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Is the Future Mapped Out ?

  1. Feb 16, 2012 #1
    I have a basic grasp of special relativity and understand why different observers will have different perspectives on the timing of events. My question is this, if Observer 1 sees two events occuring which change in time i.e of 2 lights opposite each other with the same color which change simultaneously, so red-red blue-blue green- green, and an observer moving at speed relative to Observer 1 may see red-blue blue-green green-red due to the change in angle of his time slice

    Both obervers are justified in saying that their observations are correct ( equivalence principle ?)

    Does this mean that for a moving observer 2 the observations of observer 1 are his past, and for observer 1 they are his present and the observations of observer 2 are observer 1's future

    So in effect Observer 1's future as "already occured" as observer 2 is experencing it.

    Am i living in someone elses past and someone elses future ? If so when does the future stop being mapped out ? Is/Was the whole of all time ( past future and present) concieved at once ?

    Clearly i am blaberring now and there is a good reason why the future is not already mapped out !! Please enlighten me !!
     
  2. jcsd
  3. Feb 16, 2012 #2
    Good job, paulselhi! Not everyone notices that implication of special relativity. You have nailed it pretty well. And yes, it is all of time as described by Hermann Weyl (Einstein's close friend and colleague at The Princeton Institute for Advanced Studies).

    You are not blabbering at all, paulselhi. However, that kind of discussion is sometimes not regarded as appropriate for this forum.
     
  4. Feb 17, 2012 #3
    Note that the farther away the observers are, the greater their discrepancy, even if they are moving very slowly relative to one another.
     
  5. Feb 17, 2012 #4
    Does the speed of light limit mean that the possible future-past range may be limited?
    So if observer 1 sees an event happening there would be a limit as to how much into the past or future another observer can reside since these differences in observations will be based on the other observers speed relative to observer 1

    And as .. err.. the last poster mentioned that distance is relevant, would the size on the universe put a cap on this as well

    Thanks BobC for turning me on to Herman Weyl

    "The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line of my body, does a section of this world come to life as a fleeting image in space which continuously changes in time." Hermann Weyl, Philosophy of Mathematics and Natural Science (Princeton University Press, Princeton, 1949)

    "What has been will be again, what has been done will be done again; there is nothing
    new under the sun. Is there anything of which one can say, "Look! This is something
    new?" It was here already, long ago; it was here before our time."

    King Solomon
     
    Last edited: Feb 17, 2012
  6. Feb 17, 2012 #5
    Those are pretty heavy quotes there, paulselhi. I was familiar with the Weyl quote but did not realize King Solomon was so knowlegable about special relativity. How did you ever come up with that one? Anyway, nice work.
     
  7. Feb 17, 2012 #6
    In my opinion this is no different then saying my future experience of the sun / moon / anything not in the same location as me has happened prior to my experience of it. whoopty doo
    (yes this is Galilean transformation, but what do I care of the measure of someone elses proper time/length?)

    The next step in the concept is to realize that past, present and future are relative terms, all from the perspective of assigning an arbitrary "now".

    The only part of the future that's "mapped out" is it will become your "now" & then your "past".

    When thinking of SR you should ditch the common conceptions of past / future and now. And think more in terms of cause & effect. From there (cause & effect) consider the constancy of the laws of physics and relativity of simultinaity.

    Cause (present) and effect (future) all according to the laws of physics (mapped out). Relativity of simultinaity; "now" is a relative term, and inturn "future" & "past".
     
    Last edited: Feb 17, 2012
  8. Feb 17, 2012 #7
    I assume that is KS from Israel and not some enlightened Rasta who has been " 'erbing it out"
     
  9. Feb 17, 2012 #8
    I think he hit on something far more profound than that.

    His insight is directly related to Lorentz transformations, not Galilean. And of course no one has to care about implications of special relativity. Some are passionately curious about them and others shrug their shoulders. And no value judgement is involved.

    But, cause & effect just becomes a definition with Weyl's model. One event does not cause another event in the sense that you imply, because it is all there at once. As Weyl said, "The objective world simply is, it does not happen..."
    But, it was the relativity of simultaneity with the different angles of the slices of the 3-D cross-sections across the 4-D universe that let to his recognition of the implications of special relativity.
     
    Last edited: Feb 17, 2012
  10. Feb 17, 2012 #9
    It is not more profound, that is the problem. Thinking it is profound. The profoundness is a consiquence of having a brain, in that we can consider other FoR and think but they have a "now" momnet too and that persons "now" moment is in my past, wow cool!"

    That is my point of who cares if another observers proper time length is of different measurements then mine, what is the implication in the context of past & future? The words their "now" is my "past"? It is of no consiquence to cause/effect.

    So no his insights are not strickly lorentz, it is strickly about overextending a "now" momment to beyond local, where it starts to loose meaning in the more common sense of "now".

    These are measurements we are talking about. You're stretching this to the universe is static via words such as "now" past future.

    A quote such as "The objective world simply is, it does not happen..." without strickly defining the words I think is nonsense.

    The universe IS 4D, there are no "now" "slices", so anything you derive from these "slices" is not speaking of the "nature" of a 4D continuum.

    The direction of this discussion is transparent bobc2,
     
    Last edited: Feb 17, 2012
  11. Feb 17, 2012 #10
    He/she needs to recognize the implication of SR in this context is redefining the concepts of past, present & future. Specificaly "now" / relativity of simultinaity.
     
  12. Feb 17, 2012 #11
    From "The Fabric Of The Cosmos" - Brian Greene (pg 141 hard copy):

    "A particular moment can no more change in time than a particular location can move in space: if the location were to move, it would be a different location in space; if a moment in time were to change, it would be a different moment in time."

