QED questions

  • #1

Main Question or Discussion Point

I was curious about two things. I just started reading Feynman's QED book this week. He starts by using an analogy of a clock spinning and that determines the direction (which I think is being analogous to the amplitude of the frequency?)

My first question is, as a layperson, what causes the amplitude to change when light hits on a specific angle? Does this have to do with the light interacting with the particles of the surface material?

My second question is, if light in terms of QED is only explicable in terms of particles, can this also explain electrons only existing as particles and never as waves? I don't study QED at a university level, and only have a background in chemistry. In my classes that I took I was always taught about the particle/wave duality. QED changes this idea about light, does this also change that idea about the electrons?

-Rob
 

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  • #2
Simon Bridge
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I was curious about two things. I just started reading Feynman's QED book this week. He starts by using an analogy of a clock spinning and that determines the direction (which I think is being analogous to the amplitude of the frequency?)
Not exactly.
Notice that the "clock" is attached to a particle not a wave.
The length of the arrow is the "probability amplitude" - and this is usually a fixed value.
The area of the circle swept by the arrow is (proportional to) the probability of detecting the particle.

The "clock" is actually a "complex phasor" - so it involves math you may not have met yet: involving exponential numbers and the square-root of minus one. Feynman's description here is pure analogy and should not be taken too literally.

My first question is, as a layperson, what causes the amplitude to change when light hits on a specific angle? Does this have to do with the light interacting with the particles of the surface material?
Statistics - there is a probability that the photon will be transmitted and a probability it will be reflected. Ultimately, yes, we tend to think of it as being due to the interactions between the photons and the material. In a way it described the result of the interactions.

However you need to distinguish been the probability amplitudes Feynman is talking about and the amplitude of the "light wave" that you are used to. The "clocks" are not describing a light wave.

My second question is, if light in terms of QED is only explicable in terms of particles, can this also explain electrons only existing as particles and never as waves?
Electrons do display wave-like behaior though so it is not clear what you are thinking of here.

All particles exhibit wave-like behavior, all the time, following the rules of QED. Most of the time the wave behavior is too small to spot - but under special circumstances we can make it big enough and there it is!

It is easy to set up the special circumstances for light, in fact we do it by accident all the time, but it's not so easy for electrons.

QED provides a set of rules for working out how much of what sort of behavior to expect under different circumstances - but it does not tell you what is "really" going on.

I don't study QED at a university level, and only have a background in chemistry. In my classes that I took I was always taught about the particle/wave duality. QED changes this idea about light, does this also change that idea about the electrons?
It does - QED is a particle theory, so everything is particles of some kind that, due to their statistics, can sometimes exhibit phenomena that is well described using the maths associated with waves.

At HS level, one way of thinking about wave-particle duality is by analogy with the old story about the blind men and the elephant. You know the one: one guy gets the front, examines the trunk, and notices it is like a snake so he concludes that and elephant is a kind of snake; the other guy gets the back and examines the tail and notices it is like a rope and concludes that an elephant is a kind of rope. The two get together and compare notes, check each others results etc, and come to the conclusion they are both right and the elephant exhibits rope-snake duality.
 
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