After Draqs response on ''photons must have a mass because of momentum,'' made me realize that some people still hold to that belief. Does the photon have mass? The short answer is no. Some people find it difficult to comprehend a photon as being massless because it can be deflected by the gravity of, let us say a star. One way to explain this is by saying light couples to gravity, because light also generates curvature and curvature is the equivalence of gravity. However, there is too little amount of light to have any major gravitational effects in our universe, except for perhaps 32 years after big bang, when the universe was flooded in light particles. For those who like math, here are some more reasons why the photon does not have mass. Some people like to say that the photon has mass because the photon has energy \(E=hf\) , where (h) is 'Planck’s constant' and (f) is the frequency of the photon. Thus, they tend to assume that because it has energy (E) it must have mass (M) because of Einstien’s mass-energy equivalence equation \(E=Mc^2\) They also say that the photon has momentum, and momentum is related to mass \(p = Mv\) where (v) is velocity and (p) is for momentum. Yet, you cannot justify it having mass using this argument. This is actually 'relativistic mass' - which is nothing but the measure of energy which will change with velocity. It isn't actually mass, even though mass and energy are related. In physics jargon, the mass of an object is called its 'invariant mass,' and the photon has no invariant mass. Now, a massless particle can have energy and it can have momentum, simply because mass is related to these through the equation \(E^2 = M^2 c^4 + p^2 c^2\) , which is subsequently zero-mass for a photon because \(E = pc \) for massless radiation (remember, c means the speed of light). So yes, the photon has momenta and energy, and can deliver a punch out of it when it hits a surface, but it doesn't have mass. Now... a strange situation can arise if light is trapped inside a container. If light is trapped inside of a box with mirrors inside of it, so that it cannot escape, (now the mirrors would need to be cold enough so that the mirrors do not absorb the light-energy), the total momentum is said to be zero, but the energy is not - thus, the light can contribute a very small amount of mass to the box! Now, one can say that the light in the box must have mass to even add any mass to begin with - but actually, it is more accurate to say it contributes to the mass - but do not use this as some kind of justification that light indeed has mass. That is simply not true. A photon can decrease the invariant mass value of \(E/c^2\) each time a system emits a photon... likewise, a system can increase its invariant mass by a value of \(E/c^2\), if it absorbs a photon particle.
The short answers are often becoming blunt...Please Register or Log in to view the hidden image! Concerning the mass of photon (i.e. both so called rest mass, both the relativistic one) please see my note here.
Ok. Could you please explain exactly what you mean by the bold portion of your statement above? Specifically, how the temperature of a material affects the absorption of a photon. Thanks.
Thank you, whoever it was (I suspect Ben... don't know why... just a hunch) who Latexed my terrible presentation. Now, a massless particle can have energy and it can have momentum, simply because mass is related to these through the equation shown. E^2 = M^2c^4+P^2c^2 Since the momentum rasied to the second power and the mass rasied to the second power, are perpendicular to the equal of energy. If it was mass as the equality variable, then we would find: M^2 = P^2c^2 + E^2c^4 Showing that the the momentum rasied by the second power and the energy raised to the second power are now perpendicular to matter, as shows simply: M=E/c >-< E=Mc^2 Likwise we can change them to mean the same reason of logic for even momentum: P^2 = E^4c^2 + M^2c^2 This must be incontrivertible proof, totally and infallibly empiracle evidence that shows that his equivalances where totally correct: All are symmetrial, and inter-linear simultaneously. The affirmation here of this is an invariant, through the contingant that they can all be unified into these configurations, and not one paradox arises.
Ben can asnwer this better than me better than me. What i do know, is that the relativistic mass, adds to a confined material system, a measure of kinetic energy, ahich contributes to the mass of the box..Hope this helps.Please Register or Log in to view the hidden image!
No, unfortunately, it dosen't help. I'm not interested in the trapped photon per se. Per our earlier conversation, statements like the one you made make me question your basic background in physical science. Now, as usual, I may be wrong, but even at absolute zero, electrons still maintain their normal quantum behavior about the atomic nucleus. This in no way would impede the absorption of a given photon, which is a function of an outer electron being excited to a different energy level (absorption) by the photon (assuming that the photon is not energrtic enough to knock the electron completely free, as in the photoelectric effect). Temperature is a measurement of the kinetic energy of the molecules of a material. So, my concern remains...
This should help our incongruity. Does light have mass? Light is composed of photons, so we could ask if the photon has mass. ... Therefore the light adds a small contribution to the mass of the box. ... math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html
Umm... Reiku? Are we having a language issue? Reread my post and tell me what you think I'm asking about. And it has nothing to do with the mass of the photon/box system. Ok?
''Temperature is a measurement of the kinetic energy of the molecules of a material'' And energy of photons have a temperature, which ads to the systems kinetic value.''
God sorry... I missed that Please Register or Log in to view the hidden image! The reason why, is because Photons have a temperature about them. If the mirrors are warm, they tend to absorb photons rather than reflecting them. Thergo, by making sure the mirrors are quite cold, it would give the photon a bigger chance to not be absorbed.
Ok then. Now that we're on the same page... I disagree with this. A photon has an energy about it. That energy can excite an atom, raising its temperature (kinetic motion of atoms and molecules). I'm not trying to quibble here. But if I can't agree with even your basic assertions, then where are we? This is a statement, not an explanation. Needless to say, I disagree. My claim is that the "temperature" of an atom has nothing to do with whether it absorbs a photon or not. If I'm really wrong, I'd love to know why.
The mass equivalent of the photon times its velocity equals its momentum. If any experiments prove that light pressure works against a light sail, and the pressure can be measured, then we will know the mass equivalent of the energy of the photon.
