Light - What exactly is happening?

[cowmoo32]

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? (I would think not because technically there is no change in velocity) 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?

[DaleSpam]

That is correct, light does not accelerate to c, it is always going at c from beginning to end.

[cowmoo32]

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?

[rede96]

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 a good question!

From a physics point of view, I have often wondered exactly that. How do we see? From my limited understanding, we have to 'see' the object at the same time the photon enters the eye. Otherwise we could say that the photon carries the image.

Anyway, look forward to getting the answer on this one.

[evankiefl]

What a good question!

From a physics point of view, I have often wondered exactly that. How do we see? From my limited understanding, we have to 'see' the object at the same time the photon enters the eye. Otherwise we could say that the photon carries the image.

Anyway, look forward to getting the answer on this one.

We actually see the object slightly after the light enters the retina. The retina picks up the light source and sends a nerve impulse through the optic nerve to the brain, where it is then perceived. I think about what I am perceiving and what is happening and they are largely different things. It causes me to constantly redefine what it means to see.

[DaleSpam]

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?Are you familiar with Maxwell's equations, and in particular vacuum solutions to Maxwell's equations?

[cowmoo32]

I'm somewhat familiar, but it's been years since I took the class that covered them. I'll read up and get back to you.

[HallsofIvy]

Although you can, in quantum theory, talk about photons as particles, in relativity it is better to think of light as waves- "ripples" in the electromagnetic field. Such ripples immediately move at c just as, when you throw a rock into a pound, the ripples move immediately with whatever the wave speed is for that pond (it depends upon the depth). There is no acceleration because there is nothing "physical" actually moving with the waves.

[Naty1]

So what exactly is occurring?

nobody knows: what we do observe is that as you stated light is instantaneous at speed c.
So when you turn on a light bulb and an electron is excited to a higher energy state, when it falls to a lower energy (different quantized energy) it emits radiation.....light....but why and what is happening?? Quantum theory suggests quantum states but who ordered that??

[cowmoo32]

nobody knows: what we do observe is that as you stated light is instantaneous at speed c.
So when you turn on a light bulb and an electron is excited to a higher energy state, when it falls to a lower energy (different quantized energy) it emits radiation.....light....but why and what is happening?? Quantum theory suggests quantum states but who ordered that??
Ahhh there's the answer I was looking for. Amazing, we still don't know what light really is. I remember being fairly young (6 or 7) and trying to think of something that was truly 2D and I arrived on light shining on a wall; I guess something at the quantum scale is about as close to 2D as you can get.

[DaleSpam]

Well, if you want posts that protest complete ignorance then Naty1 is your go-to-guy. He asserts ignorance even when something is well understood.

The fact is that there is no EM phenomena that QED doesn't accurately describe, so the claim that we don't know what is happening is wrong.

[Drakkith]

Ahhh there's the answer I was looking for. Amazing, we still don't know what light really is. I remember being fairly young (6 or 7) and trying to think of something that was truly 2D and I arrived on light shining on a wall; I guess something at the quantum scale is about as close to 2D as you can get.

What? Nothing about light or the particles at a quantum scale is 2d. And yes, we really do know what light is just as much as we know what an electron or proton is.

[cowmoo32]

Well, if you want posts that protest complete ignorance then Naty1 is your go-to-guy. He asserts ignorance even when something is well understood.

The fact is that there is no EM phenomena that QED doesn't accurately describe, so the claim that we don't know what is happening is wrong.

What? Nothing about light or the particles at a quantum scale is 2d. And yes, we really do know what light is just as much as we know what an electron or proton is.I know it isn't actually 2D. I was saying something at the quantum scale is as close to 2D as you could get, in that it's extremely small/thin. I understand there is no such thing as a physical 2D space

Guess it's time to read up on QED then. Would someone mind giving me the short version of what exactly happens in the transmission of an EM wave? I think the part that is confusing for me is that no medium is required.

[DaleSpam]

Guess it's time to read up on QED then. Would someone mind giving me the short version of what exactly happens in the transmission of an EM wave? I think the part that is confusing for me is that no medium is required.You certainly don't need QED for that, Maxwell's equations are entirely sufficient. The short version is that even without a medium an E field which is changing in space makes a B field which is changing in time, and vice versa.

[Drakkith]

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

[DaleSpam]

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

[cowmoo32]

As long as you associate the "medium" only with geometric properties like distance and time and not with material properties like density and velocity.That clears things up; I was stuck thinking about something with a density.

[Drakkith]

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

That is pretty much what I was thinking. :biggrin:

[Goolds]

Photons have no mass, so it is impossible for them to accelerate. This means as soon as they are created, the travel at 'c'.

[PhilDSP]

Doesn't the value of evaluating the Stress Tensor harken back to the Maxwell-Faraday concept that ultimately EM parameters are in some way suspended and propagate in space because there is some sort of tension there? If so, that seems more like a medium than nothingness (that nothing provides the basis for EM wave propagation - only the numbers in a mathematical equation) ?

