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

Simple Special Relativity

  1. Jul 21, 2006 #1
    Hello, I am currently doing an essay on time and special relativity.

    I just have a few questions,

    1) In many places, they say that time slows down when travelling very, very fast because of the image in the attachment.

    I understand that if the clock is moving, then the thing hitting either plate is going to have to travel further (the hypotenuse), and hence take longer, but because light always has to travel at the same speed, then time needs to slow down.

    But what I don't understand is why is the hitting of either plates "time", is this an analogy? If it was just a normal clock with no physical moving parts then surely this would be wrong?

    2) Secondly, if a space ship was travelling at 1000 m/s in space and it had its lights on, then it maybe common sense that the light would travel at 300,001,000 m/s.
    But of course because the speed of light is constant, the light can not travel any faster.

    Would I be right to say, because of s = d/t
    And because the speed has gone up, then the time must increase to cancel the initial increase?

    Attached Files:

  2. jcsd
  3. Jul 21, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Use the principle of relativity
    to compare the behavior of this "light clock" with a "normal clock" placed next to it.
  4. Jul 22, 2006 #3
    Hello, thanks for the reply.

    But I don't quite understand what you mean?

    Which of the things to describe why time dilates is correct above?
  5. Jul 22, 2006 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Time slows down relative to a clock that is moving at a different speed.

    Presumably, the clock is "ticking" each time the object hits the plates, thus determining the time. Where have you ever seen a "normal clock with no physical moving parts"?? Of course, when we say "time slows down" (relative to some observer) we mean that everything slows down: the observer, person A, sees the observed, person B, breathing slower, heart beat slower, etc. Of course is B is also observing A, he see's A's clocks running slower, heart beat, etc. slower. The situation is completely symmetric.

    Why would it be common sense to say that? You seem to be assuming "if A is moving at speed u relative to me and B is moving at speed v relative to A, then B is moving at speed u+ v relative to m". What reason do you have to assume that?
    Last edited: Jul 22, 2006
  6. Jul 22, 2006 #5
    Blah Blah
    You are not paying attention to what you are ‘seeing’.
    Your attachment is not a good one and in my opinion is just WRONG, or at least incomplete.
    It is not making clear what it is measuring when it says 10 units of time on a ship when it looks yellow and 8 units of time when it looks red.
    Let us assume it takes 10 units for light to cross from one side of the light clock to the other side. Then the number on the red ship is clearly wrong – time does not change on the ship just because it is moving! They don’t have two clocks on board one the “stays correct” and one that slows down. Light still travels that exact same distance in the same 10 units as before, NOT the 8 units you show in your diagram.

    What you’re not paying attention to or showing in your diagram is how long you as the observer see time passing, you need to post that on your diagram as well.
    When the ship is yellow you are ‘seeing’ the light travel the distance straight up in 10 units of time.
    But when the ship has turned red you see the light travel a longer distance along (the hypotenuse) as you are “seeing” it. Now since light speed is fixed the total time you measure for this is 12.5 units to cover that longer distance.
    Therefore as you see it ‘your clock is running faster’ or the ‘ship clock is running slower’ – it is just easier to describe it as the ship clock is slow “relative” to yours. The next important part to be clear on and explain in your homework essay is any observer on the ship we say exactly the same thing about your clock!

    Note: Work out how far up (the hypotenuse) you get in 10 units of your time, and what that means on the ship.

    Also note: there is an area in the forums here for getting help on homework problems like this; a mentor/advisor might move this thread there where it belongs. For future reference it is in the top forum under Science Education.

    Good luck on your essay.
  7. Jul 22, 2006 #6
    Ok, thanks for both of your replies, I have read alot of sources, like from Wikipedia, How things work etc...

    And I still don't understand the crucial thing.

    I have already got examples and what happens, to an observer moving at a slower speed than the other person moving fast, the person moving fast seems to be doing everything in accelerated motion. But he sees the slower moving person doing everything in slow motion (is that right?)

    So why does time slow down when you move?

    I know its a big question but I just don't understand some of the things on Wikipedia and I often miss the point.

  8. Jul 22, 2006 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  9. Jul 22, 2006 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    And, once again, time does not "slow down when you move" because all motion is "relative". You can always choose a coordinate system in which an object is or is not moving. Time slows down for an object as it is observed from a coordinate system relative to which that object is moving. The whole point of "relativity" is that you have to be constantly specifying what the motion is "relative" to!
  10. Jul 22, 2006 #9
    Sorry but not all motion is relative. As soon as you apply a force all the odds are off. :smile:
  11. Jul 22, 2006 #10
    Huh? ........
  12. Jul 23, 2006 #11

    I've read alot up.

    So I can explain time slowing down purely in terms of the light clock?

    Would this be correct, if somebody is stationary relative to the moving light clock, they will see the light bouncing up and down vertically but also moving horizontally, and hence, the light moves diagaonally which is a greater length to the original up and down.

    But because light is always seen at the same speed to all observers, then because of the equation, d = c x t , then time will have to increase if the distance has because C cannot increase.

    Is that correct as to why time dilation occurs?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Simple Special Relativity
  1. Special relativity (Replies: 1)