Light - What exactly is happening?

[TrickyDicky]

How does it show up in QED?

See section 7.5 of "An Introduction to Quantum Field Theory" from Peskin and Schroeder.

 

[Dale]

See section 7.5 of "An Introduction to Quantum Field Theory" from Peskin and Schroeder.

I don't have it. Can you summarize? Does the vacuum energy only show up via the EFE, or directly in the EM field? Or are you just talking about a gauge transformation of the potentials which attributes an energy to the vacuum but has no measurable effect?

 

[TrickyDicky]

I don't have it. Can you summarize? Does the vacuum energy only show up via the EFE, or directly in the EM field? Or are you just talking about a gauge transformation of the potentials which attributes an energy to the vacuum but has no measurable effect?

Photon self-energy shows up directly in the EM field and it has measurable effects, I think it is a small part of the Lamb shift splitting and according to WP it was observed experimentally in 1997 using the TRISTAN particle accelerator in Japan.
I can't summarize it any better as I'm no expert in QFT by any means and I have enough of a hard time understanding it myself to even try and explain it correctly to others, but I'm sure in the QM forum there will be lots of people that can explaint it to you.

 

[Dale]

Photon self-energy shows up directly in the EM field and it has measurable effects

Sure, but AFAIK that is an EM self-interaction or an interaction with another static EM field. I don't think that you need to attribute density to empty spacetime because of that.

 

[TrickyDicky]

Sure, but AFAIK that is an EM self-interaction or an interaction with another static EM field. I don't think that you need to attribute density to empty spacetime because of that.

You must have missed my point, I don't attribute an energy density to empty spacetime because of vacuum polarization. What I said is that vacuum polarization, like is the case for instance with the Lamb shift, spontaneous photon emission, the Casimir effect, the van der Waals bonds, etc, is a manifestation of this energy density. This was in the context of your asking how is this energy density relevant for EM in vacuum.
Since you said that this density is not a geometric property (of course it isn't), my point was that we can't skip mention of "material properties" altogether in this discussion about spacetime and light propagation.

 

[GrayGhost]

There are mathematical models and then there is reality. For some the distinction becomes quite blurred. In all relativity theories, “space” is defined by crude assumption. The choices being mathematical convenient. My opinion of “space-time” is that it is a mathematical construction, a creature of the mind. The “space” being referenced relating to a fuzzy concept that in some way captures the attribute of distance.

Our concept of “space” is incomplete.

Interestingly put. I might add, that our notion of time is incomplete. With all this incompleteness, it does not take a leap of faith to assume space, time, and its curvatures may well be aspects of an underlying medium.

That the progress of things is impede in “space” vouchsafes of a “somethingness” of space. The approach being what the attributes might be of something that we can’t detect but can only infer.

I would suggest that the attributes of the medium, call it space if you wish, are defined by all that exists. Matter forms in empty spacetime, and so there are inherent properties of the medium that allow particles to form and maintain their existence. This would be true for photons as well. Thus, the properties of the medium are (in part) defined by properties of particles.

All must agree that relativity theories are missing something. String theory is a groping for a more articulate formulation of “space”. String theory being a game, nonetheless.

Indeed, the game is unification, and it is generally agreed that gravity (and thus spacetime) needs quantized for unification of gravity with GUT. If string theory makes a prediction that's upheld by experiment (that other theories have not), then it will have more solid legs to stand on. IMO, it's far more unreasonable to to assume particles of point nature than string nature.

GrayGhost

 

[Dale]

Since you said that this density is not a geometric property (of course it isn't), my point was that we can't skip mention of "material properties" altogether in this discussion about spacetime and light propagation.

I don't see how the above supports your point. To my understanding, that energy density you are talking about is the energy density of the field itself, not the energy density of some "medium" which is separate from the field. Clearly the field has energy.

QM is not an area of strength for me, so I could easily be wrong in the quantum domain, but classically there is no reason to assign non-geometric properties to space-time in order to explain EM.

 

[TrickyDicky]

I don't see how the above supports your point. To my understanding, that energy density you are talking about is the energy density of the field itself, not the energy density of some "medium" which is separate from the field. Clearly the field has energy.

Completely agree. But here you are entering into semantic distinctions because the field is only a physical property associated to spacetime, like energy for instance, it doesn't have an entity in itself without spacetime.(I mean physically, mathematically it does) But for most people a funny thing happens with fields in this context. It's like if I quantitavely describe all the material features of an object and call'em its field, and then say that this description is all that exist and declare the object either non-existent or just a geometrical abstraction without material properties even though I have just described them as its field.

