questionposter Posted April 2, 2012 Posted April 2, 2012 So I understand that any "change" in a gravitational field could only happen at the speed of light, but in GR, there's no force carrier particles that travel at a finite speed, so does that technically means an interaction between two objects in each other's fields is analogously instantaneous?
RichIsnang Posted April 2, 2012 Posted April 2, 2012 Interesting, if the sun was removed, would we continue to spin round nothing for 8 minutes?
swansont Posted April 2, 2012 Posted April 2, 2012 So I understand that any "change" in a gravitational field could only happen at the speed of light, but in GR, there's no force carrier particles that travel at a finite speed, so does that technically means an interaction between two objects in each other's fields is analogously instantaneous? No. In GR, the interaction propagates at c. There's no force carrier particle because GR is a classical (i.e. not quantum) theory.
questionposter Posted April 2, 2012 Author Posted April 2, 2012 No. In GR, the interaction propagates at c. There's no force carrier particle because GR is a classical (i.e. not quantum) theory. That's basically my first post in different words. Could you elaborate as to explain how in GR why if we consider the mass of the sun to be constant, and we are presently in it's gravitational field, while the sun is already in ours if we consider Earth's to be constant, why the interaction isn't instantaneous if there are no changes? If you think about, if you have an indentation in a net, the ball doesn't need to keep forming it, it just forms that indentation once and then as long as that ball is there the field is just "there".
swansont Posted April 2, 2012 Posted April 2, 2012 That's basically my first post in different words. Could you elaborate as to explain how in GR why if we consider the mass of the sun to be constant, and we are presently in it's gravitational field, while the sun is already in ours if we consider Earth's to be constant, why the interaction isn't instantaneous if there are no changes? If you think about, if you have an indentation in a net, the ball doesn't need to keep forming it, it just forms that indentation once and then as long as that ball is there the field is just "there". You can't establish the speed of the interaction if the situation is static.
IM Egdall Posted April 2, 2012 Posted April 2, 2012 Interesting, if the sun was removed, would we continue to spin round nothing for 8 minutes? Yes. We would also "see' the Sun for the same 8 minutes or so.
Superfusion Posted April 3, 2012 Posted April 3, 2012 "Interesting, if the sun was removed, we would continue to spin round nothing for 8 minutes? "Yes." I know some theories in physics and knowledge of how time travel would work on basic levels in all but i in no way can accurately predict and craft accurate theories. So i would like to ask, Why would we continue to spin around nothing for 8 minutes? Is EVERYTHING limited to the speed of light? Including reactions to such things as loss of gravity? I understand how we would continue to see the sun for 8 minutes due to the travel time for light from the sun, That light would continue to reach us for 8 minutes. But would every part of that reaction be delayed to the speed of light? Including earths reaction to the loss of gravity? And does this mean that gravity has a travel time as well? And i assume if so it also moves at the speed of light? Just have many curiosities in the field.
Cap'n Refsmmat Posted April 3, 2012 Posted April 3, 2012 I know some theories in physics and knowledge of how time travel would work on basic levels in all but i in no way can accurately predict and craft accurate theories. So i would like to ask, Why would we continue to spin around nothing for 8 minutes? Is EVERYTHING limited to the speed of light? Including reactions to such things as loss of gravity? I understand how we would continue to see the sun for 8 minutes due to the travel time for light from the sun, That light would continue to reach us for 8 minutes. But would every part of that reaction be delayed to the speed of light? Including earths reaction to the loss of gravity? And does this mean that gravity has a travel time as well? And i assume if so it also moves at the speed of light? Just have many curiosities in the field. Yes, that's right.
