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What is the Speed of Gravity?

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Re: What is the Speed of Gravity?

#71

Post by sp332 »

Luc wrote: post #24
Tycho wrote: post #12

Reminds me of a thought experiment that's always bugged me: if you had an incredibly long see-saw or lever, like the length of the galaxy, would movement at one end instantaneously be matched by movement at the other end? (I think the reason it bugs me is because the object itself is preposterous, but I still want to know)

If it were a physical object (say, a long steel bar), the atoms would communicate at the speed of light, so your object would bend or squish, with a wave travelling along it. Now, let's say you move an ideal laser across an ideal screen some distance away. The further the screen is, the faster the laser dot will move. At some point the dot will move faster than c... But the dot is not an object. If you think in terms…

Actually, the atoms would move much more slowly, something like the speed of sound through the material.

Re: What is the Speed of Gravity?

#72

Post by scott_s »

Jun8 wrote: post #64
sliverstorm wrote: post #28

It always seems rather low-class to take shots at Newton as if he was a fool for coming up with Newton's theory of gravity instead of Einstein's.

Maybe not "taking shots", but I also feel a general "ha, he missed that big one" attitude. Newton was such a great scientist that he made his famous "I don't make hypotheses" statement in regards to the nature of gravity. Knowing the limits of your (current) knowledge is a fundamental trait of being a scientist (and a philosopher, e.g. see Wittgenstein's motto "Whereof one cannot speak, thereof one must be silent." a…

I don't. Technology had not yet progressed to the point that one could design experiments to test relativity. And such technology depends on insights from other areas of physics.

Quoting Cecil Adams (http://www.straightdope.com/columns/read/957/was-isaac-newto...),

[He] took six thousand years of disjointed fumbling and made it into a science. Two sciences, actually, physics and to a large extent mathematics.

Feeling any "ha, he" that he missed a development 200 years later loses perspective about what he did.

Re: What is the Speed of Gravity?

#73

Post by thailandstartup »

avar wrote: post #68
thailandstartup wrote: post #65

Earlier quoted context omitted.

I'd always understood gravity to be a mutual attraction between two masses. So the thing I have difficulty understanding is how light can only be attracted to mass, but itself cannot attract mass, no matter the quantity present.

I think I'm correct in saying that the light isn't attracted to the mass, but the mass bends spacetime around it, so the path of everything traveling through that medium is bent, including the paths of photons. This effect causis gravitational lensing: http://en.wikipedia.org/wiki/Gravitational_lens

Hmm, thanks. I did a little more searching and reading about this with my new vocabulary (bending spacetime). It seems like photons also bend spacetime.

More info here. All of it contradictory :) http://www.physicsforums.com/showthread.php?t=287888

So I'm still wondering if it is proper to think of a star's gravitational field being caused not just by its matter, but also by all the photons it has pumped out over its lifetime.

Re: What is the Speed of Gravity?

#74

Post by Tycho »

philh wrote: post #25
Tycho wrote: post #12

Reminds me of a thought experiment that's always bugged me: if you had an incredibly long see-saw or lever, like the length of the galaxy, would movement at one end instantaneously be matched by movement at the other end? (I think the reason it bugs me is because the object itself is preposterous, but I still want to know)

If you move it, you're really moving some of the atoms at one end. They 'bump' into nearby atoms, which 'bump' into more atoms, and so on, and eventually the atoms at the far end get 'bumped', and the far end moves. Sound waves travel in the same way. So if you move one end, that movement propagates at the speed of sound (in whatever material the lever is made of).

Hmm, I'd take your word over mine but this doesn't fee quite right to me. What I'm thinking is, say you reverse the action so that instead of bumping one end of the see-saw, you release your grip on it having been holding it down. So in normal circumstances the other end would fall while the near end rises (of course now an external gravitational field has entered the thought experiment). But if movement is still due to the bumping of atoms , how does it work, where does it start? and if it's something else, then what, and is it still not instantaneous?

Re: What is the Speed of Gravity?

#75

Post by sp332 »

thailandstartup wrote: post #55

Something I wonder about gravity - While it is often mentioned that objects with a large gravitational force can 'bend' light (gravitational lens), can a sufficient amount of light 'pull' objects towards itself? So I wonder if a star's gravity is mostly the mass of the star itself, but also, in small part, the huge amount of light that surrounds the star on all sides, being densest nearest the star itself. There's so…

> can a sufficient amount of light 'pull' objects towards itself?

Yup. As the article says, all forms of energy cause gravitational attraction. Even light! But, as you can see from E=m * c^2, it takes an insane amount of energy to make even a little mass-equivalent. C=3 * 10^8 m/s, so c^2 = 9 * 10^16 or 90000000000000000 m^2/s^2. So 1kg of mass at rest is equivalent to 9 * 10^16 joules of energy. It's about equal to a 4.5 megaton hydrogen bomb blast.

Re: What is the Speed of Gravity?

#76

Post by thailandstartup »

sp332 wrote: post #75
thailandstartup wrote: post #55

Something I wonder about gravity - While it is often mentioned that objects with a large gravitational force can 'bend' light (gravitational lens), can a sufficient amount of light 'pull' objects towards itself? So I wonder if a star's gravity is mostly the mass of the star itself, but also, in small part, the huge amount of light that surrounds the star on all sides, being densest nearest the star itself. There's so…

> can a sufficient amount of light 'pull' objects towards itself? Yup. As the article says, all forms of energy cause gravitational attraction. Even light! But, as you can see from E=m * c^2, it takes an insane amount of energy to make even a little mass-equivalent. C=3 * 10^8 m/s, so c^2 = 9 * 10^16 or 90000000000000000 m^2/s^2. So 1kg of mass at rest is equivalent to 9 * 10^16 joules of energy. It's about equal to…

Mmmm. Okay, but a star puts out a lot of energy, over a long period of time. Is it enough that we calculate the gravitational effect of all those photons, rather than just the mass of star itself when calculating say the acceleration of the pioneer crafts?

