what propels light?

[yusufcam]

a quick question....(har)

just reading something about electromagnetic radiation and wondered..
(have limited internet access, so i posted here)

light is in motion, light source is stationary, why is light propelled at such speeds...

if it is a substance how does it reach such speeds and not ever slow and why is that then the limit of speed?

:thanx:

[GeoffW1]

Hi,

A good question which attracts interest because the detailed rigorous science of it is so obscure to most of us :question:

Firstly (as I understand it) the speed of light as an absolute is an intrinsic property of space-time. It is the natural and inevitable speed of other types of elementary particle besides the photon, for instance bosons.

Prof Brian Cox (calm, calm, ladies:D) has co-written a terrific book which has in it the most simple and elegant explanation of why c is such a basic property of space-time, "Why does e=mc^2 and why should we care". It is really worth a read.

These quasi-particles are "mediators" of different fields, that is, they make the fields known and felt to us. The photon is the mediator of all kinds of electromagnetic field effects. The undiscovered graviton would be the mediator of gravitational fields, if we could find one :lol:

Photons are without any mass at rest, so have no inertia, and then need no force to push them along to c. That speed is actually their natural state, in a vacuum anyway (they go slower in a housebrick). Indeed they are really only spoken of as particles at all because they behave that way in certain circumstances. At other times they behave as a wave. This is one of the planks (no pun) of Quantum Theory.

I've given myself a headache now, so I'll leave the rest to some real experts.

Cheers

[ballaratdragons]

Great questions Colin :thumbsup:

I await the answers with you.

[mswhin63]

I think light does not accelerates instead reaches it required speed because of its massless condition.
The term photon was given because of the way it interacts with matter (bending, relection speed limit etc) but it real nature is Electro-Magnetic radiation ie. radiation that come from a source (Supernova, energy release from a source including Radio, Gamma rays etc). Electromagnetic radiation is the key to understand lights real nature.
Correct me if I am wrong.

[avandonk]

This is a non sensible question. The OP is obviously deluded at best or a troll at worst.

He only needs to study some science to get an answer to this idiotic premise.


There are reams of information that is easily accessed by google.

Bert

[avandonk]

I just get angry when ignorant idiots cannot understand basic physics.

There is a lot of this about where you lead the ignorant public with nonsense science.

My bools_s_h_i_t alarm went off!

Bert

[avandonk]

Colin you do not exist! You are just the interaction of EM particles that constantly produce you. What is even more amazing is that you live on a rather large rock orbiting a rather large star. All this is made of the same stuff!

There are far larger stars around here somewhere. The death of these stars made the nuclei that you are made of eons ago.

If I was wrong I apologise. I do not think so, as no one can be so ignorant as you portray. Unless you just crawled out from under a rock from the proverbial primeval slime.

Bert

[GeoffW1]

OMG Bert take it easy, you will have a stroke :jawdrop:

[traveller]

Whoa Bert, strong words there. There might be a chance its an innocent question, just because we have an interest in astronomy doesnt make us all astrophysicists. :question:
Bo

[GeoffW1]

I's not too serious I believe :stupid:

[avandonk]

In my defense I put it to the jury that lately many false and dare I say it liars are doubting well established science.

I am very calm and calculating. You have no idea how icy I can treat ignorant idiots. I have no intrinsic bad feeling for ignorant idiots. It is only when they pass themselves off as experts that I get a tad testy.

Bert

[ballaratdragons]

and here I was waiting to see if anyone would give a really short simple answer to one of the most fascinating aspects of science, so I could copy it and say I know everything :lol:

[avandonk]

OK ken here we go.

Reality does not rely on matter. It is the space time fabric that is our Universe. Even where there is nothing virtual particles appear and disappear.

Time could be based on the mere interaction of even virtual particles. Without interaction there is no time. When the interactions stop, time will cease.

The fact that we all exist to even ask these questions is improbable to say the least.

I could go deeper with lots of equations but that will not explain anything.

Our real understanding is just beginning.


Bert

[casstony]

Now you're just being condescending Bert - of course we know dat stuff :)

[Dave2042]

Here's my 2 cents' worth.

I don't think it's a foolish question at all. On the face of it, if light is a particle, why shouldn't it need to be set off in motion by some force like any other particle?

I think the real answer is to see that this is actually a specific version of a very general question: what is really going on with phenomenon X?

