Originally Posted by Xei
Okay, well, let's say planet D is one light year away. So from your stationary perspective it takes one year for light to get there, and another year for it to get back  2 years.
Call your position the origin of frame S, and your twin's position the origin of frame S'. They are moving at velocity 0.8 relative to you, so in your frame, S, their path is x = 0.8t, and they reach planet D when x = 0.8t = 1 lightyear, which is when t = 1/0.8 = 1.25 years. So to find the time for their round trip from our perspective we just double this, which gives us 2.5 years.
To find the time in their frame, S', when they reach planet D, we can just use the Lorentz transformation. t' = γ(t  vx) = γ(1.25  0.8*1) = 0.45γ, and for v = 0.8 we have γ = 1/sqrt(1  v2) = 5/3. So t' = 0.45*(5/3) = 3/4 years, and again we just double this to find the time for their round trip from their perspective, which gives us 1.5 years.
Which goes back to my original post... Einstein said, and yes, it has been proven, that the speed of light is C is OUR REFERENCE FRAME. There has not been 1, notta, zilch, experiement ever done to confirm that C is constant in ALL frames. Of course we, and every experiment ever known to man, will always show light to be C. It's obvious. And it's obvious that my twin, zooming towards D @ .8C, will measure the distance to be smaller. In a way, what we have done, is take a static distance observed in one frame, and then added a dynamic element to it (my MOVING twin). And then came up with a mathematical equation to balance both frames of reference so they're valid, to both observers. It's apples and oranges. Yes, the math works, and SR explains the observer dependant nature of the universe. But, in reality, does my twin really come back younger. And again, as I stated earlier, if you use some of the math, you have to use it all. Does he comes back exponentially heavier and exponentially thinner!?


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