what propels light?

[andyc (Andy)]

cwjohn, I think your post, and third paragraph in particular, perhaps is the best direct answer to the OP's question. And Bert, there was no need for the blast of abuse at the original question either IMHO. It's an entirely good question , especially if you have not done much physics. It's also a much more difficult question than it initially appears, as shown by the range of answers!

Yes, there is indeed a lot of information out there that you can learn yourself, but on the Internet you might not know when you're being told porkies, if you don't already know better. There are pages that will really convincingly try and tell you that the Sun is made of iron, that climate change is a global conspiracy (mind you some Aussie politicians fall for that one ), that the world is going to end in seven months time, or that aliens regularly visit Earth! It's best to answer a straight question honestly, then if the questioner turns out to be a troll, they will surely show themselves to be such. And it's a teaching moment for many others lurking on the thread!

[Jaybee (John)]

Well said Andy!

[sjastro]

I have no issues with the OP's question, but many replies given are either wrong or are based on quantum mechanics as it stood in the 1920s.

Ask a physicist or mathematician as to how photons are "created" and they will explain it in the arcane language of Quantum Field Theory with its use of Lagrangians and gauge theory.

Regards

Steven

[cwjohn (Chris)]

Plainly the OP was not a physicist nor a mathematician. Any discussion involving QFT, Lagrangians and gauge theory, will lose not only the OP but also the majority of the participants.

Having studied all of these subjects in detail I fully accept their value in advancing the field, but they have their limitations, which I will not go into here, nor do they give a person not versed in advanced math any real insight into the subject matter.

Unfortunately or fortunately, depending on your perspective, one of the prerequisites of modern day physics is that the practitioners be able to demonstrate the value of their work to the public that invariably pays their salaries. This involves coming down from their ivory towers and communicating their concepts in clearly understandable language.

[Peter Ward]

Originally Posted by cwjohn

......Any discussion involving QFT, Lagrangians and gauge theory, will lose not only the OP but also the majority of the participants....

Apparently Richard Feynman, was once asked by a Caltech faculty member to prepare a introductory course on why spin one-half particles obey Fermi Dirac statistics.

He said, "I'll prepare a freshman lecture on it." But a few days later he told the faculty member, "You know, I couldn't do it. I couldn't reduce it to the freshman level. That means we really don't understand it."

Begging the question: is the latter still the case?

[cwjohn (Chris)]

I would comment further on this matter.

Einstein said and I quote "What are light quanta? Nowadays every Tom, Dick and Harry thinks he knows it, but is mistaken".

Feynman was oft quoted to say that QFT an object of his own creation tells us nothing about the real nature of the objects in question.

John Bell said and I quote "I am a Quantum Engineer during the week but a man of priniciple on Sundays". By this he meant that he used all of the mathematical accounting methods which are significant by their accuracy during his normal work, but on Sundays he considered the realism of what the mathematical accounting represented.

Much more recently Rabi described the problem thus "One actually creates a remarkable thing like an electron. Its a marvelous thing. I dont see how its made. It just appears. Its a king of materialization-- the ghost shows up in reality. You can calulate how many electrons will be produced, and with what probability. But how was it born? What is it made of? Its these kind of questions I would like to see answered."

To claim that you can know all that can be known with mathematical abstraction is a complete philosophical copout. Undoubtedly, as Steven says most physicists will resort to this in the first instance (I have done it myself many times), but when pressed most will actually energise the grey matter and devote at least some of it to consideration of the epistomological implications of the mathematical concepts they are spoon fed.

[cwjohn (Chris)]

Peter

You pipped me at the post.

Actually Feyman regularly and repeatedly made these sorts of statements. In his later years he really pushed the concept of quantum computing very hard, but he stated that "You guys had better understand how all of this works before we can really proceed further, as I sure as hell dont" I have attended many quantum computing lectures since and I very much get the impression that we still dont really know and we are not really focussed on knowing. As long as it works?

I am gratified however, that since the turn of the century, and after nearly 70 years of this attitude, things are changing and some really smart people are working on the "knowing", rather than "cataloguing".

[KenGee (Kenith Gee)]

To actualy try and answer the posters question
Insert tongue into check...
Photons are messenger particles, three of the messenger particles are massless but all have been given momentum at their creation. That means they will move at or close to the speed of light. The three fundamental forces that are propergated at the speed of light,are electromagnetic, gravity and the strong. The weak force does have some mass and that's why it whimps out after a very short distance.

[sjastro]

I wouldn’t state that mathematical abstraction is a philosophical copout.

The role of mathematics in nature is a philosophical question in itself, namely is mathematics invented or discovered.

Noether’s theorem is a powerful example of mathematics in nature. Where as mathematics was largely considered a descriptive tool for physics up until the early 20th century, Noether’s theorem tells us many of the laws of physics are a consequence of an underlying mathematical symmetry that exists in nature.
Noether’s theorem plays an important role in QFT which seeks to undercover these symmetries.

