How Does Time Dilation Affect Light Speed from a Moving Spaceship?

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Discussion Overview

The discussion revolves around the concept of time dilation and its effects on the speed of light as observed from a moving spaceship. Participants explore the implications of special relativity on light emitted in different directions relative to the motion of the spaceship, addressing both theoretical and conceptual aspects.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that if time in the spaceship is slower than in other frames, a light ray emitted behind the ship should appear to travel faster than the speed of light, c.
  • Another participant counters that from the perspective of the crew on the spaceship, their time is not slowed, and they would measure the speed of the emitted light as c.
  • A third participant notes that an observer traveling behind the spaceship would also measure the light's speed as c, regardless of their relative motion.
  • It is pointed out that the idea of time being slower in one frame compared to another is a misunderstanding; all frames are relative, and both the spaceship and an external observer experience time dilation differently.
  • Participants emphasize that both frames experience time and length contraction, which ensures that the speed of light remains constant at c in all reference frames.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of time dilation and its implications for the speed of light, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some statements rely on specific interpretations of special relativity, and the discussion includes assumptions about the frames of reference that may not be universally accepted.

Giulio B.
A spaceship is moving, then it launches a ray of light behind itself.
if time in the spaceship is slower than in other frames that are moving less fast, shouldn't the ray appear much faster than c?

this thing works with a ray launched in front of the ship, but behind, shouldn't the time go faster to let us see the ray at c speed?
thanks.
 
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Giulio B. said:
A spaceship is moving, then it launches a ray of light behind itself.
if time in the spaceship is slower than in other frames that are moving less fast, shouldn't the ray appear much faster than c?

this thing works with a ray launched in front of the ship, but behind, shouldn't the time go faster to let us see the ray at c speed?
thanks.


No. First of all "The time on the ship" relative to the ship isn't slowed at all. As far as the crew of the ship are concerned, they might as well be at rest. That's why their state is caled the "Rest Frame" of the ship. Ship time is only slowed as experienced by an observer for whom the ship is moving fast.

So the ship, at rest with respect to itself, emits a light beam in any direction, and the crew measures the beam's light speed, and whaddayaknow, they get c.
 
Someone traveling behind the ship, catching up to it with greater speed, would meet the beam of light head on and also measure the light's speed at c.

A third person watching the two ships both whiz by from a nearby planet would also measure the light's speed at c.
 
Last edited:
if time in the spaceship is slower than in other frames that are moving less fast

This isn't quite right. In SR, everything's relative. One frame can not be said to be moving faster or slower than another. From the spaceship's frame, time is moving slower both ahead and behind it. From a planet's frame (as the spaceship goes whizzing by) time is moving slower on the spaceship. Both frames see a time contraction and a length contraction that force c to be the same speed in any reference frame.
 

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