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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)

#91

Post by ars »

stcredzero wrote: post #76
ars wrote: post #43

Here's another way to understand it. The difficultly: Despite the propagation delay, gravity always points to where the object should be, instead of where it was when the gravity was "emitted". The explanation: Just like the object itself is moving, the gravitational field (or electric/magnetic field) is ALSO moving! It moves at exactly the same speed, and in the same direction as the object which created it. It's "d…

> The difficultly: Despite the propagation delay, gravity always points to where the object should be, instead of where it was when the gravity was "emitted". [citation needed] My understanding is that it points to where the object was, and that the change propagates at the speed of light as gravitational waves.

That's what you would expect, but it results in unstable orbits.

Instead as the object moves, the gravitational field it emits also moves, so you end up feeling the gravity where you would expect it to be if the field transmitted instantly.

It's when the object emitting the field changes direction that things become interesting.

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

#92

Post by sesqu »

diminoten wrote: post #90
vehementi wrote: post #88

Earlier quoted context omitted.

You said the article disagrees with RRC's post, which it does not. It seems now you're just taking issue with the progression RRC made to get to the point? Or that you think she should have gone into the paper she cited? OK. But it doesn't make claims contrary to the OP. And the thing you quoted does not disagree with the other thing you quoted.

It does disagree, for the reason stated above. It's as if RRC were going to write more, decided not to, and just skipped to the conclusion.

The part you quoted does disagree with the article, but not with the part you quoted: RRC said we can measure the delay, and it is 0. The article said we can derive the delay, and it is almost 0.

That aside, yes, RRC's comments are often lacking in one way or another. However, they're also solid attempts at explaining something ad hoc, a very valuable thing on Reddit.

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

#93

Post by mahranch »

yawgmoth wrote: post #8

Something very interesting whenever this question is asked - what is the speed of light? It is "the speed at which data propagates through space time". I think that is a fascinating truth to our universe. There is a very good discussion from r/askscience on this topic: http://www.reddit.com/r/askscience/comments/1kjp6z/does_grav...

I always liked the analogy from here: http://zidbits.com/2011/04/why-cant-anything-go-faster-than-...

The speed of light can be thought of as infinite speed in that context. Since time slows as you get closer and closer to the speed of light, it stops completely at 299,792,458 m/s (hypothetically, if you gave photons a reference frame).

Trying to go faster than light could be compared to a car slowing down for a red light. Once at the red light, the car stops. Going faster than light would be like asking the car to go slower than completely stopped. It just doesn't make sense in a philosophical or mathematical sense.

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

#94

Post by vehementi »

sesqu wrote: post #92
diminoten wrote: post #90

Earlier quoted context omitted.

It does disagree, for the reason stated above. It's as if RRC were going to write more, decided not to, and just skipped to the conclusion.

The part you quoted does disagree with the article, but not with the part you quoted: RRC said we can measure the delay, and it is 0. The article said we can derive the delay, and it is almost 0. That aside, yes, RRC's comments are often lacking in one way or another. However, they're also solid attempts at explaining something ad hoc, a very valuable thing on Reddit.

> The article said we can derive the delay, and it is almost 0.

I read the article again and couldn't pick this out - could you point this out to me please?

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

#95

Post by avallet »

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…

I am not a physicist, but couldn't this be used to transmit information faster than light? Let's say you have object A attracting object B through gravitation, if you could somehow control the inertial movement of object A to encode some information, couldn't this information be retrieved instantaneously at point B by measuring the direction of the gravitational vector pointing towards A ?

I expect the answer to be something along the line of "you can't change the inertial movement of A faster than light would take to go from A to B", but I have no idea why this would be the case. If there's a physicist nearby I'd love an explanation :) .

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

#96

Post by pdonis »

diminoten wrote: post #45
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…

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…

The linked article directly disagrees with this post (except the last paragraph, which itself disagrees with the rest of the post)

Actually, Baez's article disagrees with the last paragraph of RRC's post; in so far as it talks about what's in the rest of RRC's post, it basically agrees with it (the part at the beginning about the Wile E. Coyote stuff isn't really talked about in Baez' article, except in so far as it's supposed to illustrate that a static gravity field doesn't have to "propagate" at all).

Baez' article, and the paper by Carlip that is referenced in it (and by RRC) basically say that measurements like the binary pulsar show that this statement from RRC's post is, strictly speaking, false:

all the aberrations you might expect because of a finite speed of light end up canceling out, so gravity acts like it's instantaneous

The cancellation is not exact; that's why binary pulsar systems can lose energy by emitting gravitational waves. But it's very close to being exact, closer than the analogous cancellations for electromagnetism; that's why we don't observe "aberration" in the direction of the gravitational force to the same extent that we do for the electromagnetic force.

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

#97

Post by mistercow »

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…

>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 universe, while their corresponding trillions of particles all popped into existence right inside the sun.

Presumably, in that so-rare-that-it-definitely-won't-happened case, it would take 8 minutes for us to be able to detect that the sun had doubled in mass.

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

#98

Post by sesqu »

vehementi wrote: post #94
sesqu wrote: post #92

Earlier quoted context omitted.

The part you quoted does disagree with the article, but not with the part you quoted: RRC said we can measure the delay, and it is 0. The article said we can derive the delay, and it is almost 0. That aside, yes, RRC's comments are often lacking in one way or another. However, they're also solid attempts at explaining something ad hoc, a very valuable thing on Reddit.

> The article said we can derive the delay, and it is almost 0. I read the article again and couldn't pick this out - could you point this out to me please?

"The net result is that the effect of propagation delay is almost exactly cancelled, and general relativity very nearly reproduces the newtonian result."

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

#99

Post by ballard »

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…

Yeap.

In order to preserve information causality, spacetime cannot deform instantaneously.

i.e., an object X in a frame with relativistic speed near c compared to an observer O. O will be unable to detect X by any means until X is very close.

Also, my money is on frame-dragging and Einstein's figures being within half a standard deviation. http://arxiv.org/abs/0911.4718

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

#100

Post by chinpokomon »

aphyr wrote: post #19
stephengillie wrote: post #18

It makes me think of hanging sand in Minecraft -- you disrupt 1 column and cause it to be recalculated and fall, and this causes the columns next to it to be recalculated and fall, dominoing on until all of the hanging sand has fallen. This is the propagation in action. The propagation time is a combination of the 'tick rate' and the 'size of the granules'. In Minecraft, I'm pretty sure the tick rate is around 1/10 o…

Since c is a velocity, 1/c doesn't have units of time; it has units of time/distance. Special and general relativity (at least, as commonly formulated) also assume that space and time are isotropic and continuous, which means there is no "granule", no "smallest increment" in the field equations. That may or may not be true; we're still trying to figure out if spacetime is quantized or not.

Which is where quantum mechanics comes into the picture. If we could measure something in Planck units, we might discover that there is a smallest increment. Of course Nyquist implies that we may only be able to measure something at most, twice that unit size... but that is still on a scale far beyond our reach.
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