# The Speed of Gravity

I don’t mean 32 ft./sec/sec. I mean, does the “pull” of gravity travel at C?

Example: A super-massive star suddenly appears 930 million miles away from Earth in the opposite direction from Ol’ Sol. Do we turn right immediately or wait an hour or so?

An article I read in Analog magazine about unresolved enigmas in physics mentioned the speed of gravity. IIRC, the article put the problem this way:

Suppose you have two stars in orbit around each other, with a third star orbiting them both at a distance. Does the path of the third star follow the gravitational pull of the two parent stars in the positions where they are at that moment, or where they were at the time a beam of light left the parent stars?

Newtonian physics presumes that the “speed of gravity” is infinite. Relativity has problems with the question. Special relativity says no influence can travel faster than light; yet General relativity treats gravity as the curvature of space-time, meaning it’s already “there”, rather than a transmitted force.

To muddy the waters further, the article said that according to some calculations, the minimum speed of gravity would have to be at least several multiples of lightspeed!

Great question - one I’ve wondered about.

I know there was a thread on this subject about six months ago, that I think was started by Beeruser, but I couldn’t find it. I’ll look again tomorrow.

Off the top of my head, I’m not sure how you would design an experiment to test for the velocity of a gravity wave (yes, that’s an invitation to post).

Lumpy said:

Yeah, it’s already there, but things change: positions of bodies, accreted mass of bodies, stars disintegrate, etc. How quickly are these changes transmitted? This, for me, has always been a problem as it would seem to be near instantaneous.

Can somebody show Rysdad, Lumpy and me why the effects of a change in a gravity field must be transmitted at the speed of light (or less)?

Here’s Scientific American’s take on the subject: http://www.sciam.com/askexpert/physics/physics1.html

I’m by no means an expert but …

From what I’ve read, gravitational *attraction[/] is indeed instantaneous, for the reasons specified: it is a curvature of space-time rather than a force. For example, when you look at the sun you are seeing it at the position it was at 8 1/2 minutes ago, since it takes light that long to reach the earth, but its gravitational attraction is acting from the position it is at “right now”. I’m looking for the source of this info on the web and will post it when I find it.

The distinction this author made was that gravitational waves moved at light-speed (that’s Warp 1.0 for you Star Trek buffs) but that the attraction was instantaneous. I know, I know. I smell a contradiction too. But there’s no question of transmitting information (via gravity waves) faster than light, since the waves are relativistically well-behaved.

Now I’d better find that source before I get accused of Von Danikenism again.

“If ignorance were corn flakes, you’d be General Mills.”
The Straight Dope

Okay – here it is. If you disagree flame the author, not me! Please!

http://www.ldolphin.org/vanFlandern/gravityspeed.html*

Okay – here it really is:

http://www.ldolphin.org/vanFlandern/gravityspeed.html

I’m having trouble with my UBB code tonight.

I have pondered the same question, from a quantum-mechanical point o’ view. As I understand it, gravity is carried by sub-atomic particles called “gravitrons.” (Of course, no on has proven this yet, let alone detected a gravitron, but it’s a working theory.) These, I assume, behave like all particles and travel no faster than light.

My conundrum concerns black holes. Supposedly they are giving off so many gravitrons in such a compressed space that nothing can achieve escape velocity, not even light itself. But, if nothing can reach escape veolicity, how to the gravitrons themselves escape?

I am probably displaying numerous forms of ignorance in this post, so I will stop now. Thank you for your indulgence…

Here’s another relevant link from the sci.physics FAQ.
http://www.public.iastate.edu/~physics/sci.physics/faq/grav_speed.html

Back off, man. I’m a scientist.

Beruang

Your conjecture goes right to the heart of the question: If nothing can travel faster than the speed of light, how can black holes exert gravitational force? Or, to put it in physics-speak, “…how [do] the external fields between binary black holes manage to continually update without benefit of communications with the masses hidden behind event horizons”?

So don’t worry too much about your ignorance. You’re in good company.

## One quibble, however – it’s gravitons.

“If ignorance were corn flakes, you’d be General Mills.”
The Straight Dope

Thanks everyone for the responses. I have another question, though. I was reading Pluto’s link above, and it it, it said:

“The orbit of this binary system is gradually decaying, and this behavior is attributed to the loss of energy due to escaping gravitational radiation.”

Does this orbit decay means that the two objects are spiraling toward each other? It would seem to me that, if they are losing, um, something (can’t be mass because gravitons are massless) through radiation, then the orbit shouldn’t be decaying, it should be expanding. (Less mass = less attraction = wider orbit. See what I mean?)

I never thought about that black hole paradox before, either. That one really bugs me. How can something like that communicate beyond the event horizon?

I’ll stay tuned.

[correction]
The quote I referenced was from Alphagene’s link.

I don’t have much more to add but will point out that because of mass-energy equivalence emitting gravitational radiation will reduce the mass of an object, even though the gravitons themselves have zero mass.

Yes, I’ve wondered about this too. Here’s the OP from several months ago, as mentioned by beatle. Interesting.
http://www.straightdope.com/ubb/Forum3/HTML/000224.html

Peace,
mangeorge

I just read through the thread attached above, and it makes me “want” gravity to be a state and not a force. I can live with instantaneous effects, but not with massless gravity particles that can radiate from a singularity while photons can’t.

If someone catches a graviton, let me know. I have a few questions for it.

According to my modern physics textbook, Gravitons are far too weak to be detected on an individual basis by our instruments. Kind of a cop-out, if you ask me.

But aside from that, this is one of the big problems with physics. How does the earth “know” what the sun is doing at some instant in time when it takes information 8 minutes or so to travel between them at the speed of light? Something doesn’t add up here, and even the world’s most brilliant minds are only talking about theories.
– Sylence

I don’t have an evil side. Just a really, really apathetic one.

We seem to have a question that the undebatable answer is “No one knows for certain”.

Tell ya what…if they do find and catch a graviton, I hope the next thing they locate is the anti-graviton.

Boy, I could have some fun with those!

According to one of the links above, one reason why no finite speed of gravity seems possible is that if gravitational attraction propagated at a finite speed, it would cause lateral as well as radial effects. The net effect on two bodies orbiting each other would be transfer of momentum so that the bodies would gradually be pushed apart.

Now another enigma in the news is the supposed “cosmological constant” that seems to be causing the cosmos to expand faster than originally expected. I wonder if in fact this is an effect of a finite speed of gravity. Anyone out there with a PhD in astrophysics who could tell us if there’s anything to this idea?

Here is a sort of update (or downdate considering the age)

speed of gravity same as light

http://www.space.com/scienceastronomy/gravity_speed_030107.html

speed of gravity same as light “incorect”

http://www.space.com/scienceastronomy/gravity_speed_030116.html

I seem to recall asking a similar question in a similar thread, but can’t find it. Probably an extremely ignorant question, so I appreciate your indulgence:

Isn’t gravity, at least in classical physics, just the observable effects of the curvature of space-time? Wouldn’t it be inappropriate to speak of it as a force or a particle radiated from something?