there was this gathering I think in 2011
fairly recently of experts in quantum
foundations what quantum mechanics
really means and they took a poll they
took a poll there like 16 different
questions to ask the experts and one of
them was simply what is your favorite
interpretation of quantum mechanics or
approach to thinking about what quantum
mechanics really means and the answers
was that there wasn't even any one
approach that got 50 percent of the vote
the results were all scattered across
different possibilities the traditional
Copenhagen interpretation where the
quantum state collapses when you look at
it a more modern many-worlds
interpretation where the universe splits
into different possibilities
interpretations that add something new
to quantum mechanics like random
stochastic changes of the wavefunction
or hidden variables that we can't see
and I said it was extremely embarrassing
to physics that here it is 80 years
after we sort of established how quantum
mechanics works we still don't know what
it really means
well it's absolutely great that we have
unknown questions that's not the
embarrassing part the embarrassing part
is this particular question has been
unanswered for 80 years with very little
by the way of immediately demonstrable
progress even though it's such an
important question it seems to me that
we have not been trying to answer this
question with as much vigor as we should
what is quantum mechanics really I mean
that's what's saying what is the
universe really I mean what more
important question is there than that
my favorite interpretation is the
many-worlds interpretation of quantum
mechanics this goes back to a guy named
Hugh Everett who was a graduate student
in the 1950s at Princeton and he said
you know he had this brilliant insight
that all the complicated ill-defined
parts of quantum mechanics they're all
having to do with when you measure
something when you observe something and
he said you know what what if we just
erase those parts from quantum mechanics
so nothing different happens there's no
special role for observation or anything
like that the trick is that you the
observer or a quantum mechanical system
just like anything else so what you need
to do everett said was just a trace
carefully what the world would look like
if there were quantum systems
interacting with other quantum systems
and what he claimed happened is that
every time these systems interact with
each other the world splits you get one
world that looks like one answer was
achieved in another world in which
another answer was achieved and these
rules will never talk to each other ever
again it bugs people they have all those
universes going around but that's what
the math seems to imply so is this like
a car getting to a t-junction and
instead of turning left or right it
splits into and two cars go both ways
it's basically like that there are just
as many parts of the universe before and
after it's not like you're creating more
stuff in the universe it's kind of like
and this is just an analogy it's not
exactly like this but it's like there
were exactly two copies of the car all
along but they were precisely the same
and then they diverged when this quantum
event occurred so there's sort of all
sorts of universes sitting on top of
each other and they're splitting apart
in differentiating as time goes on
is there a finite number of these or
this is a very simple kind of basic
question to which we don't know the
answer it could be a finite number or an
infinite number it's probably an
infinite number we I say that because if
there were only a finite number of
possibilities quantum mechanically then
in an infinitely old universe all of
those possibilities would happen over
and over again an infinite number of
times and this relates back to another
important question in cosmology you know
the arrow of time why is the past
different from the future
so I think we need an infinite number of
possibilities in quantum mechanics to
explain a universe that keeps evolving
and that's what we seem to observe does
it make sense to you you're telling me
and you know it's baffling me but is it
baffling you I think it does make make
sense
actually um it is an amazing consequence
of some simple equations right but
that's exactly what we should expect
when we do physics that we should follow
what the equations tell us we should
take them seriously up until they are
not compatible with the data right until
they violate experiments so quantum
mechanics in order to fit the data back
in the 1920s people had to come for
these dramatically different ideas you
know you can't make predictions with
100% certainty and so forth and they
boiled it down to a very simple set of
equations and whatever it said in the
1950s is just take those equations
seriously just believe them they already
fit the data you just have to become
comfortable with the fact of the world
you see is a tiny tiny slice of the
whole world that exists to me equations
can have a number at the end of them or
a letter at the end of them or a
combination of letters and numbers what
can be on the other side of the equal
sign that says to you multiple universes
what's the result that says that what is
that equation look like well there
there's the the equation is the
Schrodinger equation this is the
equation that tells you how the quantum
state of the universe changes with time
and the point is that ever it's
suggested that we don't need two
different equations to talk about the
change of the evolution of the universe
we only need one we only need the
trigger equation
it in the Copenhagen interpretation the
the thing that we teach our students the
thing that's in the textbooks there's
two different ways
that the quantum state evolves it obeys
Schrodinger's equation when you're not
looking at it and then when you look at
it it suddenly discontinuously changes
and that should really bug you that bugs
me much more than a whole lot of
universes these splitting events these
moments when the universe splits what
causes that what's the tipping point
what counts as an observation that
causes this split there's a very good
question it is the major I would say
progress we have made over the past 50
years in quantum mechanics
since Everett what we understand now is
that in order to so Everett said it's
not enough to think about the quantum
system you also have to think about the
observer well we've said since then is
you also have to think about the entire
rest of the universe okay and we call
this the environment and secretly you
are interacting with your environment
all the time light is reflecting off of
you you're making noise and so forth and
in quantum mechanics that means you are
becoming entangled with the environment
your quantum state is becoming
correlated with the quantum state of
everything else and that's a process
that takes time and you can tell exactly
how long it takes
and it's that interacting with the
environment that leads to this splitting
of universes if there were no
environment then universes would not
split it's that if I have an electron
here an electron there a quantum state
where it could be there or there even
before I look at it you could say
there's a universe in which it's here
and there's a universe in which it's
there but those universes can interact
with each other right I mean they can
interfere with each other in a quantum
