When I was a child, I remember being told that in the distant past, a very learned person
might hope to understand everything that was understood.
Whereas now, because of specialization, because so much is known, that's impossible.
That one person can only understand a very small fraction of what's known.
And I really didn't believe this, I didn't want to believe this.
And I envied the ancient scholars who might have aspired to knowing everything that was
And what I meant by knowing everything that was known, or understanding everything that was
understood, is not that they knew in detail everything that happened, that they had lists
of things which they remembered, that's very far from what I meant.
I meant that they understood all the explanations that were known.
And I believe that we are not heading away from an era in which one might understand all
the explanations that are known, but towards it, because we are continually unifying and broadening
The universe that we see around us is real, but it's only one small facet of reality.
The whole of reality consists of many such universes, and they're all equally real, and
I studied physics at Cambridge, and then, and after that I was always on the borderline between
I studied quantum field theory in an astrophysics department, and then I studied, I came
to the multiverse theory in the relativity center in the University of Texas at Austin,
and then I was in the mathematical institute in Oxford.
All my research has always been on the borderline between two subjects.
I suppose the reason for that is that I'm interested in unifications and in broadening
I was in my office, I've never actually been here before.
Because I work at home, so I've never used my office.
The multiverse theory comes about as an explanation of the predictions of our best theory
Quantum mechanics makes very accurate predictions, the most accurate predictions that any
But if you want to explain why these predictions are so, how these physical events come
about, there's no alternative but to postulate that what we see around us is not the
whole of reality, that reality is much more varied and has a great multiplicity.
But why would you know these multiple universes?
Suppose you take one of the classic experiments of quantum mechanics, for instance, the
Now I can show you interference on a simple laser which we've done.
What is it that forces us to believe in parallel universes?
This is an experiment, the very old one that's been first done hundreds of years ago.
Long before quantum theory was even thought of, it just involves shining light through
This is just a piece of cardboard with little pin prick holes in it.
This is a laser pointer that we use in giving lectures.
I shine the laser through the hole and there's, we can see a little spot on the screen
You'd expect to see just, if there's one hole, you'd expect to see one spot.
You see quite a complicated pattern, it's called an interference pattern.
Suppose we, what would happen if we made a second hole?
Well, here's another place on the screen where there are two holes.
We get a completely different pattern, and through three holes is a different pattern
This experiment has been known for many decades before quantum theory was even thought of.
But the key to quantum theory is what happens when we perform this experiment with very
Suppose we take the laser and we put in front of it a dark filter, very dark filter so that,
so that very few photons actually get through, maybe only one per second.
And then we can't see that anymore, but we have instruments that can see it.
So we put the instruments in where we know the light beam is.
And when the detector is in a place where photons are arriving, it will start clicking,
If we move it over slightly to a dark place in the pattern, it will stop clicking.
Now, the surprise is that even with this one photon at a time coming through the holes,
the pattern is exactly the same as it was when we were shining the bright laser light
Well, the question is, how do we explain the fact that we see the same pattern that
we detect the same pattern on the screen when there is only one photon at a time going
through here as we did when the bright laser light was shining on it?
You see, this one photon, the one we detect, must have gone through one of those two holes.
And the part, the place where it lands on the screen, depends on whether there is one
hole, two holes, or indeed three holes, however many holes there are, each number of holes
makes a completely different pattern on the screen.
So even when there is only one photon at a time coming through, the place where it lands
And what we must conclude from this is that something is coming through those other holes
and shoving our photon aside, and that process is interference.
Well, we can do further experiments to find out what that something is.
The conclusion that we draw is that this thing behaves like light in every way we can detect
experimentally except one, and that is we can't detect it.
It is there because it pushes aside the light that we can see, and it behaves like light,
and the only reasonable conclusion is that it is light of an invisible kind.
Now that doesn't take you all the way to concluding that there are many universes, but
the thing is quantum theory predicts all this, and in the equations of quantum theory,
these invisible bits of light appear in the equations, and they are treated on it in exactly
What's more, all particles behave like this, not just light, but even the particles in
the screen, the particles that you and I are made of, they all are affected, shoved aside
by counterparts of themselves that behave exactly like those particles, but cannot be seen.
They are real matter, real energy, real light that we cannot see, and so they are an entire
That's why we call it a parallel universe, and there are many of them.
In this experiment, we can see how many holes we make, there is a different pattern.
Even when one photon is going through, there is a different pattern for six holes, for
seven holes, for a hundred holes, for a thousand holes, and that means that there must
be something travelling through every one of those thousand holes, and making a difference
And that means there must be at least thousands of parallel universes.
In fact, the theory tells us there are a lot more than that.
At the time of the Big Bang, according to quantum mechanics, all the universes came
And then with the interactions between them, interference, and other interactions caused
All the other universes are as real as the one you and I are in now.
Suppose we have a lot of different and a lot of the same universes all existing together.
You have to start thinking of yourself as not as an entity existing in one universe, but
an entity existing in the multiverse as a whole.
And so there are other copies of you and of me in other universes.
Some of them, some of these copies, are completely identical to us.
Now in those cases, it's really just a matter of words to say whether they are other
universes or whether they're the same universe if they're completely identical.
There are other universes which are very like this one, but differ only in the position
Now those universes are interfering with ours and they are producing interference effects,
which we could detect in the laboratory if we wanted to.
And there are others which are very different where the interference is so small that we
will never see them, but they form part of the explanation of the things that we do see.
And there are some universes in which I'm sitting here talking to you about something
And there are other universes where I've just got up and gone to have a cup of tea.
