The photon and Electromagnetic field

[Naty1]

How does an EM field, with both an electric and a magnetic component, each of which "disappears" produce an apparently steady photon?

A point charge has an electromagnetic field it creates...A stationary observer with respect to the charge sees and electric field; an observer in motion sees a magnetic field. But photons, presumably bumps in the EM field, don't wink in and out of existence depending on our motion or the appearance of one field component or the other. .

 

[tiny-tim]

A stationary observer with respect to the charge sees and electric field; an observer in motion sees a magnetic field. …

Hi Naty1!

I'm not sure I'm getting the point you're making …

if you're saying that the observer in motion will see only a magnetic field, and no electric field, then that's wrong …

the field he sees will still be mostly electric (E² - B² = E₀²) …

there is no observer for whom the electric field disappears.

 

[Mentz114]

"Photons" only exist at the moment they are emitted or absorbed i.e. when they interact with matter. There is no evidence ( nor any way of getting any ) that photons exist in the EM field when it is not interacting with matter.

Speculations about 'free' photons usually lead to apparent contradictions, as evidenced by your question.

 

[atyy]

https://www.physicsforums.com/showthread.php?t=251358

 

[Naty1]

if you're saying that the observer in motion will see only a magnetic field, and no electric field, then that's wrong …

there is no observer for whom the electric field disappears

Well that's contrary to another thread here...I don't remember which...I'll see if I can find it... I clearly remember the (apparent) conclusion because prior to that my understanding was the same as yours...even so the magnetic field seems to appear and disappear...right??

"Photons" only exist at the moment they are emitted or absorbed i.e. when they interact with matter

Well that's novel... and difficult for messenger particles to comply with I should think! From Fabric of the Cosmos, Brian Greene:

Photons pass completely unhindered through the Higgs ocean and so have no mass at all...When Schrodinger's equation is applied in a simple situation such as a single, isolated photons passing through a screen with two slits, it gives rise to the interference pattern...

And what about the polarization of photons, (their spin directions), wave particle duality...photoelectric effect, etc,etc,etc...

Had someone answered I was mixing quantum theory with classicial field theory I would have immediately understood...because there are always anomalies when trying to do that...

Also, the answer may be related to the following:

PARALLEL WORLDS, Michio Kaku, Pg 216

...This symmetry can be made exact if we add monopoles into Maxwell's equations.
The revised Maxwell's equations remain precisely the same if we exchange the electric field with the magnetic field and interchange the electric charge e with the inverse of the magnetic charge g. This means that electricity (if the electric charge is low) is precisely equivalent to magnetism (if the magnetic charge is high). This equivalence is called duality.

(Without monopoles the equations look almost the same.)

 

[Naty1]

I found a quote, but it was used in another thread..still looking for the thread...

Also keep in mind that electric field in one reference frame is a magnetic field in another; so a static (electric) charge on a rotating body will have a perceived magnetic field.

 

[tiny-tim]

Well that's contrary to another thread here...I don't remember which...I'll see if I can find it... I clearly remember the (apparent) conclusion because prior to that my understanding was the same as yours...even so the magnetic field seems to appear and disappear...right??

Hi Naty1!

E² - B² is a relativistic invariant.

(btw, so is E.B)

So you can't have E = 0 in one frame and B = 0 in another.

EDIT:

Also keep in mind that electric field in one reference frame is a magnetic field in another; so a static (electric) charge on a rotating body will have a perceived magnetic field.

The second half of that is correct. The first half should say "Also keep in mind that electric field in one reference frame has an additional magnetic field in another"

 

[spidey]

"Photons" only exist at the moment they are emitted or absorbed i.e. when they interact with matter. There is no evidence ( nor any way of getting any ) that photons exist in the EM field when it is not interacting with matter.

Speculations about 'free' photons usually lead to apparent contradictions, as evidenced by your question.

I am confused. Do u say photon doesn't exist when they are not interacting with matter?
Supposing photon is emitted from sun and reaches earth.So, you say photon exist when it is emitted by Sun and when it interacts with earth.In the mean time, photon doesn't exist between sun and earth.

