Radio waves propagate spherically, how comes the photon moves in a straight line

[studiot]

I'm not pretending anything.

I was talking more in general, maybe I should have used "one" instead.

Talking about radiowaves like I and I suspect most of us here think about them (classical solutions to Maxwell equations) as part of the wave/particle duality is a bit too archaic. The modern duality is inside QM entirely, so adding the fully classical view makes it more like a triality.

Fair enough. ‘One’ suggests you understand that none of our models are complete and that we should choose the most appropriate one for our purposes

I wonder if the problem here is the fact that RoGeorge has juxtaposed classical EM (in the spherical radio wave) and some particulate model (in the photon), perhaps without fully realising the implications of this.
That seems to have been like a red rag to a bull for the quantum fraternity who seem reluctant to let go and acknowledge that the answer has nothing to do with QM.

Quantum mechanics does not arise for this type of wave system, although it obeys resonance curves and requirements.

That is not correct. QM still applies to a water-wave system, quantum effects are simply utterly negligible at the macroscopic scale, so classical fluid mechanics is an extremely good approximation. The same applies to electromagnetic waves - a macroscopic EM wave can be described extremely well by classical Maxwell equations, while the underlying field also has a quantum description.

So the important distinction is not that water waves are "classical" while EM waves are "quantum", but that in the regimes we normally observe them, the relevant quantum effects are negligible for water waves and often negligible for ordinary macroscopic EM waves.

EM waves are quite different in that there is no pre-existing medium and that the medium has no mass.

The statement that the EM "medium" has no mass is a conceptual error.

An individual photon has zero invariant mass, but a system of several photons can have nonzero invariant mass depending on its total energy and momentum.

I think RoGeorge has made it quite clear that he was discussing real photons as opposed to virtual photons..
Real photons have an indefinite lifetime as a ‘particle’.

I think also we can set aside negligible effects until the basic question is answered.

Water waves.  Or any wave.  The point the metaphor I recall was making when it asked the question, "Where is the wave?" was that it's a non-nonsensical question for a wave.  A wave does not have a set of coordinates.  You can characterise the wave sure, but not actually answer "Where is the wave?", because the wave just is. A travelling continuum.  It can be inspected at a "point" and you can define that "point" however you wish, but when you do that, you are now reasoning a point, not a wave.

The more you constrain the wave towards being a point, the more it behaves like a point in our measurements.  It still doesn't tell you what the full wave continuum is doing or where it is though.

Hissenberg uncertainty principle and others all seem to circle around the same dualities of measurement.  Once you "fix" a few key variables to get an answer, the answer is somehow changed because of the question you asked.  Release those variables and it goes back to "ill-defined" or "undefinable".  Constrain it with different variables, you get a different answer.  Constrain it with the same variables and do multiple tests and often get different answers.  Yet with strong mathematical accuracy QM makes predictions of what will happen in X number of cases when you constrain some variables.  That has enough scientific and industrial applications most people stopped asking "Why?" and just roll with the numbers.

If that layman metaphor holds.  A photon is what you get if you constrain the EM wave into a point by asking "Where is it?"

Heisenberg comes into play for virtual photons whose lifetime has to be within HUP limits.



RoGeorge was asking about light from Alpha Centauri or where ever, captured on his camera.

Since no one seems to want to answer a similar question I posed here is my explanation.

The photons from Alpha Centauri are similar to the last picture in the attachment – just a random few dots that have actually reached his camera unobstructed and  on beam.

The apparently smooth spherical wavefront  is actually not smooth at the finest scales of individual photons it is made of of just enormously large numbers of them like the first picture in the attachment.

[radiolistener]

I think RoGeorge has made it quite clear that he was discussing real photons as opposed to virtual photons..
Real photons have an indefinite lifetime as a ‘particle’.

I think also we can set aside negligible effects until the basic question is answered.

I don't think "lifetime" is the appropriate concept for a real photon. If you mean the time between its emission and its eventual interaction with matter, that is simply a propagation time determined by the particular setup, not a lifetime of the photon itself.

More importantly, I don't think the photon concept even gives us a meaningful way to identify a particular photon as "the same photon" at emission and detection. We can prepare a one-photon state at the source and later observe a photon detection event at the other end, but this does not mean that we have tracked an identifiable object travelling between the two events. We cannot say that we are dealing with the same photon.

Consider pouring one litre of water into one end of a pipe and collecting one litre at the other end. The water coming out could simply be water that was already inside the pipe, rather than the particular water that was poured in. Conservation of the amount of water does not establish the identity of the material that came out.

With photons the situation is even more fundamental: identifying "the same object" presupposes an object with an individual identity that persists through spacetime, i.e. essentially a classical corpuscular picture. That is not how a photon is represented in quantum field theory.

