McKy said:I have been looking for a defined speed of gravity. Unfortunately I have gotten various answers from various sources. The two answers I have received are that the speed of gravity is instantaneous due to the affect it would have on the orbit of planets if it where not. And the other that says that it should travel at the speed of light due to relativity.
McKy said:I have been looking for a defined speed of gravity. Unfortunately I have gotten various answers from various sources. The two answers I have received are that the speed of gravity is instantaneous due to the affect it would have on the orbit of planets if it where not. And the other that says that it should travel at the speed of light due to relativity.
McKy said:the speed of gravity is instantaneous due to the affect it would have on the orbit of planets if it where not.
Actually it is not but the effect is very small since the curvature here is very small. Orbits (not including orbits with massless test particles) are generally not closed in GR exactly due to the finite speed of curvature propagation which is equal to c.George Jones said:As the Earth orbits the Sun., the position of the Sun, relative to Earth, changes. If gravity propagates at the speed of light, shouldn't the Earth feel (gravitationally) where the Sun was (according to the Earth) eight minutes ago, that is, shouldn't gravitational force be directed along the line that joins where the Earth is now to where the Sun was eight minutes ago? And if this is true, then, according to the previous paragraph, how can the Earth's orbit be a closed ellipse?
McKy said:Alright, so there are two schools of thought. Speed of c and speed far greater the c.
McKy said:Theoretically could this be tested?
McKy said:Alright, so there are two schools of thought. Speed of c and speed far greater the c.
stevmg said:Can anyone tell me why Keppler's law is right? Why do planets orbit in ellipses rather than circles?
bcrowell said:Kepler's first law is not right. It's a nonrelativistic approximation. Planets move in orbits that are only approximately ellipses. Kepler's first law is approximately right because Newtonian mechanics is approximately right, and Newton's proof of Kepler's first law is valid for Newtonian mechanics.
I don't understand what you have in mind when you refer to circles as an alternative to ellipses.
A planet goes in whatever direction you throw it in (so to speak). If you throw it perpendicular to the radius at the correct speed, it goes in a circle, but if you throw it at a different angle from the same place, or even in the same direction but at a different speed, it goes in an ellipse.stevmg said:The one question I do not understand using Newtonian logic is why would the planets degenerate into an elliptical orbit with one foci being the "main man?," i.e., with the Sun located at one if the foci?
"Perihelion" is the point of closest approach to the Sun. According to Newton's theory it should be at the same place every orbit, but in relativity it moves slightly from one orbit to the next. The effect is tiny and affects Mercury the most (where the spacetime curvature due to the Sun is highest).stevmg said:Then only planet that screws this up is mercury with some perihelion (I don't even know what that means) because it is moving faster and sime relativisic effects do take place as opposed to the slower moving outer planets which seem to obey Newton quite well.
dcwarrior said:Isn't a possible test for the "speed of gravity" to take Newtonian calculations with respect to some large and quick orbiting objects (close-orbiting binary stars?), and then re-do the same calculations using the more complicated equations using relativity and see how different results they give over various periods? Then if they do show a difference in a relevant time scale, observe the model atronomical objects at the relevant interval and see which better predicts the location of the objects at the end of the period?
bcrowell said:Kepler's first law is not right. It's a nonrelativistic approximation. Planets move in orbits that are only approximately ellipses. Kepler's first law is approximately right because Newtonian mechanics is approximately right, and Newton's proof of Kepler's first law is valid for Newtonian mechanics.
dcwarrior said:Isn't a possible test for the "speed of gravity" to take Newtonian calculations with respect to some large and quick orbiting objects (close-orbiting binary stars?), and then re-do the same calculations using the more complicated equations using relativity and see how different results they give over various periods? Then if they do show a difference in a relevant time scale, observe the model atronomical objects at the relevant interval and see which better predicts the location of the objects at the end of the period?
bcrowell said:This has been done in the case of the Hulse-Taylor system. Newtonian gravity predicts that the two stars follow Kepler's laws forever. GR predicts that they dissipate energy as gravitational waves. Observations verify this prediction of GR to high precision. The problem is that this doesn't prove that every aspect of GR (including propagation of gravitational disturbances at c) is correct. This is what I was referring to in #5. You can't test propagation at c unless you have a viable test theory that predicts something other than propagation at c. But there is no such viable theory. (Newtonian gravity hasn't been viable since about 1919.)
GR doesn't assume that gravity travels at c. Propagation of gravitational disturbances at c can be proved from GR, not the other way around. (Actually, propagation at c isn't even strictly true in GR. It's only true in the limit where the amplitude of the gravitational waves is small.) So you can't just make a version of GR where gravity@c is relaxed as an assumption, because it's not an assumption. To get a theory without g@c, you would have to figure out some other theory that was different from GR in some way, such that this change in the theory had the side-effect of changing GR's prediction of g@c. There certainly are other known viable theories of gravity, such as Brans-Dicke gravity, but none of them predict anything other than g@c.dcwarrior said:However, given that the relativistic equations (which assume gravity goes at c) seems to explain the observed motions quite well, is there any way to test what happens if you relax that assumption?
http://en.wikipedia.org/wiki/Gravitational_radiation#Orbital_decay_from_gravitational_radiationdcwarrior said:The reason the binary star example was so great to explain to lay people like me is that it's intuitively easy to understand, If two objects are circling, and gravity "moves" at a "finite speed" (say, c) then if the two objects are moving quite fast compared to c, you'd expect as a layperson that the pull object A feels from object B is based on where object B was some time ago versus where it is now, and therefore you might expect the equations to come out differently. If anyone is familiar with those equations, I assume that concept is taken into account?
