Wave -Particle dualtiy, the most convincing evidence?

lntz

hello,

i am writing a report on wave particle duality and i have included the experiments you would expect. young's inteference patterns as evidence for light as a wave, and the photo electric effect providing evidence for light as a particle. etc.

what i am concerned about is does the photoelectric effect explicitly show light as a particle?

i understand that all the energy required for a photon to be ejected must arrive all at once in a 'discrete' (i use this term because i believe it is the correct way to describe this, but i could use some clarification as to what it actually means) packet. this does seem to show that light is a particle.

also increasing the intensity of the light does not increse the kinetic energy of the emmitted electrons, but increasing the frequency of the light does(which is a wave idea of light). i know that the conclusion of my report will be that light is a quantum particle that exhibits both wave and particle like behaviour, but is the photo electric effect evidence for light as a particle, or for light as a particle and a wave?

that was rather long winded, so i have just one more question. if the photoelectric effect is used to argue that light exhibits both properties, what is a convincing piece of evidence for light just as a particle? i'm sure there must be some, because many great scientists were convinced it was for a long time!

thanks for any help you can give.
lntz.

sophiecentaur

Personally, I would say that it is not evidence of anything other than the fact that the Energy is Quantised. and what you say is quite reasonable.

One other piece of 'evidence' that is quoted to show that photons are particles is the fact that they exhibit Momentum (light pressure etc.). But there is a perfectly good Classical explanation of what happens when an EM wave hits a surface (reflecting or absorbing). This also predicts exactly the same forces so you can take your pick as to which explanation you choose (if you have to choose).

It may not be too fruitful to fret over What's really what because they're all models which sometimes apply and sometimes aren't so good. I always say that it's over optimistic to think in terms of 'understanding' anything fully. It would be a good idea to use lots of 'qualifying' words in your report, rather than being too dogmatic. (In years to come, it come back and bite you if you're not careful) "OMG how could I have written that!!!?"

sambristol

your understanding of the photoelectric effect is excellent - in particular the fact that intensity of light does not affect the energy of the emitted electrons but also recall there is a minimum wavelength to eject electrons at all.

My personal view is the best evidence of the 'particle' nature of light is the Compton effect where a photon 'hits' an electron and scatters off at an angle (with lower energy ie longer wavelength) and the electron recoils gaining the energy lost by the photon. The point is that if you treat the photon as well as the electron as a particle and consider the collision just like two billiard balls you get a equation which exactly describes (ie energy and angles) what is experientially observed.

Another experiment (which I well recall doing as an undergraduate) is to shine a very weak beam of light through a double slit (Youngs experiment) and detect the light with a photomultiplier (uses the photoelectric effect to produce a 'click' when a photon hits it). If you make the light intensity so low that only one photon can be in the apparatus at any one time a very strange thing happens - add up all the 'clicks' (ie particles of light) you still get an interference pattern which is a wave property.

A good basic level explication of this is given in Feynmans book 'QED the strange theory of light and matter'

Hope this helps
Regards Sam
PS you mention 'discrete' it just means not spread out in space and / or time

sophiecentaur

Not sure about that. Afaik, the word 'discrete' means 'a particular (energy) value'. There is no reference to the 'extent' of a photon. That is an entirely different topic for discussion and there's a lot of juice in that lemon!.

sambristol

recall Heisenberg ΔE.Δt≥h and Δp.Δx≥h we are giving the same answer in two different viewpoints

Sam

sophiecentaur

I recall Heisenberg but how does that apply to the term 'discrete'? It refers to uncertainty.

sambristol

Sorry INTZ this has only at best tangential importance to your question

Sophiecentaur,

Discrete is only a model. There are no such things as discrete energy levels or positions or momenta but they are useful (Think of Fermi's model of β decay as a point interaction ie discrete interaction before the W and Z where discovered)

Hence my reference to Heisenberg

Discrete is also used in classical mechanics - I mentioned billiard balls in my original response it is modelled as a discrete interaction ie at a precise point in space and taking zero time but I hear a sound when two billiard balls collide so 'discrete' is not possible but a useful model

