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Alexander Roth: on 9/11/14 at 16:08pm UTC, wrote Dear Cristi, Thank you for your review. I have reviewed your videos. ...

Cristinel Stoica: on 9/11/14 at 9:42am UTC, wrote Dear Alex, You used a very simple example to explain three different...

Marc Séguin: on 9/2/14 at 3:58am UTC, wrote Alexander, Your video needs to be commended, because it is quite a...

Alexander Roth: on 8/18/14 at 16:30pm UTC, wrote Hi Doug et al, I really appreciate your comment, especially on my birthday...

Douglas Singleton: on 8/17/14 at 23:48pm UTC, wrote Dear Mr. Roth, Very nice video! Using a single example you are able to...

Alexander Roth: on 7/17/14 at 20:51pm UTC, wrote Video Image Video URL ...


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August 19, 2022

CATEGORY: Show Me the Physics! Video Contest (2014) [back]
TOPIC: Special Relativity --- Just Simple Geometry by Alexander Roth [refresh]
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Alexander Roth wrote on Jul. 17, 2014 @ 20:51 GMT
Video Image

Video URL

Video Description

With nothing more than the Pythagorean Theorem we can develop an intuitive grasp for the basic time and space concepts revealed by Special Relativity. These concepts help provide a foundation for understanding the Lorentz Transformation Equations (which are not included in this video).

Video Creator Bio

My background is engineering with graduate studies in engineering and physics.

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Douglas Alexander Singleton wrote on Aug. 17, 2014 @ 23:48 GMT
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

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Alexander Roth replied on Aug. 18, 2014 @ 16:30 GMT
Hi Doug et al,

I really appreciate your comment, especially on my birthday (86). Thank you.

It hadn’t even occurred to me to consider any rotational effects. That would be far afield from the goal of producing a solid, simple, conceptual understanding of SR. I would certainly have set aside any rotational effects since that would introduce radial forces which might look like gravitation (if you were in a rotating windowless Einstein elevator) and would complicate a simple picture and drag us into GR. Same for the transverse doppler --- an un-necessary complication.

At some point I would like to add to my 15 year old video a generalization of the Pythagorean equation to become a simplified version of the Lorentz equations.

If this stuff interests you, you might find my blog at of related interest.

Thanks again,


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Member Marc Séguin wrote on Sep. 2, 2014 @ 03:58 GMT

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!


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...

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Cristinel Stoica wrote on Sep. 11, 2014 @ 09:42 GMT
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,


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Alexander Roth replied on Sep. 11, 2014 @ 16:08 GMT
Dear Cristi,

Thank you for your review. I have reviewed your videos. Both very well done and the black-hole one exceptionally interesting (breaking new ground?).



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