The answer came to him in a dream. It was the old dream which he had not had for many years.

He is riding home from school on his bicycle. He knows, as he did not know then, that he must get home, must get home at once. But the street gets ever longer, and though he pedals with all his might, he has not yet reached the corner when the explosion rocks the neighborhood. Though he knows he must not, he stops as he stopped then and looks around in confusion. When he sees the flames rising above the house before him, he remembers and he rides ...

He sees the house in flames, in pieces. He hits the curb, leaps off his bike, runs up the lawn, but Mr. Henderson, running out of the house next door, tackles him. "No, boy, no! You can't go in there!"

"But my mother --"

"No! It's too late, boy, too late!"

He jolted awake and rubbed a trembling hand across his eyes. Too late! Yes, he was much, much too late -- decades too late, in fact. He snapped on the light, put on the glasses that lay on the bedside table, and gazed blankly at the key that hung on the wall. Too late! The answer came back to him then, the gossamer strands of thought that we all bring back from dreamland.

Snatching up the pen and pad from the bedside table, he began to write, frantically gathering the thoughts even as they faded. The answer was there, he knew. Grammar, spelling, nothing mattered but to get the thoughts on paper. He had three pages of notes before the memory of the dreamtime answer faded entirely. Eagerly, he began to read them ... and his shoulders slumped in disappointment.

Gibberish!

He turned the pages over as if looking for enlightenment on their backs, but after a moment, turned them face up again. The answer was there if he could only find it. Collecting the pages, he rose and moved to his tiny study and his cluttered desk. Reviewing them slowly again, he saw that this statement was connected to that statement, and if you assumed that this over here could be proven, then ...

He pulled out one of his reference books, one to which he had been a contributor before he retired -- before he was retired, that is, a thought which still rankled. But such thoughts were irrelevant. What mattered was the answer.

The sun was shining through the west window of his study and his hand was painfully cramped when he finally came out of his trance of concentration. Massaging his hand, he looked around the study, at papers thrown to the floor, at books piled high on the desk and on the floor, at business cards that he no longer needed marking important spots in the books. Precariously balanced on one pile was an empty plate that had held yesterday's leftovers; beside the pile was an empty pitcher and a glass. He found that he was fully dressed. He smiled wryly. Yesterday, if he had found himself dressed and with evidence of food and water, but with no recollection of dressing or collecting food and water, he would have had fearful thoughts of Alzheimer's or some creeping dementia. Today -- well, that was the sort of thing he used to do when he was deep in thought. He hadn't done it since retirement ... he cut off that thought and looked back at the handwritten sheets in front of him.

It was the answer. No question about it. It relied on some obscure findings in physics on the nature of time and it could be wrong -- but it could be tested. It could be demonstrated.

I can build a time machine.

He looked around the study again. It seemed as if the world must have staggered in its orbit when he realized that, but the objects around him were quite unchanged. Only he had changed. Gathering the precious papers to him, he walked out into the living room.

Her picture hung on the south wall. Not a portrait, it was simply a candid photograph that he had had enlarged. She was looking down and to the left, smiling gently at something outside the camera's vision. He wondered, as he had so many times, what she was smiling at: a kitten? a flower? himself? "I'm coming," he told her softly. "This time I won't be too late." He thought of the key hanging in the bedroom, as one like it had hung in every bedroom since he was fifteen. It was not the original, of course -- that was in a safe deposit box -- but it was a good copy. He would take it with him in the time machine. He imagined how it would happen.

He would take the time machine back to an hour or so before the explosion. He would go to the house and knock on the door. Pound on the door, in fact. She would be napping on the sofa; that was what the police determined afterwards. She would be napping, and she would hear him pounding and come to the door. Or, if she didn't hear him, he would unlock the door and go inside and get her. He would claim that he had smelled the gas and that the door was unlocked.

It was probably better that it had taken him so many decades to find the answer. If he had come for her when he was a young man, she would have recognized him and that would have been difficult. And if he had come for her when he was in middle age, she would have been afraid of a strange man. But an old man like himself? She would not be afraid. She would be thinking of the gas instead of worrying about his intentions, and they would run out together and call the gas company.

Maybe the house would explode and maybe it would not, but she would be safe. And when his younger self came up on the bicycle, he would quietly leave her and then ... well, it didn't matter what then. He would go find a shelter and claim that he was homeless and penniless and without family that would acknowledge him, all of which would be entirely true, and he would finish his life knowing that she was living out hers.

That was the good dream that had sustained him for so long. The years and the decades had worn away his memories of her; he could summon no memory of her voice or her face. This picture was all he had and all that remained of her.

Not all.

He turned slowly and with a strange reluctance to the displays on the north wall. There were three of them, collages of photographs. The center one showed his own family. The picture in the middle was the wedding picture of himself and Myra, so long ago. They had met in a physics class and she had seen something in the painfully shy and driven young man he had been. They had married three months after they met, and their marriage had endured until her death at age seventy-three, just two years ago. The time machine will not help Myra. I have no cure for cancer. Maybe someday ...

Their marriage had produced three children: Christina, Marianne, and Victoria. Their pictures were part of the collage, along with the pictures of their eleven children, and their four grandchildren: just four so far, but another on the way.

