How big is a photon and what does it look and behave like?

[Schrodinger's Dog]

Thanks that was exactly the sort of answer I was looking for ZZ, well not exactly but pretty close.

Have you ever thought in an idle moment what it is that lies behind the interference, it would be easy to assume that lights behaviour as a particle is because it is neither a wave nor a particle(a warticle or a pave maybe) if it were in some way both then it would classicaly speaking have a solution but that is not really a very comfortable thought, or is it. If Photons have an extremely/almost infinitessemly small mass then they should be able to achieve light speed with enough energy surely? In that light speed would actually be the speed at which the particles mass cannot have enough energy to move any faster and if that is the case they could be literaly both a wave and a particle? I know it's speculation but it's interesting none the less? Could we at some point with sophisticated enough technology weigh light and prove or disprove once and for all that it is massless? If it is massless then the two slit experiment becomes even more bemusing because energy should not deflect energy like that or is there some other effect that we don't know about? Well I'll go off for a while and speculate, do carry on

Light having a very small mass does not destroy the theory, but it would explain difraction wouldn't it?

 

[ZapperZ]

Light having a very small mass does not destroy the theory, but it would explain difraction wouldn't it?

No it wouldn't. Where in diffraction theory is "mass" necessary?

Zz.

 

[Schrodinger's Dog]

Why wouldn't it. a single photon would not necessarily effect itself and would hit in a straight line. If you measure it, bounce light off it then it would deviate? The Energy of light itself would have too small a mass to deviate light
I didn't say it was necessary I just said it would help to explain difraction, photons would in fact bounce off each other. And nothing precludes this as far as I can see from being true.
I've even thought of an experiment to test the validity of this premise, you could use the fact that without mass light should be bent by gravity x amount if it has mass it will be bent by y amount y-x can be used to give the mass of light. If there is no difference then light is massless. Could we use this to test the masslessness of light too? The only thing stopping this from working is lights mass may just be too small to have any visible effect over distance. In which case back to square one.

 

[ZapperZ]

Why wouldn't it. a single photon would not necessarily effect itself and would hit in a straight line. If you measure it, bounce light off it then it would deviate? The Energy of light itself would have too small a mass to deviate light
I didn't say it was necessary I just said it would help to explain difraction, photons would in fact bounce off each other. And nothing precludes this as far as I can see from being true.
I've even thought of an experiment to test the validity of this premise, you could use the fact that without mass light should be bent by gravity x amount if it has mass it will be bent by y amount y-x can be used to give the mass of light. If there is no difference then light is massless. Could we use this to test the masslessness of light too? The only thing stopping this from working is lights mass may just be too small to have any visible effect over distance. In which case back to square one.

This is getting very confusing.

I don't need OTHER photons to produce a diffracton pattern. I can shoot ONE PHOTON AT A TIME at a slit. After I've done this a gazillion times, I REGAIN the diffraction pattern that I know and love from doing this the normal way.

So where is light boucing off each other, which is in itself a rather complicated effect, to produce such well-defined diffraction pattern?

Zz.

 

[Schrodinger's Dog]

Think of waves of water molecules in a tank, what pattern do the produce from bouncing off each other, do you see any inteference patterns that may be similar with light?

 

[ZapperZ]

Think of waves of water molecules in a tank, what pattern do the produce from bouncing off each other, do you see any inteference patterns that may be similar with light?

Interference pattern you see from water waves is NOT due to water molecules bouncing off each other! It is due to the interference of a COLLECTIVE effect known as WAVES!

Again, I brought up the issue of shooting one photon at a time and regaining the diffraction pattern. Where is the "collision" between photons there?

Photon-photon scattering cross-section is EXTREMELY SMALL! In fact, under QED, photon-photon scattering at optical range is practically non-existent! The photon-photon collider that is being envisioned is being designed for gamma range where the scattering cross-section is appreciable enough to be detected.

So I do NOT buy the explanation of photons bouncing off other photons as the explanation for diffraction. This makes zero sense if one just consider on how to explain why a smaller slit would cause a wider spread of the diffraction pattern, especially when such a change does not alter the photon density in the incoming light.

Zz.

