I Charged particles and electromagnetic waves in transparent media

[member 743765]

Since charged particles like electrons are the source of electromagnetic waves how could an electron in water for example moves faster than the speed of light inside water c/n I mean how could the source moves faster than the field it creates because then the fields velocity will become larger and hence not a constant

 

[PeterDonis]

charged particles like electrons are the source of electromagnetic waves

Yes, but that doesn't mean they move faster than the waves do. They don't.

could the source moves faster than the field it creates because then the fields velocity will become larger and hence not a constant

I'm not sure where you're getting this from. What background do you have in electrodynamics?

 

[member 743765]

I'm not sure where you're getting this from. What background do you have in electrodynamics?

I believe that a source of any wave cannot moves faster that the wave it creates logically because the wave will become faster so it's speed is no longer a constant in that medium and I'm talking here about a continuous wave

 

[renormalize]

I believe that a source of any wave cannot moves faster that the wave it creates logically because the wave will become faster so it's speed is no longer a constant in that medium and I'm talking here about a continuous wave

Why do you think that's logical? Consider a supersonic aircraft traveling in the medium of air: when it breaks the sound barrier isn't it traveling faster than the sound waves in the atmosphere created by its engines?

 

[member 743765]

Why do you think that's logical? Consider a supersonic aircraft traveling in the medium of air: when it breaks the sound barrier isn't it traveling faster than the sound waves in the atmosphere created by its engines?

Yes but remember that the. Resulted shockwave is faster than sound and this is logical because if the source moves faster than the wave it emits it over run the wave fronts but it continuously radiate so the speed of the wave becomes faster and hence it's no longer a characteristic of the medium

 

[jbriggs444]

so the speed of the wave becomes faster

The speed of the wave is basically unchanged. The speed with which the sides of the shock "vee" advance is the speed of sound. The speed with which the point of the shock "vee" advances is the speed of the aircraft. But that is not the speed of the wave. That is the speed of something moving mostly crosswise with respect to the wave.

You might also check out Cherenkov radiation.

 

[renormalize]

Yes but remember that the. Resulted shockwave is faster than sound and this is logical because if the source moves faster than the wave it emits it over run the wave fronts but it continuously radiate so the speed of the wave becomes faster and hence it's no longer a characteristic of the medium

What do you mean? If I'm hovering motionless at some distance directly behind a supersonic aircraft, the engine sound I hear is due to ordinary soundwaves propagating at the speed-of-sound in air. Do you believe otherwise?

 

[member 743765]

What do you mean? If I'm hovering motionless at some distance directly behind a supersonic aircraft, the engine sound I hear is due to ordinary soundwaves propagating at the speed-of-sound in air. Do you believe otherwise?

Why did u choose behind ? Also shouldn't the medium characteristic changes near the aircraft at supersonic speed?

 

[PeterDonis]

I believe that a source of any wave cannot moves faster that the wave it creates logically because the wave will become faster so it's speed is no longer a constant in that medium and I'm talking here about a continuous wave

This is simply wrong.

It looks to me like you need to take the time to learn electrodynamics.

 

[Dale]

how could an electron in water for example moves faster than the speed of light inside water c/n

Usually they enter the water already traveling faster than c/n.

I believe that a source of any wave cannot moves faster that the wave it creates logically

Have you never heard of a supersonic aircraft? There is certainly nothing logically problematic with the source of a wave traveling faster than the wave. Ordinary boats do it all the time too.

 

[member 743765]

Usually they enter the water already traveling faster than c/n.

Have you never heard of a supersonic aircraft? There is certainly nothing logically problematic with the source of a wave traveling faster than the wave. Ordinary boats do it all the time too.

I'm asking how the speed of wave is constant (depends on the medium characteristics ) and at the same time it's source can move faster

 

[Dale]

I'm asking how the speed of wave is constant (depends on the medium characteristics ) and at the same time it's source can move faster

The source can move faster or slower. It doesn’t matter. A wave is not a projectile. Its speed doesn’t come from a push given from its source. The speed comes from the rate that changes in the medium propagate. Except in the near field the source is largely irrelevant

 

[member 743765]

The source can move faster or slower. It doesn’t matter. A wave is not a projectile. Its speed doesn’t come from a push given from its source. The speed comes from the rate that changes in the medium propagate. Except in the near field the source is largely irrelevant

When u said that waves are not projectiles I had a question .
Imagine that Im on a car that moves at constant v and have a radioactive substance that emits a photon vertically shouldn't the photon have a horizontal component relative to earth to conserve momentum since it's a mass content that removed from the car? So is that completely true even if we talk about photons?