Re: What is the Speed of Gravity?

#77

Post by lutorm »

ewjordan wrote: post #42

The Sun, as it is right now, won't have its gravity affect Earth for another 8+ minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8+ minutes ago! (Weird, isn't it?) I'm almost positive this statement is incorrect. Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predic…

I don't think it's incorrect. But for constant velocities it is canceled out because the relative motion causes things to also appear in a different direction. This happens with ordinary light, also. Stars appear in a different place to us because of our relative motion (they appear to be located more "forward" in our direction of motion).

As as example, assume there is a star directly perpendicular to our motion (and that the Earth moves in a straight line). Due to the vector addition of our motion and the light travel direction, it appears to us that the star is located slightly forward of perpendicular (typically by about 1/100 of a degree) Now assume the star were to disappear. During the light travel time the star would have time to move backwards as seen by us (due to our forward motion) so that at the moment it disappears, it appears to be located perpendicular to us.

In the context of gravity, this effect (it's called aberration) exactly cancels and the net effect is that the gravitational attraction is in a direction different from the actual location of the attractor such that it appears that gravity is instantaneous.

This only works for constant velocities, once you have accelerations it becomes more complicated. And it's not a relativistic effect at all, it's present for all waves with finite propagation speeds. You can do this experiment with boats making waves and get the same result.

Edit: And your charge example is not so good. For Galilean invariant theories it's only relative motions that matter. There is no effect if the two are moving with the same velocity. (Plus, once there are relative motions between the charges, there will be induced magnetic fields which affect the dynamics.)

Re: What is the Speed of Gravity?

#78

Post by wnoise »

ewjordan wrote: post #58
wnoise wrote: post #44

Earlier quoted context omitted.

> Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predicted" and the direction of the force is adjusted accordingly - as long as a body is not accelerating, a purely attractive or repulsive force will be pointing at its current position, not its position 8 mins ago. Absolutely correct for E&M. This is because it's a vector field, and the velocity o…

Ah, that's interesting, I never realized the rank was what determined the amount of "prediction" that a field does. That's something that should have been obvious on degrees-of-freedom considerations, but I never thought about it in that way before. That means my bootstrap-runaway argument must be flawed in some way, because a scalar field can't encode any velocity information, and as a classical field equation, it w…

Hmm. Now I need to think about it a bit more.

Suppose we have a scalar field.

For Galilean relativity, a sphere expanding at a given speed will remain so after a boost, but will have a net velocity. Densities will remain the same. A moving source shouldn't matter -- any material will itself will set the rest-frame, and must to have a non-infinite propagation velocity. Whereas, emitting particles, these will have the velocity of the source. Ah! if it's a complex wave equation, you can put a varying phase on everything encoding the velocity. But moving to a complex field is in some sense putting more degrees-of-freedom in.

For Einsteinian relativity, a sphere expanding at light speed remains a sphere in any reference frame, but the density along it changes. A moving source should have that same pattern, so it would seem that local differences (i.e. derivatives) could "point out" the velocity of the source. This behaves the same for particles being emitted. Is there a first-order Lorentz invariant scalar wave-equation, or only second-order, using the d'Alembertian? And that seems to introduce another degree-of-freedom as well.

I'm still really confused. How do pions behave? Is it reasonable to consider them as a fundamental scalar field when they're modeled as pairs of quarks? Particularly when they have excited states that are spin-1?

Re: What is the Speed of Gravity?

#79

Post by Luc »

Tycho wrote: post #57
Luc wrote: post #49

Earlier quoted context omitted.

For fun: the moon has a radius of about 1730 km. At full moon we can see about half of the moon, so the distance from one side to the other is about pi * 1730 km. So how fast do you need to move a laser pointer across the moon surface to make the dot attain the speed of light? http://www.google.com/search?q=%28%281730000+*+pi%29+m%29+%2... A bit less than 2/100 of a second. That seems quite feasible really! (of cours…

So say the dot was fairly large and you had a vantage point on the moon near where the dot sweeps by... What would you actually see??

I think philh already explained it pretty well. In your question you suppose the laser dot to be 'fairly large', which I think would complicate things (the shape of the dot 'smearing out' as it goes faster). For me, philh's explanation works best if you imagine the laser pointer to be a cannon of photons, emitting one photon after another at a very high rate and in a single direction, with the electrons of the moon's surface always reflecting the photons towards your eye (perhaps a bunch of strategically placed mirrors...).

Re: What is the Speed of Gravity?

#80

Post by avar »

thailandstartup wrote: post #73
avar wrote: post #68

Earlier quoted context omitted.

I think I'm correct in saying that the light isn't attracted to the mass, but the mass bends spacetime around it, so the path of everything traveling through that medium is bent, including the paths of photons. This effect causis gravitational lensing: http://en.wikipedia.org/wiki/Gravitational_lens

Hmm, thanks. I did a little more searching and reading about this with my new vocabulary (bending spacetime). It seems like photons also bend spacetime. More info here. All of it contradictory :) http://www.physicsforums.com/showthread.php?t=287888 So I'm still wondering if it is proper to think of a star's gravitational field being caused not just by its matter, but also by all the photons it has pumped out over its…

It doesn't make any sense to define a gravity field by photons. They're massless, so they don't attract anything, and most of the photons a star has produced over its lifetime are speeding away across the universe billions of light years away from it.

Only things with mass can attract other objects with the gravitational force. Photons aren't in that category.

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