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Q & A: speed of gravity

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Most recent answer: 01/22/2014
Q:
So I have read that gravity, and theoretical graviton particles, travel at the speed of light. Yet, things like galaxies are massive, some hundred thousand light years across. How does gravity, or the gravitons, get from the the object being orbited (for instance the black hole at the middle of the Milky Way) to the farthest out objects orbiting (in this instance the farthest out stars) in time to still effect them. Shouldn't the objects own motion taken it out of the effective field by that time? Or does gravity not travel in straight lines? Am I just not understanding this correctly?
- Gerrit Bruhaug (age 18)
Billings, Mt, USA
A:

Those galaxies have been around for a lot  longer than hundreds of thousands of years. There's been plenty of time for the gravitational fields from each part to reach all the other parts. if a couple of stars in the galaxy collided and started spinning around quickly, it would take a while for the little gravitational ripples to reach other parts. The main effect, which just would come from their total mass, wouldn't change and thus would already be present. Likewise if some stars collapse into a black hole, some ripples will get sent out in the process, but the mass and the overall gravitational effect at a distance were already there.

Mike W.


(published on 11/12/2013)

Follow-Up #1: Is gravity instantaneous?

Q:
I wonder that why is the effect of GRAVITY Instantenous , inspite its caused because of the transfer of Gravitons at the speed of light? And as we know that Black Hole doesn' t even allow photons to pass under Event Horizon so why does it even allows Gravitons to pass i.e. its own gravity??(this ques. might seem absurd) & What if we sent NEUTRINO's at a speed greater than that of light would it pass Event Horizon?
- Swarit Shatma (age 16)
Shimla, H.P., INDIA
A:

Gravity isn't instantaneous. It propagates at the speed of light. As we explain above, the gravity (or field of virtual gravitons, if you prefer) is already present outside the black hole before the black hole forms. 

As for the neutrinos, they can't travel faster than light. The reports you may have heard about that were mistaken, caused by a loose connector in some electronics.

Mike W.


(published on 01/22/2014)

Follow-up on this answer.