[Drakkith]

I don't believe the propagation of an EM wave requires any tension in a medium. The fields themselves are alternating back and forth, not any medium.

[PhilDSP]

But if there is no stress, any local concentration of energy or any other parameter couldn't exist - it would immediately diffuse into the background, wouldn't it? There would be nothing to support a finite speed of propagation or diffusion.

[Drakkith]

Stress of what? Why would stress be required? This isn't a wave like in water or air. It is composed of fields. The electromagnetic field of an electron needs no medium to work. Why would a photon?

[PhilDSP]

Actually the wave equations are not limited to the propagation of fields but pertain to potentials and energy, though energy indirectly because it flows in different directions than the movement of the fields.

In considering the movement of a photon, its transit will be the path of least energy resistance according to the Langrangian or Hamiltonian, won't it? If there were no stress of some type propelling it, and no type of relaxation of that stress which determines its path, then what would otherwise cause it to form and move where it moves? Sure, other physical models might work also, but to not entertain any model is pretty much saying nothing about the causation of EM propagation. And saying nothing is not particularly good science in my opinion.

[GrayGhost]

Stress of what? Why would stress be required? This isn't a wave like in water or air. It is composed of fields. The electromagnetic field of an electron needs no medium to work. Why would a photon?

Well, that's one viewpoint. It might seem that the medium is not required to exist, because the determination of electromagnetic field propagation doesn't require it. I have always found it interesting how some prefer to believe fields extend themselves by themselves unto themselves thru the nothingness. Sounds more akin to magic IMO. The fact that the medium is not required to define EM field propagation does not lead the medium doesn't exist. So while it is true that a photon's location or speed is determinable w/o description of any medium, it is an assumption (at best) that an electron (or photon) needs no medium to work. In fact, it is more likely that fields exist only because the medium exists, and that the nature of fields (in part) defines the properties of the very medium. But then, that's just my opinion.

From what I've read, both Maxwell and Einstein believed in a medium. Since neither could prove its existence and thus define it, they both left it unsaid ... except that Einstein stated any aether was superfluous to his theory. That doesn't mean it does not exist though, nor that it is more likely that it doesn't exist. Maxwell left it at fields alone, and Einstein left it at space and time, because they are measurable and thus definable. 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

[Drakkith]

Believe what you want. I thought the view that there was a medium was proved incorrect about 70+ years ago?

[DaleSpam]

I have always found it interesting how some prefer to believe fields extend themselves by themselves unto themselves thru the nothingness. Sounds more akin to magic IMO.Excellent strawman argument.

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.

[bcrowell]

From what I've read, both Maxwell and Einstein believed in a medium. Since neither could prove its existence and thus define it, they both left it unsaid ... except that Einstein stated any aether was superfluous to his theory. That doesn't mean it does not exist though, nor that it is more likely that it doesn't exist. Maxwell left it at fields alone, and Einstein left it at space and time, because they are measurable and thus definable.
As far as I know, this is 100% inaccurate as a depiction of Einstein's thought. You may have gotten this impression from secondhand descriptions of the following paper:
A. Einstein, "Über den Äther," Schweizerische naturforschende Gesellschaft 105 (1924) 85
original text - http://www.wikilivres.info/wiki/%C3%9Cber_den_%C3%84ther
English translation of [Einstein 1924]- http://www.oe.eclipse.co.uk/nom/aether.htm
commentary by John Baez on [Einstein 1924] - http://web.archive.org/web/20070204022629/http://math.ucr.edu/home/baez/RelWWW/wrong.html
In any case, it strikes me as kind of sad to debate what Einstein really thought. We might as well carry around Mao's little red book, or quote Genesis and Aristotle at each other.

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

[danR]

But if there is no stress, any local concentration of energy or any other parameter couldn't exist - it would immediately diffuse into the background, wouldn't it? There would be nothing to support a finite speed of propagation or diffusion.

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.

[ZirkMan]

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.
Exactly, great point. And also why it keeps traveling in the direction of the original momentum transfer? It cannot be because of its mass because light does that too and it's massless. It must be something more fundamental.

[GrayGhost]

Excellent strawman argument.

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.

OK, so spacetime is something as opposed to nothing, yet it is not a medium of any sort. 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? That is, they exist only because the medium exists? I consider spacetime a medium, which I tend to refer to as the spacetime continuum, and assume it something as opposed to nothing. I do not make the assumption that anything devoid of material property is nothing. The medium is what gives rise to anything material, assuming the variations and configurations of the medium within the medium exceed some required threshhold, a threshhold dictated by an inherent property of the very medium. So, all particles "are of the medium" to begin with. Classical mediums possess electric and magnetic constants, so why should spacetime be the one thing that possesses such constants and not be a medium? It's simply not of classical fluid nature. I'm not so sure there is any difference between what I am saying and wht you are saying, yet one is right and the other is not. We may never know, or at least in our lifetimes. Granted though, mention of a medium was "left out" in Einstein's work (far as I know), although his curved spacetime has the signature of a medium written all over it.

