time, instantly? Or does gravity travel at the speed of light? Or slower? |
Ted
hunter
Username: Ted
Post Number: 396
Registered: 5-2004
| | Posted on Thursday, March 23, 2006 - 7:52 am: | |
My garbage bag of a memory says that it is hypothesized that gravity travels at the speed of light, but that it is not proved at this stage.
Again the voices in my head say that gravity is transmitted by gravity waves, but gravity waves have not yet been demonstrated convincingly, though lots of experimenters would dearly love to be the first to publish positive results.
Any one with reliable info?
Ted
Do not go gentle into that good night...Rage, rage against the dying of the light
Benjamin Disraeli: "The Jews are a nervous people. Nineteen centuries of Christian love have taken a toll." |
Thales
gatherer
Username: Thales
Post Number: 91
Registered: 9-2003
| | Posted on Thursday, March 23, 2006 - 12:07 pm: | |
That pretty much agrees with what I have read about it. An unchanging gravitational field simply exists, but any changes are supposed to be transmitted by gravitons, which are supposed to travel at the speed of light.
But what else would happen? Some time ago I read that the Earth would cool to about 3 Kelvins in a few minutes, and I simply don't believe that. My guess is that it would be something like one degree per hour, unless the process is somewhat slowed down by the heat stored in oceans.
I think that most people would die during the second 24 hour period.
Heisenberg was here ... or over there ... well, maybe somwhere else ... |
Catfriend
gatherer
Username: Catfriend
Post Number: 122
Registered: 12-2005
| | Posted on Thursday, March 23, 2006 - 12:25 pm: | |
A basic rule (if we accept the general relativity) is that information (any kind of information!) can't be transferred faster than c, simply impossible (if, again, we accept information-travelling is affected by the Lorentz transformation). So, in this case, yes - gravity moves even a bit slower than light (yet another assumption I'm using is that the amount of information we're talking about right now is finite. Otherwise, the exact speed of light will be possibe to reach.)
Just a brief comment about those impossibilities -
the so-called Lorentz gama is:
1 / sqrt(1-(v^2/c^2)), where c is the speed of light and v is the speed of the object being discussed. Thus, if v=c, we have division by zero, and if v>c, we have a complex number due to the root of negative.
However, new voices are being heard, voices claiming the general relativity can't be applied to all kinds of information. Tachions and quantum entanglement are all beyond my level of understanding, so I'll leave that to somebody else, more wise and learned
One Ring to rule them all
One Ring to find them
One Ring to bring them all
And in the Darkness bind them! |
Cavebear
cave painter
Username: Cavebear
Post Number: 2527
Registered: 9-2003
| | Posted on Friday, March 24, 2006 - 2:24 am: | |
Yet, how could the speed of gravity be tested? It's possible to initiate or measure the exposure of a light source. It can be discretely created or observed. Yet, we cannot suddenly create a large gravity source for an experiment.
It is an interesting question.
Machiavelli was pretty devious. For a guy... |
Scott
flint knapper
Username: Scott
Post Number: 1305
Registered: 5-2003
| | Posted on Friday, March 24, 2006 - 6:11 am: | |
FTL velocities violate Special Relativity, but not Lorentzian relativity Catfriend.
Gravity is for all intents and purposes instantaneous, that is there is no delay. Its velocity would be close to infinite in practical terms. General relativity supposes that gravity is not a force to be propagated through space-time, but rather a geometric function of curved space-time, hence; Catfriend, gravity has no counterpart force directly proportional to v/c.
I typed in 'speed of gravity' into google and got this page. Really informative!
Speed of Gravity
Scott
~ ~ ~ ~ ~ ~ ~ ~
Ces gens, Jondalar, ils sourient. Ils me sourient. - Ayla |
Scott
flint knapper
Username: Scott
Post Number: 1306
Registered: 5-2003
| | Posted on Friday, March 24, 2006 - 6:22 am: | |
A counter-argument is here. It seems to me that they are simply measuring the speed of light, not gravity!
