Light - What exactly is happening?

[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?

 

[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

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

 

[Dale]

I am familiar with the 10 digit, decimal notation system

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.

 

[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.