# Thread: Speed of light + electron drift

1. ## Speed of light + electron drift

I am not sure where this question is going and I am pretty sure that my terminology is wrong. But let me at least start and I will correct my question as we go along.

Say a spaceship was travelling at near the speed of light relative to us.
Now inside of the spaceship you have wires that provide energy to the engine that is under the hood in the front of the space craft. The space craft is travelling directly away from us and towards its "front" ( the engine is the farthest point away from us, the rest of the spaceship closer ).

Without energy ( electricity ) the engine can not work.

When the spaceship accelerates and now is near the speed of light, the electrons in the spaceship still need to move around ( at the local speed of light ) and drift towards the front at a rate suffient enough to power the engines. However since it moving at near the speed of light I would think that the natural tendency of the electrons would drift backwards towards us.

Another way to look at this is that electrons move around in somewhat arbitrary motion at the speed of light, the drift usually would push from source to sink, however since it is travelling at the speed of light wouldnt an artificial sink be produced?

2. Originally Posted by tommac
When the spaceship accelerates and now is near the speed of light [...]
Relative to what?

3. Originally Posted by 01101001
Relative to what?
Say a spaceship was travelling at near the speed of light relative to us.
Relative to any observer really.

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The speed of the spacecraft (relative to us or anyone else) has no
effect on the motion of electrons or anything else in the spacecraft.

-- Jeff, in Minneapolis

5. Originally Posted by Jeff Root
The speed of the spacecraft (relative to us or anyone else) has no
effect on the motion of electrons or anything else in the spacecraft.

-- Jeff, in Minneapolis
Huh ? So only the outside is relative to us? But the inside doesnt count?

What if the wires were on the outside?

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The spaceship's length is contracted along it's relative velocity vector. To the electrons on board, however, if they had to travel 30' to get from the battery to the engine before, they have 30' to travel now.

7. Originally Posted by tommac
Huh ? So only the outside is relative to us? But the inside doesnt count?
It's not really a matter of inside/outside, it's more of a matter of the velocity is the same as the spacecraft (for both the outside observer and the inside observer).
Same speed, same clock.
Different observers, different clocks and different yardsticks. But; SOL observed the same.

8. Originally Posted by mugaliens
The spaceship's length is contracted along it's relative velocity vector. To the electrons on board, however, if they had to travel 30' to get from the battery to the engine before, they have 30' to travel now.
The electrons need to travel in the direction of travel which is directly away from the observer.

The thing is that this is similar to the flashlight on a train right except that our perception of the electrons movement would be a drift back in our direction as compared to the rest of the spacecraft.

9. Originally Posted by NEOWatcher
It's not really a matter of inside/outside, it's more of a matter of the velocity is the same as the spacecraft (for both the outside observer and the inside observer).
Same speed, same clock.
Different observers, different clocks and different yardsticks. But; SOL observed the same.
OK let me go over one more time.

We are the observer located somewhere behind the spaceship the engine is in the front of the spaceship and the battery is in the back of the ship ( aft or whatever ) ... the electricity needs to move from the back of the ship to the front of the ship. As the spaceship moves near teh speed of light. Each electron moving in a random / arbitrary direction would from our vantage point not be able to move very fast or maybe not at all towards the front of the ship ( because of relativity (for example the light shiningfrom a train moving at the speed of light example ) , however it could move freely and even accelerated towards the back of the spacecraft. All of this from our vantage point.

Now inside the ship, you have a battery which pushes the electron drift towards the engine and everything is fine and normal.

From OUR vantage point the electricity would appear to be running in reverse current.

10. Originally Posted by tommac
... Each electron moving in a random / arbitrary direction would from our vantage point not be able to move very fast or maybe not at all towards the front of the ship ( because of relativity (for example the light shiningfrom a train moving at the speed of light example ) , however it could move freely and even accelerated towards the back of the spacecraft. All of this from our vantage point. ...
That's the problem.
On the spaceship, the electron has gone from a speed of zero*, to a speed of C no matter which direction it is going.
Off the spaceship, the electron has gone from a speed* of <C to a speed of C.

In other directions, the only thing that changes is the angle of the electron moving away from the observer. It is still C for either.

