Parallel universes

If you are not prepared to sweep time travel paradoxes under
the carpet, there is an alternative possibility in which neither
the no choice paradox nor the something-from-nothing paradox
need ever appear. The price to pay may be too high for you to
accept though, despite the inevitable desensitization you must be
undergoing towards some of the absurdities you have already met
in this book. What I amabout to describe is going to sound crazier
than anything else you have met so far, yet it is based on a highly
respectable, if unconventional, interpretation of the weird results
of quantum mechanics.
I have already briefly mentioned at the beginning of Chapter 4,
whendescribing the nature of light, that quantum mechanics ranks
even higher than relativity in terms of its importance as a scientific
discovery which has affected our everyday lives. The problem
is that nobody really understands what it is telling us about the
world of the very small which it describes so accurately. This
will of course sound rather strange. How can a theory which
we do not understand be so successful? The answer is that it
predicts the behaviour of the very building blocks of matter—
not just the atoms, but the particles that make up the atoms
(the electrons, protons, neutrons) as well as light photons and
every other subatomic particle you care to name (and there are
many)—with incredible accuracy. Quantum mechanics has led us
to our current very precise understanding of how these ‘quantum’
particles interact with each other and connect up to form the
world around us. Yet, at the same time quantum mechanics forces
upon us a view of the subatomic world which goes totally against
our common sense. Physicists have now had three quarters of
a century to come to terms with this and have come up with a
number of possible interpretations of what must be going on down
at the quantum level. But there is still no overall consensus as to
which is the correct interpretation, if indeed there is only one.
The interpretation that has held sway for most of the
twentieth century is known as the Copenhagen interpretation (as
it originated at the institute in Copenhagen where one of the
founding fathers of quantum mechanics, Niels Bohr, worked) Its supporters take a very pragmatic view of the whole issue by
claiming that we should not worry about trying to understand
what is happening down at the scale of atoms which is so far
removed from our everyday world. For instance, we have no right
to expect a photon of light to behave in a way that makes sense to
us. If light appears one minute to have the properties of a stream of
particles and the next those of a wave then so be it. All that matters,
say the Copenhagenists, is that quantum mechanics works. The
mathematics agrees beautifully with what we see around us in the
real world, so why beat ourselves up over it?
Such a view has, until the last ten to twenty years, been the
majority one. Most practising physicists have been happy (well,
maybe happy is not the right word) to use the tools of quantum
mechanics—abstract symbols and mathematical techniques rather
than spanners and screwdrivers you understand. They have been
prepared to leave the pondering and musings about the deep
meaning of it all to the philosophers.
The differences between the interpretations are to do with
how we describe what is ‘happening’ to a quantum particle, such
as an electron, when we leave it alone to do whatever electrons
like doing when left alone. If we measure a certain property of
an electron, such as its position, speed or energy at a particular
moment, then quantum mechanics will tell us what we are likely
to find. However it tells us nothing about what the electron is
doing when it is not being observed. This would not be a problem
if we could trust electrons (and all other quantum particles) to
behave sensibly, but they don’t. They will disappear fromthe place
they were last seen and spontaneously reappear somewhere else
that should, by rights, be inaccessible to them. They exist in two
places at once, they tunnel through impenetrable barriers, travel in
two different directions at once and even have several conflicting
properties simultaneously. But the moment you look to see what
is going on, the electron will suddenly start behaving itself again
and nothing will look out of sorts. However, the unavoidable
conclusionswehave to draw fromthe results of our observations is
that the electron was most definitely doing something very strange
indeed when we weren’t looking. In fact all quantum particles behave in ways which would be quite impossible if they obeyed
the same rules as everyday objects we are used to.
Since quantum mechanics only ever tells us what to expect
from the results of our observations we must appeal to something
else if we insist on trying to understand what is going on when we
are not looking. This is what I mean by different interpretations. I
have so far only mentioned the Copenhagen interpretation, which
for many has been considered the standard one. It is the one
which almost all textbooks on quantum mechanics use and which
is taught to all physics students. But there is a growing consensus
that its time is up. Its supporters may still argue that it is purely
a matter of philosophical taste which interpretation we choose
but the truth is that a growing number of physicists (not quite
a majority yet) are looking for something deeper.
One of the alternative explanations to the Copenhagen view,
which is of particular interest to time travel fans, is known as the
many-worlds interpretation. According to this view, as soon as a
quantum particle, anywhere in the Universe, is faced with a choice
of two or more options, the whole Universe splits into a number of
parallel universes equal to the number of options available to the
particle. There are, according to this view, an infinite number of
universes which differ from our own to a greater or lesser degree
depending on how long ago they split off from ours, and each
universe is just as real as our own. In many of these universes
there exist carbon copies of you. In some you are a billionaire
business tycoon, in others you are living rough on the streets. In
many others you have turned out very much like you have in our
Universe apart from some minor details. For many years, this sort
of thing has been the stuff of fiction, and for many physicists it will
remain that way. There is no experimental evidence whatsoever
that parallel universes exist since we cannot make contact with
any of the others, but the truth is that it has its good points as a
scientific theory and explains away much of the strange behaviour
of the quantum world. But at what price? Physicist Paul Davies
has remarked that the many-worlds interpretation is cheap on
assumptions (a point in its favour) but expensive on universes.
If you are meeting this idea for the first time it might seem
impossible that there is even any roomfor all these other universes. After all, it may well be that our own universe is itself infinite.
Where can all the others be? The way to visualize this is to think
of our Universe as an infinitely extended flat sheet by throwing
away two of its dimensions (remember we have to go from fourdimensional
spacetime down to a two-dimensional sheet). Now
the other parallel universes can be thought of as stacked above
and below our own. There is room enough for all with enough
dimensions.
