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An Advanced Reader’s Picture Book of Comparative Cognition

By Ken Liu

My darling, my child, my connoisseur of sesquipedalian words and convoluted ideas and meandering sentences and baroque images, while the sun is asleep and the moon somnambulant, while the stars bathe us in their glow from eons ago and light-years away, while you are comfortably nestled in your blankets and I am hunched over in my chair by your bed, while we are warm and safe and still for the moment in this bubble of incandescent light cast by the pearl held up by the mermaid lamp, you and I, on this planet spinning and hurtling through the frigid darkness of space at dozens of miles per second, let’s read.

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The brains of Telosians record all the stimuli from their senses: every tingling along their hairy spine, every sound wave striking their membranous body, every image perceived by their simple-compound-refractive light-field eyes, every molecular gustatory and olfactory sensation captured by their waving stalk-feet, every ebb and flow in the magnetic field of their irregular, potato-shaped planet.

When they wish, they can recall every experience with absolute fidelity. They can freeze a scene and zoom in to focus on any detail; they can parse and re-parse each conversation to extract every nuance. A joyful memory may be relived countless times, each replay introducing new discoveries. A painful memory may be replayed countless times as well, each time creating a fresh outrage. Eidetic reminiscence is a fact of existence.

Infinity pressing down upon the finite is clearly untenable.

The Telosian organ of cognition is housed inside a segmented body that buds and grows at one end while withering and shedding at the other. Every year, a fresh segment is added at the head to record the future; every year, an old segment is discarded from the tail, consigning the past to oblivion.

Thus, while the Telosians do not forget, they also do not remember. They are said to never die, but it is arguable whether they ever live.

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It has been argued that thinking is a form of compression.

Remember the first time you tasted chocolate? It was a summer afternoon; your mother had just come back from shopping. She broke off a piece from a candy bar and put it in your mouth while you sat in the high chair.

As the stearate in the cocoa butter absorbed the heat from your mouth and melted over your tongue, complex alkaloids were released and seeped into your taste buds: twitchy caffeine, giddy phenethylamine, serotonic theobromine.

“Theobromine,” your mother said, “means the food of the gods.”

We laughed as we watched your eyes widen in surprise at the texture, your face scrunch up at the biting bitterness, and then your whole body relax as the sweetness overwhelmed your taste buds, aided by the dance of a thousand disparate organic compounds.

Then she broke the rest of the chocolate bar in halves and fed a piece to me and ate the other herself. “We have children because we can’t remember our own first taste of ambrosia.”

I can’t remember the dress she wore or what she had bought; I can’t remember what we did for the rest of that afternoon; I can’t recreate the exact timbre of her voice or the precise shapes of her features, the lines at the corners of her mouth or the name of her perfume. I only remember the way sunlight through the kitchen window glinted from her forearm, an arc as lovely as her smile.

A lit forearm, laughter, food of the gods. Thus are our memories compressed, integrated into sparkling jewels to be embedded in the limited space of our minds. A scene is turned into a mnemonic, a conversation reduced to a single phrase, a day distilled to a fleeting feeling of joy.

Time’s arrow is the loss of fidelity in compression. A sketch, not a photograph. A memory is a re-creation, precious because it is both more and less than the original.

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Living in a warm, endless sea rich with light and clumps of organic molecules, the Esoptrons resemble magnified cells, some as large as our whales. Undulating their translucent bodies, they drift, rising and falling, tumbling and twisting, like phosphorescent jellyfish riding on the current.

The thoughts of Esoptrons are encoded as complex chains of proteins that fold upon themselves like serpents coiling in the snake charmer’s basket, seeking the lowest energy level so that they may fit into the smallest space. Most of the time, they lie dormant.

When two Esoptrons encounter each other, they may merge temporarily, a tunnel forming between their membranes. This kissing union can last hours, days, or years, as their memories are awakened and exchanged with energy contributions from both members. The pleasurable ones are selectively duplicated in a process much like protein expression—the serpentine proteins unfold and dance mesmerizingly in the electric music of coding sequences as they’re first read and then re-expressed—while the unpleasant ones are diluted by being spread among the two bodies. For the Esoptrons, a shared joy truly is doubled, while a shared sorrow is indeed halved.