As I think I've observed in other threads, while this is a reasonable question to ask of physics, it's a question that physics gave up on trying to answer along time ago. The best physics can do is come up with equations that well describe observable quantities.

Particularly since the advent of quantum mechanics and relativity (both involved in the question asked), the underlying processes are so removed from our day-to-day reality, that a 'sensible' explanation corresponding to our day-to-day reality seems a hopeless task.

So, I think the answer to the question posed is not much more than this. Light just travels at the speed of light, with nothing 'starting it off' - that's just how light is, and if it doesn't make sense then it doesn't make sense.

[yusufcam]

interesting....

thought i'd ask in case there was a simple answer, no real motive other than that.

just came back from the illawara air show, plenty of light (sunshine) propelled down there today, thats for sure (forgot to take a hat).

wow, those old piston engine planes make the most interesting sounds (like four cycylinder harleys through a rock concert some noted).

a good day (missed the steamfest, last week).

anyway thanks

[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. :question:

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 !!!

[bartman]

Dark energy draws ( sucks) them along?.............
:P:thumbsup::P:thumbsup:
:D
Bartman

[Jaybee]

Geez Bert
Its a good job Colin didn't ask you in person you would have taken his head off with a single strike! It was just a simple question and he said he had limited internet access, so google may not have been an option...This ignorant idiot also wouldn't have minded a simple answer in simple terms, as clearly I'm simple, because I couldn't follow your replys at all except to know they where rather scathing and personal. I re-read Colins question and sorry, I didn't get the feeling that he was trying to pass himself off as an expert, I re-read it again and I couldn't find the bit where he slammed the publishings of established science...I guess I am just too trusting as I thought that it was a genuine question, still do by the way, and even if it wasn't, wouldn't a well thought out answer have been better and more helpful, even if only to younger less seasoned readers, than an angry tirade?? just my 2 cents...please don't hurt me.. :)

[marki]

Here you go, not my words :P

"From the source, light travels in all directions. To understand how it travels, let us consider a ray of light travelling on a rectilinear path and see what all phenomena occur on that path.

At a definite instant of time, at various points on the path of propagation of light, there are electric intensity vectors in a plane perpendicular to the path of propagation of light ray. Their magnitudes at various points are such that their tips when joined form a wave. At the same instant, there is a similar looking magnetic intensity wave in a plane perpendicular to the plane of the electric intensity vectors and also to the path of propagation of the light ray. Beyond some distance from the source, both these waves are in phase.

After an extremely small time interval, if we again join the tips of the intensity vectors, we get a new wave similar to the previous one but slightly displaced in the direction of light propagation. Thus light travels in the form of an electromagnetic wave. We discussed light propagation on a rectilinear path, but it travels in all directions from the in the same way reaching the surface of a sphere of radius equal to the distance travelled by light if the medium is homogeneous".

Gotta love physics :P:D

Mark

[ballaratdragons]

I didn't understand a word of that :confused2:

I must be tired because it wasn't going in, and about halfway through I wanted to stop reading. But I made myself read right through to the end, and it still made no sense to me.

I guess I just don't have a 'physics' mind even though I am fascinated by it :shrug:

[Paduan]

There is a great rubbish bin in the universe that the Physicists dump all of the "unexplainable" it is called the Quantum world, it is a place where the impossible meets the improbable and comes out as an answer that is indecipherable to 99.9% of the population. so when a physicist has a little hissy fit at your less responsive grey matter remember there will always be things that are alien to them. Things that you take for granted are in the Quantum world to most people bordering on Genius

[Jaybee]

Thank you Mark for your answer....I'm afraid I'm with Ken though...I didn't understand one bloody word of it! :doh: :confused2:

[barx1963]

I am no expert but here goes. As the space through which light (or any electromagnetic radiation) expands it stretches the wavelength of that light. This is how we get redshift. The photons that were emitted at the time of the big bang are still all around us, but their wavelength has been stretched to microwaves.
Obviously the total energy that was in those photons is still there, its is filling a vastly greater volume of space, so its temperature is much lower.
The above is the explanation as I have read it but summarised in my own words. The thing I am not sure about is as follows. A percentage of these photons would have been absorbed by matter in the 13 billion years since the big bang. For example the snow you see on old TVs that were out of tune was partly the CMB. So the photons that hit your TV antenna have been absorbed and are no longer out there. So these photons have been happily hitting stars, planets, interstellar dust and gas, people, aliens and TV antennas for 13 billion years, give or take.
As the temp of the CMB is one of the main planks supporting the age of the universe, wouldn't that make these estimates a bit inaccurate?
I am aware that the age of the universe is supported by other lines of reasoning, but this is something I haven't been able to get an answer to.