GrayGhost

[bcrowell]

OK, so spacetime is something as opposed to nothing, yet it is not a medium of any sort. 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? That is, they exist only because the medium exists? I consider spacetime a medium, which I tend to refer to as the spacetime continuum, and assume it something as opposed to nothing. I do not make the assumption that anything devoid of material property is nothing.

You haven't defined what you mean by "nothing," "medium," or "devoid of material property."

[GrayGhost]

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

A tall order, so I'll give the short version ...

everything ... the medium and all the variations and configurations of the medium within the medium.

exists ... that which is perceptable, or deducable from perception.

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

GrayGhost

[GrayGhost]

You haven't defined what you mean by "nothing," "medium," or "devoid of material property."

Hmmm. OK ...

nothing ... no medium of any kind, of classical nature or not.

medium ... see last post.

devoid of material property ... any spacetime region of no rest mass.

GrayGhost

[khemist]

Spacetime is not a medium. From what I gather, it is a 4 dimensional geometric manifold.

[DaleSpam]

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 wich 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.

[DaleSpam]

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".

[DaleSpam]

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.

[DaleSpam]

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". :biggrin:

[DaleSpam]

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

[DaleSpam]

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.

[DaleSpam]

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.

[DaleSpam]

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.

[DaleSpam]

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

[DaleSpam]

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."

[DaleSpam]

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.

[DaleSpam]

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"... :biggrin:

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 physic 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? :smile:

[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? :smile:

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?:smile:"


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.

[DaleSpam]

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.

[DaleSpam]

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?

[DaleSpam]

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.

[DaleSpam]

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.

[DaleSpam]

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.

[DaleSpam]

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 cSo 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?

[GrayGhost]

That seems true, but could that "creation process" be cut short? say in half, or 1/8?

Once the energy no longer exists within the atomic structure to maintain an electron in the conduction band, it falls back from conduction to valence band and a photon emits into the surrounding vacuum. The mechanism is the electromagnetic interaction. I'd venture it has no choice, once the drop to lower energy commences. We know that electron orbs must be separated as a function of Planck's constant. If electrons do begin a transition w/o completion, returning back to it current orb, I'm wondering if this would even be provable? Not sure. So in answer to your question, I'd say no.

At what point would it not be enough time for a photon of visable light to be created?

I do not think it's ever about "having enough time". It's strictly a matter of energy requirements.

GrayGhost

[DaleSpam]

Ah I see, that was no quip.
...
Sorry, for the brash reply in my previous posts Dalespam, I think we appreciate physics in different ways and for different reasons.No worries, a small misunderstanding on both sides is all.

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.Luckily this concept is probably something that you can learn without formal classes. I am just trying to gauge the right level to present it at so that it is helpful rather than intimidating. Are you familiar with regular vectors and components of vectors? Any calculus, geometry, and/or linear algebra?

[DaleSpam]

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.No, Drakkith and HallsOfIvy are correct, EM fields can exist in vacuum. These are known as vacuum solutions to Maxwell's equations. This is, in fact, how it was determined that visible light is an EM wave, and was critical to the development of radios.

[DaleSpam]

Aha, but there seems to be the same problem with the "material properties" concept and how you distinguish them from geometric properties.Hi TrickyDicky, I have been reflecting on our exchange for the last few days, and I think that I agree with you on this point now, even from a purely classical perspective. Specifically, I think that this post of mine is a little wrong:I think we can leave it at geometric properties in this context. your distinction is important for the EFE, but not Maxwell's equations.I don't think that the EFE can be ignored when discussing properties of spacetime, and the EFE clearly link the "geometric" property of spacetime curvature with the "material" property of stress-energy. While for many solutions of the EFE the stress-energy can be entirely attributed to "stuff", there do exist vacuum solutions where the only source of stress-energy is spacetime itself.

Of course, Maxwell's equations work perfectly well in flat spacetime with 0 stress-energy outside of the EM field. So the stress-energy of spacetime is not required for EM, and I don't think that it is helpful to discuss curvature and the EFE here. However, I now think that my statement that spacetime has only geometric properties is wrong as you pointed out.

[Drakkith]

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

Can you explain why the creation of a photon with a measurable wavelength would require a non instant transfer of energy? Or why you think that it would?

[cowmoo32]

No, Drakkith and HallsOfIvy are correct, EM fields can exist in vacuum. These are known as vacuum solutions to Maxwell's equations. This is, in fact, how it was determined that visible light is an EM wave, and was critical to the development of radios.I think you misunderstood me. I understand that an EM field can exist in vacuum. Drakkith said earlier that the EM field is the medium through which light travels. My question was if that is truly the case, then what allows light to travel through a region where no EM field exists?

