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. So classical physics posits a BU. Before worrying about QM, let's see what the classical picture has to say about whether things can change in a block universe. Change is defined as something being different at different times - say the position of the Earth relative to the centre of the galaxy (it traces out a wobbly spiral like a spring as it follows the Sun around an almost circular orbit around the galactic centre every quarter of a billion years). Does the fact that the Earth's orbit is a spiral embedded in space-time prevent the Earth's position from changing? Clearly not. It changes all the time. The same applies to any other changes that we observe. A person changes as they get older - in the relativistic view these are cross sections through their world-tube (or "lifeline" as Robert Heinlein put it). Particles move through space - they trace out 4 dimensional world lines, but they can still move. Everything we observe takes place in a manner that can be placed within a space-time continuum such that a "god's eye" view (or the relevant equations) would see it as static. But of course *we* don't see it like that. This appears to be the source of the problem a few people have with this concept, however - they appear to confuse the god's eye view with ours. But of course we're embedded in space-time - along for the ride. So of course we see change all the time. QM, perhaps a bit boringly, goes back to the Newtonian view. Space and time are a background arena in which wave functions evolve with time - which is of course a process that can be mapped out within a 4D manifold. Indeed the equations involved are determinstic, and the famous quantum probabilities have to be added "by hand" - so this is rather close to the Newtonian view, apart from the ad hoc wave-function collapses. Fortunately, Everett gave us a completely deterministic view - the wave function evolves in a multiverse - so the block "universe" of QM is instead a block multiverse, but otherwise it is a deterministic process embedded in a space-time manifold. Lastly, the past is an excellent example of a block universe. It is unchanging, with events embedded in it. If anyone wants to consider what the concept means, think about the past as the example of choice, a 13 billion year long block universe. Change appeared to happen to the people embedded in it, but we have a "god's eye" view of the past, and can see that they were just experiencing different points along their world-tubes. And people in the future will be able to see that the same was true of us. -- 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.