And if a photon never interacts with matter, there is no particular event at which its "lifetime" starts or ends. In fact, a photon that never interacts with matter cannot be detected in the first place, so claims about the "lifetime" of such a photon are not experimentally testable. At that point we are no longer discussing an experimentally observable property of a photon.

[studiot]

I think RoGeorge has made it quite clear that he was discussing real photons as opposed to virtual photons..
Real photons have an indefinite lifetime as a ‘particle’.

I think also we can set aside negligible effects until the basic question is answered.

I don't think "lifetime" is the appropriate concept for a real photon. If you mean the time between its emission and its eventual interaction with matter, that is simply a propagation time determined by the particular setup, not a lifetime of the photon itself.

More importantly, I don't think the photon concept even gives us a meaningful way to identify a particular photon as "the same photon" at emission and detection. We can prepare a one-photon state at the source and later observe a photon detection event at the other end, but this does not mean that we have tracked an identifiable object travelling between the two events. We cannot say that we are dealing with the same photon.

Consider pouring one litre of water into one end of a pipe and collecting one litre at the other end. The water coming out could simply be water that was already inside the pipe, rather than the particular water that was poured in. Conservation of the amount of water does not establish the identity of the material that came out.

With photons the situation is even more fundamental: identifying "the same object" presupposes an object with an individual identity that persists through spacetime, i.e. essentially a classical corpuscular picture. That is not how a photon is represented in quantum field theory.

And if a photon never interacts with matter, there is no particular event at which its "lifetime" starts or ends. In fact, a photon that never interacts with matter cannot be detected in the first place, so claims about the "lifetime" of such a photon are not experimentally testable. At that point we are no longer discussing an experimentally observable property of a photon.

I think RoGeorge has made it quite clear that he was discussing real photons as opposed to virtual photons..
Real photons have an indefinite lifetime as a ‘particle’.

I think also we can set aside negligible effects until the basic question is answered.

I don't think "lifetime" is the appropriate concept for a real photon. If you mean the time between its emission and its eventual interaction with matter, that is simply a propagation time determined by the particular setup, not a lifetime of the photon itself.

Thank you for your answer, I can’t tell whether you are agreeing or disagreeing so taking your points one at a time:-

Lifetime is one of those terms that actually stands for a whole class of concepts. This is why I simply said indeterminate.
But we have to call it something. Have you alternative suggestion ?
But yes I mean the time between creation and annihilation. This can in fact be determined for the photons from Alpha Centauri in RoGeorge’s camera, but not for the ones that miss Earth completely.

More importantly, I don't think the photon concept even gives us a meaningful way to identify a particular photon as "the same photon" at emission and detection. We can prepare a one-photon state at the source and later observe a photon detection event at the other end, but this does not mean that we have tracked an identifiable object travelling between the two events. We cannot say that we are dealing with the same photon.

We know when the photon arrives and can calculate when it must have started its journey.
Despite what Showman says, we also know its size limit since it must obey Heisenberg. This is exceedingly small since it must be confined to a volume within the HUP in order for the energy to be transferred, again within the HUP time constraints, to the absorber/detector.

This is basic QM.

Consider pouring one litre of water into one end of a pipe and collecting one litre at the other end. The water coming out could simply be water that was already inside the pipe, rather than the particular water that was poured in. Conservation of the amount of water does not establish the identity of the material that came out.

This is why your analogy does not ‘hold water’.
I have suggested a better classical one at the end.

With photons the situation is even more fundamental: identifying "the same object" presupposes an object with an individual identity that persists through spacetime, i.e. essentially a classical corpuscular picture. That is not how a photon is represented in quantum field theory.

Basically agreed.
This is more advanced QM than already noted and involves some extra postulates.
These involve the “Indistinguishability of identical particles” postulate and new state vectors for systems.
This subject does not usually appear in elementary treatments.
A really good mathematical treatment is given in Merzbacher “Quantum Mechanics”, Wiley, Chapter 20 page 508.
His book is good because it goes into the supporting background as well as, rather than concentrating on, the mathematics (Bra /Ket or whatever)

And if a photon never interacts with matter, there is no particular event at which its "lifetime" starts or ends. In fact, a photon that never interacts with matter cannot be detected in the first place, so claims about the "lifetime" of such a photon are not experimentally testable. At that point we are no longer discussing an experimentally observable property of a photon.

Basically correct for a virtual photon, but we are dealing with real photons, remember.
And as I have already mentioned, the time between their appearance and disappearance is governed by the HUP and exceedingly short.

Real photons are always the result of a material event so must have a definite start time but I agree there is no defined end time.
.

Again returning to the OP.

Pressure provides a good analogy for what is happening.