[nitsuj]

No worries, a small misunderstanding on both sides is all.

Luckily this concept is probably something that you can learn without formal classes. I am just trying to gauge the right level to present it at so that it is helpful rather than intimidating. Are you familiar with regular vectors and components of vectors? Any calculus, geometry, and/or linear algebra?

I am familiar with the 10 digit, decimal notation system :smile:

You're generous with what you know Dalespam, but from a math perspective it would be wasted on me.

I took two mandatory post secondary statistics courses (quantitative methods) which imo could be considered an introduction to (useful) mathematics.

In fact of all that post secondary business education, those statistic courses fundamentally changed my reasoning far more remarkably then the other courses. I would suspect physics education has the same effect of remarkably changing reasoning.

[nitsuj]

I do not think it's ever about "having enough time". It's strictly a matter of energy requirements.

GrayGhost

Ah, so you say that the process from start to finish takes time, but that it is either enough energy to "excite" an electron out of its comfort zone or not. The amount of time the process takes is a consequence of the amount of EM energy being radiated (length of wave). (Edit: after reading Drakkith's post 105, I'll wait before swallowing this pill)

[bcrowell]

I think you misunderstood me. I understand that an EM field can exist in vacuum. Drakkith said earlier that the EM field is the medium through which light travels. My question was if that is truly the case, then what allows light to travel through a region where no EM field exists?

The EM field is not the medium through which light travels. The EM field is the thing that measures the vibration of the wave. Light doesn't travel *through* an EM field, it *is* an EM field.

[Drakkith]

I think you misunderstood me. I understand that an EM field can exist in vacuum. Drakkith said earlier that the EM field is the medium through which light travels. My question was if that is truly the case, then what allows light to travel through a region where no EM field exists?

I don't remember saying that, and if so, then I was wrong. A photon consists of an EM field(s), it does not propagate through one.

[nitsuj]

I don't remember saying that, and if so, then I was wrong. A photon consists of an EM field(s), it does not propagate through one.

It was Hallsofivy{'s} statement in post 94. But it is a one liner and may be out of context.

[PhilDSP]

The EM field is not the medium through which light travels. The EM field is the thing that measures the vibration of the wave. Light doesn't travel *through* an EM field, it *is* an EM field.

We should probably keep in mind too that it's not just the fields that are fluctuating. The potentials (at least the vector potential) vary synchronously with the fields as does, no doubt, energy. The fields seem to be observables or quasi-observables that give one view of what light is, but not the only possible view.

[GrayGhost]

Ah, so you say that the process from start to finish takes time, but that it is either enough energy to "excite" an electron out of its comfort zone or not. The amount of time the process takes is a consequence of the amount of EM energy being radiated (length of wave). (Edit: after reading Drakkith's post 105, I'll wait before swallowing this pill)

The question is ... will you take the red pill, or the blue pill :)

GrayGhost

[DaleSpam]

I am familiar with the 10 digit, decimal notation system :smile:

You're generous with what you know Dalespam, but from a math perspective it would be wasted on me.OK, I will try to sketch the outlines without much detail.

The key idea for modern relativity is geometry. Basically, we consider our universe to be four-dimensional, three dimensions of space and one dimension of time, called spacetime. In this spacetime, a "point particle" traces out a line (its position at each point in time) called it's worldline. Then physical quantities are related to geometric quantities in this spacetime. For example, the time that a clock reads is equal to the length (spacetime interval) along its worldline.

Similarly, mass is the length of a special kind of vector called the four-momentum. In normal Euclidean geometry the only way for a vector to have zero length is for it to be the zero vector. But in space-time the four-momentum vector can also be zero if it is going at c. So the only way for a photon (no mass = zero length) to have energy and momentum is to go at c.

[nitsuj]

OK, I will try to sketch the outlines without much detail.

The key idea for modern relativity is geometry. Basically, we consider our universe to be four-dimensional, three dimensions of space and one dimension of time, called spacetime. In this spacetime, a "point particle" traces out a line (its position at each point in time) called it's worldline. Then physical quantities are related to geometric quantities in this spacetime. For example, the time that a clock reads is equal to the length (spacetime interval) along its worldline.

Similarly, mass is the length of a special kind of vector called the four-momentum. In normal Euclidean geometry the only way for a vector to have zero length is for it to be the zero vector. But in space-time the four-momentum vector can also be zero if it is going at c. So the only way for a photon (no mass = zero length) to have energy and momentum is to go at c.

Thanks Dalespam, I will look into Four-Momentum. It does seem like it will be interesting.

[bobc2]

Thanks Dalespam, I will look into Four-Momentum. It does seem like it will be interesting.

nitsuj, I'm not sure whether this kind of presentation will be useful to you, but here's a try (sorry for such an extended post).

