twin paradox

This is a animated short describing the popular version of the twin paradox. It will be followed by several more of a similar nature that will build to a description of the details of what exactly happens in the twin paradox. This is prompted by several associates who have asked me to attempt a better version than is currently available (even) on the better science YouTube channels - where it continues to be misconceived.

Note: for now I am not treating this as an exercise in experimental physics. This is a discussion of what the theory says, not whether it is true. I may get the the latter issue later.

[captainhd]

Loving the videos! Are you a physicist or a physics student?

[theobligatepedestrian]

It has been complained that the paradox/resolution is not clear enough. I accept that criticism and will attempt to fix it by adjusting this video. The full resolution will come in later videos in this list. But the basics are that -- the paradox is that given in relativity motion is relative and time dilation comes from motion, why can we not say that Alice moves away and comes back, thus concluding that Alice should be younger. This is most apparent if we put Alice and Bob in to two saucers. The resolution is that not all motion is relative. But, this will require several of these short videos to explain.

[Josiejen]

I have a couple of comments on both the video and the comments below. There is talk below of "real time" being like an atomic clock. Also that the result might be different between an atomic clock and a mere analogue clock. Both of these miss the fact that it's the rate of change of time itself that is varying. The accuracy of the measuring device is irrelevant. And this leads on to the premise of the video itself. It's a topic I'm fascinated with and I've listened to many "solution" videos which have been subsequently debunked as incorrect - and I agree with them. I do not currently know the "true" solution. The closest I can get so far is this. We do not have seperate Space and seperate Time, but a single conglomeration of the two called SPACETIME (it may have been Einstein who coined the term "fabric" - to mean they are inextricably linked and must be considered together, never separately). My current understanding is that if you're in an inertial reference frame (for example, the earth in the classic Twin Paradox), then your passage through the Time part of SPACETIME is at a maximum (1 second per second, one hour per hour, one year per year etc), and your passage through the Space part of SPACETIME is at a minimum (you are in an inertial reference frame). But the twin in the flying saucer is NOT in an inertial reference frame, because he has relative velocity for some duration. For want of a better expression, he converts some of his passage through Time (hence his time passes more slowly) in order to move through Space. If you take this to the extreme and he moved at the speed of light (forget the practicalities), he wouldn't move through time at all, since all of the available SPACETIME was being used up to move at the maximum possible linear velocity. In summary, at zero velocity (inertial reference frame) time passes at it's maximum possible rate, and at the speed of light time passes at it's minimum possible rate (zero). Hence, any relative velocity will reduce the passage of time and the closer you get to the speed of light the slower time passes. Next, the classic twin paradox is NOT a real paradox at all. A paradox is something that cannot be resolved. It should be called the APPARENT Twin Paradox, because there is a totally conventional solution. It only appears to be a paradox until you start to understand relativity. The real paradox to me, is that linear velocity is supposed to be relative. The twin in the flying saucer is supposed to be entitled to consider himself completely static the entire round trip. From his perspective it is the Earth and the entire universe that moves away from him when he fires his rocket. When he rotates his saucer to return, from his perspective he's not moving at all - it's the entire universe that rotates 180° and then moves back towards him. Since he has remained static for the entire round trip it should be the twin on the "moving" Earth that is younger. And since the flying saucer twin was "static" the entire time (from his perspective) he will be the older one (or dead). This is the real paradox, because it just doesn't happen in real life (time passes more slowly for an atomic clock flown around the world in a jet, than one that remained static). And I've not heard a convincing explanation to date. I do know the answer isn't acceleration, since there are scenarios where acceleration is avoided completely and there is only constant velocity (outward and return) and the passage of time slowing down still occurs. Maybe my earlier description of SPACETIME above is the real solution, and it's relative movent through space that makes the flying saucer twin younger and not the other way round. I look forward to any solution you present.

[nightbot]

I have this question from everyone - Isn't change of age relative to time and not age or physical appearence??? Say you and your brother are twins and one goes at the speed of light and other is here on earth then the age of your brother who goes would be slowly passing WRT Time and not his body.... He wont become older than you as per earth time but he would become old wrt time as a phenomenon??? Does this make sense to you?