 

[inha]

Think of waves of water molecules in a tank, what pattern do the produce from bouncing off each other, do you see any inteference patterns that may be similar with light?

Similiar? Perhaps. That does not imply any sort of connection between the two very very very different phenomena though.

 

[Schrodinger's Dog]

you have balls running together they pass through a slit due to gravity(they are in a vertical column with slits at the centre of the column and cups to catch the balls at the bottom, which slits difract the balls the most the larger ones or the smaller ones, if we have the same number of balls but vary the size of slit which slits effect the balls difraction the most? Now if we have a superposition of each ball does it effect the balls path? The light hits the detector in a straight unbroken line from a single photon unless we measure it then it difracts?

i'd try the experiment with 1,10,15,20,30 balls on varying slit sizes, if there was no change in difraction with the slit sizes with 10 balls i would be awfuly shocked, I would suspect the difraction would be the same regardless of how many balls you used too? Not sure having not done the experiment

 

[ZapperZ]

you have balls running together they pass through a slit due to gravity(they are in a vertical column with slits at the centre of the column and cups to catch the balls at the bottom, which slits difract the balls the most the larger ones or the smaller ones, if we have the same number of balls but vary the size of slit which slits effect the balls difraction the most? Now if we have a superposition of each ball does it effect the balls path? The light hits the detector in a straight unbroken line from a single photon unless we measure it then it difracts?

Will you please do the rest of us a favor and LOOK UP the diffraction effects? Figure out how the diffraction patterns changes with respect to the slit size, frequency of light, etc. You should know this already before you start requesting for an explanation, much less formulating one yourself.

And then, try, if you can, not to ignore my point that you CAN get diffraction patterns by shooting one photon at a time. In fact, you can get INTERFERENCE pattern by shooting one photon at a time! This nullifies as clearly as one can get your photon bouncing guesswork.

Zz.

 

[Schrodinger's Dog]

well that was quite patronising it's not the size of slit that counts it's the number of slits more slits means more interference. The same with balls in my gravity experiment. same thing would happen, increase the speed of the balls what happens. Also it says that single photons interfere with them selves because of super position can you tell me any reason why if they were both a wave and a particle at the same time this still wouldn't be the case?

 

[ZapperZ]

There is already an FAQ in the General Physics sub-forum on this question of Wave-Particle picture. This is on top of all the already discussed threads.

If anyone ever question on why I get rather annoyed by one of these things, please take a very close look at what has transpired here because it illustrates a very difficult and tedious task that one ALWAYS has to do in this situation. When you offer an answer to a question, you then end up having to explain the answer, and then end up explaining THAT answer. At some point, this goes on indefinitely and I don't care if you're a saint, it DOES get very tiring.

Do NOT be offended if someone tells you that you need to do some background studying/reading at some point! It means that your ability to comprehend the answer requires that you have some pre-requisite knowledge at some level. You have to do SOME of the work in putting an effort to understand something - it isn't just gimme, gimme, gimme. I shouldn't have to dig back down to now having to explain why wave-particle duality is only in NAME, not in principle, as far as QM is concerned. There is NO DUALITY! <shock rings through the forum>

So now, we went from photon "size" and property, to photon having mass, to photon going through a slit, to photon "wave-particle". Wouldn't it have been a lot more effective to start from ground zero and look at basic physics first?

If not, I give up!

Zz.

 

[Schrodinger's Dog]

Well the point I was trying to make is light could have mass, no reason why it can't and if it does it would mean that difraction would be because of interference on 2 levels at once, it also means the photon has a size, all I was saying was that you can't discount a photon having mass or otherwise and if it does have mass it might in part explain difraction as not just energy affecting energy but particles affecting particles. So we're agreed you have no idea whether a photon has mass since you can't prove it either way. And it's size could be very small so asserting it has no size is rather unproven? or was I reading a different thread? Or can you tell me why QM precludes a photon having mass? simple point? But obviously very hard to explain?

 

[inha]

Or can you tell me why QM precludes a photon having mass? simple point?

Very simple. Particles with any mass can not travel at the speed of light. Photons on the other hand do travel at the speed of light.