I wanted to emphasize the geometric approach suggested by DaleSpam as well as the concept that all vector components (momentum, force, velocity, etc.) transform the same as the displacements. So, we can use the same space-time diagram we use for 4-D displacement components to see how the other vector components work going from one inertial coordinate system to another.

http://i209.photobucket.com/albums/bb185/BobC_03/Four-Momentum_doc.jpg
http://i209.photobucket.com/albums/bb185/BobC_03/Four-Momentum_doc2.jpg
http://i209.photobucket.com/albums/bb185/BobC_03/Four-Momentum_doc3.jpg
http://i209.photobucket.com/albums/bb185/BobC_03/Four-Momentum_doc4.jpg
http://i209.photobucket.com/albums/bb185/BobC_03/Four-Momentum_doc5.jpg

[GrayGhost]

Can you explain why the creation of a photon with a measurable wavelength would require a non instant transfer of energy? Or why you think that it would?

The way I would put it, is that the energy transfer between atomic structure and spacetime is "continuous", assuming the process has commenced. I would not think that the photon forms in its entirely, instantaneously. The EM field is the photon. EM exists at only speed c in vacu. Therefore, I do not see the EM field instantly manifesting itself in spacetime across a region. I would figure the photon takes a duration to complete its corpuscular formation.

I've been looking for this topic on the web, and have not had much luck. Let me ask you ... consider green light of 500ns wavelength. Even if the green photon could form instantly, would a photon of that wavelength be able to form in its entirety, instantly in spacetime at speed c, within the tiny area between electron orbs that produces it?

GrayGhost

[bobc2]

Can you explain why the creation of a photon with a measurable wavelength would require a non instant transfer of energy? Or why you think that it would?

Drakkith and GrayGhost, when you talk about the behavior of an individual photon, I'm sure you know that you are now working in the realm of Quantum Mechanics and we have this strange Heisenberg Uncertainty Principle. In order to conceive of a very precise and detailed process of the creation of a photon you would have to know both position and momentum simultaneously--and also be able to specify the evolution of precise energy values simultaneously with knowledge of precise times. The Uncertainty Principle denies that as a possiblity--both experimentally and theoretically (and as a concept). You could in principle know the momentum precisely, but then the position would be completely undetermined. You could know the energy precisely, but then the time corresponding to that energy would be undetermined. That's just the way QM is.

Example: The energy of a photon is E = hf (energy equals Plank's constant times frequency). So, to have a precise fixed value of energy, you would require an infinitely long (time wise) sine wave having the required frequency. Anything other than an infinitely long sine wave will have more frequencies superimposed yielding uncertainty in the energy. But, the location in time of an infinitely long sine wave is undetermined. You could superimpose an infinite number of sine waves to create a traveling infinitessimally narrow pulse, allowing you to see the peak of the pulse at a very precisely measured time, but the collection of different frequencies would result in complete uncertainty for the energy.

Whether there is some undiscovered physics that would allow a description of the creation of a photon, I'm not sure anyone can say with 100% certainty (Einstein of course fought that description of physics for years, without success). Feynman says that just seems to be the way nature works, and he doesn't see any prospects of ever finding physics to be different from that.

In that light it would seem fruitless to pursue a detailed photon creation process. If you want to understand the macro world and creation of electromagnetic waves, then you could perhaps study the creation of a water wave, dropping a spherical steel ball into the water. But, here you have enormous numbers of water molecules all participating together (and the ball has billions of atoms)--just as you have enormous numbers of electrons oscillating in an antenna to create electromagnetic waves.

[Drakkith]

I understand that bob. I'm not trying to figure out the exact details of the creation of a photon, I'm just sticking to the idea that it forms instantly somehow. Or maybe that it's formation is unable to be determined to take a finite time or not.

[bobc2]

I understand that bob. I'm not trying to figure out the exact details of the creation of a photon, I'm just sticking to the idea that it forms instantly somehow. Or maybe that it's formation is unable to be determined to take a finite time or not.

It seems like you still run up against the Uncertainty Principle. You seem to be asking for knowledge that the photon was created in an infinitessimal increment of time, which leaves the frequency (energy) largely undetermined.

[Drakkith]

It seems like you still run up against the Uncertainty Principle. You seem to be asking for knowledge that the photon was created in an infinitessimal increment of time, which leaves the frequency (energy) largely undetermined.

How does knowing whether a photon is created instantly or not have anything to do with UP? We aren't measuring momentum, position, frequency, or anything like that. And I have yet to see any reason why the instantaneous creation of a photon is not possible. I honestly don't know the answer, but I know that nothing I've ever read or heard has said that photons are created in a finite amount of time. Does anyone have a reference or anything?

[DaleSpam]

How does knowing whether a photon is created instantly or not have anything to do with UP?There is an energy-time uncertainty principle.
http://en.wikipedia.org/wiki/Uncertainty_principle#Energy-time_uncertainty_principle

[Drakkith]

There is an energy-time uncertainty principle.
http://en.wikipedia.org/wiki/Uncertainty_principle#Energy-time_uncertainty_principle

Thanks Dale.

