Posted: Sep 12, 2011 6:26 am
mizvekov wrote:eric8476 wrote:what makes a photon a particle if it is not mass? why isn't it energy? or is that for the next nobel prize?
A photon does have an associated energy, it just does not have a rest mass.
It follows from relativity that there is a relationship between mass and energy, that they are essentially the same thing.
It's just that when you calculate how much energy a particle has, most particles have a non-zero energy when they are at rest. These include the most significant components of ordinary matter, like protons and electrons.
This energy is really equivalent to what is known as rest mass, or what is simply known as mass in everyday usage, and you can calculate this relationship with the famous e = mc2 formula.
On the other hand, when you do the same calculation for some other particles, like the photon, you are left with no energy when they are at rest.
More to the point: E=mc2 is a simplification at rest for a more complicated formula that relates energy and momentum with rest mass.
A photon has energy and momentum. When you use that formula, the rest mass is zero. This can only happen if the photon always moves at c