 

[Jonathan Scott]

I am confused. Do u say photon doesn't exist when they are not interacting with matter?
Supposing photon is emitted from sun and reaches earth.So, you say photon exist when it is emitted by Sun and when it interacts with earth.In the mean time, photon doesn't exist between sun and earth.

A photon is a way of describing a quantized transfer of electromagnetic energy and angular momentum. However, while the energy is in flight, it isn't really a collection of independent particles but rather a flow of waves containing the total amount of energy. When those waves interact with something, they will transfer quantized amounts of energy of the appropriate frequency. If the energy density is very low, then there may be a one to one relationship between photons being emitted and received, but various physical effects such as interference demonstrate that one cannot assume that the emitted and received photons were actually in some sense the same particle.

However, it's quite common to describe the flow of electromagnetic waves as a stream of photons anyway in cases where the distinction doesn't really matter.

 

[Naty1]

Jonathan posted..

However, while the energy is in flight, it isn't really a collection of independent particles but rather a flow of waves containing the total amount of energy. When those waves interact with something, they will transfer quantized amounts of energy of the appropriate frequency.

I'm not sure I agree entirely with the first sentence; however it's largely immaterial as we are into the wave (classical field theory) versus particle (quantum mechanical) viewpoint...The difficulty is akin to the double slit experiment...electromegnatic waves in some situations appear to be continuous and wavelike, in other situations discrete quantum like particle effects are manifested...it's a good concept to remember, but has been debated for about 90 years...

 

[DrGreg]

"Photons" only exist at the moment they are emitted or absorbed i.e. when they interact with matter. There is no evidence ( nor any way of getting any ) that photons exist in the EM field when it is not interacting with matter.

Speculations about 'free' photons usually lead to apparent contradictions, as evidenced by your question.

It's true that we can measure photons only by emitting them or absorbing them. But to conclude that they "do not exist" between those events seems a very perverse use of the English language.

You say that there is no evidence that they exist, but the evidence is that they are emitted (i.e. come into existence!) and are absorbed (i.e. cease to exist!).

I think what you meant to say is that we can't tell exactly where a photon is between its creation and destruction. Quantum theory tells us the photon takes all possible routes -- so it can be in many places at once -- and the best we can do is describe it via a wavefunction.

 

[Naty1]

Quantum theory tells us the photon takes all possible routes -- so it can be in many places at once -- and the best we can do is describe it via a wavefunction.

Now that I can buy into!

 

[Naty1]

Here's a quote from wikipedia,
http://en.wikipedia.org/wiki/Electromagnetic_field

which supports Tiny tim's post...so I'm back to my original understanding...thanks tim...

In the past, electrically charged objects were thought to produce two types of field associated with their charge property. An electric field is produced when the charge is stationary with respect to an observer measuring the properties of the charge and a magnetic field (as well as an electric field) is produced when the charge moves (creating an electric current) with respect to this observer. Over time, it was realized that the electric and magnetic fields are better thought of as two parts of a greater whole — the electromagnetic field.

(my boldface added)

 

[Naty1]

So nobody has answered my original question yet...If a characteristic of the EM field is observer dependent, why not the photon?

Rather odd...but maybe because the photon only moves at lightspeed, that locks in the characteristic which might otherwise change...

 

[tiny-tim]

How does an EM field, with both an electric and a magnetic component, each of which "disappears" produce an apparently steady photon? …

So nobody has answered my original question yet...If a characteristic of the EM field is observer dependent, why not the photon?

Rather odd...but maybe because the photon only moves at lightspeed, that locks in the characteristic which might otherwise change...

Hi Naty1!

The EM fields which produce photons have E = B (and E.B = 0) (I think: ) …

and if E = B in one frame, then E = B in every frame …

so, unlike an electron, a "photon-field" will look the same in any frame.

 

[Mentz114]

It's true that we can measure photons only by emitting them or absorbing them. But to conclude that they "do not exist" between those events seems a very perverse use of the English language.

You say that there is no evidence that they exist, but the evidence is that they are emitted (i.e. come into existence!) and are absorbed (i.e. cease to exist!).

I think what you meant to say is that we can't tell exactly where a photon is between its creation and destruction. Quantum theory tells us the photon takes all possible routes -- so it can be in many places at once -- and the best we can do is describe it via a wavefunction.

