If I switch on a nuclear reactor on earth, at what speed would the (small) effect of the reduction of the earth’s mass travel out to the rest of the universe?
I seem to remember information cannot travel faster than light, so that puts a top end on the speed. I just wonder if gravity is a speed of light kind of thing, or maybe much slower. Has anyone measured it?
First off, I don’t know the answer, and I really need to get back to the office, but, quickly…
I learned, as we all did, the nothing can exceed the speed of light. But gravitational attraction is regarded as instantaneous.
There’s a fellow named Tom Van Flandern at the University of Maryland who published an article in Physics Letters A (12/21/98) that claims the speed of the propagation of gravitational force must be at least twent billion times faster than the speed of light. Whoohoo! That’s fast!
That’s all I’ve got time for now. Try a web search on that guy’s name.
I have no idea.
But here is what I’ve gathered:
Gravity, under our current model, does not emit any particles (or at least no one has found any). Instead, it is a warping in space-time caused by mass. The common model is a stretched rubber sheet, with a bowling ball representing the sun, then various other fruits and seeds of varying sizes representing the planets. Bowling ball = big warping. Fruits and sees = smaller warping. That’s a two dimensional (sorta) model. Gravity does the same thing, but in three (or four, or more, who knows?).
So. . .
It would appear that “gravity” would travel at the same time as the mass causing the warping.
Gravity, like all the other forces, travels at the speed of light. As we orbit the sun, we’re responding to the gravity “emitted” ~8 minutes ago by the sun. Similarly, when you open your eyes, it doesn’t take 8 minutes to see the sun; you see the photons it emitted 8 minutes ago. So, in that sense, response to gravity is “instantaneous”.
Generally speaking, nothing that can carry information can travel faster than the speed of light. Since one could (in theory) modulate gravity waves by moving large masses relative to one another, you can be pretty sure on general principles that it can’t travel any faster than the speed of light.
Of course, if you could show experimental evidence that some information-carrying phenomenon could travel faster than the speed of light, then I’m going to suck up to you because I would love a free trip to Stockholm.
Here is the answer from the Relativity FAQ:
Since the question’s been answered already, I just wanted to throw in a personal story. Back in high school in the 70’s I used to wonder this same thing. This being B.C. (before Cecil) I wrote to Isaac Asimov and asked him. He wrote me back and said pretty much the same thing Mjollnir and SingleDad did. I still have the letter, and his business card which he enclosed.
Just as the photon is the electromagnetic field particle, it has been hypothesized that gravity has a “gravitational field particle”, called the graviton. If it exists (and it hasn’t been detected), it would have (probably) zero rest mass and would (probably) propogate at the speed of light, just like the photon. See Graviton Question
As others have already pointed out, gravitational interactions travel at exactly the same speed as electromagnetic interactions such as light. One example of a way that you might use gravity to transmit information, is this: Suppose you have two rigid, parallel wires with a massive bead on each, like this:
------O------
------O------
Now, we move one of the beads:
------O------
---------O—
This will cause the other bead to move, so as to be as close as possible to the first
---------O—
---------O—
However, it will not respond in this manner until a short time after we move the first bead, that time lag being equal to the time it takes light to travel that distance.
Another fun fact: Anything, not just gravity and light, that has some definite speed must travel at the speed of light. Things that don’t travel at c, like sound, must always travel relative to some medium.
Maybe this is because of my limited understanding of relativity but would the reactor experiment cause no net change in the earth’s gravitational field/space warping? Matter is not being destroyed, only converted to energy.
If gravitational force is propagated by a particle (graviton), then wouldn’t that require some revision of current thinking on black holes? Would black holes then emit light, because the photons carrying the light would be travelling as fast as the gravitational force that would pull them back into the black hole?
Padeye: Immediately, you’re correct. You’re merely converting the energy of matter to a different form of energy; both have identical graviation.
However, “energy” energy converts quickly to heat, whereas “matter” energy generally stays put. Once you convert the energy to heat, it will soon radiate away, reducing the mass-energy of the earth.
MC: The gravity of the black hole “warps” space-time. The propagation of this warpage is at the speed of light, but once warped, space-time stays warped. Once a particle enters this already warped space-time, at a certain point all of its possible futures lead to the singularity at the center.
Unfortunately, it’s crap. I don’t think he’s associated with any University either. I found his paper and it makes sense intuitively, but he apparently disregards much of general relativity in proving his point. I read some of his posts on Usenet (via deja.com) and I concluded that his reasoning is incorrect. I think if you do an altavista/google search for his name, you’ll find a webpage that refutes him.
Herre’s a link to the article: The Speed of Gravity - What the Experiments Say. I analyzed this paper in some depth during a thread a few months ago … involving the V word …
The major problem in the paper is that he’s mixing relativistic analysis with Newtonian analysis; although relativity includes Newtonian mechanics, and Newtonian mechanics is sufficiently accurate for the vast majority of practical analyses, when you try to use some of the relativistic equations and some of the Newtonian equations in one model, the result is meaningless. He assumes a central force field (Newtonian) and then tries to analyze that situation using relativity.
