Light - What exactly is happening?

[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.

 

[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?

 

[Dale]

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?

 

[Dale]

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.

 

[Dale]

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

 

[Dale]

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

 

[Dale]

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

 

[Dale]

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

 

[Dale]

whatever "at-once" means at the quantum level.

Well said.