What are photons?

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Are photons the things which are constantly emitted from electric charges in all directions, which then interact with other electric charges, or are photons something else?

If photons are constantly emitted from electric charges in all directions, and photons are particles, as the ring of photons spreads out, wouldn't there be gaps in the ring of photons?
 

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  • #2
Drakkith
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Photons are not particles in any sense of the normal use of the word. Instead, they are little packets of energy that an EM wave interacts with matter via. This just means that when an EM wave (such as light, infrared radiation, radio waves, etc) interacts with matter, it doesn't give up energy continuously as we might expect, but does so in 'bursts'. The amount of energy in each photon depends on the wavelength of the EM wave. Shorter wavelength EM waves put more energy into each photon. For example, a photon of an EM wave with a wavelength of 700 nm (corresponding to red light) has half the energy that a photon of an EM wave with a wavelength of 350 nm (corresponding to the extreme violet end of the spectrum) has.

Photons are not constantly emitted from electrical charges. You've probably heard of this as a result of the popularization of 'virtual particles'. Virtual particles are a way of describing the interaction between objects in Quantum Field Theory (QFT). They are nothing like what you're thinking and bear little actual resemblance to what most 'pop-science' shows, books, and articles have portrayed them as. They certainly aren't emitted from electrically charged objects like you're imagining. I wouldn't worry about them.
 
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  • #3
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Well, electric field is constantly emitted from charges in all directions right? I was thinking that photons were the same thing as electric field.
 
  • #4
Drakkith
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Well, electric field is constantly emitted from charges in all directions right? I was thinking that photons were the same thing as electric field.

We say the electric field is 'emitted' from the charge, but there isn't anything actually moving. A 'field' is a way of describing how things interact. For an electric field, we can point to any location in space and label it with values for the direction and magnitude of the field. The direction of the field is simply the direction a charged particle would be accelerated in if we placed it at that point, and the magnitude is how strong the force is. It happens to be a fact that the closer you are to an electrically charged particle, the stronger the field is. That's why we say that the field is 'emitted' from the charge.
 
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We say the electric field is 'emitted' from the charge, but there isn't anything actually moving.
But electric fields are emitted, and do actually move through space at the speed of light. If electric fields did not travel through space then information transfer would be instantaneous, but it is not.
 
  • #6
Dale
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You are thinking of electromagnetic waves. The fields do not move in any meaningful sense, but changes in the EM field do propagate and carry information and energy.
 
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The fields do not move in any meaningful sense, but changes in the EM field do propagate and carry information and energy.
So you're saying that EM field exists independent of charge? I thought EM field is emitted by charge. I thought that you can't have EM field without charge.
 
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Drakkith
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So you're saying that EM field exists independent of charge? I thought EM field is emitted by charge. I thought that you can't have EM field without charge.

In reality we can never encounter a situation where there are charges without a field, or a field without charges. Saying that the field is 'emitted' by the charges is mostly because of how the initial EM theory was developed back in the 1800's. It's one of those things that just sticks around. Usually it helps people understand and visualize what's going on, but not always.
 
  • #9
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Light is an oscillating EM field, and light travels through space.

Are you saying that EM field is a dimension like space, it exists everywhere, and light is just a disturbance in the EM field, the same way that gravity is a disturbance of space?
 
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  • #10
sophiecentaur
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Light is an oscillating EM field, and light travels through space.

Are you saying that EM field is a dimension like space, it exists everywhere, and light is just a disturbance in the EM field, the same way that gravity is a disturbance of space?
You appear to be having a problem with an apparent conflict between EM waves travelling and EM Fields not travelling (?). Take an analogy with waves on a lake. The water doesn't travel about the lake but the surface waves (variations of height and displacement ) do travel. If there is a change in water level (=Field), this step change will take time to propagate over the surface but then, no more wave until you introduce another change. There is pretty much a direct correspondence with the EM situation.
 
