What we experience is always a computation performed by our brain and
the best theories we have will not allow one to definitely say that you
are not a brain in a vat. Another thing is that you are always located
in your present moment, so any notion of the past is just a theory, you
cannot distinguish between a fictitious past and what you think is the
real past. Everything you know now about what you think is the past is
just a consequence of what exists in your brain right now.
Then invoking physics to argue that the concept of an external universe
including its history is correct is not going to work in a rigorous way,
because according to quantum mechanics there will be contributions to
the amplitude of your present observations coming from anomalous
histories. While these contributions are suppressed by astronomically
large factors, if you write down a theory where you make them exactly
zero, you get a flawed theory that will violate unitary time evolution.
The difference between, say, 10^(-10^1000000000) and 0 is actually a big
deal here, it proves that our existence is not confined to some naive
classical notion of a single universe. The picture that we instead get
is radically different than the one painted by most physicists who
ignore the small details and argue on the basis of the flawed notion of
"for all practical matters".
A more natural picture that is consistent with all of physics (all =
including the astronomically small amplitudes), is the idea that what
exists are just algorithms. An algorithm can be specified by
mathematical rules, it specifies the relation between input and output.
From the point of view of an algorithm, a universe may appear to exist,
but what really exists is that algorithm. Other algorithms then also
exist but they have to be considered as different universes. So, what
exists is a multiverse where each element is an algorithm in some
specific computational state.
In particular, this means that you are your own universe, and one second
ago you were a different person, so that person is a different universe.
Time evolution is an information preserving mapping between different
universes, there is nothing that really evolves or changes.
Also, the person you think you were when you were a small child is the
same child that ended up being persons that are totally different from
you, including people who died centuries ago. In fact, I claim that if
you pick two random persons, say, Genghis Khan and Albert Einstein then
under inverse time evolution the child they were will converge to each
other and eventually merge with each other.
This is true even for mothers and their children, the fact that the
mother gave birth to her children is not a problem because a fetus in a
womb cannot detect in which womb it is. As you go back in time, what it
is aware of and what its mother was aware of when it was a fetus become
identical.
When we were born, we wouldn't immediately have known about modern
technology, so the experiences for the newborn baby do not localize it
in time all that much. If you were born in the year 1960, you could just
as well have born in the year 1800, 1200 or 2260. This means that you
were born in all these possibilities, you started to branch out into
the different possibilities only after you became aware of these facts.
But, of course, it's not consistent to leave other animals out of the
equation. In principle one has to consider all possible algorithms, some
of which are implemented by the brains of spiders, T-Rex or some alien
animal that doesn't exist here on Earth.
Saibal
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