QM is not an area of strength for me, so I could easily be wrong in the quantum domain, but classically there is no reason to assign non-geometric properties to space-time in order to explain EM.

Clasically you might be right, but maybe due to the use of the concept of field in the way described above.

 

[PhilDSP]

Photon self-energy shows up directly in the EM field and it has measurable effects, I think it is a small part of the Lamb shift splitting and according to WP it was observed experimentally in 1997 using the TRISTAN particle accelerator in Japan.
I can't summarize it any better as I'm no expert in QFT by any means and I have enough of a hard time understanding it myself to even try and explain it correctly to others, but I'm sure in the QM forum there will be lots of people that can explaint it to you.

I guess it's ironic that Peskin and Schroeder's book is called an "introduction". Section 7.5, "Renormalization of the Electric Charge" is a good example. They delve into a highly detailed and technical description of a crucially important aspect of QED and QFT but only briefly touch on each item or consideration without possibly explaining all of the basic issues of each item. I guess that makes it an introduction but not really a "primer" in the sense of giving you all the essential information you need to put everything in place.

Being absolutely no expert on this, I'll venture a super simplified reduction of 7.5 which someone with more knowledge should comment on and correct. The basic issue is that because the photon has self energy but no mass, the effective strength of the field seen by a scattered electron is shifted depending on its distance from the interacting virtual positron. A radiative correction term is required. The mathematically simple way of accounting for that is to develop a scheme of charge renormalization which involves the procedure of dimensional regularization (of spacetime) Is that somewhat like a "transformation" on top of the LT? (It certainly sounds like it to me).

A more physical picture is that the charge normally seen by other charges at super-atomic distances is much less than the actual charge due to screening effects of a virtual electron-positron cloud. As a second charge approaches the first, the bare charge becomes more and more manifest. This is what is known as "vacuum polarization". The change-over distance is roughly the Compton wavelength.

Interestingly enough Peskin and Schroeder give this high level description (p. 255): "We can interpret the correction term as being due to screening. Ar r > 1/m, virtual e+e- pairs make the vacuum a dielectric medium in which the apparent charge is less than the true charge. At smaller distances we begin to penetrate the polarization cloud and see the bare charge."

 

[Dale]

Completely agree. But here you are entering into semantic distinctions because the field is only a physical property associated to spacetime, like energy for instance, it doesn't have an entity in itself without spacetime.(I mean physically, mathematically it does)

By this logic everything in physics "is only a physical property associated to space-time". And then we are back to Gray Ghost's definition of medium. Which is fine by me, as long as you recognize that it is a tautology.

 

[TrickyDicky]

By this logic everything in physics "is only a physical property associated to space-time".

By what logic? I simply transcribed the definition of Field from the WP. Does everything in physics have the same definition as fields? Might as well be, but then it would indeed be a tautological definition.

 

[TrickyDicky]

... A radiative correction term is required. The mathematically simple way of accounting for that is to develop a scheme of charge renormalization which involves the procedure of dimensional regularization (of spacetime) Is that somewhat like a "transformation" on top of the LT? (It certainly sounds like it to me).

AFAIK, dimensional regularization is a step previous to the renormalization proper that makes the ultraviolet divergences of Feynman integrals in 4D Euclidean space converge in less dimensions. It has nothing to do with LT that I know.

 

[Dale]

By what logic? I simply transcribed the definition of Field from the WP. Does everything in physics have the same definition as fields? Might as well be, but then it would indeed be a tautological definition.

The only reason that I know that the EM field could be considered "a physical property associated to spacetime" is the fact that there are dE/dt and dE/dx terms in the differential equations governing the EM field. If that is all it takes to make something "a physical property associated to spacetime" then that qualifies just about everything in physics.

Did you have something else in mind with that comment? If so, then in your opinion how does the EM field differ from other physical quantities such that the EM field is "a physical property associated to spacetime" and other things are not?

It is just not obvious to me how you can make a concept of spacetime as a medium for the EM field that isn't a simple tautology. That said, I am ok with the concept.

 

[TrickyDicky]

Did you have something else in mind with that comment? If so, then in your opinion how does the EM field differ from other physical quantities such that the EM field is "a physical property associated to spacetime" and other things are not?

It is just not obvious to me how you can make a concept of spacetime as a medium for the EM field that isn't a simple tautology. That said, I am ok with the concept.

Great, some tautologies are cool.

