How do you know that you are a person who has lived your life, rather than a mind full of artificial memories, moment by moment feigning a reality that doesn't actually exist?
This may sound absurd, but it has kept generations of top cosmologists awake at night. They call this the Boltzmann brain paradox.
Its namesake, Ludwig Boltzmann, was a 19th-century physicist working at a time when scientists were passionately debating whether the universe existed in infinite or finite time.
Boltzmann's main claim to fame was for revolutionizing thermodynamics,
the branch of physics that studies energy. He proposed a new interpretation of entropy, a measure of the disorder of a system. Glass is an ordered system, while broken glass is random.
The second law of thermodynamics states that closed systems tend to deform: you will not see broken glass return to its original state. Boltzmann's insight was applying statistical reasoning to this behavior.
He found that a system evolves in a more disordered state because it is more likely. However, the opposite is not impossible, it's just unlikely that we'll ever witness things like scrambled eggs being raw.
But if the universe exists for an infinitely long time, highly unpredictable events will occur, including complex objects made up of random combinations of particles.
So what does it look like in a hypothetical infinitely old universe?
In this unremarkable piece of almost nothing,
about eight octillion atoms randomly combine to form a replica of a thinker made of pasta. It dissolves instantly.
Here, these particles suddenly form something like a brain. It is filled with a lifetime of false memories leading up to the present moment, when he sees a video that says the same words before decaying.
And finally, by random fluctuations, all the particles in the universe converge into a single point, and a whole new universe erupts spontaneously.
Of the last two, which is more likely?
The mind, for all its complexity,
is a blur compared to the rest of the universe. Every single universe created by random fluctuations has the same odds as stacks of insta brains.
So by this reasoning, it seems highly likely that everything you believe is actually a brief illusion, which will soon fade. Boltzmann did not reach this limit in his thinking.
Minds themselves were introduced by later cosmologists based on his work. But he, just like everyone else, was quite sure that he himself was not just a temporary mind.
So the paradox was: How can they be true and the universe be eternal?
The resolution is one that is taken for granted today:
that our universe has not existed forever, but that time and space began with a Big Bang.
So the paradox is over, right?
Well, maybe not. Over the past century, scientists have found evidence supporting the Big Bang theory everywhere we look.
Yet while we know the Big Bang happened, no one knows what happened before it or what caused it.
Why did the universe begin in such a highly ordered and impossible state?
Is our universe in an endless cycle of creation and destruction?
Or are we in one of the many universes spread across the multiverse?
In this context,
the Boltzmann paradox has received renewed interest from contemporary cosmologists. Some argue that the leading models for where the universe came from still indicate that Boltzmann brains are more likely than human brains, suggesting something is wrong.
Others counter that minor changes to cosmological models will avoid this problem, or that Boltzmann's mind cannot actually be formed physically.
Some researchers even tried to calculate the probability of the brain breaking out of random quantum fluctuations to think a single thought.
They obtained this incredible number which is one decimal place to 10 which is about a billion times larger than the number of stars in the universe.
Boltzmann's mental paradox,
despite its absurdity, is useful because it creates a bar to which models must rise.
If, compared to such numbers, the current state of the universe is very unlikely, then something is definitely wrong with the model. Unless you're wrong...
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