We have been travelling for eleven months, ship-time, and the window has not changed in weeks.
It is a small window, no wider than my two hands. When we began, it showed stars. Real stars, scattered and indifferent, the way they look from the observation deck of any station in the inner system. Then the stars began to move.
Not the stars themselves. Us.
We are at nine-tenths of light now, and I have stopped trying to name what is out there.
We have been travelling for eleven months. Before that, we spent four years training together — long days in simulators, quiet evenings in the mess hall, the small rituals that build between two people who are preparing for something they cannot fully imagine.
I know the sound of his footsteps in a corridor. I know the way he holds a cup of tea when he is thinking. I know what he looks like when he is pretending to be brave.
That is what I brought with me.
In front, the universe has gathered itself into a single point.
It is not bright the way a sun is bright. It is concentrated — a small, fierce white, the colour of something that has been compressed past its natural state. The books call it the headlight effect. Light that would normally arrive from all directions is being folded forward into a cone. The whole sky is collapsing into one small diamond ahead of us.
Beside it, the colour is shifting. Blue, then a blue that hurts to look at, then something past blue that I have no word for. The instruments tell me it is ultraviolet. My eyes tell me it is the colour of an ending.
Behind us, there is nothing.
Not black. Not empty. Nothing. The light from home — from Earth, from the sun, from every star we have ever known — has been stretched so thin by our speed that it has slipped below the visible and into silence. If I turned around, I would see a tunnel of darkness where the universe used to be.
I do not turn around.
The ship is quiet. The life support hums. The artificial gravity holds us at a comfortable fraction of Earth's pull, and my coffee stays in its cup, and my heart beats at its ordinary rate, and nothing about my body tells me we are moving at all.
That is the strange part. The physics books warned us about this. Inside the ship, nothing changes. Our clocks tick forward at the same rhythm they ticked on Earth. We age by the same amount each day. Time does not slow for us — it slows for everything else.
Somewhere behind us, in the tunnel of nothing, the Earth is aging faster than we are. If we turned back now, we would find a world we no longer recognise. If we went far enough, fast enough, we could watch the sun burn through its last fuel, watch the galaxy go quiet, watch the last stars gutter out into the cold.
We are running from the heat death of the universe.
We will not outrun it.
There is another window in this ship. It is not the forward one. It is smaller, and it faces inward, into the cabin, and it does not show the universe at all. It shows us.
I have been sitting beside it for an hour. He is asleep in the chair across from me. His hand is resting on the armrest, fingers slightly curled, the way they curl when he is dreaming about something he will not remember in the morning.
We are the same age we were when we left. That is what the clocks say. But I do not feel the same. I feel older in a way that has nothing to do with time. I feel older the way a person feels older after watching something enormous happen and surviving it.
He asked me once, before we left, why I was coming.
I did not have a good answer then. I have one now.
I am here because at nine-tenths of light, at the edge of the diamond, with the universe folding itself into a single point ahead of us and disappearing into silence behind us, there is nothing left to hold onto.
Nothing except him.
I have read about the speed of light. Not the equations — the poetry of it. A photon is emitted and absorbed in the same instant. It has no time. It exists nowhere in between. If we could reach that speed — we cannot, the mathematics forbids it — we would stop being matter and start being something else.
We would become light.
We would become information.
We would become one.
But we cannot reach it. We can only approach it. And as we approach it, the universe ages around us, and we stay young, and we stay together, and we stay here.
That is what the window shows me.
That is what I have been looking at for the last hour.
His hand moves. He is waking.
He looks at me, and then at the forward window, and then back at me. His eyes are wide and tired and lit by the diamond ahead.
"It is different," he says.
"Yes."
"Are we close?"
"Closer than yesterday."
He does not ask what will happen. We have both read the same papers. There are things that cannot be described in the language of this universe, and passing the speed of light is one of them. If it can be done — and physics says it cannot — we would have to become something else to survive it.
We would have to become something that has no name.
He takes my hand. His fingers are cold. Mine are too.
"If it happens," he says, "and we come out the other side, and the universe reassembles itself around us —"
"Yes?"
"I will look for you first."
I do not answer. There is nothing to say. The universe is ending and beginning at the same time outside the window, and all we can do is watch.
But I hold his hand, and I do not let go.
And that is enough.
That has always been enough.