By the time they part, they each have absorbed the experiences of the other. It is the truest form of empathy, for the very qualia of experience are shared and expressed without alteration. There is no translation, no medium of exchange. They come to know each other in a deeper sense than any other creatures in the universe.

But being the mirrors for each other’s souls has a cost: by the time they part from each other, the individuals in the mating pair have become indistinguishable. Before their merger, they each yearned for the other; as they part, they part from the self. The very quality that attracted them to each other is also, inevitably, destroyed in their union.

Whether this is a blessing or a curse is much debated.

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Your mother has never hidden her desire to leave.

We met on a summer night, in a campground high up in the Rockies. We were from opposite coasts, two random particles on separate trajectories: I was headed for a new job, driving across the country and camping to save money; she was returning to Boston after having moved a friend and her truckful of possessions to San Francisco, camping because she wanted to look at the stars.

We drank cheap wine and ate even cheaper grilled hot dogs. Then we walked together under the dark velvet dome studded with crystalline stars like the inside of a geode, brighter than I’d ever seen them, while she explained to me their beauty: each as unique as a diamond, with a different-colored light. I could not remember the last time I’d looked up at the stars.

“I’m going there,” she said.

“You mean Mars?” That was the big news back then, the announcement of a mission to Mars. Everyone knew it was a propaganda effort to make America seem great again, a new space race to go along with the new nuclear arms race and the stockpiling of rare earth elements and zero-day cyber vulnerabilities. The other side had already promised their own Martian base, and we had to mirror their move in this new Great Game.

She shook her head. “What’s the point of jumping onto a reef just a few steps from shore? I mean out there.”

It was not the kind of statement one questioned, so instead of why and how and what are you talking about, I asked her what she hoped to find out there among the stars.

other Suns perhaps


With thir attendant Moons thou wilt descrie


Communicating Male and Femal Light,


Which two great Sexes animate the World,


Stor’d in each Orb perhaps with some that live.


For such vast room in Nature unpossest


By living Soule, desert and desolate,


Onely to shine, yet scarce to contribute


Each Orb a glimps of Light, conveyd so farr


Down to this habitable, which returnes


Light back to them, is obvious to dispute.

“What do they think about? How do they experience the world? I’ve been imagining such stories all my life, but the truth will be stranger and more wonderful than any fairytale.”

She spoke to me of gravitational lenses and nuclear pulse propulsion, of the Fermi Paradox and the Drake Equation, of Arecibo and Yevpatoria, of Blue Origin and SpaceX.

“Aren’t you afraid?” I asked.

“I almost died before I could begin to remember.”

She told me about her childhood. Her parents were avid sailors who had been lucky enough to retire early. They bought a boat and lived on it, and the boat was her first home. When she was three, her parents decided to sail across the Pacific. Half-way across the ocean, somewhere near the Marshall Islands, the boat sprung a leak. The family tried everything they could to save the vessel, but in the end had to activate the emergency beacon to call for help.

“That was my very first memory. I wobbled on this immense bridge between the sea and the sky, and as it sank into the water and we had to jump off, Mom had me say goodbye.”

By the time they were rescued by a Coast Guard plane, they had been adrift in the water in life vests for almost a full day and night. Sunburnt and sickened by the salt water she swallowed, she spent a month in the hospital afterwards.

“A lot of people were angry at my parents, saying they were reckless and irresponsible to endanger a child like that. But I’m forever grateful to them. They gave me the greatest gift parents could give to a child: fearlessness. They worked and saved and bought another boat, and we went out to the sea again.”

It was such an alien way of thinking that I didn’t know what to say. She seemed to detect my unease, and, turning to me, smiled.

“I like to think we were carrying on the tradition of the Polynesians who set out across the endless Pacific in their canoes or the Vikings who sailed for America. We have always lived on a boat, you know? That’s what Earth is, a boat in space.”

For a moment, as I listened to her, I felt as if I could step through the distance between us and hear an echo of the world through her ears, see the stars through her eyes: an austere clarity that made my heart leap.

Cheap wine and burnt hot dogs, other Suns perhaps, the diamonds in the sky seen from a boat adrift at sea, the fiery clarity of falling in love.

