Light - What exactly is happening?

[TrickyDicky]

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

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

 

[TrickyDicky]

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

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

 

[Dale]

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

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

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

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

 

[TrickyDicky]

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

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

Great, some tautologies are cool.

 

[bobc2]

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

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

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

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

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

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

 

[GrayGhost]

Great, some tautologies are cool.

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

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

Any ideas?

GrayGhost

 

[PhilDSP]

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

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

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

 

[Drakkith]

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

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

Any ideas?

GrayGhost

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

 

[PhilDSP]

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

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

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

 

[TrickyDicky]

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

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

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

I'm more interested in the second OP question:

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

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

 

[TrickyDicky]

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

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

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

 

[GrayGhost]

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

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

GrayGhost

 

[nitsuj]

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

From a kosher physicist definition yes.

 

[nitsuj]

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

GrayGhost

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

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

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

 

[nitsuj]

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

Off topic sorry,

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

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

 

[Drakkith]

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

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

 

[GrayGhost]

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

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

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

Or, is the transition considered instant?

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

GrayGhost

 

[Drakkith]

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

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

Or, is the transition considered instant?

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

GrayGhost

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

 

[GrayGhost]

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

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

GrayGhost

 

[nitsuj]

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

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

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

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

 

[Quickless]

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

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

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

How is this?

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

 

[Dale]

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

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

 

[Dale]

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

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

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

 

[nitsuj]

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

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

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

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

 

[nitsuj]

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

what does that mean?

 

[Dale]

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

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

 

[GrayGhost]

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

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

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

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

GrayGhost

 

[nitsuj]

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

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

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

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

 

[Dale]

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

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

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

 

[nitsuj]

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

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

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

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

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