We can quote a Feynman or Einstein about the real nature of science being obscured by mathematics, yet the alternative viewpoint exists as expressed by the Nobel laureate Eugene Wigner’s work “The Unreasonable Effectiveness of Mathematics in the Natural Sciences" or Max Tegmarks almost mystical view of a mathematical universe, just to name two.

Utimately we can judge whether QFT is nothing more than an exercise in abstract mathematics or a theory that adds to our knowledge which is manifested through an advancement of technology and other sciences.
There is absolutely no doubt to the answer.
The application of QED (quantum electrodynamics) to solid state physics led to the development of the computer. We wouldn’t be having this discussion otherwise.
QFT has helped explain phenomena such as superconductivity which had physicists stumped for over a century.

Regards

Steven

[cwjohn (Chris)]

Steven

I wouldn’t state that mathematical abstraction is a philosophical copout.

I should have qualified this more precisely. and have added "when pertaiining to quantum mechanics"

We can quote a Feynman or Einstein about the real nature of science being obscured by mathematics, yet the alternative viewpoint exists as expressed by the Nobel laureate Eugene Wigner’s work “The Unreasonable Effectiveness of Mathematics in the Natural Sciences" or Max Tegmarks almost mystical view of a mathematical universe, just to name two.

Yes, it is a matter of some philosophical debate.

The application of QED (quantum electrodynamics) to solid state physics led to the development of the computer. We wouldn’t be having this discussion otherwise.

Of course this is so, just as QFT will lead to the development of the quantum computer. Having spent time in some of the solid state labs doing the reseach your refer to I can tell you that QED was the last thing on their minds. Controlling doping levels to achieve desirable effects in mixtures of semiconductor materials was what it was all about. I believe also if you closely quizzed a typical physicist working on quantum computers about the essential deterministic reality of the workings of their system they would be relatively silent on this issue.

I guess this raises the question as to what is physics and what is engineering, but let us not go there.

QFT has helped explain phenomena such as superconductivity which had physicists stumped for over a century.

I agree with absolutely everything you say here if you replace the word "explain" with "describe".

I certainly understand and respect your point of view and also understand it is the most commonly held one within the physics fraternity. My simple point was that to claim that a suitable explanation was not available in other than mathematical terms, or that physics or physicists are in a more general sense satisfied with mathematical descriptions only, are not supportable statements.

[Dave2042 (Dave)]

Originally Posted by cwjohn

Steven

I agree with absolutely everything you say here if you replace the word "explain" with "describe".

.

A fascinating debate, but I think the above comment really demonstrates the semantic problem I've mentioned earlier.

In the highly abstract arenas of QM and GR, it's not entirely clear (to me at any rate) what "explain" means, and whether it means something different from "describe". My (fairly out-of-date) experience is that many physicists use the word "explain" on a day-to-day informal basis to describe (ha ha) what they do. Then when pressed about what's really going on, there is a tendency to acknowledge that "explain" might overstate things a little and retreat to "describe".

My personal view is that good physics always does something more than just describe, but provides some level of insight into why phenomena occur the way they do. For example, the QM explanation of superconductivity is not just an equation that describes it, there is the physical insight of how pairs of electrons link up and change how they interact. This seems to me to be beyond mere description.

It's a bit like the words "prove" and "show". Informally, they really mean the same thing. Only when you start arguing about fine technical details of epistemology do you need to start worrying about the difference.

[Peter Ward]

Originally Posted by cwjohn

........

I agree with absolutely everything you say here if you replace the word "explain" with "describe".

An elegant insight.

Models can describe extremely accurately and predict a physical event, but that said, the actual goings on are often not known, particularly at the quantum level, as fairy dust might well be the real mechanism

[avandonk]

I still rest my case as Peter W is now proposing fairy dust as a mechanism. Where I come from it is butch man dust!

Bert

[avandonk]

Originally Posted by Peter Ward

Apparently Richard Feynman, was once asked by a Caltech faculty member to prepare a introductory course on why spin one-half particles obey Fermi Dirac statistics.

He said, "I'll prepare a freshman lecture on it." But a few days later he told the faculty member, "You know, I couldn't do it. I couldn't reduce it to the freshman level. That means we really don't understand it."

Begging the question: is the latter still the case?

If you ever had to give a series of lectures to very smart young people about esoteric problems you would know that you cannot fudge it. You are always left with your own self evident level of ignorance or lack of deep understanding.

Bert

[Peter Ward]

Originally Posted by avandonk

I still rest my case as Peter W is now proposing fairy dust as a mechanism. Where I come from it is butch man dust!

