George Ellis |
In an
earlier post in this blog, published in November 2014, I mentioned that
there are six independent theories about the multiverse, almost all of them
incompatible with each other. In a recent article titled Theory Confirmation and Multiverses published
in the book Why Trust a Theory?,
edited by Radin Dardashti, Richard David and Karim Thébault (Cambridge
University Press, 2019), George Ellis updates the different multiverse theories.
He does not mention six, as I did five years ago, but nine, although he has
left out one of the six I mentioned in my post (Smolin’s), perhaps because this
theory has been abandoned in the meantime. The nine theories are:
- The multiverse in the universe: Undetectable parts of our universe, beyond our event horizon,
could have physical properties different from ours.
- The chaotic inflationary multiverse: If our universe went through an inflation phase and then slowed
down, there could exist many other similar bubbles in a much larger
universe.
- The multiverse of the M theory: This is an extension of string theory that adds an additional
dimension, which would separate the various universes in that multiverse, each
of which is called a brane.
- Cyclic universes: I did not mention
this version, because the different universes are not separated in space,
but in time (each one follows the previous one and gives rise to the
next). Cyclical universes were fashionable in the 80s, but today they don’t
seem to be in favor.
- The string theory landscape: It is more
or less the same as multiverse number 3, but without the branes.
In my post I joined these two theories into one.
- Ramifications of the quantum mechanics wave function: This theory, which was proposed by Hugh Everett III in 1957, is
what I called the quantum multiverse in time. It is also
called the many worlds interpretation of quantum mechanics.
According to this theory, everything that can happen, must happen in one
of those universes.
- Holographic projections: This is a
new proposal, which assumes that our universe is the holographic
projection of a multi-dimensional universe over three dimensions. If there
is a projection, there could be others, which would give us a multiverse.
- We live in a simulation: Our universe
is not real, but a computer simulation. I spoke about this in another
of my posts. This theory has been disproved by recent
research (2018).
- Tegmark’s mathematical multiverse: Everything that can exist, exists somewhere.
Ellis adds a tenth alternative possibility:
- There is only one universe. Therefore
the multiverse does not exist, it’s just a figment of the imagination of some
physicists. He immediately adds that the available evidence and Occam’s
razor favor this possibility.
Ellis rightly points out that the existence of so many alternative
theories about the multiverse, none of which can be shown to be false, while several
are incompatible with the others, rather than making the existence of a multiverse
more plausible, makes it less plausible:
…if
they conflict with each other, why should any of them be true?
All those theories are impossible to prove. In his words:
…we
cannot test them, because we don’t have observational or experimental access to
any other universes that may exist… Multiverse theories claim to tell us what
is true in regions for which no observation data whatever are available! These
theories are not modest in their aim.
Faced with this situation, some supporters of multiverse theories claim
that science should relax the criteria to decide whether a theory
is scientific or not. Rather than demanding that the theory can be proved false
by a (possibly mental) experiment, we should resort to arguments such as: if
we don’t have anything better, this theory must be true.
Ellis proposes an interesting test that would distinguish whether a
theory is really scientific or philosophical (ideological):
What
data/observation would lead you to abandon the multiverse model? If the answer
to this question is ‘none’ then the theory is dogma, not
science. Because multiverses can explain anything, the answer is, indeed,
likely to be ‘none’.
The final conclusion of Ellis’s article is devastating:
The
kinds of arguments given… are not adequate for claiming the theory is a
well-established scientific result. Having a well-developed mathematical theory
is not sufficient.
The same post in Spanish
Thematic thread on Multiverse and Fine Tuning: Preceding Next
Manuel Alfonseca
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