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Author Topic: Is a photon a monopole?  (Read 2688 times)

Offline petm1

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Is a photon a monopole?
« on: 24/09/2013 04:21:03 »
Why is a photon not considered to also be a monopole?
« Last Edit: 25/09/2013 18:58:30 by chris »


 

Offline Pmb

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Re: photons
« Reply #1 on: 24/09/2013 04:53:34 »
Why is a photon not considered to also be a monopole?
Because the magnetic field of the photon is not the same as that of a monopole, which it must if its to be a magnetic monopole. This is similar to asking why a photon is not considered to also be an electric charge. It doesn't have the field of that of an electrically charged particle.
« Last Edit: 24/09/2013 04:55:24 by Pmb »
 

Offline evan_au

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Re: Is a photon a monopole?
« Reply #2 on: 26/09/2013 01:08:32 »
A photon consists of an electromagnetic wave.
It consists of varying electric fields (+ & -) and varying magnetic fields (north and south), which oscillate at very high frequencies.
  • Because it consists of both + & - electric fields in equal strength, it is not an electric monopole.
  • Because it consists of both north & south magnetic fields in equal strength, it is not a magnetic monopole.
We know of many examples of electric monopoles, eg electrons (-) and protons (+). At this time, there are no known examples of magnetic monopoles (but that hasn't stopped some physicists looking for them, in cosmic ray debris, for example).
 

Offline Pmb

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Re: Is a photon a monopole?
« Reply #3 on: 26/09/2013 21:41:38 »
Quote from: evan_au
A photon consists of an electromagnetic wave.
I'm afraid that you have it backwards. An electromagnetic wave consists of photons, not the other way around.
 

Offline evan_au

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Re: Is a photon a monopole?
« Reply #4 on: 28/09/2013 22:20:18 »
I venture to suggest that both statements are true:
  • Maxwell's differential equations for the electromagnetic wave apply on all scales, down to zero field strength.
  • In Einsteins' s interpretation of the photoelectric effect, there is a minimum possible energy for light; ie electromagnetic waves are quantised. We call this quantum of light a photon.
  • So at high field strengths, the electromagnetic wave consists of many photons.
  • At the lowest possible field strength (as represented by an isolated photon), Maxwell's equations still apply, so a photon consists of an electromagnetic wave. 
 

Offline JP

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Re: Is a photon a monopole?
« Reply #5 on: 29/09/2013 15:29:43 »
I venture to suggest that both statements are true:
  • Maxwell's differential equations for the electromagnetic wave apply on all scales, down to zero field strength.
No, that isn't true.  Photons are funny creatures that make up electromagnetic fields, but each individual photon is not a solution to Maxwell's equations.  There is, in fact, non-classical light which cannot be described in terms of classical (Maxwell's) equations (http://en.wikipedia.org/wiki/Nonclassical_light).
Quote
  • At the lowest possible field strength (as represented by an isolated photon), Maxwell's equations still apply, so a photon consists of an electromagnetic wave. 
For the same reason above, a single photon does not satisfy Maxwell's equations.  Most large collections of photons collectively do satisfy Maxwell's equations, but this isn't a requirement.
 

Offline alancalverd

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Re: Is a photon a monopole?
« Reply #6 on: 29/09/2013 16:08:47 »
We use the term monopole to denote something with a signed property (charge, magnetic field) and (usually) an opposite. Hence a magnetic or electric monopole has meaning if not existence. But what is the signed property of a photon? What does an antiphoton do?
 

Offline JP

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Re: Is a photon a monopole?
« Reply #7 on: 29/09/2013 16:22:42 »
Alan, a monopole doesn't have to be signed.  You can have a gravitational monopole, which would be a point of mass.

But a monopole represents the distribution of a field coming off a point source, much as gravity comes off a point mass or electric field comes off a stationary point charge.  This field is by definition spherically symmetric.  For a photon to be a "monopole" it would have to emit a spherically symmetric field, which it doesn't.  For it to be a monopole field, it would have to look like the static field coming off an electric charge, which it also doesn't.
 

Offline Pmb

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Re: Is a photon a monopole?
« Reply #8 on: 29/09/2013 16:48:22 »
Alan, a monopole doesn't have to be signed.  You can have a gravitational monopole, which would be a point of mass.

But a monopole represents the distribution of a field coming off a point source, much as gravity comes off a point mass or electric field comes off a stationary point charge.  This field is by definition spherically symmetric.  For a photon to be a "monopole" it would have to emit a spherically symmetric field, which it doesn't.  For it to be a monopole field, it would have to look like the static field coming off an electric charge, which it also doesn't.

Thanks, JP!! I feel as though a giant weigt has been liftet right off my shoulders. Whew!
 

Offline alancalverd

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Re: Is a photon a monopole?
« Reply #9 on: 29/09/2013 17:40:26 »
Point taken. Sort of.

To my mind a gravitational monopole is signed, in that the divergence of its field is always nonzero, but I'll admit that, unlike sci-fi writers, I can't think of anything with the opposite sign.
 

Offline petm1

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Re: Is a photon a monopole?
« Reply #10 on: 30/10/2013 03:26:47 »
Thanks for the replies. I was thinking of a photon with all of its changes being in space, unchanging in time, could only be one pole at any single time.
 

Offline evan_au

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Re: Is a photon a monopole?
« Reply #11 on: 30/10/2013 07:52:22 »
Quote from: JP
There is, in fact, non-classical light which cannot be described in terms of classical (Maxwell's) equations

Ok, now I understand that there is such a thing as non-classical light.
Question: Would one isolated classical photon obey Maxwell's equations?
 

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Offline acsinuk

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Re: Is a photon a monopole?
« Reply #12 on: 30/10/2013 09:50:43 »
Shrunk
We need to look at non-classical light in a 3D way. At any instant in time dt where-abouts in the magnetic envelope is the AC pulses apparent charge?? And further what shape is that magnetic envelope?
CliveS
 

Offline JP

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Re: Is a photon a monopole?
« Reply #13 on: 30/10/2013 14:47:44 »
Quote from: JP
There is, in fact, non-classical light which cannot be described in terms of classical (Maxwell's) equations

Ok, now I understand that there is such a thing as non-classical light.
Question: Would one isolated classical photon obey Maxwell's equations?

It depends how you define a classical photon.  There's technically no such thing, since photons are quantum mechanical by nature.  However, you can break a field into rays, which can be forced to satisfy Maxwell's equations either independently or as a collection.  Rays are the closest thing we have to "classical photons."
 

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Re: Is a photon a monopole?
« Reply #13 on: 30/10/2013 14:47:44 »

 

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