einstein's special theory of relativity
combines space and time into one dynamic
unified entity space-time
but if time is connected to space can
the universe be anything but
deterministic
and does that mean that the future is
predestined
[Music]
in the last episode we saw that we could
think of the unified space-time in terms
of the block universe
an a-temporal entity that sort of just
exists
in a block universe of einstein's
relativity there's no way to clearly
define the present and so no way to
cleanly separate the future from the
past
the notion of the present is relative to
us it looks like a particular slice
through the block universe
but different observers will slice the
block at different angles that depend on
their velocities
that means that for every observer it's
possible to imagine another observer who
lives in their definition of the present
but for whom your future is already the
past or for whom your past is their
future
but how does this give us a
deterministic universe
well if we accept a unique reality for
those other observers in our present and
we accept that their perception of the
present is as valid as ours
then all of the future is defined
that seems to support the philosophical
stance of eternalism the idea that all
space and time exist from a sort of
god's eye view from outside of both
space and time
and it suggests a deterministic universe
where every slice through the block
universe is completely determined by
every other slice either behind it or
ahead of it they are connected by the
unalterable laws of physics so here's
the big question given that relativity
is definitely a good description of
space and time what would it take for
our future to remain undefined
it seems we have two options either the
entire block universe is defined past
present and future or everything outside
our past light cone is undefined
including other observers and the room
around you in what you would normally
think of as the present and the only
aspect of the present that exists is a
vanishingly small patch of space time
around your own brain in order to even
try addressing which of these options is
right
we need to take into account something
we've totally ignored so far
where relativity is the key to
understanding the universe of the large
and the very fast we also need to
understand the universe of the very tiny
we need to see what quantum mechanics
says about determinism and how it plays
with relativity and the block universe
according to quantum mechanics physical
systems parts of the universe evolve as
way functions not as well-defined
reality but rather as distributions of
probability that describe all possible
states that that chunk of the universe
could be in
if it were to be observed the wave
function can be thought of as a state in
which all physically possible realities
intermingle
countless extremely precise and
frequently replicated experiments tell
us that the universe really does exist
in this indeterminate state in certain
circumstances
the wave function is real
the big question is what causes the
transition from this world of quantum
indeterminacy to the solid singular
reality of the macroscopic world
we've talked about this profound
weirdness in many episodes including how
to interpret it
now exploring those interpretations
again may help us understand the state
of the universe beyond our immediate
selves and the immediate moment one of
the earliest and still most accepted
interpretations of all this weirdness is
the so-called copenhagen interpretation
which states that a quantum system is
literally in a state of undefinedness
until observed
at which point the wave function
collapses into a defined state all
particles take on defined properties and
one actual reality is chosen from the
many possible ones at least as far as
the heisenberg uncertainty principle
allows
the copenhagen interpretation gives us a
non-deterministic universe the wave
function evolves in a precise way
perfectly defined by the equations of
quantum mechanics but at the moment of
measurement a single reality is randomly
selected from the distribution of
possible realities the dice are thrown
when the wave function collapses that
means it's fundamentally impossible
to predict the state of a system after
observation another popular
interpretation of quantum mechanics is
the many world's interpretation which
simply states that the wave function
never collapses instead all possible
states of the quantum system continue to
exist
rather than choosing one reality out of
many when we observe a quantum system we
sort of just become part of one of the
realities and lose track of the others
the many worlds interpretation gives us
a deterministic universe or rather a
deterministic multiverse the way
function never collapses it evolves
deterministically forever the illusion
of randomness is that we find ourselves
in the one branch of the wave function
corresponding to the reality we perceive
but we aren't randomly placed in that
branch
we just are a part of that branch
so how do these interpretations connect
with what relativity tells us about past
future and present
let's start with copenhagen
it's tempting to equate everything
outside our past light cone with the
unobserved wave function you can imagine
that light cone sort of just plowing
through the undefined universal wave
function collapsing it as it goes
or at least the parts of that wave
function for which signals actually
reach our awareness that leaves your
future light cone undefined a
non-deterministic void with room for
your non-determined choices
but now let's say we believe that other
observers in the universe can also
collapse the same universal wave
function with their observations well no
problem so far you can imagine this
fleet of observers collapsing the
universe all the way up to what you
perceive as the present your future is
intact
but wait we saw last episode that your
impression of the present is relative
a fast-moving observer defines the
present very differently to you in fact
any part of space-time not in your
future light cone is potentially the
past for another observer in your
present
and that observer can imagine a third
observer on their different present for
whom your future light cone is in their
past light cone so it should have
already been collapsed the upshot is
that with no way to define an absolute
definition of the past and if we believe
in other observers there's no way to
keep the wave function of your future
from being collapsed
before you get there
now there are subtleties in this
wavefunction function collapse idea
copenhagen doesn't specify what causes
the collapse it could be observers as in
the von neumann wigner interpretation
that's the hardest to gel with a
non-deterministic universe unless you're
the only observer in the universe more
physicists prefer the idea of a
non-conscious interaction causing the
collapse in that case uncollapsed wave
functions tend to exist only in
microscopic pockets or in very special
circumstances
but that doesn't help uncollapse your
future
no the only totally coherent way for a
non-deterministic wave function collapse
interpretation like copenhagen to give
you an uncollapsed future is if you're
the only being doing the wave function
collapsing let's move on to many worlds
this interpretation sits a bit better
with the relativity of past present and
future your light cone sweeps through
the global wave function but it doesn't
collapse that wave function rather it
selects from it
sometimes the quantum multiverse of many
worlds is depicted as this branching
tree in which the realities multiply as
time progresses
in the block universe picture it's
