Light - What exactly is happening?

[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.

 

[Drakkith]

Well said.

I agree!

 

[bobc2]

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

Perhaps the closest we could get to instantaneous would be within a Planck time.

 

[Drakkith]

Perhaps the closest we could get to instantaneous would be within a Planck time.

Closest we could get to measuring it, or calculating it, or what?

 

[bcrowell]

Yes, I was referring to the actual published theory, in 1915. I do realize that Einstein deliberated over a medium for the years thereafter.

No, this is incorrect. Einstein wrote a 1924 paper in which he made the philosophical point that although relativity killed off the luminiferous aether as the supposed medium of electromagnetic vibrations, it still imbued the vacuum with specific physical characteristics, such as curvature and energy. The basic point of the paper is that we can't decide, purely based on philophical ideas like Mach's principle, whether the vacuum has its own properties; we actually have to go through the usual scientific cycle of theory and experiment in order to find out the answer. Internet kooks love to misinterpret and overinterpret this paper, or to misrepresent it by saying that Einstein referred to GR in general, throughout his career, as an aether theory.

A. Einstein, "Über den Äther," Schweizerische naturforschende Gesellschaft 105 (1924) 85

original text - http://www.wikilivres.info/wiki/Über_den_Äther

English translation of [Einstein 1924]- http://www.oe.eclipse.co.uk/nom/aether.htm

commentary by John Baez on [Einstein 1924] - http://web.archive.org/web/20070204022629/http://math.ucr.edu/home/baez/RelWWW/wrong.html

 

[GrayGhost]

No, this is incorrect. Einstein wrote a 1924 paper in which he made the philosophical point that although relativity killed off the luminiferous aether as the supposed medium of electromagnetic vibrations, it still imbued the vacuum with specific physical characteristics, such as curvature and energy. The basic point of the paper is that we can't decide, purely based on philophical ideas like Mach's principle, whether the vacuum has its own properties; we actually have to go through the usual scientific cycle of theory and experiment in order to find out the answer.

Yes, I see I used the word "deliberated", which was a mistake on my part. I meant that he "debated" its existence over the years thereafter (in favor of). My point was that "aether or medium" was not mentioned in the original published theories of Maxwell or Einstein, even though they both believed one existed (of sort) at that time. Einstein's medium (of course) not being the traditional classical aether Maxwell had assumed.

Wrt your reference ...

http://www.oe.eclipse.co.uk/nom/aether.htm" :Now, it might be claimed that this concept covers all objects of physics, for according to consistent field theory, even ponderable matter, or its constituent elementary particles, are to be understood as fields of some kind or particular ‘states of space’. But it must be admitted that such a view would be premature, since, thus far, all efforts directed toward this goal have foundered.​

Thanx for this reference bcrowell. I happen to be one of these folks, those who have the opinion that all objects are some state of the medium. I do agree that any opinion is premature in the lack of enough proof. Nonetheless, everything is premature until proven true, or otherwise. I must say though, it is very surprising IMO that in the past 96 years, it remains premature.

GrayGhost

 

[harrylin]

Post 28 deals with the historical information you mentioned and posts 28 - 64 deal with the semantics of the word "medium" in this context.

OK I see - yes indeed, that refers to a different but similar remark by GreyGhost, and it already contains part of my reply. Note that my reply was just about a historical fact related to the OP's question, it's not about semantics.

Cheers,
Harald

 

[harrylin]

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

Ah yes OK, his theory as first published only discusses observables and not possible (meta)physical explanations. That's the safest thing to do.

PS. I see nothing wrong with "deliberated": his 1920 discourse may certainly be called a "thoughtful, careful, or lengthy consideration". - dictionary.com

 

[harrylin]

[..] I happen to be one of these folks, those who have the opinion that all objects are some state of the medium. I do agree that any opinion is premature in the lack of enough proof. Nonetheless, everything is premature until proven true, or otherwise. I must say though, it is very surprising IMO that in the past 96 years, it remains premature.

GrayGhost

A century isn't much if you think of other things like that: didn't atoms take more than a millennium to be proven beyond reasonable doubt?

Cheers,
Harald

 

[GrayGhost]

A century isn't much if you think of other things like that: didn't atoms take more than a millennium to be proven beyond reasonable doubt?

Given we've already been thru the dark ages, consider me anxious :)

GrayGhost

 

[GrayGhost]

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.

I've always thought Sir Penrose believed in the infinite quantum simultaneous states of the wavefunction. Given such, it would seem that he does not disagree with non-objectivity at the quantum realm. If I recall properly, he is trying to determine why uncertainty vanishes at some level just above the size of some certain small molecules (ie a mass threshhold). At said size threshhold, the wave nature of particles vanishes in the double slit experiments. Penrose seems to think that the collective (local) gravity field of a particle system (at said threshhold) becomes unstable, collapsing the wavefunction into a single state. If's he's right, and establishes the theory accordingly, he believes he might merge GR with QMs.

GrayGhost