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  • #11
Dale
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So you're saying that EM field exists independent of charge?
I never said such a thing. What I would say is that there are solutions to Maxwell's equations which describe EM fields without any charge or current. These are called vacuum solutions and include plane waves.

I thought EM field is emitted by charge. I thought that you can't have EM field without charge.
There are vacuum solutions to Maxwell's equations in classical EM and there are photon-only Fock states in QED. In both theories you can have EM fields without charge.
 
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A Photon is basically a particle that represents a quantum of light or electromagnetic radiation.
 
  • #13
Drakkith
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Are you saying that EM field is a dimension like space, it exists everywhere, and light is just a disturbance in the EM field, the same way that gravity is a disturbance of space?

No, I don't know how you got that out of anything that's been said in the thread.
 
  • #14
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No, I don't know how you got that out of anything that's been said in the thread.
Yes, it reminds me of this exchange between the troll and the king and queen.

 
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  • #15
sophiecentaur
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So you're saying that EM field exists independent of charge? I thought EM field is emitted by charge. I thought that you can't have EM field without charge.
Don't forget that the 'originating, accelerating charge' could be hundreds of millions of light years away from the oscillating fields that you observe as they arrive at Earth. How relevant is the connection that you say, exists if the wave was generated a short time after the big bang?
 
  • #16
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No, I don't know how you got that out of anything that's been said in the thread.
You said that EM field does not travel, only changes in EM field travel.

Then sophiecentaur said that EM waves travel through space the same way that water waves travel through water.

Okay, I understand how water waves propagate through water. A wave needs a medium to propagate through. What medium does EM wave propagate through? The only way I can make sense of this information is if EM field exists everywhere independent of charge, not created or emitted by charge, and EM waves propagate through the EM field, the same as water waves propagate through water.

To clarify, I don't believe the above paragraph is correct, the above paragraph is me trying to make sense of the information that EM field doesn't travel through space.

I think that EM field does travel through space, and is constantly emitted by electric charge.
 
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  • #17
Dale
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A wave needs a medium to propagate through. What medium does EM wave propagate through? The only way I can make sense of this information is if EM field exists everywhere independent of charge, not created or emitted by charge, and EM waves propagate through the EM field, the same as water waves propagate through water.
Waves don't need a medium, but otherwise this is essentially correct. The only caveat is that the value of the field can be 0 in some locations.
 
  • #18
davenn
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Waves don't need a medium.......,

did you really mean to say EM waves don't need a medium ?
 
  • #19
sophiecentaur
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Then sophiecentaur said that EM waves travel through space the same way that water waves travel through water.
That was, as I stated, an analogy and nothing to do with the existence or not of a medium. It is the disturbance (energy) that moves from place to place and nothing else.
did you really mean to say EM waves don't need a medium ?
What's wrong with that? I thought the existence of an aether was not part of current Physics.
 
  • #20
davenn
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What's wrong with that? I thought the existence of an aether was not part of current Physics.

you misunderstand ... no one is talking about the existence of aether ... no need to bring it up ;)

well waves in water were also being spoken of and in that has a medium
the way dalespam worded it may have confused the OP even more since he was already having problems with the basic concepts ;)
I just wanted it clarified for the OP's sake

Dave
 
  • #21
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That was, as I stated, an analogy and nothing to do with the existence or not of a medium. It is the disturbance (energy) that moves from place to place and nothing else.
Imagine a vacuum with one electron in it. Now imagine a second electron magically pops into existence some distance from the first electron. Are the electrons instantaneously repelled from each other, or does the electric field have to travel through space at the speed of light?

Pretend that both electrons are at absolute zero, there are no EM waves, only non-oscillating electric field.
 
  • #22
sophiecentaur
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We are singing from the same hymn sheet then. :)
 
  • #23
Dale
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Now imagine a second electron magically pops into existence
We cannot use science to answer a question whose premise is based on magic, especially magic which is designed to violate a key law like the conservation of charge.
 