tempting to depict this as the entire
block multiplying with every quantum
event but that's not quite right a
division in the block universe can only
propagate as quickly as the result of
that quantum event can become known
that can happen at the speed of light so
we can also think of the splitting as
proceeding in advancing light cones
really what's happening here is that
parts of the universe become entangled
with each other correlated at a quantum
level so we have these spreading webs of
entanglement each such web defines a set
of properties of the universe correlated
with some prior quantum decision with an
independent web existing for all
different possible decisions here we're
bringing in the concept of decoherence
and quantum darwinism which we've
covered before decoherence is an
explanation for how the world can become
separate from each other
when you shift from simple systems like
an atom to complex systems like a cat
according to the decoherence explanation
the quantum multiverse isn't a cleanly
divided set of alternate realities
rather it's a tangled network of
alternate sub-realities
nonetheless as your light cone moves
forward in time you encounter these
entanglement networks and you have to
choose between them your reality is the
particular set of entanglement networks
that you interact with
and there are other realities with
different networks that contain
different versions of you i should add
that the light cone picture isn't ideal
here because entanglement can travel
faster than light however light speed
signaling is the surest way to transfer
quantum correlations so this is still a
useful picture so what does all of this
say about determinism and the reality of
your past and future well interesting
things actually
the evolution of the wave function is
deterministic that means all future
branching of the wave function of your
present by which i mean the entanglement
network that you currently belong to
is predefined
what isn't defined is your own
experience of that future branching
you will be the thread of conscious
experience that travels one of those
branches you'll also travel the others
but each version of you will only feel
like you travelled one of them
this interpretation many worlds plus
decoherence perhaps gives us the most
concrete notion of an objectively
existing present it's that part of the
global weight function that you're
connected to via entanglement and that
shares your timestamp of now
but the notion of now remains relative
there are still other observers in your
slice of reality for whom you're now is
their past or future copenhagen and many
worlds aren't the only interpretations
of quantum mechanics in town we should
make an honorable mention to the de
broglie bone pilot wave theory this is
an entirely deterministic interpretation
that doesn't have multiple realities
just a wave function that guides
particles in a perfectly determined way
defined by bohmian mechanics of course
we've talked about all of this before
the universe of pilot wave theory is
really a block universe
but unfortunately no one has
convincingly figured out how to make
bohmian mechanics work properly with
special relativity so it's a very simple
block universe and it's not a good
representation of ours there are other
interpretations that deserve mention but
we're out of time
so
does the future already exist according
to quantum theory
the answer depends on your favorite
flavor of quantum interpretation
perhaps we ride the dice of copenhagen
into an unknown future or surf the
splitting quantum multiverse into all
futures of which you will become just
one
of course we still haven't got to why
time needs to progress in one direction
at all
or how time even arises in the first
place
but we'll have plenty more time for time
another time on space time hey everyone
last week we introduced the idea of the
relativity of simultaneity and the block
universe and what this might tell us
about the reality of the past and future
at least i think it was last week the
whole time things very muddled in my
head these days but let's see what you
had to say anonymous makes some
important points i'll summarize in
einstein's relativity the definition of
simultaneous events is well relative it
depends on relative velocities so does
that mean that the idea of a now slice
through the block universe is
meaningless
well maybe but two observers can
certainly construct a map of events
across the universe that they would
define as happening simultaneously at a
given instant in time their slice of now
and compare those maps
even though they have to wait until the
light reaches them to do so
so the now slice has a precise meaning
what it does not have is the property of
clearly and currently existing to the
exclusion of other slices in the block
universe and that's where the relativity
of simultaneity connects to eternalism
if you can ascribe actual existence to
one slice of the block universe to one
observer's definition of the present
then you should ascribe existence to the
slice of the present as defined by any
potential observer on that first slice
and that fills the block universe past
and future the alternative seems to be
to not ascribe reality to any observers
present but then what exists just the
one observer at any rate the relativity
of simultaneity doesn't prove eternalism
rather it shows that presentism or the
idea of the growing block
interpretations are
incoherent
and speaking of presentism
carl stanlin raises a point that
highlights another objection that i have
to the concept
if only the incident exists what is that
instant
is it a very short stretch of time like
one plank unit of time or is it a truly
instantaneous stretch of zero seconds
as much as physicists hate to talk about
consciousness i think it's relevant to
bring up here our conscious experience
exists in fact it's the only thing we
can observe directly but that experience
almost certainly is emergent from the
action of our neurons and those neurons
take time to fire and communicate across
the brain
so something that currently exists our
consciousness is smeared out over at
least 100 milliseconds of time at least
that length of the present should be
thought of as real you might argue that
our conscious experience rides the
leading edge of the brain's action
emerging in a true instant synthesizing
prior brain activity but for that to be
a true zero time instant then it would
need to be the instantaneous state of
the brain the momentary spatial
configuration of its particles at that
instant that gives rise to our conscious
experience so then does a frozen brain
just experience the current incident
eternally
most probably not rather consciousness
surely requires dynamical processes a
flow of information
that means that at least a short chunk
of the block universe has a real current
existence much larger than the planck
time and if that much time can exist
then why not all of it guillermo
makasani takes issue with my
characterization of einstein as the
smartest man in the block universe
dude i said block universe not universes
rick is the smartest man in the
multiverse also i'm pretty sure einstein
was another renegade from the citadel of
rix the resemblance can't be a
coincidence and for those uninitiated
into rick and morty there are other
funny pop culture references in the
comments drake gives a nice douglas
adams quote time is an illusion
lunchtime doubly so grandpa's place
reminds us of doctor who's experience of
time as a big ball of wibbly wobbly
tommy womey's stuff but really you
should also just go and watch rick and
morty think of it as a co-requisite
study course to pbs space time wobble
dub dub
you