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Adnan Nasir

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Why is the rest mass of a photon zero?
« on: 26/01/2010 15:30:03 »
Adnan Nasir asked the Naked Scientists:
   
Why is the rest mass of a photon zero?  Doesn't this violate the law of conservation of mass?

What do you think?
« Last Edit: 26/01/2010 15:30:03 by _system »


 

Offline Soul Surfer

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Why is the rest mass of a photon zero?
« Reply #1 on: 26/01/2010 19:33:03 »
Photons are pure energy and have to travel at at the speed of light and cannot be slowed down.  The fact that light travels slower through glass than through a vacuum is not that the light is travelling any slower it is just that the photon is continually being absorbed and re-emitted by the atoms in the glass and each absorption and re-emission causes a slight delay in its passage

The relativistic equations of motion also say that any particle with mass cannot travel at the speed of light.  Neutrinos have a tiny mass and travel at almost the speed of light and this causes a small difference in the arrival time of neutrinos and light from supernovae at large distances.  This difference has been detected

If the speed of photons was in any way variable it would cause blurring in short term events at great distances.  this does not happen.
 

Offline Simple

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Why is the rest mass of a photon zero?
« Reply #2 on: 04/02/2010 15:04:11 »
According to Einstein energy and matter are two sides of the same.
The photons travelling at the speed of light through space, dark and unseen until it hits something at its road, then activated in the form of energy.
So it is not an empty space as it seems to us, but filled with photons travelling stealth in all imaginable directions.
Even the dark holes are slowly emitting energy from its polar regions.
To me as an outsider of science it looks as if the universe is in everlasting circulation of energy.
It is, as it looks to me, just continuously changing into new fractal energy centers.
Am I completely wrong about being somewhat doubtful about the idea of "big bang" as it is generally explained?
 

Offline lightarrow

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Why is the rest mass of a photon zero?
« Reply #3 on: 04/02/2010 19:27:14 »
Why is the rest mass of a photon zero?  Doesn't this violate the law of conservation of mass?
That law is not valid in physics.

About the first question: in relativity you have E2 = (mc2)2 + (cp)2
E = energy
m = mass
p = momentum

But for a photon p = E/c (it comes from electromagnetic theory) --> m = 0.
« Last Edit: 04/02/2010 20:02:36 by lightarrow »
 

Offline JP

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Why is the rest mass of a photon zero?
« Reply #4 on: 05/02/2010 08:28:52 »
Why is the rest mass of a photon zero?  Doesn't this violate the law of conservation of mass?
That law is not valid in physics.

Conservation of mass is something that's only approximately valid in classical physics (if you don't take into account relativity or quantum mechanics).  Outside of that, conservation of energy generally replaces it, since energy is the more fundamental quantity.  (From E=mc2. you can write mass in terms of energy, for example.)
 

Offline Ron Hughes

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Why is the rest mass of a photon zero?
« Reply #5 on: 06/02/2010 16:50:28 »
Curious question, if I accelerate a charged particle with respect to me it emits radiation. If I accelerate along with the accelerated charged particle does it still emit radiation with respect to me? I would think not but would like to hear others response.
« Last Edit: 06/02/2010 16:53:54 by Ron Hughes »
 

Offline yor_on

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Why is the rest mass of a photon zero?
« Reply #6 on: 06/02/2010 21:11:43 »
If you're thinking of a photon Ron, it won't radiate as far as I understand.
And I agree :) Photons are strange ::))
 

Offline lightarrow

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Why is the rest mass of a photon zero?
« Reply #7 on: 06/02/2010 23:08:51 »
Curious question, if I accelerate a charged particle with respect to me it emits radiation. If I accelerate along with the accelerated charged particle does it still emit radiation with respect to me? I would think not but would like to hear others response.
This question is very difficult. I have read answeres to it in other forums but I sincerely have understood them only partially. What I think to have understood is that if you are very close to the charge (more than the wavelenght of the radiation) you don't measure any radiation, otherwise you do. (But I'm not sure).

Edit: obviously that more should instead be less.
« Last Edit: 11/02/2010 13:22:24 by lightarrow »
 

Offline Geezer

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Why is the rest mass of a photon zero?
« Reply #8 on: 07/02/2010 00:07:17 »
I believe Soul Surfer is quite correct. Photons are simply energy, so they have no rest mass. If they had non-zero rest mass, we would have to modify the speed of light. Photons can exhibit some particle-like behaviours, but I think it only tends to confuse things when we assume they actually are particles.
 

Offline yor_on

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Why is the rest mass of a photon zero?
« Reply #9 on: 07/02/2010 15:27:34 »
Ron, that was a really good question. I kind of love it.
I have a lot of headache pills left :)
I'm assuming that your thoughts wasn't related to photons at all but to particles with restmass (matter)?

