Q.e.d.

[Guest antony]

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[snoopy]

Hi Anthony,

 

Quantum Electrodynamics is quite difficult.

 

It will probably be easier to deal with your points in reverse order.

 

Color (american spelling not colour)

 

Color was introduced as the strong interaction in quarks analog to charge in the electromagnetic force. The term was introduced to label a property of quarks which allowed apparently identical quarks to reside in the same particle

eg 2 up quarks in the proton 2 down quarks in the neutron.

 

A proton is made up of 2 up and 1 down quark to allow three particles to coexist and satisfy the Pauli exclusion principle a property with three values was needed.

 

The idea of three colors like blue, red and green light making up white light was made up and these combinations were only allowed in colourless combinations (white light).

 

They are not real colours.... color is a property not a palette.

The property can be considered as a "color charge" with only colorless combinations allowed.

 

The anti-quarks have "anti-color" so mesons can be colorless by having a red and anti-red combination.

 

Proof that they must come in 3 colors comes from the omega-minus baryon

which consists of three strange quarks.

 

Since quarks are fermions with spin 1/2 they have to obey the pauli exclusion principle and cannot exist together in identical states so the "color" property must have 3 values.

 

Gluons carry the color force between quarks and are considered bi-colored having a unit of color and a unit of anti-color so more than one color is changed at the same time in quark interactions which only happens inside the particle not outside.

 

 

Your first question is not that clear I have re-read it several times and I am still not getting to what you want answered and its getting late and I gave myself a headache answering that last question.

 

I will have another go at it tomorrow.

 

Cheers

:)

[Guest antony]

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[Turtle]

When ‘light’ is shone upon a potentially reflective surface, in simple terms, the majority of photons reflect at an angle equal to the angle with which they originally shone upon the surface.

 

My query concerns the photons that reflect with an angle other than that with which they approached. ...Many thanks

antony

 

...Your first question is not that clear I have re-read it several times and I am still not getting to what you want answered and its getting late and I gave myself a headache answering that last question.

 

i'll have a go at it. :doh: the confusion appears to arise in the conjunction of the phrases 'majority of photons' & 'reflect at an angle equal to the angle with which they originally shone upon the surface.'

 

photons not reflected are either transmitted through the material whose boundary is the surface or absorbed by the material. for the reflected photons, the angle of reflection is always equal to the angle of incidence. :doh: :)

[CraigD]

Welcome to hypography, antony! If I may, I’ll take a few of your several questions.

Does the Gluon have definitive size?

Short answer: it’s not meaningful to speak of the size (ie: volume) of a boson, the kind of particle a gluon is. They don’t follow Fermi-Dirac statistics, as fermions (electrons, quarks, etc) do, so any number of them may, in classical terms, “occupy the same space”, defying the usual techniques for approximating size (eg: counting how many of a thing fits in a container of given volume). In quantum mechanical terms, bosons, like fermions, are wave equations, but unlike fermions, their wave equations can’t be used to statistically describe a meaningful classical “size”.

 

Photons are also fundamental (gauge) bosons, so the same applies to them.

You mention that it carries a color & and an anti-color so I presume so…

It’s important, as snoopy aptly explained in his previous post, that the “color” attribute of quarks is just an arbitrarily selected term (like their names, “up”, “down”, “charm”, “strange”, “top/truth” and “bottom/beauty”). It has nothing to do with light frequency/wavelength, the usual physical quantity equated with the term “color”.

…. I am still unclear if a photon of ‘white light’ has a definitive size

There’s no such thing as a photon of white light. The color white is not a “primary color”. It’s not associated with a particular frequency, as are colors such as red (about 450 THz), blue (about 650 THz) and green (about 570 THz), but a perceptual effect caused by our eyes or other sensing devices receiving photons of many frequencies at the same time.

- if a photon of ultra violet is the smallest photon of ‘visible light’ due to its short wavelength, then does that mean a photon of white light is the length of a red photon (which encompasses X-wavelengths of the other colors)?

