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Re: Still frames and twins

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  • lasermazer
    ... what I meant was ... back again to the same ... against each other s, and ... explanation to why one of the ... If they re moving at constant velocity in a
    Message 1 of 15 , Sep 30, 2003
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      --- In Fabric-of-Reality@yahoogroups.com, "Eric Cavalcanti"
      <eric@f...> wrote:
      > Thanks, I'll (try to) take a look at those. About the acceleration,
      what I meant was
      > the initial and final accelerations, when they depart and come
      back again to the same
      > reference frame. Without that, they cannot check the clocks
      against each other's, and
      > that (the assymetry of the accelerations) is the usual
      explanation to why one of the
      > twins is younger in the well-known thought experiment.
      >
      > -Eric.

      If they're moving at constant velocity in a closed universe, then
      they can just check that their clocks are synchronized when their
      paths cross, when they'll be at the same point in space and time.
      That way there's no need for either to accelerate.

      Since the answer in the links was that the topology of the
      universe defined a preferred reference frame, I'd guess that if
      their clocks were synchronized the first time their paths crossed,
      then their clocks would still be synchronized the next time their
      paths crossed, *if* they were both moving at the same speed
      relative to the preferred frame. If not, their clocks probably
      wouldn't stay synchronized.

      Jesse
    • Alan Forrester
      ... I think that s a rather deceptive way of putting it. A slightly better way of putting it is that all clocks are physical systems and so must be subject to
      Message 2 of 15 , Sep 30, 2003
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        --- Eric Cavalcanti <eric@...-rio.br> wrote:

        > According to relativity, there is no preferencial frame to describe the
        > universe. So,
        > when we say that the multiverse is composed of many "still frames" of
        > possible
        > universes, what does it physically mean? A given event may be in
        > different instances
        > of these frames, depending on the observer, and I believe that
        > attributing physical
        > reality to all of them separately seems nonsensical.

        I think that's a rather deceptive way of putting it. A slightly better way
        of putting it is that all clocks are physical systems and so must be
        subject to quantum gravity whatever that turns out to be like. In the
        meantime we have to guess based on what we do know. First, relativity tells
        us that spacetime is the gravitational field and that the only thing that
        matters as far as measurements on it are concerned are the relative values
        of the gravitational field and any other fields that happen to be around,
        there is no absolute value with respect to some externally defined
        standard. This is basically the same as the whole flow of time doesn't make
        sense thing. If there is a flow of time then it makes sense to say that if
        we stretch the universe so that all of the little bumps and swirls (not
        that I want to blind you weith science here :-)) are twice as far apart
        then the universe will be different, or if we speed up everything in the
        universe so that it goes twice as fast then we have a different universe.
        This is NOT to say that time is not real according to GR, versions of GR
        that try to do away with time suffer from a problem in that they have to
        pick out a preferred time coordinate since in such versions of GR one has
        to pick out a preferred slicing of the universe in which the arrangement of
        physical stuff (including the gravitational field which in this case boils
        down to being plain old 3D space) gives a reading of the 'time' and so time
        is just an emergent consequence of arrangments of stuff, but there are an
        infinite number of such slicings and no good way to pick out only one of
        them as significant.

        See

        http://philsci-archive.pitt.edu/archive/00000221/index.html

        One approach to quantum gravity - canonical quantum gravity - may eliminate
        this feature of GR and give rise to a single unique slicing, according to
        Julian Barbour. I'm not at all convinced that Barbour is right and I rather
        think that he has selected canonical quantum gravity on the grounds that it
        might give rise to the property he wants rather than because it contains
        good explanations. I think spacetime will persist in quantum gravity,
        rather than being broken up into space slices or whatever, but I might be
        wrong.

        What about quantum physics? Well, it seems likely that whatever it winds up
        being clocks will be quantum systems and so there will be more than one
        distinct subsystem of the multiverse entangled with a system with a given clock reading (as well as other observables). Or one could say in quantum physics there is no preferred history just as in GR there is no preferred reference frame.

        What bearing does this have on the question? Well, if Barbour's right then we will just have one preferred frame. If I'm right, although I'm guessing wildly, then quantum gravity will feature a multiverse that can be viewed from a number of frames none of which are preferred, the many reference frames doesn't change the fact that there is just one multiverse just as it doesn't change the fact that there is one universe under GR.

