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Offline percepts

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what is the nature of a photon ?
« on: 07/03/2013 04:32:05 »
Just asked this on radio five live but ran out of time and didn't get to the bottom of it.

I keep hearing that light is massless which confuses me. So my question is:

Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something. Further more, if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy if einsteins theory e = mc^2 is applicable to them. Unless my basic understanding of physics is completely wrong then that can't be the case which is why I have to ask how can light be massless or is this statement just being used to state the case at the point of detection. i.e. the residue of the electromagnetic wave hitting something is what we call a photon and has no mass but does have energy?

thanks


 

Offline JP

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Re: what is the nature of a photon ?
« Reply #1 on: 07/03/2013 14:40:02 »
Just asked this on radio five live but ran out of time and didn't get to the bottom of it.

I keep hearing that light is massless which confuses me. So my question is:

Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something. Further more, if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy if einsteins theory e = mc^2 is applicable to them. Unless my basic understanding of physics is completely wrong then that can't be the case which is why I have to ask how can light be massless or is this statement just being used to state the case at the point of detection. i.e. the residue of the electromagnetic wave hitting something is what we call a photon and has no mass but does have energy?

thanks

Good questions.  I'll try to answer them in order:
1) In quantum mechanics, "particle" means something different than a little tiny packet of something that moves through space along well-defined trajectories.  A photon is a particle in the quantum sense, in that it's the smallest piece of energy you can extract from an electromagnetic wave.  At a detector, it also interacts at a point.  It is not particle-like in a classical sense in that when it travels from point A to point B it does so in a wavey way that's spread out over all space, not along a single trajectory.

2) The actual equation for a particle is not necessarily E=mc2, but rather E2=m02c4+(pc)2, where p is momentum.  There are two important issues in this equation: first, m0 represents what's called the invariant mass of a particle, which for photons is zero.  (Defining mass usually leads to a debate on this forum, since there are several possible definitions in relativity, but invariant mass is usually what physicists mean when they say the mass of a photon is zero).  In this case, E2=(pc)2, and a photon does have momentum, so there's no problem.
 

Offline evan_au

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Re: what is the nature of a photon ?
« Reply #2 on: 07/03/2013 14:55:05 »
Quote
if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy

Quote
The Planck constant was first described as the proportionality constant between the energy (E) of a photon and the frequency (v) of its associated electromagnetic wave. This relation between the energy and frequency is called the Planck relation:

    E = hv    (see http://en.wikipedia.org/wiki/Planck_constant)

So a photon traveling through space does have energy, provided its frequency > 0 (ie not red-shifted into oblivion).
When the photon strikes an an object, that energy typically drives an electron into a higher orbit, which can trigger chemical reactions, generate electricity in a solar cell, cause the object to get hot, etc.
 

Offline Pmb

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Re: what is the nature of a photon ?
« Reply #3 on: 07/03/2013 15:31:19 »
Quote from: percepts
I keep hearing that light is massless which confuses me.
There has been a debate on the definition of mass for several decades. What has happened is that particle physicists, the major “users” of special relativity, who study the intrinsic properties of particles only speak of what’s called the rest mass aka proper mass aka invariant mass of the particle. This is the mass you know from Newtonian mechanics. However those physicists who aren’t particle physicists but who specialize only in relativity more often seem to use the term to refer to what is called the “inertial mass” of a particle (a more complete name of what you know of from Newtonian mechanics). Cosmologists have their own ideas too since they use mass densities. When it comes to more complex systems, i.e. macroscopic bodies and bodies under stress, the simple notion of invariant/proper mass is insufficient.

There is a write up on the mass of a photon here
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

I myself have researched this subject and written extensively on it. Please see
http://arxiv.org/abs/0709.0687
http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htm
http://home.comcast.net/~peter.m.brown/sr/invariant_mass.htm
http://home.comcast.net/~peter.m.brown/sr/long_trans_mass.htm


Quote from: percepts
So my question is:

Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something.
A photon is referred to as a “particle” but it’s a quantum mechanical particle, not a classical particle. When one detects a photon one finds it located at a single place and not spread all over the place like a wave. However there are wave properties of a photon since a beam of photons will interact with each other even when the photons arrive at the detectors one at a time! It’s a complex issue as you can imagine.

The most general thing used to describe matter is the stress-energy-momentum tensor. I've discussed the inertia of stress in that paper above which is on an archive at Cornell University. Enjoy! Please let me know if the above links are useful. If not then I'll keep that in mind when/if I modify them.
 

Offline lightarrow

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Re: what is the nature of a photon ?
« Reply #4 on: 08/03/2013 13:55:25 »
Further more, if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy if einsteins theory e = mc^2 is applicable to them.
E = mc2 should be used only by vaccinated, adult, well knowledged in physics people. Popular books which use it improperly shoud be burned...  :)
That equation is WRONG in your case.
It is correct only for STATIONARY BODIES.
Is light stationary? Of course not.
 

