Photons- no mass?

[crazy legs]

How do scientists know this? Thanks.

[Clanad]

According to The Citizen Scientist, photons can never be at rest. They have no existence at rest, and so they have no "mass" or any other property at rest. Physicists say that a photon has no "rest mass." A photon can't be stopped and weighed. It exists entirely as a self-propagating bundle of electromagnetic fields. As the electric field collapses, it generates a magnetic field that's in a slightly different position. The electric field disappears when it transfers all its energy to the growing magnetic field. When that transfer has taken place, there is no longer anything to build the magnetic field. It then begins to collapse and to create an electric field in another slightly different position. When the cycle repeats, this bundle of electromagnetic energy has moved a bit and it turns out that the distance moved divided by the time it took for the cycle to take place is always exactly the same. This is the speed of light (c) The laws of physics determine where the magnetic field appears when the electric field collapses. You can't keep the electric and magnetic fields oscillating in place, because that violates the laws of physics. You can transfer the energy out of the field and into something else--that is, you can absorb and destroy a photon--but you can't stop it.
And yet a photon does have mass, because the mass-energy relationship still holds. For instance, if a photon has greater than about 2 MeV of energy, it can turn into two electrons (well, an electron and a positron). And one can reverse the process as well. One can bang an electron and positron together and have them disappear and be replaced by a photon that contains all the energy--mass energy, potential energy, and kinetic energy-- of the original particles. So, a photon can have potential mass, depending on the quantum state... The equation pertinent to the discussion is momentum=constant*frequency...

[jake-the-peg]

I think that saying that a photon has mass because it can convert into an electron positron pair that have mass is a bit confusing..

A bit like saying I have a newspaper because I have money in my pocket that I can go into a shop and spend on a newspaper!

A bit of history might help:

For many years people had fought about what light was then a rather bright chap called James Clerk Maxwell first worked out that an alternating electric and magnetic field would propergate through a vacuum at the speed of light at which point the penny dropped and people saw that light was this alternating electromagnetic field. No mass just fields.

However somewhat confusingly light is emitted in small lumps - quanta - which we call photons and these can carry momentum - despite having no mass.

Arthur Compton first showed this and got the Compton effect named after him and a Nobel prize for his trouble.

Now we also know that photons have no mass because they travel at the speed of light. Special relativity tells us that anything with any mass would become infinitely massive at the speed of light - anything that travels at this speed must therefore have no mass.

The interesting one at the moment is still the neutrino - this particle comes in three flavours and it is believed by most that none of them have any mass and that they travel at the speed of light - however it is possible that they do have a very very very tiny mass that we've never been able to measure.

If they do this might explain some of the universe's dark matter because there are so many of them that they'd add up to a huge amount of mass