        Alan
      • Eric Cavalcanti
        ... From: Alan Forrester To: Sent: Tuesday, September 30, 2003 10:56 PM Subject: Frames the
        Message 3 of 15 , Oct 1, 2003
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          ----- Original Message -----
          From: "Alan Forrester" <alan_forrester2@...>
          To: <Fabric-of-Reality@yahoogroups.com>
          Sent: Tuesday, September 30, 2003 10:56 PM
          Subject: Frames the multiverse and stuff (was Re: Still frames and twins)

          > > According to relativity, there is no preferencial frame to describe the
          > > universe. So,
          > > when we say that the multiverse is composed of many "still frames" of
          > > possible
          > > universes, what does it physically mean? A given event may be in
          > > different instances
          > > of these frames, depending on the observer, and I believe that
          > > attributing physical
          > > reality to all of them separately seems nonsensical.

          (skipping through most of the message...)

          > What bearing does this have on the question? Well, if Barbour's right then we will
          > just have one preferred frame. If I'm right, although I'm guessing wildly, then
          > quantum gravity will feature a multiverse that can be viewed from a number of
          > frames none of which are preferred, the many reference frames doesn't change the
          > fact that there is just one multiverse just as it doesn't change the fact that
          there is
          > one universe under GR.

          So, if you are right, it turns out that the many slices wouldn't really have any
          physical existence per se. It would be just an imperfect picture, right? All there
          would be is the whole 4-D multiverse.

          It seems to me that the whole business of trying to describe the multiverse (or even
          the universe, if that is the case) in a reference frame where the time and positions
          of events are well-defined is wrong in the first place, so maybe it should break down
          at some level. The fact that we cannot reach a single description for something as
          "simple" as "where and when" an event happened seems to me that we are missing
          something more profound. The description we have available is imperfect, and I don't
          think that we will ever reach a true understanding of the thing in itself without
          getting rid of these artifices.

          Does anybody know of any attempt to describe reality in a wild different way, that,
          for example, does not use concepts such as space or time, or somehow derive them from
          other concepts? I know this seems too crazy, but I have this inner feeling that such
          an explanation should exist, i.e., one that tackles the question "what IS space and
          time?".

          Or am I missing something?

          -Eric.
        • scerir
          ... Carl Friedrich Freiherr von Weizsaecker, best pupil of Heisenberg, once said (as Heisenberg reported): Nature is earlier than man, but man is earlier than
          Message 4 of 15 , Oct 2, 2003
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            Eric Cavalcanti:
            > Does anybody know of any attempt to describe reality in a wild different
            > way, that, for example, does not use concepts such as space or time,
            > or somehow derive them from other concepts?

            Carl Friedrich Freiherr von Weizsaecker, best pupil of Heisenberg,
            once said (as Heisenberg reported):
            "Nature is earlier than man, but man is earlier than natural science".

            I remember that Von Weizsaecker (since 1955) pointed out the
            possible existence of 'abstract' and maybe 'potential' primordial
            entities, named 'Urs'.
            http://arxiv.org/abs/quant-ph/9611048
            http://arxiv.org/abs/quant-ph/0309183

            Recently it turned out that these 'Urs' are, more or less,
            the qubits.
            http://arxiv.org/abs/quant-ph/0212084
          • SimonJAnthony@aol.com
            In a message dated 02/10/03 01:26:10 GMT Daylight Time, eric@fis.puc-rio.br ... Leibniz proposed Monads which others (eg Smolin, Barbour) more recently have
            Message 5 of 15 , Oct 2, 2003
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              In a message dated 02/10/03 01:26:10 GMT Daylight Time, eric@...-rio.br
              writes:


              > Does anybody know of any attempt to describe reality in a wild different
              > way, that,
              > for example, does not use concepts such as space or time, or somehow derive
              > them from
              > other concepts? I know this seems too crazy, but I have this inner feeling
              > that such
              > an explanation should exist, i.e., one that tackles the question "what IS
              > space and
              > time?".
              >

              Leibniz proposed Monads which others (eg Smolin, Barbour) more recently have
              taken as a philosophical foundation for their views. S&T are supposed to
              arise due to the relationships between the Monads, which Leibniz thought of as a
              collection of souls. I suspect that Smolin & Barbour might use different
              terminology.


              [Non-text portions of this message have been removed]
            • Gary Oberbrunner
              ... I think this has been DD s point all along, and I agree. Recall the quote Other times are just special cases of other universes. The ways we choose to
              Message 6 of 15 , Oct 2, 2003
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                > So, if you are right, it turns out that the many slices wouldn't really have any
                > physical existence per se. It would be just an imperfect picture, right? All there
                > would be is the whole 4-D multiverse.

                I think this has been DD's point all along, and I agree. Recall the quote
                "Other times are just special cases of other universes." The ways we choose
                to slice it are up to us. The block multiverse is the only physically
                existing thing. Also, division into distinct universes (slices) works most
                of the time, but there are plenty of edge cases where it does not, e.g. in
                the region of black holes and other SR-related cases.

                -- Gary
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