---Quote---
Apparently, the principle of equivalence...
Consider a man standing on a scale on earth. The scale reads, say, 170 pounds. Consider a man in completely empty space in a rocketship accelerating at 9.8m/s^2. The man is standing on a scale. The scale reads 170 pounds. Now, consider the exact same situation but now with a point charge stapled to the man's nose.

If the principle of equivalence holds then... well, shouldn't the stationary point charge in a gravitational field be radiating? Regardless of whether or not it "should", it does not.

---End of quote--------By olgranpappy

So according to the principle of equivalence they (it) don't.

This one might be the one to read first before the other papers. It' presents the ideas in a easy format. Accelerating electrons don't radiate

This is a paper I liked. Does A Uniformly Accelerating Charge Radiate

And here is a pdf  "Radiation from a Charge in a Gravitational Field" 



"It is found that the “naive” conclusion from the principle of equivalence - that a freely falling charge does not radiate, and a charge supported at rest in a gravitational field does radiate - is a correct conclusion, and one should look for radiation whenever a relative acceleration exists between an electric charge and its electric field. The electric field which falls freely in the gravitational field is accelerated relative to the static charge. The field is curved, and the work done in overcoming the stress force created in the curved field, is the source of the energy carried by the radiation. This work is done by the gravitational field on the electric field, and the energy carried by the radiation is created in the expence of the gravitational energy of the system."

And if so the problem may lie with the idea of radiation. And if radiation will be shown only to exist in its 'interactions' with? 'virtual photons' f.ex then what is this universe and what makes it seem as 'existing'?

The arrow of time I would say. And that arrow is, as I see it, a direct effect of matter, and matter and space and the arrow dances so well :) But what does the arrow first and for all? Yep, it gives us distances. It's a direct effect of having a temporal dimension. As soon as you have an arrow there will be a 'distance' coming into being.

So, welcome to the 'crank division' :)

This one is interesting too.
synchrotron-radiation

« Last Edit: 07/02/2010 16:26:51 by yor_on »
 

Offline Geezer

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Why is the rest mass of a photon zero?
« Reply #10 on: 07/02/2010 20:54:08 »
erm, might our photons have gone tangential? If this is turning into a discussion about electrons and other particles, would it not be better to discuss that in another topic?
 

Offline Ron Hughes

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Why is the rest mass of a photon zero?
« Reply #11 on: 07/02/2010 22:42:53 »
Geez, you are right but since Adnan's question has been answered I thought it would be alright to discuss a related question.

yor, an accelerated charged particle radiating should not be in doubt since it has been proven over and over again. Suppose we put a detector in the G machine at NASA and get it up to say 12G's would it detect radiation? I don't think so and if it doesn't that means radiating particles obey the laws of general relativity which I find extremely interesting.
« Last Edit: 07/02/2010 22:45:06 by Ron Hughes »
 

Offline yor_on

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Why is the rest mass of a photon zero?
« Reply #12 on: 07/02/2010 23:34:05 »
Ron I'm unsure what you mean here?
That an accelerated particle radiate?

The energy seen, as I understands it, is a direct result of the acceleration, depending on frame of reference/observation? As for what the charge might be I don't know, the concept of charge is a hard one to understand for me :) Charge

Doesn't tell you much new huh :)

I have this very nice guy 'Skulls in the stars' blog here though. He's real good on most subjects he writes on (He wrote some really nice stuff on Black Holes too, if I remember right.) and he presents a very nice historical view about the subject with just a tiny bit of math in it. Acceleration without radiation  It's really worth reading.

And if you liked that? invisibility  There you can find part 2 and 3 if you look at his posts. He has a real capacity to make difficult subjects understandable. Not as me at all?

Could he be cursed?
« Last Edit: 07/02/2010 23:57:39 by yor_on »
 

Offline Geezer

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Why is the rest mass of a photon zero?
« Reply #13 on: 07/02/2010 23:48:40 »
Geez, you are right but since Adnan's question has been answered I thought it would be alright to discuss a related question.

yor, an accelerated charged particle radiating should not be in doubt since it has been proven over and over again. Suppose we put a detector in the G machine at NASA and get it up to say 12G's would it detect radiation? I don't think so and if it doesn't that means radiating particles obey the laws of general relativity which I find extremely interesting.

I think might be better to start a new topic if you can. Then there is potentially less confusion about whether or not the question was answered, and more people might be inclined to participate in a different discussion. No biggie though.
 

Offline lightarrow

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Why is the rest mass of a photon zero?
« Reply #14 on: 08/02/2010 15:43:13 »
Agreed.
 

Offline Ron Hughes

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Why is the rest mass of a photon zero?
« Reply #15 on: 10/02/2010 19:59:51 »
yor, those sites confirm what I said. I'll take the discussion to the new theories section, the subject will be " The Photon ".
 

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Why is the rest mass of a photon zero?
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