Prologue: Quantum mechanics

Disclaimer: I don't own FF and chances are I never will.

Warning: Very eventually slash.

AN: My beta is Quiet N Cryptic. My sincerest thanks to you and your hard work.

What is it like to be Gaia? She can be neither young nor old for there is no comparison. If there are others like her, they spin through the space too far away, around their own distant stars. She is made of many lives and many memories, interloping and contradicting, experiencing myriad truths. She is made of parts that die and are recycled, like cells in a body.

Gaia is suffering from a cancer and dying because of a parasite. The cancer is humanity, cells gone rouge, drilling into her, taking her, using and polluting her, but she loves that tumor as a part of her own body. It isn't lethal yet. She can afford to wait just a little bit longer, give them time to learn. They do not know what they are doing. The parasite, the Calamity from Outside, is not part of her and she holds no affection towards her. But how can she destroy something that can survive the cold vacuum of the space, attacks of cosmic rays and magnetic storms that rage around neutron stars? She had buried the parasite alive after it drove her Cetra, the brightest parts of her, to the brink of extinction. But even that was not enough.

What is it like to defend your living body from a parasite? For Gaia there are no doctors to issue medicines. In the Lifestream the Cetra still live; for Lifestream is beyond time and space. There, Gaia holds a box in her power. It isn't actually a box, but there is no better metaphor for it. A piece of history and the collective consciousness of the Cetra are what she conceals in it and then closes the lid. She doesn't want to summon the Weapons and this is her last measure to prevent the necessity.

This generation is the end of the Cetra, but she has saved the best for the last. In the Midgard slums eleven years old Aeris Gainsborough turns in her sleep and frowns, dreaming of memories that may yet become.

Cloud woke up to see a ceiling that was not his own. It seemed vaguely familiar, especially the small crack between the boards that looked like an eye, but he couldn't remember where he had seen it before. True, these days his memory still had more holes than Nibelheim cheese, but that was about his past. He should have remembered how he had tonight ended up in wherever the ceiling was. He sat up, but the body wasn't his. It wasn't his anymore. Cloud woke up in a body that belonged to a child in a room that belonged to a child. In the corner was his toychest, on the nightstand beside his bed a cup of water, the quilt draped over him was made of blue and yellow and green swatches by his mother.

Cloud buried his nose into the warm, woolen quilt; it smelt like the soap his mother made in the big, black cauldron. He hadn't remembered it having this sharp a smell.

"I'm dreaming," he said aloud. He much preferred a dream like this to the dreams he often suffered from, dreams of Nibelheim burning, Hojo, or Aeris in the church, skewered through with a madman's blade. He sat up just because he could. This was the first lucid dream he'd ever had. Maybe he could go to see his mother sleeping in the bedroom next to his; her face had blurred in his memories a long time ago until he could only recall her kind, blue eyes. The floor was chilly under his bare feet and the moonlight through the window lit the room just so; the details seemed sharper than in any dream. He could admire it like art.

"You're not dreaming, I am." With the dear, longed, never forgotten voice came green light and Cloud turned around.

Aeris shone like the Lifestream itself as she extended her hand. Cloud had long ago learned to associate the colour green with Mako, but Aeris made it both unearthly and natural, so full of life. Oh, the irony of that. His eyes were stinging and he had to take measured breaths to keep the tears from falling. It was like in Hojo's lab. It's just pain, it will be over if you wait long enough. Inhale, exhale, in steady rhythm. It's just pain. Aeris's face was still so clear and honest; she moved like she had always moved, but her hand went through his chest with a small jolt. Inhale, exhale.

"I missed you," he whispered to the dead flower girl. It was odd how he had to look up at her now and he was terribly conscious of how underdeveloped this body was, how his centre of gravity was off and his arms were weak. She tried to touch him again, with no better success, but her smile was more happy than sad.

"I've missed you too, but now I come with good news. It doesn't have to happen like it happened." She tilted her head slightly, thinking what she had said.

"Er, allow me to rephrase that. Have you ever heard of Faremis' cat?" Her voice was full of anticipation and he felt bizarre, dreaming of her coming back to him to talk about somebody's pet. Maybe this was his subconscious being subtle and using metaphors for once.

"No, I haven't. Who is Faremis?" he kept the conversation going. He would have talked about weather, chocobo racing, the price of good glassware in Gongaga, Wutaiian guerrilla tactics, anything.

