How Does the Speed of Light Remain Constant in a Moving Spacecraft?

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Discussion Overview

The discussion revolves around the concept of the constancy of the speed of light, particularly in the context of a moving spacecraft. Participants explore the implications of this principle, comparing it to classical mechanics and addressing common misconceptions related to relative speeds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the constancy of light speed, questioning whether it behaves like a ball thrown inside a moving spaceship, suggesting that if light were affected by the ship's motion, it would appear to travel faster to an outside observer.
  • Another participant clarifies that the speed of light is invariant and cannot be used to measure one's speed, emphasizing that all observers will measure light at the same speed regardless of their own motion.
  • A further response corrects the initial assumption about adding speeds, noting that at low speeds, velocities appear to add linearly, but this changes significantly as speeds approach that of light.
  • One participant introduces the concept of time dilation and length contraction as consequences of approaching light speed, explaining that conventional Newtonian physics does not apply in these scenarios.
  • Another participant mentions the Doppler effect, explaining how it affects the frequency of light without altering its speed, providing a simplified explanation of the phenomenon.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the constancy of light speed, with some clarifying misconceptions while others maintain their confusion. The discussion remains unresolved regarding the intuitive understanding of light speed in relation to classical mechanics.

Contextual Notes

Participants highlight limitations in applying Newtonian physics to relativistic speeds, and there are unresolved assumptions about the nature of speed measurements in different frames of reference.

JKaufinger
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I know there is an answer to this somewhere, so please forgive me for not being able to see it :)

When people say that light will always be measured at a constant speed, no matter where you are, what do they mean?

Say we are in a spaceship which is standing still. Inside the spaceship, we throw a ball at 5 m/s. Now the spaceship goes to a fast speed. We throw the ball, and inside the ship, we still see it as 5 m/s.

So are they saying that if you are in the fast-moving spaceship, you will measure light the same as in the standing still space ship, just as you would the ball? That doesn't make any sense, because that would imply that light, like the ball, is affected by the inertia of the ship. It also implies that if the ship is moving at 100 m/s and the ball is 5 m/s from inside the ship, then to the outside, the ball is 105 m/s. Thus, if the light was also affected, then that means that the light would be going at c+5 m/s to the outside observer.

You see where I have this confusion?

Thanks.
 
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JKaufinger said:
When people say that light will always be measured at a constant speed, no matter where you are, what do they mean?
You can never know what speed you are moving at if you can't see some background object to measure against. So by this principle you can't use a measurement of the speed of light to tell your speed - whatever speed you are doing you will measure the same speed for light

It also implies that if the ship is moving at 100 m/s and the ball is 5 m/s from inside the ship, then to the outside, the ball is 105 m/s.
No it doesn't - speeds don't actually add like that. At low speeds they almost do so the speed would be something like 104.99999999 m/s. As things get closer to the speed of light this effect is bigger.
At the speed of light the speed is the same measured by any observer.
 
In layman's terms, you can't apply conventional physics (Newtonian) to speed of light or anything that approaches such high speed. Einstein showed that speed of light (c) is constant and as you approach this limit, time-dilation and length-contraction get introduced. If an object were to reach the speed of light, which is impossible by the way (anything with mass can't), then in theory, it would have zero-length in the direction it's heading, gain infinite amount of mass, and literally be turned into pure infinite energy.

We are so used to relatively slow speeds in our everyday-experiences that we tend to automatically think in terms of additive velocities. What happens instead in high-speed situations is that "Doppler-Effect" shifts the light spectrum to lower frequency when the object is moving away from the observer (and vice-versa) while the speed of light remains constant. That is as simple as I can explain it.
 

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