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Absolutely everything you think about yourself and the universe could be an illusion.

As far as you know, you are real and exist in a universe that was born 14 billion years ago

and that gave rise to galaxies, stars, the Earth, and finally you.

Except, maybe not.

You may actually not exist for real

but be the dream of a dead universe – you and everything you think exists.

Crazy as it sounds, this may be an unavoidable consequence

of our best scientific theories about the universe.

Ok, this is a bit much – let’s start at the beginning.

We need to understand three concepts for this idea to make sense!

1. The Arrow of Time

What distinguishes the past from the future? Put a drop of red ink into a glass of water

and you see the ink spread until it fills the container but never the opposite:

colored water where ink spontaneously concentrates and becomes a drop at the surface again.

Time always seems to flow in the direction in which the ink spreads.

But if you take a microscope, all you see will be a swarm of molecules colliding at random

– there are no rules, no forwards and backwards.

Every individual motion that happens can occur in reverse.

But we perceive a sort of arrow of time that makes things happen in one direction

– how does this phenomenon occur?

Well, this arrow of time is not actually fundamental, but a matter of probability.

When ink molecules spread to fill a glass,

there are many different “slots” of space they can occupy,

and therefore many different possibilities to combine them.

And just like your chances of winning the lottery grow the more tickets you have,

the probability that ink molecules will end up filling the glass is much higher

than the probability that they’ll concentrate in just one spot.

So it is not that the ink forming a drop again

is forbidden by the laws of physics, it’s just extremely unlikely.

To see it you’d have to wait about 10 to the power of 100,000,000,000,000,000,000,000 years

– a 1 followed by one hundred sextillion zeros. If you had this much time to spare, eventually

by pure random chance, you’d see a red blob form again. Actually, with enough time,

you could see any shape forming. Like, for example, a small, red, soggy brain.

Ok, let’s move on to idea 2.

2. The far future of the universe

Our universe was born 14 billion years ago in the Big Bang, it expanded and evolved

to give rise to the myriads of galaxies and things. In other words, the universe is kind of

a glass of water with a lot of ink doing stuff – it has an arrow of time.

But the universe is not a static glass, it seems to be getting bigger at an ever increasing speed

because of dark energy. Basically, everything in it is getting more and more diluted.

In about 100 trillion years, the last star will die. Then few interesting things will happen

for the next few decillions, vigintillions and googols of years.

Eventually the universe will be a dark place fully dominated by dark energy

– a rapidly expanding ball of pure space almost devoid of matter.

You might think that this would lead to the ultimate death of everything

but dark energy has one last surprise for you.

In a universe dominated by dark energy, space expands so dramatically

that it creates a “cosmic horizon” around you:

a border beyond which nothing will ever be able to reach you, not even light.

So for every practical purpose, the universe has become a glass of finite size

about 36 billion light-years wide, surrounded by an impassable cosmic horizon.

Such a universe glass is basically a giant black hole turned inside out.

We know that, due to quantum effects, all black holes emit a tiny amount of particles

– a phenomenon known as “Hawking radiation”. And so does our inside out black hole.

In the end, this radiation will fill the universe glass with particles again.

At this point, so far in the future that giving you a number has no more meaning,

we’ve reached the true final state. The universe has now become a closed box

full of particles at an extremely low but finite temperature. And since they have a temperature,

they undergo random motions. Or in other words: a glass filled with water and ink

and an infinite amount of time ahead. Things are about to become interesting again.

3. Typing Monkeys & Fake Universes

Eternity is a long, long time.

So now even the most extremely unlikely things can happen.

The fluctuating particles are bumping into each other over and over and over again

creating every combination of particles that’s possible.

They’re like a monkey typing at random on a typewriter.

Almost all of the time it types gibberish.

But with enough time, eventually it will write the first acts of Hamlet.

And with even more time, the complete works of Eminem.

If ink in our universe glass generates any random arrangements of particles, what could they be?

Well, a spontaneous fluctuation could give rise to a planet.

Or to a galaxy. Or even to a lot of them. So maybe our universe has

already ended and all we see around us is a “pop-up universe” – not a universe

that evolved from a Big Bang, but one that fluctuated into existence by pure chance.

And that, like the drop of ink, will only exist for a while before dissolving again.

Being random, pop-up universes could be similar to ours but with funny glitches.

In some of these universes dinosaurs are riding snails. In another, stars are made of blueberries.

In another you are wearing a funny hat. Scientists in such universes wouldn’t understand

those glitches, so maybe the greatest mysteries of physics are just nonsense bugs

of our pop-up universe. But not all possible fluctuations of our dead universe

have the same probability of occurring. Smaller fluctuations are much more probable

than bigger ones. A planet is more likely than a galaxy.

But you know what is even way more likely? A human brain.

Are you actually just a brain?

You think, therefore you exist. But what else do you truly know?

In the end, your brain is just interpreting signals from your senses

and creating a world that you experience.

So technically, you could be just your brain that thinks the world is real. And if we follow

the logic of the ink in the universe glass, in particular, you could be a disembodied

brain that, just by chance, emerged in a dead universe with your complete set of knowledge

and memories. This is a pretty bizarre idea but if we do

the math, it is kind of pretty solid. Let’s compare the number of brains inside bodies

in a living universe with the number of naked brains in a dead universe.

Let’s go really big and imagine that a total of 100 quadrillion humans will live around Earth.

And that the same amount of people will live around every star in the universe.

If we add this together we get about 10 to the power of 41 brains inside bodies that will exist.

However, in a dead universe that has had enough time to explore all possible fluctuations

and that will exist forever, the number of naked brains that would emerge is well…

infinite. So the probability that you are a floating brain

is not only vastly larger than the probability that you are a real human. It is so inconceivably larger

that we can’t even meaningfully quantify the difference. How do you compare a number

to infinity? So… are you a floating brain that exists

for one moment in time, then basically forever passes and then you exist for another moment in time?

Maybe not even in that order? Maybe your life happens backwards and you just don’t notice?

Maybe you have lived trillions of times already.

Are you the dream of a dead universe?

Really? Like, really?

Well, probably not. First of all, there are a few loopholes. For example, dark energy

could behave completely differently from what we think today and lead us to another future.

Or maybe our dead universe will be too motionless to allow the creation of brains, even with

infinite time… Or maybe the universe will end up dying in another way.

Our understanding of the cosmos doesn’t have a solid enough foundation for anyone to worry

if they are real or not. Loopholes aside, if you were a fluctuating brain,

all the laws of physics stored in your brain would have originated at random and

shouldn’t bear any relation to the real world. But we just used those laws to prove

that you are a floating brain! So even if you believe that you are a floating brain,

you’d have to admit that you have no good reason to believe that you are actually

a floating brain. Ok. So this hallucinatory trip might teach

us something about our theories about the universe. But in the end it is just a really

weird exercise in what you can do with physics. An exercise of what brains in bodies are able to think about.

So don’t worry – you are not a dream of the dead universe. Probably.

At kurzgesagt we love discussing topics like

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