Malcolm

PS Bert, there are a lot of people who don't understand basic physics(myself included, but I am getting better!). Getting angry is not the answer, save it for the genuine hoaxers and bull crap artists out there. I think the OPs question was reasonable and interesting and deserved a little more respect.

[marki]

Don't worry Ken and John, niether do I :D.

Mark

[KenGee]

Bert may have called troll a bit early, but fully understand where he is coming from. Science is hard so if you wan to learn be ready to study and work hard. There are to many trolls trying to prentend they have done the work.

The fact that the answers don't come easy is the very reason these men and women are my hero's.

[Peter Ward]

Yes...I have read pretty much the same. Problem I have with that is due the particle/wave nature of light.

As a particle, the photon's oscillation frequency slows from gamma-ray frequencies to micro-waves over time, yet, from a relativistic point of view, for the photon, no time at all has elapsed since the big-bang, and as far as it is concerned space hasn't got any bigger at all, yet its energy has decayed. :shrug:

[Jaybee]

Kenny
I must be as thick as two short planks, because I seriously do not understand why the OP's question got such a tirade of abuse. Isn't the foundation of this forum based on the unity and generosity of its members? The one thing I found here is how quickly members come to the aid of one another. If one does not know the answer, they only have to ask, and information comes from those that know and who are only too happy to share the knowledge.
From what I can gather the OP asked a simple question that deserved a simple answer. Well an answer of sorts, I don't think this question even has a simple answer...He had limited resources available so with what he had, chose a well respected and trusted source, but got a slap in the face. I agree that Science is hard, and needs to be worked at and studied, but isn't it up to the ones that we would call teachers, to teach, if asked? If he had asked this question in a University Lecture hall, would he be berated, or would the Teacher try to the best of his ability, to provide an answer that would not only benefit the OP but the rest of the class? Isn't 'study' seeking answers to things which we do not know? In any way we can? Nowhere in the OP's original post does it give the impression that he has pretended to have done the work. Clearly he hasn't done any, otherwise he wouldn't have needed to ask the question in the first place.

[JB80]

I must be a bit thick too John, don't worry it's not just you.
Here I was thinking an interesting discussion was in danger of breaking out but I shouldn't of been worried.

Thankyou to those who have treated the question seriously.

[shane.mcneil]

Hi all.

This is still called the Astronomy and Amateur Science form isn't it? I probably don't even rate as an amateur. I've learnt heaps from the friendly people hear. Still not sure what the answer to the question is though. I'll re-read the answer again. :)

[avandonk]

I am sorry maybe this will help.

It was James Clerk Maxwell that published his seminal work of four equations that related magnetic fields to electric fields in the late 1800's.

A dynamically changing magnetic field produces a changing electric field orthogonal to itself which then produces a changing magnetic field also orthogonal to itself and so on ad infinitum. We call this Electromagnetic Radiation or light or radio waves or xrays or gamma rays or lots of other labels. They are all the same thing only the frequency of oscillation or wavelength is different.

Quantum effects only control the means of production. For example an electron orbiting a nucleus even though it is a charge that is accelerating does not emit EM radiation. It is only when the electron changes from one quantised orbit to another that it emits or absorbs a quantum or photon of EM radiation.

Large masses like the Sun or Galaxies can affect the trajectory of photons by the localised distortion of space time due to the large mass present. The photon merely follows the straight line path though space time. It is not deflected by the gravitational field.

The curious structure of our Universe is such that even when there is no matter present or even photons virtual particles appear and disappear. The laws of physics seem to be independent of any localised matter or photons or anything else.

The real scary thing is these laws exert their 'authority' even when there is nothing to detect. Each of the infinite number of possible Universes are thought to all behave like this but only the basic laws are different. If the laws of our Universe were only slightly different we would not be here to observe it.

We can only tell the passing of time by observing events such as Sunrise and Sunset. Or the vibration of atoms in our atomic clocks and Pulsars etc.

I could write a book but that has been done and it is still happening.

Bert

[GrampianStars]

Ah! "OB1" left out chin stubble & nose hair ;)

[ballaratdragons]

So, the effect of Einstein Rings using the term 'Gravitational Lensing' is innaccurate.
Would it be 'Space-Time' Lensing'?

