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Thread: Does gravity have speed?

  1. #1
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    Does gravity have speed?

    Kind of a weird question but my thinking is that if gravity from a black hole traps light does that make gravity faster than light? Gravity from a black hole definitely "escapes" the black hole while light can not? Is gravity actually a thing or is it more of an effect?
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Gravitational waves move at the speed of light. Gravitons hypothetically would move at the speed of light.
    That doesn't stop a black hole having a gravitational field, any more than the finite speed of light prevents a black hole having an electric field.
    Under GR, the curvature of spacetime exists outside the black hole event horizon. Under some hypothetical quantum gravity, the force of gravity would presumably be mediated by virtual gravitons, in the same way the electromagnetic force is mediated by virtual photons. Virtual particles are not constrained by the lightspeed limit, since they pop into existence as mediators of their parent fields.

    Grant Hutchison

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    Is there "aberration of gravity"? As you rotate around a primary, does the primary "feel" as if it were slightly ahead of where it actually is?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    Is there "aberration of gravity"? As you rotate around a primary, does the primary "feel" as if it were slightly ahead of where it actually is?
    Except under extreme conditions, not so's you'd notice. Otherwise the solar system would be unstable over a period of centuries.
    Under Relativity, if we travel at constant velocity, and look at the propagation of electromagnetic radiation, we see light sources apparently displace towards our direction of travel, but we experience the force generated by electromagnetic fields as coming from their "instantaneous position" rather than their "retarded position". That is, the electromagnetic field equations under special relativity conspire to make the force from an electromagnetic field moving with constant velocity act as if it were propagating instantaneously. Teleologically, this has to be the case, because otherwise we'd be able to tell which of two bodies was moving and which was at rest by the force between them, and that would immediately break Relativity.
    Under GR, things get even more complicated, and the field equations conspire to make the gravitational force point to the instantaneous (rather than retarded) position of a source that is moving with constant acceleration. So aberration shows up only in higher-order terms like the rate of change of acceleration. And that's what causes things like the perihelion precession of Mercury, and the gravitational radiation from binary neutron stars.

    See Steven Carlip's paper Aberration and the speed of gravity for more on the topic.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Gravitational waves move at the speed of light. Gravitons hypothetically would move at the speed of light.
    That doesn't stop a black hole having a gravitational field, any more than the finite speed of light prevents a black hole having an electric field.
    Under GR, the curvature of spacetime exists outside the black hole event horizon. Under some hypothetical quantum gravity, the force of gravity would presumably be mediated by virtual gravitons, in the same way the electromagnetic force is mediated by virtual photons. Virtual particles are not constrained by the lightspeed limit, since they pop into existence as mediators of their parent fields.

    Grant Hutchison
    These gravity waves you speak of, as soon as you say wave I think of something like a wave in the ocean. So if I was able to see the gravity wave from the side in a 2D xy orientation it would look like a sine wave. X being time and Y being strength. Am I correct in assuming a gravity wave means that the gravitation strength increases and decreases with time?
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    These gravity waves you speak of, as soon as you say wave I think of something like a wave in the ocean. So if I was able to see the gravity wave from the side in a 2D xy orientation it would look like a sine wave. X being time and Y being strength. Am I correct in assuming a gravity wave means that the gravitation strength increases and decreases with time?
    They're called gravitational waves; gravity waves are something different.
    It's not quite as you imagine.
    Suppose a gravitational wave swept through your body from front to back. What you'd experience is a top-to-bottom compression and a side-to-side stretch, followed by a top-to-bottom stretch and a side-to-side compression. (Though the axis of course doesn't need to be, and in general won't be, so neatly aligned with your body axes.) That's the pattern that propagates as a wave at lightspeed. You can see a little animation of the effect at Einstein Online.

    Grant Hutchison

  7. #7
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    Quote Originally Posted by grant hutchison View Post
    Gravitational waves move at the speed of light. Gravitons hypothetically would move at the speed of light.
    That doesn't stop a black hole having a gravitational field, any more than the finite speed of light prevents a black hole having an electric field.
    Under GR, the curvature of spacetime exists outside the black hole event horizon. Under some hypothetical quantum gravity, the force of gravity would presumably be mediated by virtual gravitons, in the same way the electromagnetic force is mediated by virtual photons. Virtual particles are not constrained by the lightspeed limit, since they pop into existence as mediators of their parent fields.

    Grant Hutchison
    This answer doesn't really answer my question. Do gravitational waves, electro-magnetism, light, electricity ALL travel at the same speed? Which is more than my original question but it doesn't seem probable that they ALL travel at the same speed. Each is different. To me this is just a generic answer. Can their speed be measured?
    I know that I know nothing, so I question everything. - Socrates/Descartes

  8. #8
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    Yes to all in a perfect vacuum. Yes the speed can be measured. light and all EM slow down in a medium light glass, gases and special arrangements where light can actually stop. The gravitational waves detected from far away cosmic events did arrive at the same time as EM observations. You could guess that gravity might be slower but it cannot be faster, otherwise you could signal faster than light. It turns out to be the same speed, but the vacuum part is important. forgot to add electrons are slower, they have mass.
    Last edited by profloater; 2021-May-07 at 03:36 PM.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
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  9. #9
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    Quote Originally Posted by DaCaptain View Post
    This answer doesn't really answer my question. Do gravitational waves, electro-magnetism, light, electricity ALL travel at the same speed? Which is more than my original question but it doesn't seem probable that they ALL travel at the same speed. Each is different. To me this is just a generic answer. Can their speed be measured?
    Well, that doesn't seem to bear much resemblance to the question you actually asked.
    Gravitational and electromagnetic waves (including, obviously, light) travel at the speed of light. These speeds can be measured, and we know the measurements are identical to within a tiny fraction (something like 10-15 the last time I looked). The forces mediated by gravitational and magnetic fields behave, to some extent, as if they are being transmitted faster than light, but changes in those fields are transmitted at the speed of light.
    Electricity is the movement of electrons in a conductor, so doesn't have much connection to the above, and is certainly slower than light.

    The "reason" gravitational waves and electromagnetic radiation are both transmitted at the same speed is because gravitons and photons are massless, and massless particles are constrained to travel at the speed of light. (Masslessness is a theoretical assertion for gravitons, of course, since they haven't been detected and we are inferring their properties from what we know about gravity.)

    ETA: Here's the report I was recalling. Simultaneous measurements of the arrival time of gamma rays and gravitational waves from a neutron star merger. We know how fast gamma rays travel, because we know how fast electromagnetic radiation travels, and now we can compare the speed of gravitational waves with that of electromagnetic radiation.

    Grant Hutchison
    Last edited by grant hutchison; 2021-May-07 at 04:15 PM.

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