This question is not about a moving clock, but about the frame of reference.

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Discussion Overview

The discussion revolves around the implications of traveling to Proxima Centauri at relativistic speeds, specifically focusing on time dilation as described by Special Relativity. Participants explore how time is perceived by a traveler on a spaceship moving at 99.99999% the speed of light compared to observers on Earth, as well as the behavior of light in different frames of reference.

Discussion Character

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

Main Points Raised

  • One participant suggests that a traveler to Proxima Centauri would measure the time to be shorter than 4.2 years due to relativistic effects, which could make the journey seem less daunting.
  • Another participant agrees that this is a prospect for interstellar travel, implying acceptance of time dilation effects.
  • A question is raised about the behavior of light emitted from a moving spaceship, with one participant asserting that the traveler would see the light escaping at the speed of light, while an observer would see it overtaking the ship slowly.
  • One participant emphasizes that the speed of light remains constant across reference frames, which is a fundamental postulate of Special Relativity, and suggests further reading on the topic.
  • Another participant elaborates on the measurement of light's speed, stating that one cannot see light escaping from a laser pointer and must rely on reflections to measure its speed, linking this to the concept of a Frame of Reference.
  • A clarification is made regarding the measurement of time aboard the spaceship, indicating that the local time would be shorter than 4.2 years due to the shorter distance traveled as perceived from Earth, rather than a difference in the ticking of the clock itself.

Areas of Agreement / Disagreement

Participants generally agree on the principles of Special Relativity and the constancy of the speed of light, but there are differing interpretations regarding the implications of these principles for time measurement and perception from different frames of reference. The discussion remains unresolved on some aspects, particularly regarding the perception of light and time by the traveler versus observers on Earth.

Contextual Notes

The discussion includes assumptions about the nature of time and light in relativistic contexts, and there are unresolved questions about how these concepts apply to different frames of reference. The implications of these assumptions on measurements and perceptions are not fully explored.

@nonymous
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I found out that Proxima Centauri is 4.2 light years away from earth. If someone was on a voyage to this star via space ship, would this person measure the time to be shorter than 4.2 years if their spaceship was traveling 99.99999 percent the speed of light, assuming that the 4.2 years is measured on earth? Wouldn't this make the voyage a lot less daunting for the traveler?
Doesn't a moving clock move slower?
 
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Yes, this is one of the prospects for possible interstellar travel.
 
What if he shines a laser pointer out of the front windshield? Does he see it escaping him at the speed of light? I think yes. However the observer sees it just overtaking the ship barely. This seems too bizarre...
 
Everyone always agrees, regardless of reference frame, that light is always going the same speed, the speed of light. It is from this single postulate (and one other smaller one) that all of Special Relativity is derived.
I would suggest this as a place to read up on special relativity and its implications: http://en.wikipedia.org/wiki/Introduction_to_special_relativity
 
@nonymous said:
What if he shines a laser pointer out of the front windshield? Does he see it escaping him at the speed of light? I think yes. However the observer sees it just overtaking the ship barely. This seems too bizarre...
Actually, you cannot see light escaping from you. Once it has left the laser pointer, it's gone and you'll never see it again or have any awareness of its progress, unless it hits something and illuminates it and reflects back to you. Then what you see is a result of the light making a round trip from your laser pointer, to an object, and back to you. So the only way you can measure the speed of light is to have it reflect off of something and measure how far away that something is (with a ruler) and measure how long the round trip took (with a clock or other timing device) and then you can calculate the "average" speed of light during the round trip. But you have no idea whether the light took the same time to get to the object as it did to get back to you.

The second postulate of Special Relativity defines those two times to be equal for any inertial measurement and this is the basis for a Frame of Reference. So when we say that the light propagates away from a high speed spacecraft at c, we mean that it is defined to be traveling at that speed according to the rest frame of the spacecraft and according to the same definition of a different rest frame for the earth, it is also traveling at c. It's not the least bit bizarre once you grasp what a Frame of Reference is.

Einstein's 1905 paper introducing relativity is a good place to learn about this.
 
...would this person measure the time to be shorter than 4.2 years...

You mean "measure the [local] time aboard their own spaceship shorter than 4.2 years..." right? That's because they travel a shorter distance for the trip than was initially measured from earth...not because the person sees her own clock ticking differently.

Time on Earth passes as normal...but is not observed (measured) as such from the
fast moving spaceship. Only when Earth clocks are compared with spaceship clocks upon return would observers recognize different elapsed times.
 

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