Why do we say photons travel ?

[mkbh_10]

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 )

 

[Astronuc]

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.

 

[DaveC426913]

...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...

 

[russ_watters]

... 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.

 

[Shackleford]

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.

 

[Loren Booda]

Is the vacuum not a medium?

 

[DaveC426913]

Is the vacuum not a medium?

No. A medium is, by definiton, a substance. A vacuum, by definition, is not a substance.

 

[Shackleford]

No. A medium is, by definiton, a substance. A vacuum, by definition, is not a substance.

Right. In absence of a medium, there could be a vacuum.

 

[Danger]

Actually, in the absence of a medium, there has to be a vacuum. Those are pretty much the only available options.

 

[lightarrow]

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?

 

[Astronuc]

The electromagnetic wave IS the photon.

 

[mkbh_10]

but isn't the em wave already present everywhere in UNIVERSE

 

[Shackleford]

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.

 

[DaveC426913]

but isn't the em wave already present everywhere in UNIVERSE

What makes you say this?

 

[kwestion]

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)

 

[kwestion]

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?

 

[Nickelodeon]

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.

 

[lightarrow]

The electromagnetic wave IS the photon.

I wish it was so simple...

 

[lightarrow]

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 wavelenght 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.

 

[DaveC426913]

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 wavelenght 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?

 

[Mentz114]

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.

 

[kwestion]

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:

[...] In the absence of interactions, there are no photons.[...]

[...] 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.

 

[mkbh_10]

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)

 

[lightarrow]

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).

 

[lightarrow]

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?

 

[mkbh_10]

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

 

[DaveC426913]

If transmitter and receiver are 100 meters apart does it change anything in the reasoning?

I have no idea. I was merely asking.

 

[kwestion]

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.

[...] 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.

 

[Mentz114]

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.

 

[edziura]

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.

 

[Loren Booda]

It never exhibits its wave and particle natures simultaneously.

Good point. I had never fully considered that. I guess it depends in part on what is meant by "exhibits."

Can one realize a photon's pure wave nature from its nonsimultaneous and pure particle nature?

Is a wave packet a single photon imaginary in part, or perhaps always a multiplicity of photons?

Might wave packets interact with other wave packets?

 

[axawire]

... if you try to observe the wave nature of a photon and then try to observe its particle nature... how do you know you are still looking at the same photon? don't they all look pretty similar...

I would say science is about measurement, all we can know is what we measured in a particular interaction and we can only try to guess (educated) what happens between measured interactions...

 

[Mentz114]

Loren Booda asks,

Is a wave packet a single photon imaginary in part, or perhaps always a multiplicity of photons?

I'm not sure what you mean by 'imaginary in part'.

It might seem intuitive that you can produce a photon by chopping a light beam into a very short pulse of light. The result will have a spread of frequencies ( Fourier modes ) and looks mathematically like a wave-packet, a wave packet being a superposition of many close frequencies. It is also localizable like a particle. But it is controversial to refer to that as a photon.

Single photon sources are notoriously difficult to make, and I haven't seen completely convincing experimental results, but I may not be up-to-date with the technology.

axawire, I agree with your view that we can only guess what goes on between obervations.

 

[Loren Booda]

Is a wave packet a single photon imaginary in part...

In other words, can't the photon before measurement be represented as a vector oscillating in complex space? A Fourier transform, such as accompanies a measurement, might convert a wave packet into a photon, or a wave into a wave packet.

Single photon scintillations have been observed for decades.

 

[kwestion]

Branching this question

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).

I think it might be beneficial to mkbh_10's question if there is agreement on terminology. I've posted this question as "When is a photon a photon"