 

[Schrodinger's Dog]

Ok but the speed of light is defined by light nothing can go faster than light, not even light then, would that be so hard to accept? Light having no rest mass is not 100% certain is all I'm saying, so saying it has no size is not 100% certain either. That is the point. It's irrelevant to modern QM but none the less I think by logic you can see that light can have a size it's unlikley but not beyond the realms of QM theories; I don't personally believe it does but then I can't prove that.

 

[ZapperZ]

Well the point I was trying to make is light could have mass, no reason why it can't and if it does it would mean that difraction would be because of interference on 2 levels at once, it also means the photon has a size, all I was saying was that you can't discount a photon having mass or otherwise and if it does have mass it might in part explain difraction as not just energy affecting energy but particles affecting particles. So we're agreed you have no idea whether a photon has mass since you can't prove it either way. And it's size could be very small so asserting it has no size is rather unproven? or was I reading a different thread? Or can you tell me why QM precludes a photon having mass? simple point? But obviously very hard to explain?

I could have sworn we have gone through this already.

Try finding something in physics that you can PROVE! Asking me to prove that a photon has no mass is meaningless! This isn't mathematics where something can always be proven via a set of self-evident axioms! I can't prove Newton's Laws. I can show that it is VALID under the right cirumstances! There's a difference. Your insistance that I prove something is getting to be very annoying. I would like YOU to prove something in physics. Go on! Tell me something in physics that has been proven!

As far as the test for the postulates of SR, here you go if you're too lazy to dig this from my Journal

1. Severe Limits on Variations of the Speed of Light with Frequency;, B. Schaefer, PRL v.82, p.4964 (1999).

Also see Physics News Update report at http://www.aip.org/enews/physnews/19...t/pnu432-2.htm .
This is the most accurate measurement to-date that c is independent of frequency/wavelength. If photons have any mass, or if c isn't a constant, this would manifest itself as a variation in speed at different frequencies. So far, none has been detected.

2. http://www.aip.org/enews/physnews/20...t/pnu484-1.htm .

This is the most recent and accurate determination that the speed of light is independent of the speed of the source.

3. http://www.aip.org/enews/physnews/2002/split/590-1.html .

Again, this is the mostp recise test yet that the speed of light is independent of the direction of propagation.

4. Tests of Relativity Using a Cryogenic Optical Resonator;, C. Braxmaier et al., PRL v.88, p.010401 (2002).

Ether? What ether? This is the most precise determination to-date that the speed of light is independent of the velocity of the lab frame. The experiment used a version of the famous Morley-Michealson interferometer called the Kennedy-Thorndike test. You may read the Physics News Update report at http://www.aip.org/enews/physnews/2002/split/571-1.html .

5. Tests of Lorentz Invariance using a Microwave Resonator;, P. Wolf et al., PRL v.90, p.060403 (2003).

6. New Limit on Signals of Lorentz Violation in Electrodynamics;, J.A. Lipa et al., v.90, p.060403 (2003).

or read here http://physicsweb.org/article/news/7/2/12

In the 14th Feb. issue of Phys. Rev. Lett., there is not one, but TWO new experimental results that put a severe limit on any possible violation of the Lorentz transformation (which is built-in in Special Relativity). These two experiments present the most accurate result so far that c is velocity and earth-orientation independent. You may read the summary of one of this result at the AIP Physics News Update: http://www.aip.org/enews/physnews/2003/split/623-2.html or better yet, read the actual papers in PRL.

7. J. Luo et al., PRL v. 90, p.081801 (2003).

A report on a new measurement for the upper limit of a photon mass. In other words, if photons do have a mass, it can't be any larger than this, which is the current best resolution of our instruments to detect such a thing. This again put severe constraints on anyone claiming that photons do have a mass. http://www.aip.org/enews/physnews/2003/split/625-2.html

...

11. Muller et al., PRL v.91, p.020401 (2003).

This time, the evidence comes from the most accurate measurement to date of the uniformity of c using a modern version of the infamous Morley-Michealson experiment. Using cryogenic optical resonators, they measured for the possible anisotropy in the speed of light for over a year (as the Earth moves through space in its orbit around the sun and thus, changing its orientiation). The showed with unprecedented accuracy that the upper limit for any possible variation in c would have to be lower than 2.5 x 10^-15, which is 3 times more accurate than previous measurements.