 

[bobc2]

When a source begins to emit light, what exactly is occurring to produce an instantaneous velocity of c?

It might be a little easier to accept if you picture the photon as a 4-D filiament-like object in 4-D space. Physics doesn't seem to provide a detailed process for the creation of the initial end of the filament (and maybe there is no further detail to understand).

If we're talking about quantized photons, would it be be appropriate to say there is zero acceleration? (I would think not because technically there is no change in velocity)

Again, a 4D picture of the photon worldline is helpful. The 4D object exists as a filament-like structure oriented at a 45 degree angle for all observers. Now, if I draw a 45 degree straight line on a piece of paper, would you be asking if the first couple of points on the line had infinite acceleration in order to yield the 45 degree orientation?

Or if we refer to the light as a wave function, is the wave simply propagating with a velocity of c? Or if we use the term wavicle how is its behavior described?

It might help to specify whether you are focusing on just one photon, or whether you are thinking of classical light wave propagation, where perhaps billions of photons are participating in a beam of light.

 

[GrayGhost]

Great, some tautologies are cool.

Great, now that that's out of the way, I was wondering how you yourself would speculate on the OP's question ...

When a source begins to emit light, what exactly is occurring to produce an instantaneous velocity of c?

Any ideas?

GrayGhost

 

[PhilDSP]

AFAIK, dimensional regularization is a step previous to the renormalization proper that makes the ultraviolet divergences of Feynman integrals in 4D Euclidean space converge in less dimensions. It has nothing to do with LT that I know.

No, no theoretical connection. I meant that dimensional regularization seems to be employed in a way that is analogous to the LT. Lorentz worked toward finding a mathematical procedure that would get him from his variation of the Maxwell equations to solutions of the wave equations that gave the same optical results as Fresnel's theory. He used the LT to adjust the initial conditions for the differential equations so that the sought for solution could easily be obtained.

Dimensional regularization seems to be a similar procedure. There is the theoretical and experimental value for charge that is expected but the Klein-Gordon and Dirac equations (or rather the QED langrangian) don't produce those values unless the initial conditions are shifted.

 

[Drakkith]

Great, now that that's out of the way, I was wondering how you yourself would speculate on the OP's question ...

When a source begins to emit light, what exactly is occurring to produce an instantaneous velocity of c?

Any ideas?

GrayGhost

I think that the idea of a source "beginning to emit light" is incorrect. Light is emitted instantaneously and it has no choice but to propagate at c

 

[PhilDSP]

When a source begins to emit light, what exactly is occurring to produce an instantaneous velocity of c? If we're talking about quantized photons, would it be be appropriate to say there is zero acceleration?

You can think of radiation as pure displacement current (energy moving at its natural unrestrained velocity) Within an atom the energy movement is restrained and affected by the charges of the electrons and protons. Under the influence of the charges, the energy flow becomes conductance current which has a velocity less than c. But the energy is of the same type in both cases. When an electron shifts its orbital a portion of the energy becomes unrestrained and that portion of the conductance current becomes displacement current after the energy is accelerated from less than c to c.

The atom sort of leaks energy when an orbital shift occurs.

 

[TrickyDicky]

Great, now that that's out of the way, I was wondering how you yourself would speculate on the OP's question ...

I'll try to speculate without being "overly speculative"...

The QED explanation involves vacuum fluctuations and transitions from atom excited states to "stationary states".I think any more detailed discussion belongs in the quantum physics forum.
The thing is QED doesn't have an answer to the OP's question in the terms it is asked, because QED admits the creation of photons (creation and anihilation operators for photons), so the question of accelerating to c doesn't even arise, since the particle is created with a velocity of c. But then again this is already been said with other words by several posters and it was even moreless explicit in the OP's phrasing.

I'm more interested in the second OP question:

So what exactly is occurring? Is a wave propagating at c and what we see as light just some sort of EM disturbance? I understand how light is reflected off of objects and into our eyes, letting us see, but what is going on between the source and the destination?

What goes on between the source and the destination could be explained by considering an isolated source, and using Schwarzschild space, in this setting the EM wave radiated would be at infinity, given the fact that this space is asymptotically Minkowskian so it is bounded by Minkowski space at infinity and light follows a null geodesic so it lives at spatial infinity. It is made finite only upon detection.

 

[TrickyDicky]

No, no theoretical connection. I meant that dimensional regularization seems to be employed in a way that is analogous to the LT. Lorentz worked toward finding a mathematical procedure that would get him from his variation of the Maxwell equations to solutions of the wave equations that gave the same optical results as Fresnel's theory. He used the LT to adjust the initial conditions for the differential equations so that the sought for solution could easily be obtained.

