Are photons really particles or just a misconception?

[bhobba]

To go back to jorgdv's original question, long wavelength electromagnetic radiation does indeed behave as waves, as described by Maxwell's equations. There is a discontinuity in how e-m radiation is modeled, depending on the scale: long wavelength radiation interacts with matter as described by classical field theory and short wavelength radiation interacts with matter as described by quantum theory

There is no discontinuity - all is modeled by QED. But the approximation of EM is good enough for long wavelength radiation.

Thanks
Bill

 

[lightarrow]

There is no discontinuity - all is modeled by QED. But the approximation of EM is good enough for long wavelength radiation.

I don't think that "long wavelenght" is enough for appoximating EM field as a classical field: if the intensity is very low, you have one photon at a time even with very long wavelenght, or at least it should be so.

--
lightarrow

 

[bhobba]

I don't think that "long wavelenght" is enough for appoximating EM field as a classical field: if the intensity is very low, you have one photon at a time even with very long wavelenght, or at least it should be so.

Yes - but at our current technology such is undetectable.

The bottom line here is QED models everything - but in some cases EM is good enough. Exactly what those cases are isn't really germane.

Thanks
Bill

 

[H_A_Landman]

Crudely, Quantum Mechanics is about treating classical particles as waves, and Quantum Field Theory is about treating classical waves as particles. That it's incredibly accurate (when we can actually manage to do the integrals!) is evidence that this point of view has some merit.

On the experimental side, both the photoelectric effect and the Compton effect argue strongly that when light interacts with matter, it behaves "as if" it were made of discrete packets of energy and momentum. Einstein's original (German) paper uses this exact wording: "als ob". He doesn't say that light *is* particles, he says it behaves *as* *if* it were made of particles.

The classical wave description of light fails for many reasons. It can't explain entangled photons, or lasers (which depend on photons being bosons), or Bose-Einstein condensates of photons. And yet, if you have a large number of photons and can ignore the quantum effects, it's perfectly fine for designing (e.g.) the radio antennas in your cell phone. Like Newtonian gravity, it's approximately right most of the time, but can't possibly be exactly right.

 

[bhobba]

Crudely, Quantum Mechanics is about treating classical particles as waves, and Quantum Field Theory is about treating classical waves as particles

I know you said crudely but I can't really agree with that even in a crude way. Its a difficult thing though because what people take away from something as its 'essence' varies widely.

My take would be the following. At rock bottom QM has nothing to do with waves - wave-functions yes - waves no (note waves and wave-functions are not the same thing eg waves are not complex valued). The real essence of QM is its the most reasonable generalised probability model that allows continuous transformations between so called pure states:
http://arxiv.org/pdf/quant-ph/0101012.pdf

Ordinary non relativistic QM is what you get when you assume position is an observable and the Galilean transformations - see chapter 3 Ballentine - QM - A Modern Development.

QFT starts from a field, breaks that into small blobs, describes those small blobs by ordinary QM, then let's the blob size go to zero to get a continuum. When you do that something interesting happens - mathematically its nearly the same as the harmonic oscillator from ordinary QM:
https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

It turns out the creation and annihilation operators is the exact formalism of particles.

Thanks
Bill

 

[vanhees71]

I don't think that "long wavelenght" is enough for appoximating EM field as a classical field: if the intensity is very low, you have one photon at a time even with very long wavelenght, or at least it should be so.

Well, it's a tautology, but you have one photon at a time if and only if the state of the electromagnetic field is prepared to be a one-photon state. Any other state this is not the case. A very dim laser light is still a coherent state, whose superposition in terms of occupation-number (Fock) states consists mostly of the vacuum, but also states of all photon numbers.

That's why bhobba is right in #40 in saying that it is always wrong to claim one can think of the electromagnetic waves as consisting of particles. That's even "wronger" than thinking of massive quanta as classical particles. In the latter case, there is a classical limit of the quantum description in terms of classical particles. For massless quanta with spin $\geq 1$ it's not even possible to define a position observable. So there is no classical limit in terms of classical particles at all.

 

[f95toli]

Yes - but at our current technology such is undetectable.

I think it depends on what you mean by "long wavelength". Several groups around the world are now routinely working with single photons (Fock states) with frequencies of a few GHz, meaning the wavelengths involved are several cm. There are several working designs for emitters but building "click" detectors for these frequencies is still challenging, but we are very nearly there (at the moment most people use linear detectors, meaning you need to average for quite a while if you want to measure correlations functions).

It is an interesting field to work in, on my desk I have books and papers about QM as well as books about microwave engineering and you end up having to shift your "perspective" depending on the problem you are working on

Hence, I agree that there is no such things as a "classical "limit. There is certainly no reason why you couldn't work with photons of frequencies of say a few hundred MHz, i.e. essentially FM radio.

 

[vanhees71]

Wow, that must be challenging. I guess it's mostly single-photon states in the microwave range trapped in a "cavity" (quantum dot). Or do you really mean free single-photon states in the cm-wavelength range? If so, how do you produce single-photon states with so long wavelengths in practice?

 

[f95toli]

Wow, that must be challenging. I guess it's mostly single-photon states in the microwave range trapped in a "cavity" (quantum dot). Or do you really mean free single-photon states in the cm-wavelength range? If so, how do you produce single-photon states with so long wavelengths in practice?

We mainly use superconducting qubits. They can be used both to create states in a cavity (which in our case is simply a microwave resonators) and to "launch" single photons into a microwave transmission line. Hence, most of the equipment we use is just standard microwave components (e.g. a 90 degree hybrid is the microwave analogue of a half-silvered mirror). meaning once the single photons leave our sample they travel in standard microwave coax, using normal SMA connectors etc.
Of course we have to keep our sample and most of the components cold (about 10mK) to avoid being swamped by thermal photons, but otherwise it is a pretty normal microwave setup.

 

[lightarrow]

How are single photons detected? Which kind of detector is it?

--
lightarrow

 

[H_A_Landman]

The Schroedinger Equation is a wave equation. That it contains a square root of -1 is weird, from the viewpoint of classical wave equations, but doesn't really change its nature. Wave functions come out of the SE as its solutions in particular contexts, so it is not entirely unreasonable to view them as derived and not primary. (QM is similar in some ways to Hamilton-Jacobi theory in classical mechanics, where particles are modeled as wavefronts, and the H-J action plays a similar role to QM's phase.) So anyway, QM uses a wave equation to describe the "motion" of classical "particles", which is what I meant by treating classical particles as waves.

I don't think we really disagree about what QFT is. The harmonic oscillator in standard QM describes a particle in a particular, simple potential. It has quantized energy levels that are equally spaced. You can go up or down a level using the raising or lowering operators. In QFT, we just model a field as a bunch of QHOs and interpret the energy levels as representing number of particles (occupation number), and thus the operators get renamed as creation and annihilation operators and are viewed as adding or subtracting a particle. (The mechanical analogy here is phonons.) In this sense, the field (which was classically described by wave equations) is modeled as a set of particles instead.

Getting back to the original question, if the field is electromagnetic, then the particles are photons, and at least in QED photons are as real as any other particle.

 

[vanhees71]

Photons are massless quanta with spin 1. It is dangerous to depict them as "particles". There is no classical particle limit for photons, at least not a very intuitive one, as far as I know.