The speed of light and causality

[Boltzmannbrain]

8 minutes ago, Genady said:

I watched the video. I did not see an explanation why it actually slows down. It just says it does. Why the sum of these two waves, let's call them primary and secondary, moves slower than the primary one?

Go to 7:10 in the video.  You simply add the amplitudes of the waves.  For example, you see when the crests and a troughs align; you get 0 amplitude, or just a flat line, and so on.  So it is just what happens when slow and fast waves combine mathematically.

[Genady]

1 minute ago, Boltzmannbrain said:

Go to 7:10 in the video.  You simply add the amplitudes of the waves.  For example, you see when the crests and a troughs align; you get 0 amplitude, or just a flat line, and so on.  So it is just what happens when slow and fast waves combine mathematically.

Right, it shows that the amplitude changes. It does not explain why the speed changes, though, does it?

Also, there are no slow and fast waves there, as both waves are electromagnetic and thus both move with the speed of light. Right?

[geordief]

10 minutes ago, Genady said:

Right, it shows that the amplitude changes. It does not explain why the speed changes, though, does it?

I was wondering whether the direction of the resultant wave  differs from the other 2 waves and if that might cause a slow down.

And what about the interactions  with  particles  .Do they slow down the wave ?

10 minutes ago, Genady said:

 

Also, there are no slow and fast waves there, as both waves are electromagnetic and thus both move with the speed of light. Right?

Is what I though,too but the waves from the electron are not moving in the same direction as the light wave ,are they?

Edited February 19, 2023 by geordief

[Boltzmannbrain]

4 minutes ago, Genady said:

Right, it shows that the amplitude changes. It does not explain why the speed changes, though, does it?

If you add both waves all throughout, you will see that the new wave's troughs and crests move slower.  It is just the outcome from adding the amplitudes of both waves.

Quote

Also, there are no slow and fast waves there, as both waves are electromagnetic and thus both move with the speed of light. Right?

Good question.  I am not sure how that works.  The only thing I can think of is that the wave he is referring to is the disturbance of the electric field that the electron makes as it accelerates in space.  In that case it would only travel as fast as the electron travels, speaking in classical terms.  But I really have no idea.

[Genady]

22 minutes ago, geordief said:

I was wondering whether the direction of the resultant wave  differs from the other 2 waves and if that might cause a slow down.

The direction of the resultant wave is different. Why the speed would be different, though?

 

22 minutes ago, geordief said:

And what about the interactions  with  particles  .Do they slow down the wave ?

I don't see how they could.

 

22 minutes ago, geordief said:

Is what I though,too but the waves from the electron are not moving in the same direction as the light wave ,are they?

They are not. They are moving in all directions, I think.

 

Another questions in relation to this. Let's assume for simplicity that the primary wave is monochromatic. Is the resultant wave monochromatic? If so, what is different about it, i.e., wavelength, frequency, or both?

[swansont]

1 hour ago, Boltzmannbrain said:

What do you mean?  The photons are the light.

Yes. And photons get absorbed and emitted, by virtual states (so there is no energy or momentum imparted) and this takes time. So the photons travel at c, but the light takes longer to traverse the medium - it slows down.

In the classical view, the permittivity of a medium is larger than the vacuum value; the EM field can’t oscillate as it does in free space - because it’s interacting with the electrons in its vicinity - so it slows down and has a shorter wavelength

[Genady]

36 minutes ago, Boltzmannbrain said:

If you add both waves all throughout, you will see that the new wave's troughs and crests move slower. 

I don't believe it. I'd love to see that math.

 

16 minutes ago, swansont said:

And photons get absorbed and emitted, by virtual states (so there is no energy or momentum imparted) and this takes time.

The Fermilab video linked above says that this explanation is wrong. Do you think they are wrong?

[MigL]

35 minutes ago, swansont said:

the EM field can’t oscillate as it does in free space - because it’s interacting with the electrons in its vicinity - so it slows down and has a shorter wavelength

This would be the effect Dr Lincoln is referring to in the video, better explained by our own Dr Swansont.

This is still a classical interpretation and  I had thought the photon 'takes time to be absorbed/emitted' with conservation of momentum considerations was a good QM interpretation.
I guess it's time to research another interpretation.

[swansont]

48 minutes ago, Genady said:

The Fermilab video linked above says that this explanation is wrong. Do you think they are wrong?

Where does he say that? (time stamp)?

 

My problem with what I saw was already raised elsewhere - he states that the wave from the electron motion travels at a different speed, but that’s just a circular explanation. If the E field oscillations travel at c, then why would the other oscillations travel slower? He just states this with no explanation. 

 

[Genady]

16 minutes ago, swansont said:

1 hour ago, Genady said:

The Fermilab video linked above says that this explanation is wrong. Do you think they are wrong?

Where does he say that? (time stamp)?

4:51

 

17 minutes ago, swansont said:

My problem with what I saw ... He just states this with no explanation.

This is my problem, too. I'm looking for a better explanation, if there is one.

[Boltzmannbrain]

3 hours ago, Genady said:

I don't believe it. I'd love to see that math.

You don't even need the math to know it slows down.  Just look at how the waves' amplitudes add and cancel each other out.  Start at 7:06.  

 

[swansont]

2 hours ago, Genady said:

4:51

He’s talking about resonant scattering - i.e. absorption with an allowed transition - which is not the same thing. He’s right that that explanation doesn’t work, for reasons he gives, but that’s not the QM explanation being offered. So one can make the case that he’s debunking a strawman.

Absorption by a virtual state doesn’t permit transfer of energy or momentum to the atom; the only possibility is for the photon to continue on along the same path. (Which, again, is not the case he discusses)

[Genady]

26 minutes ago, Boltzmannbrain said:

You don't even need the math to know it slows down.  Just look at how the waves' amplitudes add and cancel each other out.  Start at 7:06.  

 

But he is talking there about two waves moving with different speeds. It is not the case here. Here we have two electromagnetic waves, and both have the same speed in vacuum. How do they add up to a slower wave?

[Boltzmannbrain]

3 minutes ago, Genady said:

But he is talking there about two waves moving with different speeds. It is not the case here. Here we have two electromagnetic waves, and both have the same speed in vacuum. How do they add up to a slower wave?

I thought he was indirectly saying that the wave from the electron is slower.  But then of course the question is how that EM wave got slower.  I have no idea.  I will try to find out though. 

[Genady]

28 minutes ago, Boltzmannbrain said:

I thought he was indirectly saying that the wave from the electron is slower.  But then of course the question is how that EM wave got slower.  I have no idea.  I will try to find out though. 

I have an idea how interference of two waves moving with the speed c can result in a wave moving with a speed v < c, in principle.

The two waves don't travel on the same line, like in the video, but at an angle, like here:

The waves from A and from B interfere in such a way that the resulting wave appears in C. Each one moves with the speed c, but the resulting wave propagates with a slower speed v.

[Boltzmannbrain]

2 minutes ago, Genady said:

I have an idea how interference of two waves moving with the speed c can result in a wave moving with a speed v < c, in principle.

The two waves don't travel on the same line, like in the video, but at an angle, like here:

The waves from A and from B interfere in such a way that the resulting wave appears in C. Each one moves with the speed c, but the resulting wave propagates with a slower speed v.

Good idea!  Maybe this is the answer.