What makes me go into multiple copies in multiple universes?
You already exist in multiple copies, initially they are all identical.
And then when a moment of choice happens, both the microscopic one and one made by you
consciously, these identical copies become different from one another.
So suppose you're walking in Oxford and you have to choose to go left or right?
In that case, for instance, it could be that in half the universes I go left and in
And could you say that the universe splits up as soon as you go left or right?
It's not a matter of splitting, this terminology of splitting was the way that the many
universes theory was originally introduced when they thought of there being a universe
But nowadays it's better, we find it better to think of there being just a certain number
of universes, perhaps an infinite number, already there.
And then half of them do one thing and half do another.
How do I know in which universe I am or how do I even know who I am, what I am if there
That is the same question that one of a pair of identical twins might say if there are
two identical twins called Joe and Jack and Joe might ask, well why am I Joe and him
Well, the answer is if you were Jack and he was Joe, you'd still be asking this question.
The thing is the fact of the matter is there are two of you and they are identical or
they were identical until they started to become different by having different things
And it's the same with you and your counterparts in other universes.
They start off identical, when they're identical it doesn't make any difference whether
But then when they start becoming different it is worth calling them different versions
Some of them in more distant parts of the multiverse will be so different from you that
We have to get used to the fact that new advances in science get further and further away
If you want science merely to predict the outcomes of experiments without telling you why,
then you don't need to believe in multiple universes.
In fact, you don't believe need to believe in multiple planets.
You don't need to believe that anything outside this room exists if you only want to predict.
And if you want to explain, then you must adopt the explanation which meets the facts and
in quantum mechanics there is only one explanation that meets the facts and that is the
We can see the behavior of things like photons being affected by things we can't see.
Our only choice is to say it behaves as if it were affected by those or to say it really
And it only makes sense to say it's really affected because something that doesn't exist
Somebody who is busy with logic and mathematics and quantum theory, it looks rather messy
Yes, well you have to remember this is my place of work.
I sit at my computer and write the books that I've been writing and then when I'm working
on other kinds of research, if I receive some papers, then my filing system is my floor.
Like immortality or perpetual motion, time travel is one of the enduring fantasies of the
But until recently, the view held by scientists has been that time travel is impossible.
Not because there is any law of physics for bidding it, but because if it were possible,
Ever since childhood, I wanted to travel in Doctor Who's Time Machine.
More recently, I made a filming which I had the privilege of travelling in the actual
TARDIS of Doctor Who to explain how the many universes interpretation of quantum mechanics
solves the problems which in science fiction are usually thought to exist with time travel.
The obstacles, the paradoxes, which are usually thought to prevent time travel, actually
do not prevent it if we perform a proper quantum mechanical analysis.
Does physics give the possibility of time travel?
The first is, is it physically possible to build a time machine that is a path from the
And the answer there is it's still an open question, but as far as we know, there are
physical processes which would allow the tearing of the fabric of space time in such
a way that there would be a pathway into the past.
This would involve something like a rotating black hole, some very violent event in space
The other half of the question is, if we had such a path, would it be possible actually
to travel on it and what would then happen in regard to the paradoxes?
For instance, what if we try to change the past?
Would the past be as it is recorded or would it get changed?
If, for instance, I go into the past and prevent myself from ever entering the time machine
or from even building the time machine, then what would happen?
If so, who was it that went back into the past to stop me?
There's apparently a paradox and the resolution of the paradox is that when one does
things like that, when one goes into the past and changes things, the result of that is
that one has actually gone into the past of a different universe.
I don't think I would because when you travel in time, one thing we do know is that you
So at the very least what would happen is that I would end up in a universe where there
was another copy of me, the copy who hadn't set out in a time machine, and in that
universe there'd be two copies of me, and in my original universe there'd be no copies
That would mean that all my friends and the people I know in this universe, they would
see me entering the time machine and not come out.
And from their point of view, I would be lost forever.
There would be another group of the same people in another universe.
They would see one copy of me, and they would see a second copy arising coming out of
And from then on, they would be in the company of two copies of me.
And they might not consider that to be an improvement on one.
The fact that we can't directly detect parallel universes is to some degree an accident
If you wanted to detect the motion of the Earth with your human senses, you wouldn't
The way we detect the motion of the Earth is, for instance, by taking a long pendulum,
Foucault's pendulum, and letting it hang for a while.
And in the cause of a few hours, we see the plane of its rotation changing.
And this is because the plane actually stays the same, and the Earth is rotating underneath
So this is an instrument which detects the rotation of the Earth.
If, by chance, our senses had contained an instrument equivalent to this pendulum, we would
In a similar way, if our senses happen to work through quantum interference, then we would
be able to feel directly the influence of parallel universes.
So saying that you don't feel other universes is like the inquisition saying to Galileo
that they don't feel the Earth moving beneath their feet.
Well, the reason why they don't feel the Earth moving beneath their feet is because Galileo's
But if they do a different experiment with the Foucault's pendulum or with telescopes,
they will indirectly be able to tell that the Earth is moving.
The core of every scientific advance is a better explanation.
Now quantum mechanics and the many universe interpretation are our best explanation of the
In fact, I would say they are the only current explanation of the physical world at the
We don't understand it fully yet, and I don't think one ever understands a theory fully
One day there will be a successor to quantum theory, just as quantum theory itself is
the successor to classical theories of physics.
And then we will understand the full implications of quantum theory and where it went wrong.
And I would expect, again, on the basis of what usually happens in science, we would expect
that future theory to be even strangers in quantum theory.