The Massless Photon

MeJennifer

To me that is very unfortunate, I very much appreciated your explanations on GR issues. Let me know which forum you will post in the future by private email.

ZapperZ:
Zapper, I fail to understand your frequent harshness to people here. You are supposed to be a mentor but instead you come over as a very arrogant person.

ZapperZ

It may appear that way, but I have to impose the rules uniformly. I had asked for the opinions of other Mentors on the issue of citing one's own website as if it is a "reference". It is unfair to require others to cite valid references while we look the other way for some people.

And as for being "arrogance", we ALL get accused of being such a creature at one time or another. Try looking in another thread in this very subforum and you'll see that pmb has been accused of being one himself. So like SR itself, such a term is highly "relative".

Zz.

pmb_phy

Then the rules are inadequate for my purposes and I find that I no longer wish to post under these restraints.

pmb_phy

Hi Jen - I'll very much miss you and others and this forum. But ZapperZ goes to far or the rules are pretty dumb. In either case I'm not going to place myself in such an environment. I'll PM my e-mail address to you and where I will head to after here. Please pass it on to those who wish to know. You're a sharp person so I'd hate to loose track of you and the others.

Jen (May I call you that?) - Please find this rule that ZapperZ claims to exist. I can't believe it was written to be interpreted the way ZapperZ interprets it. In anycase its people deleting and editing what I post that I find highly offensive. If ZapperZ thinks its possible to give a complete explaination of a subject which requires a ton of explaining to fully comprehend in a single thread that he's totally out there. E.g. To explain the concept of mass in relativity sufficiently it took me 17 pages of well thought out writing to do that. And IT would be silly to keep posting it again and again when people ask the same question. Its better to write the derivation/explanation up and post it on my web site.

Best wishes

Pete

bernhard.rothenstein

I think that there are professions where harshness should be avoided. Teachers should be a good example for learners offering information and constructive criticism.

Lelan Thara

Can you clarify one thing for me? When you say that a particle with no rest mass "can only travel at light speed" - do you mean the speed of light in a vacuum, or the speed of light within whatever medium is in question? Photons do travel at less than the speed of light in a vacuum when in a denser medium, right?

Here's another question - I've always heard that a massive object that accelerated to the speed of light would acquire an infinite mass. So a photon leaving, say, glass and entering a vacuum would accelerate to c - and if it possessed mass, become infinitely massive - right? How do you square this with the possibility of photons having a relativistic mass?

Since there is no such thing as a perfect vacuum - is it possible that no photon ever actually reaches c?

As for your "rest mass/relative mass" thread wars - let me just say that I assume you guys have been asked to answer questions similar to mine more times than you care to remember. I really appreciate that you folks have had the patience to answer my questions. (And I do hope I don't bore you all into a thread war. )

pervect

I take it that you don't necessarily agree that "mass is a mess"? Or perhaps you are simply reserving judgment? Interestingly enough, Okun himself makes similar remarks in the paper you cite, which is available online.

http://arxiv.org/abs/hep-ph/0602037


Anyway, to express my POV in a sometwhat different manner:

Energy and momentum can be defined independently of mass, via Noether's theorem. This is somewhat technical, but serves as a perfectly adaquate defintion. Hopefully, people have some intuitive ideas of what energy and momentum are, and how they can be measured.

Mass can then be regarded as a quantity that is derived from energy and momentum.

This avoids some of the issues that come up when mass is viewed as a "quantity of material", an idea that originated with Newton, but was soon regarded as unsatisfactory. Philosophically, one builds on a stronger base by taking the opposite tack, and defining energy and momentum in a manner that does not rely on the "problematical" concept of mass.

I'm not sure who first pointed out the problems with Newton's original defintion - but the unit for the quantity of matter (as in counting the number of particles) is really the mole, not the kg. In the context of GR, for example, if one takes two 1kg masses and place them together, the total mass of the system will be very very slightly different than 2kg, due to the gravitational interaction of the components - at least if one uses one of the usal GR defintions of mass, such as the ADM, Bondi, or Komar defintions.

nakurusil

The photon travels at c, always. Even in materials with the refraction index n>1. In such material the photons get repeatedly absorbed and re-emitted. This process [tex]appears[/tex] to slow down the photons when , in reality, between every emission and absorbtion the photons travel at c.
This is why, when they exit the material, they are not accelerated to c, they are simply re-emitted at c.
As to the perfect vacuum, the interstellar space approaches it very closely.

Lelan Thara

Wow. That is completely news to me. Thanks very much, Nakurusil.

pmb_phy

Show me where I said "My web poage is a reference" rather than "On this web page I derive..."??

Also, where are these rules you're refering to so I can at least read for myself whether what you're claiming I'm doing is really violating the rules? I'm nearly 99.99% sure that they as are you claim and for that reason I wouldn't post here in the future. Exactly how many mentors are there which you claim to agree that web pages with basic/simple derivations may not be linked to? How many did you ask? How many disagreed with you?

pmb_phy

This is quite incorrect. I just looked at the rules I agreed to abide by when I signed up. The state exactly the following
Nothing in this has anything to do with your claims. Show me where in that set of rules where posters agree to cite valid sources and where the term "valid source" is defined. Otherwise it appears to me that that you're simply being malicious and abusing your mentor privilages.

If you refuse to cite these rules you claim exist then I've decided conntinue posting at the urge of my fellow posters, God bless'm.

Other forums which are operated like this and use the same forum structure have no such rules. To be precise a moderator there told me this
Pete

pegleg bates

Doc, that equation doesn't clear up the mystery of why a photon is massless, because p = mv. You are back to the original problem. Both components have mass, therefore both momentum and energy would equal 0.

Doc Al

In relativity, p ≠ mv. (Where I use m for the rest mass.) In that formula I provided, both E and p are non-zero for a photon, while m = 0. (Note that this thread is years old.)

pegleg bates

Got it. I know that this thread is old, but it conveniently came up when I searched for this question.