    "Under close scrutiny, the flowing river of time more closely resembles a giant block of ice with every moment forever frozen into place."
     
  13. Feb 17, 2012 #12
    Special relativity gives us many "now slices." But the "now slices" for different observers moving at different velocities cut the 4-D universe at different angles--that was the clue that led to the understanding of a 4-dimensional continuum.

    That's why Brian Greene, in his book "The Fabric Of The Cosmos" spent so much time developing the loaf of bread analogy with the slices of bread sliced at different angles. He associated physical reality (the entire 4-dimensional universe) with the whole loaf of bread and associated the different slices with the different 3-D universes that different observers live in at any given instant of time (different "now slices").

    And that's why so many physicists have been captivated by the concept of the block universe. Paulselhi, I think you would be interested to google "block universe."
     
    Last edited: Feb 17, 2012
  14. Feb 17, 2012 #13
    That's a really good point, e^(iPi)+1=0 (how did you come up with that interesting handle?). This was dramatized in Roger Penrose's famous Andromeda Paradox shown below (my graphics version).

    Bill and Ruth walk past each other. Since they are in relative relative motion, they are living in different instantaneous 3-D worlds. In Bill's world the Andromeda leaders are meeting to discuss whether to attack earth. In Ruth's world they have already made their decision and the attack is under way.
    AndromedaParadox_bob2.jpg
     
    Last edited: Feb 17, 2012
  15. Feb 17, 2012 #14
  16. Feb 17, 2012 #15
    As if the analogy you quote is the one that got me to stop "learning" SR from Brian Greene's poetic physics books.

    I'm pretty sure he "spent so much time developing the loaf of bread analogy" because it sells books. It only confuses the reality that physics "describes".

    Lastly, if this is how you believe it is (block universe) then this perspective of a block universe has to also be true for every single bit of anything there is in the universe (that's subject to SR type physics) does it not? Given the relativity of simultaneity this can't be true.
    How could anything move? (note spacetime is isotropic)
     
    Last edited: Feb 17, 2012
  17. Feb 17, 2012 #16

    PeterDonis

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    Actually, no, this is just the principle of relativity.

    No. For the time ordering of events to be observer-dependent, the events must be spacelike separated; each must be outside the other's light cone. That means there can be no causal connection between the two events. So what observer 2 is experiencing can't directly be observer 1's "future", because observer 1 can causally affect his future.

    As nitsuj has commented, Brian Greene's talk about the "block universe" obfuscates this crucial point.
     
  18. Feb 17, 2012 #17

    PeterDonis

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    But this is only true if the distinction between Bill's world and Ruth's world has no causal consequences; in other words, Bill can't causally affect the Andromeda leaders' decision any more than Ruth can, even though in Bill's world the decision hasn't happened "yet" but in Ruth's world it has "already happened". So this presumed distinction actually has no physical content.
     
  19. Feb 17, 2012 #18
    Actually, Brian Greene does one of the better jobs of describing what's going on with special relativity for the layman--and also for some physicists who have not thought too much about some of its implications.

    Yes. And of course it is.

    That's the whole point. Nothing does move in the block universe model. And it's the relativity of simultaneity that implies the block universe.
     
  20. Feb 17, 2012 #19
    This is a popular objection to the block universe model. However, it is totally irrelavent. Theoretical physicists are interested in a model that represents the entire universe. The range of modeling would certainly not be limited to one's personal light cone.

    And when paulselhi was talking about observer1's "future" I think he was referring to the future 3-D cross-section cut across the 4-D universe (not the directly experienced future that he actually experiences with light transmittal information). Just as when talking about an observer's "Now", a convention is typically adapted that assumes we've accounted for all of the distracting details such as speed of light time delays before actually "seeing", etc. Brian Greene makes all of that very clear in his book. So, we know what he is talking about when he talks about the "future" and the instantaneous 3-D cross-section (or slice of the loaf) that an observer "sees" in language that is understood in the context of Greene's discussion. And we know what Roger Penrose is talking about when he compares the events in the andromeda galaxy for Bill and for Ruth in our sketch--the inability of Bill or Ruth to influence events is irrelavent to the fundamental issues related to 4-dimensional existence. These issues are very profound for many physicists.

    We easily have a concept of the sun existing "right now", regardless of not receiving light for another few minutes. We have a concept of stars existing "out there" even though it may be years before their existence is confirmed with the arrival of the light.

    We don't ignore knowledge about the content of the distant reaches of the universe because it is not in our light cone. We don't ignore the theory of quarks because they are not directly observable. We pursue string theory even though it may never be possible to confirm the theory by experiment.

    The block universe is implied by special relativity. It is a logical model. That does not make it a fact of external objective reality. It's not a concept embraced by every physicist. Some physicists don't even embrace any kind of external objective physical reality.

    No. The block universe concept is the crucial point. The outside-of-light-cone issue is a red herring.
     
    Last edited: Feb 17, 2012
  21. Feb 17, 2012 #20
    The physical content is manifest directly and is profound. Whether Bill or Ruth can influence the Andromeda decisions has nothing to do with whether events really exist in Andromeda. Physicists are interested in external physical reality that is independent of observers. What exists "out there" does not depend on observers. The existence of the early universe did not and does not depend on observers. The existence of objects on the other side of the moon does not depend on observers. Just because the event of a rock rolling down the side of a mountain on mars "right now" cannot be affected by us does not have anything to do with the reality of the existence of the rock or the event.

    The no-physical-content presumption just because there is no possibility of intervention from outside the light cone is a red herring argument.
     
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