The entry in the Relativity FAQ that’s already been posted indicates how this approach is incorrect.
Mass warps space, directly proportional to the amount of mass. The instant that mass becomes energy, the warpedness(?) of space changes. Speed of light constraints are only relevant for something traveling through space.
If two 10lb balls are orbiting each other in space, and one of them magically radiates a pound of nuetrinos or something, at that instant the mutual gravitational relationship, based on the warpature of space, is different.
Hence, the orbits of the balls are changed instantaneously
The “information” about the weight loss doesnt have to travel to the other ball, at whatever speed. Where the ball is, thats information, speed limits apply. Where the ball is gonna be, thats instantaneous.
How fast is Gravity? Throw your friend off a building and see how long it takes him to hit the ground. As he thuds into the ground the electrostatic field (on the ground) reacts much faster than the gravity does. 
“The “information” about the weight loss doesnt have to travel to the other ball, at whatever speed. Where the ball is, thats information, speed limits apply. Where the ball is gonna be, thats instantaneous.”
FreakFreely, with all due respect, what the heck are you talking about? It sounds like you’re saying that position is a quantity, whereas velocity is not, or there’s some fundamental mathematical distinction between them.
I remember that I used to assume that Gravity was instantaneous, too. But then I realized that I’d been making an assumption, and that many of those Scientists who do things like this are a lot smarter than me. It’s my understanding that, as SingleDad and Chronos have mentioned, Gravity is right now believed to travel at c, but that sensors do not yet exist accurate enough to show this experimentally. I am rather sure that a speed limit on Gravity does not conflict with black hole theories. Just because Gravity travels at the speed of a photon does not mean that it’s subject to all the same laws as a photon.
Also, about the whole destruction of mass thing. When you fire up a nuclear reactor, as has already been pointed out, you’re not decreasing the gravitational pull of the Earth one iota. The thing is, and I think I have this right, mass and energy don’t get converted between each other. Mass and energy are equivalent. To say a particle has m mass or mc² energy are the same thing. Photons do have mass. I guess you could even have two photons orbiting each other. Another form of what we call energy, other than photons, is kinetic energy, the energy of something moving. But you remember from Special Relativity that when you speed something up, it gains mass? Well then, when you give an object Kinetic Energy, you are in fact giving it mass. Again, mass and energy are equivalent.
The gravitational interaction between A and B is entirely the warping of space due to thier respective mass. Gravity is the warping of space, not an effect of it. The warping of space by mass is not “information” that needs travel through space, it is space itself. Hence, the amount of time necessary for a change of mass to affect the warping of space is precisely the amount of time necessary for the transition between mass to energy.
Ball A, having a change in mass, does not send a message to Ball B “change your orbit”. Ball B changes its orbit immediatly because the space-warping through which it travels is changed immediatly by the mass change of A. If Uncle Albert is right, why do we need to dream up some “gravitons” to transmit gravity? When a light beam is warped around a gravity field, do gravitons travel to the light beam and bounce back? You gotta be kidding.
Sorry, that’s information. If what you claim is true, what prevents us from wiggling ball A in just the correct way to make ball B change it’s orbit in time to the latest gangsta rap song? That is certainly information in the Shannon sense, even if you don’t think that it’s information in the everyday sense {grin}.
There’s no need for gravitons in today’s formulation of General Relativity. But …
General Relativity has been tested and investigated to an incredible degree. The Standard Model of particle physics (quantum mechanics) has also been tested and investigated to an incredible degree. Both produce absolutely and incredibly precise and unquestionably correct results ** except in extremely unusual situations where they both must be taken into account**. In those situations, the results are not merely incorrect, they’re meaningless. Something is wrong, and figuring out what is wrong and doing something about it is *the unsolved question in today’s physics.
It seems most likely that gravitons are part of what is needed to unite the two theories.
Yup, it’s almost certain that gravitons travel to the light beam and back. Nope, not kidding. There’s no requirement that the universe “make sense”. It’s been proven that, when quantum interactions are being considered, the universe absolutely does not “make sense” in terms of our common sense and everyday experiences.
One common illustration is two ice skaters throwing bowling balls back and forth. It’s easy for us to imagine the effect; whenever a skater throws or catches a bowling ball, their motion changes. But this is purely “repulsive force”; a skater that throws a bowling ball recoils from the thrown ball, and a skater that catches a bowling ball picks up some motion in the direction the ball was moving. So the skaters move apart.
I don’t know of any real-world analogy that illustrates how gravity (an attractive force) or electromagnetism (attractive or repulsive depending on the charges involved) is transmitted by the exchange of “particles”. But that’s not a failure of the scientific model; it’s just an indication that quantum stuff is really, really, really weird. It’s weirder than a snake’s suspendors.