
English: 
I recently gave a talk, here at the Ri,
 on Quantum Mechanics
which, I was very pleased to see,
got a big response.
However, some people,
some viewers of that talk
expressed some bafflement,
or worse
about an analogy
that I used during it.
And so, I wanted here
to go through that analogy
 in a little more detail,
hopefully a little more clearly, 
and explain what it's really about,
and what it's really
trying to show.
It's an analogy
 to try to understand
the quantum phenomenon
 called entanglement.
And I want, first of all, to point out
that the analogy isn't mine.
It's not something
that I've invented.
It's adapted
from an analogy
 that was devised in the late 1990s
by two physicists, Sandu Popescu and Daniel Rohrlich.
And they were trying to understand
the implications of this quantum property of entanglement.
And I'll come back, at the end, to what it was that they were exploring.

English: 
I recently gave a
talk here at the Ri
on quantum mechanics, which
I was very pleased to see
got a big response.
However, some people,
some viewers of that talk,
expressed some bafflement
or worse about an analogy
that I used during it.
And so I wanted here to
go through that analogy
in a little more detail,
hopefully, a little more
clearly, and explain
what it's really about
and what it's really
trying to show.
It's an analogy to try
to understand the quantum
phenomenon called entanglement.
And I want first
of all to point out
that the analogy isn't mine.
It's not something
that I've invented.
It's adapted from
an analogy that
was devised in the late 1990s
by two physicists, Sandu
Popescu and Daniel Rohrlich.
And they were
trying to understand
the implications of this quantum
property of entanglement.
And I'll come back
at the end to what it
was that they were exploring.

English: 
Now, I think some people had the impression
that this was just a very complicated way
of talking about entanglement.
It's not doing that.   This analogy is actually
doing something a little more than that.
But I'll talk, first of all,
about what entanglement is.
It's what happens
when any two quantum particles interact.
It has to happen.
And what results from that
 is that those particles,
once they've interacted, they are "entangled",
and it means that their quantum states are interdependent.
There's a correlation
between them,
so that if one is
 in one particular state,
then the other one has to have
some other particular state,
depending on the kind of entanglement
 that they have.
So, the classic example
 would involve, say, 2 electrons.
And electrons have
a quantum property called spin,
which you don't
need to know anything about,
other than that
 the spin of an electron can have 2 values.
It's a bit like a sort of
quantum heads or tails.

English: 
Now I think some people
had the impression
that this was just a very
complicated way of talking
about entanglement.
It's not doing that.
This analogy is actually
doing something a little more
than that.
But I'll talk first of all
about what entanglement is.
It's what happens when any two
quantum particles interact.
It has to happen.
And what results from
that is that those
particles, once they've
interacted, they are entangled.
And it means that their quantum
states are interdependent.
There's a correlation
between them,
so that if one is in
one particular state,
then the other one has to have
some other particular state,
depending on the kind of
entanglement that they have.
So the classic example would
involve, say, two electrons.
And electrons have
a quantum property
called spin, which you don't
need to know anything about,
other than that the spin of an
electron can have two values.
It's a bit like a sort of
quantum heads or tails.

English: 
An electron can be
spin up or spin down.
And if the electrons
become entangled,
...then...
this may create a situation
where those two spins are correlated,
such that if one of the electrons
 has a spin up,
then the other one
must have a spin down.
Now, this is a prediction
of Quantum Mechanics.
And it was pointed out {that this property}...
this...   this phenomenon of entanglement CAN happen.
It was pointed out
 by Albert Einstein, in 1935.
And he figured
 that there was something wrong about it.
I explained in the talk
 how we can think about entanglement.
In some ways, this correlation
 between the 2 states of electrons
in some ways, it's a bit like having 2 gloves,
 a left-handed and a right-handed glove.
Now, if you imagine
 that you have those 2 gloves,
and you send them out to 2 people
 on different sides of the world, maybe.
And these 2 people
 - we're going to meet them again shortly -
are called
Alice and Bob.
Amm...  And they know
 that these gloves are a pair.

