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Brian Greene talks about spacetime as a loaf of bread. https://www.youtube.com/watch?v=eR8DYZzmin0

His analogy does seem to imply block time, so I was wondering if the theory of General Relativity does actually espouse block time? As in, everything has already happened. Or as in, "the whole of spacetime is already there, now".

I hope my question wont be confused as a philosophical musing like the ones you guys hate, but I was actually wondering if there is anything in the theory that actually says such a thing. And if not, then why? Like, if I were to say such a thing, how would you disprove what I said? Why would Brian Green give this analogy then?

Qmechanic
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DLV
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    Related: http://physics.stackexchange.com/q/53272/ – dmckee --- ex-moderator kitten Sep 27 '15 at 01:14
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    The whole of spacetime is the whole of spacetime. What would it mean for it to be "there now"? If you're asking whether all of spacetime is contained in a single spacelike slice of spacetime, then of course the answer is no. If not, what does "now" mean? – WillO Sep 27 '15 at 02:16
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    Not all physicists hate philosophy. I believe Greene (note the 'e' on the end of his name) here is simply explaining his subscription to an eternalist interpretation. I think the question is a good one, but IMO already has a pretty good answer by Ben Crowell in the question marked by @dmckee. Also note that to talk about things exactly as Greene says, one needs a solution to GR to have so called timelike Killing fields to have the implied universal time co-ordinate implied in Greene's exposition. The standard model of cosmology allows such a beast, but not all solutions in GR do. Even so .... – Selene Routley Sep 27 '15 at 02:56
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    ... an Eternalist interpretation still stands, through (the somehwhat technical) mechanisms explained in Roy Simpson's Answer here. – Selene Routley Sep 27 '15 at 02:59

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One doesn't need to go to GR; SR is enough for the argument.

Eternalism (the block theory of time) states that the past, present and future exist.

This should be contrasted with Presentism which states that only the present exists.

To see what this means, consider as is normal in physics, time as a dimension in analogy to space; but whereas we have the physical ability to move however we wish in three dimensions of space, we cannot do that with time; I cannot walk into the past, and nor the future; and nor is there some putative time-machine that will allow me to do this; this motivates Presentism - we are always at the present moment of time; this also is consistent with our direct experience as opposed to how we think about space and time, especially theoretically.

In Classical Mechanics, with its Newtonian notion of Absolute Time we are granted absolute simultaneity: everywhere can have exactly the same time; it's a requirement - it must have, not it 'can have'; hence presentism is a possibility, as is Eternalism.

So we have two options for an ontology of time.

However, it's suggested by the Rietdjik-Putnam argument that this is not possible when Relativity is factored into this classical picture ie in SR; since every event at 'present' has its own plane of simultaneity, and these need not coincide. Hence Presentism is ruled out, leaving Eternalism.

Which probably means Presentists have some work to do ...

Mozibur Ullah
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  • Yeah, you've made me remember doing Minkowski diagrams in introductory SR and seeing different "cuts" for different perspectives of simultaneity. However, if this is so obvious why would there be any debates on this? (As the comments below my question suggest). – DLV Sep 27 '15 at 03:34
  • Partly for the reasons I elaborated above; it's a more natural position working from direct experience. – Mozibur Ullah Sep 27 '15 at 03:44
  • There's also a growing block theory that says that the Past and Present exist but not the Future; and McTaggarts argument on the un-reality of time, and there's one also by Barbour. – Mozibur Ullah Sep 27 '15 at 03:47
  • @MoziburUllah: I'll be very disappointed if the future doesn't exist. I've been hoping to spend some time there. – WillO Sep 27 '15 at 16:20