# How soon that a force affect another object?

Imagine this scenario: I have 2 objects in vacuum without any force exerted upon them not even a possible gravitational force between them. Now if one of them gets a gravitational or magnetic force, how long from the moment the object gained this force to the other object to be affected by it? Is it instantaneous? Is it the same as speed of light?

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Good question. A magnetic force would certainly be the speed of light, but the gravitational force, I'm not so sure. – Ataraxia Mar 17 '13 at 17:01
According to wikipedia, the gravitational force would also be the speed of light. I'm not sure why though, so I'll leave it to someone who knows general relativity to explain. – Ataraxia Mar 17 '13 at 17:03
Bosons are particles which carry force. Photons are force carriers of EM field, W and Z bosons for weak force and gluons are carriers of strong force. But this is mere information. I guess the exact answer you seek will be given by someone well versed with these particles. – Pallavi Roy Mar 17 '13 at 17:15
Possible duplicates: physics.stackexchange.com/q/5456/2451 and links therein. – Qmechanic Mar 17 '13 at 17:16
That question only adresses gravitational force. Is it the same for all kind of forces? – Leonardo Marques Mar 17 '13 at 21:40

## 1 Answer

In your comment you ask:

That question only addresses gravitational force. Is it the same for all kind of forces?

Generally speaking a force transmitted by massless particles like the photon and graviton obeys an inverse square law while a force transmitted by massive particles falls off exponentially with distance. This means that only the forces transmitted by massless particles are long range and therefore that any long range force propagates at the speed of light.

So for any macroscopic objects the interaction between them is going to travel at the speed of light.

Trying to assign a speed to the weak and strong nuclear forces is a bit complicated. At longish range, i.e. within a few proton diameters, the strong force is transmitted by massive mesons and therefor propagates at less then the speed of light. However the underlying force is transmitted by massless gluons. This doesn't give rise to a long range inverse square law because of confinement. The weak force is transmitted by massive W and Z bosons at low energies, but above the electroweak transition these become massless and in principle the electroweak force is long range.

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could you point me to some good source of information about long and short range forces? This concept is new for me – Leonardo Marques Mar 18 '13 at 13:22
hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html is a good start and follow the links from it. – John Rennie Mar 18 '13 at 15:32