*I know electrons aren't really zero, but localized to a battery, you can effectively call it zero for this example.

11. Originally Posted by NEOWatcher
Off the spaceship, the electron has gone from a speed* of <C to a speed of C.
Still confused. Off the spaceship, the electron goes from <C to C in the direction of motion and goes from -<C to C =~ delta 2C in the opposite direction.

Spaceship ----->

E1 <-------------Spaceship->E2

E2 electron when moving with the direction of the spaceship
E1 when moving oposite of the spaceship

E1 > E2

All other angles will follow a percentage of this tendency. with 90% moving with the speed of the spaceship.

Electrons would be flying off the back of the spaceship.

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## Huh?

Originally Posted by tommac
(snip)

From OUR vantage point the electricity would appear to be running in reverse current.
Why would you think that? A fundamental part of relativity is that after applying the appropriate math, all observers can agree about what is happening.

In general, be cautious about 'thought experiments'. It's the devil to see what's wrong with them if they are not very carefuly constructed at the outset. That's what 0110etc's comment is about. And, for example, there is an opportunity here in your question to open the door to rigidity, inertia, and reference frame questions, which the big guys are still arguing about.

Briefly, the electrons in the conducting cable are not sloshing around like a liquid, and could care less how fast the ship is going., and for that matter, the real liquid(s) in the plumbing will behave the same way. Remember, even under acceleration, the reaction is the same as in a gravitational field, and we do not have to stir the electrolyte in our car batteries to get the electrons off the bottom before starting (double rear window VW's of the '50's don't count).
Last edited by John Mendenhall; 2008-Nov-03 at 06:13 PM. Reason: clarity

13. Really the wire part doesnt matter all of that much. We can take any electron in or on the spaceship. Let me re-ask the question.

How does an electron act relative to an observer that is travelling at relativistic speeds towards or away from it? This assumes that the electron will spend some time moving towards the observer and other time moving away from the observer. ( and other times moving perpendicular to the observer etc ... )

The electron can not travel much faster in one direction.

Would the electron slow down in all directions? ( I dont think this can be accomplished without having 3d time ).

Originally Posted by John Mendenhall
Why would you think that? A fundamental part of relativity is that after applying the appropriate math, all observers can agree about what is happening.

In general, be cautious about 'thought experiments'. It's the devil to see what's wrong with them if they are not very carefuly constructed at the outset. That's what 0110etc's comment is about. And, for example, there is an opportunity here in your question to open the door to rigidity, inertia, and reference frame questions, which the big guys are still arguing about.

Briefly, the electrons in the conducting cable are not sloshing around like a liquid, and could care less how fast the ship is going., and for that matter, the real liquid(s) in the plumbing will behave the same way. Remember, even under acceleration, the reaction is the same as in a gravitational field, and we do not have to stir the electrolyte in our car batteries to get the electrons off the bottom before starting (double rear window VW's of the '50's don't count).

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Tommac, the foundation of special relativity is that electromagnetic phenomena work the same for all observers, independent of their state of motion. So that answers your question: electricity works just fine aboard the ship, in a way indistinguishable from electricity anywhere else.

Your specific question about a free electron in motion within the ship is one about addition of velocities under special relativity. The electron can move relative to the ship with any velocity less than lightspeed, in any direction. An outside observer will, in general, measure its velocity in one direction as being different from its velocity in another direction. The electricity will work properly in either case.

Please don't try to get into a discussion of your notion of 3D time in Q&A.

Grant Hutchison

15. tommac,

Various responders have answered your questions as clearly as I could have, if not better, and yet you persist in believing that our perception of an object's direction of motion in the spacecraft would be reversed if the spacecraft is receding from us fast enough.

I conclude from this and from many other of your posts that your understanding of the basic physics is very deficient, to an extent that cannot be corrected by short answers in a forum such as this one. I would recommend that you study some good textbooks on the topics involved. There are people in this forum who can make some good specific recommendations.

By the way, electrons in a typical circuit driven by a battery or a generator do not travel anywhere near the speed of light.

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Originally Posted by tommac
...our perception of the electrons movement would be a drift back in our direction as compared to the rest of the spacecraft.
No, it would not.