12. M. Fullekrug, PRL v.93, p.043901 (2004).

We have another experimental evidence for the constancy of the speed of light - this time coming from very low frequency radio waves in the frequency range of 5 to 50 Hz. Again, this measurement places the upper limit on the photon rest mass (if any) at less than 4 x 10^-52 kg (yikes!).

13. This continues an earlier collection of experimental observations (listed here) that are consistent with the postulates of Special Relativity. This time the report comes from a workshop paper on the measurement of the isotropy of the speed of light using the cosmic microwave background radiation. This measurement claims the most stringent measurement on any possible anisotropy of c of our observable universe.

http://arxiv.org/abs/astro-ph/0410742

I will put it to you that each one of these indicates that there are MORE verification on the validity of SR's postulates than there are for "photons have mass".

Zz.

 

[Schrodinger's Dog]

I agree, but my point is prove it beyond reasonable doubt that was the only point I wanted to make. Therefore at the start of this thread when this question arose if you just said well we can't know 100% that light has no mass but it is extremely likely that it does and very unlikely that it doesn't if that is the case then etc,etc,etc. Might save yourself a lot of arguments.

 

[vanesch]

you have balls running together they pass through a slit due to gravity(they are in a vertical column with slits at the centre of the column and cups to catch the balls at the bottom, which slits difract the balls the most the larger ones or the smaller ones, if we have the same number of balls but vary the size of slit which slits effect the balls difraction the most? Now if we have a superposition of each ball does it effect the balls path? The light hits the detector in a straight unbroken line from a single photon unless we measure it then it difracts?

Entirely. The superposition of balls (especially the *vertical* superposition of balls) influences the diffraction pattern obtained by the slits. In fact, it is a not very well known fact, but exactly this argument was used by Einstein to try to invalidate quantum theory, and it was Bohr who answered him: the Sagnac effect of general relativity. Indeed, the Sagnac effect (also known under a slightly different angle, the Unruh effect) slightly changes the rate of the flow of the arrow of time in the gravitational field, and as such puts the eigentime of the different balls slightly out of phase, which result in the interference pattern of the balls with a coherence length equal to their inverse Compton wavelength. However, to avoid gravitational collapse, the no hair theorem states that there is a lower limit to the size of the balls that can diffract. Given the fact that photons must satisfy a similar relationship, this gives us already a lower limit on the size of the photon, and hence, through the Hawking-Galerkin theorem, a lower limit on the mass of the photon (well, the Majorana photon, of course, because the Dirac photon, through its Zitter motion, can annihilate its effective mass term in the jigsaw mechanism of the Higgs field).

However, the above reasoning is 1) not well known by most physicists, and 2) leads in practice to such a ridiculously low under bound, that it will be very hard if not impossible to determine either the size or the mass of the photon. As such, the approximation of the massless photon is usually taken to be exact, and this works for all practical purposes...


ZZ, I want my pin.

 

[ZapperZ]

I agree, but my point is prove it beyond reasonable doubt that was the only point I wanted to make. Therefore at the start of this thread when this question arose if you just said well we can't know 100% that light has no mass but it is extremely likely that it does and very unlikely that it doesn't if that is the case then etc,etc,etc. Might save yourself a lot of arguments.

Sorry, but this is OBVIOUS to anyone who has studied even elementary SR! It is why Einstein himself formulated that E = pc for photons! This is clearly in the introduction page of ANY lesson on modern physics, even online texts have such a thing! Refer to the hyperphysics page if you don't believe me! I didn't realize that I have to always qualify what is in the TEXTBOOKS as being "extremely likely"!

Reasonable doubt? Did you know that the most convincing evidence that the postulates of SR is correct is NOT from some esoteric experiements, but rather from the electronics that YOU are using? Every single second, you are validating a number of SR's postulates! How? The semiconductors that you are using in your solid-state transistors require RELATIVISTIC CORRECTIONS to account for the band structure. Without that, the theoretical band structure does NOT fit the experimental results, and we have no clue on why they work!

