On 5 February 2014 10:05, meekerdb <[email protected]> wrote: > On 1/31/2014 11:05 PM, LizR wrote: > > There seems to be a bit of confusion about this idea. Some people on the > list seem to abhor the idea of a block universe, but when they attack the > concept, they invariably go for straw men, making statements like "change > can't happen in a block universe" (which are obviously nonsense, or > Einstein et al would hardly have entertained the idea in the first place). > > So, I'd like to maybe clarify what the idea means, and give them a > proper target if they still want to demolish it. > > A block universe is simply one in which time is treated as a dimension. > So Newtonian physics, for example, specified a block universe, in which it > was believed (e.g. by Laplace) that in principle the past and future could > be computed from the state of the present. The Victorians made much of time > being the fourth dimension, probably most famously in Wells' "The Time > Machine". This was the Newtonian concept of a block universe, and was > generally treated quite fatalistically (Wells didn't indicate that history > could be changed, for example). > > Then special relativity came along and unified space and time into > space-time. The reason SR gives rise to a block universe is the relativity > of simultaneity. You can slice up space-time in various ways which allow > two observers to see the same events occurring in a different order. Hence > there is no way to define a "hyperplane of simultaneity" that can be agreed > upon by all observers as being a present moment. This indicates that > space-time is a four-dimensional arena in which events are embedded. > Indeed, I have never heard of an alternative explanation of the relativity > of simultaneity that gets around this result - if it's correct, space-time > is a block universe, that is to say, time is "just" another dimension. > > > I think you are reading too much into special relativity. What makes SR > different is that only spacetime interval is meaningful, whereas for Newton > both duration and distance were separately invariant. Sometimes you can > work problems just in terms of the 4-intervals and 4-momenta and not > introduce any coordinate system. The coordinates (t,x,y,z) that give a > block-time picture are in principle dispensable (and they can even be > misleading when t is identified as "the real time"). It's like giving > locations on Earth latitude and longitude coordinates instead of citing a > string of transit measurements and triangulations to give the relative > location of two things. The latter is too messy to calculate with. But > notice, that is exactly what GPS does. It locates every GPS receiver in > spacetime in terms of 4-space triangulation relative to known orbital > events. Then the receiver converts that to lat and long for our > convenience. > > So I would say that treating time as a dimension is a good way of looking > at things, but it's not *forced* on us by SR. >
Well obviously nothing's *forced* on us by SR - apparently it can even be made compatible with absolute time, at least for some people! So ..... you're saying that looking at space-time as dimensional isn't forced on us by SR, correct? (I assume, given that SR unifies space and time, you aren't just separating out time, but saying this is true of space-time, yes?) Well, of course this is suggested by some TOEs (like comp). But that would be an unnecessary complication in this discussion, imho. Still, from a "Machean" viewpoint this may be true, and Einstein was keen on Mach's ideas. Or at least Mach would say you don't *need* any *particular* coordinate system to do calculations in SR - the coordinate system you use for a given calculation is relative to the system you're using, making it arbitrary in terms of the rest of the universe. But you are apparently making a stronger claim. You said we don't need a coordinate system at all, we can just use 4-momenta and 4-intervals - so using those doesn't imply or define a 4D coordinate system? So what do they imply? Do they (hehe) just operate in a vacuum? Is this some sort of (I think it was Julian Barbour) -ish vision of geometry swimming around unsupported by any external framework? Maybe you could be more precise, I'm afraid my knowledge of SR is limited to things like the website I linked to, which tend to use space-time, light cones, worldlines etc as part of the furniture. But anyway, I don't see the connection with block universes. The argument from SR to a block universe involves the relativity of simultaneity, so unless you are saying *that *isn't a consequence of SR, I'm not sure what the relevance of all this is. (Well, unless it's just to give fuel to people who will twist what you're saying around to say there's *no *implication of a 4D manifold in SR, of course. Are you just troll-baiting?) -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
