The Movement of Electricity in a Space Ship

Click For Summary

Discussion Overview

The discussion revolves around the movement of electricity in a spaceship traveling at relativistic speeds, particularly focusing on the implications of Einstein's mental experiments. Participants explore how observers inside and outside the spaceship perceive the operation of electrical devices, such as light bulbs, under the constraints of relativity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that nothing can exceed the speed of light relative to a "rigid point," and that electricity travels through a wire at the speed of light in a vacuum.
  • One participant questions whether an observer inside the spaceship (O1) would see a light turn on while an observer on Earth (O2) would not, suggesting a conflict in perceptions due to relativistic effects.
  • Another participant corrects the assumption that velocities add in a straightforward manner, emphasizing the need for the relativistic velocity addition formula, which ensures that resultant speeds do not exceed the speed of light.
  • Some participants express uncertainty about whether the light would be on for O1 and off for O2, with one participant suggesting that both observers would eventually see the light turn on, albeit with delays due to signal propagation.
  • There is a suggestion that the scenario is analogous to shining a flashlight forward in the spaceship, raising further questions about the consistency of observations between different frames of reference.
  • One participant introduces the concept of length contraction, proposing that the distance electricity must travel to reach the light bulb is reduced from the perspective of a stationary observer on Earth.

Areas of Agreement / Disagreement

Participants express differing views on how observers perceive the operation of the light bulb, with no consensus reached on whether O1 would see the light on while O2 would not. The discussion remains unresolved regarding the implications of relativistic effects on electrical signals.

Contextual Notes

Participants highlight limitations in assumptions about velocity addition and the propagation speed of electricity in different media, indicating that these factors may affect the conclusions drawn from the thought experiment.

DonB
Messages
71
Reaction score
0
Hello. I am an occasional poster here with little background but a big interest in the mental experiments (ME) of Einstein and others. Of particular interest to me are factors related to what I call the Classic Spaceship Mental Experiment. [mentor's note: a link to an unacceptable reference under the Physics Forums rules has been removed]

From previous discussions (some here) I understand that it is generally accepted (and correct me if I'm wrong) that:

1) Nothing can exceed the speed of light relative to any given "rigid point" (Einstein's term);

2) In a vacuum electricity travels through a wire at the speed of light.

That being the case, for a spaceship traveling at/near the speed of light relative to the earth, if an observer (O1) inside the spaceship turns on an electrical switch allowing electricity to flow from a battery located in the rear of the spaceship to a light bulb in his compartment, he will see the light come on since the electricity is traveling within the ship at c relative to him. However, an observer (O2) on the Earth arguably will not be able to see the light come on since it would require electricity to travel at 2c relative to him.

Now let's take this ME to a more down to Earth scenario. For a jet traveling at mach one, the electricity that runs the length of the fuselage is c relative to the pilot (O1). But according to our two givens above, since electricity can't travel at >c relative to O2, it must travel at <c relative to the fuselage as observed by O2 (i.e., no faster than c - mach-one)? So O2 would here see variations in lights and/or instrument readouts due to the abnormal speed of electricity relative to the jet? And would only the speed of electricity flow in the direction of flight be 'warped,' but the flow from the light back to the battery be normal?

The ramifications of the two givens seem to demand unworkable scenarios. So where is my thinking wrong on the ME above?
 
Last edited by a moderator:
Physics news on Phys.org
The main mistake is assuming that velocities add the same way as they do in the Galilean transform. The correct formula is called the relativistic velocity addition formula. That always returns a value less than or equal to c.

A minor mistake is the idea that electricity travels through a wire at c. Light travels through a vacuum at c, but EM waves will travel slightly slower in a wire depending on the material properties. This is a minor point because you could replace all of your thought experiment wires with thought experiment lasers in vacuum for a very minimal cost.
 
I appreciate the insights and corrections, Dale. But it still doesn't tell me if the light will be on for O1 yet off for O2 -- and if so, how that is possible. (I'm such a novice. Sorry.)
 
DonB said:
I appreciate the insights and corrections, Dale. But it still doesn't tell me if the light will be on for O1 yet off for O2 -- and if so, how that is possible. (I'm such a novice. Sorry.)
Subject to the signal propagation delays, each will see the light come on in due time.
 
DonB said:
But it still doesn't tell me if the light will be on for O1 yet off for O2 -- and if so, how that is possible.
Either the light turns on or it doesn't, and that has nothing to do with any observers moving at any speed anywhere - if the light is going to turn on, it's going to turn on no matter who is watching it. If one observer calculates that the light turns on and the other one calculates that it does not... Then one of them has miscalculated. As others have already mentioned, in this particular case the miscalculation is the result of not using the correct formula for addition of velocities - if you search this forum for references to "relativistic velocity addition" you will find many good explanations.
 
DonB said:
But it still doesn't tell me if the light will be on for O1 yet off for O2 -- and if so, how that is possible. (I'm such a novice. Sorry.)
In the scenario you described it will turn on in both O1 and O2 frames. The relativistic velocity addition formula explains how that can be compatible with the "speed limit" of c.
 
Is this not the same question as if a person in the back of the ship shines a flashlight forward? Or if the ship turns its headlights on?
 
Ben123 said:
Is this not the same question as if a person in the back of the ship shines a flashlight forward? Or if the ship turns its headlights on?
It is indeed the same question.
 
Thanks everyone for the input!
 
  • #10
I believe the length of the spaceship contracts, so the electricity doesn't have as far to go to reach the bulb (from the point of view of a stationary observer on Earth).
 

Similar threads

Undergrad Gravity at Schwarzschild Radius of a Black Hole
  • · Replies 17 ·
Replies
17
Views
4K
High School Spacetime interval question involving a Pendulum in a Space Ship
  • · Replies 7 ·
Replies
7
Views
1K
High School Traveling through space at zero km/s
  • · Replies 11 ·
Replies
11
Views
935
Undergrad The Einstein Clock aka Light Clock
  • · Replies 34 ·
2
Replies
34
Views
3K
High School Is time an illusion?
  • · Replies 5 ·
Replies
5
Views
2K
Physics of ISV (Interstellar Vehicle) in Avatar Series
  • · Replies 4 ·
Replies
4
Views
1K
Undergrad Can Time Travel Into the Future Really Happen?
  • · Replies 19 ·
Replies
19
Views
3K
Undergrad Effects of time dilation for near-speed-of-light travel
  • · Replies 65 ·
3
Replies
65
Views
12K
Undergrad Basic Special Relativity question but the math isn’t working
  • · Replies 17 ·
Replies
17
Views
2K
Undergrad Is the concept of a one-way speed of light meaningful and measurable?
  • · Replies 25 ·
Replies
25
Views
5K