"Gast Faremis was my father. He was also the head of Shin-Ra's Science Research Department that originally discovered Jenova, which he later came to regret deeply, but he had other, less apocalyptic merits in his résumé, too. He applied quantum mechanics to a living being. In his original thought experiment he describes how one could, in principle, transform a superposition inside an atom to a large-scale superposition of a live and dead cat by coupling cat and atom with the help of a so-called diabolical mechanism…" Cloud's felt like he would either laugh or cry any moment. Not enough to find himself in his ten-year-old body in a town that apparently wasn't burnt and dead and Aeris as a ghost, she expected him to follow that?

"Superposition?" he asked weakly.

"The superposition principle," Aeris explained with a sympathetic smile and a ghost-touch to his lips, "says that the way to describe the world is to assign such a complex number to every possible situation, and that the way to describe how things change is to treat these numbers mathematically as if they were probabilities. Gast proposed a scenario with a cat in a sealed box, where the cat's life or death is dependent on the state of a subatomic particle. The Mideel interpretation, which by the way is the correct one, says that the cat remains both alive and dead until the box is opened." Both dead and alive, this was beginning to hint hope, even if he had absolutely no idea how.

"Is this real?" Because if someone could pull coming back from death off it would be Aeris. She had summoned Holy and stopped Meteor, single-handedly saving the world. She had done more good than he ever had, so why did she have to be the one to go in the first place?

"This is real," she told him with understanding voice and Cloud felt his knees betraying him. He dropped down to the floor, he could have written long tomes of pain, but he had never known joy could be such a shock it almost made you throw up. He forced his body to obey and stood again right away, looking deep into Aeris' eyes. They were even greener now, and this felt much too real to be dream, didn't it? It had to, because if this was a dream it was cruelty beyond anything Jenova and Hojo together had managed to cook up.

He was not going to let her down ever again. He wasn't.

"You can be alive, too?" he asked, truly afraid of the answer. He was clad in thin pyjamas and without the quilt around him the room was chilly enough to freeze such wistful notions. In real life the Nibelheim nights were cold and the dead didn't come back. She shone green like life and still so much like the seawraiths he had heard Cid telling tall tales of. There was no scent he could detect and his Soldier instincts were screaming mirage.

"Better than that, my heart. If you do everything right it may never happen at all. A system stops being a superposition of states and becomes either one or the other when an observation takes place. As long as the box is closed, the system simultaneously exists in a superposition of the states of "decayed nucleus and a dead cat" and "undecayed nucleus and a living cat", and only when the box is opened the wave function collapses into one of the two states. Cloud, when they found the Calamity's remains, Gaia locked this piece of history into a box to protect herself. It happens both ways as we speak."

Cloud hadn't paid much attention to what she said after the "it may never happen" part. He wasn't going to understand it without a full quantum mechanics lecture with all trappings and he had much better things to do now than learning physics.

There was no way his mind could concoct scientific jargon like that.

"What do I do now?" he asked eagerly. How to be the living cat instead of the dead cat? How to escape his childhood hero's madness, his mother's death, Hojo's scalpel, Zack's death, his beloved's death? He would have done anything.

"Humans are made of memories, my heart. In fact, you are not an adult Cloud in a child's body, but a child with the adult's memories, memories of the bad course of events. It's all about probabilities, you see. Like I said, the way to describe how things change is to treat the situation's numbers as if they were probabilities. Since there is a much higher chance of things turning out badly, the Cetra are stacking the odds in our favour. You have seven days to change the world so that the cat will live. After that, the memories will be gone and you will be a normal child again." She smiled to him like she used to, and his heart ached with longing and loss. If none of it had ever happened, she would never be his at all, but Zack's, who had fallen through her roof first. But she would be alive and so would everyone else; much more than he had dared to dream in his most private, deepest sleep. That was what mattered. This was the anything he would do.

"Kiss me one more time. I love you, and have loved you since I fell through your roof," he whispered and Aeris knelt to press her immaterial lips against his, ran her tongue right through his and if anything it felt like an electric shock. Her eyes were sad also.

"Zack may love me well if things go right, but you will still have loved me the best, even if you never get to do so." She was gone with the light and he was cold, but determined.

Seven days. He had seven precious days and he was going to make the most of them if it killed him.

Gaia knows entropy. Entropy is energy broken down in irretrievable heat, the partial loss of the ability of a system to perform work due to the effects of irreversibility. Entropy is the slow death of the universe. Energy can not be destroyed, but it can be irretrievably lost. There will be an end to all.

But this is not the end. Even entropy can be warded off by the power of creation, and love is creation. Love can create life. Love can create one precious chance.

AN: I didn't come up with this on my own; I'm borrowing Schrödinger's cat here.