# The Naked Scientists Forum

### Author Topic: Can electrons travel faster than light speed?  (Read 6151 times)

#### Europan Ocean

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##### Can electrons travel faster than light speed?
« on: 12/03/2013 18:02:09 »
If a 1 km long rod of silver was travelling at, say, 99.5% light speed and a current was sent through the rod in the direction of the travel, what would happen to those electrons? Surely they can't travel beyond light speed?
« Last Edit: 16/03/2013 22:25:17 by chris »

#### yor_on

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##### Re: Electrons and light speed?
« Reply #1 on: 12/03/2013 18:23:15 »
Depends on your definitions. Is the rod accelerating? Or is it moving uniformly? If it is moving uniformly then electrons will travel at its 'normal speed', relative that rod, as I understand it. Any uniform motion becomes unmeasurable in a black box scenario. When you think of a 'speed/velocity' you need to define it relative something, but measurements locally made on that rod will treat it as 'standing still', just as you might see Earth. Earth has all kinds of speeds, simultaneously, only depending on what you define that speed from. And yeah, relativity can blow ones mind away :)

#### simplified

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##### Re: Electrons and light speed?
« Reply #2 on: 12/03/2013 19:40:36 »
I think the electrons have slow speed in the rod.Therefore their speed is about 99.5% light speed.

#### yor_on

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##### Re: Electrons and light speed?
« Reply #3 on: 12/03/2013 19:53:57 »
Ask yourself what you think would happen if you went to the front of the rod turning on your flashlight. Would light now speed away at 00.5%  of its normal light speed, or do you expect it to behave as always? Moving at 'c'.
==

That one been tested on Earth by the way.
« Last Edit: 12/03/2013 19:56:43 by yor_on »

#### Bill S

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##### Re: Electrons and light speed?
« Reply #4 on: 12/03/2013 20:17:27 »
If you applied the relativistic velocity formula to the speed of the rod and the speed of the electron, relative to the rod,  wouldn't the combined speeds be less than c?

#### yor_on

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##### Re: Electrons and light speed?
« Reply #5 on: 12/03/2013 20:41:25 »
Not sure how you mean? Are you wondering that with different relative motion (uniform) there should be differences to how fast the electrons move in the rod? Maybe we can test it by asking what it would tell us if it was so? Wouldn't that mean that we now had a way to define a 'absolute speed' of the rod, not using any other frame to measure it relative?

The rod is our inertial object here, and whatever moves in it, does so relative it.

#### yor_on

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##### Re: Electrons and light speed?
« Reply #6 on: 12/03/2013 20:48:28 »
My view is that if you want to use 'points' 'time dilating' and 'contracting' relative other points you need to find some way to introduce accelerations in that rod. Because then you can expect the acceleration to 'propagate' inside that rod. But a uniform motion is no acceleration that I can find. So there all points should be at rest relative each other, just as the rod is defined to be 'at rest' with the gravitational field, transforming it away. It might have been this you were thinking of too?
=

Although there is one more thing, all atoms have a mass, does this mass act on other mass, and if it do, does atoms 'move', if you can define a (classical) motion to the constituents of mass you might be right there, and if you wonder there, well, that is what I'm wondering about too :). Think of something defined to move in a orbit, then it actually is accelerating at a constant (uniform) speed. So do we have this in atoms, or do we have 'orbitals' only?
« Last Edit: 12/03/2013 21:01:56 by yor_on »

#### yor_on

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##### Re: Electrons and light speed?
« Reply #7 on: 12/03/2013 21:04:51 »
The fact is that if we want to be very strict here, mass better not have any circular motions inside it, because if it does you should be able to introduce 'accelerations' in mass. the other type of acceleration though, as some particle accelerating linearly is ? not what I expect, even though I assume it to be possible too.

#### Pmb

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##### Re: Electrons and light speed?
« Reply #8 on: 12/03/2013 22:41:34 »
If a 1 km long rod of silver was travelling and 99.5% light speed and a current was sent through the rod in the direction of the travel, what would happen to those electrons? Surely they can't travel beyond light speed?
The drift velocity of electrons in a metal is very small. People tend to, incorrectly, think that electrons in current move at high speed where as the "drift velocity" is on the order of centimeters per second. It takes about an hour for a single electron to go from the " to the - terminal of your car battery.

