photons and light

[adidamu]

with reference to particle wave discussion of light , i would like to add the following thoughts which may make the things clear . As jake observed waves can be considered as a disturabance or motion of particles. HENCE, ACCORDINGLY IF WE ASSUME LIGHT AS A WAVE AND PHOTON AS PARTICLE, then one can understand duality. Photon is a particle and whose motion can be considered as light.(in other words movement of photons is to be considere as LIGHT).Then as photons, the units of light can be considere as particle and the LIGHT can be considerd as wave. I think this may clarify the position. thank you.

[Nametag]

Linkie.

And, uh, not really. Photons aren't particles moving with a wavelike motion, they're quanta of light energy that can be treated as having an observable frequency and wavelength. Jake didn't observe anything, he quoted a dictionary passage that added to his confusion. As the answer made quite clear, light is a property of space that looks like a wave when observed in some ways, and like a particle when observed in other ways.

Similarly, electrons, which act just like particles almost all the time, also have an observable wavelength and frequency, which is manifestly not due to a wavelike motion of the electron itself (if an electron were doing all that accelerating, it would be shedding all kinds of photons all the time, which they don't).

[John W. Kennedy]

As a general rule, if you think you understand Quantum Theory, you haven't been paying attention.

[tim314]

The wave isn't an oscillating motion of the photons, and moreover the fact that light is a "wave without a medium" doesn't depend on the fact that there is such a thing as photons. The wave equation for light is derivable from Maxwell's Equations, which make no reference to photons.

I find that in thinking about the wave-particle duality it's helpful to think of wave packets. An ordinary wave extends out to infinity in the forward and backwards directions, with a constant amplitude. A wave packet still oscillates like a wave, but the amplitude is greatest at some peak position and then gets smaller and smaller as you move out to infinity. As the wave packet is made more and more sharply peaked, it looks less and less like a wave and more and more like a single peak at one point. In other words, it becomes localized to a particular point in space like a particle. (On a more technical note, if we were to take the Fourier transform of the wave packet, so as to decompose it into waves of fixed wavelength, we would find that the more sharply the packet is peaked, the greater the contributions from more and more different wavelengths. That is, the wavelength of the packet becomes less and less clearly defined as its location becomes more and more clearly defined).

Basically, a particle is a wave packet in one limit, and a wave is a wave packet in the opposite limit. In quantum mechanics, measurement changes the thing that you're measuring. Thus if you try to measure the position of a particle of light (that is, a photon), you'll cause the wave packet to look more like the sharp peak, and if you try to measure the wavelength of the light you'll cause the wave packet to look more like a wave. Measuring both at the same time is impossible (as we know from the Heisenberg uncertainty principle).

Even without quantum mechanics, we find that classical wave packets obey some sort of uncertainty principle, in that they can't have a precisely defined position and wavelength at the same time. The difference with quantum mechanics is that measuring one or the other causes that property to become sharply defined, and the complementary property to become correspondingly more uncertain.

[ReuvenB]

Originally Posted by John W. Kennedy

As a general rule, if you think you understand Quantum Theory, you haven't been paying attention.

As an undergrad taking Quantum Mechanics II, this is more true than most people can hope to understand.

[Chronos]

Even without quantum mechanics, we find that classical wave packets obey some sort of uncertainty principle, in that they can't have a precisely defined position and wavelength at the same time. The difference with quantum mechanics is that measuring one or the other causes that property to become sharply defined, and the complementary property to become correspondingly more uncertain.

That, and the fact that in quantum mechanics, the wavelength is associated with the momentum.

[dracoi]

Originally Posted by John W. Kennedy

As a general rule, if you think you understand Quantum Theory, you haven't been paying attention.

I think that's right on. Quantum mechanics "makes sense" only from a mathematical standpoint.

[dropzone]

Screw quantum mechanics! If you still believe that light can be, partially, a wave, you are back at beliving in phlogiston, which goes (NOT FAR ENOUGH) to explaining Dark Matter.

Youse guys are stuck with the 19th Century, whether you like it or not.

[John W. Kennedy]

Nope. Light isn't a wave, but it isn't not a wave, either. Else how to interpret

• diffraction
• Snell's law
• interference
• and the plain fact that radio is man-created waves?

No, the best understanding of light quanta continues to be that of the wag who, in the tradition of Good King Rilchiam, dubbed them "wavicles".

