Dear Scientific Types: Question about the propagation of gravity

[anarch]

I apologize if this has been beaten to death in here already...

I'm just curious as to what the consensus is on whether gravity propagates instantaneously, or whether the force of gravity is subject to the "universal speed limit" of c that applies to electromagnetic phenomena.

On the one hand we have the Kopeiken/Fomolont experiment of a couple years ago described here:

http://physics.about.com/cs/gravity/a/speedofgravity.htm

This purports to measure the speed of gravity, but then confuses me by bringing gravity waves into the discussion. I don't think that's quite the same thing, and I don't think there was ever any controversy as to the speed of gravity waves (if they can carry information, they cannot travel faster than light). And anyway, it comes up with a value of 1.06*c, or 6% faster than the usual speed of light in a vacuum...with a margin of error of 20%.

On the other hand, quite a few reasonable-sounding people have said that this experiment measured not the "speed of gravity", but the speed of light...which, not surprisingly, comes in right around c with the margin of error.

So...yeah, anybody want to help me out here? Feel free to explain it as you might to a child. I apologize in advance for my general ignorance.

[tridim]

1. When I die, I want to come back as a quantum being

It's probably the only way to really know what's going on behind the scenes.

[anarch]

7. we are all quantum beings...

we just have trouble seeing things clearly :)

[bonito]

2. Well

Im in a black hole right now and everything sucks.

[anarch]

13. and black holes...there's another good question

How can black holes have gravity (propagating at the speed of light) if the escape velocity from the black hole is greater than the speed of light (i.e. even light gets sucked into the black hole)?

[mahatmakanejeeves]

3. It is not instantaneous

There was something about this in the paper a few weeks ago.

Imagine the sun suddenly disintegrates. If gravity is instanteous, earth goes flying out of orbit. Why, why, goes the cry on earth. After, no one can see any difference for 19 minutes, or however long it takes for light coming from sun to reach earth. Everything looks hunky-dory. Gravity travels at or nearly light speed.

Sorry, can't come up with article right away.

Best way to avoid sun explosion? Vote rethug. Dems are weak on sun explosion. 9-11, stay the course, 9-11.

[anarch]

6. but the sun we see is always 8.3 minutes old...

It is actually, "right now", about 20 arc seconds East of wherever it is visible in the sky, and that is the point the planet actually orbits...I think. I'm still confused...if the sun winked out of existence, we wouldn't see anything change for 8 minutes, but I'm having trouble picturing the Earth continuing to orbit nothing...I mean, it would fly off in a tangent of the last actual position of the sun, and thus apparently continue its orbit, but only apparently, from our perspective. :shrug:

[Turbineguy]

4. I love DU

My roommate in college was extremely clever (he still is and we are close friends).

I once asked him a question regarding electrical engineering (a subject that caused many to flunk out ot that school).

He gave me an answer so convoluted that I realized If I had understood the answer, I would have never asked the question in the first place.

None the less, I passed the course and obtained my Degree

I cannot answer your question. But I think I can say with absolute certainty: This is a question that would never be asked on Free Republic.

Thank you and all those who challenge us.

[anarch]

12. I sometimes wish I'd studied something like electrical engineering

instead of being a "Liberal Arts" person. Then I probably wouldn't be kept up at night by questions like this one. :D

[Turbineguy]

14. At the risk

of starting a mutual admiration society.....

Some of the best people I worked with were "Liberal Arts" majors.

[longship]

5. Einstein's General Theory of Relativity

Gravity propogates just like every other force in the universe, at light speed.

This is more than just a consensus. Unless major tenets of the G-TOR are falsified (*very* unlikely to ever happen) there's no need to doubt this fact.

[anarch]

9. but I thought GR explained gravity as a geometric effect,

and not a propagating force at all? Certain components of gravity must certainly travel at the speed of light (gravity waves, gravitational radiation), but I'm still not seeing how the actual force of gravitation can be both a light-speed phenomenon and a geometric feature of space-time...

(still confused)

[Salviati]

15. One thing that may aid your confusion...

It's more of a philosophical point than a physics one.