  • #24
Drakkith
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Imagine a vacuum with one electron in it. Now imagine a second electron magically pops into existence some distance from the first electron.

We cannot use science to answer a question whose premise is based on magic, especially magic which is designed to violate a key law like the conservation of charge.

To elaborate on Dalespam's post, were aren't just giving you a hard time about this. We don't live in a universe where charges and fields have existed separately or where we can create charges from nothing, violating several conservation laws, so we cannot trust that what we think would happen is actually accurate.
 
  • #25
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we cannot trust that what we think would happen is actually accurate.
Yes we can because what you think will happen is informed by experiments backed up by evidence.
 
  • #26
Drakkith
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Yes we can because what you think will happen is informed by experiments backed up by evidence.

Not in a case of magically appearing electrons it's not.
 
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Imagine a vacuum with one electron in it. Now imagine a second electron magically pops into existence some distance from the first electron. Are the electrons instantaneously repelled from each other, or does the electric field have to travel through space at the speed of light?

Pretend that both electrons are at absolute zero, there are no EM waves, only non-oscillating electric field.

Instead of coming up with an impossible situation, you could consider a realistic situation. You have two electrons and one is moved from one position to another. This is a similar question to what you posted, but it still respects the continuity equation of EM fields. In this case the disturbance in the field will propagate at the speed of light (my recollection is that there are some unexpected things that can happen under particular constraints, but E&M wasn't my strongest area of physics). The field is always there, but the magnitudes and directions of the field change in time.

It might be better to discuss something like this that doesn't break any physical laws, but still gets at some of your questions.
 
  • #28
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Instead of coming up with an impossible situation, you could consider a realistic situation. You have two electrons and one is moved from one position to another. This is a similar question to what you posted, but it still respects the continuity equation of EM fields. In this case the disturbance in the field will propagate at the speed of light (my recollection is that there are some unexpected things that can happen under particular constraints, but E&M wasn't my strongest area of physics). The field is always there, but the magnitudes and directions of the field change in time.

It might be better to discuss something like this that doesn't break any physical laws, but still gets at some of your questions.
Yeah I was trying to think of a better situation, but i got distracted and did something else.

Your situation works. Imagine two electrons in a vacuum, they are being repelled by each other, then one electron is moved to a new position. Is the other electron instantly repelled away from the new position? The answer is no it isn't, because the EM field has to travel through space at the speed of light. It isn't EM waves which are traveling, it's the EM field itself.
 
  • #29
Dale
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Again, the field does not have a velocity.

Look at Maxwell's equations. No velocity.
 
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Again, the field does not have a velocity.

Look at Maxwell's equations. No velocity.
Then why are the electrons not instantly repelled?
 
  • #31
Drakkith
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Then why are the electrons not instantly repelled?

What do you mean 'instantly'? They've always been in an electric field. As they approach each other the force between them increases because of the electric field between them. There is nothing moving here except the particles.
 
  • #32
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What do you mean 'instantly'? They've always been in an electric field. As they approach each other the force between them increases because of the electric field between them. There is nothing moving here except the particles.
Two electrons in a vacuum, electron 1 and electron 2. They are repelling each other. Electron 1 is moved to a new position. Is electron 2 instantly repelled from the new position of electron 1? Or does it take some time before electron 2 is repelled from the new position?
 
  • #33
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X0gfP.png

In this picture it takes some time for the EM field to affect things from the new position. The EM field has to travel through space.
 
  • #34
Drakkith
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In this picture it takes some time for the EM field to affect things from the new position. The EM field has to travel through space.

If I'm understanding that diagram right, the charge was moving to the left and was then accelerated to the right. The change in the field is an EM wave and moves at c.
 
  • #35
Dale
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Then why are the electrons not instantly repelled?
Because Maxwell's equations say that they won't. All with out attributing velocity run the speed.
 

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