Essays on the far future of intelligence
← All essays The guide

The limits of intelligence · part 3 of 4 · by Bill Guan

A mind a billion light-years wide would get about ten thoughts

The big question

Could a single mind ever stretch across the universe?

It could try, and it would barely get started. You're about to see how the speed of light and the expanding universe tear a cosmic mind apart, and why the far future of intelligence looks like a scattered network of local minds instead.

Picture a single mind a billion light-years wide, its thoughts carried between galaxies at the speed of light. Max Tegmark makes a claim in his book Life 3.0 that I've thought about on and off for years: a mind like that would have time for roughly ten thoughts in total before dark energy, whatever is making the universe expand faster and faster, broke it into disconnected pieces 1.

It sounds like a riddle, but the reasoning behind it is simple, and it says something real about the shape the far future of intelligence has to take.

Your guess first

Of all the galaxies our telescopes can see, what share could a message sent from Earth today ever reach?

The universe is carrying its galaxies out of reach

HOW MUCH CAN WE EVER REACH?WHAT WE CAN SEEradius 46.4 billion light-yearsabout 400 billion galaxiesLESS THAN5%usWHAT WE COULD EVER REACHradius 16.5 billion light-yearsabout 17.5 billion galaxiesred arrows: every year the reachable zone shrinksby about 1 light-year and loses about 3 galaxies

In 1998, two teams of astronomers measuring distant exploding stars found that the expansion of the universe is speeding up, a discovery that won the 2011 Nobel Prize in Physics 2. Whatever drives that acceleration is what physicists call dark energy.

The consequence for anyone hoping to reach far-off places is harsh. Toby Ord, a philosopher at Oxford, worked through the numbers in 2021. The observable universe, everything whose light we can see, has a radius of about 46.4 billion light-years, while the affectable universe, everything a signal leaving Earth today could ever reach, has a radius of only about 16.5 billion light-years 3. In his words, "Less than 5% of the galaxies we can currently observe could ever be affected by us, and this is shrinking all the time" 3. He counts roughly 17.5 billion galaxies we could still reach, out of about 400 billion we can see.

The window is closing as we speak. Every year, "the radius of the affectable universe will shrink by about one light year and about 3 galaxies will slip forever beyond our causal reach" 3.

A mind that big gets one thought every billion years

TEN THOUGHTS, THEN SILENCEgold dot: one signal, crossing at light speedTHOUGHT 1a billion yearsper thoughtTHOUGHT 5space keepsstretchingTHOUGHT 10the pieces arecut off for good

A mind can only coordinate as fast as a signal can cross it. A brain does it in about a millisecond, and a computer the size of the Earth would need about 42 milliseconds, while a mind a billion light-years wide needs a billion years for every single crossing, even with signals moving at the speed of light, the fastest anything can go. On those timescales the universe itself stops being a fixed stage and starts changing underneath the mind.

That's what breaks the cosmic mind. Each of its thoughts takes a billion years, and over a handful of those billion-year stretches the expansion drags its far-flung parts beyond reach of each other, so after about ten thoughts the pieces can never talk again.

Cosmic intelligence has to think locally

AN ARCHIPELAGO OF MINDSa blueprinta whole mind, as a signaleach island: a fast, dense mind of its ownMESSAGES TAKEMILLIONS TO BILLIONSOF YEARS

The obvious conclusion is that anything occupying a big chunk of the universe would have to rely on local calculations, with only occasional long-distance messages. A second idea follows that I still find striking. If a civilization can rearrange matter at will, the only thing it ever needs to send across the cosmos is information, so blueprints and even whole minds can travel as signals, and the matter at the other end gets rebuilt into whatever is needed.

My assessment: the picture that falls out of this is something like an archipelago. Each island is a fast, dense mind working mostly on its own, and the islands exchange occasional messages that take millions or billions of years to arrive, which means no single will spans the whole thing in real time.

Dark energy also ended the dream of thinking forever

There's a second casualty of dark energy, and it's an older and more beautiful idea. In 1979 the physicist Freeman Dyson argued that intelligent life could keep thinking forever on a finite supply of energy by gradually slowing its thoughts and hibernating between bursts of activity. He concluded that "life and communication can continue for ever, utilizing a finite store of energy, if the assumed scaling laws are valid" 4.

Dyson was working in a universe that expands forever while slowing down. Once the acceleration was found, Lawrence Krauss and Glenn Starkman reran the analysis. In an accelerating universe, space is filled with a faint background glow that never cools below a fixed temperature, which means a thinker can't keep stretching its energy by running colder and colder 5. Their conclusion: "the total information recoverable by any civilization over the entire history of our universe is finite, and assuming that consciousness has a physical computational basis, life cannot be eternal" 5. To be fair, they add that it "can be astronomically long" 5, so what's left is a final budget of thoughts for the whole universe and a shrinking region in which to spend it.

A colony you can never call home

Here's what keeps pulling me back to this. Picture a civilization that sends a colony to a galaxy a billion light-years away. By the time a message arrives asking how things are going, a billion years have passed, and by the time the reply comes back, two billion. The people who launched it have no way to supervise it or call it home, and eventually the expansion cuts the connection for good.

Whatever that colony becomes, it becomes on its own. Over billions of years of silence its culture and maybe even its values will drift in directions nobody at home will ever hear about, so the far future, if it's full of minds at all, may be full of strangers who share an ancestor and little else.

Sources

  1. Max Tegmark, Life 3.0 (2017), ch. 6, as paraphrased by Eann Patterson, "Slow deep thoughts from a planet-sized brain," Realize Engineering (2020-03-25). realizeengineering.blog/2020/03/25/slow-deep-thoughts-from-a… [TBD: replace with direct book quote]
  2. The Nobel Prize in Physics 2011, awarded "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae." www.nobelprize.org/prizes/physics/2011/summary/
  3. Toby Ord, "The Edges of Our Universe" (2021). arxiv.org/abs/2104.01191
  4. Freeman Dyson, "Time without end: Physics and biology in an open universe," Reviews of Modern Physics 51, 447 (1979). journals.aps.org/rmp/abstract/10.1103/RevModPhys.51.447
  5. Lawrence Krauss and Glenn Starkman, "Life, the Universe, and Nothing: Life and Death in an Ever-Expanding Universe," Astrophysical Journal 531, 22 (2000). arxiv.org/abs/astro-ph/9902189