# Conceptual help: matter waves and light waves

1. Nov 13, 2012

### channel1

Ok so I'm coming to terms with the following:

1 - Matter is not a wave, nor does it propagate as a wave. There is no physical wave, amplitude, etc. The probability of the position of matter varies in a wave-like pattern. It is often called a wave because physicists have no explanation for why there is a probability in the position, so they just describe the whole thing as a wave.

2 - The same goes for light.

Is this correct, and can anyone elaborate a bit on these concepts? I was utterly confused in class (my community college taught us that matter and light are waves, the book at my university says the same but our professor and TA clarified otherwise and I'm just mind-bombed because these are concepts that I struggled with at my community college)

2. Nov 14, 2012

### Simon Bridge

I believe I have just the man for the job: Richard Feynman (vids) on QED.
... watch all of them.

It is possible to formulate the physics in terms of waves or particles (which each have properties not present in the classical namesake). Feynman prefers the particle description because that is what you detect ... all the energy arrives in a lump, and, sometimes, the lump arrives as soon as the equipment is switched on (i.e. too soon for a wave to have built up enough energy.)

3. Nov 14, 2012

### PeterO

One could argue that until a microscope with sufficient resolution to see a clear picture of the nucleus of an atom is developed, we will not know exactly what it is.
We do, however, know a great deal about how atoms, and subatomic particles for that matter, behave in given circumstances.
That same can be said of light - we know a lot about how it behaves, but cannot be certain of what it is composed of - if light is composed of anything tangible.

A great debate developed over whether light was made up of particles, or was a wave.

The wave model was doing very well, especially when it came to diffraction and interference, but fell short when the photoelectric effect was observed and later explained.

We still use a particle model to explain/predict some behaviour, and the wave model for other behaviour.

During the development of the models for light - there appeared a momentum property of the light - with the momentum proportional to the wavelength of the light [assuming it was a wave].
We already knew the momentum of a particle is given by mv [mass times velocity] which offered the possibility of an object with momentum, being associated with a particular wavelength. The mass of your average atom also meant that the "wavelength" would be extremely small. However, "reasonable" wavelengths were associated with tiny objects like electrons.

When beams of electrons were passed through crystals, the scatter pattern which occurred exactly matched the pattern when appropriate "light beams" were passed through the same crystal [the frequency of the "light" puts it that part of the spectrum we call x-rays rather than the visible-light part of the spectrum"]

The only way to explain/predict the possible direction an emerging electron could take, was to do an analysis of an equivalent wave.

So, the wave model for matter is not suggesting that moving matter is bouncing up and down like the surface of the ocean - but only that its possible paths after being scattered can be accurately predicted if we use a wave model, using the appropriate wavelength for the momentum of the particle.
We are not saying [I am not sure we even know] why the particles only head off in certain directions, but at least we are able to predict the directions very accurately.

I hope that helps.

4. Nov 14, 2012

### channel1

Once again, my favorite physicist to the rescue... Thanks I haven't seen his QED vids yet

5. Nov 14, 2012

### Simon Bridge

I don't think the standard model is thought of as either a particle or a wave theory... but the firld of study is called "particle physics".
Until QED. The path the electron took could not be determined at all - just where it ended up. The classical assumption was for a direct path from one or other slit to the screen/detector.
You seen those lectures (in the link)?

BTW: you get the interference pattern from matter because the schrodinger equation is a special case of the helmholtz equation. That is also why the matter-wave analogy works so well.

http://arxiv.org/pdf/quant-ph/0703126.pdf
... there are a number of issues with the treatment in this paper - like: author neglects to state the approximations being used, fails to resolve the measurement issues, and the system is described using a hidden-variables approach. However, the interference is derived from Dirac formalism without reference to matter waves - which is the key point here.