Light - What exactly is happening?

[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 threshold dictated by an inherent property of the very medium. So, all particles "are of the medium" to begin with. Classical mediums possesses 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.

 

[Dale]

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

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

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

 

[PhilDSP]

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

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

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

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

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

 

[TrickyDicky]

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

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

 

[Dale]

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

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

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

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

 

[TrickyDicky]

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

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

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

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

 

[Dale]

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

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

 

[TrickyDicky]

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

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

 

[Quickless]

There are mathematical models and then there is reality. For some the distinction becomes quite blurred.

In all relativity theories, “space” is defined by crude assumption. The choices being mathematical convenient.

My opinion of “space-time” is that it is a mathematical construction, a creature of the mind. The “space” being referenced relating to a fuzzy concept that in some way captures the attribute of distance.

Our concept of “space” is incomplete.

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

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

 

[Dale]

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

No, I wouldn't.

 

[TrickyDicky]

No, I wouldn't.

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

 

[Dale]

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

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

 

[TrickyDicky]

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

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

 

[Dale]

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

 

[TrickyDicky]

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

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

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

 

[Dale]

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

How does it show up in QED?

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

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

 

[TrickyDicky]

How does it show up in QED?

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

 

[Dale]

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

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

 

[TrickyDicky]

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

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

 

[Dale]

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

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

 

[TrickyDicky]

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

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

 

[GrayGhost]

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

Our concept of “space” is incomplete.

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

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

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

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

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

GrayGhost

 

[Dale]

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

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

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

 

[TrickyDicky]

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

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

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

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

 

[PhilDSP]

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

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

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

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

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

 

[Dale]

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

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