Superfusion Posted April 3, 2012 Posted April 3, 2012 Yes, that's right. Thank you for the clarification. I have a lot to learn and i am looking forward to it. Another thing i can factor in when i create equations with such factors.
questionposter Posted April 4, 2012 Author Posted April 4, 2012 (edited) CHANGES in a gravitational field take time, but for reasons I don't understand completely, swan said that a static situation like, doesn't take time? Happens instantaneously? Like, how much time does it take to be able to be in a gravitational field? Because in relativity, if your in a static gravitational field, and the situation is "static", then the correlation between the gravitational fields is I guess instantaneous or is just established by a correlation, but in QM that situation actually requires force carrier particles to travel distance over time, and this simply sitting in a gravitational field is not an instantaneous or undefined action like in GR since particles would need to travel distance over time in order for it to happen. Edited April 4, 2012 by questionposter
swansont Posted April 4, 2012 Posted April 4, 2012 CHANGES in a gravitational field take time, but for reasons I don't understand completely, swan said that a static situation like, doesn't take time? Happens instantaneously? Like, how much time does it take to be able to be in a gravitational field? Because in relativity, if your in a static gravitational field, and the situation is "static", then the correlation between the gravitational fields is I guess instantaneous or is just established by a correlation, but in QM that situation actually requires force carrier particles to travel distance over time, and this simply sitting in a gravitational field is not an instantaneous or undefined action like in GR since particles would need to travel distance over time in order for it to happen. If it's static, then using the term instantaneously is a non-sequitur. I don't know how a quantum theory of gravity would look, but here's one possibility: the graviton exchange is a steady state. The flux of gravitons is the same as a it was an arbitrary time in the past, so the interaction is constant. If this changes, it will take 8 minutes for that change to reach us, or any other point that's the same distance from the sun.
questionposter Posted April 4, 2012 Author Posted April 4, 2012 If it's static, then using the term instantaneously is a non-sequitur. I don't know how a quantum theory of gravity would look, but here's one possibility: the graviton exchange is a steady state. The flux of gravitons is the same as a it was an arbitrary time in the past, so the interaction is constant. If this changes, it will take 8 minutes for that change to reach us, or any other point that's the same distance from the sun. I think the main difference is one is a correlation and one is a causation...
swansont Posted April 4, 2012 Posted April 4, 2012 I think the main difference is one is a correlation and one is a causation... How so?
michel123456 Posted April 4, 2012 Posted April 4, 2012 It is not physically possible to make the mass of the sun disappear just like that. The sun may blow out or shrink, its mass will not disappear.
questionposter Posted April 4, 2012 Author Posted April 4, 2012 How so? One is just, if your in the field, your in the field and the reaction keeps instantaneously happening, whereas in QM you need particles which travel distance over time at a finite speed to continuously sustain a static field.
swansont Posted April 5, 2012 Posted April 5, 2012 One is just, if your in the field, your in the field and the reaction keeps instantaneously happening, whereas in QM you need particles which travel distance over time at a finite speed to continuously sustain a static field. I don't see how that makes one correlation and the other causation, and the view that the reaction is instantaneous is wrong. If you think of it in terms of field lines, i.e. classically, the field lines still take time to propagate, which is conceptually the same as saying the curvature takes tim to propagate. If you changed the scenario and instead had a planet pop into existence, it would feel attraction immediately under both models. Space is already curved where it appears, and the virtual particles are already there. But that still doesn't mean the interaction speed is infinite.
questionposter Posted April 17, 2012 Author Posted April 17, 2012 (edited) I don't see how that makes one correlation and the other causation, and the view that the reaction is instantaneous is wrong. If you think of it in terms of field lines, i.e. classically, the field lines still take time to propagate, which is conceptually the same as saying the curvature takes tim to propagate. If you changed the scenario and instead had a planet pop into existence, it would feel attraction immediately under both models. Space is already curved where it appears, and the virtual particles are already there. But that still doesn't mean the interaction speed is infinite. No what I'm saying is that it doesn't take time for a gravitational field to exist where it is already existing in GR, but in QM, you need to constantly have virtual particles traveling distance over time to interact with atoms which is why it makes more since for gravity to be emitted from the surface of a black hole since time is stopped at the event horizon itself. In GR, if you stopped time, an already existing gravitational field would continue to exist, but in QM, but you stopped time, virtual particles couldn't travel distance over time and thus gravitational fields would stop effect you if you could remain completely still, unless I'm missing some weird time symmetry thing. Edited April 17, 2012 by questionposter
swansont Posted April 18, 2012 Posted April 18, 2012 No what I'm saying is that it doesn't take time for a gravitational field to exist where it is already existing in GR, but in QM, you need to constantly have virtual particles traveling distance over time to interact with atoms which is why it makes more since for gravity to be emitted from the surface of a black hole since time is stopped at the event horizon itself. In GR, if you stopped time, an already existing gravitational field would continue to exist, but in QM, but you stopped time, virtual particles couldn't travel distance over time and thus gravitational fields would stop effect you if you could remain completely still, unless I'm missing some weird time symmetry thing. No, I don't think that's right. The virtual particles already exist, too. If you have an electric/magnetic/gravitational field the field exists regardless of the presence of a target particle. If one appears, it feels an instantaneous interaction. In the QM picture, it would interact with virtual particles that had been emitted prior to its appearance.
