You know, it's funny. I LITERALLY just had an online chat with a cosmologist yesterday evening and one of the things he said to me was that matter, as we currently know it, may not actually exist at all. Fields exist, and particles such as Protons, Neutrons and Electrons are merely excitations of those fieldsI agree with @Halc on this, but I think you need to sit down and think what you mean by the word exists.
We generally accept that the table your computer is sitting on exists.
A few hundred years ago no one would have known that it was made of molecules, or of the existence of atoms, electrons, etc. Knowing about those does not change the fact that the table exists, maybe not in the form everyone thought, but still the same at the macro level. I am surprised at your cosmologist, describing Protons, Neutrons and Electrons as merely excitations of those fields really doesnít answer or change anything, itís just a lower level of detail. We donít describe a table as merely molecules.
As Ian Hacking said of electrons "if you can spray them, then they are real."
At any rate though, I do understand most of what you're saying. Mostly that we have very little idea what light actually is.Actually we we know a great deal about what light is, how it works etc. It is another of those ďmerelyĒ excitations of a field, and that means we can understand it in ways we couldnít before.
Hi ES. I would like to offer an alternative view on this:
The next important thing is that you have probably read or been taught that momentum = mass x velocity. This is probably why you are concerned about the photon having 0 mass but still having a non-zero momentum.I may be misreading what you say, but it implies that relativistic mass was Ďdevelopedí in response to the Ďrest massí of the photon being zero, and hence to maintain the concept of momentum for the photon.
There are at least two ways we can address this issue. The first is to say that many physicists were also troubled about this. It's a very good question to ask and something that does seem quite puzzling.
Physicist's were sufficiently determined to maintain this simple concept of momentum that they developed a quantity called "relativistic mass". They accepted that the invariant mass of a photon wasn't anything you could ever really measure, it certainly wasn't going to be measured as the mass of the particle when it was at rest in some inertial frame. So they determined that the invariant mass wasn't something that should be used in that formula momentum = mv.
The concept of mass varying with relative velocity predates special relativity and the concept of the photon, coming from the work of Lorentz and others. Lorentz was trying to work within a stationary aether theory and postulated that the measuring apparatus designed to detect movement relative to the aether was length contracted (Fitzgerald contraction) and so could never detect the movement. This led to his famous transforms.
There was also parallel work on the concept of electrostatic mass, that a charged body is harder to accelerate than an uncharged one, and this electrostatic mass increases with velocity. Lorentz was working on an electron theory and applying this electrostatic mass via his transforms he developed the concept of relativistic mass (both longitudinal and transverse). Interestingly he also changed the original Newtonís law that ďforce = rate of change of momentumĒ into the form we know today, F=ma.
What is really interesting is that Poincarť took the Lorentz transforms and gave them the form we use today. He also showed they were the result of principle of least action, showed that what we call the spacetime interval is invariant, suggested c might be an unsurpassable limit, suggested a clock synchronisation method using light, and suggested gravitational waves might exist. He apparently decided that developing the work would be too much effort for no useful result. So near!
Einstein originally took on the term relativistic mass in his early papers when he showed his famous E2=(m0c2)2+(pc)2 , but spoke against its use later.
Maintaining the concept of momentum for the photon was never an issue, as long as you believe that momentum is conserved (which thou shalt). If the photon has momentum as it leaves the atom, then the atom should recoil, which it does, and that can be measured. Similarly momentum is transferred at the receiving end.