# Does Electrical or Magnetic field attract photons?

1. Dec 11, 2014

### Ozgen Eren

I have heard that photons are attracted by gravity. Does this apply to electrical or magnetic fields?

2. Dec 11, 2014

### jerromyjon

No because photons have no charge polarity.

3. Dec 11, 2014

### Ozgen Eren

Yeah that makes sense. Do you know with what acceleration gravity pulls them? They ought to have some sort of mass to have a finite acceleration in response to gravity's force?

4. Dec 11, 2014

Staff Emeritus
In Newtonian gravity, which is wrong, photons are massless and are not affected by gravity. In GR, the gravitational "charge" is not mass but stress-energy, and for the a photon this is not zero.

5. Dec 11, 2014

### jerromyjon

Gravity does not "pull" them. Gravity bends space and the photon continues on a straight path through curved space. Photons are never accelerated, only the frequency is affected, which confirms they have no mass.

6. Dec 11, 2014

### Ozgen Eren

Okay so how can they have a nonzero momentum?

7. Dec 11, 2014

### jerromyjon

Because they have energy.

8. Dec 11, 2014

### Ozgen Eren

Having energy does not imply having momentum, any object in rest has potential energy and zero momentum.

9. Dec 11, 2014

### jerromyjon

What does a photon at rest have?

10. Dec 11, 2014

### Khashishi

If you simply asked if photons are attracted by EM fields, I would simply say no, and that would be the end of it. But since you specifically mention gravity, the answer is yes (for a certain definition of "attract"), very weakly, since electric and magnetic fields have mass-energy and generate a gravitational field. In Gaussian units,
$E=\frac{1}{8\pi}\left(\mathbf{E}^2+\mathbf{B}^2\right)$

11. Dec 11, 2014

### Staff: Mentor

Photons are not at rest. They are moving at c and have energy and momentum. Yes, they are massless, but this does not prevent them from having momentum, you just need to use different laws with them, not the laws that govern bodies with mass.

12. Dec 11, 2014

### Staff: Mentor

In relativistic mechanics, the general relationship between energy, momentum and mass is E2 = (pc)2 + (mc2)2, where m is what some people call "rest mass". (Physicists generally call it simply "mass".) For a photon, m = 0 so E = pc or p = E/c.