[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.

[tomcam]

Gotta be honest it doesn’t look a day over 10^12 years old when it’s sober and had a good night’s sleep

[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?

[Zircom]

You haven't experienced a whole lifetime. Every moment up until the current [whatever the smallest unit of time really physically possible is] is just a memory of an experience. Subjectively from your perspective there's absolutely no difference between having actually experienced all that and merely having memories of having experienced all of it. You could materialize into existence for exactly one moment and believe you've lived an entire lifetime, but in reality you're just a brain floating in space experiencing one brief moment of a lie told by the chance arrangement of the molecules that make up your being.

[cricalix]

Now that's an existential crisis right there. There's no way to determine if I've literally flashed into existence and am remembering this sentence, and am thinking of how to continue it, versus actually existing on the timescale of a human life.

Urgh.

Then again, does it matter? Probably not.

[mikelevins]

There's another way to look at it: you can think of yourself as having just come into existence this instant, having inherited all memories and evidence of any previous existence.

Now you are blessed with the incredible gift of spontaneously coming into existence, plus a vast treasure trove of inherited experience (both good and bad) that you can explore and try to understand, and of inherited skills and knowledge that you can put to use however you choose.

[travisjungroth]

I came to the same realization at about 12 years old. The mechanics were different than the Boltzmann brain. I saw it as that there are only two things that define the current universe: matter and energy. How could I know the universe wasn’t _just_ created? Every atom of my house, every atom of my body, created or placed in just the spot it is now, with just the energy it has now. My memories from five minutes ago are exactly what a brain that looks like mine would “remember” from five minutes ago.

Looking back, this realization became a feeling and this feeling became the backdrop for much of my growth as a person. Sometimes I was filled with fear. Sometimes I would shed all responsibility because it didn’t feel real.

At the best times, it fills me with a profound sense of agency, balanced with responsibility. It’s like teleporting into someone else’s body. Some things you choose to play along with, taking care of “his” mother. Other things you abandon, changing direction in your life as you need, despite what anyone might say.

[pooper]

At the risk is starting the obvious, you only really experience an instant though. Everything else is I a memory.

[zogomoox]

It's more likely a Boltzmann brain would be a microscopic quantum computer, dreaming up planets and people and you to get over the boredom of being alive for a picosecond.

[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.