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When sun is gone, gravity of light, which one faster?

  1. Sep 13, 2007 #1
    I know that it took approximately 3 minutes for lights to travel from sun to earth. Let say the sun is gone now. Like erased, click-n-drag out of the universe in an instant! So until 3 minutes later, no one on earth would realize it, because nothing travel faster than light! But, could I argue that at the instance the sun is gone, we wont feel the gravity of the sun anymore, which surely could be detected by some sort, and so we know that the sun is gone!
  2. jcsd
  3. Sep 13, 2007 #2
    the force of gravity moves at the speed of light so it would reach the earth at the same time as the light would. if you want to see this in action watch the elegant universe video
  4. Sep 13, 2007 #3
    what he said, but I believe its is 8 minutes
  5. Sep 13, 2007 #4
    Kind of funny that some people today still seem to be unwittingly asking the exact same question that produced GR..
  6. Sep 13, 2007 #5
    Yes, it is approx 8 minutes for light to travel from Sun to earth.

    Now, assuming the Sun were Massive enough to collapse to a black-hole (I know it's not massive enough!), then how would the space-time curvature, already present, be up-dated?
    I have heard Physicists talk on one-hand about s-t curvature and the other hand about Gravitons. Surely they are mutually incompatible?
    Any Gravitons would be unable to 'get-out' of the Black-Hole to update the s-t curvature.
  7. Sep 13, 2007 #6
    Gravitons are weak-field perturbations -- ripples on an ocean. Truly massive change, like a collapsing black hole, follows different dynamics. A better question is this: to an observer safely parked in the asymptotic region of a Schwarzchild metric, in-falling particles never cross the event-horizon in the proper time of the observer. Now consider a spherically collapsing star... as the boundary of the star contracts towards the event-horizon, it too should never appear to cross it from the point of view of the external observer... so do black holes actually ever form from a collapsing star scenario?
  8. Sep 13, 2007 #7


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    See How does the gravity get out of the black hole? from the Usenet Physics FAQ on John Baez's site. Also see the subsequent question What happens to you if you fall into a black hole? which talks about how a black hole can be considered a sort of "frozen star" from the perspective of observers on the outside since they should theoretically measure it taking an infinite amount of time for anything to reach the event horizon...this should help explain what they mean in the first answer when they say "In this sense the black hole is a kind of "frozen star": the gravitational field is a fossil field. The same is true of the electromagnetic field that a black hole may possess."
    Last edited: Sep 13, 2007
  9. Sep 13, 2007 #8
    Quote from link above:

    If light always travels at c, how is it taking 'longer and longer'?
    I see that light (and any other information) will be frequency shifted to be less energetic upon reaching the observer, but they still travel at c. The only other option is they don't emerge at all in the sense they are red-shifted so much as to be un-observable.
    If the light 'goes black' then any other information 'goes black'--- not there.

    Also, anything entering the Black-Hole does go in---- in it's own frame. It is just the outside observers that get the view described in the link.

    Link quote:
    I suppose that answers the question then? But then says in reference to the Electric fields that are allowed outside the Black hole:

    Implying that Gravitons, should they exist would be Virtual-Gravitons and travel >c .
  10. Sep 13, 2007 #9
    You are mixing two incompatible theories here. Large scale Space-time warping as defined by GR does not use “Gravitons” which are part of the Standard Model based on QM. And QM has not been able to deny GM by experimentally detecting a “Graviton”, at least not so far.

    Also, if you instantly replace the Sun with an equal mass Black Hole, which is possible on paper using GR. GR would show no change at all in the Space-time warping in our solar system, except for the areas inside the original size of the sun. That is how GR Space-time warping works without any dependence on gravitons for large scale issues like this, while small scale particle theories have so much trouble solving large scale issues.

    The only change based on GR is there would be no more sunlight. I don’t expect the conflict between GR and QM, can be resolved in this thread.
  11. Sep 13, 2007 #10


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    On the original question of the sun disappearing. A detailed analysis of this scenario gives the result that the sun can't disappear - this would violate several important conservation laws that are "built into" GR.

    It's similar to the way that charges can't disappear in classical electromagnetism - they are conserved.

    Therefore, you can't sensibly ask "what happens if the sun disappears", at least not in the context of GR, any more than you can ask "what happens if a charge disappears" in classical electrodynamics.

    What you can do (in principle, it's still not practical) is blow the sun up, and detect the resulting gravity waves. To do this, you have to blow the sun up in a fashion that's not spherically symmetrical. If you blow the sun up in a spherically symmetrical manner, except for very small effects related to the rotation of the sun, there would be no gravity waves, and no effect on gravity at the Earth until the ejecta actually reached Earth's orbit.

    If you blow up the sun in a non-spherically symmetrical manner (say you split it into two parts, one part goes to the celestial north at a high velocity, the other part goes to the celestial south), gravity waves will reach the Earth in about 8 minutes. You'd still need fairly sensitive instruments to detect them, however.

