|
Post by speakertoanimals on Sept 7, 2010 19:24:33 GMT 1
Newton -- gravity depends on masses of earth and sun, and distance between them.
Distance between then WHEN? If NOW, then the pull of the sun on the earth is directed towards where the sun is NOW, which is not the same as where we see the sun to be (light takes time to travel).
Hence we either have information as to the position of the sun (ie where gravity pulls the earth) as being transmitted instantaneously, or that simple picture is wrong.
If the pull is directed towards where the sun WAS, then we still have a problem, since then the pull of the earth on the sun is directed towards where the earth WAS as well. So, we have a problem, because if there is this delay, then the two pulls don't line up.
think instead of two equal mass planets orbiting each other, at opposite sides of a circle. If A gets pulled towards where B was, the pull doesn't go towards where B is now, and through the centre of the circle. ditto for pull on A due to B. hence from point of view of forces, we have two forces acting, which exert a torque. Which is wrong, it would contradict what Newton said about action and reaction being equal and opposite, which would mean angular momentum would no longer be conserved, and would give orbits that contradicted simple laws of planetary motion that Kepler worked out.
It would be wrong in spades, in effect, this eight-minute lag between us and the sun. but we can't just fudge it up by saying can cheat and know about where things are instantaneously, because that would contradict relativity, which also agree with experiment.
So, problem solved by the fact that in relativity, we have forces that have a lightspeed time delay built in, but they don't just depend on position and distance, but speed as well. Hence if I know the position of the sun and its speed eight minutes ago, I can predict roughly where it is now, which is why gravity manages to have a delay, yet still agree with Prof Newton as regards action and reaction.
Lesson: General relativity says gravitational forces depend on mass and motion, WITH lightspeed time delay. Hence for starters, the gravitational field of a rotating body like the earth should be slightly different to field for non-rotating earth (frame dragging) and Gravity Probe B has been trying to measure this very tiny effect.
|
|
|
Post by abacus9900 on Sept 7, 2010 20:01:19 GMT 1
But doesn't the sun create a permanent curved space geometry that the earth simply follows? In other words, the earth does not have to 'wait' for the sun to emit gravitational waves - they are already in place.
|
|
|
Post by kiteman on Sept 7, 2010 20:37:32 GMT 1
In the Newtonian model, gravity is seen as a kind of ray, shining out to affect things.
If that is true, and gravity "travels" at c, then it will take about 8 minutes for the Sun's pull to "reach" Earth.
In that time, the entire Solar System is moving through space, so by the pull "reaches" Earth, it will be acting towards the point where the Sun was when it "shone" out the gravity.
That would mean the earth's orbit should be centred on a point X miles behind the Sun's actual location, instead of on the Sun.
Clearly, this is not the case.
That means that gravity is not a ray. It is not carried by any particles.
Gravity is, instead, a distortion of space-time. The familiar "gravity well" image. This well does not radiate out, any more than "depth" radiates out from a hole in the ground.
When talking about "speed of gravity", we are really talking about the speed of ripples in the "surface" of the well because of rapid movements of massive objects (hence the comment about binary pulsars by Abacus).
|
|
|
Post by abacus9900 on Sept 7, 2010 20:55:49 GMT 1
Yes, I can relate to that description, Kiteman. Thanks.
|
|
|
Post by Progenitor A on Sept 7, 2010 20:58:54 GMT 1
In the Newtonian model, gravity is seen as a kind of ray, shining out to affect things. If that is true, and gravity "travels" at c, then it will take about 8 minutes for the Sun's pull to "reach" Earth. In that time, the entire Solar System is moving through space, so by the pull "reaches" Earth, it will be acting towards the point where the Sun was when it "shone" out the gravity. That would mean the earth's orbit should be centred on a point X miles behind the Sun's actual location, instead of on the Sun. Clearly, this is not the case. That means that gravity is not a ray. It is not carried by any particles. Gravity is, instead, a distortion of space-time. The familiar "gravity well" image. This well does not radiate out, any more than "depth" radiates out from a hole in the ground. When talking about "speed of gravity", we are really talking about the speed of ripples in the "surface" of the well because of rapid movements of massive objects (hence the comment about binary pulsars by Abacus). I don't think so. There is no evidence that gravity propagates in waves. Such waves have never been detected. In fact such waves are the Holy Grail of the Unified Field Theory. If gravity does not propagate, then no 'speed' is invloved. There is no 'yesterday' and tomorrow' with gravity. All observations show it as a static constant that variies only with mass. We are not talking of ripples because such gravity ripples have never been detected.