experiment but once I look at it and I
see the electron there or there and then
all of my degrees of freedom all my
information get entangled with the
environment around me then these two
possibilities can't interfere with each
other anymore they've gone their
separate ways
decoherence has occurred in the
technical jargon and that's something we
think we understand on the basis of
equations when you and I talked we
talked about this in analogies like
turning the steering wheel of a car and
going left
right but these aren't the events we're
talking about are there these events are
something much smaller and much more
fundamental
yeah the kinds of events that lead the
universe to split or simply observations
of quantum systems and that's a little
bad way to put it because observation
makes it sound like the existence of a
conscious observer is somehow important
to quantum mechanics and it's completely
not that that's an utterly bogus road to
go down
what an observation simply is is
bringing one system into contact with
another so that what this system is
doing becomes entangled becomes related
to what this system is doing so if this
system is spin up or spin down and this
system is a little arrow then you bring
them in contact and the arrow becomes
correlated with what the spin is doing
that's when the universe splits you've
told me where you stand on this you
favor this you know the multiple worlds
thing that didn't win the survey did it
no it did not
I'm in the plucky minority on this one
the winner although again nothing got a
majority but the plurality 42% of the
vote was the Copenhagen interpretation
which is sort of the traditional
textbook interpretation and it's good
enough for government work the
Copenhagen interpretation that's the one
that says that when the cat is in the
box and I open it up then suddenly the
state of the cat changes to either being
alive or dead depending on which one I
observe and if I observe it alive then
the part of the cat that was dead ceases
to exist it is erased from the books of
reality and in that approach all of
these questions we've just been talking
about become very very difficult to
answer if I open the box but I didn't
look in does the that branch of the
wavefunction go away what if a virus
observes the cat you know is the cat
observing it's all these questions that
sort of make our minds confused they're
really difficult questions in the
Copenhagen interpretation in the Everett
interpretation every quantum system is
on equal footing there's nothing special
that causes wave functions to collapse
forty-two percent forty-two percent
believe it and I suspect you know now
I'm going beyond saying true things and
saying my personal opinion which I
happen to
is true I think it's just because people
haven't thought about it very hard and
that's not to say if you think about it
very hard you're led to the many-worlds
interpretation but you are letting the
Copenhagen interpretation when you think
about it very hard you know the more
people worry about how quantum mechanics
really works the less likely they are to
accept the Copenhagen interpretation
these are physicists being so many these
are people who do this for their job how
can they not think about it well it's
the difference between being a racecar
driver and a mechanic right in order to
drive the car really well and really
fast you don't necessarily need to know
all of its inner workings it might be
helpful you might make an argument that
knowing the best knowing the car the
best you can makes you a better driver
but not necessarily right so working
physicists don't always need to
understand the interpretation of quantum
mechanics they just need to know how to
work with it right how to solve the
equations that it gives you no one
disagrees about what the predictions of
quantum mechanics really are unless you
have an interpretation that changes
those predictions in some way so I think
it's embarrassment personally because
why are you doing physics if you're not
interested in how the world really works
also a lot of bars on that graph they're
certainly yeah there's a handful let me
just very quickly mention some of the
possibilities one is something of an
ingenious but sort almost certainly
wrong idea called the gr W theory after
the three people who invented it whose
names I can't pronounce so gr W says
that wave functions do collapse but not
when you look at it there's just a
certain chance per unit time that the
wave function of any one particle will
spontaneously collapse randomly and
unpredictably and then if that particle
is part of a bigger system it brings the
whole system along with it and so
basically it's a way that you can
enforce big systems to always look
classical even when small systems obey
the rules of quantum mechanics it's an
ingenious idea another famous one is the
Bohm interpretation of quantum mechanics
which actually goes back all the way to
Louis de burrell who was one of the
founders of quantum mechanics and that's
a hidden variables theory in other words
they say that the wavefunction is not
all the information there is you can
only make statements and predictions
about probabilities because you don't
have all the
information there is to have there are
sneaky variables that you don't know
about people recoil at the idea of that
but it is a theory that that makes sense
it makes unambiguous predictions and
finally I mean not maybe not finally but
one other approach is a more epistemic
approach which means an approach that
treats the wavefunction not as something
real but just as a bookkeeping device
for talking about your information your
knowledge about the system so it is a
purely informational approach to quantum
mechanics I'm not an expert in those
areas I don't see why that would help me
understand when wave functions collapse
even if you say that it's not real but
it's certainly one of the things that
got votes in that surveying you think
this is embarrassing what's it going to
take to fix it are we looking for
evidence or proof or a great
communicator or evangelist who's going
to make people sing a lot yeah I need to
be clear the thing that is embarrassing
is not that people disagree with my
favorite interpretation but that we as a
community don't agree what it will take
well that's a very good question it's
going to I think it depends on what the
right answer is if the right answer is
that we really need a better theory that
some new dynamics like in the grw theory
is actually involved then maybe what we
need is an experiment you know these are
theories that are truly different from
conventional quantum mechanics and
therefore make experimentally testable
predictions and we're testing them very
few people expect to see the quantum
mechanics is wrong but if it is that
would be a huge step forward obviously
if it's not that then a lot of it is
just you know convincing a bunch of
skeptical hard-nosed physicists that the
unpalatable consequences of your
favorite interpretation of quantum
mechanics need to be taken seriously
whether it's hidden variables or many
many universes out there that you will
never see everyone agrees on which
direction of time is yesterday in which
direction is tomorrow for example we can
remember things that happened yesterday
none of us can remember things that
happen tomorrow even though things will
happen