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The Tick-Tocks are the only uranium-based life forms known in the universe.

The surface of their planet is an endless vista of bare rock. To human eyes it seems a wasteland, but etched into this surface are elaborate, colorful patterns at an immense scale, each as large as an airport or stadium: curlicues like calligraphy strokes; spirals like the tips of fiddlehead ferns; hyperbolas like the shadows of flashlights against a cave wall; dense, radiating clusters like glowing cities seen from space. From time to time, a plume of superheated steam erupts from the ground like the blow of a whale or the explosion of an ice volcano on Enceladus.

Where are the creatures who left these monumental sketches? These tributes to lives lived and lost, these recordings of joys and sorrows known and forgotten?

You dig beneath the surface. Tunneling into the sandstone deposits over granite bedrock, you find pockets of uranium steeped in water.

In the darkness, the nucleus of a uranium atom spontaneously breaks apart, releasing a few neutrons. The neutrons travel through the vast emptiness of internuclear space like ships bound for strange stars (this is not really an accurate picture, but it’s a romantic image and easy to illustrate). The water molecules, nebula-like, slow down the neutrons until they touch down on another uranium nucleus, a new world.

But the addition of this new neutron makes the nucleus unstable. It oscillates like a ringing alarm clock, breaks apart into two new elemental nuclei and two or three neutrons, new starships bound for distant worlds, to begin the cycle again.

To have a self-sustaining nuclear chain reaction with uranium, you need enough concentration of the right kind of uranium, uranium–235, which breaks apart when it absorbs the free neutrons, and something to slow down the speeding neutrons so that they can be absorbed, and water works well enough. Creation has blessed the world of the Tick-Tocks with both.

The byproducts of fission, those fragments split from the uranium atom, fall along a bimodal distribution. Caesium, iodine, xenon, zirconium, molybdenum, technetium… like new stars formed from the remnants of a supernova, some last a few hours, others millions of years.

The thoughts and memories of the Tick-Tocks are formed from these glowing jewels in the dark sea. The atoms take the place of neurons, and the neutrons act as neurotransmitters. The moderating medium and neutron poisons act as inhibitors and deflect the flight of neutrons, forming neural pathways through the void. The computation process emerges at the subatomic level, and is manifested in the flight paths of messenger neutrons; the topology, composition, and arrangement of atoms; and the brilliant flashes of fissile explosion and decay.

As the thoughts of the Tick-Tocks grow ever more lively, excited, the water in the pockets of uranium heats up. When the pressure is great enough, a stream of superheated water flows up a crack in the sandstone cap and explodes at the surface in a plume of steam. The grand, intricate, fractal patterns made by the varicolored salt deposits they leave on the surface resemble the ionization trails left by subatomic particles in a bubble chamber.

Eventually, enough of the water will have been boiled away that the fast neutrons can no longer be captured by the uranium atoms to sustain the reaction. The universe sinks into quiescence, and thoughts disappear from this galaxy of atoms. This is how the Tick-Tocks die: with the heat of their own vitality.

Gradually, water seeps back into the mines, trickling through seams in the sandstone and cracks in the granite. When enough water has filled the husk of the past, a random decaying atom will release the neutron that will start the chain reaction again, ushering forth a florescence of new ideas and new beliefs, a new generation of life lit from the embers of the old.

Some have disputed the notion that the Tick-Tocks can think. How can they be said to be thinking, the skeptics ask, when the flight of neutrons are determined by the laws of physics with a soupçon of quantum randomness? Where is their free will? Where is their self-determination? Meanwhile, the electrochemical reactor piles in the skeptics’ brains hum along, following the laws of physics with an indistinguishable rigor.

Like tides, the Tick-Tock nuclear reactions operate in pulses. Cycle after cycle, each generation discovers the world anew. The ancients leave no wisdom for the future, and the young do not look to the past. They live for one season and one season alone.

Yet, on the surface of the planet, in those etched, fantastic rock paintings, is a palimpsest of their rise and fall, the exhalations of empires. The chronicles of the Tick-Tocks are left for other intelligences in the cosmos to interpret.

