# Gravity Vs. Speed of Light

We all know that nothing can go faster than light. This is because as something approaches the speed of light, its mass will approach infinity.
What if something was falling towards a immense body very very far away. Of course the object will accelerate and gain velocity and get close to the speed of light. Its mass would increase. Gravity increases with mass so why would this object not be able to reach the speed of light?

ShayanJ
Gold Member
We all know that nothing can go faster than light. This is because as something approaches the speed of light, its mass will approach infinity.
What if something was falling towards a immense body very very far away. Of course the object will accelerate and gain velocity and get close to the speed of light. Its mass would increase. Gravity increases with mass so why would this object not be able to reach the speed of light?
This is based on some misconceptions about several things and is wrong.
At first, mass increase is not that much of a real effect. Its, like time dilation and length contraction, something that happens relatively. I mean, A sees B in motion and so sees his mass increasing and B sees A in motion and so sees his mass increasing. A and B notice nothing about themselves because they both think they're at rest.
Second, people don't use relativistic mass and mass increase nowadays. They're useless troublesome notions. Its just that the formula for energy and momentum depend on velocity and are frame-dependent, as they were in Newtonian mechanics. So the last paragraph should be said about energy and momentum.
Third, You're talking about gravity here and so its out of SR. SR is only a special case of GR for a space-time which is empty of large amounts of mass and energy concentrated in such a small region to cause considerable gravity.
Fourth, in GR the effect of gravity on objects, doesn't depend on their mass. Imagine a hose hanging from top of a building. We can throw little balls inside it and they go along the hose until they reach the ground. Now if I bend the hose, all the balls' paths takes the shape of the hose no matter what is their mass or size. Its the same with gravity. Gravity is the change in the shape of space-time and matter has to follow that shape when moving (in space and time) and it really doesn't matter what is its the mass!

Ibix