 

[Ibix]

Imagine that Im on a car that moves at constant v and have a radioactive substance that emits a photon vertically shouldn't the photon have a horizontal component relative to earth to conserve momentum since it's a mass content that removed from the car?

Presuming you mean the source emits vertically in the car's rest frame and are asking if the light will have a horizontal component in the Earth's rest frame, yes that is correct.

So is that completely true even if we talk about photons?

Yes. Photons are definitely not projectiles.

Note that there are significant differences between light in vacuum and light in a medium. The wave interacts with the medium and its speed is fixed relative to that. So with something like a supersonic aircraft thereis no wave propagation directly forwards because there's no way for that to work, but wave propagation to the sides and backwards works as normal. It's similar with light from sources travelling faster than the speed of light in the medium - there are some clear diagrams on the Wikipedia page on Cerenkov radiation.

 

[snorkack]

Is it generally the case that wave speed depends not only on the medium, but also on the amplitude (and probably history of amplitude, too)?
That is, the shockwave in front of a supersonic projectile propagates faster than sound in front of the projectile, where its amplitude is large and increases the speed of sound locally, but only at the speed of sound further away where the wave has spread out and is at a lower amplitude? And by analogy, an electromagnetic wave propagates faster than light in the medium close in front of the charged particle, where the electrostatic field of the particle is so strong as to overwhelm the polarization of the medium and increase the speed of light, but merely at the speed of light further away, where the wave has spread out and is no longer strong enough to change the speed of light?

 

[weirdoguy]

And by analogy, an electromagnetic wave propagates faster than light in the medium close in front of the charged particle, where the electrostatic field of the particle is so strong as to overwhelm the polarization of the medium and increase the speed of light, but merely at the speed of light further away, where the wave has spread out and is no longer strong enough to change the speed of light?

This is a word salad. Besides, moving particles field is not static.

 

[Nugatory]

Is it generally the case that wave speed depends not only on the medium, but also on the amplitude (and probably history of amplitude, too)?

No, that is generally not the case. As long as it is a wave the speed depends on the frequency and wavelength. The amplitude matters only if it is great enough to change the physical properties of the medium enough that it we no longer have a wave.

 

[Dale]

shouldn't the photon have a horizontal component relative to earth

Yes. The photon will indeed have a horizontal component. But note that its speed will still be c. A wave’s direction is definitely affected by the motion of the source, even in the far field. But its speed depends on the medium only.

So is that completely true even if we talk about photons?

Yes.

 

[Herman Trivilino]

I believe that a source of any wave cannot moves faster that the wave it creates

Have you ever ridden in a motor boat or ship, or even just watched one move past you? The bow wave created is a perfect example of a wave that was created by a source moving faster than the wave.

Even swimmers can do this, taking advantage of the trough behind the wave to help them breathe.

https://en.wikipedia.org/wiki/Bow_wave

 

[snorkack]

No, that is generally not the case. As long as it is a wave the speed depends on the frequency and wavelength. The amplitude matters only if it is great enough to change the physical properties of the medium enough that it we no longer have a wave.

When water waves reach shallow water, the depth of the water and therefore the propagation speed of the wave will depend on the amplitude, or rather the phase, of the wave. With the result that the crests, consisting of deeper water and therefore travelling faster, will catch up with the troughs - eventually breaking.
Is surf a "wave" or not? Eventually it recedes and is then overtaken by next wave, so it is still at least periodic.
But I somehow doubt that the fundamental electromagnetic field strength is inclined to cease to be single valued, the way water level ceases to be single valued in a plunging breaker!

 

[Herman Trivilino]

But I somehow doubt that the fundamental electromagnetic field strength is inclined to cease to be single valued, the way water level ceases to be single valued in a plunging breaker!

At any location the water level has only one value.

 

[snorkack]

At any location the water level has only one value.

See https://man.fas.org/dod-101/navy/docs/swos/cmd/xpd/SP6-2-8/sld010.htm
At locations 3-5, a vertical line from bottom to sky, at a range near the wave front, passes from water below to air above twice, and the third time is from air below to water above. Which of these is "the" water level?

 

[Herman Trivilino]

Okay. I see what you mean now. I can't think of an analogous situation for an em wave, though.