I still don’t understand how light can have momentum and yet have no mass. As I understand, p = mv, which would mean for photons, p = 03*10^8 m/s = 0. So what gives? How does light have momentum, yet not have mass?
p = mv is a classical formula, and does not hold for very large values of v. In relativity, p is defined to be something a little different, and that’s what physicists mean when they say momentum.
OK, there’s been big disagreements about this on SDMB before, but photons have relativistic mass which is a function of their energy (E=mc^2). Of course you could argue that relativistic mass isn’t really mass at all.
The important thing is that photons have no rest mass.
This is what confuses me. So photons have relativistic mass, which comes from their energy. How is this different to normal mass? How come photons can possess relativistic mass, and travel at light speed, but the reason why we can’t travel at light speed is because as we go faster we have more relativistic mass?
I appreciate that the only way the equations solve out to prove photons have no mass, but i’m really looking for an intuitive understanding of why, rather than a mathematical proof.
Well mooka, photons have a finite energy, therfore a finite relativistic mass. To accelrate a massive object to light speed from below light speed it takes an infinite amount of energy therfore giving them an infinite mass. Relativistic mass is a function of the protons kinetic energy, the rest mass of an object is most probably a function of it’s gravitational potential energy.
Photons have never accelrated up to light speed, they always travel at that speed is perhaps one way to understand it.
Just to throw a monkey wrench in all this, I recall a Scientific American article from many years ago about the consequences of photions having finite mass. I don’t recall it, to tell you the truth, but I do recall that they treated it in a much more serrious manner than a brush-off. They showed how Maxwell’s equations would have to be altered, and the sizes of correction terms. I haven’t kept up with it, obviously, so I don’t know if something came along to unhinge their deductions or not, but it’s worth looking up.
No-ones proved from observation that photons have zero mass, every so often you hear that someone has observed a new, lower, upper limit for the mass of a photon. The best evidence with have for photons having zero mass is that the equations which requie them to have zero mass work.
I don’t pretend to know much about quantum physics, quarks, and the like. But don’t quantum physics and relativity disagree with one another??
The “photons have zero-mass” works as a plug, so the model we’ve built of particle interactions mirrors what we see through experiments. So until someone comes up with the unified theory, we have to models which seem to work in some situations, but disagree with each other at the same time.
There is a difference between models and reality.
No, this is incorrect.
If photons had slightly non-zero mass, then they would have slightly different speeds according to their energies. If the mass is small enough, these would all be very close to the speed of light. However, for astronomically distant light sources, the difference in speeds would make for measurably different arrival times. When a star goes supernova, for instance, we would see the X-rays arrive first, then the UV, then the red, etc. But we don’t see that. They all arrive at the same time. Therefore, they must have zero mass.
I caught a photon in my hand and weighed it on my bathroom scales, and I can indeed confirm that photons have zero mass.
gr8guy, thanks (belatedly) for the explanation. Makes sense.
MC Master of Cermonies, I’ve always wanted a pet photon. Did you tame it after you captured it, or is it still wild?
Yes I’ve trained it to perform the two slit experiment and also to play dead (like schrodinger’s cat)
Not quite. We can’t say that they all arrive at exactly the same time; there are limits to the precision of our experiments. All we can say is that they arrive at awfully darn close to the same time, which implies that their mass is awfully darn small. Specifically, we know that the mass of the photon is less than about 210[sup]-16[/sup] eV (at least, as of July 2000: There may be slightly better information since then). For comparison, the lightest particles known to have nonzero mass are the neutrinos, estimated to be about 2 eV, and the electron, at 5.1110[sup]5[/sup] eV. Incidentally, the best bounds for the photon mass come from measurements of electrostatic fields, not from delay measurements.
It also doesn’t cut it to say that photons must be massless since that’s required by unbroken gauge symmetry. That’s fine to say, but how do we know that gauge symmetry is unbroken? Again, there are limits to our experiments, and there’s no way to tell the difference between “massless photon” and “photon with darn friggin’ tiny mass”.
If photons do, in fact, have mass, then they would travel slightly slower than c. c is not defined to be the speed of light; it’s defined as the upper limit on speed for particles with worldlines forward in time. It may be that there are actually no particles in nature which travel at c, but you can still define c.
By the way, quantum mechanics has no problem at all with Special Relativity, which is what we need here. It’s only when you try to combine QM with the more advanced General Relativity (which deals with gravity) that you have problems.
Indeed, the Particle Data Group bound that Chronos is quoting is primarily based on a rather cute method.
(Incidentally, that paper discusses the Maxwell-Proca equations, the modification of the Maxwell equations that CalMeacham half-remembered from Scientific American.)
Umm, of course. But that doesn’t alter the fact that gauge invariance is the widely accepted best guess for “why don’t photons have mass?” Taking that as a different question to “what is the mass of a photon?”
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Achernar thanks for clearing up my problem there. Course, this means I lose a minor bet, but since the other guy isn’t on SDMB do I have to mention it?
Q.E.D. how do you put in super- and sub-scripts?
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