We regard SR with a high enough validity that we USE it for practical applications! A lot of people take this for granted or just ignorant about such facts, but yet, this is THE most convincing evidence that something is valid.

Zz.

 

[ZapperZ]

ZZ, I want my pin.

Not just yet, vanesch. There's a condition that has yet to be met!

Zz.

 

[Schrodinger's Dog]

I think the reson this argument went on for so long was because what you asserted to him you asserrted 100% to be true. It seems to me that he gave up posting when you explained some source material, but the confrontational nature of your posting made him continue to post long after you'd made the point. It does take a great deal of patience to explain some of th ideas in QM but if you say things like we know and site references your average student has no chance of accesing then your argument gets lost because we have no idea what your talking about you then say something like did you even read bla bla bal and we say no I can't get acces to it, and then we come up with something based on what we can actually read and you say my god din't I already explain this, then you say. Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice. No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do. That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.

 

[inha]

I think the reson this argument went on for so long was because what you asserted to him you asserrted 100% to be true. It seems to me that he gave up posting when you explained some source material, but the confrontational nature of your posting made him continue to post long after you'd made the point. It does take a great deal of patience to explain some of th ideas in QM but if you say things like we know and site references your average student has no chance of accesing then your argument gets lost because we have no idea what your talking about you then say something like did you even read bla bla bal and we say no I can't get acces to it, and then we come up with something based on what we can actually read and you say my god din't I already explain this, then you say. Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice. No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do. That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.

Any university student will have access to scientific journals. Even if one is not an university student but there's an university nearby can go to it's library. Someone not bothering to check the references provided is not the fault of the one who gave the references.

 

[ZapperZ]

I think the reson this argument went on for so long was because what you asserted to him you asserrted 100% to be true.

What exactly did I asserted to be 100% true? Where did I do this?

My incursion into this thread was to ILLUSTRATE why the question "how big is a photon" is similar to asking "how painful is purple?". Or did you forget that? How did I say what to be 100% true?

It seems to me that he gave up posting when you explained some source material, but the confrontational nature of your posting made him continue to post long after you'd made the point. It does take a great deal of patience to explain some of th ideas in QM but if you say things like we know and site references your average student has no chance of accesing then your argument gets lost because we have no idea what your talking about you then say something like did you even read bla bla bal and we say no I can't get acces to it, and then we come up with something based on what we can actually read and you say my god din't I already explain this, then you say. Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice. No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do. That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.

Er.. as far as the "pin" is concerned, you have no clue what vanesh is up to, so try not to practice the same thing that you've been doing in here, which is jumping onto something that you barely know about.

Secondly, ignorance is not an excuse as a license to formulate things. That's like Bugs Bunny able to float in air because he said he never learned about gravity. When you ASKED me, not about SR's postulate, or the validity of SR's formulation, but its APPLICATION in the form of the observation of diffraction effects and "mass", I PRESUMED that you ALREADY know ALL about the postulates and what they mean! ANYONE can read up on this and you don't need special web access. Hyperphysics website has a wealth of info on this! But then you started asking for PROOFS! This is hysterical! What form of "proofs" do you want? How does one prove a physics theory or concept? If you want indications if they are valid, then I cite EXACT SOURCES. I don't just TELL you. I do not expect you to buy everything I said without backing it up. I tell everyone here the very same thing, that you need to be more discriminating when someone tells them something and to demand credible sources! That's what you got! I gave you exact citations on various papers in REPUTABLE JOURNALS. This is how we do physics! I treated you with enough respect that I EXPECT you to double check on my claims and the sources!

Instead, you complain that you can't get access to them! Oy vey!

Zz.

 

[Schrodinger's Dog]

The point is though if your trying to explain something to someone and they say I ahve no idea what your talking about because I can't read the sources, don't be surprised at why people then say something that goes against your sources, it's because they have no idea what you are talking about or how you came to that conclusion. It may be how the Physics world does things. But if a teacher walked into a classroom and said turn to page 40, And there were no books on the desks, don't be surprised when the students learn nothing. You also have a habit of missing the point a lot too.