[Chronos]

No, the best understanding of light quanta continues to be that of the wag who, in the tradition of Good King Rilchiam, dubbed them "wavicles".

Honestly, I've never been fond of that term. The image it always calls to mind for me is frozen waves hanging from the eave of a house.

[Half Man Half Wit]

What's wrong with just calling them quanta, and accept their behaviour as just the way quanta behave? In fact, I believe a case could probably be made that macroscopic particle- and wavelike behaviours are simply some form of approximation to the behaviour of quanta, and that thus both do not clash paradoxically in the quantum world, but in fact simply experience their natural unification -- a classical object behaves only particle-like when we can ignore internal displacements and phononic excitations and the like, and waves in a classical medium are similarly statistical descriptions of, for instance, the 'bouncing balls' of ideal gas theory.

[John W. Kennedy]

Originally Posted by Half Man Half Wit

What's wrong with just calling them quanta, and accept their behaviour as just the way quanta behave?

Nothing except the unfortunate fact that only a few 'umble mathematicians are willing to do so.

In fact, I believe a case could probably be made that macroscopic particle- and wavelike behaviours are simply some form of approximation to the behaviour of quanta, and that thus both do not clash paradoxically in the quantum world, but in fact simply experience their natural unification -- a classical object behaves only particle-like when we can ignore internal displacements and phononic excitations and the like, and waves in a classical medium are similarly statistical descriptions of, for instance, the 'bouncing balls' of ideal gas theory.

A pretty thought, but, alas, it won't do. Even a single quantum exhibits wave/particle duality.

[Exapno Mapcase]

The Age of Entanglement: When Quantum Physics Was Reborn, by Louisa Gilder is several hundred pages of nothing but a history of how physicists have been driven crazy thinking about how to - and how not to - interpret these results. Well, Bohr and his disciples just accepted the fact and said not to think about it, while a line through Schroedinger, Einstein, Bohm, and Bell went to great lengths to say that it must have meaning.

The book is written for the lay reader - it is structured with many conversations and debates between physicists, taken from their letters and works, a device that doesn't work for everybody although it probably will help most non-physicists wade through the arguments more easily - but it also contains more than most want to know about the history of the subject.

If you want to see how thinking about QM built from the very beginning, this is a great book. If you just want a summary of what the thinking is today, look elsewhere.

[Half Man Half Wit]

Originally Posted by John W. Kennedy

A pretty thought, but, alas, it won't do. Even a single quantum exhibits wave/particle duality.

Yes, of course, but how does that impinge on what I said? I was arguing that the quantum behaviour is fundamental, and that we can only describe macroscopic behaviour as being either wave or particle-like because we can generally ignore everything else/it isn't manifestly present (I probably should have been more explicit about the fact that by classical particle, I meant [the idealisation of] some extended thing like for instance a baseball or something; else, there'd hardly be any internal structure and excitations thereof to forget about).

[John W. Kennedy]

Originally Posted by Half Man Half Wit

Yes, of course, but how does that impinge on what I said? I was arguing that the quantum behaviour is fundamental, and that we can only describe macroscopic behaviour as being either wave or particle-like because we can generally ignore everything else/it isn't manifestly present (I probably should have been more explicit about the fact that by classical particle, I meant [the idealisation of] some extended thing like for instance a baseball or something; else, there'd hardly be any internal structure and excitations thereof to forget about).

And then you went on to say

simply experience their natural unification -- a classical object behaves only particle-like when we can ignore internal displacements and phononic excitations and the like, and waves in a classical medium are similarly statistical descriptions of, for instance, the 'bouncing balls' of ideal gas theory.

which is broken.

[Half Man Half Wit]

Originally Posted by John W. Kennedy

And then you went on to say

which is broken.

I really don't mean to get on your case here, but could you elaborate? It's a useful idealization leading to an approximation good enough for most practical purposes, which was my point really -- that our everyday picture is the approximation, that it's not the case that two fundamentally different classical behaviours are in paradox in the quantum world, but that it is in effect simply a failure of the imagination because we're only used to the macroscopic. I don't intend to say that gas is actually made up of those little balls, but that the assumption that it is leads to a good description of pressure waves. Or take a solid, where you approximate the potentials as harmonic oscillators and get a linear Hooke's law force as a result -- the point being that there really isn't any clear cut wave/particle dichotomy, they're both different approximate descriptions of the underlying quantum behaviour, which simply doesn't conform to either the one or the other -- and should not be expected to!