Light can't travel at the speed of light?

[croghan27]

This link might cast some light on your enquiries.http://www.wired.com/science/discoveries/magazine/15-11/st_alphageek#

Hehehehehe ... it may cast light on my enquiries - even as it certainly makes murky the answers.

Is it the temperature that causes the 'delay' or the constituents of the BCE?

What is this BCE made of ... can I get some at the local Home Hardwear?

If temperature comes from the vibration of molecular/atomic sized particles and the closer to absolute zero you get is the closer to lack of vibrations, does this cause the 'light' or the energy of the photon, to delay its passage through the BCE?

I take it that: Her first trick was slowing a pulse of light to a crawl — 15 mph as it traveled through the BEC. was not referring to the photon itself (having been told they go at 'c' all the time) but of the admittance and rejection of the impulse provided by the photon through the BEC. But how does Pro. Hau 'freeze' a beam and then 'release' it?

Looks like the reporter was dazzled by reporting the story - as am I, in reading it.

(Is this a thread drift?)

 

[jtbell]

no it doesn't , that is just the time lag of the photons hitting atoms, getting absorbed and then re-emitted , but they always travel at c .

That simple "time lag between absorption and re-emission" picture is incorrect. I suggest you take a look at post #4 in the "Physics Forums FAQ" at the top of this very forum.

 

[cragar]

I see , interesting

 

[physics pfan]

There is a severe misunderstanding here on what is meant by the word "particle" as applied to photons especially. Would you still insist on the same criteria for photons if we instead call it "light corpuscular"? Would you also then want a light corpuscular to have mass?

So we instead call it a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical "particle", the latter of which has (i) a definite position (ii) a definite BOUNDARY in space of what it is (i.e. you can see where a ping pong ball ends), etc. This is not what a photon is. We didn't call it a "particle" because it has those properties. We call it a particle because its energy comes in quantized clumps. That's it!

And if you want to argue that "Hey, doesn't energy implies mass via E=mc^2?", then I'll point to you our FAQ that has dealt with this misunderstanding so many times already.

Moral of the story: never get hung up on the ordinary English words that we use to describe something in physics. The actual physics description of it is the overriding principle, and the words we use to describe it is a very weak attempt at understanding nature, and as can be seen here, is filled with unintended connotations.

Zz.

Dr. Z:

You have added a lot to this forum and I am in awe of your knowledge. But a couple of small points regarding your post.

First you say: "So we instead call it [the photon] a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical 'particle'..."

And then you say: "We call it a particle because its energy comes in quantized clumps. That's it!"

My guess is you wrote the latter statement in haste. We are really dealing here with the wave-particle duality of the photon which is a rather deep matter. If said photon of quantized energy terminated by spreading its energy and momentum over all available space paths then I don't think we would call it a particle; we would probably call it a "wave."
But of course the photon doesn't terminate over multiple locations; it terminates only at one space location. So we call it a "particle" because we have adopted the classical model that an entity of kinetic energy traversing space is a PROJECTILE that has a single space point of impact/termination. Whether this "projectile" model is valid for the photon is open to question [although not by most physicists], but that is another question.
But I would respectfully suggest that you reconsider the assertion that for the photon particle == quantized energy.

Physics Pfan

 

[ZapperZ]

Dr. Z:

You have added a lot to this forum and I am in awe of your knowledge. But a couple of small points regarding your post.

First you say: "So we instead call it [the photon] a "particle" because when we make some measurements of it, it tends to behave as if it is our ordinary, classical 'particle'..."

And then you say: "We call it a particle because its energy comes in quantized clumps. That's it!"

My guess is you wrote the latter statement in haste. We are really dealing here with the wave-particle duality of the photon which is a rather deep matter. If said photon of quantized energy terminated by spreading its energy and momentum over all available space paths then I don't think we would call it a particle; we would probably call it a "wave."
But of course the photon doesn't terminate over multiple locations; it terminates only at one space location. So we call it a "particle" because we have adopted the classical model that an entity of kinetic energy traversing space is a PROJECTILE that has a single space point of impact/termination. Whether this "projectile" model is valid for the photon is open to question [although not by most physicists], but that is another question.
But I would respectfully suggest that you reconsider the assertion that for the photon particle == quantized energy.

