Awesome demonstration! Mr. Greene's enthusiasm in teaching, his ability in simplifying troublesome concepts and also the quality of the animations used is truly top class! It's unbelievable that this super high-quality content is available free!!

I’ve found this site recently and very much enjoy your explanations on various physics topics The twin paradox has been of interest to me since I was in a Special Relativity (SR) class over 60 years ago. At that time, the Minkowski FT diagram was used to explain the paradox, and even though I could follow the math, I was never satisfied with the explanation. About 15 years ago, I thought I had a nice explanation of why there is no paradox, which also gave exact calculations. I found you have explained this extremely well graphically in your modules “the twins communicate.” This not only explains why there is no paradox, it explains what the clocks are seen to be doing during the trip. There is no need to include accelerations or general relativity (like many videos claim) to provide a solution. While these can affect clock rate, they cannot account for the twin paradox. Time dilation is all that is needed. I had a couple of comments, which you may or may not have already noticed, but would make a nice addition to the video. One is that the age ratio of the twins at the end of the trip is exactly the time dilation factor, gamma, = 1/(sqrt(1-v^2/c^2)), no matter what v is. The one who turns around ages less The other is the use of the term “time jump”, when explaining simultaneity. Many videos interpret this as meaning there’s a clock jump somewhere in the twin paradox, which is incorrect as demonstrated in “the twins communicate”. The explanation there is right-on, In my opinion.

It is fascinating to see graphically the relative progression of each observer's experiences: At the turn around point in Gracie's journey she has aged five years and sent out 5 annual greeting messages but received only 1 from George. George on the other hand has aged thirteen years and sent out 13 annual greeting messages but received only 2 from Gracie. But what an eventful homecoming and reunion!