# Rest mass of the photon

1. Feb 11, 2012

### zachfoltz

I'm new to physics, so I'm sorry if this is a dumb question. If a string cannot be point like, it has measurable spacial dimensions, then it must have mass. Well if a string was vibrating at the resonant frequency of a photon, and at rest it must have mass and therefore a nonzero rest mass. This is obviously wrong because according to SR a photon must have 0 rest mass. what am I getting wrong here?

Thanks

2. Feb 11, 2012

### LBloom

You're right, a string does have a finite spacial extent. However, just because it is not point like does not mean it has to have mass. In fact, when bosonic string theory was first being studied, people discovered objects with negative mass squared. The presence of tachyons (objects with imaginary mass) showed that the vacuum was unstable and showed the need for fermions.

It certainly is strange to think about, but string theory does have objects that are massless but not pointlike. To me a massless point like object is just as strange when you think about it!

3. Feb 11, 2012

### mathman

A naive way of looking at is that strings are 1 dimensional, so they don't necessarily have mass. Current theory seems to say that mass results from interaction with the Higgs field. Without Higgs nothing would have mass.

4. Feb 11, 2012

### zachfoltz

Your right, that is pretty weird as well!

But that raises the question, if a point like particle has no mass, it "doesn't exist", or at least isn't observable. However if a string doesn't have mass, do we treat it the same as a point particle? or does it exist spatially but have no mass?

Thanks for the help it's much appreciated!

5. Feb 12, 2012

### mathman

Current state of physics theory is such that any answer to your questions are very speculative.

6. Feb 13, 2012

### ohwilleke

One way to get some particles that are massive, while others are massless, is to conceptualize the distinction as related to an orientation to timeline v. spacelike dimensions respectively.