A photon has a definite frequency, and, since it travels at a constant speed, a definite wavelength. However, higher frequency/shorter wavelength photons are not in any way “physically smaller” than lower frequency/longer wavelength ones. Because their energy – the amount of work they can do – is directly proportional to their frequency, in a sense, shorter wavelength photons are “bigger” than longer wavelength ones, though “bigger” as I use it here means “more consequential” rather than referring to a length or volume.

 

Although photons with frequencies lower than visible red ones are often termed “infra-red” (meaning “under red”), “red” is not considered an overall term for all EM radiation frequencies lower red’s. Rather, ranges of frequencies are given names related to the terminology of disciplines concerned with them. For example, at about 300 GHz, the term “far infra-red” gives way to the term “extremely high frequency”, a naming convention that continues down to the range of lowest detectable frequencies, “extremely low frequency”, with a few odd adjectives such as “ultra”, “super”, and “voice” appearing. Many diagrams labeled with these terms are available, such as the one in this wikipedia article.

[Guest antony]

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[snoopy]

Hi Antony,

 

First of all I have to apologise for spelling your name incorrectly it was late and I had a young baby daughter to look after as well as typing up a reply to your question so I hope you will forgive me on that one.

 

To Turtle : First of all thanks for trying taking on some of the burden ! :) of answering the questions. But you are taking a classical view of photons in your reply and I think Antony was asking how QED views this particular exchange. But again thanks for trying to take on some of the burden.

 

To Craig D: Even more thanks to you for taking on a even larger explanatory burden and doing it in a very clear way. :)

 

Back to Antony

 

Gluons have zero mass and spin 1 and quark interactions are confined to 10^(-15) meters. Its not experimentally possible to indicate what "size" they are as they are so very much smaller than an electron. Suffice to say they are very small indeed, they are considered for theoretical reasons to have zero mass experiments confirm they must have either zero mass or very near zero mass.

 

As I said in my previous post "color" is analogous to "charge" in electromagnetism as far as I know its only referred to as "color" but then again I dont know everything. :)

 

Back to the light questions now...

 

In classical physics a photon always take the shortest distance between two paths (experiments confirm this to be true) the problem with this is how does the photon know before it sets off on its journey which is the shortest path ?

 

QED attempts to explain this by suggesting that the photon simply travels over all possible paths and the wave function "collapses" to the shortest path which is then detected by a detector. In QED a photon can go faster than c or slower than c but when you add up the sum of all line integrals it averages out to c.

 

You might think this is a bit of a maths fudge but it works extremely well and experimentally confirmed to a very high degree of accuracy.

 

I am indeed familiar with Feynmann his lectures and his book and I think I know what you are trying to get at now.

 

Feynmann diagrams are based on Langrangian mechanics and use complex numbers. If you are not familiar with complex numbers this could get quite tricky and you might need to post on the mathematics section of hypography. In real numbers the square root of -1 cannot be found, in complex numbers this is solved as 1 at an angle of 90 degrees.

 

If this is not clear to you then you need to learn more about complex numbers.

 

Each path is assigned a complex valued probability amplitude and the actual amplitude we observe is the sum of all amplitudes over all possible paths. In a great deal of the paths photons are cancelled out in destructive interference collisions. I think this is what you were getting at in the first post.

 

These collisions cancel themselves out in much the same way as classical wave mechanics.

 

Other paths are found not to be the shortest and collapse to the shortest possible path.

 

So in QED the light bouncing of a reflective surface will take all possible paths

some individual photons will cancel each other out in destructive interference others will not take the shortest path but taken as whole they will collapse to the shortest possible path at an average speed of c with a high degree of probability that they will hit the detector and lower probablities they will choose another path.

 

Im hoping this about covers it.

 

(If not please post again as I said at the beginning QED is quite difficult but by no means give up.)