According to that page:

A state that only exists for a short time cannot have a definite energy. To have a definite energy, the frequency of the state must accurately be defined, and this requires the state to hang around for many cycles, the reciprocal of the required accuracy.

If I understand this correctly, this refers to measuing the energy of the photon OR the electron. I can't see how this has any relation to the creation of a photon being instaneous or not.

It seems like you still run up against the Uncertainty Principle. You seem to be asking for knowledge that the photon was created in an infinitessimal increment of time, which leaves the frequency (energy) largely undetermined.

Sure. I can agree with that. But I don't see how this applies to the discussion. Are we all on the same page here? What does anything in the last few posts have to do with the time it takes to create a photon?

[bobc2]

How does knowing whether a photon is created instantly or not have anything to do with UP? We aren't measuring momentum, position, frequency, or anything like that.

It's not just whether you have performed a measurement. The Uncertainty Principle implies that the photon cannot exist with both momentum and position defined precisely, nor can it exist with both energy and time defined precisely. This prohibits you from conceiving a detailed evolving process that results in creation of a photon.

Recall the double slit experiment. The single photon seems to somehow travel thru both slits in the form of a wave which results in either constructive interference, cancellation or partial reinforcement, depending on the position along the screen. In this situation it is difficult to talk about the creation of a photon particle.

The double slit experiment remains perhaps the most mysterious phenomenon of nature (equal or greater to the differing cross-section views of observers in Special Relativity theory).

Roger Penrose seems to believe that particle wave functions are the realities of nature, whereas many other physicists feel there is no objective reality for a particle until the wave function collapses. This all makes for difficulty in imagining the details of how a photon is created.

Having said that I like DaleSpam's earlier post that simply asserts that the photon appears instantly with velocity c. He commented in a later post that it does not accelerate to c, because if it is not moving at velocity c in the first place then it is not a photon.

And I have yet to see any reason why the instantaneous creation of a photon is not possible. I honestly don't know the answer, but I know that nothing I've ever read or heard has said that photons are created in a finite amount of time. Does anyone have a reference or anything?

I am cerainly sympathetic with your feelings about this. You are right, you will not find any literature presenting a scenario for the evolving of a photon over a finite amount of time. The closest thing to that would be to present the creation of a photon as the creation of the wave having some wavelength. Then, you must wait for at least the time period corresponding to one wavelength ( W = c/f , where W is wavelength and f is the frequency of the photon).

When I do space-time diagrams of photons and massive particles, I'm picturing a block universe with every elementary particle there as 4-D flilments extending along their respecive 4th dimensions. This is perhaps in conflict with QM (especially the Copenhagen interpretation), but I assume at the submicroscopic QM level you would see fuzziness in the filament structures, such that an observer would have no way of computing the future positions with perfect precision.

But I don't see the inability of physics to compute the future as a road block to having the block universe there with all of the infinitessimal detail. Observers could still come up with QM while at the same time the block universe has everything set in concrete, including whatever QM level fuzziness is required to satisfy our observation of QM phenomena. This does of course have an impact on very significant philosophical issues leading to considerations outside of present day physics. As the great physicist DeWitt of Chapel Hill and University of Texas said, "There is much more to reality than physics."

(By the way, I can't believe I misspelled Planck's name in an earlier post)

[danR]

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? (I would think not because technically there is no change in velocity) 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?

Going back to this original question, if it hasn't been brought up previously, I would conjecture a realist/classical answer to this question: in the case of an electron losing energy, the standing wave representing the electron's 'vibration mode' in a particular orbital configuration 'stands' nonetheless as the self-interference pattern of electric and magnetic fields already shifting values across the orbital's dimensions at the speed of light. The wave is 'standing', but its components are not.

So the matrix that gives birth to the photon already has exactly that velocity; acceleration is not involved. That may not sit well with a quantum explanation though.

[Drakkith]

I appreciate the QM lesson bob, but I am aware of all of that and I will readily agree that we cannot know whether light is created instantly or not anyways. :)

[harrylin]

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?

Yes, we may regard a light wave as some sort of EM disturbance; as a matter of fact that is just how light is treated in relativity theory. That concept was even the physical basis of the light postulate.

Cheers,
Harald

[harrylin]

Believe what you want. I thought the view that there was a medium was proved incorrect about 70+ years ago?

Why did you think that? 2011 - 70+ = ca. 1940... what do you think happened around or before that time?

[DaleSpam]

I appreciate the QM lesson I am by no means a QM expert, but IMO this whole conversation simply reflects a discomfort with basic QM wierdness. In QM things change from one state to another state without transitioning gradually through intervening states all the time. This is no different, it is perfectly standard QM fare, regardless of whether or not the creation of the photon can be pinned down to some instantaneous duration (which is also standard QM wierdness).