Physics Pfan

In a superconductor, the wavefunction for the Cooper pairs, which are paired electrons, are non-localized. This means that the cooper pairs, at any given instant before a measurement, are "spread out" all over the relevant space of the superconductor. Yet, these are "particles", meaning that these are composed of "electrons". It illustrates the case where even such "particles" CAN have the situation similar to what you deem as "classical wave", meaning they are spread out over all space the way a classical wave should.

It also means that photons can do the same thing even when we consider them as "clumps" of energy (and spin and momentum). So your assertion that only "wave" should be able to exhibit this spread in location doesn't hold true, because we have seen similar behavior in "particles" as well.

What people are missing here is that, once something can be described quantum mechanically, then hanging on to our classical biases no longer works! This is true be it for something we classically thought to be a wave (light), all the way to something we thought to be a classical particle (electrons, protons, neutrons, buckyballs, etc.). And the fact that these are consistently described via ONE single formulation (QM) and where all the wave-like and particle-like observations can be derived using just that one formulation, should dispel the myth that there is such a thing as a "wave-particle duality" in QM. There is a wave-particle duality in classical physics and in our minds, but there isn't any in QM! How can there be when you use the same set of description for everything?

Zz.

 

[threadmark]

If you are referring to the quadradic mean then yes that’s a formulation, but if your referring to quantum mechanics then no its not a formulation it’s a principle.

 

[ZapperZ]

If you are referring to the quadradic mean then yes that’s a formulation, but if your referring to quantum mechanics then no its not a formulation it’s a principle.

This makes zero sense.

Zz.

 

[threadmark]

The fact is, Niels Bohr gave us the description of the complimentary. being you need wave and particle functions to describe light. Look it up, its fact. you can also look up Luis de Broglie how stated
E=hv ; p=hv/c
On the left is the equation belonging to particles. And on the right are properties relating frequency/waves. even to measure the particle property of momentum you need to know the wave property called frequency

 

[Born2bwire]

The fact is, Niels Bohr gave us the description of the complimentary. being you need wave and particle functions to describe light. Look it up, its fact. you can also look up Luis de Broglie how stated
E=hv ; p=hv/c
On the left is the equation belonging to particles. And on the right are properties relating frequency/waves. even to measure the particle property of momentum you need to know the wave property called frequency

Except that Bohr and deBroglie ended up being wrong. Their ideas were the precursors to modern quantum mechanics. In particular, deBroglie's matterwave is a bit egregious because it only works with single particles in vacuum. If you were to place potential barriers, like a quantum well, then deBroglie's equation does not work.

ZapperZ cannot be more explicit in his statements here. Quantum mechanics has no wave-particle duality. It treats light, electrons, etc. all equally. The wave-particle duality is a consequence of trying to define quantum behavior within the limitations of classical physics. The behavior of quantum mechanics was observed prior to the development of a mature theory with such things as the photoelectric effect and the behavior of atoms.

 

[threadmark]

His 1924 doctoral thesis, Recherches sur la théorie des quanta (Research on Quantum Theory), introduced his theory of electron waves. This included the wave-particle duality theory of matter, based on the work of Albert Einstein and Max Planck on light. The thesis examiners, unsure of the material, passed his thesis to Einstein for evaluation who endorsed his wave-particle duality proposal wholeheartedly; de Broglie was awarded his doctorate.

 

[SpectraCat]

His 1924 doctoral thesis, Recherches sur la théorie des quanta (Research on Quantum Theory), introduced his theory of electron waves. This included the wave-particle duality theory of matter, based on the work of Albert Einstein and Max Planck on light. The thesis examiners, unsure of the material, passed his thesis to Einstein for evaluation who endorsed his wave-particle duality proposal wholeheartedly; de Broglie was awarded his doctorate.

Yep, that's a great story .. I tell it all the time to my students ... but what is your point? That was almost 90 years ago ... quantum theory has evolved just a little since then. You seem to have gotten huffy because ZZ and some others didn't give appropriate credit to the founders of QM when making statements. Please. This stuff is in the mainstream, and is part of the introductory chapters of thousands of textbooks .. it is public domain common knowledge, and THAT is the greatest testament to the founders of QM, that their theory has persisted and evolved for almost 100 years into "the most successful theory in physics".

 

[Born2bwire]

His 1924 doctoral thesis, Recherches sur la théorie des quanta (Research on Quantum Theory), introduced his theory of electron waves. This included the wave-particle duality theory of matter, based on the work of Albert Einstein and Max Planck on light. The thesis examiners, unsure of the material, passed his thesis to Einstein for evaluation who endorsed his wave-particle duality proposal wholeheartedly; de Broglie was awarded his doctorate.

