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Photons
May 21, 2011 15:29:03 GMT 1
Post by principled on May 21, 2011 15:29:03 GMT 1
I believe that in a post sometime back (can't find the actual post at the moment), STA said that photons are mass less (ie has no rest mass). Since that post I have been pondering this and seek some clarification.
Now, as I understand it, a photon travels at the the speed of light. I also believe that photons can have various energy states.
When we look at Newtonian mechanics we can see that having no rest mass would indicate that the photon requires no force to accelerate it and so has no energy. This would also seem to be the case in the equation E=MC^2 as well.
So, how do we know that the photon has energy and how can we measure it?
Physicist seem to "get round the problem" ;)by talking about relativistic mass, which can increase without limit, whereas rest mass (or mass less) is fixed. This is an interesting concept, which I would like to understand further.
Any takers?
Regards P
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Photons
May 21, 2011 18:50:26 GMT 1
Post by nickcosmosonde on May 21, 2011 18:50:26 GMT 1
The photon has energy but no mass. Personally, I find it never helps to think of "the photon" as some sort of particle. I mean, never. It's a wave of energy, always, and if you think of it like that you never run into any contradiction. It's true you can say much the same about any particle to some extent - the sifference is that some waves are in some way enclosed around themselves, and it's this enclosure that produces the phenomena we experience as "mass".
The analogy of a wave in the ocean is I think entirely valid and apposite. The mass of the ionic water is not the wave - the wave is a non-material energetic phenomenon in itself, setting the various water particles in motion, but in itself something separate and independent of all those material motions.
It's difficult, I'm with you. We're long indoctrinated to think of the world in terms of things and solid objects, and it's hard to lose it. But seemingly there it is - that's the reality. Waves of energy. Some persist and circularise in spatio-temporal location, others dissipate in energetic freedom.
There's a deeply satisfying analogy - I think it's more than analogy - with mathematics, or more generally logic. Between measurement, or ratio, and number. These are abysmal matters.
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Photons
May 21, 2011 20:06:52 GMT 1
Post by principled on May 21, 2011 20:06:52 GMT 1
Thanks for the quick reply Nick. Let me sleep on what you've said and I'll get back to you. Regards P
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Photons
May 21, 2011 21:08:21 GMT 1
Post by mak2 on May 21, 2011 21:08:21 GMT 1
Consider a particle with rest mass. An electron, for example. Its mass increases as its velocity increases. Special relativity has the formula
m(v)=m(0)/squareroot(1-v^2/c^2)
where m(0) is rest mass and m(v) is the mass when moving with relative velocity v. m(v)-m(0) is the relativistic mass.
If v was equal to c the mass would become infinite, showing that a particle with rest mass can never reach the speed of light, c.
As photons do travel a the speed of light, they must have zero rest mass but they do have energy and therefore relativistic mass.
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Photons
May 22, 2011 10:00:29 GMT 1
Post by principled on May 22, 2011 10:00:29 GMT 1
Mak2 Thanks for your post. I'll combine your post and Nick's in my response once I've had time to digest them (hopefully tonight or tomorrow). Regards P
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Photons
May 22, 2011 16:42:33 GMT 1
Post by nickcosmosonde on May 22, 2011 16:42:33 GMT 1
I think I'm in agreement with mak. My only quibble - and it's not really a quibble, but perhaps the most profound conundrum in physics - is with the precise meaning of:
"As photons do travel a the speed of light..."
There's something deeply wrong with this intuitive formulation. There's a conceptual contradiction in the application of our pre-relativistic notions of "travel" and "speed" to light, not helped by the unnecessary confusion dragged over from the quantum physical paradigm of a "photon" as some sort of "particle".
It's clear that in any ordinary understanding of "travel" and "speed", we're forced by SR to the conclusion that light doesn't "move" at all, at any "speed". Rather, it is the universe of mass that is "moving".
I know that this sounds utter nonsense. Possibly, it is. But we're running up against the limits of our conecptual understanding here - the boundaries between what we mean by our fundamental ultimate abstract concepts like space and time, motion and energy, and what we actually empirically observe.
