The Fabric of Reality David Deutch

partition the multiverse in the manner of Figure 11.6, into separate
spacetimes, or into super-snapshots each of which entirely determines the
others. So we know that the classical concept of time as a sequence of
moments cannot be true, though it does provide a good approximation in
many circumstances — that is, in many regions of the multiverse.
To elucidate the quantum concept of time, let us imagine that we have sliced
the multiverse into a heap of individual snapshots, just as we did with
spacetime. What can we glue them back together with? As before, the laws
of physics and the intrinsic, physical properties of the snapshots are the only
acceptable glue. If time in the multiverse were a sequence of moments, it
would have to be possible to identify all the snapshots of space at a given
moment, so as to make them into a super-snapshot. Not surprisingly, it turns
out that there is no way of doing that. In the multiverse, snapshots do not
have ‘time stamps’. There is no such thing as which snapshot from another
universe happens ‘at the same moment’ as a particular snapshot in our
universe, for that would again imply that there is an overarching framework
of time, outside the multiverse, relative to which events within the multiverse
happen. There is no such framework.
Therefore there is no fundamental demarcation between snapshots of other
times and snapshots of other universes. This is the distinctive core of the
quantum concept of time:
Other times are just special cases of other universes.
This understanding first emerged from early research on quantum gravity in
the 1960s, in particular from the work of Bryce DeWitt, but to the best of my
knowledge it was not stated in a general way until 1983, by Don Page and
William Wooters. The snapshots which we call ‘other times in our universe’
are distinguished from ‘other universes’ only from our perspective, and only
in that they are especially closely related to ours by the laws of physics.
They are therefore the ones of whose existence our own snapshot holds the
most evidence. For that reason, we discovered them thousands of years
before we discovered the rest of the multiverse, which impinges on us very
weakly by comparison, through interference effects. We evolved special
language constructs (past and future forms of verbs) for talking about them.
We also evolved other constructs (such as ‘
if… then…’ statements, and
conditional and subjunctive forms of verbs) for talking about other types of
snapshot, without even knowing that they exist. We have traditionally placed
these two types of snapshot — other times, and other universes — in
entirely different conceptual categories. Now we see that this distinction is
unnecessary.
Let us now proceed with our notional reconstruction of the multiverse. There
are far more snapshots in our heap now, but let us again start with an
individual snapshot of one universe at one moment. If we now search the
heap for other snapshots that are very similar to the original one, we find that
this heap is very different from the disassembled spacetime. For one thing,
we find many snapshots that are absolutely identical to the original. In fact,
any snapshot that is present at all is present in an infinity of copies. So it
makes no sense to ask how many snapshots, numerically, have such-and-

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such a property, but only 
what proportion of the infinite total have that
property. For the sake of brevity, when I speak of a certain ‘number’ of
universes I shall always mean a certain proportion of the total number in the
multiverse.
If, aside from 
variants of me in other universes, there are also multiple
identical 
copies of me, which one am I? I am, of course, all of them. Each of
them has just asked that question, ‘which one am I?’, and any true way of
answering that question must give each of them the same answer. To
assume that it is physically meaningful to ask which of the identical copies is
me, is to assume that there is some frame of reference outside the
multiverse, relative to which the answer could be given — ‘I am the third one
from the left…’. But what ‘left’ could that be, and what does ‘the third one’
mean? Such terminology makes sense only if we imagine the snapshots of
me arrayed at different positions in some external space. But the multiverse
does not exist in an external space any more than it exists in an external
time: it contains all the space and time there is. It just exists, and physically it
is all that exists.
Quantum theory does not in general determine what will happen in a