(my bolding).

I think that strengthens my case. The wave function for a photon is not a Schroedinger equation in the normal sense. I quote from (1)

This choice of normalization reflects the fact that a photon has no mass that can be localized at a point; rather it has only helicity and energy, and the energy cannot strictly be localized at a point. The square of the wave function gives the probability density for energy, not particle location

So the wave function does not describe a particle, even in the loosest sense.

(1)"The Maxwell wave function of the photon" M. G. Raymer and Brian J. Smith.
In proc. SPIE conference Optics and Photonics, The Nature of Light: What is a Photon?" (San Diego, Aug. 2005)

 

[jtbell]

The EM fields which produce photons have E = B (and E.B = 0) (I think: )

Electromagnetic waves have E = cB (more precisely $|\vec E| = c |\vec B|$ at all points.

 

[tiny-tim]

oops!

Electromagnetic waves have E = cB (more precisely $|\vec E| = c |\vec B|$ at all points.

Hi jtbell!

As you've probably guessed, I learned my electromagnetism with c = 1, and I keep forgetting to put the c back in.

hmm … then I suppose it's E² - c²B² which is invariant?

 

[DrGreg]

I think that strengthens my case. The wave function for a photon is not a Schroedinger equation in the normal sense.

So the wave function does not describe a particle, even in the loosest sense.

I'm out of my depth with the fine details of quantum theory, but if I understand you correctly you are saying that a photon behaves like a particle when it is emitted and absorbed, but not in between. Phrased like that, that makes sense, but I wouldn't say a photon "doesn't exist" between those events, just that it isn't very particle-like. (To my way of thinking, "wavefunction" and "photon" mean pretty much the same thing.)

 

[lightarrow]

I'm out of my depth with the fine details of quantum theory, but if I understand you correctly you are saying that a photon behaves like a particle when it is emitted and absorbed, but not in between. Phrased like that, that makes sense, but I wouldn't say a photon "doesn't exist" between those events, just that it isn't very particle-like. (To my way of thinking, "wavefunction" and "photon" mean pretty much the same thing.)

Sorry if I come into your discussion.
If the photon is defined as the object representing the particle-like behaviour of light, and if that particle-like behaviour doesn't exist between source and detector, then the photon...shouldn't exist there.
(I have put two "if" and a "should" because in the past I have almost been banished from this forum for having wrote exactly that statement (without "if" and with "does" instead of "should") ).

 

[DrGreg]

Sorry if I come into your discussion.
If the photon is defined as the object representing the particle-like behaviour of light, and if that particle-like behaviour doesn't exist between source and detector, then the photon...shouldn't exist there.
(I have put two "if" and a "should" because in the past I have almost been banished from this forum for having wrote exactly that statement (without "if" and with "does" instead of "should") ).

... but is that the generally accepted definition of a photon? (Please provide references, preferably on-line.)

 

[Jonathan Scott]

Sorry if I come into your discussion.
If the photon is defined as the object representing the particle-like behaviour of light, and if that particle-like behaviour doesn't exist between source and detector, then the photon...shouldn't exist there.
(I have put two "if" and a "should" because in the past I have almost been banished from this forum for having wrote exactly that statement (without "if" and with "does" instead of "should") ).

Even if you can't exactly say where it is in between, if you emit a photon and one end and receive a photon at the other, and the intensity is not large enough not to cause overlapping or interference effects, the quantum properties of the photons being emitted will be passed to the receiver, so it is reasonable to describe this as a photon in between too. However, you can for example send one photon through two slits (or a diffraction grating) and see that the path is modified apparently by interference with itself, which is more wave-like than particle-like.

However, I think this is a topic for the Quantum theory forum rather than relativity.

 

[Mentz114]

If you take a cup of water from the ocean, you have a cup of water and an ocean. When you pour the water back into the ocean, does the 'cup of water' still exist ?

When the EM field interacts with matter, the energy is exchanged in 'cupfuls'. It does not mean that the energy only exists in 'cupfuls'.

Sorry to use such a childish analogy - feel free to shoot me down.

 

[DrGreg]

If you take a cup of water from the ocean, you have a cup of water and an ocean. When you pour the water back into the ocean, does the 'cup of water' still exist ?