English: 
An electron can be
spin up or spin down.
And if the electrons
become entangled,
then this may create a situation
where those two spins are
correlated such that if one of
the electrons has a spin up,
then the other one
must have a spin down.
Now this is a prediction
of quantum mechanics.
And it was pointed out that
this phenomenon of entanglement
can happen.
It was pointed out by
Albert Einstein in 1935.
And he figured that there
was something wrong about it.
I explained in the talk how we
can think about entanglement.
In some ways, this
correlation between the two
states of electrons,
in some ways,
it's a bit like having
two gloves, a left-handed
and a right-handed glove.
Now if you imagine that
you have those two gloves,
and you send them out to two
people on different sides
of the world, maybe.
And these two people-- we're
going to meet them again
shortly-- are called
Alice and Bob.
And they know that
these gloves are a pair.

English: 
And so, as soon as Alice
receives her glove, and opens the package,
and finds she has the left-handed glove,
then she knows,
right away, that Bob
must have the right-handed glove,
because there is 
this correlation between them.
There's nothing magical
 about how she gets that information.
It's just simple logic.
However, here's the complication
in Quantum Mechanics.
Because Niels Bohr, the Danish physicist
 who pioneered early Quantum Mechanics,
suggested that, 
in the case of quantum particles,
it's not the fact that they have this property, 
whatever it is, spin or whatever,
all the time.
They only have that property,
 with a fixed value, when we observe it.
So, for these 2 entangled photons {electrons},
if we think of sending those out
to Alice and Bob,
Bohr said:
While they're going outwards,
they DON'T HAVE
 a fixed orientation of their spin.
All we can say is that
those spins are correlated.

English: 
And so as soon as Alice receives
her glove and opens the package
and finds she has the
left-handed glove,
then she knows
right away that Bob
must have the right-handed
glove, because there is
this correlation between them.
There's nothing magical about
how she gets that information.
It's just simple logic.
However, here's the complication
in quantum mechanics.
Because Niels Bohr, the Danish
physicist who pioneered early
quantum mechanics,
suggested that,
in the case of
quantum particles,
it's not the fact that they have
this property, whatever it is,
spin or whatever,
all the time.
They only have that
property with a fixed value
when we observe it.
So for these two
entangled photons,
if we think of sending
those out to Alice and Bob,
Bohr said, while
they're going outwards,
they don't have a fixed
orientation of their spin.
All we can say is that
those spins are correlated.

English: 
So, if Alice then measures her electron,
and finds it has spin up,
then Bob's will have spin down.
But that spin wasn't determined
until Alice measured it.
And this is where Einstein felt
 there was a problem with entanglement,
because it seemed to indicate that
 the act of Alice measuring her electron spin...
somehow affected Bob's spin.
So that once Alice had found
the spin had to be spin up,...
somehow, magically
 - or "spookily," as Einstein said -
that seemed to sort of transmit
some kind of influence to Bob's spin, 
to make sure that it was spin down.
Alice could equally
 have measured her electron
and found that it had spin down, 
in which case Bob's would be spin up.
So, there seemed to be this - what Einstein called
 "a spooky action at a distance" - implied...
by Bohr's idea
 of Quantum Mechanics.
Einstein and two colleagues,
Podolsky and Rosen,

English: 
So if Alice then
measures her electron
and finds it has spin up, then
Bob's will have spin down.
But that spin wasn't determined
until Alice measured it.
And this is where
Einstein felt there
was a problem with
entanglement, because it seemed
to indicate that the act of
Alice measuring her electron
spin somehow
affected Bob's spin.
So that once Alice had found
the spin had to be spin up,
somehow, magically-- or
"spookily," as Einstein said--
that seemed to sort of
transmit some kind of influence
to Bob's spin to make sure
that it was spin down.
Alice could equally have
measured her electron
and found that it had
spin down, in which case,
Bob's would be spin up.
So there seemed to be
this, what Einstein called,
"a spooky action at a
distance" implied by Bohr's
idea of quantum mechanics.
Einstein and two colleagues,
Podolsky and Rosen,

English: 
suggested in 1935
that, actually, there
has to be some
alternative to this,
because spooky
action at a distance
shouldn't be allowed in physics.
Einstein had showed that it
is impossible for any signal,
any information to be
transmitted faster than light.
And so you can't have
this instantaneous action
at a distance.
There has to be some time for
a signal to span the distance.
And so Einstein suggested that
what must be going on instead
is that, all along,
these two electrons had
some property that somehow
fixed their spins already.
It's just that it was a property
that we couldn't measure.
He called them hidden variables.
So you couldn't find out
in any experiment which
of the two possible spins
Alice's electron had
as it was going towards her.
But nevertheless, it was fixed.
And so that then
reduces the situation
to being like the left-handed
and the right-handed glove,
which were left-handed
and right-handed

English: 
suggested in 1935 that, actually, 
there has to be some alternative to this,
because "spooky action at a distance"
shouldn't be allowed in Physics.
Einstein had showed that it is impossible
 for any signal, any information
to be transmitted faster than light.
And so, you can't have
this instantaneous "action at a distance".
There has to be some time
 for a signal to span the distance.
And so, Einstein suggested that,
what must be going on instead,...
is that, all along,
these two electrons
had some property
 that somehow fixed their spins already.
It's just that it was a property
that we couldn't measure.
He called them "hidden variables".
So you couldn't find out,
in any experiment, which of the 2 possible spins...
Alice's electron had as it was going towards her,
but, nevertheless, it was fixed.
And so, that then reduces the situation to being
like the left-handed and the right-handed glove,...

English: 
which were left-handed and right-handed
 all along, in transit.
So, there were
 these 2 possibilities
for what entanglement was about,
Bohr's view and Einstein's view.
The trouble was,
there was no obvious way
 of distinguishing between them,
because they both
predicted the same outcome,
which was that we would measure
— or Alice and Bob would measure —
that there are
 these correlations that exist
between the spins
 of the 2 entangled electrons.
How do we know if that's due to hidden variables,
 or due to something else,
that Bohr was suggesting?
That changed in 1964, 
when the Irish physicist, John Bell,
suggested an experiment.
It was...  In that case, at that stage,
 it was just a thought experiment
that he said 
would allow us to distinguish
between these 2 possibilities.
And it's John Bell's experiment
that these boxes,
 these quantum boxes, are mimicking.

English: 
all along in transit.
So there were these
two possibilities
for what entanglement was about,
Bohr's view and Einstein's
view.
The trouble was, there was no
obvious way of distinguishing
between them, because they both
predicted the same outcome,
which was that we
would measure--
or Alice and Bob would measure--
that there are
these correlations
that exist between the spins of
these two entangled electrons.
How do we know if that's
due to hidden variables
or due to something else
that Bohr was suggesting?
That changed in 1964, when
the Irish physicist John
Bell suggested an experiment.
In that case, at that stage, it
was just a thought experiment
that he said would allow us to
distinguish between these two
possibilities.
And it's John Bell's experiment
that these boxes, these quantum
boxes, are mimicking.
Personally, I've never
seen an explanation

English: 
of John Bell's experiment
that is at all easy to follow.
So instead of trying to
explain John Bell's experiment,
that's what these boxes are for.
So here's how this
box analogy works.
There are these two boxes.
They are machines,
slot machines,
into which you can put a
coin and get out a toy.
So you can put it
in either-- they
will take either one-pound
coins or two-pound coins.
And out will come one
of two types of toy,
either a rabbit or a dog.
And there are particular
rules for each kind of machine
that will tell you, if you
put in a certain coin, then
you will get out a
certain animal, OK?
And we have to figure
out combinations
of which coins give which
kind of animals in order
to satisfy three rules.
And I'm just going to
postulate these rules.
But of course, they've
actually been carefully chosen
so that they replicate
the kind of situation

English: 
Personally, I've never seen
an explanation of John Bell's experiment
that is at all easy to follow.
So, instead of trying to
explain John Bell's experiment,
that's what these boxes are for.
So, here's how
 this box analogy works.
There are these 2 boxes.
They are machines,
slot machines,
into which you can put a coin
 and get out a toy.
So you can put it in either...   they will take
 either 1-pound coins or 2-pound coins.
And out will come
one of 2 types of toy, 
either a rabbit or a dog.
And there are particular rules
 for each kind of machine,
so that if you put in...   that will tell you,
 if you put in a certain coin,...
then you will get out a
certain animal, OK?
We have to
figure out
combinations of which coins
 give which kind of animals,
in order to satisfy 3 rules.
And I'm just going
 to POSTULATE these rules.
But, of course, they've actually
 been carefully chosen
so that they replicate
the kind of situation

English: 
that Quantum Mechanics imposes
in John Bell's experiment.
And the rules go as follows.
The first rule is very simple,
that if Alice puts in
 a 1-pound coin into her box,
it will spit out a rabbit.
The second rule is that,
if both Bob and Alice
put in 2-pound coins
into their boxes,
then the boxes will produce
1 rabbit and 1 dog.
And it doesn't matter
which way 'round that is, but they will have
that combination.
The third rule is that
any other combination of coins,
other than two 2-pound coins,..
will produce
 either 2 rabbits or 2 dogs.
So we have to find
inputs and outputs
that satisfy these 3 rules.
So what can they be?
Well, let's work through them.
We know already what the output of Alice's box
 has to be if she puts in a 1-pound coin.
It has to be a rabbit. 
That's the first rule.

English: 
that quantum mechanics imposes
in John Bell's experiment.
And the rules go as follow.
The first rule is very
simple, that if Alice
puts in a 1-pound coin into her
box, it will spit out a rabbit.
The second rule is that
if both Bob and Alice put
in 2-pound coins
into their boxes,
then the boxes will produce
one rabbit and one dog.
And it doesn't matter
which way 'round that is,
but they will have
that combination.
The third rule is that
any other combination
of coins other than
two 2-pound coins
will produce either two
rabbits or two dogs.
So we have to find
inputs and outputs that
satisfy these three rules.
So what can they be?
Well, let's work through them.
We know already what the
output of Alice's box
has to be if she puts
in a 1-pound coin.
It has to be a rabbit.
That's the first rule.

English: 
Now, when we think about it,
this means that,
 no matter which coin,
- whether a 1-pound or 2-pound -
Bob puts into his box,
it has to produce also a rabbit.
That's because,
if Alice has produced
 a rabbit with 1 pound,
then
the only way
 we can get a dog
is that   
if they both put in 2 pounds.
So Alice already put in 1 pound.
So, Bob's box has to produce a rabbit
 in both of those cases,
with a 1-pound
 or a 2-pound.
So, we've almost
already figured out
 what our rules have to be.
So, all we need to figure out now is
 what a 2-pound coin in Alice's box will produce.
Well, let's think about it.
If she puts in a 2-pound,
Bob puts in a 2-pound, too.
We've also got the second rule, 
which says that
two 2-pound coins have to produce
a rabbit and a dog.
So that must mean
 that a 2-pound in Alice's box produces a dog.
And then we've satisfied
the second rule.

English: 
Now when we think about it,
this means that no matter which
coin-- whether a
1-pound or 2-pound--
Bob puts into his box, it
has to produce also a rabbit.
That's because if Alice has
produced a rabbit with 1 pound,
then the only way we can get
a dog is that if they both put
in 2 pounds.
So Alice already put in 1 pound.
So Bob's box has to produce a
rabbit in both of those cases,
with a 1-pound or a 2-pound.
So we've almost already figured
out what our rules have to be.
So all we need to
figure out now is
what a 2-pound coin in
Alice's box will produce.
Well, let's think about it.
If she puts into a 2-pound,
Bob puts in a 2-pound, too.
We've also got the
second rule, which
says that two 2-pound
coins have to produce
a rabbit and the dog.
So that must mean that a 2-pound
in Alice's box produces a dog.
And then we've satisfied
the second rule.

English: 
The trouble with that
is that those [outputs]
inputs and outputs
violate the rules
in another case.
Because for a 1-pound
and a 2-pound,
we've got this combination
of rabbit and dog.
But we're only meant to get that
if they both put in 2 pounds.
So in one time,
 out of the 4 possible permutations,
the rules are violated.
And no matter
how you try and do this,
no matter how you try
 and think of different combinations,
you will find that you can never do better
 than 3 times out of 4.
Now, if you think
 that you have found a solution
that satisfies these 3 rules
 all the time, in all 4 cases,
it's probably that you've come up
 with a solution like this one.
A solution in which, let's say,
Alice's box
alters its output
depending on
 which coin Bob put in.
There is no physical way,
if these boxes are unconnected
— there is nothing
 that passes between them —

English: 
The trouble with that is
that those inputs and outputs
violate the rules
in another case.
Because for a 1-pound
and a 2-pound,
we've got this combination
of rabbit and dog.
But we're only meant to get that
if they both put in 2 pounds.
So in one time out of the
four possible permutations,
the rules are violated.
And no matter how
you try and do this,
no matter how you try and think
of different combinations,
you will find that you can never
do better than three times out
of four.
Now if you think that you have
found a solution that satisfies
these three rules all the
time in all four cases,
it's probable that
you've come up
with a solution like this one,
a solution in which, let's say,
Alice's box alters
its output depending
on which coin Bob put in.
There is no physical way--
if these boxes are
unconnected, there
is nothing that
passes between them--
there is no physical
way in which

English: 
we can build a machine that does
that, that somehow magically
or telepathically knows
what the other person has
put in into their box.
So that's not going to work.
However, there is a way that
we can allow that to happen,
which is that we produce
a physical connection
between the boxes that
sends a signal between them
so that Bob's box, for example,
receives a signal telling it
what Alice has put into her box.
And then it might alter
its output accordingly.
It's perfectly possible to
produce a mechanism like that.
The trouble with
that is that Bob
has to wait until the signal
is received from Alice's box,
or until she's put in her coin
and the signal has been sent.
He has to wait until
that's happened
before he puts in his coin so
that his box knows what to do.
That takes some
finite amount of time.
Even if the signal is travelling
at the speed of light,

English: 
there is no physical way
 in which we can build a machine
hat does that, that somehow
 magically or telepathically knows
what the other person
 has put in into their box.
So that's not going to work.
However, there is a way
that we can allow that to happen,
 which is that we produce
a physical connection
between the boxes,
 that sends a signal between them.
So that
So that Bob's box, for example,
receives a signal telling it
 what Alice has put into her box.
And then it might
alter its output accordingly.
It's perfectly possible
 to produce a mechanism like that.
The trouble with that
 is that Bob has to wait
until the signal
is received from Alice's box,
or until she's put in her coin
and the signal has been sent.
He has to wait
 until that's happened
before he puts in his coin,
 so that his box knows what to do.
That takes some
finite amount of time.
Even if the signal is travelling
at the speed of light,

English: 
it's still going to take
some time to get there.
So that's not going
to work if we're
trying to make
boxes that satisfy
these rules instantaneously,
when Bob and Alice put
in their coins at
exactly the same moment.
We want an instantaneous
solution to this problem.
So that's never going to happen.
At least, it's never going to
happen for classical boxes.
If these are quantum boxes--
if we allow them this property
of quantum entanglement
so that their inputs and outputs
can be correlated in some way--
then we can do better.
And in fact, we know
exactly how much better
we can do, because the laws of
quantum mechanics-- the rules,
the mathematical
equations-- allow
us to calculate exactly
how much more often
we can satisfy the four rules
compared to the classical case.
So in the classical
case, we can only
get it three times out of
four, 75% success rate.

English: 
it's still going to take
some time to get there.
So,
that's not going to work
if we're trying
 to make boxes that satisfy
these rules instantaneously,
when Bob and Alice
put in their coins
 at exactly the same moment.
We want an instantaneous solution
 to this problem.
So that's
 never going to happen!
At least, it's never going to
happen for classical boxes.
If these are quantum boxes,
if we allow them this property
of quantum entanglement,
so that the {2 boxes can be...}
their inputs and outputs
can be correlated in some way,
then,...we can do better.
And in fact, we know exactly
 how much better we can do,
because the laws of
quantum mechanics,
— the rules, 
the mathematical equations —
allow us to calculate
 exactly how much more often
we can satisfy the...the 4 rules,
compared to the classical case.
So, in the classical case, we can only get...
get it 3 times out of 4, 75% success rate.

English: 
Quantum Mechanics tells you
that you can get roughly 85% success rate.
And why...   why 85?
Well, that is...   is just a number
that the equations give you.
It's actually, 
more precisely, it's, ahh,...
it's a number that involves
the square root of two.
We don't need to worry about that.
It's just approximately 85%.
But I will come back
to why 85% later.
So, Quantum Mechanics allows you to do better
 if these two boxes are entangled.
And this is really
what Bell's experiment
was allowing you to do.
You did
 the equivalent measurement
with 2 particles
that were entangled.
And Bob and Alice
 were making measurements,
making choices about 
how they make those measurements,
and seeing how strong
 the correlation was between them.
According to Classical Physics,
 you could only get a 75% correlation.
But the same was true,
John Bell showed,
of Einstein's hidden variables.
It was only if Quantum Mechanics
 went beyond that,
as Bohr suggested it did,

English: 
Quantum mechanics tells you
that you can get roughly 85%
success rate.
And why 85?
Well, that is just a number
that the equations give you.
It's actually, more
precisely, it's
a number that involves
the square root of two.
We don't need to
worry about that.
It's just approximately 85%.
But I will come back
to why 85% later.
So quantum mechanics allows
you to do better if these two
boxes are entangled.
And this is really
what Bell's experiment
was allowing you to do.
You did the
equivalent measurement
with two particles
that were entangled.
And Bob and Alice were making
measurements, making choices
about how they make
those measurements,
and seeing how strong the
correlation was between them.
According to classical
physics, you could only
get a 75% correlation.
But the same was true,
John Bell showed,
of Einstein's hidden variables.
It was only if quantum
mechanics went beyond that,
as Bohr suggested
it did, that you

English: 
could do better and get 85%.
Well, as I say this was
just a thought experiment
for John Bell.
But very soon, physicists
realised that you could do it
for real.
You could create two
entangled particles
and make the measurements.
And this was done.
It was first done in
the 1970s and then more
definitively in the 1980s.
And since then, it's been
done many, many times
in many, many different ways.
Every single time, the
result has been very clear.
The classical limit or
the hidden variables limit
of 75% correlation between the
particles is always exceeded.
You get this 85% success.
And so this suggests
that Einstein's idea
that there are these things
called hidden variables, which
fix the properties
of quantum particles
before they're measured,
this doesn't apply.
It seems that Bohr was right.
Now this doesn't imply, as
you might sometimes hear,

English: 
that you could do better,
 and get 85%.
Well, as I say, this was
 just a thought experiment for John Bell.
But very soon, physicists realised
 that you could do it for real.
You could create 2 entangled particles
 and make the measurements.
And this was done.
It was first done in the 1970s,
and then, more definitively, in the 1980s.
And since then, it's been done many, many times,
 in many, many different ways.
Every single time, 
the result has been very clear.
The classical limit,
 or the hidden variables limit
of 75% correlation between the particles
 is always exceeded.
You get this 85% success.
And so, this suggests
that Einstein's idea
that there are these things
called hidden variables,
which fix the properties
of quantum particles
before they're measured,
this doesn't apply.
It seems that Bohr was right.
Now,...
this doesn't imply, 
as you might sometimes hear,

English: 
that there really is
 some "spooky action at a distance".
Many times, when experiments
like this have been done,
the newspaper headlines
have proclaimed
that Einstein is proven wrong,
and "spooky action at a distance" is real.
Spooky action at a distance was 
what Einstein's interpretation of entanglement involved.
But, actually, it is a better way
 to think about entanglement
to say something
 a little different.
And one way to think about it is to say that,
 once the 2 boxes become entangled,
they are no longer
 separate objects!
So that what happens over here
is completely independent
 of what happens over here.
They are, in some quantum sense, 
the same object.
And that remains the case,
 no matter how far apart they are.
Even if the particles
 that you measure
were separated,
were on the opposite sides of the galaxy,
they remain, in some sense,
a single quantum entity.
Another way of thinking
 about that, is to say
that there is some kind
 of sharing of information between them.

English: 
that there really is some
spooky action at a distance.
Many times, when experiments
like this have been done,
the newspaper headlines
have proclaimed
that Einstein is proven
wrong and that spooky action
at a distance is real.
Spooky action at a distance was
what Einstein's interpretation
of entanglement involved.
But actually, it is a better
way to think about entanglement
to say something a
little different.
And one way to think about it
is to say that once the two
boxes become entangled, they are
no longer separate objects, so
that what happens over here is
completely independent of what
happens over here.
They are, in some quantum
sense, the same object.
And that remains the case no
matter how far apart they are.
Even if the particles that
you measure were separated,
were on the opposite
sides of a galaxy,
they remain, in some sense,
a single quantum entity.
Another way of
thinking about that
is to say that there
is some kind of sharing
of information between them.

English: 
And this is really what Popescu
and Rohrlich's quantum boxes
were about.
It was expressing this situation
in terms of a kind of sharing
of information.
Physicists call this
quantum nonlocality.
And it's distinct
from this notion
that somehow making a
measurement on this particle
is transmitting information--
is transmitting a signal--
to the other particle to fix
what the value of its property
is.
That doesn't happen.
If it did happen, it would
violate special relativity,
as Einstein suggested.
But quantum mechanics
tells you something else.
It tells you that there
is this property called
quantum nonlocality, which is
very hard to find words for,
but that is a real
property of the world.
So if you see headlines saying
spooky action at a distance
is real, don't believe them.
Now I want to come
back, finally,
to what Popescu
and Rohrlich were
trying to do with their boxes.
Because one physicist,
Yakir Aharonov,

English: 
And this is really what Popescu and Rohrlich's
 quantum boxes were about.
It was expressing this situation
in terms of a kind of sharing of information.
Physicists call this
quantum nonlocality.
And it's distinct
from this notion that, somehow,
making a measurement
 on this particle is transmitting information
— is transmitting a signal — to the other particle,
 to fix what the value of its property is.
That doesn't happen.
If it did happen, 
it would violate special relativity,
as Einstein suggested.
But Quantum Mechanics
tells you something else.
It tells you that there is this property,
 called quantum nonlocality,
which is very hard
 to find words for,
but that is a real property
 of the world.
So, if you see headlines saying
"spooky action at a distance" is real, don't believe them.
Now I want to come back,
 finally, to what
Popescu and Rohrlich
 were trying to do with their boxes.
Because

English: 
one physicist, ahh,...
Yakir Aharonov, suggested, umm...that…
…perhaps what we're seeing here,
 with this quantum nonlocality, is kind of
a stretching of the normal laws
 of cause and effect,
as far as 
special relativity will allow.
umm... So that somehow,
 Quantum Mechanics is sort of…
…almost violating the spirit of special relativity, 
without violating it actually in practice,
without actually allowing you
 to transmit information faster than light.
Perhaps Quantum Mechanics
 is doing this
you know, right up to the limit
 of what is physically possible.
Well, Popescu and Rohrlich thought,
Let's see about that!
Can we imagine a case?
in which there is
 some kind of entanglement
that does better
 than Quantum Mechanics,
that does better than 85%?
And they thought about it, and they came up
 with this idea of these two boxes,
that, they showed,
could have a set of rules
 that, without violating special relativity,

English: 
suggested that perhaps what
we're seeing here with this
quantum nonlocality is kind of
a stretching of the normal laws
of cause and effect, as far as
special relativity will allow,
so that somehow, quantum
mechanics is sort of almost
violating the spirit of special
relativity without violating it
actually in practise,
without actually allowing you
to transmit information
faster than light.
Perhaps quantum
mechanics is doing
this right up to the limit of
what is physically possible.
Well, Popescu and Rohrlich
thought, let's see about that.
Can we imagine a
case in which there
is some kind of
entanglement that
does better than
quantum mechanics, that
does better than 85%?
And they thought about it.
And they came up with this
idea of these two boxes
that they showed could
have a set of rules that,
without violating
special relativity,

English: 
allows you 100% correlation.
It's physically breaking
no laws, let's say,
that we know of to have
a situation like this--
if you like, a kind of
super-quantum correlation.
And so the question becomes,
why isn't the world like this?
You see, we often think
about quantum mechanics
as something sort of
added on or something
different from
classical mechanics.
Classical mechanics give
you this sort of 75% success
rate in this case.
Quantum mechanics
does something more.
So we kind of think,
where does that come from?
Why does quantum mechanics
allow you to do these things
that you can't do classically?
Popescu and Rohrlich approached
it from another angle.
Because they show
that, actually, things
could be even, if you like,
more quantum than they are.
And so why aren't they?
And so that raises
a new question.
If we could understand
why quantum mechanics is
limited in what quantum
nonlocality can do--

English: 
allows you a 100% correlation.
It's physically
breaking no laws, let's say,
Ahh... that we know of,
 to have a situation like this.
A kind of...   if you like, a kind of
super-quantum correlation.
And so the question becomes,
why isn't the world like this?
You see, we often think
about Quantum Mechanics…
as something sort of added on,
 or something different from Classical Mechanics.
Classical Mechanics give you
 this sort of 75% success rate in this case.
Quantum Mechanics
does something more.
So we kind of think, you know,
where does that come from?
Why does Quantum Mechanics
allow you to do these things
that you can't do classically?
Popescu and Rohrlich
approached it
 from another angle.
Because they showed, that actually things could be,
 even, if you like, MORE quantum than they are.
And so why aren't they?
And so, that raises
a new question.
If we could understand
why Quantum Mechanics is limited
in what quantum
nonlocality can do,

English: 
limited to this 85%
figure instead of 100%--
if we could understand
that, then maybe
we would understand
a little more
about why it is that the world
has these quantum properties
that it seems to have at the
level of fundamental particles.
If you still find this
analogy a little bit
confusing, then probably
what you need to do
is to look it up in
my book, Beyond Weird,
which talks about
this and more and goes
into more detail about what
quantum entanglement does
and doesn't mean, and also about
how we're starting to make use
of it in quantum technologies,
like quantum computing
and quantum cryptography.
And let me remind you,
if you haven't already
subscribed to the
Ri YouTube channel,
then you should do that.

English: 
— limited to this 85% figure, instead of 100% —
if we could understand that, 
then maybe
we would understand
a little more
about why it is that the world
has these quantum properties
that it seems to have
 at the level of fundamental particles.
If you still find this analogy
 a little bit confusing,
then probably
what you need to do
is to look it up in
my book, "Beyond Weird",
which talks
 about this and more, and goes
into more detail
 about what quantum entanglement
does and doesn't mean, 
and also about how
we're starting to make use of it
 in quantum technologies,
like quantum computing,
and quantum cryptography.
And let me remind you,
 if you haven't already
subscribed to
 the Ri YouTube channel,
then you should do that.