Regards

Sam

Cthugha

Photon antibunching is typically considered the most convincing evidence for the particle nature of light. That effect is pretty simple. If you have a single photon source and direct its emission via a beam splitter toward two detectors, they will never "click" simultaneously. The absence of these simultaneous clicks is antibunching (first demonstrated in resonance fluorescence by Kimble, Dagenais and Mandel).

sophiecentaur

I don't see where you get that conclusion from. You have two photons which interact with the two detectors. Are you saying that there is some sort of 'exclusion' mechanism at work? Of course, only one detector can register any one photon so there must be two for two detectors to react. These two detectors can't be in the same place so how can there be any reason to associate what happens at one with what happens with the other? You are assuming that these are coherent photons?
The abstract of the paper you quote mentions evidence of quantisation and not of particles. I am not a subscriber so I couldn't read the full article.

Cthugha

To be honest I do not understand your answer.

Where do you get two photons from? For a single photon source you need one photon at a time.

Of course? In a purely wave model there will always be the possibility of both detectors firing, even at low intensities.

Of course they are not at the same position. They are at two different output ports of a beam splitter. If they were coherent, both detectors could fire. That kind of experiment is explicitly performed to distinguish between coherent light fields and other light fields (thermal or in this case single-photon Fock states).

Maybe the Rev. Mod. Phys. overview article from Paul written in 1982 is a more comprehensive overview (Rev. Mod. Phys. 54, 1061–1102 (1982)), although I do not know whether it is available for free. At least it gets to the point directly in the abstract, saying "A review is given of recent theoretical studies devoted to the problem of generating radiation fields that exhibit the opposite of the well-known bunching of photons observed in light from thermal sources, the so-called antibunching effect. It is made clear that this phenomenon reflects the corpuscular nature of light and, hence, cannot be interpreted in terms of classical electrodynamics, needing, instead, the quantum-mechanical formalism for its description."

I am still pretty puzzled where your problem is. This is THE standard experiment performed to identify and demonstrate single-photon sources which is routinely performed in labs all over the world and has been performed and published more than 1000 times.

sophiecentaur

I am drawing the distinction between 'Particles' and 'Quanta'. That paper certainly points out that waves are not enough to explain certain phenomena but it doesn't imply anything about 'little bullets'. That is my 'problem'. I have never seen anything to convince me that photons are enough 'particle-like' to justify the term. They are 'special' enough to justify a separate term, surely. My theory is that people want a nice cuddly term for them and they hang onto it because of that.

Cthugha

But 'particle' is not meant to imply 'little bullets'. 'Particle' just means that the energy exchange between the electromagnetic field and something else happens in a quantized manner, not that a photon should be interpreted as a point-like particle

jtbell

In hindsight, one can argue plausibly that it was not a good idea to carry over the word "particle" from the classical world to the quantum one, because it carries along too much ontological baggage that leads to misconceptions about how photons, electrons, etc. behave and what they "really are."

But that ship sailed many decades ago, and it's probably impossible to call it back to port now.

sophiecentaur

Exactly. Doesn't that highly important idea warrant being given its own name? It has confused generations of people into a far too mechanical view of the things. You are in a small minority of people who don't actually think in terms of 'little bullets'.

Cthugha

I fully agree that the terminology can be confusing for laymen. However, technology terms often use words known from everyday usage and places them in a different context. I suppose the problem is that the terminology has grown historically and it might be pretty complicated to change it. I suppose you would need an authority in the field and many agreeing followers to change the terminology used. While it would indeed be a good thing from a pedagogical point of view to change the wording, I am afraid the probability of that really happening is rather small.

sophiecentaur

A lot of stuff on PF just happens to be at that level where these terms cause most aggro.

Karbort

Hey, long time reader, first time poster here.

If it were not a particle (i.e. non-quantized energy exchange), how would the energy exchange occur? Does the difference lie in the energy transfer not being quantized (e.g. any amount of energy can be transferred, without being limited to only changes between quantized states) or does it lie in the transfer not occurring "instantly"?

Thanks!