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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)

#111

Post by gliese1337 »

stcredzero wrote: post #78
jswhitten wrote: post #61

Earlier quoted context omitted.

How would that modulate a gravity field? If you're thinking that the matter and antimatter have gravity and that gravity disappears when they annihilate because their mass is gone, that's not the case. All energy and momentum produces gravity, not just mass.

You could only modulate gravity as fast as you could move energy close to you or far away from you. An implausibly powerful gamma ray flashlight flicking on and off would let you do this. Get the beam powerful enough, and it will have a gravitational field.

Get the beam powerful enough and it will have meaurable gravitational field. All beams of any intensity technically do contribute some, albeit infinitesimal, component to the gravitational field.

It's a fun exercise to calculate just how strong a laser beam you'd need to have to make a black hole entirely out of photons.

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

#112

Post by gliese1337 »

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…

Typical Sci-Fi wormholes have that problem, but there are nevertheless valid GR solutions that look a heckuvalot like wormholes. And they do obey all those conservation laws. One can think of it as the wormhole mouths being objects (made out of space rather than matter) that accumulate and lose conserved quantities like mass and momentum and charge as objects enter or leave them, or you can think of it as lines of force getting stretched out to pass through the wormhole mouths whenever an object moves through them, since the fields can't just shear off.

Incidentally, this generally means that you have to be careful about balancing the mass flow in each direction through your wormhole, lest one mouth develop negative mass (and presumably antigravity) from too much stuff leaving it and the other mouth end up shrouded in a black hole.

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

#113

Post by nkuttler »

jswhitten wrote: post #61
yeureka wrote: post #46

I always wondered about this. If changes in gravity propagated faster than light, could you build a transmitter by modulating a gravity field by colliding matter with anti-matter?

How would that modulate a gravity field? If you're thinking that the matter and antimatter have gravity and that gravity disappears when they annihilate because their mass is gone, that's not the case. All energy and momentum produces gravity, not just mass.

There was this one russian http://en.wikipedia.org/wiki/Eugene_Podkletnov#Podkletnov.27..., but nobody has yet managed to replicate his results.

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

#114

Post by RuggeroAltair »

yeureka wrote: post #46

I always wondered about this. If changes in gravity propagated faster than light, could you build a transmitter by modulating a gravity field by colliding matter with anti-matter?

Just to make it clear. If gravity was propagating faster than light, then general relativity for how we conceive it would have to be reformulated. It would be similar to the problem of if neutrinos were actually going faster than the speed of light (they're not).

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

#115

Post by radarsat1 »

tedsanders wrote: post #87
ok_craig wrote: post #80

Earlier quoted context omitted.

> To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously. Not after a very small interval of time, but absolutely instantaneously. As in zero time elapses between dropping and falling. > This is fairly earthshaking, really. Because it imp…

No, I think you're right. RobotRollCall sets up a straw man with the Wile E Coyote example. Electric attraction travels at the speed of light, yet you would feel it right away after walking off a cliff.

In fact you "feel" gravity all the time, even when you're not falling... in the mechanoreceptors in your feet.

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

#116

Post by juliangamble »

In general relativity gravity propagates at c. The rate of orbital decay of binary pulsars is, among other factors, dependent on the speed of gravity. The in-spiral rate of one binary pulsar system has been measured and found to agree with the rate predicted by general relativity to within a 0.2% margin of error.

Gravitational waves haven't been directly measured yet though, so there's no direct confirmation. With multiple detectors currently in operation if a signal is detected and able to be tied to a specific location in space the timing delays between when its received between the two primary LIGO detectors (Livingston, LA, USA and Hanford, WA, USA) and the VIRGO detector (PISA Italy) should allow for estimating a propagation speed.

Advanced LIGO, expected to begin collecting data in 2014 is expected to be able to detect a number of signals so hopefully the question will be settled in a few years. However, there might not be any published results for a while after it goes active. The search for pulsar spindown signals with LIGO data is done via the Einstien@Home distributed computing project; and in prior runs several years passed between when the first part of the data set was collected and when papers on it were finally published. http://physics.stackexchange.com/a/26743/9521

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

#117

Post by RuggeroAltair »

Steve Carlip used to be my advisor in grad school and his style of writing conveys completely his beautiful way of explaining (and thinking about) deep questions in physics.

One think that I believe is important to point out is that this short essay is focusing on the experimental aspect of the measurement of the speed of gravity. It gives an explanation on why this is even an intelligent question to answer and why the answer we commonly accept is that gravity moves at the speed of light. I will never have better words than him so I won't add anything to what he said.

What I would like to point out, based on some comments I read here, is that this essay is not talking about whether or not theoretically would be possible to have a speed of gravity faster than the speed of light.

General relativity and special relativity have been tested on several aspects, and they pretty much are in agreement with all experimental constraint. There is no other theory that can explain everything we see (some theories tend to explain a few things, but not all of them, or they are too vague).

A theory in which the speed of gravity is faster than the speed of light isn't unconceivable per se, but no one has been able to write a completely consistent one yet, mostly because it would have large consequences on what special and general relativity imply for cosmology and particle physics (and causality). If gravity moved faster than the speed of light then we'd have to 'fix' a lot of problems coming down to paradoxes and stuff like that.

So take this essay as a way to appreciate how elegant and fine some experimental questions on gravity (and physics) are.

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

#118

Post by jimmahoney »

fragsworth wrote: post #63
Semiapies wrote: post #51

Earlier quoted context omitted.

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…

The gravitational field points at where the object would be, given its current velocity. This is not to say that the object will actually end up there . If the velocity changes, the gravitational field changes direction according to the new velocity. The change propagates at the speed of light.

Yes.

This is a case when a picture (well, a GIF animation) is worth lots of words.

Look at http://jimmahoney.net/pulse_acceleration_field_lines.gif .

(I made this from the "Moving Charge" java physics applet at http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/Mo... . It's a demo of the physics of electric charges, not general relativity, but the essence of the idea is the same even if the field equations are different. This follows directly from the central idea of relativity: the physics of uniform constant motion is the same as no motion, if you're moving along at the same speed.)

The point charge (mass for gravity) in the center is first moving slowly to the right, then suddenly changes direction to move slowly to the left. (The small red vector indicates velocity.)

The white lines represent the direction of the field, that is, the direction an object would be pulled.

Near the point source, the field lines point to where it is. Far away, the news hasn't arrived yet that it's motion has changed, and so they point to where it would have been. During the brief acceleration when the source changed direction, the field lines connecting these two regions are "kinked" strongly - that's the radiation, which propagates outward at the speed of light.

A second object would feel a sideways pulse as the wave (kink) passed by.

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

#119

Post by yeureka »

jswhitten wrote: post #61
yeureka wrote: post #46

I always wondered about this. If changes in gravity propagated faster than light, could you build a transmitter by modulating a gravity field by colliding matter with anti-matter?

How would that modulate a gravity field? If you're thinking that the matter and antimatter have gravity and that gravity disappears when they annihilate because their mass is gone, that's not the case. All energy and momentum produces gravity, not just mass.

My understanding of physics is very poor, I was under the impression that only mass produced gravity.

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

#120

Post by davedx »

aidenn0 wrote: post #103
mistercow wrote: post #97

Earlier quoted context omitted.

>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 univers…

That can't happen without them annihilating very quickly. Otherwise it would be violating conservation of mass.

What about Hawking radiation? [1] Then one of the pair escapes and the other falls into a black hole.

[1] http://en.wikipedia.org/wiki/Hawking_radiation

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