# Wave-Particle Duality (QED)

1. Jun 30, 2015

### KiNGGeexD

I have been reading Richard Feynmans Quantum Electrodyamics and quite early in the first chapter he asserts that Photons are particles. His reasoning that as you decrease the intensity of light incident on a photomultiplier the clicks which the multiplier make become less frequent but equally loud. He doesn't go into much more depth on this issue and assumes particles in there "corpuscular" interpretation. However I always imagined light as quanta. Packets of photons, so is it not the case that decreasing intensity just decreases the number of quanta?

Feynman states there are properties which support his assertion and I was hoping Physics Forums could shed some light on them, excuse the pun

2. Jun 30, 2015

### HomogenousCow

Wave particle duality is an outdated concept, in qft everything is fields. Once you quantize those fields, there are some states with definite momentum which can be interpereted as particles.
People should really stop teaching wave particle duality.

3. Jun 30, 2015

### KiNGGeexD

I'm just going into my honours year so wave-particle duality was a concert introduced! Any helpful resources?

4. Jun 30, 2015

### scarletpete

I am trying to understand the exact problem of particle wave duality. Perhaps somebody can correct me but it seems to me an electron is a negatively charged particle, i dont think most people would disagree. However, it has wave associations only when a voltage is applied to create a flow of electrons. Only then has it an electro magnetic field surrounding it in concentric circles, which rotate around an electric cable or beam of electrons in a direction as described by Maxwell's corkscrew rule. If the carrier frequency of electrons is modulated as in Radio, the radiating electric field causes the associated electromagnetic field, which is at right angles to the electric field, to reflect the electric field wave pattern. This electromagnetic field travels infinite distances the same as gravity and these waves react easily with other electromagnetic or magnetic influences.
Recapping: an electron has no wave properties until a voltage is applied, but then it has.

5. Jun 30, 2015

### Staff: Mentor

6. Jun 30, 2015

### WannabeNewton

If I had a dollar for every time someone outside of PF preached this as dogmatically, I would have zero dollars.

7. Jun 30, 2015

### atyy

You will find the concept coherently formalized when you study quantum theory properly. The quantum particle is a fundamental concept in quantum mechanics. Unlike a classical particle, a quantum particle does not have simultaneously well-defined position and momentum. However, it is like a classical particle in that it can be counted using the integers. However, there is a wave equation that governs the probabilities of experiments performed on the particle(s).

8. Jul 1, 2015

### KiNGGeexD

Yea I assumed anything quantum would obey the Uncertainty principle, it all seems rather vague at times. I suppose no one really knows

9. Jul 1, 2015

### scarletpete

I must admit I know nothing of quantum theory, but I do know how Televisions And Radios are designed and work. On tried and trusted physics formulas from people like: Faraday, Ohm, Watt, Maxwell, Lenz, and many more. I will however study more about quantum mechanic before I criticise it.

10. Jul 1, 2015

### phinds

No one really knows what? That quantum objects are just that and not classical particles or waves?

11. Jul 1, 2015

### Feeble Wonk

Good luck King... If you are just being introduced to these ideas, and aren't going to be using them professionally, I have a piece of advice (as a non-physicist)... Learn the formulas, run the numbers and call it a day. OR, prepare to have your world view significantly altered for the rest of your life.

12. Jul 1, 2015

### atyy

There is no problem of "particle-wave duality". There is only a problem of "wave-particle duality". Kidding!

There is no problem of either. "Wave-particle duality" is a loose term from early days when quantum phenomena were first being seen to describe that the phenomena resembled how classical particles and classical waves behaved. In those days, quantum theory was only known as incoherent pieces of the puzzle. Nowadays, we do have a quantum theory in which the pieces are coherently assembled. Some people don't like the term "wave-particle" duality because it comes from a time when people did not fully understand quantum theory. Others, like me, think the term is fine, since the early apparently disparate pieces have now been properly synthesized. Regardless, we all agree that we now have a quantum theory that makes complete sense.

So there are no classical particles. There are only quantum particles, which differ from classical particles in that they do not have simultaneously well-defined positions and momenta. There are also no classical waves. Quantum particles are described by equations that have wave-like properties and particle-like properties, and yet are different from classical particles and classical waves.

Last edited: Jul 1, 2015
13. Jul 1, 2015

### KiNGGeexD

No one really know what electrons are, they have never been observed, along with a lot of constituent parts of matter (strings) it was a general comment which I regret ;)

14. Jul 1, 2015

### KiNGGeexD

As far as quantum objects I suppose it's hard to say what they really are except from mathematical abstractions

15. Jul 1, 2015

### KiNGGeexD

That is all fine, I'm aware I have only scratched the surface of the quantum world. However we do not have particle equations? We have wave equations? Or is this a matter of mere naming

16. Jul 1, 2015

### phinds

I don't know what a "particle equation" would be in this case. The wave equation gives the probability of finding the a particle if you measure for a particle.

