The Twin Paradox Explained and Resolved





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Published on Nov 29, 2012

This video explains what the twin paradox is. An identical twin travels very fast and when she returns her twin who stayed on Earth has aged more. This is due to special relativity. The paradox is trying to understand why the Earthbound twin ages more than the twin who travels in the rocket. Why not the other way around? One standard explanation is that the symmetry is broken. That explanation, by itself, is not good enough. This video goes into more detail.

The Teaching Company (a.k.a. The Great Courses) has this video course that will explain relativity to just about anyone.

Einstein's Relativity and the Quantum Revolution: Modern Physics for Non-Scientists, 2nd Edition

This book might also be helpful.

Relativity Simply Explained

I was not clear in the video what I meant by "seeing" your twin's clock run slow. When they are approaching each other and getting high frequency pings you could argue that they are seeing each others clock run fast, and this is true in one sense of the word "see," in fact the one we usually use, one involving photons hitting us. When I say they each "see" each others clocks run slow I mean if they could see it instantly, without having to wait for the light to travel to them.

When we look at a clock on the wall that is 10 feet away we are actually seeing it as it was 10 nanoseconds in the past. In the video when I say "see" I mean we pretend we don't have to wait for light to get to us. For Earth twin to "see" rocket twin's clock run slow when she is near the star we have to pretend that we don't have to wait the 10 years for the light to get to Earth. When we do this Earth twin "sees" rocket twin's clock run at the same slow speed both before the turn around and after the turn around. So for "seeing" (in quotes) we don't have the Doppler effect or delay of light travel.

When you take a relativity class they do this by imagining placing synchronized clocks everywhere in your frame of reference which is moving at a constant rate relative to some other object of interest. In your frame of reference you are not moving (and all your clocks are not moving) and that object is. You then "see" the single moving clock (the one attached to the moving object) by comparing it to the clock in your frame of reference that is next to it at the time.

When rocket twin changes reference frames at the turn around she must have a new set of synchronized clocks. Something happens at this event which changes things in a way I don't fully understand.


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