People often believe that physics is searching for a description of what the universe is, this is in fact wrong. Physics is searching for a description of what the universe is like.

GR tells us that gravity is like a geometric effect, but this geometry is not what gravity is, it is what it is, and we're just trying to find models to describe how it behaves. This geometrical model of gravity works astonishingly well on the macro scale, but does not provide an adequate way to combine it with the standard model of the other forces, so while this model is admirable in it's scope, it is still incomplete, as it does not entirely describe what gravity is like at all scales.

If, after lots of hard work, we do come up with the long sought for "theory of everything", that describes all of the possible interactions in the universe at all scales to any degree of accuracy, it still wouldn't tell us anything about what the universe really is, after all, it could just be the flying spaghetti monster controlling everything with his noodly appendage, as long as he conspired to play by all the rules that we've discovered.

Any theory is ultimately just a model, and currently, all the models we have now, although they are fantastic in their range of applicability, are ultimately incomplete.

[anarch]

17. That's a very good point,

and I think you may have just won me over as a FSM convert....

:D

[Ready4Change]

21. Never doubt his noodly appendage.

:)

[longship]

16. GTOR is a theory of Gravity.

The explanation is simple. When an event happens to change a gravitational field, the distortion of space-time which is a result of that change travels like ripples in a pond at the speed of light.

[Salviati]

11. It propagates at c because the "graviton" is (should be) massless

like the photon, which is the carrier of E&M. Massless particles like this must always travel at c, but one of the fundimental forces, the weak force, has massive carriers, so that it's force must propigate below c.

(to depart from the "classical" wave-like way of looking at these fields, and look at the quantum nature of them that is...)

[Salviati]

8. Gravity propagates at a speed of c`

To make an analogy to the E&M case, the gravitational field is like the E field, and gravity waves are similar to E&M waves (i.e. light). When the source of an E field changes, it needs to "update" the field as to it's new configuration. E.g. if the source gets closer to you, the field needs to get stronger. These updates propagate out at c, and in fact, if the source is constantly changing, requiring a constant stream of updates, it is this stream of updates that we call E&M radiation.

Similarly, gravity waves can be thought of as the mechanism by which the gravitational field gets updated as to what it should look like given the state of the source at the time that the wave was emitted. So, if you'll buy that gravity waves should travel at c, then it follows that "the gravitaional field" will also propagate at c.

[anarch]

10. OK -- gotta think about that one for a while

Although this assumes that gravity is a field effect, like electromagnetism, not a part of the fundamental structure of the universe...and of course, there's that nagging question of how gravity fits in with quantum theory...

But I do need to ponder this....

[Ready4Change]

18. Think of it this way.

Think of the force of gravity as being like a Slinky, with the Sun holding one end and the Earth holding the other.

Say the Sun lets go.

The Earth is still holding it's end, and still feeling a force while the Slinky is recoiling, pulling itself out to the Earth, which is pretty much clueless until the Slinky arrives and can no longer exert a force. Then the Earth goes flying off in a straight line, rather than continuing its circular orbit.

That all ignores little technicalities like how to make a sun sized mass just cease to exist with no trace.

What is more realistic is to imagine the sun "plucking" the gravitational slinky, and the sound of the plucking travelling along the length of the slinky until it reaches the Earth.

[anarch]

19. my problem is,

when I picture this, I picture most of the slinky disappearing instantaneously, at the same time as the sun...leaving only the component that represents Earth's gravitational pull on the sun. I can then picture that part of the slinky reverberating, like a wave, at the speed of light.

The plucking analogy I can see, but I can't get past thinking that this represents various kinds of radiation, and not what I am calling "gravity"...although I'm starting to suspect that what I'm after is something even more fundamental to the way the universe is put together, so maybe I should call it something else....

[Ready4Change]

20. There ya go.

'I can't get past thinking that this represents various kinds of radiation, and not what I am calling "gravity"'

It is thought that the force of gravity is propogated by a particle called a graviton, which is so small that the energies required to detect it are beyond us at this time. People are looking for it's effects. The problem there is that gravity is such a relatively weak force that to "make" it also takes large amounts of energy. So it's a tough nut.