[harrylin]

[..] Granted though, mention of a medium was "left out" in Einstein's work (far as I know), although his curved spacetime has the signature of a medium written all over it.
GrayGhost

In fact he discussed it in several talks and papers, at least in 1918, 1920 and 1924. He considered his "curved" space-time to be a description of properties of a medium. At that time he also regarded the constant c as such a property: logically a wave propagation constant c cannot exist without a medium that has that property.

[DaleSpam]

harrylin, you are about 9 pages late to the conversation. We have already discussed this at length. It depends entirely on your definition of "medium", and becomes purely semantic.

[harrylin]

harrylin, you are about 9 pages late to the conversation. We have already discussed this at length. It depends entirely on your definition of "medium", and becomes purely semantic.

Dalespam, GreyGhost's "as far as I know" statement was incorrect and I quickly read through those pages before replying. Of course I may have overlooked it, so please give the number of the post in which the same information was given to him.

Harald

[DaleSpam]

Post 28 deals with the historical information you mentioned and posts 28 - 64 deal with the semantics of the word "medium" in this context.

[rahulbling]

sorry for the interruption
but how exactly can a photon knock out an electron, if it is considered as a particle, then it has to lose some energy, and slow down to an extent to which you can predict its movement

[DaleSpam]

When a massless particle loses energy it changes frequency, not speed.

[GrayGhost]

I am by no means a QM expert, but IMO this whole conversation simply reflects a discomfort with basic QM wierdness. In QM things change from one state to another state without transitioning gradually through intervening states all the time. This is no different, it is perfectly standard QM fare, regardless of whether or not the creation of the photon can be pinned down to some instantaneous duration (which is also standard QM wierdness).

Well, I'm not denying quantum weirdness, nor the QM wave equation. However, how is it that we know the electron's wavefunction is not collapsed during the band transitioning mechanism? If its not collapsed, I would agree the photon forms instantly in spacetime ... although then the wavelength of the photon should be compared against the portion of atomic structure that produces it, as a verification of reasonable. If the wavefunction is collapsed during electron transitioning, then the formation of a photon over duration seems reasonable, because the electron always has a definite location in space and time as it makes the transition.

When a photon arrives at the electron, could it not be said that the electron's wavefuntion collapses, given that observing something causes a wavefunction to collapse? Could the reverse be true when a photon is released with the electron as it drops back to valence band?

It may well be impossible to ever know ?

GrayGhost

[GrayGhost]

In fact he discussed it in several talks and papers, at least in 1918, 1920 and 1924. He considered his "curved" space-time to be a description of properties of a medium. At that time he also regarded the constant c as such a property: logically a wave propagation constant c cannot exist without a medium that has that property.

Yes, I was referring to the actual published theory, in 1915. I do realize that Einstein deliberated over a medium for the years thereafter.

GrayGhost

[DaleSpam]

However, how is it that we know the electron's wavefunction is not collapsed during the band transitioning mechanism?In the end we would know because experiments agree with the results of calculations that use the uncollapsed wavefunction and do not agree with the results of calculations that use the collapsed wavefunction. Unfortunately, I don't know the QM literature well enough to cite any experiments to that effect, but QM is extremely well-tested so I would be stunned if they did not exist. The only reason that we have to bother with any of this quantum weirdness is because it works experimentally.

[GrayGhost]

In the end we would know because experiments agree with the results of calculations that use the uncollapsed wavefunction and do not agree with the results of calculations that use the collapsed wavefunction. Unfortunately, I don't know the QM literature well enough to cite any experiments to that effect, but QM is extremely well-tested so I would be stunned if they did not exist. The only reason that we have to bother with any of this quantum weirdness is because it works experimentally.

Well, that being the case, I'd have to accept that the photon would be considered to form at-once ... whatever "at-once" means at the quantum level. If the electron jumps the bands in zero time, then it stands to reason the photon should be formed at-once. Difficult to swallow, but QM is a rock solid theory, so.

GrayGhost

[DaleSpam]

whatever "at-once" means at the quantum level.:smile: Well said.

[Drakkith]

:smile: Well said.

I agree!

[bobc2]

Well, that being the case, I'd have to accept that the photon would be considered to form at-once ... whatever "at-once" means at the quantum level. If the electron jumps the bands in zero time, then it stands to reason the photon should be formed at-once. Difficult to swallow, but QM is a rock solid theory, so.

GrayGhost

Perhaps the closest we could get to instantaneous would be within a Planck time.

[Drakkith]

Perhaps the closest we could get to instantaneous would be within a Planck time.

Closest we could get to measuring it, or calculating it, or what?

[bcrowell]

Yes, I was referring to the actual published theory, in 1915. I do realize that Einstein deliberated over a medium for the years thereafter.

No, this is incorrect. Einstein wrote a 1924 paper in which he made the philosophical point that although relativity killed off the luminiferous aether as the supposed medium of electromagnetic vibrations, it still imbued the vacuum with specific physical characteristics, such as curvature and energy. The basic point of the paper is that we can't decide, purely based on philophical ideas like Mach's principle, whether the vacuum has its own properties; we actually have to go through the usual scientific cycle of theory and experiment in order to find out the answer. Internet kooks love to misinterpret and overinterpret this paper, or to misrepresent it by saying that Einstein referred to GR in general, throughout his career, as an aether theory.