Apart from the sheer extravagance of requiring an infinite
number of universes, some physicists claim that the many-worlds
interpretation also does away with free will. Here is how it works:
Whenever you are faced with any kind of choice, say touching the
tip of your nose and not touching the tip of your nose, and you
choose (you think freely) not to touch it, what will have happened
is that the Universe split into two and there will now be a parallel
universe in which you did touch your nose. You are conscious
of having taken one of the available pathways, but there will be
another version of you in a parallel universe who is conscious of
having made the alternative choice.
The many-worlds interpretation of quantum mechanics was
proposed by an American physicist by the name of Hugh Everett
III in the 1950s and, despite not catching on at the time, has recently
been favoured by a growing number of cosmologists who feel it is
the only viable interpretation when applying quantum mechanics
to describe the whole Universe.
At the same time, the idea of parallel universes has been
exploited by science fiction writers who have recognized that
it rescues them from time travel paradoxes. More recently, the
Oxford physicist David Deutsch has developed his own version
of the theory and points out that if we wish to take the possibility
of time travel into the past seriously then we are forced to take
the many-worlds interpretation seriously. I should explain that
Deutsch is a strong supporter of the many-worlds idea, which he
refers to as the multiverse interpretation (the term ‘multiverse’
implies the multitude of all universes, the totality of reality). I
am not convinced by this view, but I cannot rule it out. For what
it’s worth, my favourite interpretation of quantum mechanics is one due to the physicist David Bohm which requires just the one
universe thank you very much.
So how does the many-worlds view cope with the paradoxes
of time travel? As I mentioned in option three when attempting to
explain the possible alternatives available to John Connor in the
Terminator paradox, a time traveller will not travel into the past
of his or her own universe—not surprising given that there is an
infinite number of pasts to choose from—but into that of a parallel
universe.
According to Deutsch, who takes the block universe idea
literally in his book The Fabric of Reality, the Universe does not
divide up into multiple copies of itself at the moment we are faced
with a choice. Instead, there are already an infinite number of
parallel universes out there. At the moment of choice we are just
following one particular pathway, like a train going through a
complicated junction. This means that the future is open since
there are many options available to us, but so is the past. Our own
spacetime is just one of an infinite number of pasts and futures.
Travelling into the past in Deutsch’s multiverse is no different to
the way we would normally get carried along into the future. We
simply follow a time loop into one possible past.
Deutsch’s approach is just one of a number of versions of
the many-worlds interpretation. I will re-examine our time travel
paradoxes within the more conventional version of the manyworlds
theory—if you can ever call an infinity of parallel universes
conventional. The no choice paradox no longer applies since we
are free to alter the past as we wish since it will not be our past.
Events in the parallel universe we have travelled to need not turn
out the way they did in our own universe. John Connor can now
kill his mother and stop himself from being born in that universe
while the mother back in his own universe survives. Of course
if he does fail to kill her then he will be born. There will now
be one universe (soon to proliferate due to all the other quantum
choices that are going on) in which John Connor grew up, hopped
into a time machine and disappeared forever. There will also be
another universe in which there is either one or two ‘John Connors’,
depending on whether he succeeds in killing his mother or not. If he does not kill her the two Johns will live side by side but
separated in age by however far back he has travelled in time.
One thing to remember though is that the chances of John finding
his way back to his own universe, or even one very much like it,
are very small indeed. There are simply too many to choose from.
The something-from-nothing paradox can also be explained
away. The Mona Lot painting was of course sent back from the
future of a parallel universe and was painted by the Leonardo in
that universe. Leonardo does not even need to send the painting
back to the past at the allotted time. He can keep it. After all, even
if he does send it back it would be a third Leonardo who will find
it in his time machine.
Even the problem of creating multiple copies of ourselves
by looping repeatedly around in time is resolved. If there ends
up being 100 copies of you in one universe this just means that
there are 99 other universes from which you have disappeared.
Conservation of mass and energy no longer applies to each
universe separately but to all universes taken together.
One of the advantages of the original version of the manyworlds
interpretation, in which the Universe splits only when you
are faced with a choice, is that it gives us an arrow of time which
points in the direction of increasing number of universes. There
are always more universes in the future than the past. But surely,
you might think, isn’t this rule violated when we allow for time
travel into the past? If I travel back today into the yesterday of a
parallel universe I cause it to start splitting according to the choices
I can make once I am there. But how can that parallel universe
have started splitting yesterday before I even made the decision
to travel back? Is this another form of the no choice paradox? It
seems as though the parallel universe I am about to travel to must
know in advance that I will be arriving and making certain choices,
thus forcing me to travel back to that universe, and making those
choices.
Again, the many-worlds interpretation offers a neat way out.
General relativity allows a way of connecting up our Universe
with a parallel one which may not necessarily involve time travel
into the past, as we shall see in the next chapter when I introduce wormholes. A time machine is of course one way of making this
connection. The mistake in the previous paragraph is to think
that this connection is made the moment we travel back in time.
It is not. It is made the moment the time machine is created
(or switched on) allowing for the possibility of time travel to any
universe which splits off from ours subsequent to the moment
the time machine is switched on. At the instant of switching on
there will be universes in which versions of us start arriving. This
is because, even if we decide not to use the time machine but to
destroy it instead, it is too late. Having been faced with this choice,
our Universe splits and there is another universe in which we did
not destroy the time machine but used it instead. Insomeuniverses
we travelled back to the very earliest moment possible (just after
it was switched on). In others, we travelled back to a later time.
Even while you are still strapping yourself into the time machine
the Universe is splitting, due to all the other choices it is making
everywhere else in space, and there will therefore be an infinite
number of ‘you’s travelling back in time! I think I’ll stop typing
now and go and lie down for a while.