 

Cheers :)

[Farsight]

Antony: If I can chip in my twopennyworth, IMHO Quantum ElectroDynamics is a most interesting subject. I can't profess expertise in it, and there are some aspects of it that I dislike. But I think it's an extremely worthy topic. I'd recommend you stick with Quantum ElectroDynamics before you look at Quantum Chromodynamics, where the "color" is somewhat unfortunate. To indicate why, at the risk of embarrassing myself, here's an excerpt from something I've been working on:

 

Come with me on a journey. It starts with basic concepts. Concepts so basic that you’ve never actually thought about them. It ends with Albert Einstein’s dream, a world of pure marble geometry. This is what he was working on, and never finished. Godel handed him the key when they were together at the Institute for Advanced Study in Princeton. It’s both disturbing and it’s wonderful. So elegant and so simple, and we were so so close. What a crying shame that Richard Feynman, the great explainer, turned down that Princeton position after the war.

 

 

Feynman won joint Nobel Prize for Quantum Electrodynamics in 1965. The gap between QED and Einstein’s twilight work is so easily bridged. If only Einstein had passed on the torch to Feynman. Maybe we would have had a unified theory of everything by now, a theory that takes the best Quantum Electrodynamics and blends them into a new improved Relativity. A Relativity that actually explains what gravity is, that tells us how to master it. But it didn’t happen. So our Rocket Science remains complicated, and difficult. Too difficult.

 

 

After the Challenger disaster it was Feynman who told NASA they’d been fooling themselves about safety, and about O rings. It’s incredible how they could get it so wrong. You wonder how people can get it so wrong. I think that if Feynman had gone to Princeton, and talked to Einstein, he would have realised that this applied to other things. And things would have turned out different..

[Guest antony]

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[CraigD]

Maybe it is better that you think all is neat & tidy with QED (& your explanation), for me personally, at least three fundamental issues have been raised, that question other aspects of physics. However I find myself at a place I have visited previously, a place where I have no motivation to discuss the issues I mention further.

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So go ahead, I will withhold because the system does not offer me dignity, nor the motivation to continue, and you can destroy yourselves, by blackmailing each other and not allowing yourselves any spiritual tranquillity/freedom.

Antony, would you like to have this thread closed, preventing more posts to it?

 

I think I speak for everyone here when I say it’s not the intention of hypography to offend you dignity, or blackmail or deny anyone spiritual tranquility and freedom. For the most part, we’re all here to discuss and increase our understanding of science.

[Guest antony]

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[Guest antony]

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[Erasmus00]

Firstly, an obvious issue, if the theory predicts that light can travel faster and slower than c, but that overall it travels at c, then where does the ‘slower’ manifest itself? Does the overall beam/particle of light arrive to the receiver slower than c, making up for the faster speed of the other particles that are required to explore all possible routes?

 

A beam of light/particles is, necessarily, a collection of "real" or "on-shell" photons. This means that they necessarily travel at c. Its important to realize that thinking of light usually means thinking of CLASSICAL light, or large scale light behavior. This is always real photons, which travel at c.

 

Secondly, if we imagine the experiment I mentioned before, where we have a thin sliver of the reflective surface to the left of the receiver, angle of incidence/reflection, and even to the left of the emitter, and we then release a short burst of ‘white visible light’ (ie from red to violet in the electromagnetic spectrum), we would see the Cancelling Out process taking place, with no photons registering at the receiver.

 

I think maybe some diagrams or something would help. I've read this and your firsts posts in this thread and still can't understand what you are trying to get at.

-Will

[Guest antony]

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[InfiniteNow]

Maybe I am better writing this up in an essay, as I am not aware how to include diagrams herein (my illustrations don’t have a web address!).

You may need to wait until after 10 posts, however, you can either put the web address (url) of the picture into image tags like this:

 

[img=http://www.MyImageAddressHere.jpg]

 

Or, when replying, go the the Advanced Reply editor (either by clicking on "New Reply" or after clicking the quick reply button click "Go Advanced."

 

From there, in the menu at the top is a little paper clip. Click that and a new window will popup asking you for the location of the file you wish to upload. Either locate the picture file from your computer OR give the address of the file from the web, and click upload.

 

Once confirmed, you'll close the upload window, then click the paper clip again and your file will be there. Just click that and it will be added to the body of your post.