Scott
~ ~ ~ ~ ~ ~ ~ ~
Ces gens, Jondalar, ils sourient. Ils me sourient. - Ayla |
Thales
gatherer
Username: Thales
Post Number: 92
Registered: 9-2003
| | Posted on Friday, March 24, 2006 - 3:24 pm: | |
"Gravity is for all intents and purposes instantaneous"
which is what I meant with "an unchanging gravitational field simply exists" (gravitons are not associated with an unchanging field), but it seems to me that a change in a gravitational field qualifies as information, and therefore can't travel at a speed faster than c.
Heisenberg was here ... or over there ... well, maybe somwhere else ... |
Catfriend
gatherer
Username: Catfriend
Post Number: 123
Registered: 12-2005
| | Posted on Friday, March 24, 2006 - 4:04 pm: | |
Hmmmm great (yet confusing!) two sites, Scott!
So, the main question seems to be whether gravity is a constant field, "simply existing", or a kind of information.
Let me propose the following experiment concerning information-propagation in a seemingly "simply existing object":
Suppose you have an iron rod, 300,000 kms long. You pull one end - automatic logic suggests the other end will move instantly - after all, matter is definitely something constantly existing! However, at second thought, the other end moving would be some sort of information transferred faster than c... And indeed, the iron rod would stretch and change so that the far end will "know" of the pull after some time, not at t=0.
Ok, I know this experiment is not exactly analogous to gravity, yet I think the basic idea of matter (or an unchanging field) twisting itself to keep the "c velocity" barrier intact is quite similar.
Let me know if what I'm saying is BS
One Ring to rule them all
One Ring to find them
One Ring to bring them all
And in the Darkness bind them! |
Cavebear
cave painter
Username: Cavebear
Post Number: 2529
Registered: 9-2003
| | Posted on Friday, March 24, 2006 - 11:58 pm: | |
Wouldn't a long rod of any material stretch physically if pulled from one end suddenly? That isn't so much "time", as it is just the physical properties of the matter.
However, if you had a material that would not physical stretch (call it "unobtanium"), would the opposite end immediately move the same as the pulled end? That's the question, right?
Machiavelli was pretty devious. For a guy... |
Catfriend
gatherer
Username: Catfriend
Post Number: 125
Registered: 12-2005
| | Posted on Saturday, March 25, 2006 - 7:20 am: | |
Yes, I meant some sort of unobtainium.
As I understand it, there isn't any kind of interaction which is, strictly speaking, instantaneous. All processes take some minimal time to work.
One Ring to rule them all
One Ring to find them
One Ring to bring them all
And in the Darkness bind them! |
Cavebear
cave painter
Username: Cavebear
Post Number: 2548
Registered: 9-2003
| | Posted on Sunday, March 26, 2006 - 8:58 pm: | |
So, OK, if unobtainium didn't stretch or compress, that might make it seem to move FTL.
Actually, I had an experiment in mind. The rod of unobtanium with transitters at both ends. A receiver set to measure the distance to each transmitter. One end of the rod gets moved. The receiver will measure the positions and movements of both ends.
Might the rod move end to end so uniformly as to be immediate?
OK, any matter that dense would probably have to immediately collapse into a sphere. So lets say a sphere of black hole matter. If you could hit it on one side, would the movement effect the other side FTL?
I don't expect we really know the answer (or will be able to answer such a question for millenia), but it is interesting to think about.
Still, I have trouble thinking of gravity as being other than instananeous (but mostly because matter can't be created in a spot immediately).
Machiavelli was pretty devious. For a guy... |
Thales
gatherer
Username: Thales
Post Number: 93
Registered: 9-2003
| | Posted on Monday, March 27, 2006 - 7:54 am: | |
"matter can't be created in a spot immediately"
In theory it can, because matter can be made from energy, but I believe our current abilities are limited to subatomic particles, and not many of those at the same time, not enough to create an easily detectable gravitational field.
Heisenberg was here ... or over there ... well, maybe somwhere else ... |
Pine
flint knapper
Username: Pine
Post Number: 1174
Registered: 5-2003
| | Posted on Wednesday, March 29, 2006 - 5:01 pm: | |
Guys, even if the other end of the unobtainium rod moves instantaneously with the side you are pulling, it will take time for you to see the other end moving towards you due to speed of light - the moment you start pulling the other end is observed at rest for 1 more second, then it starts moving. So the rod will seem to be stretched, then 1 second after your end comes to rest it will seem to resume its original size. Conversely to an observer at the other end it will seem to contract as that end moves away from the observer while your end remains in place for one more second.
