Curtailed length and the twin paradox

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Discussion Overview

The discussion revolves around the concepts of length contraction and the twin paradox in the context of special relativity. Participants explore the implications of high-speed travel on perception and aging, as well as the conditions under which these phenomena occur.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions how length contraction can be observed if an object moves at extremely high speeds, suggesting that at such speeds, the object would appear as a wave rather than a discernible shape.
  • Another participant clarifies that the term "see" may be misleading, proposing that "measure" would be more appropriate, and explains methods to determine the length of a fast-moving object.
  • A calculation is provided indicating that a 30m airplane would shorten by approximately 0.02 millimeters at high speeds.
  • Participants discuss the twin paradox, with one questioning the relative nature of velocity changes between the twins, suggesting that both could be seen as changing their velocities relative to each other.
  • Another participant asserts that acceleration is absolute in special relativity, noting that only the Earth twin remains in a single inertial frame during the journey, while the traveling twin does not.

Areas of Agreement / Disagreement

Participants express differing views on the nature of velocity changes in the twin paradox, with some arguing that the Earth twin and the spaceship twin experience changes in velocity relative to each other, while others maintain that only the spaceship twin undergoes significant changes in inertial frames. The discussion remains unresolved regarding the implications of these perspectives.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about perception at high speeds and the definitions of inertial frames. The mathematical steps involved in calculating length contraction are not fully explored.

Quan Chi
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Hello,

I've got two quick questions.

Why is it so that they say "if a object moves extremely fast then we see it curtailed/shortened a little bit"? But if it moves that fast that the length even may be noticeable, how on Earth do we SEE it shortened when it moves THAT fast past us? If a spaceship flies by in the speed of light, we see nothing but a wave. I might be wrong here, I hope I am, that's why I want to know why people still say that we See it shortened.

And besides, if an airplane flew by us even as slow as 1000 km/h. How much exactly does the aeroplane shorten then? I have no time to calculate, but was it less than a millimetre..


____________________
Second problem
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Why is it so that if one twin steps on a spacecraft and goes for a flight in the speed of light and after returning his twin brother is the one who became older? I mean isn't it the same if the spaceship stays put and the whole universe went for a flight in the speed of light?


Thanks for your replies.
 
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The word "see" is a bit misleading here, as what we would "see" regarding an object traveling at near c speeds depends on more factors then just the object's length contraction.
"Measure" or "determine", would be better choices. For instance we could lay out a measuring stick right next to the objects path, and as it passes by note at what points of the measuring stick the endpoints of the object are opposite at a given instant.
Or we could time how long it takes between the front and back end of the object to pass a given point and determine its length from its known speed.

For a plane of 30m in length, you would get a shortening of about.02 millimeter.

As for the second question, this is the standard "twin Paradox" which has been discussed a lot on this board already try doing a search on the term. The upshot is that the twin in the Spaceship makes changes to his velocity and the Earth twin does not.
 
Oh yes, I checked all the topics in this Relativity forum, but still... the spaceship twin makes changes to his velocity in relative to what? To the Earth twin? But it would be the same if the Earth twin made the changes in His velocity in relative to the spaceship twin.
 
Quan Chi said:
Oh yes, I checked all the topics in this Relativity forum, but still... the spaceship twin makes changes to his velocity in relative to what? To the Earth twin? But it would be the same if the Earth twin made the changes in His velocity in relative to the spaceship twin.
Acceleration is absolute in SR, just like in Newtonian physics. If you accelerate, you feel G-forces; the traveling twin feels them, but the Earth twin does not (ignore gravity since it's not dealt with in SR). So only the Earth twin stayed in a single inertial frame throughout the journey, and every inertial frame will say that the Earth twin maintained a constant velocity while the traveling twin did not.
 
This helped me a lot. Thanks, guys.
 

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