Consider a litre of gas at some reasonable pressure P1.
We see this as a force smoothly spread over the entire surface of the container.
This corresponds to the smooth description of the radio wavefront.
Yet we know from the kinetic theory that this smooth pressure is actually made up of a very large number of momentum transfers form moving molecules.
In the same way the photon model suggests that the wavefront is actually made up of and even larger number of smaller particles called photons, capable of transferring specific amounts of energy in a macroscopically smooth manner.

Now let the gas expand or exhaust some from the container.
Eventually we will reach the situation of loosing that smoothness and being able to detect individual strikes.
This is the same situation with RoGeorge and his Alpha Centauri camera observations.

Note I am not claiming the gas model is identical to the photon model. There are differences.
But I do suggest it is useful in this case.

[radiolistener]

I don't think "lifetime" is the appropriate concept for a real photon. If you mean the time between its emission and its eventual interaction with matter, that is simply a propagation time determined by the particular setup, not a lifetime of the photon itself.

Thank you for your answer, I can’t tell whether you are agreeing or disagreeing so taking your points one at a time:-

Lifetime is one of those terms that actually stands for a whole class of concepts. This is why I simply said indeterminate.
But we have to call it something. Have you alternative suggestion ?
But yes I mean the time between creation and annihilation. This can in fact be determined for the photons from Alpha Centauri in RoGeorge’s camera, but not for the ones that miss Earth completely.

A lifetime is a characteristic of an unstable state undergoing decay. A free photon has no spontaneous decay or annihilation process, so it does not have an intrinsic finite lifetime.

If a light signal travels from Alpha Centauri to Earth for 4.37 years, those 4.37 years are simply the propagation time between two physical events, not the lifetime of a photon. 

Moreover, knowing the emission and detection events does not tell us how long the actual emission and absorption processes themselves take. Those are interactions between the electromagnetic field and matter, and their temporal extent depends on the particular source and detector.

Modern physics does not describe a photon as a small corpuscular object travelling through space, so the analogy with gas molecules is not really appropriate here. Experimentally we have a preparation/emission event at the source and a detection/absorption event at the receiver.  The existence of the two events alone does not establish a classical "lifetime" or an identifiable object persisting between them.

We know when the photon arrives and can calculate when it must have started its journey.
Despite what Showman says, we also know its size limit since it must obey Heisenberg. This is exceedingly small since it must be confined to a volume within the HUP in order for the energy to be transferred, again within the HUP time constraints, to the absorber/detector.

This is basic QM.

The Heisenberg uncertainty principle does not give a photon a minimum physical size or a maximum volume.

The relation between position and momentum uncertainty is a constraint on the uncertainties of a quantum state; it does not mean that a photon is a small object that must be confined to a particular volume.

So HUP does not provide a universal "size limit" for a photon, nor does it imply that a photon must occupy a particular volume in order for its energy to be absorbed.

Basically correct for a virtual photon, but we are dealing with real photons, remember.
And as I have already mentioned, the time between their appearance and disappearance is governed by the HUP and exceedingly short.

Real photons are always the result of a material event so must have a definite start time but I agree there is no defined end time.
.

Again returning to the OP.

Pressure provides a good analogy for what is happening.

Consider a litre of gas at some reasonable pressure P1.
We see this as a force smoothly spread over the entire surface of the container.
This corresponds to the smooth description of the radio wavefront.
Yet we know from the kinetic theory that this smooth pressure is actually made up of a very large number of momentum transfers form moving molecules.
In the same way the photon model suggests that the wavefront is actually made up of and even larger number of smaller particles called photons, capable of transferring specific amounts of energy in a macroscopically smooth manner.

Now let the gas expand or exhaust some from the container.
Eventually we will reach the situation of loosing that smoothness and being able to detect individual strikes.
This is the same situation with RoGeorge and his Alpha Centauri camera observations.

Note I am not claiming the gas model is identical to the photon model. There are differences.
But I do suggest it is useful in this case.

I don't think the gas analogy is really appropriate here.

At low intensity, individual photon detection events can indeed be observed. But this does not mean that the classical wavefront is literally composed of a large number of tiny particles in the same sense that a gas is composed of molecules. The statistics of photon detection are a consequence of the quantum state of the electromagnetic field and its interaction with the detector.

So the gas analogy can be useful as a very rough analogy for the transition from a smooth macroscopic measurement to individual detection events, but it becomes misleading if it is taken to imply that a radio wave is physically a collection of photon "strikes" in the same way that gas pressure is a collection of molecular collisions.

[MrAl]

Hi,

Lifetime of a photon is an interesting thing to think about.  Even the propagation time is interesting because it depends where we think it starts and where we think it ends.
A photon from a star that forms in the core can take hundreds of thousands of years to just each the surface of the star.  Our sun estimates are something like 50k to 300k years, yet it only takes about 8 minutes to reach the earth once it leaves the surface of the sun completely.
The reason for the delay is because of the diffusion process which we don't have once it leaves the surface.