17. Jul 1, 2015

### Strilanc

Come now, I count at least two dollars.

18. Jul 1, 2015

### KiNGGeexD

But as far as the properties of said particle which we have a probability for?

19. Jul 1, 2015

### atyy

It is mostly a matter of naming. One could say we have wave equations for particles. It is best to learn the formalism properly, and just get used to the fact that different people use different naming conventions.

20. Jul 2, 2015

### KiNGGeexD

Because if we call it a particle we collapse the wave function to a single point? So it cannot have a probability to be "everywhere" like quantum theory boasts

21. Jul 2, 2015

### HomogenousCow

When you measure the position of a particle it collapses to a single point. Prior to said measurement (the standard interpretation of QM states that) the particle exists in a "spread out" state over an area.

22. Jul 2, 2015

### Lews

First - I'm just a guy (geophysics undergrad - many years ago), so I'm just giving my ideas - not necessarily correct or complete.

I'm struggling with this as well.- along with fields. Here's what I see so far. Feyman says a photon is a particle in QED - forget waves. Sean Carroll says forget particles and focus on fields instead. A particle is just the result of a collapse of the uncertainty of position and momentum. But he's really speaking of probability fields - with the probability defined but QED theory (probability amplitudes based on time oscillations). If we think of each location in space of a photon at a particular time, it has a certain probability to be in another location in space at another time - each of these other locations has a probability of being there depending on the distance. If you think about it, areas of equal distance from the starting point creates a sphere. So the probability fields are infinite numbers of spheres, representing the same distance from the starting point. What seems to matter is time not distance, but since the photon is traveling at constant speed (c), if the medium traveling through is uniform, then location in space means the same amount of time has elapsed. Each of these probability spheres is really what a field is defined as. An aside - recognizing it as fields helps conceptually and mathmatically, but I don't think it necessary helps with the concept that Newton struggled with - action at a distance.

But there's also another wave concept besides wave-particular duality - probability waves - that makes no sense to me if my definition of a wave is correct - interaction with adjacent particles/positions.

Here's what I mean. What's the difference between a wave and a field? I think the answer is that a wave interacts with its neighbors. Like dominoes or water. That's the thing that Sean Carroll and others leave out in layman talks they give. Waves "propagating" through the field. Amber waves of grain move in the same direction from a force (the wind) but they do not push on each other - well maybe a little but it's primarily the wind. Feyman says photons and electrons are not acting as a wave to explain QED. He says it's been tested - I believe him but if so that means that each photon is not interacting with it's neighbor. Predicting whether a photon from one source location will arrive in another location is independent of other photons. Each photon carries an internal mechanism - a probability mechanism that oscillates from zero to a max probability - who knows how this works. It's independent so it can't be a wave. It's a disturbance in space-time field that can be thought of as a particle. And it becomes that particle only when we bother to measure it; otherwise it is just a probable disturbance in a particular location in the space-time field - with the probability changing in spheres of equal probability throughout the universe.

What's the conclusion of my rambling? Modern physics says that waves are out. Even the concept of probability waves is misleading. Unless and until we discover some interaction between particles. But I think physic still think of light "propagating" through space, when QED says it's more about probabilities. I don't think you can think propagating without waves - so the real duality is you use whichever concept works best as an approximation given the problem you're trying to solve, but the best understanding of the universe is not-wavelike, but a weird probability situation. There are instances where you can use probability computation to solve the problem but other times wave works as well and might be easier mathmatically.

To all of you that know more than I do (a huge number) - please remember to use terms like "misled" "lacks complete understanding", etc., as opposed to "moron" "feather brain" etc. Partly I'm writing this to get it clear in my own head - and things are very fuzzy there most of the time.

Lews

23. Jul 2, 2015

### KiNGGeexD

I'm comfortable with all of that. I know about wave equations and particles however I just wasn't sure about Feynman's logic

Last edited: Jul 2, 2015
24. Jul 2, 2015

### KiNGGeexD

Or rather his assertion that photons are particle-like

25. Jul 3, 2015

### atyy

No, a quantum particle is "everywhere" or "potentially anywhere". If we measure its position we collapse the wave function to a single point.

Another way to see the wavelike nature of a quantum particle is to use Feynman's picture that a quantum particle takes all possible paths, whereas a classical particle takes only one path. The effects of quantum mechanics require Planck's constant for their description. When we make Planck's constant smaller and smaller in the quantum formalism, the quantum formalism becomes closer and closer to the classical formalism in which the particle takes only one path.