- Einstein's Special Theory of Relativity
combines space and time
into one dynamic unified entity, spacetime.
But if time is connected to space,
can the universe be anything but deterministic?
And does that mean that the future is predestined?
("PBS Space Time" theme song)
In the last episode,
we saw that we could think of the unified spacetime
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 cleanly 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 spacetime
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 wave 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 undefined-ness 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-Worlds 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 wave 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 spacetime,
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 wave function collapse idea.
Copenhagen doesn't specify what causes the collapse.
It could be observers
as in the Von Nuemann-Wigner Interpretation.
That's the hardest to gel with a non-deterministic universe,
unless you are the only observer in the universe.
More physicists prefer the idea
of a non-conscious interaction causing the collapse.
In that case, un-collapsed wave functions
tend to exist only in microscopic pockets
or in very special circumstances.
But that doesn't help un-collapse your future.
No, the only totally coherent way
for a non-deterministic wave function
collapse interpretation like Copenhagen,
to give you an un-collapsed future
is if you are 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 are 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 traveled 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 wave function
that you are 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 are 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
de Broglie-Bohm 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 Bohmiam 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 thing's 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 observer's 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 Stanland raises a point
that highlights another objection
that I have to the concept.
If only the instant exists, what is that instant?
Is it the 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 a hundred 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 instant 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 plank time.
And if that much time can exist then why not all of it?
Guilherme Marquesani 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 Rick's.
The resemblance can't be a coincidence.
And for those uninitiated it 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, timey-wimey stuff."
But really you should all just go
and watch "Rick and Morty".
Think of it as a co-requisite study course
to "PBS Space Time".
"Wubba lubba dub, dub."
(majestic orchestral music)