# Are photons massless?

1. Jul 14, 2010

### Kurushimi

This is just a random inquiry that's confusing me. I remember hearing at one point that the reason no object could travel at the speed of light is that an object with mass going at the speed of light would require an infinite amount of energy to get moving that fast. And that photons could do it because they were massless. But, then, I also recall hearing that photons aren't truly massless...which seems contradictory to me. I can't remember my sources. Was I misinformed about one (or, perhaps both) of these?

2. Jul 14, 2010

### skycastlefish

I'd like to know too. I'm on a desparete journy to understand the difference between energy (ei light) and matter-form energy.

3. Jul 14, 2010

### the_house

Yes, to the best of our knowledge photons are exactly massless, which means they travel at c (which we refer to as "the speed of light" only because photons appear to be massless).

Any massive object will have a speed that is strictly less than c no matter how much energy it has.

4. Jul 14, 2010

### Parlyne

There are two answers to this question. The theorist's answer is "yes, the photon is massless. Were it not, the elecric potential energy of a charged particle would vary like $\frac{1}{r}e^{-m_\gamma r}$ instead of just $\frac{1}{r}$, among other effects." The experimentalist's answer is "probably. Our best measurements of the photon's mass are consistent with 0 and the upper bound they set is 15 or 16 orders of magnitude smaller than any other known mass."

5. Jul 14, 2010

### Staff: Mentor

I would say this as "yes, to within experimental error".