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<blockquote data-quote="Umbran" data-source="post: 7547195" data-attributes="member: 177">Okay, finished a thing.&nbsp; I can take a moment on this.&nbsp; The trick is to not think of this as &quot;one twin stays at home, and one flies out for a while&quot;.&nbsp; Think of it as &quot;one twin stays at home, the other goes to a place and comes back&quot;Imagine that we have twins.&nbsp; One stays on Earth, and the other is sent to Alpha Centauri, 4 light years away.&nbsp; They will travel at 0.8c.&nbsp; We will assume that the time to reach that speed, and to turn around and re-accelerate toward Earth, are negligible.&nbsp; This is not true, but it is demonstrative.&nbsp; Numbers have been chosen to make them easy to work.From the point of view of Earth (Earth's rest frame), the ship is traveling a total of 8 light years, at 0.8c, and so will come back in 10 years - everyone on Earth will age 10 years before the ship returns.&nbsp; (People on Earth expect that time on the ship will be reduced by a factor&nbsp; ϵ = sqrt (1 − v^2 / c^2) .&nbsp; ϵ=0.6, so the ship will experience 6 years of time.)Now, the rest frame of the ship is *moving* with respect to the rest frame of Earth.&nbsp; And time dilation is not the only effect of travel near the speed of light!&nbsp; We talk about time more, because time is mysterious, but distance along the line of motion is also contracted!When the ship is flying, it looks to them like the distance to Alpha C. is, compared to that measured on Earth, contracted by the same ϵ=0.6.&nbsp; So, the distance to them is only 2.4 light years each way!&nbsp; And, to travel the round trip of 4.8 light years at 0.8c takes... 6 years!People on the ship *agree* that they will experience less time on a trip there and back again.-----------This is a standard example.&nbsp; It may also be seen on Wikipedia if you want a different statement of it in a larger context.<a href="https://en.wikipedia.org/wiki/Twin_paradox#Specific_example" target="_blank">https://en.wikipedia.org/wiki/Twin_paradox#Specific_example</a></blockquote>

[QUOTE="Umbran, post: 7547195, member: 177"] Okay, finished a thing. I can take a moment on this. The trick is to not think of this as "one twin stays at home, and one flies out for a while". Think of it as "one twin stays at home, the other goes to a place and comes back" Imagine that we have twins. One stays on Earth, and the other is sent to Alpha Centauri, 4 light years away. They will travel at 0.8c. We will assume that the time to reach that speed, and to turn around and re-accelerate toward Earth, are negligible. This is not true, but it is demonstrative. Numbers have been chosen to make them easy to work. From the point of view of Earth (Earth's rest frame), the ship is traveling a total of 8 light years, at 0.8c, and so will come back in 10 years - everyone on Earth will age 10 years before the ship returns. (People on Earth expect that time on the ship will be reduced by a factor ϵ = sqrt (1 − v^2 / c^2) . ϵ=0.6, so the ship will experience 6 years of time.) Now, the rest frame of the ship is *moving* with respect to the rest frame of Earth. And time dilation is not the only effect of travel near the speed of light! We talk about time more, because time is mysterious, but distance along the line of motion is also contracted! When the ship is flying, it looks to them like the distance to Alpha C. is, compared to that measured on Earth, contracted by the same ϵ=0.6. So, the distance to them is only 2.4 light years each way! And, to travel the round trip of 4.8 light years at 0.8c takes... 6 years! People on the ship *agree* that they will experience less time on a trip there and back again. ----------- This is a standard example. It may also be seen on Wikipedia if you want a different statement of it in a larger context. [url]https://en.wikipedia.org/wiki/Twin_paradox#Specific_example[/url] [/QUOTE]

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