Special Relativity --- Just Simple Geometry by Alexander Roth

Dear Mr. Roth,

Very nice video! Using a single example you are able to explain various SR concepts (time dilation, length contraction, simultaneity). Two very very minor quibbles (i) the video is a bit heavy on lecturing but it seems most of the videos either lean more on the lecture side (which then more clearly explains the physics if one pays attention) or lean toward the more artistic/video oriented side (which then makes it hard or impossible to exactly understand the physics). Also in this regard I would say our own submission leans to the lecture side. (ii) At 3:49 in the video there is a statement that relativity has no effect in directions that are perpendicular to the velocity (i.e. if the velocity is in the z-direction the Lorentz transformations of the x and y directions are trivial - they are the same in both frames). In the context of the video this is absolutely correct, however there is something called the transverse Doppler effect - even if an object is moving perpendicular to some observer (e.g. an object undergoing uniform circular motion with the observer being at the center of the orbit) there will be some Doppler shift due to the fact that the clock on the object and clock at the center of the circle run at different rates, which will lead to a frequency shift for any signal sent from the object to the observer.

But again these are very minor points (actually more correctly comments). This was a very well made and well explained video.

Best luck in the contest,

Mike, Max, Dan, Simon, Doug

Alexander,

Your video needs to be commended, because it is quite a challenge to present all 3 relativistic effects in less than 9 minutes! The way you do it is quite original, and it is certainly interesting how we can get the time dilation effect simply by considering the vertical motion of the light bubble in the frame of the spacecraft. I have been teaching introductory special relativity to 19-year old students for over 20 years, and I know that what is particularly difficult to explain comes next: the symmetry of time dilation (that makes clock D indicate 1.15 microsecond when it passes clock C reading 1 microsecond), and the de-synchronization of the moving clocks relative to the Earth's frame. As a specialist of relativity, I can follow your demonstration step by step and all is well, but I wonder how the neophyte reacts to your explanation of the "1.15" seconds... and the fact that clock D counter-intuitively starts running before its sister clock B reaches the tower is probably even harder to swallow. However, I don't really know how it could be explained more satisfactorily within the setup of your presentation and the length of your video. :)

I hope your video does well in this contest. Good luck!

Marc

P.S. Thank your for reviewing (and presumably rating) part 1 of my trilogy of videos "This Is Physics". If you have time to view, leave a critique and rate the other two parts (using your author code), it would be quite appreciated! I think many of the participants in the contest have not yet realized that a video needs to get AT LEAST 10 COMMUNITY RATINGS to be considered for the final round, so I have started to actively ask for ratings...

Dear Alex,

You used a very simple example to explain three different relativistic effects, in a very nice video! Congratulations, and allow me to give you the 10th vote, so that you can qualify to the final. If you have time, I would appreciate if you will watch and rate my videos The puzzle of quantum reality and Can a black hole keep a secret?.

Best regards,

Cristi