Light - What exactly is happening?

[TrickyDicky]

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

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

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

 

[GrayGhost]

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

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

GrayGhost

 

[nitsuj]

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

From a kosher physicist definition yes.

 

[nitsuj]

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

GrayGhost

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

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

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

 

[nitsuj]

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

Off topic sorry,

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

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

 

[Drakkith]

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

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

 

[GrayGhost]

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

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

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

Or, is the transition considered instant?

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

GrayGhost

 

[Drakkith]

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

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

Or, is the transition considered instant?

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

GrayGhost

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

 

[GrayGhost]

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

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

GrayGhost

 

[nitsuj]

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

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

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

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

 

[Quickless]

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

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

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

How is this?

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

 

[Dale]

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

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

 

[Dale]

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

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

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

 

[nitsuj]

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

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

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

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

 

[nitsuj]

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

what does that mean?

 

[Dale]

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

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

 

[GrayGhost]

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

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

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

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

GrayGhost

 

[nitsuj]

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

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

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

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

 

[Dale]

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

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

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

 

[nitsuj]

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

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

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

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

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

 

[Dale]

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

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

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

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

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

 

[GrayGhost]

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

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

GrayGhost

 

[nitsuj]

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

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

Ah I see, that was no quip.

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

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

 

[HallsofIvy]

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

No, the medium is the electromagnetic field.

 

[Dale]

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

Yes, it is just shorter to write.

 

[cowmoo32]

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

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

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

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

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

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

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

No, the medium is the electromagnetic field.

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

 

[Drakkith]

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

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

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

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

 

[cowmoo32]

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

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

 

[GrayGhost]

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

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

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

GrayGhost

 

[nitsuj]

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

GrayGhost

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

 

[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

 

[Dale]

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?

 

[Dale]

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.

 

[Dale]

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?