malawi_glenn said:
nope the general question was "how exactly does mass "become" energy?"
One guy claimed that photons, electromagnetic waves, are "pure" energy. But where is such definition stated and motivated?
The title of the thread is the reverse. (How exactly does energy "become" mass?)
To answer that question we first need to define what we mean by the terms mass and energy. General we associate mass with the properties of influencing a gravitational field and with having momentum, but a photons can have both those properties and yet it is often stated that a photon is massless. The only form of mass that is officially recogised is rest mass. A single photon has no rest mass, because it can not be identified with an inertial rest frame.
Wikipedia says this:
"Matter creation is the process inverse to particle annihilation. It is the conversion of massless particles into one or more massive particles. This process is the time reversal of annihilation. Since all known massless particles are bosons and the most familiar massive particles are fermions, usually what is considered is the process which converts two bosons (e.g. photons) into two fermions (e.g., an electron-positron pair).
Photon pair production Because of momentum conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by momentum conservation law when in the presence of another particle (it may be another photon or other boson, or even a fermion) which can share photon's momentum. Thus, matter can be created out of two photons, for example (this is the process inverse to annihilation)."
I think the above article is what the OP is getting at. How can photons with no rest mass be turned into particles and anti-particles with rest mass?
The article identifies photons as bosons which are defined as massless particles. If a photon is massless and yet has energy, can it not be described as pure energy?
It also defines fermions as matter particles. In other words they have a property that photons do not have. Namely, the property of having mass. Mass is in effect energy that is confined and for which a rest frame can be identified. All particles that have no mass have to travel at the speed of light and all particles that have mass, can not travel at the speed of light and that is the main distinction between pure energy and mass.
malawi_glenn said:
To me, photons are one form of energy, mass is one form of energy. Energy can not be created or destroyed, only converted into different forms.
While no one will argue with fact that energy can not be created or destroyed, it can also be said that rest mass can not be created or destroyed either. For example if a nuclear bomb is detonated, the rest mass of the system is the same before and after the nuclear bomb detonated, just as the total energy of the system is the same before and after the bomb detonated.
What changes is the nature of individual particles. Some individual particles that had the property of rest mass before the explosion, now have the property of being able to travel at the speed of light and not having an identifiable rest frame as an individual particle. Just to make things clear as to why the rest mass of the system is the same before and after the detonation, it should be made clear that that a pair of photons going in opposite directions, can have an identifiable rest frame (and associated rest mass) as a pair, in which their total momentum is zero.