17. Here is a follow-up, after reading the following article:
http://hyperphysics.phy-astr.gsu.edu...ic/ohmmic.html

The electrons in a wire jiggle at high speed, but only a very small fraction of c. With no voltage along the wire, their mean position is virtually stationary.

When a current is flowing, a drift velocity is superimposed on the jiggling motion. This drift velocity is very slow, perhaps millimeters per second in a typical case.

18. Originally Posted by Hornblower
Here is a follow-up, after reading the following article:
http://hyperphysics.phy-astr.gsu.edu...ic/ohmmic.html

The electrons in a wire jiggle at high speed, but only a very small fraction of c. With no voltage along the wire, their mean position is virtually stationary.

When a current is flowing, a drift velocity is superimposed on the jiggling motion. This drift velocity is very slow, perhaps millimeters per second in a typical case.
Yes this is the drift velocity. However the jiggle is at speeds close to C.
They are at the speed of the electron. The drift is slow but the electrons in the wire are moving quickly.

Take a hose of water for example. If you turn on the water for an empty long hose it will take some time for the water for flow through the hose and come out the other end. If you fill the hose then turn on the water, it will take about the speed of sound in water of latency before the water comes out of the other end of the hose.

It is very similar for electron drift. You have a wire, you put an electron on the wire and at near speed of light an electron comes out of the other end of the wire ... the catch is that it is not the same electron. The time it takes the electron that you put on one end of the wire to come out the other side is very slow and that is the drift.

This drift is from a lower potential on one side than the other ( I think ).

The analogy from my school was the drunk man on a hill. The drunk man stumbles in a random direction. But since there is a hill it is more likely that he will stumble in the downward direction than an upward direction.

In my example above the electrons from a distant observer would all be more likely to travel to the rear direction of the spaceship. When aboard the spaceship the electrons are travelling stumbling towards the front.

19. Originally Posted by grant hutchison

Please don't try to get into a discussion of your notion of 3D time in Q&A.
I appologize, I just hinted at it in my state of confusion.

What I am confused about is the microscopic phenomena within the macroscopic object moving at relativistic speeds.

I may have confused it in my wording. What I am asking is:

If electron movement is arbitrary around a nucleus and the nucleus is travelling at relativistic speeds does how is the electron movement effected in order not to violate the laws of special relativity?

20. After all these months and innumerable questions you still seem to have a basic misunderstanding of physics. Either that or you are subtly trying to push an ATM concept.

Please take Hornblowers advice, you can't get a physics education via questions on an Internet forum.

21. Originally Posted by tommac
Yes this is the drift velocity. However the jiggle is at speeds close to C.
They are at the speed of the electron. The drift is slow but the electrons in the wire are moving quickly.

Take a hose of water for example. If you turn on the water for an empty long hose it will take some time for the water for flow through the hose and come out the other end. If you fill the hose then turn on the water, it will take about the speed of sound in water of latency before the water comes out of the other end of the hose.

It is very similar for electron drift. You have a wire, you put an electron on the wire and at near speed of light an electron comes out of the other end of the wire ... the catch is that it is not the same electron. The time it takes the electron that you put on one end of the wire to come out the other side is very slow and that is the drift.

This drift is from a lower potential on one side than the other ( I think ).

The analogy from my school was the drunk man on a hill. The drunk man stumbles in a random direction. But since there is a hill it is more likely that he will stumble in the downward direction than an upward direction.

In my example above the electrons from a distant observer would all be more likely to travel to the rear direction of the spaceship. When aboard the spaceship the electrons are travelling stumbling towards the front.
Your reasoning continues to be a mystery to me. Suppose the electrical system is such that the crew sees the electrons jiggling, and drifting away from the tail and toward the nose. If I could observe the electrons from afar as the craft runs away at close to c, I would expect to see them jiggling and drifting more slowly as a result of the time dilation, but the drift still would be toward the nose. Please explain in appropriate detail why you think otherwise.

22. Originally Posted by Hornblower
Your reasoning continues to be a mystery to me. Suppose the electrical system is such that the crew sees the electrons jiggling, and drifting away from the tail and toward the nose. If I could observe the electrons from afar as the craft runs away at close to c, I would expect to see them jiggling and drifting more slowly as a result of the time dilation, but the drift still would be toward the nose. Please explain in appropriate detail why you think otherwise.
OK at least I think we are on the same page now. Thanks for being patient.