Maybe I can say that "photons are not real" in a more casual sense in that they seem to exist in a reality all of their own.  How can we reconcile that with our own reality depends on how we want to.

[studiot]

Hi,

Lifetime of a photon is an interesting thing to think about.  Even the propagation time is interesting because it depends where we think it starts and where we think it ends.
A photon from a star that forms in the core can take hundreds of thousands of years to just each the surface of the star.  Our sun estimates are something like 50k to 300k years, yet it only takes about 8 minutes to reach the earth once it leaves the surface of the sun completely.
The reason for the delay is because of the diffusion process which we don't have once it leaves the surface.

Maybe I can say that "photons are not real" in a more casual sense in that they seem to exist in a reality all of their own.  How can we reconcile that with our own reality depends on how we want to.

Diffusion not gravity ?

Interesting I had not heard of this, but I don't find cosmology/astrophysics very interesting.

[MrAl]

Hi,

Lifetime of a photon is an interesting thing to think about.  Even the propagation time is interesting because it depends where we think it starts and where we think it ends.
A photon from a star that forms in the core can take hundreds of thousands of years to just each the surface of the star.  Our sun estimates are something like 50k to 300k years, yet it only takes about 8 minutes to reach the earth once it leaves the surface of the sun completely.
The reason for the delay is because of the diffusion process which we don't have once it leaves the surface.

Maybe I can say that "photons are not real" in a more casual sense in that they seem to exist in a reality all of their own.  How can we reconcile that with our own reality depends on how we want to.

Diffusion not gravity ?

Interesting I had not heard of this, but I don't find cosmology/astrophysics very interesting.

Hi,

Well I suppose gravity could have an effect since it is strong, but diffusion is probably a better way to look at it because the photon does not travel in a straight line it bounces around a lot in effect.  It's quite amazing.

[MrAl]

If you put two floating plungeres in a large pool of water and start bouncing one up and down.  The other one starts to bounce up and down.

No water "flowed".  Energy was transfered.  This is exactly what waves do.

It was the way particle duality was put to me that rachetted one more click of understanding.  The simple point was "Here" is a particle.  You can give it a point reference and say, "There is the particle".  The diagram was replaced by a wave.  Question: "Where is the wave?"....  oh.... ah.... I see.

Technically water did flow.  Volume under the curve does not magically appear.

So go on, in what way is your analogy compared to the OP question ?

According to duality, photons and radio waves are the same thing.

Plungers and water waves are not.

Im just bouncing analogies onto analogies.  Most of the physics involved are analogies themselves and proudly so.  Approximations that facilitate modelling nature and producing useful outcomes.

Nobody believes there really is a little particle with a "e" T-Shirt on it.  Seriously.

Hi,

That is the amazing thing about the quantum world.  There are no analogies for some quantum phenomena because we always try to boil it down to a classical explanation.  There's nothing we can do about it, we have to just "accept" that the quantum world works differently than in our usual world.

If an alien came to visit us from some distant planet, would we try to come up with a formula for aliens?
Maybe, but we'd most likely just accept it as is.  That would be something new also that we would have to just accept.

What seems amazing to me is HOW different it can be.  It's almost incredible, unless you factor in the fact that we simply don't know everything about the universe, yet.  Then it's not as amazing because we know that new things will come up that we don't really understand the way we normally understand things.

Ok, so where did my Fosters go...

With these sorts of replies, I don't see that people are serious about the subject.


I agree that the model of one phenomenon exhibits differences from the model of a different phenomenon.
Why would anyone expect it to be otherwise ?
But it is also true that there is often commonality between pairs of models that allow one to be used as analogs for the other  - in the right circumstances.

For instance both classical wave theory and QM have an uncertainty principle.
The classical version starts with Parseval's theorem and several  pages of maths later ends with the conclusion that   Δt Δω  ≥  1/2,
when considering non recurrent waveforms.

So here we can see some commonality between classical and quantum.

However in the classical model the wave variable, φ,  has a physical meaning in out material universe.

On the other hand the wave variable in quantum theory does not correspond to any physical variable, certainly not probability as some proclaim.
In fact in only takes a few lines of maths to prove that Ψ cannot represent probability which is positive definite, while Ψ has positive and negative parts.

Nevertheless quantum theory cannot do without classical underpinning since the forces and energies involved are classical quantities ( non relativistic or relativistic as appropriate).

Another difference often overlooked is that classical analysis looks for solutions to the wave or other equations since its variable, φ, represents some material world quantity.

Quantisation looks for the zeros of the solutions to the corresponding quantum equations to meet energy requirements.