|
|
|
Post by kiteman on Sept 7, 2010 21:11:01 GMT 1
I was attempting to clarify Speaker's post of 3:44pm today, to satisfy Abacus' request.
|
|
|
Post by abacus9900 on Sept 8, 2010 10:59:30 GMT 1
Not directly no, but observations of the orbital decay of a pair of binary pulsars due to a loss of gravitational radiation give a good indication of the speed of gravitation since the rate of the orbital decay depends on the speed of gravitational energy loss. This can be worked out mathematically and has been found to be equal to the speed of light to within 1%.
|
|
|
Post by speakertoanimals on Sept 8, 2010 13:02:59 GMT 1
If we have curved spacetime, then the delay thing applies to what happens if the state of the body causing the curvature changes -- how fast does that ripple propagate outwards.
The other stuff was about what happens if we pretend that space is flat, and gravity is a force. In that case, we do have a speed, the speed of Newtonian gravity. The point being that even though it is only an approximation to the real case, it still better make sense as an approximation, and not do things like violate the conservation of angular momentum, which the forces not quite obeying action and reaction would do.
|
|
|
Post by Progenitor A on Sept 9, 2010 7:34:27 GMT 1
Not directly no, but observations of the orbital decay of a pair of binary pulsars due to a loss of gravitational radiation give a good indication of the speed of gravitation since the rate of the orbital decay depends on the speed of gravitational energy loss. This can be worked out mathematically and has been found to be equal to the speed of light to within 1%. But what is this orbital decay? Are they getting closer or separating? I assume, as you talk of 'gravitational radiation' - as if the gravity is somehow leaking away-, then the decay is such as to increase the distance between them? But before the concept of 'gravitational radiation' is invoked, other possible causes must be considered. The pulsars, as we well know radiate massive amounts of energy. This radiation wil reduce the mass and lead to orbital decay. It might be countered that the decay is far greater than the loss of mass due to energy radiation. But how about if they are radiationg the far more massive Dark Energy that is also hypothesised? The fact remains, as you have said, that gravity waves are simply an hypothesis that has no (direct) evidence to support it
|
|
|
Post by Progenitor A on Sept 9, 2010 7:38:07 GMT 1
If we have curved spacetime, then the delay thing applies to what happens if the state of the body causing the curvature changes -- how fast does that ripple propagate outwards. Yes that is succinctly put. However such ripples have not been observed The other stuff was about what happens if we pretend that space is flat, and gravity is a force. In that case, we do have a speed, the speed of Newtonian gravity. The point being that even though it is only an approximation to the real case, it still better make sense as an approximation, and not do things like violate the conservation of angular momentum, which the forces not quite obeying action and reaction would do. Not half as succinct and not at all clear (to me) I am afraid
|
|
|
Post by speakertoanimals on Sept 9, 2010 10:54:14 GMT 1
If we have curved spacetime, then the delay thing applies to what happens if the state of the body causing the curvature changes -- how fast does that ripple propagate outwards. Yes that is succinctly put. However such ripples have not been observed The other stuff was about what happens if we pretend that space is flat, and gravity is a force. In that case, we do have a speed, the speed of Newtonian gravity. The point being that even though it is only an approximation to the real case, it still better make sense as an approximation, and not do things like violate the conservation of angular momentum, which the forces not quite obeying action and reaction would do. Not half as succinct and not at all clear (to me) I am afraid whether they have been observed is not is not the point. The point is that gravity is an observed effect, that changing the state of the gravitating body changes the field, hence it is then necessary to ask -- at what speed do these disturbances propagate? If infinite, Newton was right and special (let alone general) relativity is incorrect. If finite, at what speed, and can such a finite speed explain observations. The fact that we cannot directly observe the disturbances doesn't mean that they don't exist, even if they travel at infinite speed.
|
|