And I just need to note that my suggestion of 3d time stems from a clear confusion about time-dilation. So I need again to delve into this clarification in hopes of not agitating people who will jump at the chance of saying I am trying to promote ATM here.

As the space ship moves near the speed of light it will stretch in the direction of motion and you will have dilated time to keep the speed of light constant.

There is no stretching / contracting in the perpendicular directions.

( am I right so far? )

Say we have two flashlights one pointing foward and one backwards ( stern/aft ) The light shining forwards is travelling slower relative to the speed of the spaceship from our view point than the light from the aft.
t1 dSLaft=0 dSLstern=0
x---LightLight---x
x---Spaceship---x
t2 dSLaft=2 dSLstern=0
x--Light--Light--x
x----Spaceship--x
t3 dSLaft=4 dSLstern=0
x-Light----Light-x
x-----Spaceship-x
t4 dSLaft=6 dSLstern=0
xLight------Lightx
x------Spaceshipx

So time needs to move faster in the region towards the aft and slower in the region towards towards the stern. In order for light to appear that it is moving at the speed of light relative to it. Are we in agreement here? Or am I lost? If time slows everywhere then the light moving off the aft will not be able to be kept constant.

Lets start with this small point and we can discuss more. Please let me know if we are in agreement.

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Tom,

"Aft" means "at, near, or toward the stern". In the rear. The bow is forward.

-- Jeff, in Minneapolis

24. Originally Posted by tommac
This drift is from a lower potential on one side than the other ( I think ).
From high, to low. Think, waterfall.
The analogy from my school was the drunk man on a hill. The drunk man stumbles in a random direction. But since there is a hill it is more likely that he will stumble in the downward direction than an upward direction.
On a hill he won't stumble in a totally random direction. Or is that what you mean?
In my example above the electrons from a distant observer would all be more likely to travel to the rear direction of the spaceship. When aboard the spaceship the electrons are travelling stumbling towards the front.
Why would they be more likely?

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Tom,

To simplify your question, can you have the spaceship just coasting at
whatever speed, not accelerating? You could still have electrons moving
around doing something other than running the engine. There may be a
complication if the spaceship is accelerating.

-- Jeff, in Minneapolis

26. From my reading of the OP and followups, I assumed that we were talking about observations once speed had been obtained, rather than during the acceleration.

27. If the speed has already been attained, then the answer is very straightforward. You start by noting that for the observer in the ship, the circuit is performing perfectly normally, in every way. Then to transform to observers outside the ship, you just subtract the velocity of the observer from the instantaneous velocity of every electron, but you do that in the relativistic way of combining velocities, such that no velocities ever exceed c. This is also what grant hutchison said. Yes, the direction of the electron motion can change for outside observers, but the action of the current will not change at all-- the outside observer will always be able to transform back to the on-board frame, and in so doing will always see a perfectly normal circuit.

If you fly over a football game in a high-speed aircraft, the game may look a bit odd at first, what with all those people running backward and so forth, but your mind will quickly understand that a perfectly normal football game is being played there. Relativistic corrections do not change that basic truth-- indeed, preserving that truth is the whole purpose of relativistic corrections.

28. Originally Posted by Jeff Root
Tom,

"Aft" means "at, near, or toward the stern". In the rear. The bow is forward.

-- Jeff, in Minneapolis

Hah ... OK cool! Good thing I dont drive a boat.

29. Originally Posted by hhEb09'1
From high, to low. Think, waterfall. On a hill he won't stumble in a totally random direction. Or is that what you mean?Why would they be more likely?
Well what I was thinking at the time was the the electrons relative to us would travel in the rear direction at a speed of 2x the speed of light away from the ship while not travelling much at all in the direction of the BOW.

Similar to shining a light in the front and the back of a train moving relativistically.

30. Yes, thanks ... I way over complicated the question and I agree with the simplification below.

Originally Posted by Jeff Root
Tom,

To simplify your question, can you have the spaceship just coasting at
whatever speed, not accelerating? You could still have electrons moving
around doing something other than running the engine. There may be a
complication if the spaceship is accelerating.

-- Jeff, in Minneapolis