However, there are interesting potential benefits. While it's a very weak force, gravity also acts over tremendously long ranges. It could be the ultimate in communication methods at or below the speed of light.

It might also be the only way to get a "window" view into the alternate universes suggested by many theories. Verification of those could fundamentally change a great deal of our understanding of just what is going on in our little neck of the woods.

BTW, I'm not a researcher. I've just read a few books. (And stayed at a Holiday Inn. :) ) I've heard from a few researchers here on DU who can probably step in if I'm getting something wrong here.

[Meldread]

22. Erm I might be wrong, but...

I suck at math and all that stuff, and I don't have a degree in physics or anything so take what I am about to say with a grain of salt.

I find that I understand things best through examples, and when I think about gravity having some sort of time delay it makes sense. Think of it this way, let's say you have a string with an object tied to the end of it. You begin spinning in a circle, this would cause the object to rotate around you. This is, to the best of my understanding, similar to how gravity works. The larger object (you) exerts its force onto a smaller object (the object at the end of the string), and the string can represent the invisible force of gravity. Alright, now lets say the string near your hand snaps while you are still spinning.

Would the object at the end of the string react instantly? I wouldn't think so - there would be a delay - the object would still be spinning around you (the larger object) after the string had broken. Increase the distance and the force is more noticeable.

Increase the distance to roughly 93 million miles (the distance from the Sun to Earth) and imagine the string breaking. Keep increasing the distance and the longer it will take to have a reaction, while keeping in mind that supposedly nothing can move faster than the speed of light.

So lets say that the sun magically disintegrates instantly one day. What effect would that have on Earth? Well I think the effect would be similar to the string example. It would, in my mind, depend upon the distance from the object, where the smaller object was located in relation to the larger object, how much gravitational force the larger object exerted onto the smaller object, and the other gravitational forces in the area as they would likely have some effect.

For some reason in my mind I picture it kinda like a giant invisible wave. Larger object disappears and the wave is largest at it's starting location and the further away it gets the smaller the wave until it has little to no significance compared to the objects it touches.

[rman]

23. Gravity waves are variations in local gravity;

caused by such events as colliding black holes - it is a 'gravity ripple effect'. It is not the 'medium' of gravity in the way that photons are the 'medium' of radio waves and light.

Attempts are under way to measure these gravity waves:
http://www.spacedaily.com/news/gravity-05f.html
http://216.239.59.104/search?q=cache:NAwHLIWqG20J:faculty.physics.tamu.edu/allen/Gould-gravity-waves.ppt+measure+%22gravity+waves.%22&hl=en&lr=lang_nl|lang_en
(html version of powerpoint presentation at Texas University http://faculty.physics.tamu.edu/)

[anarch]

24. right - and at this point not yet observed, but predicted by theory

But then what is the 'medium' of gravity?

I guess I'm trying to reconcile my (admittedly hazy and uninformed) understanding of General Relativity with observed phenomena such as the non-coincidence of the maximum observed change in the gravitational field with visual observation of light during a solar eclipse. That is, the "gravitational eclipse" doesn't happen at the same time as the observed solar (light) eclipse.

I don't know quite how to say it, but I'm starting to form the opinion that gravity really isn't a force at all (although certain effects of it are observable as force-phenomena), but simply a property of mass/energy possessed by the universe as a whole. We observe it as a force, but that is a matter of perspective.

I guess I've also always been confounded by the fact that the limitations placed on the observer by the relativistic delay in the transit of light--and thus all information--makes it very difficult to talk about where any object in the universe really is, "right now". I mean, we can only know about anything relative to our position, and yet we know that observed objects like distant stars and galaxies are "right now" somewhere else entirely from where they appear, or may not even still exist in the universal "right now".

I guess that's why it's called "relativity", and here I am trying to artificially impose an impossible viewpoint that encompasses the entire universe, as it might be seen from outside...which is really meaningless to us, since we only have information that exists within our particular, relative "light cone".

Well, now I think I've succeeded in confusing myself even further.