Photon Duration & Wavelength: Exploring the Mysteries

[Averyuniquename]

Perhaps this a question that make no sense or has no answer but...
I understand an EM wave is not like the carrier of an AM station but rather similar to a Keyed Morse Code transmission. In other words bust of trains ow waves
If my description is correct
How long the burst last?
How many wavelenghts it contains?
I know the photon energy is determined by the frequency but there are different solutions.
Is the energy determined by the length of duration, or by the amplitude of the EM field?
What is the order of magnitude of the EM field amplitude?

 

[Nugatory]

I understand an EM wave is not like the carrier of an AM station but rather similar to a Keyed Morse Code transmission. In other words bust of trains ow waves

You're right, the question "How long is a photon?" doesn't make sense. But neither does this part - radio transmissions like the carrier of an AM station are EM waves.

If my description is correct
How long the burst last?
How many wavelenghts it contains?
I know the photon energy is determined by the frequency but there are different solutions.
Is the energy determined by the length of duration, or by the amplitude of the EM field?
What is the order of magnitude of the EM field amplitude?

The behavior of the EM field in a wave is described by Maxwell's equations from classical physics; photons aren't involved and introducing them only adds confusion. The length of a burst of EM radiation, the frequencies in it, and the amplitude of the EM field all depend on the source that's generating the wave.

 

[sophiecentaur]

You have asked "that question" that causes people most trouble when they want photons to be 'familiar' objects. Photons are not like that. They are Quantum Particles and not little bullets (or even long ones). They have no Extent and you could say they only exist whilst they are interacting at each end. The Energy is carried by a wave. You just can't think classically; you don't need to. The 'length of time' i think has to depend on which particular system they interact with - like the bandwidth and rise times of a conventional receiver. But that's much too classical really.
And welcome to p f btw.

 

[Averyuniquename]

Thanks for your answers
I wrote "I understand an EM wave is not like the carrier of..." Sorry but that is not what I meant to write and confused people.
I wanted to write Photon instead of EM Wave.
If an atom emit a photon something must be able to be said about.
Is a Photon a EM wave? We talk about wave length and frequency. Can we say a Photon has a polarization? Does it interact instantly or take a finite time to that?

 

[George Jones]

What quantity is used to characterize a photon?

 

[Averyuniquename]

What quantity is used to characterize a photon?

Energy and frequency or are you implying there are no characteristics ascribed to it, other than speed?

 

[George Jones]

Is energy related to (spatial) momentum?

 

[Averyuniquename]

I understand you think you are posting stimulating questions but you are not helping. If I posted a inappropriate question, like what gender are angels, then stir me in a direction that leads to better understanding.

 

[Nugatory]

If you're trying to ask what a photon is, you could give this thread a try: https://www.physicsforums.com/threads/what-is-a-photon.879128/#post-5522356

And yes... "How long is a photon?" is indeed like asking about the gender of angels.

Often when people first hear that photons are "particles" they form a mental model that light is made up of photons the way a flowing river is made up of water molecules, that there's a stream of photons moving from the light source to whatever is illuminated by that light. That mental model feels natural, but it's hopelessly misleading. A better (but still imperfect - there's no substitute for a proper quantum electrodynamics textbook) model says that photons appear when electromagnetic radiation interacts with matter.

Feynman's "QED: The strange theory of light and matter" is the only layman-friendly equation-free description of QED that I know of. A key point in Feynman's treatment is the that the theory is not about photons moving through space from one point to another, but about the probability of a photon being detected (that is, it delivers a quantum of energy to something) at a given point in space.

 

[Khashishi]

Is energy related to (spatial) momentum?

Yes, for a photon in vacuum, E = cp
Photons also have angular momentum.

 

[Averyuniquename]

If you're trying to ask what a photon is, you could give this thread a try: https://www.physicsforums.com/threads/what-is-a-photon.879128/#post-5522356

And yes... "How long is a photon?" is indeed like asking about the gender of angels.

Often when people first hear that photons are "particles" they form a mental model that light is made up of photons the way a flowing river is made up of water molecules, that there's a stream of photons moving from the light source to whatever is illuminated by that light. That mental model feels natural, but it's hopelessly misleading. A better (but still imperfect - there's no substitute for a proper quantum electrodynamics textbook) model says that photons appear when electromagnetic radiation interacts with matter.

Feynman's "QED: The strange theory of light and matter" is the only layman-friendly equation-free description of QED that I know of. A key point in Feynman's treatment is the that the theory is not about photons moving through space from one point to another, but about the probability of a photon being detected (that is, it delivers a quantum of energy to something) at a given point in space.

