The Boltzmann brain paradox - Fabio Pacucci
How do you know you’re a person
who has lived your life,
rather than a just-formed brain
full of artificial memories,
momentarily hallucinating a reality
that doesn't actually exist?
That may sound absurd,
but it’s kept several generations
of top cosmologists up at night.
They call it the Boltzmann brain paradox.
Its namesake, Ludwig Boltzmann,
was a 19th century physicist
operating in a period when scientists
were passionately debating
whether the universe had existed
for an infinite or finite time.
Boltzmann’s main claim to fame was
revolutionizing thermodynamics,
the branch of physics that studies energy.
He put forward a new interpretation
of entropy,
which is a measure
of the disorder of a system.
A glass is an ordered system,
whereas a shattered glass is disordered.
The second law of thermodynamics states
that closed systems tend towards disorder:
you won’t see a shattered glass
return to its pristine state.
Boltzmann’s insight was applying
statistical reasoning to this behavior.
He found that a system evolves to a more
disordered state because it’s more likely.
However, the opposite direction
isn’t impossible,
just so unlikely that we’ll never witness
things like scrambled eggs turning raw.
But if the universe exists
over an infinitely long time,
extremely unlikely events will happen,
including complex things forming
out of random combinations of particles.
So what does that look like in a
hypothetical infinitely old universe?
In this unremarkable stretch
of near-nothingness,
about eight octillion atoms randomly
come together to form a replica
of the Thinker made of pasta.
It instantly dissolves.
Over here, these particles suddenly form
something like a brain.
It’s filled with false memories
of a lifetime up to the present moment,
when it perceives a video saying
these very words,
before decaying.
And finally, by random fluctuations,
all the particles in the cosmos
concentrate in a single point,
and an entire new universe spontaneously
bursts into existence.
Of those last two,
which is more likely?
The brain, by far—
despite all its complexity, it’s a blip
compared to an entire universe.
Every one universe produced by random
fluctuations has equivalent odds
to heaps upon heaps of insta-brains.
So by this reasoning,
it seems extremely more likely
that everything you believe to exist
is actually a brief illusion,
soon to be extinguished.
Boltzmann didn’t get quite
that far in his thinking;
the brains themselves were introduced by
later cosmologists building on his work.
But they, like just about everyone else,
were pretty sure that they themselves
weren't just ephemeral brains.
So the paradox was: how could they be
correct and the universe be eternal?
The resolution is something most take
for granted today:
that our universe
has not existed forever,
but rather time and space
started with a Big Bang.
So that’s the paradox over and done with,
right?
Well, maybe not.
In the last century, scientists have found
evidence supporting the theory
of the Big Bang everywhere we look.
Yet while we know
that the Big Bang happened,
no one knows what, if anything,
preceded and caused it.
Why did the universe begin in such
an extremely ordered, and unlikely, state?
Is our universe in an unending cycle
of creation and collapse?
Or might we be in one of many universes
expanding within a multiverse?
In this context, Boltzmann’s paradox
has found renewed interest
by contemporary cosmologists.
Some argue that leading models
for where the universe came from
still imply that Boltzmann brains
are more likely than human brains,
suggesting something’s amiss.
Others counter that slight modifications
of the cosmological models
would avoid the problem,
or that Boltzmann’s brains
can’t actually physically form.
Some researchers even attempted
to calculate the probability
of a brain popping out of
random quantum fluctuations
long enough to think a single thought.
They got this incredible number
whose denominator is 10 to a number
about a septillion times larger
than the number of stars in the universe.
The Boltzmann brain paradox,
despite its absurdity,
is useful because it creates a bar
that models have to rise to.
If, compared to numbers like this one,
the current state of the universe
is exceedingly unlikely,
something in the model is
almost certainly wrong.
Unless you’re the one who is wrong...