As the Tick-Tocks flourish, they also deplete the concentration of uranium–235. Each generation consumes some of the non-renewable resources of their universe, leaving less for future generations and beckoning closer the day when a sustained chain reaction will no longer be possible. Like a clock winding down inexorably, the world of the Tick-Tocks will then sink into an eternal, cold silence.

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Your mother’s excitement was palpable.

“Can you call a realtor?” she asked. “I’ll get started on liquidating our stocks. We don’t need to save anymore. Your mother is going to go on that cruise she’s always wanted.”

“When did we win the lottery?” I asked.

She handed me a stack of paper. LENS Program Orientation.

I flipped through it. …Your application essay is among the most extraordinary entries we’ve received… pending a physical examination and psychological evaluation … limited to the immediate family …

“What is this?”

Her face fell as she realized that I truly did not understand.

Radio waves attenuated rapidly in the vastness of space, she explained. If anyone is shouting into the void in the orbs around those distant stars, they would not be heard except by their closest neighbors. A civilization would have to harness the energy of an entire star to broadcast a message that could traverse interstellar distances – and how often would that happen? Look at the Earth: we’d barely managed to survive one Cold War before another started. Long before we get to the point of harnessing the energy of the Sun, our children will be either wading through a post-apocalyptic flooded landscape or shivering in a nuclear winter, back in another Stone Age.

“But there is a way to cheat, a way for even a primitive civilization like ours to catch faint whispers from across the galaxy and perhaps even answer back.”

The Sun’s gravity bends the light and radio waves from distant stars around it. This is one of the most important results from general relativity.

Suppose some other world out there in our galaxy, not much more advanced than ours, sent out a message with the most powerful antenna they could construct. By the time those emissions reached us, the electromagnetic waves would be so faint as to be undetectable. We’d have to turn the entire Solar System into a parabolic dish to capture it.

But as those radio waves grazed the surface of the Sun, the gravity of the star would bend them slightly, much as a lens bends rays of light. Those slightly bent beams from around the rim of the sun would converge at some distance beyond.

“Just as rays of sunlight could be focused by a magnifying glass into a spot on the ground.”

The gain of an antenna placed at the focal point of the sun’s gravitational lens would be enormous, close to 10 billion times in certain frequency ranges, and orders of magnitude more in others. Even a 12-meter inflatable dish would be able to detect transmissions from the other end of the galaxy. And if others in the galaxy were also clever enough to harness the gravitational lenses of their own suns, we would be able to talk to them as well—though the exchange would more resemble monologues delivered across the lifetimes of stars than a conversation, messages set adrift in bottles bound for distant shores, from one long-dead generation to generations yet unborn.

This spot, as it turns out, is about 550 AU from the Sun, almost 14 times the distance of Pluto. The Sun’s light would take just over three days to reach it, but at our present level of technology, it would take more than a century for a spacecraft.

Why send people? Why now?

“Because by the time an automated probe reached the focal point, we don’t know if anyone will still be here. Will the human race survive even another century? No, we must send people so that they can be there to listen, and perhaps talk back.

“I’m going, and I’d like you to come with me.”

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The Thereals live within the hulls of great starships.

Their species, sensing the catastrophe of a world-ending disaster, commissioned the construction of escape arks for a small percentage of their world’s population. Almost all of the refugees were children, for the Thereals loved their young as much as any other species.

Years before their star went supernova, the arks were launched in various directions at possible new home worlds. The ships began to accelerate, and the children settled down to learning from machine tutors and the few adults on board, trying to carry on the traditions of a dying world.

Only when the last of the adults were about to die aboard each ship did they reveal the truth to the children: the ships were not equipped with means for deceleration. They would accelerate forever, asymptotically approaching the speed of light, until the ships ran out of fuel and coasted along at the final cruising speed, towards the end of the universe.

Within their frame of reference, time would pass normally. But outside the ship, the rest of the universe would be hurtling along to its ultimate doom against the tide of entropy. To an outside observer, time seemed to stop in the ships.

Plucked out of the stream of time, the children would grow a few years older, but not much more. They would die only when the universe ended. This was the only way to ensure their safety, the adults explained, an asymptotic approach to triumphing over death. They would never have their own children; they would never have to mourn; they would never have to fear, to plan, to make impossible choices in sacrifice. They would be the last Thereals alive, and possibly the last intelligent beings in the universe.

