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Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?
« on: 31/03/2020 17:17:53 »One thing about this so-called paradox that is worth a little thought touches on the very basics of how we think about time travel.In no sense has he traveled into Emily's past. He just experienced 20 yrs less time than she did between their separation and rejoining. Relativity never claims that Dan travels into the past, only that the time interval he measured between the two events was shorter. Time passed differently for the two. If Dan left Emily when both their calendars read 2100, then when they met up again, Emily's calendar would read 2150, while Dan's would read 2130.
Dan and Emily are twins, and Dan makes the obligatory journey, after which he will be 20 yrs younger than his sister. Fast forward to the point of return in (say) the year 2150. Dan has aged 20 yrs less than Emily; he is at the stage she would have been at 20 yrs earlier. It is easy to think of this as his having travelled 20 yrs into his sister’s past, but is that the wrong way of looking at it? Both are at the same point in time – 2150. In what sense has Dan travelled into the past?
When They meet up again, it is 50 yrs into Emily's "future" and 30 yrs into Dan's future.
When you state that they are are the same point in time, 2150. you are implying some type of absolute universal nature to time, and that 2150 is the "real" time. It is only 2150 by the Earth clocks and this no more the "real time" than the 2130 according Dan's clocks. It is just that when they meet up again, Dan agrees that by the Earth clock it is 2150 (twenty more years passed for Earth than did for him.)
There is the time as measured by Emily, and there is the time as measured by Dan. and there is no more meaning to time than that in this scenario.
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Again, the SR twin scenario does not claim any travel "into the past".
Consider another scenario. This time there is no journey into space, but in 2150 Dan uses a time machine to travel back 20 yrs. He meets Emily. She is as she was in 2130. He has travelled 20 yrs into her past and she is younger, not older, as is the case in the former scenario.
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Once again. SR does not imply "time travel" other than the time travel we all experience everyday from this moment to the next. What SR does say is the there is no universal meaning to "the passage of time". Every inertial reference frame measures time by its own independent standard, and that is the only meaning to "time" there is.
Dan comes back from space younger than Emily. Pop Sci books and on-line discussions provide the "hitch-hiker" with abundant explanations for that. However, if, instead of the high-speed journey, Dan had been placed in some sort of “stasis chamber” in which his development and ageing had been halted for 20 yrs, the result in 2150 would be the same as in the first scenario. He would appear to be 20 yrs younger than Emily. So, would we claim that he had time-travelled?
It's like the notions of left and right. Everyone's "left" and "right" is unique to them. If we are standing next to each and not facing the same direction, my "left" will not be the same as your "left'. And there is no "universal" concept of "leftness".
Putting Dan in a stasis chamber, while he stays at rest with respect to Emily might, for Dan, "seem" the same personally, but only If Dan were not allowed to measure what was happening outside his chamber. It might "mimic" the end result of Relativistic effects, but it wouldn't be the same.
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