Dimensional regularization seems to be a similar procedure. There is the theoretical and experimental value for charge that is expected but the Klein-Gordon and Dirac equations (or rather the QED langrangian) don't produce those values unless the initial conditions are shifted.

I see now what you mean, I find it a very significant parallelism.

 

[GrayGhost]

I'm more interested in the second OP question: What goes on between the source and the destination could be explained by considering an isolated source, and using Schwarzschild space, in this setting the EM wave radiated would be at infinity, given the fact that this space is asymptotically Minkowskian so it is bounded by Minkowski space at infinity and light follows a null geodesic so it lives at spatial infinity. It is made finite only upon detection.

True, but a proton is made finite by collison as well, assuming they do not decay. The difference is that the proton experiences the flow of time, while the photon should not. IMO, explain why time progresses as it does, while showing how it relates to speed c activity in vacu, and one will be much closer to a satisfactory answer of the OP's questions.

GrayGhost

 

[nitsuj]

As long as you associate the "medium" only with geometric properties like distance and time and not with material properties like density and velocity.

From a kosher physicist definition yes.

 

[nitsuj]

I'd submit that everything that exists is of the very medium. If no medium, the electron could not exist let alone work. But then, I haven't been able to prove it either, so :)

GrayGhost

bcrowell - " To make this a meaningful statement, you'd have to define terms like "everything," "exists," and "medium.""

Grayghost said "of the medium" which makes it "meaningful" (which is subjective right?).

Said differently, everything that exists within spacetime is a different state of spacetime. A laughable comment in an SR/GR forum, but the physics discussion I'm sure doesn't end there.

 

[nitsuj]

The more fundamental mystery that cuts across all these kinds of waves and waving, and even the propagation of particles, is the conservation of momentum. Why doesn't a golf ball go any old way when I hit it? It would make life more interesting.

Off topic sorry,

Is conservation of momentum what allows a spaceship to orbit Earth and not all of the sudden stop moving relative to umm anything and Earth continues on flying through the galaxy along with the solar system, leaving the once orbiting spaceship behind?

If so, those astronauts have a lot of faith in this so called "conservation of momentum". What if it stops conserving all of the sudden?

 

[Drakkith]

If so, those astronauts have a lot of faith in this so called "conservation of momentum". What if it stops conserving all of the sudden?

Billions of everyday examples of nature following this rule alongside millions of more scientific observations and finally thousands of people over the course of at least a few centuries acknowledging that the rule has never been broken. So "what if" scenarios are simply not realistic currently. But IF it did stop...well the results would be obvious.

 

[GrayGhost]

I think that the idea of a source "beginning to emit light" is incorrect. Light is emitted instantaneously and it has no choice but to propagate at c

Hmmm. Well, I'm not so sure. Got a question for you ...

How much time does it take an electron to transition the gap from conduction band? It's a finite time, yes? If so, then it seems that there exists a process whereby the photon commences formation, builds, and completes formation ... even though it travels at c during the entire process and thereafter. Yes?

Or, is the transition considered instant?

I think the OP was interested as to WHY the photon would exist at speed c even while being formed, ie no acceleration. So, what process could do that, and how.

GrayGhost

 

[Drakkith]

Hmmm. Well, I'm not so sure. Got a question for you ...

How much time does it take an electron to transition the gap from conduction band? It's a finite time, yes? If so, then it seems that there exists a process whereby the photon commences formation, builds, and completes formation ... even though it travels at c during the entire process and thereafter. Yes?

Or, is the transition considered instant?

I think the OP was interested as to WHY the photon would exist at speed c even while being formed, ie no acceleration. So, what process could do that, and how.

GrayGhost

As far as I know the jump between energy levels is instant. But I'd really like someone more experienced in this area to take a shot.

 

[GrayGhost]

As far as I know the jump between energy levels is instant. But I'd really like someone more experienced in this area to take a shot.

I seem to recall that from undergrad school too, but it's been awhile. Even if the electron is assumed to jump instantly, it would seem to me that the electromagnetic interaction related to the jump cannot occur at once, for otherwise the photon would have no wavelength. However, I'm just speculating here, so.

GrayGhost

 

[nitsuj]

"If so, those astronauts have a lot of faith in this so called "conservation of momentum". What if it stops conserving all of the sudden?"