And deBroglie did not develop a complete theory, again his ideas were direct precursors to modern quantum mechanics. Much of the foundations of quantum mechanics were worked out through the last half of the 1920's. Schroedinger developed the non-relativistic wave equation after deBroglie presented his electron waves. Heisenberg developed his formulation after deBoglie. In modern theory, deBroglie's ideas are a historical footnote. The next large development was quantum field theory and its application in quantum electrodynamics which was developed initially in the 1940's and resurfaced in the 1950's. All of these theories do not have an explicit wave-particle duality. In QM and QFT, we treat the photon, electron, gravitron, etc. all with the same physics. The phrase "wave-particle" is merely a stop-gap that attempts to bridge the properties assigned to classical waves and classical particles to how a field and its assciated particle behaves in quantum mechanics.

And really, if you are going to post word for word other people's work, you should cite where you took the passage from, in this case Wikipedia.

 

[DaveC426913]

And really, if you are going to post word for word other people's work, you should cite where you took the passage from, in this case Wikipedia.

Yeah. Threadmark: technically, that's plagiarism. Use the quotes feature.

 

[threadmark]

I think we need to stick to the facts,( Light can't travel at the speed of light?)
Well to answer this question, we need to ask what is light and what is it traveling in. empty space? , glass? Its 299,792,458m/s if E=mc^2. if light can't travel at the speed of light then tell us where Einstein got his formula wrong.

 

[ZapperZ]

I think we need to stick to the facts,( Light can't travel at the speed of light?)
Well to answer this question, we need to ask what is light and what is it traveling in. empty space? , glass? Its 299,792,458m/s if E=mc^2. if light can't travel at the speed of light then tell us where Einstein got his formula wrong.

It was croghan27 who made the faulty connection that all "particles" must have mass, and since light is a "particle", how can it travel at c! The rest of us have been trying to correct that by showing that a "particle", especially in the QM concept of light, need not have a mass! That's why a photon can travel at c!

Einstein did not get Special Relativity wrong. None of the responses that many of us have done here indicated that SR is wrong.

Now which part of that did you not understand?

Zz.

 

[threadmark]

Hey dude, I refer to the original question of this post. i study the history of physics, from Newtonian physics/classical physics and quanta. My books may be old but they still are relative today. It confused me when you stated it acts like a particle. So I post my books information to get feedback but you still say I am wrong. I came at you like I did because the information I have known for years had been protested by zapperz. I have taught this history to people.

 

[SpectraCat]

Hey dude, I refer to the original question of this post. i study the history of physics, from Newtonian physics/classical physics and quanta. My books may be old but they still are relative today. It confused me when you stated it acts like a particle. So I post my books information to get feedback but you still say I am wrong. I came at you like I did because the information I have known for years had been protested by zapperz. I have taught this history to people.

No one said the *history* was wrong ... they just pointed out that things have evolved a bit since the inception of QM, and people now know that the complementarity principle is basically superfluous, and was just a way of rationalizing the confusing results of QM to an audience that had only ever known CM. If you start out learning QM early, it still seems weird, but it's easier to come to grips with. It also means that we (scientists) are more willing to accept that the QM description of "particles" is complete without need for analogy to classical concepts of waves or particles.

 

[ZapperZ]

Hey dude, I refer to the original question of this post. i study the history of physics, from Newtonian physics/classical physics and quanta. My books may be old but they still are relative today. It confused me when you stated it acts like a particle. So I post my books information to get feedback but you still say I am wrong. I came at you like I did because the information I have known for years had been protested by zapperz. I have taught this history to people.

Too bad you don't understand the present!

If you continue to provide what are essentially a physics question, but with outdated historical answers, then you've provided erroneous answers. I'm shocked you didn't implicate any Caloric theory along the way to smack down Thermodynamics. You also seem to read things that aren't there. No one who responded to the OP even suggested that SR is wrong, yet you somehow implicated as such and merely added to the non-relevant discussion.

I strongly suggest you wait until a relevant historical question comes up, or hang around the History forum, before you jump in with your "expertise".

The OP no longer seems to respond or participate in this discussion, so unless he/she contact me for further clarification, I will assume that this has been sufficiently addressed. So this thread is done. This thread was never meant as a "historical discussion of outdated quantum physics".

Zz.