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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)

#51

Post by Semiapies »

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…

Now, here's the thing that confuses me.

Say some advanced civilization can move around some large mass - an asteroid, a planet, a black hole, whatever, we'll just call it "the big mass". They can, at will, fly it back and forth between two distinct positions we'll call 0 and 1.

And let's say some distance away - a light-year, say - they have an facility where they can measure with excruciating precision the force of gravity on a test mass. After isolating out all other known gravity sources, they can use the remaining vector of gravitation force to compute the current position of the original big mass - and whether it's at position 0 or position 1.

Assuming they can drag around the big mass from one position to the other in a short amount of time, shouldn't the people at the remote facility be able to detect where the big mass is long before light could reveal its position? Couldn't they use those observations to receive a low-bandwidth, but faster-than-light message?

How does relativity prevent that?

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

#52

Post by VladRussian2 »

ars wrote: post #44
jessriedel wrote: post #40

Earlier quoted context omitted.

Why would the fact that dropped objects start falling instantaneously show that signals travel instantaneously, either pre- or post-GR? It just shows that the message "fall if you are dropped" is just being continuously emitted by the Earth, including before the object was dropped.

That's exactly what it shows. The question becomes what if the object moves - how long does it take to notice the new location of the gravity.

it is called gravitational wave. The GGP strangely didn't mention it. The gravitational wave is a strange creature as it is a disturbance of space-time, thus question of its speed, distance by time, becomes a bit murky - like what distance? what time? - depending on how you answer the questions (and don't forget, talking about astronomical scales we start to face astronomical factors like noticeable spacetime expansion) - you get corresponding result.

>>"fall if you are dropped" is just being continuously emitted by the Earth, including before the object was dropped.

it is gravitational field of Earth. If Earth were to move with acceleration (as any movement without acceleration is as good as not moving at all by suitable choice of coordinates) the object would start to fall toward previous "retarded" location of the Earth, until the gravitational wave caused by the accelerated move of Earth "updates" the gravitational field at the position of the object.

>The question becomes what if the object moves - how long does it take to notice the new location of the gravity.

a moving object gets instantaneous value of gravitational field at its new position (while this value may be already "obsolete" like in the above mentioned accelerated movement of Earth)

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

#53

Post by jessriedel »

ars wrote: post #44
jessriedel wrote: post #40

Earlier quoted context omitted.

Why would the fact that dropped objects start falling instantaneously show that signals travel instantaneously, either pre- or post-GR? It just shows that the message "fall if you are dropped" is just being continuously emitted by the Earth, including before the object was dropped.

That's exactly what it shows. The question becomes what if the object moves - how long does it take to notice the new location of the gravity.

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.

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

#54

Post by protomyth »

chiph wrote: post #7

So, the FTL gravity pulses used in David Weber's Honorverse series (sci-fi space opera) aren't possible. :(

Well, a hand-wavy explanation could be the gravity interacts with hyperspace[1] allowing the detector to pick it up FTL.

1) it seems his version of hyperspace has a lot of interaction with gravity (e.g. gravity waves like The Deep)

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

#55

Post by prawn »

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…

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.

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

#56

Post by icegreentea »

TallGuyShort wrote: post #29
mturmon wrote: post #17

People may be interested in experiments to detect these waves. From the Wikipedia page on LIGO: "LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. [...] At the cost of $365 million (in 2002 USD), it is the largest and most ambitious project ever funded by the NSF. Observations at LIGO began in 2002 and end…

I'm surprised the term "waves" (a la electromagnetism) is used instead of "fields" (a la magnetism). Can anyone explain that?

Waves (such as EM waves) imply the change in some field, which is what they are trying to detect. After all, we already detect gravitational fields - after all, we are standing on the earth - what they're trying to detect are very specific changes to gravitational fields, thus waves.

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

#58

Post by vehementi »

jessriedel wrote: post #53
ars wrote: post #44

Earlier quoted context omitted.

That's exactly what it shows. The question becomes what if the object moves - how long does it take to notice the new location of the gravity.

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!

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

#59

Post by vehementi »

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…

That... doesn't contradict anything.

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

#60

Post by ars »

Semiapies wrote: post #51
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…

Now, here's the thing that confuses me. Say some advanced civilization can move around some large mass - an asteroid, a planet, a black hole, whatever, we'll just call it "the big mass". They can, at will, fly it back and forth between two distinct positions we'll call 0 and 1. And let's say some distance away - a light-year, say - they have an facility where they can measure with excruciating precision the force of…

It takes a while for the test mass to notice that the big mass moved.

So the test mass is pulled in a direction where the big mass no longer is, but rather where it was.

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