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SpaceTime.. Why can we access past but not future events

  1. Feb 15, 2016 #1
    If general relativity, space and time are postulated as a 4D non euclidean space that exist absolutely. All past, present, and future events are static moments in this model. In this model, the present moment can access and has knowledge of past events, but cannot access or have knowledge of future events (at least not in my experience). It is also stated that other observers can have access to events in my future that I myself do not have access to.

    My access to events in the past is not constrained in any way.. but it is bounded by the limit of the speed of causality into the past. Some events in the past are simply too massively far away and have been pushed even farther away by the inflation of space itself.

    I find this model has a HUGE ('uge if you are donald trump) burden of credibility as it cannot even begin to explain why visibility into the space 'SpaceTime', is uni-directional. If it postulates that events are visible in my future to other observers yet those events are not visible to me, when I can see so many events into the past yet cannot see into my future, even if others can.

    Hmmm..... no idea how to think about this.
  2. jcsd
  3. Feb 15, 2016 #2


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    Words like "exist absolutely" and "static moments" don't have any well-defined meaning in this context. Your question would be improved IMO if you removed this material.

    Your question actually doesn't have any relativistic elements. Newtonian spacetime can also be described as a 4-dimensional space. (There is one difference between the way this plays out in Newtonian spacetime and relativity, which is that in relativity we have a light cone, and the question becomes why we can remember the past light cone but not the future light cone. But this doesn't fundamentally change much about your question or the answer to it.)

    Your question asks about the psychological arrow of time, which is what allows us to remember the past but not the future. The psychological arrow of time originates from the thermodynamic arrow, which in turn originates from the (currently unexplained) fact that the big bang had low entropy.

    A pretty good review article on this topic is this:
    Callender, Craig, "Thermodynamic Asymmetry in Time", The Stanford Encyclopedia of Philosophy (Fall 2011 Edition), Edward N. Zalta (ed.), http://plato.stanford.edu/archives/fall2011/entries/time-thermo

    The reason that the arrow of time requires explanation is not that spacetime can be described as a four-dimensional space. The reason it requires explanation is that the microscopic laws of physics are essentially time-reversal symmetric. "Essentially" is because the weak force violates this symmetry, although I don't think that really affects any of the argument. The Callender article addresses this.
  4. Feb 15, 2016 #3


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    In addition to the other nonstandard terms that bcrowell mentioned, this one is also a little confusing. If "access to" means "can receive information from" or "can be caused by" then the same thing happens in relativitistic and non relativistic physics. In both cases causes always preceed effects. The relativity of simultaneity does not change that.
  5. Feb 15, 2016 #4

    Mister T

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    Events in your future haven't happened yet. No one has access to them.

    Events in your future can be simultaneous with events in my present, but then they'd be causally disconnected from me. In other words I can't access them in the following sense: They are separated from me in space in such a way that even at the fastest speed possible notification of them won't reach me before they happen to you. I therefore certainly wouldn't be able to be present when they occur, either.
  6. Feb 15, 2016 #5


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    As Dale has pointed out, the word "access" doesn't have any well-defined meaning here. This isn't a physical argument, just a description of how non-physicists would view the question.

    It's true that there are events outside the light cone, but I don't see any logical connection between this and the OP's question.

    Rather than trying to reinvent the wheel, you might want to read the review article I linked to in #2. It's not as though nobody has ever thought about these issues before.
  7. Feb 15, 2016 #6


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    Where is this stated?
  8. Feb 15, 2016 #7
    Perhaps, what he is trying to express is that there are some future events which are inside someone's light cone but not inside his light cone.
  9. Feb 15, 2016 #8


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    Since it's non-standard, I'll provide my own interpretation: if by "access" to actually interact with, no. If he means in memories or in photos, that doesn't qualify as "access" to me. Those events and the viewing of those events is gone. The photos and memories are just records. However:
    I don't see any cause for concern here. That's just relativity of simultaneity. If you are uncomfortable with it, just remember that you only ever "access" your own present and you can choose to interpret that all events you see (the act of seeing) are in your present regardless of when the signals were sent.
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