If you want to know how to find velocities given in one frame given the values in another please see http://home.comcast.net/~peter.m.brown/sr/velocity_trans.htm

#### Europan Ocean

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##### Re: Electrons and light speed?
« Reply #9 on: 13/03/2013 02:03:52 »
The rod is travelling at a velocity that is an estimate, so that the rate of travel of electrons through a silver rod plus the rate of travel of the rod exceeds light speed. The current going forward.

Yes on the same point, what if light is emitted from the front? And from the rear shining backward?

I was inspired by the idea from an astro physicist of it not being plausible to accept that in dense matter such as in a star, for sound to travel faster than light.

#### yor_on

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##### Re: Electrons and light speed?
« Reply #10 on: 13/03/2013 05:53:09 »
How will sound travel through a infinitely dense material at all? Will it treat the material as if it 'wasn't there'? That reminds me of the theory on forcefields and a roll of toilet-paper. It never rips of where it should, which leads to suggest that a forcefield is best created by using them holes. and furthermore leads me to the conclusion that a roll of toilet paper should become 'infinitely strong', if we just use the holes, ignoring the paper.
=

Sorry, got sidetracked, slightly cheeky subject I admit.
(Just shows you that at TNS, physics will go to any depth, unraveling reality.)
« Last Edit: 13/03/2013 06:22:32 by yor_on »

#### Europan Ocean

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##### Re: Electrons and light speed?
« Reply #11 on: 14/03/2013 02:39:18 »
Well you know, sound doesn't travel on the moon, unless through the surface ground. It goes through our atmosphere at mach 1, but in the deep oceans whale songs can be heard from hundreds of kms away. Indians would listen for trains on hot days, by putting their ears to the iron. The more dense the matter, the better sound travels through it, and stars are very noisy, explosive...!

#### bizerl

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##### Re: Electrons and light speed?
« Reply #12 on: 14/03/2013 04:59:32 »
When a voltage is applied to something, is there any one particular electron that travels from one terminal to another? I had a (totally unfounded) idea that the effect was more like Newton's cradle where the electrons jumped between the copper (or in this case, silver) atoms.

If we were watching this rod shoot past on Earth and could somehow "see" the electrons "flowing" (in whatever way they flowed) through the rod, it would seem to flow much slower through the rod due to time dilation. The same way that someone running really fast across the top of the rod would seem to be running in slow motion.

If we were travelling on the rod, or at the same speed next to the rod, and we were watching the electrons, they would move just as fast as electrons should move, however the rod would also be at rest in this frame, so it is still slower than c.

I think.

In terms of sound, that's just stuff banging into eachother back and forth. The reason it doesn't propogate through space is there is not enough for it to bang into something else. The reason we can't "hear" a supernova is because not enough particles are being displaced through space for the compression wave to affect the air molecules next to our eardrum. I suppose the denser the material, the easier it is for particles to interact and the compression waves to propogate.

I have heard (on this forum) of something called Cherenkov Radiation which is when something travels faster than light through a medium. So I suppose it is possible for sound to travel faster than light in certain situations, just not in a vacuum.

#### lightarrow

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##### Re: Electrons and light speed?
« Reply #13 on: 14/03/2013 13:07:59 »
If a 1 km long rod of silver was travelling and 99.5% light speed and a current was sent through the rod in the direction of the travel, what would happen to those electrons? Surely they can't travel beyond light speed?
rod's speed: Vr
electrons' speed with respect to the  rod: Ve

electrons' speed with respect to our frame: V

V = (Vr + Ve)/(1 + Vr*Ve/c2)

Example:
Vr = 0.995c;
Ve = 0.9c (irrealistic in a metal but just as a "gedankenexperiment")
-->  V = (0.995c + 0.9c)/(1 + 0.995*0.9) =~ 0.9997362c.

The equation: V = (Vr + Ve)/(1 + Vr*Ve/c2) is valid for every speed, low or high; try with low speeds and tell me which is the result and the approximating equation.

#### yor_on

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##### Re: Electrons and light speed?
« Reply #14 on: 15/03/2013 00:59:15 »
True EO :)

But what I'm wondering about there is what properties one could expect from a infinitely dense material. Would sound treat it as some ideal description of mass? meaning no 'vibrations' between its 'property' at all, or would it really 'vibrate' to sound?