He stepped closer to the picture, searching for the picture of Jodie. She was his oldest great-grandchild and had presented the collage to him. Terribly shy and conscious of the importance of the task, the five-year-old had bungled the presentation and dropped the collage on his foot, then burst into tears. Her father had then made things worse by assuring his grandfather that she was a brilliant child who was already learning to dance and read music. The mortally embarrassed little girl had fled to her mother and had not dared to approach her great-grandfather for the rest of his birthday party.

Ah, there was her picture: a sweet little girl with a nervous smile. Some kind hand had written the genealogies of each descendant under the picture to spare an old man's memory: Josephine Sanders, daughter of John Sanders, son of Christina Sanders. He suspected Marianne's hand; she had always been a thoughtful child.

So: Josephine Sanders, daughter of John Sanders.  The family liked to call John "Doc John" because he was the only "real" doctor among them. There were eight PhDs, counting himself, but only one MD, and that was John. He and Janet had met in the emergency room when he was a student and she brought her mother in with a broken nose. When Josephine was born, her mother had sent numerous presents for the baby -- and the rake that had broken her nose, sporting a large pink bow and a note reading "The luckiest rake anyone ever stepped on."

So: John Sanders, son of Christina Sanders. Christina had married Daniel Sanders on the rebound. Her first husband -- he couldn't remember the lecher's name and didn't care to -- had betrayed her with an unknown number of women. When she learned the truth, she had had to go through the humiliating process of being tested and treated for venereal disease. She never told her father which disease, but he knew that at least it could not have been HIV or herpes, neither of which are curable. To escape the pain and the shame, she had fled halfway across the country, fetching up in Memphis, where she took a job as a school administrator. Daniel had worked alongside her for years, patiently gaining her friendship and, at last, her love. Their marriage was still solid, he thought, looking at their wedding picture in the collage, and they had raised four good children.

His eyes went back to Jodie. She looks nervous. She should be nervous. Her great-grandfather wants to kill her.

No! Never to kill! Only to save my mother!

But part of his mind was still working on the implications.

I was planning to go into biochemistry. I never thought of physics. When my mother died, I went into physics because I wanted to study the nature of time. If she hadn't died ...

The good dream was coming back to him, but it was beginning to be a nightmare.

He would pound on the door and wake her. They would run to a neighbor's house to call the gas company. Then his younger self would ride up and he would leave them together. She would be alive and then ...

And then his younger self would never consider going into physics. He would go into biochemistry and he would never meet Myra. She would meet another young physics student and she would never know of the life she might have had. He would meet another woman and they would have children, but they would not be the same children. There would never have been a Christina, and she would never have met the lecher and never have moved to Memphis; she would never have met Daniel and John would not have been born. Janet would have brought her mother to the emergency room and some other young man would have treated her, and maybe some other family would have received the rake.

There would be no Jodie.

He continued to stare at Jodie's picture, his mind whirling. He could tell his younger self ... but even if he knew the importance of going into physics and meeting Myra, their lives would not be the same -- my mother bouncing her first grandchild on her knee, that same gentle smile on her face -- and they could never produce the same children. Never the same grandchildren, never the same great-grandchildren. Never Jodie. And not just his own descendants.

He tore his gaze away and looked at the other two collages, similar displays showing the descendants of his brother and sister. And besides them, how many of his students had met and married other students in his classes? How many had come to his university to study under him who might have studied elsewhere? What of their children and grandchildren?

But there will be others! There will be children who never existed because she died! What about them?

His gaze returned to Jodie. The number of possible descendants of any woman is unthinkably, unimaginably huge. Out of all those possibilities, Jodie was the one possibility that had been actualized. Did she not have the right to remain actual? Those other might-have-beens -- what right had he to take away her actuality for their benefit? To save his mother would not be to kill Jodie -- no, nothing like that. She would disappear back into the realm of probability. She would not die because she would never have lived. No one would even know that she ever might have lived --

No one will know but me.

He would know. He would know when he brought his mother out of the house that his children, his grandchildren, his little Jodie, were gone forever -- had unhappened. He would know that Jodie would never learn to read music, would never learn to dance, would never drop a picture on his foot ...

He looked down at the papers in his hand and found that they had somehow been crumpled in his fist. He laid them on the nearest coffee table and smoothed them carefully.

The Uncertainty Principle bites everyone. Even here. If these figures are correct, no time machine, not even the best of them, will ever be able to go back more than about eighty years. I would have time. Sixty years since the Explosion, maybe five years to build the machine. I would have plenty of time. If I don't have a heart attack or something, but I'm in good shape.

He was trying not to think of the implications, but still his eyes went back to Jodie's picture. He stared at it for a long moment and then turned back to his mother's picture. She was the possibility that was actualized out of the unthinkable number of people that could have lived. She had had her moment of reality -- too short and too painful, but she had had it.

Must I take away the reality of your descendants to spare you?

He looked down and found that the papers were in his hand, crumpled again. This time he did not smooth them.

This field is obscure. I'm the world's leading expert in this field and I almost couldn't get it. It might be that no one else will think of it for a hundred years.

He turned back to Jodie.

If it is not discovered again for a hundred years, it will be too late to make you unhappen. You will be safe. All of you will be safe. If it is not discovered again. If no one sees these papers.

Swiftly, not wanting to think of what he was doing, he strode to his study and hastily pushed the papers -- all of them, even the scribblings of his dreamtime thoughts -- into the shredder. As an after-thought, he pulled all the bookmarks out of his books and reshelved them, somehow putting them in the right places despite the tears that blurred his vision. At last it was done. He returned to the living room to face his mother.

And her smile.