MigL Posted April 18, 2012 Posted April 18, 2012 Charge is conserved by a black hole also, Questionposer, so if your logic was correct its field would not be able to esape the event horizon, but it does. However, some have theorized that mass, charge and angular momentum as well as entropy are conserved on the event horizon.
questionposter Posted April 18, 2012 Author Posted April 18, 2012 (edited) No, I don't think that's right. The virtual particles already exist, too. If you have an electric/magnetic/gravitational field the field exists regardless of the presence of a target particle. If one appears, it feels an instantaneous interaction. In the QM picture, it would interact with virtual particles that had been emitted prior to its appearance. I'm not seeing how that works though, could you be more specific? Because I think even in modern theories virtual particles still have many properties or real particles, such as their speed limit being light. http://en.wikipedia....irtual_particle If they are massless but are comprised of energy, like light, wouldn't they only be able to travel at the speed of light and nothign more or less? Also if one appears but there's an instantaneous reaction, shouldn't changes also happen instantaneously? And how could virtual particles exist without a particle to generate them? If you have an electric/magnetic/gravitational field the field exists regardless of the presence of a target particle. Edited April 18, 2012 by questionposter
swansont Posted April 19, 2012 Posted April 19, 2012 I'm not seeing how that works though, could you be more specific? Because I think even in modern theories virtual particles still have many properties or real particles, such as their speed limit being light. http://en.wikipedia....irtual_particle If they are massless but are comprised of energy, like light, wouldn't they only be able to travel at the speed of light and nothign more or less? Also if one appears but there's an instantaneous reaction, shouldn't changes also happen instantaneously? No, there's no problem here with what I said. And how could virtual particles exist without a particle to generate them? Never said there wasn't. I said the field exists regardless of any target particle, i.e. another particle in the interaction. e.g. the gravitational field of the sun exists independent of any planets orbiting it. Gravitons from a quantum theory would exist at some distance r, having been emitted by the sun earlier.
questionposter Posted April 19, 2012 Author Posted April 19, 2012 (edited) Never said there wasn't. I said the field exists regardless of any target particle, i.e. another particle in the interaction. e.g. the gravitational field of the sun exists independent of any planets orbiting it. Gravitons from a quantum theory would exist at some distance r, having been emitted by the sun earlier. Well that makes sense, but...can you still be more specific as to how virtual particle interactions within a field are instantaneous when virtual particles can only travel at the speed of light? Edited April 19, 2012 by questionposter
swansont Posted April 19, 2012 Posted April 19, 2012 Well that makes sense, but...can you still be more specific as to how virtual particle interactions within a field are instantaneous when virtual particles can only travel at the speed of light? I am at a position x, and something else is at the origin. The particle at the rigid sends out a photon, and it takes a time t=x/c to get to me. But if it already sent out a photon x/c seconds ago, the photon is already here. So under the hypothetical situation where I can just pop into existence at x, I can interact with a photon right away, if it was emitted x/c seconds ago.
questionposter Posted April 19, 2012 Author Posted April 19, 2012 I am at a position x, and something else is at the origin. The particle at the rigid sends out a photon, and it takes a time t=x/c to get to me. But if it already sent out a photon x/c seconds ago, the photon is already here. So under the hypothetical situation where I can just pop into existence at x, I can interact with a photon right away, if it was emitted x/c seconds ago. Right, but if time stopped, it doesn't matter how many seconds ago a photon was emitted does it? It still wouldn't travel distance over time, and thus, how would it ever make it to you?
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