Hello again,

Well I am not sure what you are trying to say here.  First it sounds like some sort of mocking, which for one is untrue as told by the number of replies, but you don't need to mock anybody out you just need to state your case.

Anyway, it sounds like you are taking an apple and a pear and saying that they both grow on trees.  Yes they both have that in common, but it's a very vague commonality.  Now if you said (and if it was true) that pears have the same seeds as apples, THAT would be a very interesting commonality which would take some explaining.  By "same" I mean exactly the same, no difference whatsoever.

START_QUOTE
On the other hand the wave variable in quantum theory does not correspond to any physical variable, certainly not probability as some proclaim.
END_QUOTE

Where are you getting this information from?  What are you considering the "wave variable" to be?
The wavefunction is a probability function.  Check out "probability amplitudes" of the wavefunction.  Check out the square of that also.
Also check out the Born rule.

Actually I wasn't mocking anyone.

I was, however, feeling a trifle aggrieved that whilst I carefully considered and responded to points put by others,
Some others seem to be cherry picking my words out of context in order to simply tell me I am wrong, without any explanation or backup.

You for instance did this with your line about probability in the post I just quoted, in response to my claim, which included reasoning.

I seriously ask you to look at my Rochester experiment in the preceding  post and add your prediction.

Hi again,

I'll look at the experiment and see what I can note from that.  The wavefunction will be altered so the pattern should change. The point where the energy is absorbed would depend on the change in wavefunction probability amplitudes, so it should move.

As to your statement about the 'shortest pulse of light' being a photon, I think you have to add context to that because there are different ways to define a pulse of light.  For example, the shortest pulse measured to date is somewhere around 20e-18 seconds, but a photon in relativity has zero length in the direction of travel so a start and end time would be impossible to determine.

[radiolistener]

Oh and I offer a simplified definition of a photon.  The shortest (in time) possible pulse of light.

I think there is a fundamental problem with your simplified definition of a photon.

A light pulse and a photon are not the same thing. A shortest light pulse can transfer an amount of energy corresponding to a very large number of photon quanta, while a single-photon state can have a finite temporal extent determined by the particular state or mode in which it is prepared.

Shortening a light pulse does not progressively turn it into a single photon. Pulse duration and the quantization of the field are different things.

So there is no universal "shortest pulse" that can be used as a definition of a photon and the duration of a light pulse does not define a photon’s size or lifetime.

[studiot]

Hi again,

I'll look at the experiment and see what I can note from that.  The wavefunction will be altered so the pattern should change. The point where the energy is absorbed would depend on the change in wavefunction probability amplitudes, so it should move.

As to your statement about the 'shortest pulse of light' being a photon, I think you have to add context to that because there are different ways to define a pulse of light.  For example, the shortest pulse measured to date is somewhere around 20e-18 seconds, but a photon in relativity has zero length in the direction of travel so a start and end time would be impossible to determine.

Please remember that I am trying to produce a model in line with the OP's wish to use photons.
A typical photon that is created on Alpha Centauri and arrives about 4.5 years later at his camera on Earth.

It is easier to start at the camera end of the process, because there is no waiting time in an excited state to consider.
Once the photon is absorbed that is the end of its life.
The absorption time is known to be of the order of 10^-18 seconds, by direct experiments.
QM requires that when the light arrives at the camera detector it must deliver the exact amount of energy to excite the absorbing atom or molecule within that time.
Light is known to travel at  a speed of 3 x 10^8  metres per second so will have travelled a distance of 3 x 10^8  x 10^-18  metres in the absorption time.

Since this is the only light available within that time and distance it must contain the entire energy absorbed.

Any part of a light wave that arrives earlier than this will not intersect the absorbing molecule, which will not have arrived at the intersecting position.
Any any part of the light wave that arrives after this time will be too late to supply energy to the absorption.

Similar times occur for the actual emission, although it is often usual to add waiting time in the excited state before emission, which we do not need for this model.

Since light production, transmission and absorption is a flow process, the segment of that process must be the same segment that arrives at the camera 4.5 years later.

That is why I call a photon the smallest segment of that light for the purposes of this simple model.

You can complicate the model with various versions of QM probability statistics related to observed spectral line broadening, but it does not add to the concept.

This model includes a definition of ‘lifetime’, but as I said, there are alternative definitions for instance ‘service lifetime’  which are not appropriate here.

This does not address the  issue of ‘travelling in straight lines.
In point of fact the geometry of these lines is irrelevant to the argument.
The straight line travel is however consistent with the fact that both real and virtual photons include the transport of momentum in them.
Momentum is a line segment vector and therefore straight.

[radiolistener]

The absorption time is known to be of the order of 10^-18 seconds, by direct experiments.