    It turns out that if you want to get a strong gravity wave signal, you're better off imploding the sun , ideally imploding it into a black hole, rather than exploding it. That's because there's roughly an (R/r_s)^5 fall off in gravity wave emissions for objects falling into black holes, so if you count the efficiency of gravity wave emission at the event horizon as unity, at 10x the Schwarzschild radius it's down by a factor of 100,000. Since our sun is about 200,000 times larger than it's Schwarzschild radius of 3km, that puts the gravity wave emission down by a factor of [itex]\approx[/itex]10^26 compareed to a black hole.
  12. Sep 13, 2007 #11


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    Light travels at c locally in general relativity, meaning that in any small region where the spacetime curvature is negligible, a freefalling observer will measure light to move past him at c. But it doesn't necessarily travel at c globally in coordinate systems which cover large regions of curved spacetime, like the Schwarzschild coordinates which are usually used in discussing black holes (although you are free to use any coordinate system you like in general relativity, some just make the math easier than others). For an observer at a fixed distance from the event horizon in Schwarzschild coordinates, light will indeed take longer and longer to reach him the closer it is emitted from the event horizon.
    "virtual particles" of all types are used to make calculations in quantum field theory, but it isn't clear they're "real" in any physical sense as opposed to being just mathematical tools for making calculations about the outcomes of actual physical measurements. So yes, virtual gravitons might travel faster than c in a theory of quantum gravity (while non-virtual gravitons would move at c), but it's already true that virtual photons travel faster than c in quantum electrodynamics, but this isn't understood to be a violation of relativity since this will never result in any classical information being transferred faster than light. For more information on virtual particles, see this virtual particles FAQ, and you could also take a look at sections S3a - S3c of this physics FAQ by Arnold Neumaier which argues for the idea that virtual particles should not be considered to be real physical entities.
    Last edited: Sep 13, 2007
  13. Sep 13, 2007 #12
    This just does not seem to make sense in GR terms.
    Are you saying that the gravity Earth feels from the sun would change if the suns mass was imploded and contained into a space 200,000 times smaller to fit inside a 3km Schwarzschild radius.
    What would the change be for the force field at the range of earth?
    Would it make the Earth seem to be heaver or lighter as in pulling it into a smaller or larger orbit than we have now.

    I’m under the impression that if you were to rearrange the mass evenly at any locations- - lets say inside the orbit of mercury, as long as the center ant total amount of that mass was still the same as what the sun currently has; the net “gravity” imposed on Earth by a GR space-warping would be the same. Even if contained all in a single point.
  14. Sep 14, 2007 #13


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    If the sun were not rotating, you are correct in saying that there would be no gravity wave in the implosion case just as there would be no gravity waves in exploding it - without rotation, the situation is spherically symmetrical, and Birkhoff's theorem says that you won't get gravity waves from any spherically symmetrical system collapsing or expanding while maintaining it's spherical symmetry.

    However, the sun is rotating, among other things, which spoils the spherical symmetry. It is believed that gravitational collapse of a star should have a gravitational wave signature, one that will hopefully someday be able to be detected by Ligo. A google literature search finds for instance:


    One way of looking at this is to say that because of the rotation (and other factors as the original authors mention), the intial state isn't quite spherically symmetrical.

    The (R/R_s)^5 law is a very rough approximation that came out of MTW, chapter 36. So ccurrently, I believe that you'd get some very small amount of gravitational waves from blowing up the sun from these same small assymetries, but they'd be suppressed by a very large factor from the better-studied case of a realistic stellar collapse.

    This is a very crude analysis, I haven't really gone into it in any depth, however. I'm also assuming that some sort of induced stellar collapse is possible , and would have a gravity wave signature similar to a natural collapse.
  15. Sep 14, 2007 #14
    I don't see any loss in symmetry in the POV from Earth caused by the sun rotating. Maybe frame dragging, but I don't think that could be detectable at that kind of distance. So I guees in my view LIGO will never be able to detect a "Gravity Wave" which has been the case so far. When and if it does is when I'll know I'm wrong.
  16. Sep 14, 2007 #15


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    Do you think your handwavey arguments are right and all the physicists who have done an actual mathematical analysis of what GR predicts about gravitational waves from collapsing stars got it wrong, or are you disputing that GR's predictions are correct?
  17. Sep 14, 2007 #16
    What "handwavey argument" ?
    Are you disagreeing with pervect[/p] when he said I was correct?
    Do you have a Non "handwavey argument" to explain the potential exception he cited for the case of the sun rotating before “imploding” it into a blackhole ?
    What does your “mathematical analysis” tell you;
    – will the G Field at Earth be increased pulling it down to a smaller orbit?
    - Or decreased allowing to Earth to move out to a larger orbit?

