# How long until gravity has effect on something

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How long does it take gravity to reach an object and exert a pull on it?

Meaning: if say there is only one small object with little mass on one side of the universe at the moment, and then suddenly a supermassive object appears on the other side of the universe. These are now the only two objects in this universe.

How long before the supermassive object's gravity reaches the small object and begin to pull on it? Or does it happen instantly once the supermassive object exists?

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gravity travels at the speed of light. so lets say the sun completely vanished mass and all, we would still orbit a full 8 minutes before we notice a thing.

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theres no hard evidence for a specific speed for gravity, some measurements of binary neutron stars suggest that energy is being lost to the propogation of gravity as though it were moving at c, but some sources will say it's instant

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Er, didn't Einstein prove that it's NOT instant?

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GR does predict that it is not instant, and there is evidence that backs that up.

Conjecture that gravity is instantaneous (or nearly so) is almost always flawed by not understanding that the retardation is different for gravity vs light: when you see something, you are actually seeing where it was at a time d/c earlier, (and as nanogrinder notes, that's about 8 minutes for the sun) and that will apply if something is moving at a constant velocity. But gravity's retardation is tied in with acceleration — basically, if I'm moving through space 8 light-minuted from the sun I will feel gravity from where the sun is now, because it has already curved the space where I will be in 8 minutes. For this reason, the vector of where we see the sun and the direction that gravity points are slightly different; this is often misinterpreted as gravity travelling faster than light.

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Nice explanation, thanks. I've been really unclear about that differential business, but that helped quite a bit.

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light is a product, energy and in motion requires no source for travel.

gravity is an effect, when the source is gone, so is its effect.

the last seen effects of energy light from the sun, takes 8 minutes, while the actual source is gone. if the source from an effect is gone why should that effect continue. just a thought...

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Well that's an interesting thought, but I think we're really more interested in real-world physics than speculative philosophy here (in this particular thread). (Or am I just misunderstanding Jackson33?)

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light is a product, energy and in motion requires no source for travel.

gravity is an effect, when the source is gone, so is its effect.

the last seen effects of energy light from the sun, takes 8 minutes, while the actual source is gone. if the source from an effect is gone why should that effect continue. just a thought...

How would you get rid of the source?

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maybe move the source far away, say the radius between the objects was approaching infinity, then the limit of 1/ifinity^2 would be aproaching zero, which in a sense wouldn't make the object disapear (conservation laws) but would make it's effect zero

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Agreed. But you can't move the source faster than c, so you can't get rid of whatever gravity is still present.

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• 2 weeks later...

True,

Many theorists believe the universe is expanding FTL. Does this not mean it is in the nature of the fabric of space-time to react Superluminaly to things?

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If you believe in Einstein's theory of relativity, gravity is not like a photon or a beam of photons, but instead is more like a depression in the fabric of space - if I understand correctly. I believe this to be true, so I think that if you could turn off a switch which made Earth suddenly have no gravity, a chain reaction would occur that would probably ripple through space since all objects of mass exert force on each other. I think we see that as mass is converted to energy too in star life. As star mass dissipates, the star grows due to less gravity keeping the fusion process constrained, then they die in a bang as the fusion reactions stop and gravity forces and implosion.

I think anyways?

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so wait..... do photons of light even though they have no mass do they still have a gravitational field?

if not then why is the "speed" of gravity relative to the speed of light?

maybe my logic is incorrect but it doesent seem right that the "speed" of gravity should be construed to the speed of light if light itself doesent have a gravitational field?

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