 

[ZapperZ]

The point is though if your trying to explain something to someone and they say I ahve no idea what your talking about because I can't read the sources, don't be surprised at why people then say something that goes against your sources, it's because they have no idea what you are talking about or how you came to that conclusion. It may be how the Physics world does things. But if a teacher walked into a classroom and said turn to page 40, And there were no books on the desks, don't be surprised when the students learn nothing.

But it doesn't bother you that the book IS required in the class and the students don't have it? I would have asked "Why DON'T you have a book? Wasn't it listed as required to be in this class"?

The problem here is that you'd rather be told in having-waving forms to the answers you want, rather than specific technical details. I've been trying to ween people off this and make people want MORE details. Hand-waving arguments should no longer be tolerated as being the definitive answer, it is too ambiguous. Yet, look at this thread and see how "satisfied" some people are with such things. I can find unbelievable amount of loopholes in those explanations.

I have taught college level courses before. I have to assume that the students entering my class already POSSESS a certain level of knowledge. If not, I will NEVER be able to tackle the subject matter because I always have to backtrack and answer questions that they should have known already! This is what I had to do with you in this thread. It appears that almost at every turn, we are going back into the fundamentals of not only SR, but QM! At some point, such an exercise is no longer productive and gets nowhere fast. If you want to know about QM and SR's fundamentals, ask THOSE! But if you start asking about a higher-level question that makes use of those foundations, then do not be surprised if someone tells you to go look up those foundations yourself!

And talk about missing points. You missed MY question on where I asserted something to be 100% true. Where did I make such a statement? Or can you not back up things that you say?

Zz.

 

[reilly]

SDOG
[Anyway the upshot is don't assert anything until you know the person has read the relevant material, I couldn't follow your arguments because I couldn't acces some of the stuff you'd referenced,when I made this clear you ignored it twice.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
RA I'm quite baffled. I presume you are quite serious about requiring people to make sure you have found and understood sources and relevant material before they deign to help you. The nicest thing I can say about such a statement is that it is backwards and upside down. Nobody owes you nothin.'

Also, clarity, like beauty, is in the eye of the beholder.

If you cannot cut it, go back and study, and do homework problems -- unless, of course, the author of the problems is somehow disagreeable or unreliable.

>>>>>>>>>>>>>>>>>>>>>>>.


No wonder you get into such lengthy arguments so often, your failing to acknowledge that not everyone has the source material you do.
>>>>>>>>.. Everyone, i assume, can ask questions

That on top of the fact that you seem to be saying(to most people anyway that you're right beyond reasonable doubt, not the case but it sounds that way) and you have a problem, which is why I suggested you start posting information people can actually access and not chastising them when they don't fully understand your argument because of it. I think I've made my point. And I think I should get that pin thankyou very much.
>>>>>>>>>>>>>..
(RA)
You clearly have not taken the time to learn much about the field of
physics. If you did, then you would never say much of what you do above.And, is it not odd, that more often than not, many see this forum as a great opportunity to learn, and so they "listen' and learn, and enjoy the process.

With all due respect, you have just said what many of my college sudents said upon receiving a grade less than that expected. It was my fault, not theirs. Many more, however took a somewhat different tack.


I'm wondering -- Have you ever taught physics? or anything?

Regards,
Reilly Atkinson

 

[Boffin]

Photons do have mass, but

Or can you tell me why QM precludes a photon having mass?

Very simple. Particles with any mass can not travel at the speed of light. Photons on the other hand do travel at the speed of light.

I'm jumping in here with an answer, because none of the mentors have provided a decent one to this issue in my opinion. So if you don't like the answer I guess you'll just whack me over the head. So be it.

Photons do have mass! Relativistic mass (I originally wrote relative which of course is wrong) that is. But photons do not have rest mass. And the common definition of mass in science books basically is that when the word mass is used by itself then mass=rest mass. Ideally scientists should always be saying the photon does not have rest mass, because that is what they really mean. Then the issue would not cause confusion. All energy can be said to have relativistic mass (I originally wrote relative) directly proportional to the energy. I've studied the issue quite a bit and that is the best description by experts I can find.

