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Perspective and Time Question RE: Green's Fabric of Cosmos

  1. Mar 4, 2009 #1
    Hi there...just got through chapter 6 and I'm having some issues fully grasping the concepts...

    I hope I can fully convey my question coherently. Please bear with me, I'm not a physicist! This might sound pretty dumb to you who are more experienced... ;)

    Greene is saying that the "now" slices are reliant on our perception and the speed at which we are moving. I was having trouble understanding how Chewie's movement would alter his now slices...

    Shouldn't a "now" slice encompass where everything is at a specific moment in time, regardless of our perception of when we see it? Say the universe is dark and we are blind...things are still occurring around us. Aren't Person A and Person B at locations X and Y whether we're looking at them or not?

    I know that time slows down for someone moving faster...is that why Chewie's "now" slice starts including other things?

    Sorry for all the confusion...I'm going to try to read through again, but is there another description of this that might supplement? I feel like I'm missing something...
  2. jcsd
  3. Mar 4, 2009 #2
    Yes, and that is exactly what they do. The entire point is that different observers disagree on which set of events qualifies as "now." This has nothing to do with when Chewie perceives anything. It's after all that is taken into account. There is no such thing as an agreed-upon "specific moment in time." What qualifies as a single "moment in time" depends on reference frame.
  4. Mar 4, 2009 #3


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    Basically the now slice is entirely conventional. When we see "live" coverage on TV, it is of course not "live". It took time for the light to travel from there to space to your TV. So it took place "then". When was "then"? One way to assign "then" is to say that it was "now" minus the amount of time light took to travel to you (in a straight line, not by bouncing round the world). This method of assigning "then" clearly depends on the speed of light.

    In "Newtonian" physics or ordinary intuition, if you run towards an object, that object will be coming towards you faster than if you were stationary. Curiously, this is not true for light. The speed of light, so crucial for assigning "then", is constant no matter how fast you run towards it. Because of this strange fact, the assignment of "then" is dependent on how fast you are running.
  5. Mar 5, 2009 #4
    Thanks for the replies. I'm still confused though...

    I am having a hard time seeing how now slices are affected by slow movement over vast distances. I see how time slows for objects moving near the speed of light, but I find it difficult to comprehend how time is affected by vast distances.

    Specifically, Greene lost me on page 137 (of my edition) where he's explaining how Chewie's walking away from earth causes Lincoln's assassination to be on his now list when he stands up to walk away...EVEN IF this is only added to his now list billions of years later. Then how Chewie's walking towards earth puts the 2100 presidential election on his now list.

    Is special relativity only a matter of perspective, or does it actually alter spacetime for the different observers?

    I guess what I really need is another explanation of this "now list" concept so I can get a different angle on the theory. Greene is great but sometimes his writing really loses me.

    Trying to understand this is like trying to balance spinning plates! :eek:
  6. Mar 6, 2009 #5
    Try looking at figure 5.4 a) and b) at the bottom of 137. I think that picture is there specifically to give you a pictoral answer to your question
  7. Mar 6, 2009 #6
    hg: I've read FABRIC several times and each time I get to the section you question my eyes glaze over....I still have a number of question marks on pages 134,135, 136....while I'll readily admit I don't get what Greene is explaining at a visceral level as I would like, the key is on page 134:
    in other words, distance magnifiies the distinction between two observers conceptions of now...and that includes magnifying even moderate differences in speed....

    Give it some time to sink in....I have not seen other analogous descriptions...reread again next month...and maybe again.....come back again and explain it to me!!!!
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