Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: _Stefan_ on 01/07/2009 12:46:13
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A friend asked me if photons physically move in the wave pattern that they are depicted as, and I couldn't find a source that explicitly stated this was true. One source stated that they did not, and that the oscillations are just a mathematical description, but then went on as if this hadn't been said.
So, do photons physically oscillate as depicted by sine waves, or not?
Thanks. [:)]
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It is very risky to treat photons as if they are extremely tiny particles. If they were infinitely small then they wouldn't interact with anything. So how big can you say they are?
It is unfortunate that the corpuscular theory of light was resurrected, once the wave theory was introduced.
There is no reason, at all (show me wrong.please do) for the photon to be any more than a unit of energy, corresponding to the frequency of the radiation involved. Wave theory (yes- classical) accounts for all the 'mechanical' interactions between light and matter. QM merely introduces the idea of Quanta - not necessarily bullets.
I think the little bullet model is such an attractive idea that people tend to stop thinking any deeper.
"behaves like" is not the same as "is"
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So the photon does not physically oscillate along its trajectory?
Then what is the wave supposed to depict?
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So the photon does not physically oscillate along its trajectory?
Then what is the wave supposed to depict?
The electromagnetic wave *associated* with the photon. The photon is not that.
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The wave describes where the photon is likely to turn up.
'Where' the photon is, whilst on its journey from A to B, is anyone's guess. Does it have to be anywhere or 'doing' anything?
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Thanks both of you.
Can someone please elaborate on the "association" between the photon and the wave? I keep reading that the wave is either a property of the photon, or it's the photon itself. I understand how the wave property has been observed, but not how it relates physically to the photon concept.
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There's 2 possible "waves" associated with a photon.
One kind of wave that might be associated with photons is the classical electromagnetic wave. This kind of wave has been really understood since the 1800s, and is what people mean when they say "light waves" or "radio waves." A photon is what you get out if you ask "what's the smallest unit of this field I could detect?" Quantum mechanics is really weird, however, so you can't just think of photons as being little bullets riding on the wave. That won't add up properly to give you a classical wave. The wave is somehow a weird combination of all the photons in the proper way. The proper way of adding up photons to get a classical wave is called a "coherent state," but the mathematics of it is pretty complicated.
Another wave that might be associated with a photon is the quantum mechanical probability wave that describes the photon. In quantum mechanics, you can use waves to figure out where you're most likely to see a particle when you look for it. You could do something like this for a photon as well. If you arrange these waves in the "coherent state" I mentioned above, you'll get a classical wave out. But you can arrange them differently, and get quantum mechanical objects that you couldn't see in the classical theory. A simple example is a single photon, which cannot exist by itself in the classical theory.
As Sophiecentaur mentioned, a photon is really just the answer to the question "what is the minimum unit of energy I could get from this light?" Since quantum mechanics is weird, when you find a particle that answers one question very well, you often know very little about other questions. In this case, you know a photon is exactly the thing that carries the smallest unit of energy, but you know very little else about how the photon behaves--it doesn't travel on the wave, and you don't even really know where it is in space!
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I don't think that either electromagnetic (EM) waves, or photons, have been adequately described. Yes, their behaviours have been described, but not their 'nature' - that is, what are they made out of and what is their structure.
Classical waves are not really entities in themselves but describe the characteristics of the passage of energy through a medium, so for example, when you look at a transverse wave out at sea, you're not looking at the lateral movement of the water, because the water just moves up and down but otherwise stays in the same place, but the movement of energy through the water.
With EM radiation though, there seems to be no medium involved, so what it is then that is actually 'waving' at us?
It's indisputable that EM radiation exhibit wavelike behaviour, but then it can also exhibit photon-like behaviour too, so until all these factors can be reconciled I think we really have to admit that atm, we really don't know exactly what's going on. The trouble is that it's a rather fundamental thing not to know.
