# Why are photons affected by gravity?

1. Oct 8, 2015

### ayush solanki

Photons are massless,so why does it get affected by gravity?and are neutrinos affected by gravity too?thank you.

2. Oct 8, 2015

### Chalnoth

Because General Relativity couples to energy, momentum, pressure, and twisting forces rather than mass. Gravity looks like it only couples to mass in most situations because rest mass energy is so much greater than kinetic energy, momentum, pressure, or twisting forces in most situations.

Photons don't have mass, and can't experience twisting forces, but do have momentum, pressure, and energy.

3. Oct 8, 2015

### ayush solanki

OK thank you a lot.

4. Oct 8, 2015

### Orodruin

Staff Emeritus
Note that there is a priori no reason not to believe massless objects would not be affected by gravity in the Newtonian setting. The gravitational acceleration in Newtonian gravity is independent of the mass of the object (a = g) also if we take the limit $m\to 0$.

5. Oct 8, 2015

### ayush solanki

Can you suggest an advanced book to better understand it?

6. Oct 8, 2015

### ayush solanki

Which a 16 year old can understand?

7. Oct 8, 2015

### Chalnoth

If you drop a rock near the Earth, the acceleration imparted to the rock by gravity from the Earth is completely independent of the rock's mass. You can then estimate the impact that Newtonian gravity should have on a light beam by just saying that it's an object that is traveling at speed c.

However, if you try doing this, you get the wrong answer. The actual deflection is twice what you would expect from Newtonian gravity alone. General Relativity predicts this extra factor of two.

8. Oct 8, 2015

### Orodruin

Staff Emeritus
It is also worth noting that this is for a light beam travelling perpendicular to the gravitational field. Newtonian gravity alone would also predict the light to accelerate when travelling along the gravitational field - which is also wrong, it changes in frequency but not in speed.

9. Oct 8, 2015

### ayush solanki

Thanks a lot guys.that was very helpful.