Offline lightarrow

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Re: what is the nature of a photon ?
« Reply #5 on: 08/03/2013 13:57:03 »
Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something.
Define "particle". After your answer, my reply will show you that the question was not trivial.
 

Offline lightarrow

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Re: what is the nature of a photon ?
« Reply #6 on: 08/03/2013 14:05:45 »
There has been a debate on the definition of mass for several decades. What has happened is that particle physicists, the major “users” of special relativity, who study the intrinsic properties of particles only speak of what’s called the rest mass aka proper mass aka invariant mass of the particle. This is the mass you know from Newtonian mechanics.
You compute that light has zero mass with newtonian mechanics? Or, maybe, with classical electrodynamics (Mxwell's theory)? And what about the *quantum* object "photon"? Still newtonian mechanics?
 

Offline JP

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Re: what is the nature of a photon ?
« Reply #7 on: 08/03/2013 15:05:46 »
Matt Austern expresses this entire argument nicely in two equations in the Baez link posted above:

E = mrelc2   
and
E2 = p2c2 + mrest2c2 .         

Obviously a photon has non-zero energy, and from the first equation, mrel is therefore non-zero for a photon.
The second equation does allow mrest to be zero for a photon, and there are good reasons for it to be zero in quantum mechanics. 

So how can one of these masses be zero and the other non-zero for a photon?  They represent two different physical quantities.  The reason they're both sometimes confusingly called "mass" is that for macroscopic objects moving at non-relativistic speeds (when you can use classical mechanics), both definitions reduce to the same thing that Newton would have called simply "mass."
 

Offline percepts

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Re: what is the nature of a photon ?
« Reply #8 on: 09/03/2013 01:19:57 »
Thanks all for replies. I begin to understand.
 

Offline Pmb

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Re: what is the nature of a photon ?
« Reply #9 on: 09/03/2013 02:15:42 »
There has been a debate on the definition of mass for several decades. What has happened is that particle physicists, the major “users” of special relativity, who study the intrinsic properties of particles only speak of what’s called the rest mass aka proper mass aka invariant mass of the particle. This is the mass you know from Newtonian mechanics.
You compute that light has zero mass with newtonian mechanics? Or, maybe, with classical electrodynamics (Mxwell's theory)? And what about the *quantum* object "photon"? Still newtonian mechanics?
I was referring to luxons i.e. particles which have a rest frame or travel at v << c. In any case Newtonian (inertial) mass is define as the m in p = mv. Same in relativity. I gave those links above so as to explain all of this in excrutiating detail. Anybody who reads those should pretty much be an expert on the subject by the time they're finished reading all of them.
« Last Edit: 09/03/2013 03:14:00 by Pmb »
 

Offline Pmb

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Re: what is the nature of a photon ?
« Reply #10 on: 09/03/2013 02:17:00 »
Matt Austern expresses this entire argument nicely in two equations in the Baez link posted above:

E = mrelc2   
and
E2 = p2c2 + mrest2c2 .         

Obviously a photon has non-zero energy, and from the first equation, mrel is therefore non-zero for a photon.
The second equation does allow mrest to be zero for a photon, and there are good reasons for it to be zero in quantum mechanics. 

So how can one of these masses be zero and the other non-zero for a photon?  They represent two different physical quantities.  The reason they're both sometimes confusingly called "mass" is that for macroscopic objects moving at non-relativistic speeds (when you can use classical mechanics), both definitions reduce to the same thing that Newton would have called simply "mass."
It's also because they are affected by gravity and generate a gravitational field.
 

Offline Pmb

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Re: what is the nature of a photon ?
« Reply #11 on: 09/03/2013 02:49:41 »
Quote from: lightarrow
That equation is WRONG in your case.
It is correct only for STATIONARY BODIES.
Is light stationary? Of course not.
Yeah, but as you very well know, whether it's right or wrong is a matter of what the definition of m is. If m is proper mass then it's wrong. If it's inertial mass then it's right.
« Last Edit: 09/03/2013 03:12:30 by Pmb »
 

