# Lightspeed and 0 mass

1. Mar 20, 2012

### squareroot

So, an object can move with c if and only if it has 0 mass.So howcome that light is bended by gravity? How can smth with 0 mass be bend by gravity?

Ty

2. Mar 20, 2012

### Banjy

Well,I think when the 0 mass moves,it's mass is no longer 0 according to the formula E=m*c^2

3. Mar 20, 2012

### Bill_K

Because gravity couples to energy (and momentum, and stress), not mass. A photon has zero mass but nonzero energy.

4. Mar 20, 2012

### squareroot

ty for your answers .I haven't studied relativity in school yet but from what i know gravity is mass times gravitational acc.,but.if u apply that here m is zero so G is zero

Are the formulas from classic mechanics applicable in any relativistic situation?

5. Mar 20, 2012

### Mark M

The bending of light around objects with mass, gravitational lensing, occurs because the spacetime around a massive object (such as the sun, a galaxy, etc.), is curved, and distorts the path of the light travelling near that object.

See this image for an example:

As Bill K said, it is the momentum of the photons that determines how dramatically they are curved away from their original path.

Last edited by a moderator: May 5, 2017
6. Mar 20, 2012

### Staff: Mentor

7. Mar 20, 2012

### HallsofIvy

Only as approximations in the "v much less than c" case.