All parents make choices for their children. Almost always they think it’s for the best.

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All along, I had thought I could change her. I had thought she would want to stay because of me, because of our child. I had loved her because she was different; I also thought she would transform out of love.

“Love has many forms,” she said. “This is mine.”

Many are the stories we tell ourselves of the inevitable parting of lovers when they’re from different worlds: selkies, gu huo niao, Hagoromo, swan maidens… What they have in common is the belief by one half of a couple that the other half could be changed, when in fact it was the difference, the resistance to change, that formed the foundation of their love. And then the day would come when the old sealskin or feather cape would be found, and it would be time to return to the sea or the sky, the ethereal realm that was the beloved’s true home.

The crew of Focal Point would spend part of the voyage in hibernation; but once they reached their first target point, 550 AU from the Sun, away from the galactic center, they would have to stay awake and listen for as long as they could. They would guide the ship along a helical path away from the Sun, sweeping out a larger slice of the galaxy from which they might detect signals. The farther they drifted from the Sun, the better the Sun’s magnification effect would be due to the reduction of interference from the solar corona on the deflected radio waves. The crew was expected to last as long as a few centuries, growing up, growing old, having children to carry on their work, dying in the void, an outpost of austere hope.

“You can’t make a choice like that for our daughter,” I said.

“You’re making a choice for her, too. How do you know if she’ll be safer or happier here? This is a chance for transcendence, the best gift we can give her.”

And then came the lawyers and the reporters and the pundits armed with soundbites taking sides.

Then the night that you tell me you still remember. It was your birthday, and we were together again, just the three of us, for your sake because you said that was what you wished.

We had chocolate cake (you requested “teo-broom”). Then we went outside onto the deck to look up at the stars. Your mother and I were careful to make no mention of the fight in the courts or the approaching date for her departure.

“Is it true you grew up on a boat, Mommy?” you asked.

“Yes.”

“Was it scary?”

“Not at all. We’re all living on a boat, sweetheart. The Earth is just a big raft in the sea of stars.”

“Did you like living on a boat?”

“I loved that boat – well, I don’t really remember. We don’t remember much about what happened when we were really young; it’s a quirk of being human. But I do remember being very sad when I had to say goodbye to it. I didn’t want to. It was home.”

“I don’t want to say goodbye to my boat, either.”

She cried. And so did I. So did you.

She gave you a kiss before she left. “There are many ways to say I love you.”

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The universe is full of echoes and shadows, the afterimages and last words of dead civilizations that have lost the struggle against entropy. Fading ripples in the cosmic background radiation, it is doubtful if most, or any, of these messages will ever be deciphered.

Likewise, most of our thoughts and memories are destined to fade, to disappear, to be consumed by the very act of choosing and living.

That is not a cause for sorrow, sweetheart. It is the fate of every species to disappear into the void that is the heat death of the universe. But long before then, the thoughts of any intelligent species worthy of the name will become as grand as the universe itself.

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Your mother is asleep now on Focal Point. She will not wake up until you’re a very old woman, possibly not even until after you’re gone.

After she wakes up, she and her crew mates will begin to listen, and they’ll also broadcast, hoping that somewhere else in the universe, another species is also harnessing the energy of their star to focus the faint rays across light-years and eons. They’ll play a message designed to introduce us to strangers, written in a language based on mathematics and logic. I’ve always found it funny that we think the best way to communicate with extraterrestrials is to speak in a way that we never do in life.

But at the end, as a closing, there will be a recording of compressed memories that will not be very logical: the graceful arc of whales breaching, the flicker of campfire and wild dancing, the formulas of chemicals making up the smell of a thousand foods, including cheap wine and burnt hot dogs, the laughter of a child eating the food of the gods for the first time. Glittering jewels whose meanings are not transparent, and for that reason, are alive.

And so we read this, my darling, this book she wrote for you before she left, its ornate words and elaborate illustrations telling fairytales that will grow as you grow, an apologia, a bundle of letters home, and a map of the uncharted waters of our souls.

There are many ways to say I love you in this cold, dark, silent universe, as many as the twinkling stars.