[Dave2042]

I think the point is that you get into all kinds of semantic pitfalls when you try to be precise in your language, but while using everyday terms. So in my experience, physicists tend to not talk of 'straight lines' in GR, as the term just confuses. They talk, instead, of 'geodesics', which are the paths that light follows. In a sense geodesics are like straight lines - absent matter they are straight lines. Once you add matter, space-time bends, and the geodesics change accordingly. Does this mean the geodesics are now bent, or are they still straight lines? I don't know that the question even means anything.

As for gravitational lensing, yes it's arguably a classically-based term that doesn't properly reflect the phenomenon, but my view is that it's just a label, and a neat one.

[GeoffW1]

Hi,

That wouldn't be a bad way of putting it. Bert meant (I think) that these photons don't feel gravity directly, but just follow their normal path through space-time, the very fabric of which has been distorted by the presence of matter.

Howzat? :P

[GeoffW1]

BTW,

You know that at conferences and seminars and drinking sessions between front-rank physicists, working at the very edge of all this theory, they also get upset and hysterical and abusive, and throw chairs and things?

Not Prof Pearlywhites though

Cheers

[Kal]

I think that when someone clearly explains the whats and hows regarding the expansion of the universe you might get the answer you seek :P

[GeoffW1]

We did not want to say anything :rofl::rofl::rofl:

[Jaybee]

Thanks Bert...


and yeh....thanks Geoff!:lol:

[cwjohn]

De Brogle showed that all particles exhibit a wavelike motion. Bohr cointed the bord "Complementarity" as an all purpose methodology to cover the particle/wave debates. In short whether something exhibits behaviour as a particle or wave will depend on it method of measurement.

Along these lines one can look at the world as composed of waves, or at the very least define it by wave functions mathematically. Simplistically, one could say that the natural state of any wave in a vacuum and sans mass will be to travel at the speed of light in all directions unless prevented otherwise. Thus goes the way of all photons which in common parlance when grouped together we call light. This is the general case. When we add rest mass the wave slows down and when we combine particles the wavefunctions combine in a complex arrangement which generally are beyond our computation capacity to fully appreciate, but which in the macro sense lead to classical concepts.

A photon being massless does not require classical descriptions such as inertia and therefore does not require a compelling force. Upon creation a photon will entangle with others and in bulk they will generate the electromagnetic phenomena defined by Maxwells equations. A photon however does have to be created in the first instance as discussed here. The creation of photons by "quantum jumps" has been hotly debated for a century and defies any classical analysis. The semantics we use are probably insufficient to describe the phenomena. For instance to say an electron orbits the nucleus is incorrect as the physics of this do not work. To say it is a standing wave, vis a vis Schrodinger's equation is more mathematically pleasing but nevertheless fails closer scrutiny. The best we can say is that the electron, whateve that is, has a probability of being in a certain place at a certain time, with a certain momentum or energy, but both cannot be exactly defined together, the basis of this being of course Heisenberg's uncertainty principle. Dirac made a clear point of this when he stated that his Theory of Electrodynamics assumed electrons behaved in a certain way for all "intents and purposes". Perhaps one day we will understand the actual mechanism of quantum jumps when we have a more stingent theory of everything.

As to a photon's frame of reference from the point of view of special relativity the main point is that energy is conserved. In our frame of reference energy is conserved by the balance of the expansion of the universe and the stretching of wavelength. I believe that is all that the basic physics requires.

As to the stupidity or otherwise of this question I believe the problem is the intermingling of classical and quantum mechanical concepts that creates the problem. Most classical descriptions in physics can boil down to an explanation that you can proverbially describe to your mother. Quantum mechanics however is counter intuitive. Therefore when one talks about light with a little bit or reading and some dumbing down of the calculus one can get a basic feel for Maxwells equations and their general context. However, once one delves into particles which "exist" in the order of the plank length then one has to deal with an epistomological and potentially metaphysical mindfield. Unfortunately modern media deals with a mix of classical and quantum mechanical phenomena in a mish mash of short statements deviod of any contextual basis and the result is mass confusion.

In this situation I am afraid the only way forward is either to leave the subject alone or undergo a serious amount of self education.

[andyc]

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 :thumbsup:, 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 :rofl:), 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]

Well said Andy!:thumbsup:

[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]

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]

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]

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]

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]

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]

Steven



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



Yes, it is a matter of some philosophical debate.



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.