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Photons
May 22, 2011 19:24:26 GMT 1
Post by principled on May 22, 2011 19:24:26 GMT 1
Hi Nick and Mak2 I've had a chance to read your posts and have also had a look at two sites: Wiki (of course): en.wikipedia.org/wiki/Photon Where they seem to say that the photon is both a particle and a wave, whose energy (and momentum) is dependent upon wavelength and inverse of frequency. Now IF its speed is constant (as some say), then I would assume this to mean that its mass (relativistic mass-mr) will vary according to them as well. So what determines the frequency or wavelength (and therefore the mr, I assume) that any particular photon would have? The second site (http://www.desy.de/user/projects/Physics/ParticleAndNuclear/photon_mass.html) is also interesting and easier to follow (for me) This talks about the implied rest mass limits based on "satellite measurements of planetary magnetic fields". So they seem to be saying that it is possible for a photon to have a rest mass (as small as that may be). I'm a little out of my comfort zone with the above, but hope that it will promote some more discussion. P
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Photons
May 22, 2011 20:06:59 GMT 1
Post by nickcosmosonde on May 22, 2011 20:06:59 GMT 1
Hi Nick and Mak2 I've had a chance to read your posts and have also had a look at two sites: Wiki (of course): en.wikipedia.org/wiki/Photon Where they seem to say that the photon is both a particle and a wave Yes, indeed, this has been the standard linguistic formulation since the late 1920s. But what does it mean? Light is raidtaed in discrete energetic packets, and that enrgy can impact upon matter in a localised manner, that's what it means, as far as I can tell. That's the "particle" nature of light. But otherwise, there's a side range of other observed behaviour that tells us quite explicitly that it is a wave phenomena, and is not localised at all. And so it is with other "particles" in fact. That tells me there's something amiss with the whole 19th Century notion of "particle" - and I'm not alone in this. The whole notion of momentum, now. What role does this have in an analysis of the behaviour of light? It has energetic results on impact with matter, that's all. It exhibits a measurable pressure, also. But is this what we mean by "momentum"? No. It's a carry-over from an entirely seperate paradigm, derived from mathematical equations describing the motion of mass. I think we can take this as a given, for the moment. Invariant is a better concept, empirically, though. Them? Ha! My answer - the originating motion of the mass that has emitted that energetic radiation. Stress on the "motion". But you won;t find any physicist who'd agree with that, I don't think. I'd like to hear what StA has to say about these questions. You're delving deep under anyone's current comprehension, I believe. Except of course an em wave can not possibly have a "rest mass" according to any accepted theory - in that they're agreed. There are no accepted answers in this particular zone, P, of that much I'm sure.
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Photons
May 23, 2011 15:52:56 GMT 1
Post by speakertoanimals on May 23, 2011 15:52:56 GMT 1
Except some posts seem to have forgotten that according to quantum theory, sometimes a thing will act like a particle, at other times like a wave, and sometimes like both and neither!
Quantum objects, to be precise, are neither particle nor wave, but somehow both and more than both.
So no surprise if actual behaviour seems at odds with CLASSICAL ideas of particles or of waves.
As regards a photon, it MAY have a particular frequency/energy, or it may not! Whoops, you go, that sounds weird! Except it is just like any other particle (like an electron). Depending on exactly what your experiment is, the electron may have a well-defined energy, or it may not. Same goes for photon, since energy is frequency.
People get worried, except going back to waves, having a DEFINITE frequency./wavelength just means we have an INFINITE wavetrain, hence information whatsoever as to location, since anywhere along the wavetrain is equally likely. If we have a wave which is more localised, then by fourier analysis, we have a RANGE of frequencies present, hence in photon terms, a RANGE of possible energies, but a more definite location.
There's your error! According to relativity, things with zero rest mass travel at the speed of light (in a vacuum), and ONLY at the speed of light. Hence energy equals work required to get it up to speed just doesn't apply here. They either exist (and travel at lightspeed), or they don't exist.
What determines the frequency? Well, as for any other particle, the nature of the reaction where it was created. Energy and momentum still have to balance out, and for many reactions, a RANGE of momenta and energies are possible. So same holds for photons as for any other particle. In most cases, you specify that a particular reaction occurs (A hits B or somesuch), and then you have a range of possibilities, depending on which way each of the products go, and how much energy they have.
It isn't really correct to think of 'a' photon having a range of energy states -- the problem with that is that it assumes you can label a photon, and know that the photon you had over here is the same one as turns up later, after you've done something to it to change its energy. Usual way of looking at it is that you have a photon over here at the start, then you have some sort of interaction, then a photon of a different energy comes out at the end. SAME just doesn't work.
Saying it is the SAME photon, just picked up some more energy, just isn't the right concept, it kind of assumes that you could tell if it wasn't the same! Except you can't! So makes no sense to talk of the same photon, unless you have something like, came in from the left, had this energy and and this momentum and this polarisation, nothing happened at all, and then left stage right, so might as well call it the same one since nothing happened!
Except the whole point of the quantum hypothesis is that em energy comes in discrete chunks! Another system can only ever absorb (or emit) energy in terms of one photon, two photons etc. Hence if an excited atom goes back to the ground state and emits some em energy, what should we call it but ONE photon?
Hence since we can count them, and they only are created or distroyed in whole chunks, why not particle? It's a very different concept, after all, compared to that of a wave.
We can count electrons too (think of charge), hence same concept useful, and lets call both particles.
Hence to NOT think of a photon as a particle in some sense just ignores the whole of quantum theory. In which case, you know you'e going to get daft results....................
