[snyena]

The Boltzmann brain hypothesis suggests that it would be more likely for a single brain to spontaneously and briefly form in a void (complete with a memory of having existed in our universe) rather than for the universe to come about in the manner cosmologists think it actually did.

In this physics thought experiment, a Boltzmann brain is a fully formed brain, complete with memories of a full human life in our universe, that arises due to extremely rare random fluctuations out of a state of thermodynamic equilibrium. Theoretically, over an extremely large but not infinite amount of time, by sheer chance, atoms in a void could spontaneously come together in such a way as to assemble a functioning human brain. Like any brain in such circumstances (the hostile vacuum of space with no blood supply or body), it would almost immediately stop functioning and begin to deteriorate.

By one calculation, a Boltzmann brain would appear as a quantum fluctuation in the vacuum after a time interval of 10^10^50 years. This fluctuation can occur even in a true Minkowski vacuum (a flat spacetime vacuum lacking vacuum energy). Quantum mechanics heavily favors smaller fluctuations that "borrow" the least amount of energy from the vacuum. Typically, a quantum Boltzmann brain would suddenly appear from the vacuum (alongside an equivalent amount of virtual antimatter), remain only long enough to have a single coherent thought or observation, and then disappear into the vacuum as suddenly as it appeared. Such a brain is completely self-contained, and can never radiate energy out to infinity.

Sources:

https://en.wikipedia.org/wiki/Timeline_of_the_far_future https://en.wikipedia.org/wiki/Boltzmann_brain

[tomcam]

Well then it’s equally likely that badgers the size of Earth could be forming from time to time, and since they’re the size of earth they would decay much more slowly than our puny local badgers… so shouldn’t we see the occasional Earth-sized badger through the Hubble?

Or how about the much lower odds needed for a copy of Shakespeare’s plays translated to Klingon inscribed on a giant sheet of titanium to appear spontaneously? Far more likely to happen than a Boltzmann brain, right? So should we not have found an item or two like that?

[marcosdumay]

If by "equally likely" you mean "much, much less likely, but also more than 0 probability" than yes.

> so shouldn’t we see the occasional Earth-sized badger through the Hubble?

Take another look at the time-frame on the GP's comment and remember that the Universe is ~10^13 years old.

[contradistinct]

What Hubble? If you're a Boltzmann brain, the Hubble is just an idea you have in your temporary brain, before it pops like a soap bubble.

[sillysaurusx]

You forgot the part where people suspect that we are Boltzmann brains.

[desbo]

If I’m a Boltzmann brain that can only survive for an instant, why do I experience existing for a whole lifetime?

[tomcam]

…that themselves originated from The Simulation

[andbberger]

> Like any brain in such circumstances (the hostile vacuum of space with no blood supply or body), it would almost immediately stop functioning and begin to deteriorate.

that is not such rare fluctuations are ephemeral. they are ephemeral because the dynamical system is reversible, for the same reason that the molecules in a box of gas dissipate immediately after they happen to coalesce in the corner

[lloeki]

> a single brain to spontaneously and briefly form in a void

I recall first hearing about this by being introduced to it via the "giant marshmallow hypothesis"†: it's somewhat equally likely that a giant marshmallow spontaneously and briefly form in a void. A Boltzmann brain is then merely a different arrangement of atoms and stuff.

† I don't think it has a specific name.

[marcosdumay]

A giant marshmallow is way more likely. Not all arrangements of atoms have the same probability.

[leakbang]

Intriguing yet terrifying at the same time.

[HomeGear]

You need more than a brain to have a thought. Or am I missing something?

[themeiguoren]

The anthropic principle is my favorite mind-hole to spelunk down.

[bsmith]

Oh no, not again.