When the EM field interacts with matter, the energy is exchanged in 'cupfuls'. It does not mean that the energy only exists in 'cupfuls'.

Sorry to use such a childish analogy - feel free to shoot me down.

So are you saying that if you have a stream of photons being emitted, once they are in flight you can't distinguish one from another and all you have is a composite wavefunction representing "all the photons"?

If you emit a stream of 10 photons, does that not imply that 10 photons will be later absorbed somewhere else (assuming no energy-momentum escapes to infinity)?

 

[lightarrow]

If you emit a stream of 10 photons, does that not imply that 10 photons will be later absorbed somewhere else (assuming no energy-momentum escapes to infinity)?

Yes, infact you haven't stated anything about what photons do in between (or if they are somwhere or if they exist at all).

 

[Jonathan Scott]

So are you saying that if you have a stream of photons being emitted, once they are in flight you can't distinguish one from another and all you have is a composite wavefunction representing "all the photons"?

If you emit a stream of 10 photons, does that not imply that 10 photons will be later absorbed somewhere else (assuming no energy-momentum escapes to infinity)?

I wouldn't go quite as far as the "cupful" analogy, in that in most cases the energy concentrations in the field caused by photons entering it are sufficiently distinct that they effectively represent a single photon in flight, and information can be passed in the polarization of a single photon.

However, when interference or combination effects are involved I think that the photons being received do not necessarily correspond 1-1 with those that were emitted, even though the overall quantity may be the same. I personally find this rather weird, as mathematically electromagnetic waves at normal energies basically pass through one another without interacting, yet interference effects can cause the flow of energy to be diverted into different directions!

This is only what I recall learning about the subject, and it may possibly be inaccurate or outdated. I am not an expert on photons (nor on QED), and the Quantum Mechanics forum may be a better place to find one.

 

[Mentz114]

So are you saying that if you have a stream of photons being emitted, once they are in flight you can't distinguish one from another and all you have is a composite wavefunction representing "all the photons"?

Yes. To describe the collection of 10 photons, we need only Maxwell's equations, I would think.

If you emit a stream of 10 photons, does that not imply that 10 photons will be later absorbed somewhere else (assuming no energy-momentum escapes to infinity)?

As you say, conservation of energy and momentum would demand it. Is it possible to do an experiment to decide if the quanta really are like particles ? Something like creating a field of ten photons in a high-Q cavity and then putting suitably tuned atoms or ions in the cavity at various places as detectors. Would a particle-photon model make different predictions to the non-local EM field ?

[aside]
My cups of water analogy breaks down rather badly because photons have a fixed fequency and so a collection of identical photons can only be absorbed by a suitably tuned detector.

Water is not quantised and will go in any size receptacle.

 

[lightarrow]

I wouldn't go quite as far as the "cupful" analogy, in that in most cases the energy concentrations in the field caused by photons entering it are sufficiently distinct that they effectively represent a single photon in flight, and information can be passed in the polarization of a single photon.

I have put in color the interesting phrase.
To say "polarization of a single photon" you have to refer to a specific class of QM interpretations: that of hidden variables.

 

[Primordial]

I like the ideas being addressed here, and would like to see more especially near the junction where Planck time is a peorid for one complete EM cycle, and how it would interact with an inertial reference system that would increase it's relativistic energy past the Planck max.

 

[Jonathan Scott]

I have put in color the interesting phrase.
To say "polarization of a single photon" you have to refer to a specific class of QM interpretations: that of hidden variables.

I disagree; pure states of polarization can definitely be used to transmit information. QM interpretations only become relevant when dealing with entangled superposed states.

 

[lightarrow]

I disagree; pure states of polarization can definitely be used to transmit information. QM interpretations only become relevant when dealing with entangled superposed states.

Ok, but to have pure states you have to prepare the photons, so the property "to have a well defined spin component in one direction" cannot be referred to the microscopic object "photon" only, but to the entire experimental setting.

 

[DrGreg]

I have created a new thread here in the Quantum Physics forum of this site to ask further about whether photons "exist" between emission and absorbtion. I suggest further discussion of quantum mechanical aspects should continue in that thread, and we keep this thread for discussion of any outstanding relativistic issues.