Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Albert Einstein's Theory of Relativity

  1. May 13, 2010 #1
    Hi, i'm really having a hard time grasping some of the concepts of this theory. Yes i'm not a really clever person but I enjoy learning and need abit of help. My question is probably very basic to you guys but I need some confirmation that im thinking the right away about this. I have been reading something I found on the net..

    "We have Bert and Dana. Take a bus, and put Bert on the bus. The bus goes down the road. Dana, she sits here, on the side of the road. He's in the bus and she's on her ***. And now take a rock off of the moon, and let it fall at them. It hits the air and cuts in two. The two bits burn, and then land just as Bert and Dana are side by side. One hits the dirt up the road a ways, and one hits down the road a ways. Dana sees each rock at the same time, but Bert sees one rock and then sees the next rock. Now: if Bert and Dana both see Dana as the one who is "at rest", they both will say that the two bits came down at the same time. Dana will say, "I am 'at rest', and I saw them both land at the same time, so they both did, in fact, land at the same time." And Bert will say, "I move away from the rock down the road, so when I add that fact in, I can see that if I were 'at rest', I'd have seen both land at the same time. "

    Now my line of though is that if bert is moving away from the rock it will take him longer to see it because the light has to travel a bigger distance back to his eye before he realises it's hit the ground. The rock infront will seem as though it has hit 1st because the distance is shorter to him, meaning the light will come back to his eye faster. Giving him the effect that one has hit the ground before the other one.

    Is this correct or am I thinking about this the wrong way completey? I realise that the speed of light is so fast you would hardly notice a difference but what if the bus was travelling very very fast. Then the distance would be greater and make the light travel a larger distance back to the eye, resulting in some time difference which was noticable to the human eye. Is this what this anolgy is trying to get at?

    Many thanks in advance.
    Last edited: May 13, 2010
  2. jcsd
  3. May 13, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    It's true that since Bert is moving away from one rock landing that the light from that landing takes more time to reach him. (And the light from the other rock landing, since he's moving towards it, takes less time to reach him.) After you take the light travel time into account, Bert will conclude that according to him the rocks really did land at different times.

    The reason that Bert says the rocks landed at different times is not simply because the light from one landing traveled more or less distance than the light from the other landing.
  4. May 13, 2010 #3
    Ok, I'm going to think about this while at work and try to work out the real reasons behind this. Thanks mate.
  5. May 13, 2010 #4


    User Avatar
    Gold Member

    As Doc Al pointed out, effects arising from transmission delay are not part of special relativity. The substance of special relativity comes into view only after transmission delays have been taken into account.

    Unfortunately, quite some introductions to special relativity do conflate transmission delay issues and relativistic issues. So my advice is: when reading an introduction you recognize that in some form or another transmission delay is interwoven, drop it and move on.

    A good introduction will maintain separation between taking transmission delay into account, and the things that are unique to special relativity.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook