In early spring of 1959,
a young American physicist traveled to Copenhagen.
He was just 28 years old.
His name was Hugh Everett.
He was to meet with Niels Bohr,
one of the giants in the world of quantum mechanics.
When Everett was sent to Bohr,
he challenged every part of Bohr's understanding
of quantum theory, his worldview.
What Everett presented to Bohr
was an extraordinary new theory that might be one
of the most important scientific ideas of all time.
Everett was trampling over every
belief that Bohr held dear.
Everett's theory came with strange
and outlandish implications.
It meant that there must be countless parallel universes
and many worlds just like our own,
populated by alternative versions of us.
What's interesting is if quantum theory itself,
taken realistically, tells us there are many worlds,
that's why we take them seriously.
To my view, the many-worlds,
or the Everett interpretation, if you like,
is a reductio ad absurdum.
Bohr rejected the idea
and a disillusioned Everett left academia for good.
But it was a glimpse of a new way
of making sense of quantum theory.
How does a physicist ask what is a physical
interpretation of this quantity,
there is simple-- And the intellectual
battle still rages today.
I don't think that there are two worlds.
I prefer to have a single world
with a number of internal perspectives.
But today there is a new theory emerging
that many believe will finally vindicate Everett.
These results are sensational,
if you can buy the many worlds.
(lively music)
(contemplative music)
Hugh Everett's idea that there are many worlds
may have been dismissed by Niels Bohr,
but it didn't go away entirely.
In Oxford in the '90s, Professor Simon Saunders
was plotting to bring Everett's work back
into the mainstream of philosophy.
The idea of many-worlds was now being taken seriously.
My arrival in Oxford, I had this set of ideas,
some of it written, some of it not,
but what I didn't have were people who got it,
who, as it where, could pick up those ideas and run with it.
Simon is a philosopher and physicist.
Together with Harvey Brown,
they brought together a team of academics
that is now called the Oxford group.
I think there's a certain degree of serendipity.
I think you have to have people who are creative,
think outside the box. (people chattering)
Simon belongs to this Oxford gang of philosophers
who are all enamored by this idea of the many-world,
and are preaching it around. (people chattering)
It was one of the most happy
and fulfilling moments in my life,
to find young people, indeed, even undergraduates,
responding with real talent.
Above all, David Wallace. We know something
about state-dependent solutions.
Simon Saunders nearly single-handedly
brought the Everett interpretation to the fore
in philosophy of physics.
So Simon's work here was both intellectually
very important, and I think actually,
personally very courageous, because that was a time
where in philosophy, if not in physics,
these ideas were not really taken very seriously,
and there was a certain amount of risk, I think,
in working in these areas.
Undeterred, Simon and his fellow philosophers
were brave enough to suggest the unimaginable.
What does it mean to have many worlds?
Many-worlds is difficult, it's fantastical.
In the many-worlds context, you're supposed to be
thinking, oh, here's what's going to happen,
the world's gonna split into two copies.
It does seem to have mind-boggling consequences
that there are many individuals
almost identical to oneself.
There's definitely going to be one
future copy of me that sees A,
and there's definitely going to be one
future copy of me that sees B.
(shears snipping) (playful music)
So quantum mechanics, it begins as a theory
of particles, of atoms, of molecules.
So what many-worlds is saying
is that as Simon decides to prune the roses,
so he splits into two.
In parallel universes,
one Simon goes indoors and reads a book,
whilst the other Simon gets on with the gardening.
So Everett had the basics, he had the basic idea.
The system, very much like a tree,
where innumerable ways of getting
to different paths at the top of the tree.
So we have the branching structure,
where each branch is a world.
On each branch, we have a different
physical states of affairs, just like the one now,
around us. (water hissing)
This does all seem rather bizarre.
We are familiar with the world around us,
where everything behaves as we would expect.
An apple falls from the tree to the ground
according to classical laws.
It all makes sense.
But at the very small, we see classical laws break down.
In the quantum world, particles behave in a weird way.
They can be in different places at the same time,
a cloud of possibilities.
This is the strange world of quantum theory.
Quantum theory has to count as the greatest,
most revolutionary theory ever found in physics,
and the most successful.
Describing the strange,
uncertain behavior of small particles
first seemed impossible.
But in December 1925, an Austrian physicist,
Erwin Schrodinger, retreated to an isolated
mountain cabin to consider the problem.
His great insight was to treat
the electron of a hydrogen atom not as a particle,
but as a kind of wave.
The equation he produced was the Schrodinger equation.
It tells how the state of a quantum system
changes with time, very much like how Newton's second law
describes in the classical world the path of an object
over time. (gentle music)
These changes to a quantum system over time,
he called the wave function.
When we describe an object in quantum mechanics
like an electron or a photon or something like that,
we describe it by a wave function which is something
that's really sort of spread out in space.
It's rather like a field in space.
"Think of the wave function," said Schrodinger,
"as a catalog, a directory of all the future
"possible outcomes for a quantum object."
It was this equation that shaped all of Everett's thinking.
Just take those quantum equations and run with those.
Get everything out of those.
That was Everett's intent. (traffic whizzing)
And that is exactly what he did.
With extraordinary daring, Everett said, "Quantum mechanics
"should be seen as a universal theory."
Just like Newton's law of gravity,
Schrodinger's equation should be
applied to the whole universe.
"Do this," Everett realized,
"and the mathematics perfectly described many-worlds."
Everett didn't put the many-worlds into the theory,
the many-worlds aren't an extra idea
that we bring into quantum mechanics to save it.
The many-worlds are what
quantum mechanics was saying all along.
Everett just said, "Take the theory seriously.
"Take it literally."
Everett would have been first
introduced to Schrodinger's equation
when he was an undergraduate at Princeton.
It would've been demonstrated by the two-slit experiment.
Every student of quantum mechanics comes across
the two-slit experiment with the single photon.
It is, as Richard Feynman memorably put it,
"The central mystery of quantum mechanics."
So what we're looking at here is the laser source.
Now this is a laser that can be attenuated,
turned down, to such a weak intensity
that a single photon is passing
through this apparatus at a time.
We have the two slits.
Iconic example of the strangeness of quantum mechanics
is a particle that passes through two holes.
The photon, the individual photon goes through one slit,
some interaction here, arrives at the screen,
at the photomultiplier.
It's recorded as a blip.
What would you expect to see,
given only one particle at a time
passing through the apparatus?
Well, if the photons were behaving
like, say, tennis balls,
you would expect that the balls would hit the screen
in just two places behind each slit.
But when single photons are fired at the slits,
something strange happens.
The photons are hitting the screen in a place
that would be impossible if they were traveling
in a straight line through the two slits.
So instead of two clear bands in line with the slits,
the photons create a series of smudges.
It looks as if each single photon
somehow goes through both slits at the same time,
and there is interference between the two
as if it were a wave.
We have to consider that it passes through both holes,
because if we think that it passes through one,
we don't see interference,
we don't see fringes of interference.
Everett was entirely familiar
with this device, this apparatus.
What he made of it is that if, at the very small,
quantum mechanics tells us two contradictory
states of affairs obtains at the same time,
and that is boosted up to the macroscopic level,
then big, as well as small,
can be in two places at the same time.
So we can now do two-slit experiment,
for example, with organic molecules.
So all of the evidence in favor of the microscopic
validity of quantum mechanics seems to me
to spread all the way up to the macroscopic level.
If this sounds a very simple conclusion,
then that is because simplicity
is at the heart of Everett's many-worlds.
Thinking in terms of simplicity, the key element
is just to be very simple-minded about things.
One has a rule, the Schrodinger equation,
which tells you how a something, the wave function,
evolves over time.
The Everettian story is just to say,
"Okay, let me just take that straightforwardly
"as a description of the world."
Things, even people, in two places at once.
This is the central idea that nobody
had understood prior to Everett.
To suggest that quantum mechanics
apply to both big and small
was certainly a revolutionary idea.
Before Everett, physicists had divided the world into two.
The large, familiar classical world,
and the small, weird, quantum world.
And one of the greatest physicists
of them all was Niels Bohr.
When Everett was sent to Bohr,
he challenged Bohr's views.
He challenged every part of Bohr's understanding
of quantum theory, his worldview.
For Bohr, quantum theory
only kicked in the microscopic level,
and only in a well-defined way,
given an explicit experimental context.
Bohr's philosophy was called
the Copenhagen interpretation of quantum mechanics.
The interpretation was developed by Bohr
with help from his colleague Werner Heisenberg in 1927.
It was named after the institute he directed in Copenhagen.
However, Bohr's theory may have divided the world
into big classical things like tables and chairs,
and small quantum particles such as atoms,
but he also said something else.
He said that when you look at a particle
in its fuzzy, quantum world,
it stops having an infinite number of different
possible positions and immediately appears
as a single particle in one particular place.
So in the case of the two-slit experiment,
for example, the electron goes through the slit
or the photon goes through the double slit,
it spreads out, but when we look at the screen,
we always find the electron or the photon
in one definite place.
Bohr's Copenhagen interpretation
provided an explanation for this strange behavior
and it was all to do with the particle's wavefunction,
its catalog of quantum possibilities.
Bohr said that it was the act of measurement,
simply looking at something,
that collapsed the wavefunction,
with all its many possibilities,
into just the one single outcome,
the reality around us that we actually see.
But to Everett, this made no sense, why?
For something to exist,
does someone need to be looking at it?
And Everett was not alone in this thought.
So Einstein famously asked,
"Is the moon there when nobody looks?"
And of course, he meant it rhetorically.
He was giving example of something
where any fool would concede that the moon is simply there.
I think that the moon is there.
There's no doubt that the moon is there
when we're not looking at it, right?
We're not a special object in the universe.
We're just natural creatures in a natural world.
We're just a piece of the universe,
a piece of physics like everything else.
And like everything else,
we interact with the rest of the world,
so things get a reality for us when they interact with us.
Schrodinger, in particular,
Schrodinger did not agree with Bohr.
The idea that on performing a measurement,
something radical and dramatic
and discontinuous and indeterministic takes place
on measurement, he found absurd.
What counts as a measurement?
(upbeat music)
To bring out this absurdity,
he gave his experiment involving a cat.
What Schrodinger did to the poor cat
was use it to sort of make clear how crazy sounding
the way people were talking about quantum mechanics is,
because the cat is just a very vivid way
of measuring a system.
(cat meows) A cat is locked in a box.
(cat purrs) In the box
is a radioactive substance, a flask of poisonous gas,
and hammer triggered by radiation
from the decaying uranium.
So Schrodinger said, "Right".
You couldn't do this these days.
"Wire up the possibly decayed atom
"to some poor cat
"and to some gadget that kills cats
"and so if the atom decays,
"the cat dies, (cat meows)
"if the atom doesn't decay, the cat lives."
(cat purrs)
If you take Schrodinger's cat,
that would be something where you initially have a cat
and then you do an experiment
which according to the evolution of Schrodinger's equation,
would provide what we call a superposition
of a cat being alive and dead at the same time
and that's what the Schrodinger equation evolves to,
but then you say, you do a measurement on the cat,
or look at it,
and what you see is either a dead cat or a live cat.
The idea of superposition
is at the heart of the Schrodinger equation.
The wavefunciton, remember,
describes each of the states of the superposition.
It's what makes it like a wave equation.
States superpose rather in the way that waves of water
can be added to one another.
Now, quite why looking in the box suddenly makes
just one of the states of the superposition,
say, a live cat, appear,
whilst another, the dead cat,
just blinks out of existence,
even Bohr could not explain.
It did seem to be all rather mystical.
Neils Bohr and his Copenhagen interpretation
sort of admitted it was a cheat in a sense
by saying, "You've got to treat the measuring apparatus
"as a classical device,
"so you don't treat it according to quantum mechanics."
If you did treat it according to quantum mechanics,
then the measurement would say
cat dead and cat alive at the same time,
and the thing is that the Schrodinger equation
preserves the superposition forever.
And so to Everett,
it was clear that there was no need to cheat.
He said "Just treat the meausrment apparatus
"as part of the quantum system."
This was Everett's big central idea.
Simply let the Schrodinger equation apply to everything,
the whole universe, including the experimental apparatus.
For Everett, quantum theory was to be taken
as a universal theory, a fundamental theory
and applies to everything, big and small,
so apply it to the experimental apparatus as well,
apply it to the experimentalist as well.
These ideas were anathema to Bohr.
To many physicists,
Everett's ideas were also profoundly disturbing.
If the wavefunction didn't really collapse,
then this could only mean one thing.
Every possible state of the superposition
of the cat would be preserved.
The Everett interpretation had arrived
at the fantastical idea of many worlds,
two cats, each cat in its own separate parallel world.
And imagine people, also involved in this experiment,
we have the superposition of people seeing the cat alive,
the superposition with people seeing the cat dead.
So instead of just being cat alive
and dead at the same time,
there's now cat alive, David sees living cat,
and cat dead, David sees dead cat at the same time.
It's the inevitable consequence of the equations.
Everett was prepared to run with that.
Nobody else before him had the courage to suggest this.
But the measurement paradox afflicts the recovery
of anything like the world that we see
and the mathematical formulism.
But for Simon and the Oxford group,
their support of the many-worlds interpretation
means facing the criticism of leading physicists
who simply do not buy Everett's ideas.
For Roger Penrose,
the problem with the Everett interpretation
is Schrodinger's equation itself.
If you believe that the Schrodinger equation,
or unitary evolution as it's sometimes
more correctly called if you like,
that unitary evolution is an exact law of nature,
then you are lead to universes
in which all of these
different alternatives somehow coexist.
Now the question is why don't we see that?
And this is where the problem
for this Everett-type interpretation comes in
because we don't see that kind of a world.
(people chatting)
Simon is following the steps of Schrodinger
and so what exists is this wavefunction.
This is a realistic position.
That's the thing that is out there.
It creates a number of problems
like how do you go from this wavefunction
to the actual things of the world we see,
which are chairs, shoes, trousers,
bee, air, particles, stuff.
To my view, the many-worlds,
or the Everett interpretation if you like,
is a reducto ad absurdum.
It's telling you what, if you simply believe
that the Schrodinger equation,
or unitary evolution, is exactly true of the world,
you get nonsense.
When we use a metaphor, we are used to the fact
that each of us has a different perspective on the world.
I see this room from a different perspective.
I see a chair from here.
You see a chair from there.
Another person see a chair from there,
so the complexity from reality
can be seen as an intersection of perspective
that match or don't match.
Simon want to break it up
into a multiplicity of worlds.
I prefer to have a single world
with a number of internal perspectives.
'Cause Carlo is not prepared
to apply quantum theory to everything.
I've talked about big and small.
Even more important is that it's everything.
For Carlo, there's always the observer
who is outside the system.
(upbeat music)
That's the difference.
(sad music)
Many-worlds is difficult.
It's fantastical and in a way,
there's concern that the ideas can't be taken seriously.
This has been extremely difficult for me
in those early days.
The trouble was Simon knew all too well
that Everett's theory was not without its problems.
Everett may have described the development
of the superposition of worlds,
but he could only do so through experiment.
But there was the problem.
The only way that he had of getting out
these macro superpositions
was in the context of an experiment.
It seemed it was no different from Bohr.
You needed the experiment.
You needed actually the dynamics
to be cooked up for the experiment
to give you the superposition of worlds.
That leads you to the many-worlds theory, which is my.
Everett's insight, it didn't really take
measurement itself out of the story,
so it didn't fully fulfill Einstein's hope
of a physics with no reference
to measurement or observation.
I mean, what about a world,
a universe without any people, without any experiments?
Would there not be many-worlds in that universe?
Is it just somehow with human beings
that we create these many worlds?
This made nonsense of the theory for many people.
To try to reformulate quantum mechanics
to avoid troublesome mention of measurement
or observation. But Simon had arrived
at Oxford with a set of ideas
that he believed would solve this problem
and vindicate Everett.
The 1980s had seen the emergence of a new mathematical tool
not available to Everett.
It was called decoherence theory.
The Oxford group believe that decoherence theory
could describe how the branching of worlds occurs
regardless of whether measurements are performed
and why it is that the world in which we exist
never recombines with all the other branching worlds
and why each world is oblivious to all others.
What decoherence theory taught us
was that this constant branching
is something that's happening naturally,
just as a result of physical processes all the time.
The University of Oxford,
today at the forefront of quantum computing,
is no stranger to decoherence theory.
Quantum computers work by maintaining
the quantum state of a system in a superposition of states,
each computation delicately interacting with each other.
If there's any contact between the system
and the surrounding environment,
then just as the ripples from a stone
dropped in a lake disappear into the environment,
so the interference effects,
so typical of the quantum, are washed out.
This is decoherence, and when it happens,
all quantum behavior is lost.
I took over these methods developed by others,
decoherence theory and very specifically,
the decoherent histories formulism
because that is the one that really
makes best sense of Everett's ideas
and I shared how the standard problems
that beset the theory could be solved.
Decoherence theory turns out to be the answer
of why it is that one actually gets
a plurality of macroscopic determinate worlds
out of a fundamentally, if you will,
formless, evolving wavefunction.
The whole language of worlds and branches
becomes okay when those interference effects
get washed out
and that's what decoherence does.
First and foremost, you no longer
have to make reference to experiments.
That was the most important of all.
These ideas work for any kind of macroscopic process.
There doesn't have to be a measurement process.
In fact, all it needs is a universe itself.
It is the leaking of the quantum interference effects
into the environment that makes it appear
there is wavefunction collapse.
Interference, which is the quantum phenomenon
that leads to all the paradoxical results
of the two-slit experiments and the like in quantum theory,
interference is a delicate flower
and when you're trying to create
quantum mechanical phenomena on large scales,
then very quickly that interference disappears,
and that disappearance of interference
is what we mean when we say that the worlds,
the branches in the quantum wavefunction
stop interacting with each other.
When decoherence happens,
that's when somebody in one branch
can no longer interact with the goings on in another branch.
It all sounds rather fantastical
that we are surrounded by numerous other universes
that don't interfere with each other
and that we can only see the one we ourselves are in,
but is it really so strange?
♪ Close down, make decisions now ♪
The collapse of the wavefunction
is this idea of signals in radio waves
being made at the same time.
Each of them is oblivious to all of the others
that are being made,
but we think, hang on, there can only be one signal,
so all of the others somehow have to go away.
♪ On the gulf stream ♪
Why do the others have to go away?
If they don't interfere with one another,
why couldn't they all be present at the same time?
And of course, they are.
We tune into different radio stations,
we only hear one at a time.
Each frequency has decohered from all the others.
Just as we are surrounded by different radio channels,
so we are surrounded by entire universes,
all possible versions of the superposition
all independent from each other.
(radio crackling)
The process of decohrence provided
the basis of a unique way of looking
at the superposition principle.
In other words it provided a unique account
of how these branches were described with in the theory.
If decoherence has happened,
then that's when the many-worlds interpretation
will say branching has occurred
and then its description of the situation
will be no, there are really two cats here,
one on one branch of the universe
and one on the other branch of the universe.
One of them's alive, one of them's dead.
It's all quite classical.
And it's how this classical world emerges
out of the wavefunction,
where decoherence finally answers Everett's critics.
Just like radio stations,
decoherence theory has a unique way
of showing the superposition of worlds.
The Schrodinger equation dictates
how the sequence of superpositions unfold,
but track an individual term in the superposition
from one time to the next,
and what emerges is a classical world.
Do the same for each term
and you have, like radio stations, many parallel worlds,
each obeying approximately classical equations,
each story slightly different from each other,
and it's not just Newtonian laws
that reveal themselves,
but also equations for gases, for fluids,
equations for crystals, for chemical reactions,
for air particles, for materials
like chairs, tables and books are made of
and even cats. (cat meows)
Decoherence theory gave us
macroscopic superpositions without measurements.
They're happening all the time.
It gave us the full vivdness and richness and variety
of macroscopic world
and matter in all its forms.
It took those branches
and it showed why branching structure is there,
why you don't get recombination of branches.
(woman laughing)
The contribution of Simon and his colleagues
to the idea of many-worlds is nearly done.
Bohr relied on the idea of classical experiments,
Everett replaced them by quantum experiments,
the Oxford group got rid
of the dependence on experiments altogether,
and they showed how the classical reality of our own world
arises out of the wavefunction.
Whilst the debate about many-worlds will continue on,
the Oxford group is now breaking up.
After two decades, they're now going their separate ways,
some to new posts in America,
others to retirement,
but their legacy is clear.
Cheers.
(upbeat music)
So you basically only need
things like the principle of superposition
and the basic mechanisms of decoherence
for the many-worlds story to emerge.
These results are sensational
from the point of view of realism,
if you can buy the many-worlds.
(audience applauding)
What I believe is that the only way
to interpret quantum mechanics realistically
is in terms of parallel universes.
There are parallel universes
in so far as quantum mechanics is true
and I don't know if quantum mechanics is true.