Twin paradox - which twin is older?

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    Paradox Twin paradox
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Discussion Overview

The discussion revolves around the twin paradox in the context of special relativity, exploring the implications of relative motion and aging between two twins, one traveling at high speed and the other remaining stationary on Earth. Participants examine the conditions under which age differences arise and question the nature of simultaneity and reference frames.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants propose that the twin traveling at high speed should be younger than the twin on Earth, but question whether the Earth twin could also be considered to be traveling at high speed from their own frame of reference.
  • Others argue that the age difference becomes apparent only when the twins reunite, emphasizing that the twin who accelerates (changes direction) will end up younger.
  • A later reply suggests that without a shared point in spacetime, the concepts of "younger" or "older" are not absolute, and the twin traveling at constant speed will be older upon reunion.
  • One participant introduces a metaphor comparing the paths taken by the twins to straight and bent lines in a 2D plane, suggesting that the straight path (constant velocity) results in less aging.
  • Another participant emphasizes that the twin who moves the greatest distance in the allotted time will be younger, referencing a hyperbolic relationship in spacetime that affects proper time.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the twin paradox, with no consensus reached on the nature of aging and the effects of acceleration versus constant velocity. The discussion remains unresolved regarding the absolute definitions of age in different reference frames.

Contextual Notes

Participants highlight the importance of acceleration in determining age differences, but the discussion does not resolve the complexities of simultaneity and the definitions of aging in different frames of reference.

Who May Find This Useful

Readers interested in the nuances of special relativity, the twin paradox, and the philosophical implications of time and aging in different reference frames may find this discussion informative.

maya :D
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Hi everybody! :)
can anyone help me with this idea: in the twin paradox, a twin that travels at speed of light in space should be younger than the one that remains on earth. But why can it be other way? Can we say that the twin on the Earth is traveling at speed of light compared to the twin in space, from his frame of reference? If we say that twin in a spacecraft is relatively still, than the twin on the Earth is moving. If this experiment was really possible, how could we say which twin would be older?
Thanks :-)
 
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The age difference shows up when they try to get together. If one doesn't accelerate, but the other does (i.e. turns around), then the accelerating one ends up younger.
 
Thanks, I understand that, but I don't understand if they met, why would the twin that remained on Earth been older? Why couldn't it be that all of us that remained on Earth were aging more slowly than the twin in spacecraft ? thanks:)
 
There's no absolute concept of "younger" or "older" unless the two twins are in the same point of spacetime. In order to compare their ages, we have to bring them together. When that happens, the one who's been traveling at constant speed will turn out to be older than the one who's been changing directions.
 
hamster143 said:
There's no absolute concept of "younger" or "older" unless the two twins are in the same point of spacetime. In order to compare their ages, we have to bring them together. When that happens, the one who's been traveling at constant speed will turn out to be older than the one who's been changing directions.
One consequence of this is that if the traveling twin continued traveling at constant speed, and someone strapped a giant rocket to the Earth so that it could be accelerated and catch up with the traveling twin, in this situation it would be the traveling twin who was older, not the Earth twin. As hamster143 says, whichever twin changes directions (or accelerates in any way) will end up being younger than the one who travels at constant velocity between their meetings. This is closely analogous to the fact that if you draw two paths on a 2D plane which cross at two different points, and one path is a straight line between the two points while the other bends, then the straight-line path will always have a shorter distance than the path that bends (because a straight line is the shortest distance between points in 2D space).

For more info on the twin paradox see here:

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html
 
Thanks everybody:)
 
There's been a lot of twin paradox threads lately. I'm quoting myself from one of them:
Fredrik said:
My standard answer is "Check out #3 and #142 (page 9) in this thread". Post #142 is just what Mentz and Ich are talking about. Post #3 contains a spacetime diagram that you should check out.

See also kev's argument and DrGreg's spacetime diagram here about why it doesn't make sense to say that it's the acceleration that causes the age difference.
 
Fredrik said:
There's been a lot of twin paradox threads lately. I'm quoting myself from one of them:

Maybe it is time the twin paradox had its own subforum in the Special and General Relativity forum and its own sticky. :wink:
 
  • #10
A sticky would be a good idea actually. I have felt that way for years.
 
  • #11
Fredrik said:
A sticky would be a good idea actually. I have felt that way for years.

I want to add my vote for a sticky. PLEASE !
 
  • #12
I'm adding my vote for sticky too!:)
 
  • #13
maya :D said:
Hi everybody! :)
can anyone help me with this idea: in the twin paradox, a twin that travels at speed of light in space should be younger than the one that remains on earth. But why can it be other way? Can we say that the twin on the Earth is traveling at speed of light compared to the twin in space, from his frame of reference? If we say that twin in a spacecraft is relatively still, than the twin on the Earth is moving. If this experiment was really possible, how could we say which twin would be older?
Thanks :-)

The twin that moves the greatest distance in the alotted time (which would be the same total time for both) would be the younger because motion in spacetime "subtracts" (not linearly but by a hyperbolic formula) from proper time and "youngifies" the mover. ALWAYS, ALWAYS, ALWAYS

Think of it this way - did you ever see a little boy who was always on the go and was 2 feet tall? He just never stood still long enough to grow.
 

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