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the weird rules of the subatomic world
are very very different to those of the
familiar large-scale universe a huge
outstanding question is when and why
does the weirdness of quantum mechanics
give way to classical physics one answer
to this question suggests that the
entire universe is so much weirder than
we imagined or should I say the
multiverse one of the strangest features
of the quantum description of reality is
the idea of superposition we can't
describe the most fundamental building
blocks of our universe with defined
singular properties
instead they seem to behave as
probability clouds of all properties
they might have worried to try to
measure them mathematically this is
encapsulated in the wave function over
quantum particle or system of particles
the best illustration of why we need to
describe the quantum world this way is
the famous double slit experiment we did
an episode on a check it out if you
aren't familiar with summarize a stream
of photons are electrons or even
molecules travels from some point to a
detector screen via a pair of slits
these particles arrived at the screen
distributed like the interference
pattern you expect from a simple wave
quantum mechanics very successfully
predicts this result by describing h
particles journey as a superposition of
all possible trajectories in other words
the particle simultaneously takes all
possible paths which means it passes
through both slits it tries out all
history's between launch and landing and
those many maybe histories somehow
interact with each other to determine
the most likely final destination when a
measurement is made in a sense different
possible superposed histories appear to
converge on one final outcome but what
causes that convergence in the original
Copenhagen interpretation of quantum
mechanics the active measurement was
thought to collapse possibility space
into a single real
30 at least with respect to the measured
property it collapses the wave function
that collapsed signifies the transition
between the quantum and classical realms
one of the founders of quantum mechanics
and Schrodinger found this ridiculous
and he proposed his famous
schrödinger's cat thought experiment to
highlight the absurdity it goes like
this cap is in a box with a classical
poison machine containing a radioactive
element is set to share the flask in the
event that the radioactive element
decays if that happens the cat dies that
radioactive decay is a truly quantum
process and so until it's observed it
exists in a superposition of states it
has both decayed and not decayed that
doesn't that mean that the entire
macroscopic system attached to that
quantum event is also in superposition
if so then the cat should be
simultaneously alive and dead until we
open the box but why can't the cat
collapse its own wave function and from
its point of view is the physicist
outside also a quantum blur until the
box is opened and what about the entire
rest of the universe that's not
currently being observed by physicists
or cats many adherence to Copenhagen now
have a more sensible resolution to the
paradox of schrödinger's cat it's that
quantum superposition doesn't extend to
macroscopic scales it disappears in
different quantum scale histories
diverge this is called decoherence when
the wave functions describing quantum
systems overlap sufficiently in other
words they're coherent it's possible to
get interference in the double-slit
experiment and spookley correlated
quantum entanglement measurements but
when these systems interact with their
environment
coherence is lost and parallel histories
fall out of alignment
they can no longer interact with each
other by the Copenhagen interpretation
we might say that the universe chooses
the final outcome of all those histories
it doesn't exactly choose a single
history instead chooses and end result
say particle location on the screen or
cat alive or dead pneus based on those
histories if a larger number of possible
histories lead to a given result then
it's more likely that the universe will
select that outcome the Copenhagen
interpretation says that this selection
happens in a fundamentally random way
the universe plays dice even if the dice
are weighted towards certain results is
what we would call a non-deterministic
interpretation because there's no
underlying predictability behind the
selection
however there is another way to
interpret the transition between the
quantum and classical world's what if
the wavefunction never collapses if we
could imagine a cat in a superposition
of states alive and did why stop at the
cat
what if the family of possible States
extends beyond the radioactive decay
beyond the cat and includes the observer
and indeed the entire universe to if we
open the box and find the cat is alive
it's because we're part of an entire
quantum timeline in which the
radioactive decay and subsequent
poisoning never happened but there's an
equally valid timeline in which it did
and another version of us experiencing
that this sounds outrageous but it's a
very serious interpretation of the
mathematics of quantum mechanics
it was proposed by shoot Everett in his
1957 PhD thesis entitled the theory of
the universal wavefunction it's come to
be known as the many-worlds
interpretation to outline the idea
without killing so many cats let's talk
about what this means in the context of
the double-slit experiment the