Billions of everyday examples of nature following this rule alongside millions of more scientific observations and finally thousands of people over the course of at least a few centuries acknowledging that the rule has never been broken. So "what if" scenarios are simply not realistic currently. But IF it did stop...well the results would be obvious.

Yea, the obviousness of this should have made the comment funny.

The question if that force is called "Conservation of momentum" was real.

 

[Quickless]

We have gone far a field of the original question. My take on what the OP was getting at is

1. Imagine a state with no light
2. Now add light

The instant the light is added it's measured speed is c. There is no transition from 0 to c?

How is this?

The inference is that whether we perceive it or not, light is always traveling a c. One does not create light. One creates the ability to perceive light.

 

[Dale]

Why would a newly created particle start at 0? That doesn't make any sense.

If it is 0 in some frame then it is moving at some velocity in all other frames. So it must be able to start at some non zero velocity anyway. Which velocity should that be? Obviously the one that conserves energy and momentum. For photons that is c.

 

[Dale]

Why would a newly created particle start at 0? That doesn't make any sense.

If it is 0 in some frame then it is moving at some velocity in all other frames. So it must be able to start at some non zero velocity anyway.

Which velocity should that be? Obviously (IMO) the one that conserves energy and momentum. For photons that is always c.

 

[nitsuj]

Why would a newly created particle start at 0? That doesn't make any sense.

If it is 0 in some frame then it is moving at some velocity in all other frames. So it must be able to start at some non zero velocity anyway.

Which velocity should that be? Obviously (IMO) the one that conserves energy and momentum. For photons that is always c.

Is that true that "c" conserves momentum? I conserve momentum but I don't travel at c, often.

 

[nitsuj]

One does not create light. One creates the ability to perceive light.

what does that mean?

 

[Dale]

Is that true that "c" conserves momentum? I conserve momentum but I don't travel at c, often.

Yes it is true that c conserves momentum for a photon. You clearly aren't a photon if you conserve momentum and yet don't travel at c.

 

[GrayGhost]

Why would a newly created particle start at 0? That doesn't make any sense.

If it is 0 in some frame then it is moving at some velocity in all other frames. So it must be able to start at some non zero velocity anyway.

Which velocity should that be? Obviously (IMO) the one that conserves energy and momentum. For photons that is always c.

True, but then why would the photon be generated at speed c "wrt all", with no rest frame ... in so far as the process within atomic structure that creates it?

GrayGhost

 

[nitsuj]

Yes it is true that c conserves momentum for a photon. You clearly aren't a photon if you conserve momentum and yet don't travel at c.

If you were less of a smart *** and had elaborated in a direction that made sense, is it because I have mass? (which you may interprut as "I'm not a photon")

your continuous derogatory tone taints this awsome forum, honestly why reply "Obviously, of course and clearly" when making a point. As if it makes you feel more right, at the expense of the other person being more wrong.

If I were you, I would just refrain from responding to questions that I found require a smart *** response, and would save it for face to face encounters.

 

[Dale]

If I were you, I would just refrain from responding to questions that I found require a smart *** response, and would save it for face to face encounters.

I thought it was a pretty reasonable response given your comment. In fact, it was quite restrained compared to my first impulse.

What precisely did you find out of line, given that I was responding to your previous post? Or are you allowed to make such comments and I am not allowed to respond?

 

[nitsuj]

I thought it was a pretty reasonable response given your comment. In fact, it was quite restrained compared to my first impulse.

What precisely did you find out of line, given that I was responding to your previous post? Or are you allowed to make such comments and I am not allowed to respond?

I wouldn't (and didn't) say "out of line". I said smart ***. Reminding me I'm not a photon is a smart *** answer to my question regarding "c" conserving momentum and me conserving momentum too, however don't travel at "c".

Perhaps from your perspective it was more likely I had confused myself with a photon,

as opposed to actually not knowing the aspects of the conservation of momentum.

 

[Dale]

Reminding me I'm not a photon is a smart *** answer to my question regarding "c" conserving momentum and me conserving momentum too, however don't travel at "c".

But my comment (and the entire thread) was specifically limited to photons/light:

Which velocity should that be? Obviously (IMO) the one that conserves energy and momentum. For photons that is always c.

I am sorry that I misinterpreted your sincere question as a sarcastic quip. I hope you can understand why I made that mistaken assessment given the context.

For a massless particle, like a photon, to have any energy or momentum it must travel at c. Are you at all familiar with four-vectors, especially the four-momentum?

