# Faster than light

jk4
It is possible for the electron beam in a a television picture tube to move across the screen at a speed faster than the speed of light. Why does this not contradict special relativity?

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It is possible for the electron beam in a a television picture tube to move across the screen at a speed faster than the speed of light. Why does this not contradict special relativity?
A typical modern TV has a vertical horizontal scan frequency of 96kHz which means it scans 96,000 horizontal lines per second. Assuming a generous large TV with a width of 1 meter that equates to the electron beam moving at 96,000 m/s which is orders of magnitude slower than the speed of light which is just under 300,000,000 m/s.

tiny-tim
Homework Helper
It is possible for the electron beam in a a television picture tube to move across the screen at a speed faster than the speed of light. Why does this not contradict special relativity?
Hi jk4!

Because nothing is moving across the screen.

The electrons are all moving towards the screen, slower than light.

The effect that is moving across the screen is different dots lighting up … none of them is moving.

You can achieve the same faster-than-light effect by shining a torch at the moon and wiggling it a bit!

No information is conveyed by the beam … even if you know that the first 100 "dots" are red, the very next dot may be blue!

jk4
but that statement says that the beam does move faster than light, and then asks why it's not a violation.

Doc Al
Mentor
It is possible for the electron beam in a a television picture tube to move across the screen at a speed faster than the speed of light. Why does this not contradict special relativity?
The spot on the screen is not a physical entity nor can it be used to transmit a signal faster than the speed of light.

Check out: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3").

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jk4
what do you mean by "the spot on the screen"?
I really don't know anything about how televisions work.

Doc Al
Mentor
what do you mean by "the spot on the screen"?
I really don't know anything about how televisions work.
The electron beam hits the TV screen making a spot. (That moving spot makes the TV image that you watch.) When they talk of the speed of the beam moving across the screen, they mean the apparent speed of that spot. (Not the speed of the electrons in the beam!)

jk4
ok, I understand what you mean.
However, I while I was waiting for responses, I read something saying that electrons can travel faster than light through certain mediums.
I also read that when a particle does this it emits cherenkov radiation.

Is what I read correct? and more importantly, is that what happens in a TV?

Either way, I understand the explanation you gave me, I'm just curious about how the above applies.

While thinking about this question I thought of this interesting device that appears to send signals faster than light, but I have figured out why it does not work. I thought it might be amusing for you guys to figure out why it does not work too.

Consider a circular wall with a radius of 5 light seconds. The perimeter is therefore 10 Pi or about 31.4 light seconds long. I stand in the middle with a lazer pointer and spin round once per second projecting a moving spot on the wall. That means the spot on the wall appears to be moving at 31.4c. Observer A is positioned somewhere on the wall and observer B is positioned exactly opposite observer A. Each observer sees a flash from my spinning lazer once per second. Now if observer A wants to send a signal to B it would normally take 10 seconds for the signal to travel from A to B. To speed things up I agree to stop spinning the instant I see a signal from A and B will take the absence of a signal from me as being a signal from A. The signal only takes 5 seconds to travel from A to me because I am in the middle, and I instantly stop spinning and about one second later B sees that his regular once per second flash from my lazer does not arrive and he is aware of a message signal from A in 6 seconds instead of the usual 10. Have fun ;)

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tiny-tim
Homework Helper
I read something saying that electrons can travel faster than light through certain mediums.
I also read that when a particle does this it emits cherenkov radiation.

Is what I read correct? and more importantly, is that what happens in a TV?
Hi jk4!

Yes, that's right … relativity doesn't prevent electrons from overtaking light, only from going faster than a certain speed.

But that doesn't happen in a TV … the TV tube is a vacuum (well, almost!) … the light we see when watching TV isn't emitted by the electron, it's emiitted by the phosphor dot (or whatever it is) that the electron hits.

jtbell
Mentor
The electron beam hits the TV screen making a spot. (That moving spot makes the TV image that you watch.)
Assuming you're watching a CRT TV, not an LCD or plasma.

Before long, this example will go the way of using rotating phonograph records to talk about angular velocity.

Doc Al
Mentor
LOL. Kind of like trying to explain "clockwise" to someone who has never seen an analog clock.

tiny-tim
Homework Helper
… these new-fangled phonographs …

Assuming you're watching a CRT TV, not an LCD or plasma.

Before long, this example will go the way of using rotating phonograph records to talk about angular velocity.
For the benefit of British and Commonwealth members, jtbell doesn't mean phonograph cylinders … apparently "phonograph" is a quaint American term which includes "gramophone" … see http://en.wikipedia.org/wiki/Phonograph#Terminology".

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to kev,

The signal only takes 5 seconds to travel from A to me because I am in the middle, and I instantly stop spinning and about one second later B sees that his regular once per second flash from my lazer does not arrive and he is aware of a message signal from A in 6 seconds instead of the usual 10. Have fun ;)
well, lets start at t1=0 (in your frame). You turn on the laser, standing at the center, oriented towards the 0th degree of the circle. radius is 5 light seconds long. so it will take 5 seconds for your first laser spot to appear at the 0 degree mark. by then you will have turned 5 revolutions and just ready to start your 6th one. so the t2=0 (of the wall's frame) starts when t1=5 (your frame), making an absolute time delay of 5 secs between events (related to switching the laser on/off) in both the frames. so when you stop spinning (or equivalently switch the laser off) the instant you see a message from A, B will notice this discrepancy only after 5 seconds of it happening (due to the delay). thus it will take 10 seconds for the signal to reach B from A, regardless of the troubles you took.

P.S. on why the two frames should be considered different: you're spinning at the center of the circle. assume that there is an invisible rod connecting you to a particle X straight ahead of you on the wall. so regardless of the mechanical drag that the 5 light second long rod puts on you (!), you spin at 1 rev/sec. according to basic rotational mechanics, the particle X should move at a velocity quite higher than you. thus you both are in different inertial frames.