Why do we say photons travel ?

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    Photons Travel
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Discussion Overview

The discussion revolves around the nature of photons and the concept of their travel. Participants explore whether photons can be considered to travel in the traditional sense, the implications of their properties, and the relationship between electromagnetic waves and photons. The scope includes theoretical considerations, conceptual clarifications, and some speculative ideas.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants argue that photons, while having zero rest mass, are not material particles and do indeed travel from their source to points of interaction.
  • Others propose that instead of saying photons travel, it might be more accurate to say that energy is communicated, suggesting that nothing physically travels through space.
  • A participant questions the idea of a medium, asserting that a vacuum is not a medium, while another participant affirms that electromagnetic waves do not require a medium for propagation.
  • There is a suggestion that photons may not travel at all, but rather that electromagnetic waves are generated at the source and detected at another point, raising questions about the nature of photons and their existence in the absence of interactions.
  • Some participants discuss the relationship between electromagnetic waves and photons, with one stating that the electromagnetic wave is the photon, while another expresses confusion over this assertion in light of conflicting information from literature.
  • There is a discussion about the peculiarities of photon behavior, including their existence in empty space and the conditions under which they can be said to exist or not exist.
  • One participant raises a hypothetical scenario regarding the behavior of photons in relation to radio signals and their wavelength, questioning how photons could be said to "die" at the receiver.
  • Another point raised is that light exhibits particle-like properties only when interacting with matter, suggesting that photons may not exist independently without such interactions.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of photons and their travel. There is no consensus on whether photons travel in the traditional sense or if they exist independently of interactions.

Contextual Notes

Participants highlight limitations in understanding the nature of photons, including the dependence on definitions of medium and the conditions under which photons can be said to exist. The discussion also reflects unresolved questions about the implications of electromagnetic waves and their relationship to photons.

Who May Find This Useful

This discussion may be of interest to those exploring the fundamental nature of light, the behavior of photons, and the theoretical implications of electromagnetic theory in physics.

mkbh_10
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We consider light is made up of material particles (photon), and we know photons travel at speed of light having zero rest mass so we say photons travel but how can something travel at this speed , we say electric forces are experienced by a test charge due to some other charge , the field spreads out at speed of light but it should be that nothing travels or spreads out as the information is already present there , it just gets changed/communicated to that charge , instead of saying photons travel the energy is communicated & nothing travels , energy gets communicated from one atom to other , we cannot see in absolute darkness as there is no energy to be communicated


It should be that energy is communicated as there is change in energy when light is there but it is already present in darkness too (we can't see anything as there is no change in energy )
 
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We don't consider photons as 'material', which would imply mass/matter. Photons do travel from their origin to some point of interaction with matter, e.g. electron usually or if high enough energy, a nucleus.

Consider that light travels to us from stars which are thousands to millions to billions of lightyears from us.
 
mkbh_10 said:
...it should be that nothing travels or spreads out as the information is already present there , it just gets changed/communicated to that charge..
I don't understand the above statement, but:

Photons can be blocked, redirected and intercepted. Photons can even impart their momentum upon a massive object.

If it walks like a duck...
 
mkbh_10 said:
... the field spreads out at speed of light but it should be that nothing travels or spreads out as the information is already present there , it just gets changed/communicated to that charge , instead of saying photons travel the energy is communicated & nothing travels
Even when light was thought to simply be some sort of wave traveling on a medium, it was still correct to say it "travels", just like any other wave travels despite the media having no net motion. That's what you are describing. Clearly, information (not to mention energy) is transmitted from one place to another.
 
A photon is an electromagnetic wave as opposed to an elastic wave, such as a sound wave which propagates through some medium. Electromagnetic waves do not require a medium in which to propagate.
 
Is the vacuum not a medium?
 
Loren Booda said:
Is the vacuum not a medium?
No. A medium is, by definition, a substance. A vacuum, by definition, is not a substance.
 
DaveC426913 said:
No. A medium is, by definition, a substance. A vacuum, by definition, is not a substance.

Right. In absence of a medium, there could be a vacuum.
 
Actually, in the absence of a medium, there has to be a vacuum. Those are pretty much the only available options.
 
  • #10
The OP question is quite stimulating. What if photons don't travel at all, but we have a photon generated at the source, an electromagnetic wave traveling from source to detector and another photon generated at the detector?
 
  • #11
The electromagnetic wave IS the photon.
 
  • #12
but isn't the em wave already present everywhere in UNIVERSE
 
  • #13
Danger said:
Actually, in the absence of a medium, there has to be a vacuum. Those are pretty much the only available options.

That's what I was thinking, but I wasn't sure.
 
  • #14
mkbh_10 said:
but isn't the em wave already present everywhere in UNIVERSE
What makes you say this?
 
  • #15
lightarrow said:
The OP question is quite stimulating. What if photons don't travel at all, but we have a photon generated at the source, an electromagnetic wave traveling from source to detector and another photon generated at the detector?

I like it. It should be explored. It sounds like it was. These types of semi-classical theories were viable until the 1970's until photon-correlation experiments definitively proved that the quantization was a property of the light itself (taken from wikipedia article on photon)
 
  • #16
Astronuc said:
The electromagnetic wave IS the photon.

Hello, I'm trying to reconcile this statement with an article that I read stating that "[...]experiments confirm that the photon is not a short pulse of electromagnetic radiation[...]" and also that "[...]According to our present understanding, the electromagnetic field itself is produced by photons[...]" (ephasis added)

Can you help clarify. I don't mean to hijack this thread. Maybe it should be a separate post?
 