Copenhagen interpretation tells us that
the superposition of particle
trajectories of history's merges into
the single timeline of the observers
reality many worlds says this merging
never happens those alternative
histories continue and we find ourselves
in just one of those timelines which one
well they're all equally likely but some
very similar to each other for example
many histories lead two photons landing
on the bright bands of the interference
pattern and very few to the dark bands a
weekend to find ourselves in the more
common families of history's this is a
pretty crazy notion the many-worlds
interpretation invites the idea that
reality splits into different branches
every time quantum states diverged into
different possibilities for example at
every particle interaction everywhere in
the universe
this would lead to an unthinkably large
number of alternate timelines or worlds
that contain or possible realizations of
this universe since the Big Bang it
seems extravagant to propose uncountable
eternally branching universes just to
get out of collapsing away function it's
like building an entirely new house to
avoid doing the dishes
remember the Copenhagen interpretation
itself proposes multiple worlds in the
superposition of powers or properties of
a quantum system both many worlds and
copenhagen create alternate realities
it's just that copenhagen merges them
into a single time line with its wave
function collapse the superposition of
states of many worlds can be thought of
as overlaid histories slices of a
universal wave function that diverge
from each other as the universe of balls
but none ever vanish many worlds may in
fact be the more pure interpretation of
the mathematics of quantum mechanics
because there's nothing in that math
that requires the collapse of the wave
function so many worlds is more
economical in the number of unsupported
concepts it adds to quantum mechanics
even if it isn't particularly economical
in the number of universes that predicts
now
Everett idea wasn't taken too seriously
when it was first proposed that may have
been in part because he wasn't a
well-known physicist he was just a
graduate student who all but disappeared
into military research at the Pentagon
right after graduation but another point
of resistance must be the overwhelming
existential crisis induced by the idea
of near-infinite versions of oneself
many worlds may imply that every
possible version of you exists out there
and you're just the one who happens to
be experiencing this branch of reality
every other possible life path including
those branching in different directions
from every decision you ever made
maybe just as real in fact each may be
real in vast multitudes there's no more
evidence for many worlds than there is
for other mainstream interpretations of
quantum mechanics and it is somewhat
mainstream these days with many noted
physicists being swayed by its parsimony
its economy of ideas but it remains an
interpretation and so although it is
supported by the incredibly successful
mathematics of quantum mechanics it is
not yet added a prediction that might
distinguish it from other equally
supported interpretations nor is it
complete explanation there are some
ideas about what's really happening when
these neighboring coherent histories
interact or why the wavefunction
translates to probabilities the way it
does but they are far from generally
accepted
unlike copenhagen many worlds is a
deterministic interpretation
any given time line is a predictable
chain of cause and effect it explains
the apparent randomness of quantum
mechanics with a sort of observer bias
all possibilities are chosen at every
Junction and we just happen to be seeing
the one that happened in the branch that
we occupy that adds a second possible
cause for philosophical Annie's in a
purely deterministic universe what
happens to free will if we're going to
make all possible decisions anyway why
sweat any given choice for sure but of
all those countless future branches of
reality summer going to be pretty
amazing
think of it as a
choose-your-own-adventure and steer this
version of you towards one of the more
awesome many World branches of
space-time well guys do you know who
this is this is diana from physics cool
right here on space-time to be in space
here
that's right one thing i love about your
show is all of the crazy awesome
experiments you do they are so much fun
that thing with the water vortexes blew
my mind you don't do a lot of
experiments on your show we food do
experiments okay well then I have a
challenge for you i want to challenge
you to prove that the earth is round
using an experiment
how long do I have you've got a year
starting now
challenge accepted one thing I really
like about physics cool is it explains
complex stuff in such understandable
ways i try but I'm a kind of job any
kind of guy i love those long ciety
words it's hard to keep track of them
sometimes so I the charge for you
ok i would love you to an episode in
which you explain the five most jargony
and commonly encountered modern physics
words in simple English done that I want
you to use only the 1000 most common
words in english language to do that
that's gonna be a little idea but
challenge accepted
so keep watching PBS space-time to make
sure that Matt follows up with that
challenge and keep watching physics goal
because we're gonna hold you to that
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