I don't think "the absorption time is known to be of the order of 10^-18 seconds, by direct experiments" is a justified statement. Attosecond experiments can measure time delays and temporal aspects of particular light-matter processes, but that is not the same as directly measuring a universal photon absorption time. The measured quantity also include contributions from the particular atomic or molecular process and from the measurement method.

And even if we assume that a particular interaction has a characteristic timescale of 10^-18 s, this does not mean that the photon occupies a spatial segment of length c*dt, or that all of its energy was confined to that segment immediately before absorption.

This does not address the  issue of ‘travelling in straight lines.
In point of fact the geometry of these lines is irrelevant to the argument.
The straight line travel is however consistent with the fact that both real and virtual photons include the transport of momentum in them.
Momentum is a line segment vector and therefore straight.

Regarding the "straight line" between Alpha Centauri and the detector, experimentally we have an emission event at one point and a detection event at another point. We can certainly draw a straight line connecting those two events, but this is a geometrical relation between the endpoints, not a directly observed path of the light between them.

In the simple case of two points in a homogeneous medium, the straight line is the shortest optical path between them, but note that treating this geometrical line as a physical path is an illusion. This does not mean that there is a physical line segment along which an individual photon can be said to have travelled.

Moreover, experiments show that the detection does not require such a continuous physical path to exist between the source and the detector. For example, if an obstacle is placed on the straight line between them, light can still reach the detector, even though the straight geometrical line between the source and detector is physically blocked. This illustrates that the straight line is a geometrical description of propagation in the corresponding optical approximation, not a physical line that the light literally follows.

Nor does straight-line propagation follow simply from the fact that momentum is a vector. Momentum is a vector in momentum space, not a line segment in physical space.

[Marco]

Something i wonder about Bell inequality experiments. Why don't they use single "particle" entanglement and the path degree of freedom as the default instead of trying to entangle two "particles"? Most of the experiments get too obtuse and mathematical.

[MrAl]

Hi again,

I'll look at the experiment and see what I can note from that.  The wavefunction will be altered so the pattern should change. The point where the energy is absorbed would depend on the change in wavefunction probability amplitudes, so it should move.

As to your statement about the 'shortest pulse of light' being a photon, I think you have to add context to that because there are different ways to define a pulse of light.  For example, the shortest pulse measured to date is somewhere around 20e-18 seconds, but a photon in relativity has zero length in the direction of travel so a start and end time would be impossible to determine.

Please remember that I am trying to produce a model in line with the OP's wish to use photons.
A typical photon that is created on Alpha Centauri and arrives about 4.5 years later at his camera on Earth.

It is easier to start at the camera end of the process, because there is no waiting time in an excited state to consider.
Once the photon is absorbed that is the end of its life.
The absorption time is known to be of the order of 10^-18 seconds, by direct experiments.
QM requires that when the light arrives at the camera detector it must deliver the exact amount of energy to excite the absorbing atom or molecule within that time.
Light is known to travel at  a speed of 3 x 10^8  metres per second so will have travelled a distance of 3 x 10^8  x 10^-18  metres in the absorption time.

Since this is the only light available within that time and distance it must contain the entire energy absorbed.

Any part of a light wave that arrives earlier than this will not intersect the absorbing molecule, which will not have arrived at the intersecting position.
Any any part of the light wave that arrives after this time will be too late to supply energy to the absorption.

Similar times occur for the actual emission, although it is often usual to add waiting time in the excited state before emission, which we do not need for this model.

Since light production, transmission and absorption is a flow process, the segment of that process must be the same segment that arrives at the camera 4.5 years later.

That is why I call a photon the smallest segment of that light for the purposes of this simple model.

You can complicate the model with various versions of QM probability statistics related to observed spectral line broadening, but it does not add to the concept.

This model includes a definition of ‘lifetime’, but as I said, there are alternative definitions for instance ‘service lifetime’  which are not appropriate here.

This does not address the  issue of ‘travelling in straight lines.
In point of fact the geometry of these lines is irrelevant to the argument.
The straight line travel is however consistent with the fact that both real and virtual photons include the transport of momentum in them.
Momentum is a line segment vector and therefore straight.

Hi,

Just to be clear, I never said you couldn't call it that, just that if you call it that there should be some stated assumptions to go with it, that's all.
I think your assumptions might be clear enough now though so no worries

[studiot]

The absorption time is known to be of the order of 10^-18 seconds, by direct experiments.

I don't think "the absorption time is known to be of the order of 10^-18 seconds, by direct experiments" is a justified statement. Attosecond experiments can measure time delays and temporal aspects of particular light-matter processes, but that is not the same as directly measuring a universal photon absorption time. The measured quantity also include contributions from the particular atomic or molecular process and from the measurement method.