#### yor_on

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##### Re: Electrons and light speed?
« Reply #15 on: 15/03/2013 01:02:14 »
In the first case all the 'material'. marking out a distance, seem to disappear to me, as far as 'sound' is concerned. Which is interesting :) Also it should mean that there will be no dampening of that sound. as you should be able to treat the material as a infinitely thin 'wall' as I think
« Last Edit: 15/03/2013 01:04:01 by yor_on »

#### bizerl

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##### Re: Electrons and light speed?
« Reply #16 on: 15/03/2013 01:24:55 »
True EO :)

But what I'm wondering about there is what properties one could expect from a infinitely dense material. Would sound treat it as some ideal description of mass? meaning no 'vibrations' between its 'property' at all, or would it really 'vibrate' to sound?

If we're talking about crazy infinity stuff, wouldn't an infinitely dense material be a black hole, and therefore any vibrational movement would be damped by gravity? [insert "someone just mentioned black holes on this forum again" smiley]

Also, I always saw sound as a compression wave. If something is infinitely dense, can it be compressed further in order to propogate an oscillating sound wave?

#### yor_on

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##### Re: Electrons and light speed?
« Reply #17 on: 15/03/2013 11:58:53 »
It shouldn't, should it? A very strange idea if you treat it mathematically, also thinking of it as some sort of symmetric description to there being 'nothing' stopping that sound ,if one could treat it as a infinitely thin membrane. Worthy of a good SF. And yes, it should be a 'black hole' to me too. Or maybe a 'infinite matrix' of black holes?
=

Da*n, you just destroyed my infinitely thin membrane there :)
« Last Edit: 15/03/2013 12:00:51 by yor_on »

#### Bill S

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##### Re: Can electrons travel faster than light speed?
« Reply #18 on: 18/03/2013 20:42:19 »
Infinitely dense?  Infinitely thin?  How do you define either of these?

For example; what is the difference between something that is infinitely thin and something that is not there?

#### Pmb

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##### Re: Can electrons travel faster than light speed?
« Reply #19 on: 18/03/2013 23:20:02 »
If a 1 km long rod of silver was travelling and 99.5% light speed and a current was sent through the rod in the direction of the travel, what would happen to those electrons? Surely they can't travel beyond light speed?
rod's speed: Vr
electrons' speed with respect to the  rod: Ve

electrons' speed with respect to our frame: V

V = (Vr + Ve)/(1 + Vr*Ve/c2)

Example:
Vr = 0.995c;
Ve = 0.9c (irrealistic in a metal but just as a "gedankenexperiment")
-->  V = (0.995c + 0.9c)/(1 + 0.995*0.9) =~ 0.9997362c.

The equation: V = (Vr + Ve)/(1 + Vr*Ve/c2) is valid for every speed, low or high; try with low speeds and tell me which is the result and the approximating equation.

Note: I already gave that reltion above as well as its derivation.

#### Pmb

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##### Re: Can electrons travel faster than light speed?
« Reply #20 on: 18/03/2013 23:36:49 »
Quote from: Europan Ocean
The rod is travelling at a velocity that is an estimate, ..
What does that mean? We're talking about classical mechanics of which special relativity is a sub set. As such velocity is meaningful and can be said to be exact even if we don't measure it with infinite precision.

Quote from: Europan Ocean
..so that the rate of travel of electrons through a silver rod plus the rate of travel of the rod exceeds light speed. The current going forward.
Nope. That is quite incorrect. You' thinking in terms of what's known as the Gallean transformation. Instead you should be thinking in terms of the Lorentz transformation. In any case the drift speed of electrons is  very small, on the order of cm per minute.

Nothing can travel faster than light except tachyons if they do indeed exist.

#### lightarrow

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##### Re: Can electrons travel faster than light speed?
« Reply #21 on: 19/03/2013 19:21:46 »
Note: I already gave that reltion above as well as its derivation.
I know, but in this case I wanted to give also a direct example to the OP.

#### yor_on

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##### Re: Can electrons travel faster than light speed?
« Reply #22 on: 19/03/2013 20:09:47 »
Don't really think there is any Bill, well, for this at least :) And if Bizerl hadn't destroyed my new hypothesis we could have explored that one, into infinity :) On the other hand, infinity seems a mathematical definition consisting of several magnitudes, although practically I expect them all to end the same for me, with my demise that is.

#### The Naked Scientists Forum

##### Re: Can electrons travel faster than light speed?
« Reply #22 on: 19/03/2013 20:09:47 »