Understanding the Massless Photon

[ZapperZ]

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.

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]

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.

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

 

[pmb_phy]

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.

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 explanation 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]

harshness on the forum

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.

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]

.
Turns out that certain particles have this constant equal to 0. In that case, the relationship simplifies to E = |p| c
It turns out that these things can only travel at light speed.

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 fully aggree with your first four lines.
Proposing Jammer somebody will tell you the following
High Energy Physics - Phenomenology, abstract
hep-ph/0602037
From: Lev Okun [view email]
Date: Fri, 3 Feb 2006 15:58:53 GMT (19kb)
The Concept of Mass in the Einstein Year
Authors: L.B. Okun
Comments: 19 pages, Presented at the 12th Lomonosov conference on Elementary Particle Physics, Moscow State University, August 25-31

Various facets of the concept of mass are discussed. The masses of elementary particles and the search for higgs. The masses of hadrons. The pedagogical virus of relativistic mass.
Full-text: PostScript, PDF, or Other formats
As I see, discussions on physics help take sometimes a very unpolite character. That is why I finish with
sine ira et studio

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

"There is no doubt that the problem of mass is one of the key problems
of modern physics. Though there is no common opinion even among the experts what is the essence of this problem. For most of particle theorists, as well as members of LHC community, the solution of the problem is connected with the quest and discovery of the higgs – scalar boson which in the Standard Model is responsible for the masses of leptons and quarks and their electroweak messengers: W and Z. The discovery of higgs and the study of higgs sector might elucidate the problem of the pattern of hierarchy of masses of leptons and quarks: from milli electron Volts for neutrinos to about 180 GeV for t-quark. For many physicists it is a QCD problem: how light quarks and massless gluons form massive nucleons and atomic nuclei. Still for majority of confused students and science journalists there is no difference between mass of a body m and its energy E divided by c2: they believe in the “most famous formula E = mc2”.

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]

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. )

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 $$appears$$ 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]

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 $$appears$$ 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.

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

 

[pmb_phy]

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".

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 referring 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 also in violation of our rules that you have to cite valid sources. And we define valid sources as being either peer-reviewed published works, or work that are part of standard accepted physics. Please do not do this anymore or those references will be edited out.

This is quite incorrect. I just looked at the rules I agreed to abide by when I signed up. The state exactly the following

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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

We get quacks coming over telling us Einstein is wrong and they've managed to unite all the forces without using math...if you fit into this category, which it appears you don't, don't worry about your links. This rule is made for us to be able to remove content such as quack theories...discussing accepted physics and theory is fine...what's not fine is to publish your theories on our site as a form of peer review.

Pete

 

[pegleg bates]

The complete relativistic expression for the energy of a particle is this:
$$E = \sqrt{p^2c^2 + m^2c^4}$$

Where p is the momentum. Note that for massive particles at rest (momentum = 0) that equation becomes the more familiar $E = mc^2$.

For a photon: the mass is zero, but the momentum and energy are non-zero. E = pc = hf.

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]

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.

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]

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.)

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