# Does massless particles experience gravity

1. Aug 23, 2008

### mite

From Newtonian theory massless particles wouldn't experience force of gravity.

Then from General relativity prospective gravity is ripples in space time due to a massive

body so other objects move in the straight path in this curved space-time hence this

objects appears to move in a curved path. Then what would happen if we have a massless

particle with no energy. As space time is curved (path the objects move is curved) shouldn't

it move in a curved path. If it moves in the curved path doesn't it mean that massless

particle also experience gravity. I am confused can anybody help.

2. Aug 23, 2008

### Hootenanny

Staff Emeritus
A particle with no energy doesn't exists. However, massless particles do follow a null geodesic in curved space-time.

Last edited: Aug 23, 2008
3. Aug 23, 2008

### Staff: Mentor

In Newtonian theory a massless particle may not experience a force from gravity, but it would still experience a gravitational acceleration.

f = GMm/r²
ma = GMm/r²
a = GM/r²
limit of a as m->0 = GM/r²

4. Aug 23, 2008

### mite

I dont know what null geodesic is? can u explain
can u guide me to some website on this topic

5. Aug 23, 2008

### mite

But if particle is massless then there is no force only then how can there be acceleration

6. Aug 23, 2008

### Staff: Mentor

f = m a

If m=0 then f=0 regardless of a. Therefore, in Newtonian theory a massless particle can experience an acceleration without a force.

By the way, this is not a correct way of looking at light and gravity since EM waves carry momentum, but it is only intended to illustrate the point that you need to think carefully when you are thinking about massless particles in Newtonian terms. A lot of your intuitive Newtonian ideas don't apply to massless particles any more than they apply to particles moving at a significant fraction of c.

Last edited: Aug 23, 2008
7. Aug 23, 2008

### Hootenanny

Staff Emeritus
A geodesic is a generalisation of of a straight line and a null geodesic is so called because it's arc length is zero (or the magnitude of the tangent vector is zero). If you Google "geodesic" you should find plenty of references.

Moving away from geodesics and back to your question, a massless particle in curved space-time will follow a straight line in space-time. However, it's path will 'appear' curved in 3D-space (i.e. if we just consider space at an instant in time).