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.
Brent
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