instein'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

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 world's 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