And even if we assume that a particular interaction has a characteristic timescale of 10^-18 s, this does not mean that the photon occupies a spatial segment of length c*dt, or that all of its energy was confined to that segment immediately before absorption.

Bald rebuttals without reasoning or reference to direct experiments are not acceptable.
I did supply reasoning for my description  of the model.
If you are looking for more detail, then say so, otherwise discussion cannot continue.

This does not address the  issue of ‘travelling in straight lines.
In point of fact the geometry of these lines is irrelevant to the argument.
The straight line travel is however consistent with the fact that both real and virtual photons include the transport of momentum in them.
Momentum is a line segment vector and therefore straight.

You have avoided the comment in red.

Regarding the "straight line" between Alpha Centauri and the detector, experimentally we have an emission event at one point and a detection event at another point. We can certainly draw a straight line connecting those two events, but this is a geometrical relation between the endpoints, not a directly observed path of the light between them.

In the simple case of two points in a homogeneous medium, the straight line is the shortest optical path between them, but note that treating this geometrical line as a physical path is an illusion. This does not mean that there is a physical line segment along which an individual photon can be said to have travelled.

Moreover, experiments show that the detection does not require such a continuous physical path to exist between the source and the detector. For example, if an obstacle is placed on the straight line between them, light can still reach the detector, even though the straight geometrical line between the source and detector is physically blocked. This illustrates that the straight line is a geometrical description of propagation in the corresponding optical approximation, not a physical line that the light literally follows.

Nor does straight-line propagation follow simply from the fact that momentum is a vector. Momentum is a vector in momentum space, not a line segment in physical space.

Congratulations. You are the first person I have come across that actually acknowledges the idea that many directed line segment vectors exist in another space than position space.
However that does not alter the rules of vector algebra.
Directed line segment vectors are only one type of a very large number of objects that obey the rules of vector algebra in applied mathematics, albeit that there are a very large number of line segment vectors.

Straight line  -  Here I'm not convinced that propagation is the right word as to me it implies some sort of 'spreading out'  -  Which is why I chose the word travel.
Anyway Photons are defined by the standard theory to operate that way.

[showman]

Leaving water aside, photons and radio waves are still not the same thing. There is no duality that claims that.

For those who are really interested, I recommend first this, as it's quite short https://www.classe.cornell.edu/spr/2000-02/msg0022377.html
And then perhaps something like Introduction to Quantum Optics by Grynberg, Aspect and Fabre, especially chapters 4 and 5. You can even ignore the mathematics if you don't care, as there is a lot of explaining text.
Once everyone knows about number, coherent, squeezed, one-photon multimode, and other possible states of the quantized EM field, this topic might eventually conclude.

Is there a reason you seem to want this topic to conclude?
When people talk, they often talk back and forth a lot in order to understand each other and bring out related information.  It's normal.

Because this thread is going absolutely nowhere without any common understanding. There is a certain person who has not only no idea what QED means even in the most basic level, but still claims to know everything about photons (which is coincidentally 100% QED term and 0% classical physics term) which to him are clearly still some "small parts of classical EM field".

Not only that, when given actual information how things work in reality, it is promptly rejected so there is not even willingness to learn anything, i.e. there is no back and forth but back and back and back forever with so much nonsense. Yet, it can be easily rectified by either a) if you don't care about learning QED just shut up and don't use and avoid the term photons. I already posted some pages ago a physics Nobel winner who advocates that view. About 100% of the experiments with light discussed here can be explained by (semi-)classical physics or b) learning at least something about it before confidently claiming bullshit. Again some very good references are given above, the book that I cited is authored by another Nobel winner, for experiments with photons. So read what the masters have done and learn, then come here and go back and forth about what photons do or do not if something remained unclear.

[radiolistener]

Bald rebuttals without reasoning or reference to direct experiments are not acceptable.
I did supply reasoning for my description  of the model.
If you are looking for more detail, then say so, otherwise discussion cannot continue.

Your statement was that the absorption time is known from direct experiments to be of the order of 10^-18 s. I am questioning the meaning of that quantity itself. A time delay or a characteristic timescale measured in a particular light-matter interaction is not automatically a universal "absorption time".

If you have a specific experiment that directly measures the quantity you call the photon’s absorption time, please give the reference and explain exactly what physical interval was measured. Otherwise, the statement remains unsupported.

This does not address the  issue of ‘travelling in straight lines.
In point of fact the geometry of these lines is irrelevant to the argument.
The straight line travel is however consistent with the fact that both real and virtual photons include the transport of momentum in them.
Momentum is a line segment vector and therefore straight.

You have avoided the comment in red.

I did not avoid the point about the geometry being "irrelevant".