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]

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]

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]

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]

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

[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.:screwy:

[bartman]

Awesome answer,
Thanks John!
Bartman

[sjastro]

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

Lets put that definition to the test.
I know you are a law abiding citizen and would never speed in your Ferrari.:P
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.

[Peter Ward]

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 ;)


:D

[Peter Ward]

Ah..there was some irony/humor there Steve....

That said, yes, I understand the energy has to go somewhere, and "an electron emits a photon"...we can even determine its energy / wavelength....but to labour the point *how* does an electron emit a photon?? i.e does the Photon "creation" process have a deeper mechanism other than popping into existence?

[Octane]

I just wanted to say that this has been a very interesting thread to read, thus far. Keep it going, people. :D

H

[Regulus]

Well done chris. A good answer to a good question.

[sjastro]

Peter,

First of all one needs to define what QFT tells us about photons.

QFT tells us why photons exist and why they have certain physical properties such as zero mass.
It tells us the conditions under which photons are created without providing an explanation as to how they are created.

A single photon is defined as a vector, it carries an intrinsic angular momentum as "clockwise" or "anticlockwise" spin. Observers in space would therefore see the photon differently according to their position in space. If the photon appeared the same to all the observers it would have a spherical symmetry.
While this appears to be trivial, the non spherical symmetry of a photon tells us the conditions under which they are created.
In the hydrogen atom the ground state exists in a spherically symmetrical state. If the atom is excited and the electron moved to a higher energy state, the symmetry can no longer be spherical. When the electron returns to it's ground state a non spherically symmetrical photon is emitted. This cannot happen if both the ground and excited states of the atom are spherically symmetrical.
It forms part of the selection rules for electron transistions on which quantum mechanics is built on.
There is a subtle relationship between symmetry and angular momentum.

QFT extends the notion of symmetry and allows to us explain why the photon has certain properties. I use the term explain rather than describe as QFT has also made predictions for other particles and bosons according to their symmetry that were unknown at the time, but have been subsequently discovered.



Regards

Steven

[snas]

Steven

Just wanted to say that I am glad to see you posting again. While my biologist's eye view of astronomy is coming from a much lower level than yours, I have always enjoyed the discussions created by both yourself and Craig and have even occasionally learnt something! Like I have learnt on this thread where photons come from. Would like to see Craig come back too because he was prolific starter of interesting threads!

Stuart

[cwjohn]

Steven



This does not sound like goobledygook to me, and it did not sound so when I was first taught it decades ago, and I also suspect Bert and Peter have also been educated to this extent also.

I think however, you have to consider the import of your language here.

First of all "A physicist states" implies that you speak for all physicists. This is a somewhat arrogant statement. By virtue of an honours degree in physics I suppose I am theoretically a physicist albeit not a practicing one. I would not state that a photon is "created ..." but rather that a photon is "described by the Lagrangian....".

So you say a photon is created to preserve the conservation laws. Why? Because Noether said so in 1920.

This makes perfect sense???? Why?

Nobody argues that QFT and the math of symmetry describe the Standard Particle Theory par excellence. Many emminent physicist (for example Lee Smolin) do not accept this as a reasonable end point for our deliberations. And indeed if this were to be the case there are many very smart physicists wasting their time investigating string theory and quantum loop gravity.

Perhaps you are most correct when you say



I think this is definitely true and it is an argument that goes back to Einstein and Bohr and continues to this very day. You are content to accept mathematical descriptions as reality, and indeed you have quoted the leading proponents of this proposition (a metaphysical proposition to boot). There are others (and I fall into this camp) who do not accept this as "reality" but rather seek deeper explainations. However, to lump all physicists into your way of thinking is not reasonable.

[sjastro]

Chris,



That is your assumption which is incorrect.
I’m stating the mainstream view which is supported by a vast majority of physicists who partake in research.



It makes perfect sense particularly when looked at from a historical perspective.

Noether’s theorem is the evolution of an idea starting from around the 1830s when Newtons second law F=ma was derived by defining the gravitational field as a dynamical system using a Lagrangian.
The use of a Lagrangian did not require Newtons second law to be known beforehand.
Similarly the Lagrangian of an electromagnetic field defined as a dynamic system doesn’t require photons to be known beforehand either. A photon is a prediction of the local invariance of the Lagrangian under a U(1) or rotation transformation.
If you believe that QFT is simply describing a phenomena than how do you explain the other gauge bosons which are predicted by QFT but were not observed at the time.