 

[GrayGhost]

For a massless particle, like a photon, to have any energy or momentum it must travel at c. Are you at all familiar with four-vectors, especially the four-momentum?

When you say "at c", you really mean "travel at invariant speed", yes?

GrayGhost

 

[nitsuj]

But my comment (and the entire thread) was specifically limited to photons/light:I am sorry that I misinterpreted your sincere question as a sarcastic quip. I hope you can understand why I made that mistaken assessment given the context.

For a massless particle, like a photon, to have any energy or momentum it must travel at c. Are you at all familiar with four-vectors, especially the four-momentum?

Ah I see, that was no quip.

I know nothing of "four-vectors" or the "Four-momentum". If I had the time and resource, I would be happy to become formally educated on physics.

Sorry, for the brash reply in my previous posts Dalespam, I think we appreciate physics in different ways and for different reasons.

 

[HallsofIvy]

Would it be incorrect to say that the medium is spacetime?

No, the medium is the electromagnetic field.

 

[Dale]

When you say "at c", you really mean "travel at invariant speed", yes?

Yes, it is just shorter to write.

 

[cowmoo32]

I haven't checked this thread in a few days; I thought it was dead and I was clearly wrong.

Again, a 4D picture of the photon worldline is helpful. The 4D object exists as a filament-like structure oriented at a 45 degree angle for all observers. Now, if I draw a 45 degree straight line on a piece of paper, would you be asking if the first couple of points on the line had infinite acceleration in order to yield the 45 degree orientation?

I just want to make sure I'm reading this correctly: Are you implying that light always exists? The way I read this (especially regarding the line on paper analogy) is that you're saying that it wouldn't be proper to ask if the beginning of the light (or line in the analogy) experienced an acceleration. I agree with that, but no matter if you view light as a wave or a stream of photons it must have a source. It isn't there one moment and is the next; something happened.

It might help to specify whether you are focusing on just one photon, or whether you are thinking of classical light wave propagation, where perhaps billions of photons are participating in a beam of light.

Referring to my last point stating that both waves and particles must have a source, it shouldn't matter in which way you view light, both the wave and the photon immediately achieve a velocity of c.

I think that the idea of a source "beginning to emit light" is incorrect. Light is emitted instantaneously and it has no choice but to propagate at c

So you wouldn't consider turning on a laser as "beginning to emit light"? At t₀ it was not emitting light, at t₁ it was emitting light.

No, the medium is the electromagnetic field.

So the electric field exists in a vacuum? It is my understanding that an electric field only exists around an EM source. If it does exist in a vacuum (I know I'm hitting on a quantum discussion), might it be linked to virtual particles?

 

[Drakkith]

So you wouldn't consider turning on a laser as "beginning to emit light"? At t₀ it was not emitting light, at t₁ it was emitting light.

No. I would say the actual emission of light is instantaneous. The laser obviously has a finite time while charges move and things start to happen before the first photon is emitted, but the actual emission I thought was instant.

So the electric field exists in a vacuum? It is my understanding that an electric field only exists around an EM source. If it does exist in a vacuum (I know I'm hitting on a quantum discussion), might it be linked to virtual particles?

Yes, an EM field easily exists in a vacuum. If an electron and a proton are in intergalactic space and separated by a mile without anything else in between they would still feel each others EM Field.

 

[cowmoo32]

Yes, an EM field easily exists in a vacuum. If an electron and a proton are in intergalactic space and separated by a mile without anything else in between they would still feel each others EM Field.

I understand that. You said the medium is the electric field, so let's take an area of space devoid of anything: no stars, planets, nothing. Light can travel through it, but by what means? In this area of space in a perfect vacuum there are no electrons or protons present, and thus no EM field.

 

[GrayGhost]

So you wouldn't consider turning on a laser as "beginning to emit light"? At t0 it was not emitting light, at t1 it was emitting light.

No. I would say the actual emission of light is instantaneous. The laser obviously has a finite time while charges move and things start to happen before the first photon is emitted, but the actual emission I thought was instant.

My opinion ... although each portion of the photon moves always at c during its entire creation process, and thereafter, it takes time for the photon to form from start to end. If it did not, then how could have a measurable wavelength?

GrayGhost

 

[nitsuj]

My opinion ... although each portion of the photon moves always at c during its entire creation process, and thereafter, it takes time for the photon to form from start to end. If it did not, then how could have a measurable wavelength?

GrayGhost

That seems true, but could that "creation process" be cut short? say in half, or 1/8? At what point would it not be enough time for a photon of visable light to be created?