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Post by nickcosmosonde on May 24, 2011 5:02:22 GMT 1
Except some posts seem to have forgotten that according to quantum theory, sometimes a thing will act like a particle, at other times like a wave, and sometimes like both and neither! Quantum objects, to be precise, are neither particle nor wave, but somehow both and more than both. I haven't forgotten that standard nostrum, I just don't agree with it - as I said, I don't think it ever helps to think of a photon as a "particle". Now, some "quantum objects" can be thought of in this way, agreed - but only if we modify our old classical ideas of what "particle" meant, and view them as self-enclosing waves. What is wrong with the classical ideas of waves? We may have measured its frequency/energy, or we may not have done. To assert in the latter case that it may not have one at all is by definition making an unverifiable proposition, based of course on no possible empirical evidence. You're equating what we can currently understand and predict about the future behaviour of a quantum object using mathematical equations with what the objects are in themselves. This is the standard Copenhagen Interpretation, formulated by a group of physicists fervently convinced of the truth of logical positivism mixed with a then current fashion for post-Kantian German phenomenology. I repeat, there can be by definition no possible observational evidence for this metaphysical stance, and nor does it make much rational sense. And of course all this conceptual muddle disappears if you consistently think of the photon as a wave. Why should "discrete chunks" be conceived as "particles"? Waves by their nature come in discrete chunks too. Not really. Take one end of an attached washing line or skipping rope and move it up and down, you'll get a waveform that has a discrete and definite whole number of nodes. But it's preferable to call them waves, because that's what they are. Not at all. Many quantum physicists agree with me and have no use for the concept "particle" at all. What is its useful indispensable role? What results would be different? I don't think you're right on this one.
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Photons
May 24, 2011 12:29:03 GMT 1
Post by speakertoanimals on May 24, 2011 12:29:03 GMT 1
Except that view doesn't work either!
What's wrong with the classical idea of waves is also whats wrong with the classical idea of particles -- the real world doesn't behave like that at the quantum level.
Sounds like you just don't like quantum theory! And supposed lack of empirical evidence is just daft, frankly. Same evidence for that kind of quantum superposition as there is for photon being in two places at once in the double slit case.
Waves DON'T. Take a wave equation, and basic wave is an INFINITE sinusoid, no chunkiness about it!
As regards em, we have comes in discrete chunks of energy, so you can, in effect count them. The classical wave picture doesn't explain things like black body radiation, or the photoelectric effect. So, if you are going to admit that em energy comes in discrete chunks, might as well give one such chunk a handy label.
The point being, surely, that in some cases you measure stuff as being localised in the way a particle is, and sometimes non-localised in the way a wave is. Hence just says that BOTH pictures are useful. Just that since in the classical limit things do look like particles, seem a useful place to start from.
I don't see the problem, we have two distinct classical concepts (particles, waves), neither on its own captures quantum nature of stuff, hence we have to use both to get us started. Just throwing one out entirely then leaves you having to explain how it comes back in again in the classical limit, which seems like making life harder for yourself.
If you don't think the particle concept is at all useful, what do you replace it with? Okay, I could just talk about wavefunctions, but that doesn't give most people any handle on what is going on. And when I say -- I then detect a thing here, what is the thing we are detecting? It carries energy, and momentum, and angular momentum, and possible charge and magnetic moment as well, and they all 'turn-up' at the same point at the same time (Heisenberg fuzziness not forgotten), so what is wrong with describing that in terms of a 'particle' being detected? What else are you going to call it, and why invent a new name when the old one captues what happens in that case rather well?
Except that is a standing wave, and you can't have MORe nodes with fundamentally altering the wave (changing the wavelength), which is NOT what happens with em waves, where for the same wavelength, you can have any number you want between 0 and infinity.
With travelling waves, the picture doesn't work either. The wavelength is fixed, but the amplityude (hence energy density) is still continuous, hence you can't try and use the points of a particular phase to try and make it look discrete. You also have the problem that you can't have just ONE bump or trough of a wave, you have to have the whole thing.
If you want a discrete, quantised aspect (as esxperiment says we should), we need to add a new thing.
I don't see the point of going through the intro to quantum theory, especially when you haven't said WHY you don't believe it (actually, I'm not that interested in 'belief' -- if you don't like the conventional explanation, do you have a better one to offer that works? not so far it would seem.....................).
They're neither, that is kind of the whole point.
Except if you want to get all technical, then we can just talk about quantum fields -- except we still have, buried in that, the basic creation and annihilation operators -- creators of what? Our basic quanta again, so we might as well use the particle label!
I just don't see the point -- why abandon some concepts that can be useful, even if neither actually describe the entire situation?
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Photons
Nov 18, 2011 20:40:00 GMT 1
Post by striker16 on Nov 18, 2011 20:40:00 GMT 1
That would seem to say that a photon, or whatever, varies according to what experiment is made on it. How then can we really detach experiments from particles? If I ask: What is a photon, the only way to determine this is by a measurement but a measurement is not really the photon itself but a series of events that happen between the thing we are measuring and the end result of 'events' that happen in between observing the photon and the idea of it ending up in someone's brain. We have to remember that in between measuring the actual photon (whatever that really is) and what is perceived, there are apparatus that somehow change the thing in itself (the photon) by converting wavelengths which indirectly act as an analogue to describe the photon's properties.
To me, this says we can only find out about reality by noticing 'disturbances' between the thing in itself and our brain (or mind). It's no wonder science is now finding that Einstein may have been wrong since newer, more sophisticated experiments have been designed that reinterpret what is 'out there.'
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