Either your memory is wrong or the person being quoted is incompetent, a crank, being misquoted, having his/her statement oversimplified, ...dcwarrior said:Also, I seem to recall at least one interview where I thought an astronomer said he assumes that gravity is instantaneous. I wish I could remember what he actually said.
bcrowell said:GR doesn't assume that gravity travels at c. Propagation of gravitational disturbances at c can be proved from GR, not the other way around. (Actually, propagation at c isn't even strictly true in GR. It's only true in the limit where the amplitude of the gravitational waves is small.) So you can't just make a version of GR where gravity@c is relaxed as an assumption, because it's not an assumption. To get a theory without g@c, you would have to figure out some other theory that was different from GR in some way, such that this change in the theory had the side-effect of changing GR's prediction of g@c. There certainly are other known viable theories of gravity, such as Brans-Dicke gravity, but none of them predict anything other than g@c.
dcwarrior said:A static gravity FIELD (say, between two objects not moving with respect to one another) is, correct me if I'm wrong, not necessarily particles or waves flowing from one to another but just a function of space-time? Therefore, that's why you say gravity doesn't travel at c? Because "speed" is not relevant?
dcwarrior said:Thanks, when I saw someone cited the Hulse-Taylor system I was able to see that people have been working on "relativistic celestial mechanics" for some time.
However, given that the relativistic equations (which assume gravity goes at c) seems to explain the observed motions quite well, is there any way to test what happens if you relax that assumption? (Maybe not, I understand.)
The reason the binary star example was so great to explain to lay people like me is that it's intuitively easy to understand, If two objects are circling, and gravity "moves" at a "finite speed" (say, c) then if the two objects are moving quite fast compared to c, you'd expect as a layperson that the pull object A feels from object B is based on where object B was some time ago versus where it is now, and therefore you might expect the equations to come out differently. If anyone is familiar with those equations, I assume that concept is taken into account?
Also, I seem to recall at least one interview where I thought an astronomer said he assumes that gravity is instantaneous. I wish I could remember what he actually said.
I looked at the question on some other forums and there were a lot of huffy answers about "nothing can go faster than c" and over-reliance on the circular reasoning of that one direct attempt to prove gravity propagates at c. It seems to me that those in the know could be a lot more authoritative to those who seem to think the astronomy/physics community has missed the boat on this by going straight to the work people have done on celestial mechanics, with the binary star work as a big example.
stevmg said:Even if the "pull" came earlier the pull was always there even though you are feeling the pull from some time ago, it is still a "pull." It would only make a difference at the very beginning, not after equilibrium was established.
Make sense?
Just think of running under a rain cloud. You get hit with water from eight or nine minutes before but you still get wet.
dcwarrior said:yes, but you could definitely tell the difference. If stand still your head and shoulders are wet but if you run, your whole front gets wet. The point about moving is, (and I think we'd need to consult one of the celestial mechanics folks) is that if 2 objects are orbiting quickly compared to the speed of light, at equilibrium, the pull comes from a different direction from its then-current position. Which of course may already be taken into account by the equations, but I don't know that.
DrGreg said:A planet goes in whatever direction you throw it in (so to speak). If you throw it perpendicular to the radius at the correct speed, it goes in a circle, but if you throw it at a different angle from the same place, or even in the same direction but at a different speed, it goes in an ellipse.
When the solar system first started to form, it's likely everything started off moving in circles, but the lumps of matter that later merged to form planets would have collided and interacted gravitationally, deviating from circles to ellipses.
"Perihelion" is the point of closest approach to the Sun. According to Newton's theory it should be at the same place every orbit, but in relativity it moves slightly from one orbit to the next. The effect is tiny and affects Mercury the most (where the spacetime curvature due to the Sun is highest).
stevmg said:[...]the perpendicular pull on a binary star is perpendicular to where the other star used to be, so it always "lags" behind where it should be. Wouldn't that just be another larger circle? Try and figure that one out just based on a time delay, much less relativity.
bcrowell said:There is a straightforward argument that the time lag should lead to the loss of energy to gravitational radiation. See http://www.lightandmatter.com/html_books/genrel/ch09/ch09.html#Section9.2 , subsection 9.2.1, second paragraph. Taylor and Wheeler make a similar argument in Spacetime Physics, but they phrase it in terms of Atlas doing mechanical work on two planets.
I don't think the existence of a time delay by itself is enough to imply the perihelion shift. The perihelion shift exists even for a test particle orbiting in a Schwarzschild spacetime. Since the Schwarzschild metric is static, the speed of propagation of gravitational effects is irrelevant.
For a nonmathematical explanation of the perihelion shift, see http://www.lightandmatter.com/html_books/genrel/ch06/ch06.html#Section6.2 , subsection 6.2.6, second paragraph.
stevmg said:I downloaded those two references you gave in .pdf format (you web .pdf converter on IE8)
