What pushes the EM waves in forward direction?

[sophiecentaur]

I think this site might be really useful in understanding how waves propagate:

https://phet.colorado.edu/sims/html/wave-on-a-string/latest/wave-on-a-string_en.html

The speed of the wave is totally independent of the amplitude or shape of the wave. When you change the "tension" on that applet, you change the speed of the wave.

At last someone has brought this thread to a sensible level again. We don't need the big guns to explain quite satisfactorily how waves propagate. Huygens (look him up) proposed a process which happened to apply to classical light but applies just as well to any wave. He envisaged anyone point on a broad wavefront (i.e. a plane wave traveling in just one direction) radiating energy in all directions (so-called secondary wavelets). These wavelets will add to (/ interfere with ) each other and it so happens that, in any other direction than forward, they cancel and they only enhance in that direction. The energy has only one way to go. If you take a simple wave propagating on a string and consider the phases and amplitudes along one section, starting at a few points along the string, you will find that the phases of the 'backward' secondary waves will be such as to produce a zero phasor sum backwards and add together in the forward direction.

Forget about em not needing a medium / photons / QM and all the rest. Just sort out the fundamentals of classical waves first.

 

[Neandethal00]

Sorry, but this is simply just wrong. By definition, something moving at constant velocity has zero acceleration

Boy, just a wrong word can make lots of heads spin. My sentence should have been
Something cannot "achieve" a constant velocity without an acceleration
Which is universally true. An acceleration precedes a constant velocity. If you say photons do not work that way, then we know little about photons,
or velocity of photons is not really its velocity. Can velocity be instantaneous without acceleration?

 

[Orodruin]

Boy, just a wrong word can make lots of heads spin. My sentence should have been
Something cannot "achieve" a constant velocity without an acceleration
Which is universally true. An acceleration precedes a constant velocity. If you say photons do not work that way, then we know little about photons,
or velocity of photons is not really its velocity. Can velocity be instantaneous without acceleration?

We know a lot about photons. One of the things we know is that they are a pure quantum field theoretical concept and you really should not expect them to behave as small classical balls. This also goes for electrons and other particles on small enough scales.

 

[EM_Guy]

Neandethal,
Since you are getting hung up on acceleration, it is worth pointing out that energy is discretized. I assume you are familiar with the concept of kinetic energy and its relationship to velocity. And you are obviously familiar with the classical ideas regarding the relationship between acceleration and velocity. So, this may make your head spin - (and you shouldn't just accept this - you should study and be convinced yourself of the veracity of this claim). But one of the consequences of Max Planck's work is that energy of objects is discretized. Meaning that an object can only possesses an energy that is an integer multiple of hf. Meaning that energy is not continuous. So, for example, when you push your little sister on the swing, you may have thought that her velocity could be plotted as a continuous function. At her apex, her velocity is zero. And at some point she reaches a maximum velocity, before she beings decelerating. And so, if she goes from 0 to vf back to 0, you might have thought that she was experiencing every single value of velocity that there is between 0 and vf. But in fact, she did not. She only experienced integer multiples of hf. Now, h is really, really small, so you never noticed that your sister was "skipping" a whole bunch of values of velocity as she accelerated, but she did. When you start looking at really, really small particles, and/or really, really high frequencies, hf becomes significant. And then we have to deal with the reality of quantum physics - as strange as it may seem to us.

 

[Drakkith]

EM Guy, I don't think kinetic energy is quantized.

 

[yoron]

That statement is confusing Drakkith. Had to look it up :) Would it be a oversimplification to say that in interactions (as a collision) the energy released is discrete, whereas a particle in free motion has a continuous energy?

 

[EM_Guy]

EM Guy, I don't think kinetic energy is quantized.

Why not?

 

[EM_Guy]

EM Guy, I don't think kinetic energy is quantized.

I never thought about kinetic energy being quantized until I saw these videos:

 

[Drakkith]

That statement is confusing Drakkith. Had to look it up :) Would it be a oversimplification to say that in interactions (as a collision) the energy released is discrete, whereas a particle in free motion has a continuous energy?

I don't know, honestly.

Why not?

Because as far as I know, you can increase or decrease the speed (and thus the kinetic energy) of an object by any amount.

 

[EM_Guy]

Because as far as I know, you can increase or decrease the speed (and thus the kinetic energy) of an object by any amount.

I'm just an electrical engineer. My training is 100% classical. But from my limited understanding of quantum physics, everything is quantized. Light is quantized. Electrons are quantized. It seems that everything is quantized. Or to put it another way, light has wave-like attributes. Electrons have wave-like attributes. Everything has wave-like attributes. It seems that energy is only available in discrete packets. Don't all the Rules of Quantum Mechanics apply to the macroscopic world?