Bert

Therein lies the problem Bert! ......there are so many sorts of dust...in particular Bull-dust can easily muddy an interpretation of what's really happening

[Jaybee (John)]

Right-O, well I have contemplated this since the OP posed the question and will attempt an answer. I've dumbed this down so I can understand it!
So just to help set the scene, Photons have 2 speeds, Stop, and lightspeed. Now lightspeed is their 'terminal velocity'. Its like Human's Terminal velocity (Its called 'Terminal' because if you hit something while traveling at it...well lets just say that things would break off, leaving you a bit dead..) anyway Human's terminal velocity is around 190km/h. Photons terminal velocity is around 1,080,000,000kp/h...just a tad faster. So.. Photons are generally quite happy just lurking around in the cold but they react badly to heat, which they prefer to avoid. So when you flick on your torch, energy from the battery heats up the tungsten wire, produces heat and wakes up all the Photons. Sensing imminent danger of 3rd degree burns they immediately flee away from the heat source and as they have no mass and only 2 speeds, they are instantly traveling at terminal velocity. As they have no mass and therefore no tail assembly or rudimentary rudder, they can't slow down or turn, so they just keep going in a straight line until they smash into something. Some of them will bounce around for ages until we catch them with our eyes. Of course Stars generate boggins of heat (boggins is not a scientific measurement but it does equal 'Alot') and because there are huge gaps in space, the Photons can travel great distances before they are caught. So the answer to the OP's question is (drum roll please)...... HEAT!.. .......Ta Da! ..........well it helped me anyway.

[bartman (Bart)]

Originally Posted by Jaybee

Right-O, well I have contemplated this since the OP posed the question and will attempt an answer. I've dumbed this down so I can understand it!
So just to help set the scene, Photons have 2 speeds, Stop, and lightspeed. Now lightspeed is their 'terminal velocity'. Its like Human's Terminal velocity (Its called 'Terminal' because if you hit something while traveling at it...well lets just say that things would break off, leaving you a bit dead..) anyway Human's terminal velocity is around 190km/h. Photons terminal velocity is around 1,080,000,000kp/h...just a tad faster. So.. Photons are generally quite happy just lurking around in the cold but they react badly to heat, which they prefer to avoid. So when you flick on your torch, energy from the battery heats up the tungsten wire, produces heat and wakes up all the Photons. Sensing imminent danger of 3rd degree burns they immediately flee away from the heat source and as they have no mass and only 2 speeds, they are instantly traveling at terminal velocity. As they have no mass and therefore no tail assembly or rudimentary rudder, they can't slow down or turn, so they just keep going in a straight line until they smash into something. Some of them will bounce around for ages until we catch them with our eyes. Of course Stars generate boggins of heat (boggins is not a scientific measurement but it does equal 'Alot') and because there are huge gaps in space, the Photons can travel great distances before they are caught. So the answer to the OP's question is (drum roll please)...... HEAT!.. .......Ta Da! ..........well it helped me anyway.

Awesome answer,
Thanks John!
Bartman

[sjastro]

Originally Posted by Peter Ward

Therein lies the problem Bert! ......there are so many sorts of dust...in particular Bull-dust can easily muddy an interpretation of what's really happening

So a nuts and bolts explanation of a mechanism is required otherwise the mechanism is not real.

Lets put that definition to the test.
I know you are a law abiding citizen and would never speed in your Ferrari.
Here is a hypothetical case then.

Suppose you are driving on a straight road and you plant your foot down on the accelerator.
You have the acceleration on the Ferrari and the proper acceleration as you are pushed back into your plush bucket seat.
The acceleration on the Ferrari can be explained as the end product of a chain of events starting with the conversion of chemical energy into mechanical energy in your engine. So you have a nuts and bolts explanation for the Ferrari's acceleration. Note we have gone outside the realm of physics to provide an engineering explanation for the acceleration.

What about the proper acceleration? Its "mechanism" is a result of Newton's third law for every action there is an equal and opposite reaction. It cannot be explained in terms of engineering.
The proper acceleration is due to a "fictitious" force, the effects of which however are very real.

Both accelerations are defined by mechanisms but one seems more intuitive than the other. Perhaps the term intuitive should be used instead of real.

The use of the word real smacks too much of a metaphysical argument.

A physicist states that a photon is created so that the Lagrangian for the electromagnetic field is invariant under a U(1) symmetry operation. While this may sound like goobledygook to you, Bert and 99.99% of the population, when simplified it means a photon is created to preserve the conservation laws. It makes perfect sense.

If you want a theory that tries to explain a photon as a product of a recipe such as a tinge of ingredient X and a dash of Y go consult an alchemist, it's not physics.

The physics explains why photons exist.

Regards

Steven

[Peter.M]

I am a humble chemist but I did read somewhere that the radiation from the big bang is not visible to us because it was hot enough at the time for the particles to be a plasma, and because plasma is opaque the photons were "blocked" by this.

Originally Posted by Peter Ward

Oh! Oh! I have a question.

Electron changes valence levels....and emits a photon. Paul Dirac had a lot to say about that.

What has me stumped is: what is the mechanism? Why do electrons do that? Is it simply enough to say, well, they just do.

Which leads me to my next question: condsider E=cv (energy of a photon is directional proportional to its frequency )

Where did all the energy of the (very high frequency) photons from the big-bang go? since they have apparently now cooled to very low frequency in the CBR !!!

[Peter Ward]

Originally Posted by sjastro

The acceleration on the Ferrari can be explained....
.....Steven

Steve! Eureka! It's now obvious to me why Ferrari's, at least those in motion, are predominantly rosso corsa red... Particularly when seen from the rear