I addressed the comment in orange, which is the argument you actually gave for straight-line travel: "Momentum is a line segment vector and therefore straight".

The fact that vectors obey the same algebra regardless of how they are represented does not establish a spatial trajectory. Momentum is a vector in momentum space, it is not a line segment in physical space.

So the relevant question is not whether vector algebra applies, but how you get from a momentum vector to the claim that light physically travels along a straight spatial line?

Congratulations. You are the first person I have come across that actually acknowledges the idea that many directed line segment vectors exist in another space than position space.
However that does not alter the rules of vector algebra.
Directed line segment vectors are only one type of a very large number of objects that obey the rules of vector algebra in applied mathematics, albeit that there are a very large number of line segment vectors.

Straight line  -  Here I'm not convinced that propagation is the right word as to me it implies some sort of 'spreading out'  -  Which is why I chose the word travel.
Anyway Photons are defined by the standard theory to operate that way.

Regarding your comment that you prefer “travel” because “propagation” suggests spreading out: this is just terminology. In physics, “propagation” does not necessarily mean spreading out; it is routinely used for the propagation of light, electromagnetic fields, waves, and quantum states.

More importantly, you say: "Photons are defined by the standard theory to operate that way."

If by "that way" you mean that a photon is a small object that travels along a definite straight line from emission to absorption, then that is not a definition of a photon in quantum electrodynamics. Straight-line propagation is a result of the geometric-optics description in the appropriate limit, it does not establish a classical trajectory of an individual photon.

That distinction is exactly what I am pointing out.

Interference between waves from two sources can produce local maxima and minima at different points on a detector. We can take the source position and any one of those maxima and draw a vector connecting the two points. But the existence and direction of that vector obviously does not mean that energy propagated along that straight line in the form of small objects travelling from the source to the detector.

[MrAl]

Leaving water aside, photons and radio waves are still not the same thing. There is no duality that claims that.

For those who are really interested, I recommend first this, as it's quite short https://www.classe.cornell.edu/spr/2000-02/msg0022377.html
And then perhaps something like Introduction to Quantum Optics by Grynberg, Aspect and Fabre, especially chapters 4 and 5. You can even ignore the mathematics if you don't care, as there is a lot of explaining text.
Once everyone knows about number, coherent, squeezed, one-photon multimode, and other possible states of the quantized EM field, this topic might eventually conclude.

Is there a reason you seem to want this topic to conclude?
When people talk, they often talk back and forth a lot in order to understand each other and bring out related information.  It's normal.

Because this thread is going absolutely nowhere without any common understanding. There is a certain person who has not only no idea what QED means even in the most basic level, but still claims to know everything about photons (which is coincidentally 100% QED term and 0% classical physics term) which to him are clearly still some "small parts of classical EM field".

Not only that, when given actual information how things work in reality, it is promptly rejected so there is not even willingness to learn anything, i.e. there is no back and forth but back and back and back forever with so much nonsense. Yet, it can be easily rectified by either a) if you don't care about learning QED just shut up and don't use and avoid the term photons. I already posted some pages ago a physics Nobel winner who advocates that view. About 100% of the experiments with light discussed here can be explained by (semi-)classical physics or b) learning at least something about it before confidently claiming bullshit. Again some very good references are given above, the book that I cited is authored by another Nobel winner, for experiments with photons. So read what the masters have done and learn, then come here and go back and forth about what photons do or do not if something remained unclear.

Hi,

Why worry about them?  Ignore them and just present the information you believe to be valid.  Members can still talk about that.
It always seems to make no sense to me why some members start to insist that a thread be 'closed', if that is what you are aiming for.  Once that happens, ALL previous statements almost disappear and if the topic comes up again it all has to be said all over again.  Why concern yourself with this just do your own thing and people who understand will reply with some reasoning behind it most likely.  If you really don't like the way it's going you always have the choice to not reply anymore.  You can even make a final reply stating why you don't like this thread anymore or something like that.

But more to the point of your frustration, I think what we need is some graphic images like drawings that show what is happening in various cases.  It could be very helpful because many people work better with images than words, or images with words too.  We always want to visualize what we think is happening or what actually does happen.  That could read like an encyclopedia.  Yes it takes a little more work, but think of what happens when someone posts a question here about a somewhat complicated or unusual circuit and provides no schematic ... we all have to keep wondering what circuit it actually is.  This leads to a lot of misunderstandings and back and forth questions that sometimes go for a long while.  So images are very often very important to a discussion especially in technical discussions and articles.  It's really a requirement in most cases, otherwise it sometimes sounds like just rambling on and on

If you can find some images from any books or articles you've read that could be very helpful if you post some here.  You can then describe what is going on in the images.  Videos are not that helpful if they are too long, but shorter ones could help also, although it is hard to define a 'best' video time length.