 

[Drakkith]

But from my limited understanding of quantum physics, everything is quantized.

I'm pretty sure that's not true, but I don't know enough about quantum physics to explain why.
What say you, @Orodruin ?

 

[Orodruin]

I'm pretty sure that's not true, but I don't know enough about quantum physics to explain why.
What say you, @Orodruin ?

Very much so. Free particles display a continuous energy spectrum. What is quantised is the number of particles in each energy mode, the mode of the photon field having a frequency $\nu$ can only contain an energy which is a multiple of $h\nu$.

 

[EM_Guy]

Free particles display a continuous energy spectrum.

What are "free particles?" Are electrons "free particles"? If a free particle is a whole bunch of molecules (consisting of atoms - consisting of subatomic particles), and if electrons have a dual wave-like / particle-like nature (and presumably other sub-atomic particles also have a dual wave-like / particle-like nature), wouldn't the energy of the "free particles" just be some linear combination of the quantized sub-atomic energies?

What is quantised is the number of particles in each energy mode

By "mode" do you mean "field configuration"? I do not understand what you are saying here.

the mode of the photon field having a frequency ν \nu can only contain an energy which is a multiple of hν.

I'm not sure what a "photon field" is. Is that the same thing as a group of photons all sharing the same frequency? Are there photon fields associated with electrons or groups of electrons? Electrons can only have an energy that is a multiple of hν, right?

 

[blue_leaf77]

By "mode" do you mean "field configuration"?

I believe mode here is meant as frequency. Btw sorry dude, sometimes certain same term is used differently in different disciplines. Single mode -> single frequency.

What are "free particles?" Are electrons "free particles"? If a free particle is a whole bunch of molecules (consisting of atoms - consisting of subatomic particles), and if electrons have a dual wave-like / particle-like nature (and presumably other sub-atomic particles also have a dual wave-like / particle-like nature), wouldn't the energy of the "free particles" just be some linear combination of the quantized sub-atomic energies?

By definition, free particles are particles (not atoms or molecules) that roam a space influenced by no potentials. Sometimes, this definition is generalized a bit more to include particles of positive energy, not necessarily flying in a region of no potential. Actually not everything in QM is quantized, particles having positive energy, for example free particles, have continuous spectrum. You might have heard that the energy spectrum of hydrogen atom is discrete, but that's only the energies associated to the bound states, i.e. states having negative energy - the Schrödinger equation for hydrogen atom also supports positive energy states whose energy spectrum is continuous.

 

[Noctisdark]

As Orodruin said, something in motion will stay in motion, but if you view it in another frame of reference, you might see it in rest. but the stunning thing about EM waves is that you can never see them in rest, they, in any frame,they move at c .
So what really pushes EM waves ? Let's think of them as a wave, EM waves are a moving disturbance is the EM field, suppose you put and proton and electron in the vacuum in such a way that an electron can orbit they proton, once the proton have moved, the electron could go out of the orbit, when the proton moved the electric field have changed, and the change reaches the electron at the speed of light so the EM field can't change "fast" and the change is the EM wave, so it's nothing like a water wave, it's a moving disturbance !
Let's now think of them as particles, when the proton moves,when it drops from it's excited state, it has lost some energy, where should that go ? it must go somewhere right ? by intuition you can think that it goes to another object, and the electron is a good candidate because it's the only particle that exist in our scenario, and special relativity restricts that that energy reaches the electron at an infinite speed, so it goes there by the speed of light...
They seem to propagate and move at a certain speed because special relativity limits them, but actually it's an exchange of energy/disturbance of the EM field and you can't say that something pushes them because they aren't something to be pushed .
One last thing, the photons propagates as the EM wave, it only decides to be a photon (particle) when it hits the electron, the photon is a bosons, it a particle the mediates the EM field changes at the speed of light, I may have confused you when I said particle in case you haven't studied QM or read the theory, but EM waves exhibit particle-like propetries when being measured .
Good luck, and I hope I've made it a little bit clearer !

 

[blue_leaf77]

EM waves exhibit particle-like propetries when being measured .

When being measured? You should have also specified which measurement and/or in which conditions the particle nature of light can reveal itself, we all know that not all measurements may promote the particle nature over the wave counterpart.

 

[yoron]

As I get it EM_guy, it's the mathematics that defines it. And to argue with those craves a dedicated mathematically minded person willing to get headaches, possibly even enjoying them :) When I looked it up I found those. https://www.physicsforums.com/threads/energy-of-free-particle-not-quantized.597934/ as well as http://physics.stackexchange.com/questions/38433/energy-is-quantized . To me it seems they agree with Drakkith, but one would like a easier to digest definition too. But also, as physics are taking us further and further away from a 'linear Newtonian' world into non linearity and probabilities, maybe those are the simple explanations :)