Cohen's Law: 'Unless you fail at more than 10% of the things you try, you aren't trying enough things.' |
Scott
flint knapper
Username: Scott
Post Number: 1308
Registered: 5-2003
| | Posted on Friday, March 31, 2006 - 2:52 am: | |
Or we could do the reverse, turn matter into energy, therby instantly decreasing mass. We could measure the propagation of gravity that way, though I don't know *how* we could do that withough blowing ourselves up!
Scott
~ ~ ~ ~ ~ ~ ~ ~
Ces gens, Jondalar, ils sourient. Ils me sourient. - Ayla |
Cavebear
cave painter
Username: Cavebear
Post Number: 2563
Registered: 9-2003
| | Posted on Friday, March 31, 2006 - 5:16 pm: | |
Pine - Still, the receiver equidistant from the 2 ends would detect both ends moving at the same time.
Scott - That might be the easier way to test gravity. Arrange 2 asteroids in orbit around their common center of gravity. Turn one into energy (a nuke in the center of it?). See if the other one changes its motion FTL.
Machiavelli was pretty devious. For a guy... |
Ted
hunter
Username: Ted
Post Number: 403
Registered: 5-2004
| | Posted on Friday, March 31, 2006 - 8:39 pm: | |
Cavebear - yes, but the pull is at one end of the rod.
The receiver in the middle will detect the centre of the rod moving before it receives the signal from either end of the rod that it has moved.
It is logically equivalent to giving the rod a lengthwise push from the middle. The signal from the ends will arrive after the push has been given.
Ted
Do not go gentle into that good night...Rage, rage against the dying of the light
Benjamin Disraeli: "The Jews are a nervous people. Nineteen centuries of Christian love have taken a toll." |
Cavebear
cave painter
Username: Cavebear
Post Number: 2569
Registered: 9-2003
| | Posted on Sunday, April 02, 2006 - 8:53 pm: | |
The transmitters in my thought experiment were at the ends of the rods with a receiver equidistant to the ends.
It does not matter that the speed of information takes lightspeed, only that the information received indicated that the movement of the 2 ends of the rod occurred simultaneously (which is why equidistant point matters). Yes non-equidistant points could be adjusted-for, but I want to reduce extraneous factors.
Machiavelli was pretty devious. For a guy... |
Ted
hunter
Username: Ted
Post Number: 405
Registered: 5-2004
| | Posted on Sunday, April 02, 2006 - 8:58 pm: | |
Yes, but the part in the middle moved before either of them -
Which means, of course, that to the observer in the middle, the rod moved in the middle before it was pushed at the pushing end!
Ted
Do not go gentle into that good night...Rage, rage against the dying of the light
Benjamin Disraeli: "The Jews are a nervous people. Nineteen centuries of Christian love have taken a toll." |
Cavebear
cave painter
Username: Cavebear
Post Number: 2570
Registered: 9-2003
| | Posted on Sunday, April 02, 2006 - 9:48 pm: | |
No, there was no information transmitter in the center of the rod. I have that the ends only and there is a receiver equidistant from the ends (and distance doesn't matter as long as it is known. A distance of 1 light year may be assumed for calculation convenience.
Machiavelli was pretty devious. For a guy... |
Ted
hunter
Username: Ted
Post Number: 406
Registered: 5-2004
| | Posted on Monday, April 03, 2006 - 7:31 am: | |
I'm not sure what use a receiver is if it cannot transmit the information it receives!
And in any case, you may not have put an observer there, but I have. 
And it leads to the result above, as far as I can tell.
But in any case, unobtainium is unobtainable, and there is no substance which is incompressible.
Thus any push at one end will in effect be a pressure wave, compressing the substance, and the pressure wave will travel at a significant fraction less than the speed of light to the end of the rod.
Ted
Do not go gentle into that good night...Rage, rage against the dying of the light
Benjamin Disraeli: "The Jews are a nervous people. Nineteen centuries of Christian love have taken a toll." |
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