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Time dilatation and 'now slices'

  1. Nov 13, 2011 #1
    i read brian greene's book 'the fabric of the cosmos' a long while ago, and i remember being surprised by the notion that one's 'now slice' of reality can angle toward the past OR the future of someone else's. (toward past if moving away, toward future in moving toward someone who is 'stationary'). this doesn't seem to mesh with my understanding of special relativity, because i thought time dilation occurred the same regardless of what direction you were moving-- just as long as you are moving relative to someone else. hence, the whole twin's paradox, which includes a trip away from earth, then a trip back. i would think that 'now slices' should always angle toward the past as someone moves through spacetime.

    i just watched the nova program (based on the book), and this cognitive dissonance has been reawakened in me. could some please explain, in layman's terms, how this is so? i don't question the truth of brian greene's statements, i just want to fully understand them.

  2. jcsd
  3. Nov 13, 2011 #2


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  4. Nov 13, 2011 #3
    I could be wrong... but... I believe this is an observed effect of the moving observer's surroundings relative to the observer at rest. The further the moving observer looks into space the greater further into the future (looking forward) or past (looking backwards) he will see in relation to the observer at rest. This isn't really a hard concept to grasp when you think about it.

    The clincher here is that when you look deep into space you’re looking deep into the past. The Andromeda galaxy is 2.41 Mlys away. This means that when you are looking at this galaxy, you are looking 2.41 million years into the past. Brian Greene is giving the impression that you could literally look into the future prior to the events occurring (not possible), the only truth here is that you can look into the past after the event occurs.

    If you were to move towards the Andromeda galaxy, you would observer events occurring sooner than the person who is standing still. And vice versa, if you were moving away from the galaxy, you would notice events occurring later than the person standing still. When a star explodes - this is an event. If you observer this event occurring in the Andromeda galaxy, this event occurred over 2 million years ago. Events are observable because the light from that event moves outward into space. Think about that event like a wall that is moving towards you. If you ride a bike towards that wall - you will hit the wall sooner than if you were standing still, and if you move away from the wall, it will hit you later.

    Its simple logic - not time dilation. Brian Greene merely makes it sound fancy by showing you a loaf of bread and cutting it in different angles. He doesn't mention, however, that you are already looking into the past.
  5. Nov 13, 2011 #4


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    I would suggest just drawijng a space-time diagram, scaled such that light beams always travel at 45 degree angles.

    Then just draw a moving observer, a light beam emitted from him towards some object, which gets reflected.

    The event on the moving observer's worldline that's simultaneous with the radar event is at the midpoint between the transmission and reception, and a line from said midpint to the radar event defines the "line of simultaneity" for the moving observer.

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