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New animated video explaining Special and General Relativity.

  1. Nov 30, 2011 #1
    Hi Everyone.

    I have just created an animated video on Special and General Relativity, explaining how all the conclusions are derived from the fact that light has the same speed for all observers.

    It is at:

    I put a lot of effort into making the animations, so please take a glance at this, and please pass it along to anyone who you think might be interested. I think it is great both for educational and entertainment value.


    Please note:
    Those are two underscore "_" characters in the YouTube link:
  2. jcsd
  3. Nov 30, 2011 #2
    I liked this. It's good. However one thing I would have said is that the first rule applies only to things moving at a constant velocity. I showed it to a friend who doesn't have a scientific education and the first thing he said was: That isn't true, I know my car is moving because I can feel the acceleration.

    PS. I recognise the graphics from various sources. I'm sure it's all OK but do check the copyrights. The ship is from Flash Gordon but I guess that's old enough to have expired. :-)
  4. Nov 30, 2011 #3
    Actually, I deliberately decided to leave out the phrase about constant velocity. I explain the case of acceleration 12 minutes into the video, and how you account for it by thinking that there is a gravitational field causing you to be thrown back into your seat, and causing the rest of the Universe to accelerate around you.
  5. Nov 30, 2011 #4
    And I don't think anyone is going to complain about the copyrights of the background photographs, since those images are all over the internet.
  6. Dec 1, 2011 #5


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    Are you going to modify your video based on feedback you get?
  7. Dec 1, 2011 #6
    @6:02. There are three ships moving at the same relative velocity with respect to Sarah. They are at rest wrt each other. Wouldn't Sarah measure the same time dilation for all three ships? Somehow, though, time is running faster on the left ship wrt the ship on the right. I'm a little confused.
  8. Dec 1, 2011 #7

    It depends on what kind of feedback it is. Making changes to the animations is very time consuming, so I am inclined not to make changes. Though, of course, I would make changes if, for example, someone pointed out a factual error in the video.
  9. Dec 1, 2011 #8
    She would see the time on all three ships running at the same speed. However, she and Adam would disagree on the simultaneity of events on the three ships. That is, Sarah would see the clock on the left ship running ahead of the clock on the right ship, even though she would see both clocks running at the same speed.
  10. Dec 1, 2011 #9
    Hey guys, if you enjoyed this video, can you please do me a favor and click the "like" button on it, so that people will be more likely to find it when they do a search on YouTube. One person apparently just clicked the Dislike button on it, and I need a to have a high ratio of "likes" to this one dislike so that people will be able to find it.

  11. Dec 1, 2011 #10
    Thanks, I understand. I was confused by the phrase "time running ahead".
  12. Dec 1, 2011 #11
    Ya but by that point he'd already given up because it was in HIS opinion "obviously wrong".

    In my opinion teaching is primarily a communication skill first and foremost and there's a difference between teaching and stating facts. I for example am quite good at some things but people can't learn from me because I don't know how to communicate the information to them effectively. Mind you, the guy in question does have learning difficulties so perhaps he isn't a typical student. Anyway good luck with video and hope it does well.

    edit: Let me give you an example with my housekeeper and baking. She didnt go to a very good school and wasnt taught metric. She doesn't know how volume and weight are linked and the fact that 100ml of water weights 100g. Now most orgnaic stuff is like 90%+ water so it's a fair approximation that 100ml of milk also is 100g. I tell her this, and assume it's taught.

    So when it comes to baking she sees milk + eggs. Now it is completely obvious to me that you first break the eggs into a bowl then add equal weight OR volume of milk and you do it in that order because you can pour smal amounts of milk but eggs must be added in discrete amounts being the size of the egg (about 55g).

    Then she watches a chef on TV. He breaks an egg into a glass, sees how far it comes up the glass, then adds milk to double the level in the cup. She goes "OMG that's amazing and so quick, why didnt you tell me that? I've been weighing it all the time".
    Last edited: Dec 1, 2011
  13. Dec 3, 2011 #12


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    When you give an example to illustrate the meaning of Einstein's first postulate, you have Adam tossing a ball straight up and down but the ball appears to follow roughly an upside down V as Sarah views it. Then when you illustrate Einstein's second postulate, you have Adam sending a beam of light to mirror which to him goes straight down and up but Sarah views it as following a V path. This is not an illustration of the second postulate but simply another illustration of the first postulate.

    If you are really serious about making a video that will teach relativity, you should make it clear that neither Adam nor Sarah can watch the progress of a light beam. Unlike watching the progress of a tossed ball which uses light that travels so much faster than the ball, there is nothing with which to watch the progress of a light beam. Your animation would be so much better if you had the beam of light progressing horizontally from a source at the front of the space ship to a mirror at the rear and then propagating back to Adam at the front. This will show that from Sarah's point of view, the light takes an unequal time to make the trip in one direction as it does in the other direction but Adam can't know that. All he knows is how long it took from the time the light left him until the time it got back to him. Then, knowing how far away the mirror was, he can calculate the round trip speed of the light. If you want, you can point out that he will get the same answer whether his ship is parked on the ground or traveling at a high rate of speed but you will have to bring in the length contraction to make this work.

    But to illustrate the second postulate, you want to have Adam put a clock at the rear of his spaceship where the mirror is and have him set the time on it so that when the light reflects off the mirror, it is midway between the start and stop time of the light beam at his location. This is what is meant by the second postulate and it is how Adam synchronizes all the clocks that are at rest with respect to himself.

    Do you understand the distinction I'm making?
  14. Dec 3, 2011 #13
    Even though they can't watch the progression of light, they can still measure the time elapsed between when the light is first emitted, and when the reflected light is received. So, I think that the example I used is just as good as the one you proposed.
  15. Dec 3, 2011 #14


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    Yes, and that is covered by the first postulate. If you are not going to illustrate the significance of the second postulate, then you aren't teaching Special Relativity. I just hate to see a great opportunity passed up.
  16. Dec 3, 2011 #15
    You lost me. I thought this clearly explained why time slows down for moving objects. Both Sarah and Adam measure the time it takes in between when the laser is fired, and when the reflected light comes back. And since they measure different times, due to the fact that different distances are involved, the conclusion is that their clocks are moving at different rates.
  17. Dec 3, 2011 #16


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    That's not the significance of Special Relativity. That was also concluded under Lorentz's Ether Theory prior to Einstein. Everything that you can measure and observe can be explained by either LET or SR. It's Einstein's second postulate about what cannot be measured that establishes his Special Relativity.

    Also, it's not Sarah's observation of what happens when Adam is moving and performing his light bouncing experiment that is what is important. It would be very complicated to correctly depict what is going on there because it involves relativity of simultaneity which is a conclusion of SR after you invoke the second postulate and clock synchronization and frames of reference. Instead, she needs to do a similar experiment with her own stationary clock and mirror and that's where she gets a shorter time than the time that Adam got even though the distance is the same and the light travels at the same speed in both depictions.
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