  • #17
Light is a product of oscillatory energy with associated peaks and troughs just like any other wave energy. The photon is a momentary manifestation as the wave energy accelerates to the peak, the photon then disappears then reappears as the wave energy accelerates to the trough and so on. No spatial movement of the photon - apart from which it doesn't live long enough.
 
  • #18
Astronuc said:
The electromagnetic wave IS the photon.
I wish it was so simple...
 
  • #19
kwestion said:
I like it. It should be explored. It sounds like it was. These types of semi-classical theories were viable until the 1970's until photon-correlation experiments definitively proved that the quantization was a property of the light itself (taken from wikipedia article on photon)
It could be, I don't know much about those experiments. However I wonder what could be the meaning of "photons as traveling corpuscles" in the following situation: a radio transmitter is switched on at 300 kHz; 1 meter away a receiver gets the signal. The photon has a wavelength of 1 km so it is "born" in the transmitter and "dies" in the receiver much before having extended on *one* single wave? It's very strange.
 
  • #20
lightarrow said:
It could be, I don't know much about those experiments. However I wonder what could be the meaning of "photons as traveling corpuscles" in the following situation: a radio transmitter is switched on at 300 kHz; 1 meter away a receiver gets the signal. The photon has a wavelength of 1 km so it is "born" in the transmitter and "dies" in the receiver much before having extended on *one* single wave? It's very strange.
I question whether this actually happens. If you get too close to the transmitter don't weird things happen to the reception?
 
  • #21
Everything points to the EM field being a potential to produce photons. The only time light shows particle like properties is when it interacts with matter. This always happens in quanta, which must be localised at the absorber/emitter, and also exchange momentum with the matter. In the absence of interactions, there are no photons.

Of course this may be seen as paradoxical because we can only detect a field by interacting with it, in which case it seems like a cloud of quanta.

In modern cavity experiments with ultracool atoms or ions, it is possible to inject a small amount of EM energy into a high-Q ( monochromatic) cavity, so that we could say there is 1 photon in the cavity. If we had two tuned atoms in the cavity there is a finite chance that they will both begin to absorb the 'photon', unless the field is non-local.

Photons are certainly very weird.
 
  • #22
I'm reading that photons don't decay spontaneously in empty space. I think there is an implication that photons do exist in empty space without interacting with matter.

I'm not trying to be a smart alec, but I hope you appreciate an apparent discrepency in these statements:
Mentz114 said:
[...] In the absence of interactions, there are no photons.[...]
Mentz114 said:
[...] we could say there is 1 photon in the cavity[...]

The first statement would seems to prohibit something from being called a photon inside an empty cavity.
 
  • #23
but shouldn't photons be already present everywhere in the universe , how it is that they travel , which will mean that they were not present in a place before but now they are present & after the event has happened (light switched off) they again vanish (but where)
 
  • #24
Can a photon exist in absence of interaction? Perhaps we should start a new thread about it. (My answer is no, but I'm open to every possibility).
 
  • #25
DaveC426913 said:
I question whether this actually happens. If you get too close to the transmitter don't weird things happen to the reception?
If transmitter and receiver are 100 meters apart does it change anything in the reasoning?
 
  • #26
lightarrow said:
Can a photon exist in absence of interaction? Perhaps we should start a new thread about it. (My answer is no, but I'm open to every possibility).

If they are not present in absence of interaction then where do they go after the interaction , suppose a light beam travels in space , we say it is made up of photons , so when this light beam is switched off where do the photons go , do the photons that came out of the source keep traveling towards infinity .



Now photoelectric effect must stop at a certain point ie, no extra electrons are available on surface or at a layer below the surface to get out of metal , in photoelectric effect , if a light beam is kept on even when the photoelectric effect has stopped the photons should travel towards the metal and & generate heat as no extra electrons are available to absorb photon energy & get out of metal , that means there is an aacumulation of photons which if continued will build up more & more energy which means they will certainly not travel at speed of light & loose their energy to convert into heat which is a contradiction , am i going somewhere on this part
 
  • #27
lightarrow said:
If transmitter and receiver are 100 meters apart does it change anything in the reasoning?
I have no idea. I was merely asking.
 
  • #28
mkbh_10 said:
do the photons that came out of the source keep traveling towards infinity

I think that is what PF_Mentor was getting at when asking us to think about light that has been traveling for millions or billions of years from distant stars.

mkbh_10 said:
[...] that means there is an aacumulation of photons which if continued will build up more & more energy which means they will certainly not travel at speed of light & loose their energy to convert into heat[...]

At this point, the photons can either be reflected or their energy can be completely absorbed by one of the particles after which the heat energy can be re-radiated as a photon. Accumulation seems like a possible misstatement? I did read that it could take a million years for light to travel from the core of the sun to the edge because of so many collisions, but I don't think that's exactly the topic.
 
  • #29
Some posters above have asked "what happens to photons in empty space".

Light is a transverse wave of electric and magnetic fields, satisfying the source free Maxwell equations. This wave carries energy, translational momentum and angular momentum.

When the wave interacts with matter, these properties become localised and a quantum of energy is lost or gained from the field. The matter may gain/lose energy and/or momentum.

It may therefore not be meaningful to talk of photons in free space.

This is only my opinion.
 
  • #30
Ever heard of wave-particle duality? When electromagnetic energy travels through "empty" space (you might find a book called "The Void" interesting), it behaves as if it were a wave. When it interacts with matter, it behaves as if it were a particle, referred to as a photon. It never exhibits its wave and particle natures simultaneously.
 

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