Both string theory and quantum loop gravity draw heavily on QFT. Neither theory departs from the symmetry theme of QFT and both are examples of gauge theories.
The very smart physicists still work on QFT such as Ed Witten who has made significant contributions.
Why raise string and quantum loop theories in the first place?
Neither provides the "realistic" approach you are seeking.



I have to ask you the obvious question, is there any theory in physics that satisfies your criteria of providing a realistic approach?

Regards

Steven

[cwjohn]

Steven



Well we have moved from "all physicists" to the "vast majority". I suspect when pressed most of this "vast majority" would conclude that QFT describes rather than explains. I suspect they would not accept as a suitable description of a photon as:



Here you state that a photon is in fact a "prediction" as defined by a Lagrangian etc which is a mathematical construct. You also describe it as a "vector". You also state it carries "intrinsic angular momentum" but at no stage do you state what "it" is. You also state QFT "tells us the conditions under which photons are created without providing an explanation as to how they are created."

So this "prediction" which is a "vector" is defined by a theory which says nothing about the creation of said "prediction" or "vector". Does this really make sense to you on reflection.

Furthermore you state that because QFT has predicted other particles (a fact that is beyond doubt) that ipso facto it explains "the ontological and epistomological basis of these particles". This is a logical fallacy which cannot be sustantiated on any evidential basis.

You already state you dont know how a photon is created (or QFT doesnt, I guess you may). If you believe you know what a photon is then state it simply, but not as a "prediction", or a "vector".

I thought religion was confusing???

Just kidding. I reached for one of my texts on QFT (Zee QFT in a nutshell 2003). In the intro he says



As to your question



As I have stated before all theories of physics that do not deal at the Plank level. Its not that I don't accept theories of physics at the quantum level. I certainly do, but I maintain that they are descriptive and predictive. They explain a real result, but they do not explain what is really happening. As Zee says when some brilliant person takes us beyond that barrier then we will see the next breakthrough in physics.

Just my opinion.

[Kal]

This may be what you intended to say but it is not what you initialy communicated. You would have been better off by starting with "The mainstream view of a physicist is .. "

[sjastro]

What has the mainstream view got to do with whether physicists consider QFT to being a phenomenological theory or not?
What I stated was that QFT is a mainstream theory supported by the vast majority of physicists (and that includes the string and quantum loop gravity theorists) otherwise it would not be mainstream. It has nothing to do with a physicists personal take on whether QFT explains or describes things.

You have completely taken my statement out of context.



Once again your propensity of distorting of my statements to create a strawman argument is duly noted.

What is obvious is that you have a problem comprehending the objectives of QFT.

The simplest explanation I can give is a piece of history which clearly shows the differences between a theory that describes (a phenomenological theory) and a theory that explains (a non phenomenological theory).

In the 1960s the physicist Murray Gell-Mann came up with the EightFold Way theory (http://en.wikipedia.org/wiki/Eightfold_Way_(physics)) which described the grouping of particles into octets according to their quantum numbers.
There was a missing particle but its properties were predicted according to its position in the group. The particle was discovered and its experimental properties closely matched the predictions.

The Eightfold Way theory is a rare example of a phenomenological theory in particle physics that has led to a prediction of a particle.
The Eightfold Way theory cannot explain however why the particles are arranged in octets or why each particle possesses the properties it has.

The Eightfold Way theory was a stepping stone to the development of Quantum Chromodynamics (QCD) which explains the Eightfold Way theory, all without having to invoke stories about creation which you seem to be fixated on.

Your argument is fallacious because it attempts to pidgeon hole QFT into a "creationist" theory which it clearly isn't. Creation itself isn't theorized in QFT. The "conditions" for creation however are through symmetry preserving (photons) or symmetry breaking (W and Z bosons).



"Macro" physics is much more phenomenological than quantum mechanics. Why is it that scientists require a quantum mechanical explanation for dark energy and dark matter given by your definition quantum mechanics is purely descriptive?

Regards

Steven

[cwjohn]

Steven

The discussion seems to have drifted somewhat. I do not think it is constructive to bore everyone about whether a theory is phenomological or non-phenomological.

We are not poles apart. Our differences are more philosophical than scientific.

The bottom line is that you are satisfied that you clearly understand what a particle is, and you have expressed it here. I find your definition unsatisfactory as do many others, physicists and non physicists alike, and I have expressed that here.

I am sure people will come to their own conclusions on this matter.

It was an interesting discussion nontheless.