Light - What exactly is happening?

[Dale]

Have you ever considered that while space, time, and curvature are all that is required to describe the propagation of fields, that they may be attributes (of many) of an underlying medium?

medium ... that which gives rise to all that is known to exist, including space, time, matter, and energy.

With that definition of "medium" it is tautologically true that space, time, and curvature are attributes of the "medium". Of course that doesn't tell you anything useful, but I can certainly agree that a tautology is true.

 

[PhilDSP]

It would not diffuse into the background because part of light's specifications is momentum. We have to back up to why and where the light had its inception: there was a momentum that had to be conserved, and being massless, the light's velocity has the one universal velocity for massless entities. When the self-contained electric and magnetic fields oscillate and reform, the momentum specification informs them which way to reform, and the massless specification tells it what speed.

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.

Good reasoning, though I don't quite agree with your conclusion. Momentum might not even be able to exist without 2 things: (1) some type of medium that allows its transfer from one spatial point to another and (2) some force binding the particle together (if you're considering a particle rather than a wave)

As I think Dalespam says the concept of spacetime is thought to describe in one sense how point (1) can be accommodated, though exactly how in technical detail no one has quite properly explained in my opinion.

De Broglie is probably the person who has most deeply probed the question of what momentum is. His Ph.D. thesis and related papers are available on the internet (sorry I don't have the link but searching the forum would no doubt yield the links) His book "An Introduction to the Study of Wave Mechanics" probably shouldn't be missed. But he clearly states that the wave equations for a particle are not stable in the sense that the particle's spatial extent expands extremely rapidly and would very soon diffuse into the background. (Now I'm thinking that Bohm might have more clearly stated that point) The momentum would of course also be diffused. Hence (my conclusion) a binding force of some type is required. This is no doubt related to Lorentz's, Abraham's and Poincare's observation that the total energy of an electron requires an additional binding energy which is not electromagnetic.

 

[TrickyDicky]

With that definition of "medium" it is tautologically true that space, time, and curvature are attributes of the "medium". Of course that doesn't tell you anything useful, but I can certainly agree that a tautology is true.

You seem to be saying here that since "medium" is not a well defined concept , (unless you have a physically meaningful definition of medium, in which case please share it with us) it is not useful to say that spacetme is a medium or an attribute of a medium.
I agree with that, by the same token saying that light is a type of wave that doesn't require a medium would be an empty assertion, as long as we don't define medium in a precise way.

 

[Dale]

You seem to be saying here that since "medium" is not a well defined concept

It is well defined here, Gray Ghost gave a definition. I am only saying that using the given definition it is tautologically true and adds no information.

I agree with that, by the same token saying that light is a type of wave that doesn't require a medium would be an empty assertion, as long as we don't define medium in a precise way.

I agree, hence my post 15. I prefer to talk about the specific properties that are required, and not get bogged down in a semantic argument.

 

[TrickyDicky]

I agree, hence my post 15. I prefer to talk about the specific properties that are required, and not get bogged down in a semantic argument.

Aha, but there seems to be the same problem with the "material properties" concept and how you distinguish them from geometric properties.

Spacetime is spacetime, not "nothingness". It lacks material properties like density and velocity, but it has geometrical properties like distance and duration and curvature. That is all that is needed for the fields to propagate.

Those you call geometric properties are exactly the properties that the WP page attributes to matter (anything that has mass and occupies volume:length and curvature), so you are actually saying spacetime has "material properties", not that it lacks them.
Of course in the same entry for the term "matter" it is acknowledged that "different fields use the term in different and sometimes incompatible ways; there is no single agreed scientific meaning of the word "matter".

 

[Dale]

Aha, but there seems to be the same problem with the "material properties" concept and how you distinguish them from geometric properties.

Fine, then just leave it at "geometric properties" only and skip mention of "material properties" altogether.

 

[TrickyDicky]

Fine, then just leave it at "geometric properties" only and skip mention of "material properties" altogether.

It's fine with me too
A property such as density has been mentioned in this thread, certainly spacetime has an energy density, I'm not sure if I should consider density as a geometric property, would you?

 

[Quickless]

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.

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.

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.

 

[Dale]

It's fine with me too
A property such as density has been mentioned in this thread, certainly spacetime has an energy density, I'm not sure if I should consider density as a geometric property, would you?

No, I wouldn't.

 

[TrickyDicky]

No, I wouldn't.

Too bad, then we can't leave it at "geometric properties".

 

[Dale]

Too bad, then we can't leave it at "geometric properties".

In what way is the energy density of space-time relevant for EM in vacuum? I think we can leave it at geometric properties in this context. your distinction is important for the EFE, but not Maxwell's equations.

 

[TrickyDicky]

In what way is the energy density of space-time relevant for EM in vacuum?

Vacuum polarization or self-energy (a manifestation of empty space energy) is quite relevant for Electromagnetic fields in vacuum.

 

[Dale]

How so? Where does it show up in Maxwells equations?

 

[TrickyDicky]

How so? Where does it show up in Maxwells equations?

Well, it certainly shows up in QED theory, which you'll concede that has something to do with electromagnetism. We've moved on a bit since Maxwell.

But even if it is not explicit in the Maxwell equations, (maybe just implicit in their form as PhilDSP suggested) if Maxwell himself interpreted his own equations as showing something "medium-like" in relation with EM propagation, I don't feel qualified or expert enough to contradict him.

 

[Dale]

Well, it certainly shows up in QED theory, which you'll concede that has something to do with electromagnetism. We've moved on a bit since Maxwell.

How does it show up in QED?

But even if it is not explicit in the Maxwell equations, (maybe just implicit in their form as PhilDSP suggested) if Maxwell himself interpreted his own equations as showing something "medium-like" in relation with EM propagation, I don't feel qualified or expert enough to contradict him.

As you say, we have moved on a bit since Maxwell.

 

[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.