A. Einstein, "Über den Äther," Schweizerische naturforschende Gesellschaft 105 (1924) 85

original text - http://www.wikilivres.info/wiki/%C3%9Cber_den_%C3%84ther

English translation of [Einstein 1924]- http://www.oe.eclipse.co.uk/nom/aether.htm

commentary by John Baez on [Einstein 1924] - http://web.archive.org/web/20070204022629/http://math.ucr.edu/home/baez/RelWWW/wrong.html

[GrayGhost]

No, this is incorrect. Einstein wrote a 1924 paper in which he made the philosophical point that although relativity killed off the luminiferous aether as the supposed medium of electromagnetic vibrations, it still imbued the vacuum with specific physical characteristics, such as curvature and energy. The basic point of the paper is that we can't decide, purely based on philophical ideas like Mach's principle, whether the vacuum has its own properties; we actually have to go through the usual scientific cycle of theory and experiment in order to find out the answer.

Yes, I see I used the word "deliberated", which was a mistake on my part. I meant that he "debated" its existence over the years thereafter (in favor of). My point was that "aether or medium" was not mentioned in the original published theories of Maxwell or Einstein, even though they both believed one existed (of sort) at that time. Einstein's medium (of course) not being the traditional classical aether Maxwell had assumed.

Wrt your reference ...
Einstein 1924 (http://www.oe.eclipse.co.uk/nom/aether.htm):Now, it might be claimed that this concept covers all objects of physics, for according to consistent field theory, even ponderable matter, or its constituent elementary particles, are to be understood as fields of some kind or particular ‘states of space’. But it must be admitted that such a view would be premature, since, thus far, all efforts directed toward this goal have foundered.

Thanx for this reference bcrowell. I happen to be one of these folks, those who have the opinion that all objects are some state of the medium. I do agree that any opinion is premature in the lack of enough proof. Nonetheless, everything is premature until proven true, or otherwise. I must say though, it is very surprising IMO that in the past 96 years, it remains premature.

GrayGhost

[harrylin]

Post 28 deals with the historical information you mentioned and posts 28 - 64 deal with the semantics of the word "medium" in this context.

OK I see - yes indeed, that refers to a different but similar remark by GreyGhost, and it already contains part of my reply. Note that my reply was just about a historical fact related to the OP's question, it's not about semantics.

Cheers,
Harald

[harrylin]

Yes, I was referring to the actual published theory, in 1915. I do realize that Einstein deliberated over a medium for the years thereafter.

GrayGhost

Ah yes OK, his theory as first published only discusses observables and not possible (meta)physical explanations. That's the safest thing to do. :tongue2:

PS. I see nothing wrong with "deliberated": his 1920 discourse may certainly be called a "thoughtful, careful, or lengthy consideration". - dictionary.com

[harrylin]

[..] I happen to be one of these folks, those who have the opinion that all objects are some state of the medium. I do agree that any opinion is premature in the lack of enough proof. Nonetheless, everything is premature until proven true, or otherwise. I must say though, it is very surprising IMO that in the past 96 years, it remains premature.

GrayGhost

A century isn't much if you think of other things like that: didn't atoms take more than a millennium to be proven beyond reasonable doubt? :rolleyes:

Cheers,
Harald

[GrayGhost]

A century isn't much if you think of other things like that: didn't atoms take more than a millennium to be proven beyond reasonable doubt? :rolleyes:

Given we've already been thru the dark ages, consider me anxious :)

GrayGhost

[GrayGhost]

Roger Penrose seems to believe that particle wave functions are the realities of nature, whereas many other physicists feel there is no objective reality for a particle until the wave function collapses. This all makes for difficulty in imagining the details of how a photon is created.

I've always thought Sir Penrose believed in the infinite quantum simultaneous states of the wavefunction. Given such, it would seem that he does not disagree with non-objectivity at the quantum realm. If I recall properly, he is trying to determine why uncertainty vanishes at some level just above the size of some certain small molecules (ie a mass threshhold). At said size threshhold, the wave nature of particles vanishes in the double slit experiments. Penrose seems to think that the collective (local) gravity field of a particle system (at said threshhold) becomes unstable, collapsing the wavefunction into a single state. If's he's right, and establishes the theory accordingly, he believes he might merge GR with QMs.

GrayGhost

[GrayGhost]

Perhaps the closest we could get to instantaneous would be within a Planck time.

Theoretically, and in relation to any physical meaning, I'd agree. All we need to do is increase the accuracy of the 4pi microscope (http://en.wikipedia.org/wiki/4Pi_Microscope) some 1 x 1028 times over, then we're good :)

GrayGhost