    If the current view of GR cannot explain which of those two happens and why, but only declares that a “wave” should exist – then I have ever right to consider the wave claim incomplete or "handwavey" till they shown the wave by a direct test.
  18. Sep 14, 2007 #17


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    He said "If the sun were not rotating, you are correct in saying that there would be no gravity wave in the implosion case", but then he went on to say "However, the sun is rotating, among other things, which spoils the spherical symmetry. It is believed that gravitational collapse of a star should have a gravitational wave signature, one that will hopefully someday be able to be detected by Ligo." Your handwavey argument was that a rotating star would not produce gravitational waves, because "I don't see any loss in symmetry in the POV from Earth caused by the sun rotating." But as pervect said, physicists have analyzed the case of the collapse of a rotating star, and found that according to GR gravitational waves would be generated.
    What "exception" are you talking about? He said there'd be no gravitational waves if the star was not rotating, but explained that in the case of a rotating star, its collapse would generate gravitational waves.
    It's not my analysis, its the analysis of physicists. And I don't think that the creation of gravitational waves necessarily implies any change in the average G field experienced by the Earth (although it might imply a change in the tidal forces)--this question from an online black hole FAQ says:
    Uh, why do you think the current view of GR cannot explain what happens to the G field at the Earth? I'm sure it can, even if the posters on this thread don't have that information handy for you.
    The question of what GR predicts about the collapse of rotating stars can be determined using mathematics alone. If GR predicts that rotating starts generate waves but experiment shows no waves, this would show that GR is an incorrect theory of gravity, but it wouldn't change the facts about what GR predicts. And your handwavey argument was purely based on GR, since the assumption that spherical symmetry = no gravitational waves is just a theoretical prediction of GR itself.
    Last edited: Sep 14, 2007
  19. Sep 15, 2007 #18
    ?? The exception you qouted me quoting pevect "the case of a rotating star"

    But you do have it handy and provided it by qouting them as saying

    "What if the Sun *did* become a black hole for some reason? .... The Earth and the other planets would not get sucked into the black hole; they would keep on orbiting in exactly the same paths they follow right now. ..."
    No change means no wave, and "tidal forces" forces are not some independent thing, it can only change if the gravitational field changes.

    So here you admit my "handwavey argument" is in fact based on GR.

    Look, I already said I could be proven wrong, but not by your wining about it. You and I have different standards; you chose to dogmatically accept your scientific leaders as pronouncing pure irrefutable facts, I do not and prefer the Scientific Method of doubt and test. Especially when those same leaders express my same doubts when they propose experiments like LIGO to resolve those doubts. I am simply willing predict doubts are justified and no Gravitational Waves will be detected.
    Last edited: Sep 15, 2007
  20. Sep 15, 2007 #19


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    pervect said that the collapse of a rotating star would generate gravitational waves. You said it wouldn't.
    But this quote doesn't say there would be absolutely no change in the gravitational field--it says "as long as you stay well outside the horizon, a black hole's gravity is no stronger than that of any other object of the same mass", which suggests it might be some sort of limit where the further you get from the horizon the closer the gravitational field is to that of the original star, but there might always be some slight divergence. Alternatively, I suppose it's possible that the field only differs when you're closer to the BH than the radius of the original star, outside this radius it's identical--this would be the case in Newtonian mechanics. But the quote itself doesn't clearly indicate which one it is. In any case, it's just your assumption that gravitational waves are impossible if there is no permanent change in the gravitional field--why couldn't it be true that when the star collapses, distant planets experience a transient change as a gravitational wave passes them by, then the gravitational field returns to being exactly what it was before the star's collapse?
    Yes, but it's handwavey all the same--you said "I don't see any loss in symmetry in the POV from Earth caused by the sun rotating", but clearly if GR predicts that gravitational waves are only possible in a non-spherically-symmetric situation, and physicists have done the actual calculations for a rotating star's collapse and found that GR predicts it generates gravitational waves, then you must be incorrect that the collapse rotating star is a spherically symmetric situation. And thinking about this a little more, it seems to me you're just confusing cylindrical symmetry with spherical symmetry--the first means that the situation is unchanged if you rotate around the cylinder's axis, the second means the situation is unchanged if you rotate about any axis going through the center of the sphere. And clearly if we pick an axis which is different than the sun's axis of rotation, then rotating about this axis will change the sun's axis of rotation (along with the direction of all the tangential velocity vectors), so this is not a spherically symmetric situation.
    But we're not talking about an empirical question requiring real-world testing here, we're talking about a purely mathematical question of what GR predicts. So what I'm saying is that I trust the actual detailed calculations of large numbers of trained physicists over some vague intuitive nonmathematical arguments, and if you trust these intuitive arguments over the calculations of all these physicists just because they're your intuitive arguments, then I think you're falling into the psychological trap discussed here.
    They express no doubt that GR predicts gravitational waves for a collapsing rotating star, the doubts are only over whether GR is correct in the first place. But that's not what we're discussing on this thread, since again, your argument was based on assuming that GR's "no gravitational waves without a violation of spherical symmetry" claim is correct.
    Last edited: Sep 15, 2007
  21. Sep 15, 2007 #20
    You asked for the exception that pervect & I still disagreed on and that is it – I thought that was clearly stated and I do not understand why you are confused by it.
    I don’t base anything on “violation of spherical symmetry” my observations are based on the explanations failing to describe how & why a gravitational wave will be felt by the Earth. However if the math of GR does demand that G-Waves are real then I am willing to accept that my opinion must be that GR is incorrect. I’m comfortable with that until proven otherwise, which is consistent with current science that recognizes that GR and Quantum Theories as current understood are incompatible and both cannot be correct. Therefore at least one of the two, potentially GR must be wrong.
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