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LeeE
Yes, their behaviours have been described, but not their 'nature'
I think it's becoming my crusade, in life to dissuade people that there is any ultimate 'nature' to anything. The best we can do is apply models that cover most events.
And that ain't necessarily a bad thing to have to come to terms with.
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I don't think that either electromagnetic (EM) waves, or photons, have been adequately described. Yes, their behaviours have been described, but not their 'nature' - that is, what are they made out of and what is their structure.
Classical waves are not really entities in themselves but describe the characteristics of the passage of energy through a medium, so for example, when you look at a transverse wave out at sea, you're not looking at the lateral movement of the water, because the water just moves up and down but otherwise stays in the same place, but the movement of energy through the water.
With EM radiation though, there seems to be no medium involved, so what it is then that is actually 'waving' at us?
Yes, the reasoning is flawless. The best we can say is that what 'waves' is the field: the force applied on a trial charge depends on the location of the charge in space and in time according to a sinusoidal law (or sums, or integrals of sinusoids).
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I keep reading that the wave is either a property of the photon, or it's the photon itself.
Neither one nor the other... [:)]
To simplify things (that is, if we don't want to make a description with quantum electrodynamics), the wave and the photon are just two ways to describe the same, mysterious object: light.
You have to use the wave description when you want to analyse things as interference, diffraction, ecc.; you have to use the photon description when you want to analyse things as photoelectric effect, Compton scattering, ecc.
The *only* link between the two concept is this: a photon's energy is h times the wave frequency.
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And the photon has the property of momentum, too.
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To simplify things (that is, if we don't want to make a description with quantum electrodynamics), the wave and the photon are just two ways to describe the same, mysterious object: light.
But, importantly, QED is the language that allows you to relate the two descriptions in a way that makes sense.
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LeeE
Yes, their behaviours have been described, but not their 'nature'
I think it's becoming my crusade, in life to dissuade people that there is any ultimate 'nature' to anything. The best we can do is apply models that cover most events.
And that ain't necessarily a bad thing to have to come to terms with.
Heh - if you were talking about people I'd be 100% with you, but regarding physics... well I'm not ready to admit defeat just yet.
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...The best we can say is that what 'waves' is the field:...
So far, so good, but what is a field (and I don't mean the thing that neilp and his sheepy pals hang about in) made out of?
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To simplify things (that is, if we don't want to make a description with quantum electrodynamics), the wave and the photon are just two ways to describe the same, mysterious object: light.
But, importantly, QED is the language that allows you to relate the two descriptions in a way that makes sense.
Yes indeed; it works. But it only describes the behaviour, not what it is. It's rather like saying a carnivore eats meat without saying whether it's a lion or a shark.
There's got to be a more complete description, but I don't know what it is. I do think we'll get there one day though.
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Thanks for the very interesting posts everyone!
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what is a field ?
It is a way of describing the region of influence of a mass, charge or current. People often think in terms of 'likes of force' in a field but they are not really there - just something to help you visualise it.
It is the gradient of the Potential energy at a point in space; the stronger the field, the more work is involved in moving about ('uphill or downhil') in that field. A field will cause a force to act on a mass / charge or flowing current. A field is a vector quantity, as it describes the strength and direction of the force.
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And the photon has the property of momentum, too.
Certainly, but it can derived from its energy. Even classically, momentum is E/c.
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...The best we can say is that what 'waves' is the field:...
So far, so good, but what is a field (and I don't mean the thing that neilp and his sheepy pals hang about in) made out of?
Yes, but in this way whe never finish asking. Let's say we discover that a field is made out of...X. Then, what is X made out of?
The field is a well defined property of space, we know how to measure it and that's all we need to know.
If, then, one day comes a guy named Albert who explains what is a field in terms of something else and this description is more prolific and efficient, ok; but the usefulness of concepts like "spacetime curvature" is not the fact that "it explains what a gravitational field is made out of" but that it's a more prolific and efficient (experimentally correct) description.
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I can't disagree with any of those comments, but I'm still not satisfied [;)]