 

[Dense]

In the two undergrad physics courses I took, there was no mention of QED, and relativity was only lightly touched on. So forgive my ignorance.

But what I'm getting from reading this thread is the following:

Q: How big is a photon?
A: It has no size. A photon has a location, but occupies no space.

Q: What does a photon look like?
A: It doesn't look like anything. It is a chunk of energy, not made of matter. It's hard to imagine energy by itself, because we tend to think of it in terms of its effect on matter, but that's what a photon is.

Q: What does a photon behave like?
A: A photon has some very strange and very cool behaviors. It has momentum but no mass. It only ever travels at the speed c; any slowing effect you observe is caused by absorbtion and emission of photons by matter. A single photon can be diffracted like a wave, and can behave as if that single photon passed through more than one slit at the same time and interfered with itself. There are even stranger and cooler behaviors.

Q: Why does a photon behave like that?
A: Quantum Electrodynamics is the best explanation we've got. It says a photon takes every possible path, at the same time, once it's been emitted (travelling in straight lines). Each path has a different probability, calculated with complex numbers, and the math gets funky. The weighted average of all the probabilities gives you the final observed path of the photon.

Q: How's that work?
A: This forum isn't the best place to get that answer. Here's some sources you might find helpful.


Am I sort of on target here?

 

[ZapperZ]

I'm jumping in here with an answer, because none of the mentors have provided a decent one to this issue in my opinion. So if you don't like the answer I guess you'll just whack me over the head. So be it.

OK, since you ask for it...

Photons do have mass! Relative mass that is. But photons do not have rest mass. And the common definition of mass in science books basically is that when the word mass is used by itself then mass=rest mass. Ideally scientists should always be saying the photon does not have rest mass, because that is what they really mean. Then the issue would not cause confusion. All energy can be said to have relative mass directly proportional to the energy. I've studied the issue quite a bit and that is the best description by experts I can find.

OK, NOW we're playing with words. When you say the mass of the Earth is so-and-so kg, or the mass of that object is so-and-so kg, do YOU ALWAYS have to clarify that this is the REST MASS and not the RELATIVISTIC (there's no "relative mass") MASS? Is this ALWAYS a required clarification, failure of which causes utter ambiguity? HONESTLY?

And I'll tell you what. I can play this game as well as the next person. I'll show you why EVEN the use the the term "relavistic mass" isn't kosher!

http://arxiv.org/abs/physics/0504110
http://arxiv.org/abs/physics/0504111

So take that!

Zz.

 

[ZapperZ]

In the two undergrad physics courses I took, there was no mention of QED, and relativity was only lightly touched on. So forgive my ignorance.
But what I'm getting from reading this thread is the following:
Q: How big is a photon?
A: It has no size. A photon has a location, but occupies no space.

Not quite. Recall there's a difference between an "empty set" and a "set of 0". Similarly, the question on the size of a photon doesn't have an answer "it has no size", because that means it's size is zero. In condensed matter physics, we deal with a "quantum dot" and can have several phenomena associated with such a thing. So even something with almost no size can be investigated.

No, the answer that *I* have given was that I can't answer that because "pain" was never defined with the color purple. I cannot say the answer to the question to how painful purple is that is it is not painful. That would be as meaningless as if I were to answer "yes, purple is VERY painful". Until we have a formulation regarding the SIZE of a photon and how that is defined, I have no way of answering that.

Q: How's that work?
A: This forum isn't the best place to get that answer. Here's some sources you might find helpful.
Am I sort of on target here?

From my observation on here, and in other forums, the BEST usage of a medium like this is as AN IMPETUS to study things in detail. What this means is that you ask a question, it turns out it is more complex than what you originally thought, people responding gave an idea of what is involved in the answer, and point to some references in which a clearer, more in-depth description can be found. This, to me, is the most effective usage of a forum such as this. Ultimately, the learning part has to done by the individual. You are the one who has to put in the effort. If it is tedious and difficult, then tough! If it were easy, EVERYONE would and could have done it!

Zz.

 

[Dense]

Thanks for the clarification.

So it's not even "a photon has no size," but rather "size isn't something that pertains to photons." Is that the distinction?