 

 

 

Here is one shown with the IMG tags:

 

 

Here is one shown as an attachment (which you can select by clicking to open a larger version):

 

[CraigD]

Correct me if my logic is wrong, but surely this illustrates that photons must posses a definitive size. I say so as the photon that ‘bounces’ at an angle where it would be detected by the receiver, surely must be cancelled out, not only by a photon bouncing at a contradictory angle, but more importantly a photon of the same size/internal make up (ie another white light photon). If this were not the case, how would a complete cancellation occur?

I think you’re suffering from a couple of misconceptions, leading to your conclusion that a photon must have a “definite”, classical size.

 

The first has to do with the idea that reflection is due to photons “bouncing” against reflective bodies in the way that a macroscopic ball bounces against a wall or floor. According to quantum mechanics, this is not what occurs. Rather, a photon interacts with an electron bound to an atom of the reflective body - in quantum terms, their wave functions are modified, in a classical analogy, the photon is absorbed by the electron, boosting it to a higher-energy orbit around the nucleus. For some instant, in the classical analogy, the photon no longer exists. The excited electron than emits an photon with nearly identical energy (frequency), polarization, etc, but with a different momentum vector part.

 

The second has to do with constructive and destructive interference. This wave-like phenomena is not due to any physical collision between photons, but to their quantum wave functions, which are related to their wavelength. Some arrangement that causes the possible paths of a photon to coincide at some point with path lengths differing by one half of their wavelength results in the complex conjugate of wave function at that point being zero, which equates to the classical probability of detecting the photon at that point being zero, and a dark spot appearing in the interference pattern. Points where the path lengths are an integer multiple of the wavelength result in a bright spot of constructive interference appearing. In between are many “shades of gray” of mixed descructive and constructive interference.

… I have raised this final point as it also lends support to the fact that there is a ‘white light’ photon that posses an equal amount of energy from all its relevant wavelengths, when compared to the energy a receiver needs to perceive such a thing.

There simply is not, according to either quantum mechanics or earlier theories of light, such a thing as a single quantum of light (a photon), light corpuscle or wave (according to earlier theories) with a color of “white”.

 

One of the oldest demonstrations of this is attributed to Newton, who noted that, when a beam of white light was passed through a refractive material – such as a glass prism - at a non-right angle to the surface between it and the less refractive surrounding media – typically air – the beam splits into beams of different colors. If passed though another prism, these beams cannot be further split, nor can one of them be made into white light. Only by recombining all of the colored light beams into a single one, can the beam be restored to its original white “color”.

 

Consider also that “white” is a subjective description, dependent on the observer. The subjective definition of “white” is “light without color”. To we human denizens of a clear nitrogen-oxygen atmosphere planet around a yellow star, white is the specific spectrum – average collection of photons of various frequencies - emitted by that star within the range to which our eyes are sensitive. To a terrestrial animal with a different visual range, white is defined by a different spectrum. The composition of light might be altered to a spectrum that a human could not distinguish from white, but another animal considered “colored”, or vice versa.

 

We humans can be tricked into terming a spectrum white, when in reality, it has a very different spectrum than another we believe to be the same. For example, although the white background of the computer screen on which I’m typing this appears to me to be the same color as a piece of paper under bright sunlight, if I were to examine the light from each with a spectrometer, I would find that the “computer light” spectrum to be clumped around 3 frequencies corresponding to the red, green, and blue emitters of the screen. Different kinds of screens – a flat panel vs. a CRT, or a solid-state-emitter “laser TV” screen, while appearing to have the same colors, have different spectra. Images projected by filament light bulbs through chemical photographic film have smoother spectra, while those illuminated by sunlight are even smoother.

 

For the denizens of star unlike our Sun, or a planet with an atmosphere unlike our Earth, white could be an entirely different spectrum. Our “white” could seem very “colored” to them, and vice versa, or even “black” – that is, none of the light in our visual ranges might be detectable by the other.

 

Due to atypical variations in or damage to eyes or nerves, two humans may even disagree about what constitutes white light – though, it this case, we can say that people for whom the sun looks white (or yellow) are normal, while those for whom it looks some other color, are not.

 

White is a concept, not something that can be represented by the quantum number of a single photon.

[Guest antony]

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