If you're trying to ask what a photon is, you could give this thread a try: https://www.physicsforums.com/threads/what-is-a-photon.879128/#post-5522356

And yes... "How long is a photon?" is indeed like asking about the gender of angels.

Often when people first hear that photons are "particles" they form a mental model that light is made up of photons the way a flowing river is made up of water molecules, that there's a stream of photons moving from the light source to whatever is illuminated by that light. That mental model feels natural, but it's hopelessly misleading. A better (but still imperfect - there's no substitute for a proper quantum electrodynamics textbook) model says that photons appear when electromagnetic radiation interacts with matter.

Feynman's "QED: The strange theory of light and matter" is the only layman-friendly equation-free description of QED that I know of. A key point in Feynman's treatment is the that the theory is not about photons moving through space from one point to another, but about the probability of a photon being detected (that is, it delivers a quantum of energy to something) at a given point in space.

Thanks, that was helpful.

 

[George Jones]

Yes, for a photon in vacuum, E = cp

I was trying to use the Socratic method to illustrate that photons are characterized by precise values of momenta, and thus cannot be characterized by positions along something like a classical trajectory.

 

[Averyuniquename]

I was trying to use the Socratic method to illustrate that photons are characterized by precise values of momenta, and thus cannot be characterized by positions along something like a classical trajectory.

OK thanks. So since the momentum is known the position uncertainty is infinite, therefore it can be anywhere or nowhere. It make no sense to confine it in space.

 

[George Jones]

OK thanks. So since the momentum is known the position uncertainty is infinite, therefore it can be anywhere or nowhere. It make no sense to confine it in space.

Yes, this is what I was trying to at, but I suspect that his is just hand-waving. I sometimes like hand-waving arguments, but I also like what Griffiths wrote in his elementary particles book,"When you hear a physicist invoke the uncertainty principle, keep a hand on your wallet."

At a substantially more technical level, there isn't a position operator for photons.

 

[ZapperZ]

The problem here is that this is all about the "position" operator, and not a "size" operator. There is no "size" operator here that can immediately spits out a value upon measurement.

An off-shoot of what George Jones did here is to ask you for ALL the properties and characteristics of "a photon". We can define its momentum, it's wavelength, etc... etc... But nowhere in the complete list of characteristics here is there anything resembling a "size" or volume. A photon is never defined as having such a thing.

So yes, when you ask for a length or a size, you ARE asking about the gender of angels, because a photon is never defined that way.

Zz.

 

[sophiecentaur]

I think of it this way: if a photon were generated by one particular source (a particular atom with a particular transition) then everything about that transition would need to be the same for another different atom with a different electron arrangement or the photon could be 'too long or too short' to be absorbed. But EM energy is transferred perfectly well from one system to another.
So Photons have to be identical and with a few characteristics as possible - i.e. their energy and nothing more.

 

[ZapperZ]

Perhaps this a question that make no sense or has no answer but...
I understand an EM wave is not like the carrier of an AM station but rather similar to a Keyed Morse Code transmission. In other words bust of trains ow waves
If my description is correct
How long the burst last?
How many wavelenghts it contains?
I know the photon energy is determined by the frequency but there are different solutions.
Is the energy determined by the length of duration, or by the amplitude of the EM field?
What is the order of magnitude of the EM field amplitude?

The strange thing about this whole thread, and the reason why I didn't join in the "fun" from the very beginning is that, the original post has a rather significant misunderstanding. If you read the post carefully, the part where the OP mentioned "bust (sic) of trains ow (sic) waves" and "How long the burst last", this is NOT the "size of a photon", but rather the pulse length of the EM wave!

This is what I found rather odd, because radio and TV transmission is cw, not in pulse mode. The beginning premise, at least according to me, was completely false, and it made the rest of the discussion rather strange.

Zz.

 

[sophiecentaur]

radio and TV transmission is cw, not in pulse mode.

There are plenty of 'pulses' in TV signals and even in some Audio programmes. But the times of those pulses are longer than 1ns.

I do agree that the OP was very imprecise in the way the question was formed, though. The idea that a received CW signal consists of string of photons (once it hits the receiver, that is) is not too ridiculous.

 

[DrStupid]

They have no Extent and you could say they only exist whilst they are interacting at each end.

How do you explain the interference pattern in the double-slit experiment with single photons without extend?

 

[sophiecentaur]

How do you explain the interference pattern in the double-slit experiment with single photons without extend?

No defined extent. Where are you trying to take this?

 

[DrStupid]

No defined extent.

That's something different than "no Extent".

Where are you trying to take this?

Your answer shows that you already got it.

 

[sophiecentaur]

Your answer shows that you already got it.