Offline lightarrow

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Re: what is the nature of a photon ?
« Reply #12 on: 09/03/2013 12:18:03 »
There has been a debate on the definition of mass for several decades. What has happened is that particle physicists, the major “users” of special relativity, who study the intrinsic properties of particles only speak of what’s called the rest mass aka proper mass aka invariant mass of the particle. This is the mass you know from Newtonian mechanics.
You compute that light has zero mass with newtonian mechanics? Or, maybe, with classical electrodynamics (Mxwell's theory)? And what about the *quantum* object "photon"? Still newtonian mechanics?
I was referring to luxons i.e. particles which have a rest frame or travel at v << c.
?? What do you mean?
Luxons don't have a rest frame and travel at c:

http://en.wikipedia.org/wiki/Massless_particle
Quote
In any case Newtonian (inertial) mass is define as the m in p = mv. Same in relativity. I gave those links above so as to explain all of this in excrutiating detail. Anybody who reads those should pretty much be an expert on the subject by the time they're finished reading all of them.
I wrote that comment on your post because I had the ... subtle feeling that you tried to suggest to people that invariant mass is a less modern concept than relativistic mass  :)
I believe it's the opposite...
« Last Edit: 09/03/2013 12:21:12 by lightarrow »
 

Offline Pmb

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Re: what is the nature of a photon ?
« Reply #13 on: 09/03/2013 16:41:40 »
Quote from: lightarrow
What do you mean?
Luxons don't have a rest frame and travel at c:
Sorry. I meant tardyons.

Quote
http://en.wikipedia.org/wiki/Massless_particle
Quote
In any case Newtonian (inertial) mass is define as the m in p = mv. Same in relativity. I gave those links above so as to explain all of this in excrutiating detail. Anybody who reads those should pretty much be an expert on the subject by the time they're finished reading all of them.
I wrote that comment on your post because I had the ... subtle feeling that you tried to suggest to people that invariant mass is a less modern concept than relativistic mass  :)
I believe it's the opposite...
We've already discussed that subject to death so I won't go there. Sorry.
 

Offline lightarrow

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Re: what is the nature of a photon ?
« Reply #14 on: 09/03/2013 19:54:32 »
Quote from: lightarrow
That equation is WRONG in your case.
It is correct only for STATIONARY BODIES.
Is light stationary? Of course not.
Yeah, but as you very well know, whether it's right or wrong is a matter of what the definition of m is. If m is proper mass then it's wrong. If it's inertial mass then it's right.
Are you sure? Which is the transverse relativistic mass of a photon?
 

Offline percepts

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Re: what is the nature of a photon ?
« Reply #15 on: 09/03/2013 22:55:37 »
Just asked this on radio five live but ran out of time and didn't get to the bottom of it.

I keep hearing that light is massless which confuses me. So my question is:

Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something. Further more, if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy if einsteins theory e = mc^2 is applicable to them. Unless my basic understanding of physics is completely wrong then that can't be the case which is why I have to ask how can light be massless or is this statement just being used to state the case at the point of detection. i.e. the residue of the electromagnetic wave hitting something is what we call a photon and has no mass but does have energy?

thanks

Good questions.  I'll try to answer them in order:
1) In quantum mechanics, "particle" means something different than a little tiny packet of something that moves through space along well-defined trajectories.  A photon is a particle in the quantum sense, in that it's the smallest piece of energy you can extract from an electromagnetic wave.  At a detector, it also interacts at a point.  It is not particle-like in a classical sense in that when it travels from point A to point B it does so in a wavey way that's spread out over all space, not along a single trajectory.

2) The actual equation for a particle is not necessarily E=mc2, but rather E2=m02c4+(pc)2, where p is momentum.  There are two important issues in this equation: first, m0 represents what's called the invariant mass of a particle, which for photons is zero.  (Defining mass usually leads to a debate on this forum, since there are several possible definitions in relativity, but invariant mass is usually what physicists mean when they say the mass of a photon is zero).  In this case, E2=(pc)2, and a photon does have momentum, so there's no problem.
Thanks for this reply which puts it in simplistic terms that even I can begin to understand.
 

Offline Pmb

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Re: what is the nature of a photon ?
« Reply #16 on: 10/03/2013 03:56:00 »
Quote from: lightarrow
Are you sure? Which is the transverse relativistic mass of a photon?
Yes. I'm sure. I'm confused as to why you don't know the answer. It's pretty trivial. I think you forgot to put on your thinking cap. :) Tardyons, such as electrons can be defelcted so that they accceleratet transverse to their direction. Since a photon can't be deflected it travels in a straight line so that the transverse mass is zero. The same holds for the longitudinal mass, it's zero too. What isn't zero is the relativistic mass defined as the m in p = mv by hypothesis this is a conserved quantity.
« Last Edit: 10/03/2013 04:05:05 by Pmb »
 

Offline lightarrow

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Re: what is the nature of a photon ?
« Reply #17 on: 10/03/2013 16:18:14 »
Quote from: lightarrow
Are you sure? Which is the transverse relativistic mass of a photon?
Yes. I'm sure. I'm confused as to why you don't know the answer. It's pretty trivial. I think you forgot to put on your thinking cap. :) Tardyons, such as electrons can be defelcted so that they accceleratet transverse to their direction. Since a photon can't be deflected
Why a photon can't be deflected?
Quote
it travels in a straight line so that the transverse mass is zero. The same holds for the longitudinal mass, it's zero too. What isn't zero is the relativistic mass defined as the m in p = mv by hypothesis this is a conserved quantity.
Let's say that you are right about relativistic mass of a photon: the formula E = mc2, with m as the relativistic mass, is correct for photons only, not for tardyons.
 

Offline simplified

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Re: what is the nature of a photon ?
« Reply #18 on: 10/03/2013 17:01:54 »
Relativistic mass should be = general mass - gravitational mass
Because gravitational mass is constant for any observer,it is not relativistic.
 

Offline yor_on

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Re: what is the nature of a photon ?
« Reply #19 on: 10/03/2013 22:39:58 »
Hmm :)

We're of two minds there. Relativistic gravitational mass just mean that you change your coordinate system. If you fall with the gravity, the gravity disappear for you. If a floor stops you, you find it to come back. That's how we define it as 'relativistic', meaning that it depends on you, relative it, as well as mass. If gravity was a substance then you could say you are co moving with it in a free fall. But it is no substance I know how to define, and if it also can 'distort' a space, what substance would it be?
 

Offline yor_on

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Re: what is the nature of a photon ?
« Reply #20 on: 10/03/2013 22:59:46 »
A correction. Shouldn't have used  'Relativistic gravitational mass', it's better to just call it 'gravity' because putting those words together it becomes a 'energy density'. When you get up to relativistic speeds that 'energy density' created will bend a space a according to relativity. Although you could read that as changing your 'coordinate system' too it becomes extremely tricky. If you look at particle accelerators they create particles of more mass than the two colliding on a daily basis, using a acceleration.
 

Offline Pmb

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Re: what is the nature of a photon ?
« Reply #21 on: 11/03/2013 01:30:29 »
Quote from: lightarrow
Why a photon can't be deflected?
First off I’m restraining my discussion to special relativity (SR). In SR there is nothing that interacts with a photon that can deflect it. As I recall it, and I may e wrong, I don’t consider Compton scattering to be deflecting the photon since a photon is destroyed when it hits the electron and a new one produced. Otherwise the photon can’t be deflected as an electron is i.e. via the electric field. No such field for deflecting photons exit that I’m aware of,


 
Quote from: lightarrow
Let's say that you are right about relativistic mass of a photon: the formula E = mc2, with m as the relativistic mass, is correct for photons only, not for tardyons.
That is incorrect. Some people even define relativistic mass as m = E/c2.

It can be shown that if p = mv where m = relativistic mass then it can be shown that E = mc[sup[2[/sup]. I’ve derived that relationship here - http://xxx.lanl.gov/abs/physics/0308039
 

Offline simplified

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Re: what is the nature of a photon ?
« Reply #22 on: 11/03/2013 13:44:48 »
Hmm :)

. If you fall with the gravity, the gravity disappear for you.
No,that doesn't.Gravity increases my energy.
 

Offline yor_on

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Re: what is the nature of a photon ?
« Reply #23 on: 11/03/2013 15:46:47 »
Well, We're in a gravitational field now, on Earth. And as long as you feel gravity acting on you, you can be seen as accelerating relative it, at one G, according to relativity, and also as I think. Now assume that we 'push' Earth a little, accelerating this little planet :) to then stop accelerating it. Will the light bulb inside a house now become slightly more blue shifted, as it gained a new 'relative motion', which also is a uniform motion, equal to the uniform motion before the acceleration?
 

Offline yor_on

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Re: what is the nature of a photon ?
« Reply #24 on: 11/03/2013 15:52:29 »
But yes, assuming that gravity only has to do with mass and accelerations you might formulate it that way, with a question mark regarding the way gravity at large is presumed to act in space.. But, to use it you need that floor, and that is frames of reference.

Or maybe not thinking some more. If we take planets of different mass we can define each one as 'accelerating' 'differently. Will now a same light bulb, blue or redshift, depending on what planet you connect it? Or will it present you with a same 'energy' locally measured?

After all, if you accept that you're not 'at rest' with the gravitational field, while being 'at rest' with the surface of a planet, different mass should make a difference? (If that formulation would be correct.)
=

Locally meaning you being 'at rest' with the light bulb, in each case, as being in a 'black box scenario' on that surface.

(You could then split accelerations in two, one being a local (constant) uniform acceleration, the one equal to Earths gravity according to the equivalence priciple, the other being non uniform accelerations, which Earth does not show.)
« Last Edit: 11/03/2013 16:38:26 by yor_on »
 

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Re: what is the nature of a photon ?
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