He could drift amongst the stars for a thousand years, cataloguing their number, their composition. He could sift through nebulae, stand watch as stars formed, let particles of dust from ones that had died drift through his fingers, take note of every atom in every arrangement, and it would mean nothing without someone to share it with.
Atoms conjoined in infinite variety, possibilities and impossibilities, shifting and changing, constantly flowing, torn apart and rejoined in patterns beyond conception. If he wasn't there to see, it would still continue. The vastness of it all overwhelming, dwarfing, running in the veins of everything that drew breath and everything that didn't, comprehended by none.
To stop River from ever saving him, and setting into motion her end, and her beginning, of all the mysteries of the universe, this was impossible to solve. A convoluted bond starting with the first time he saw her and the last she saw him. To save her would be to erase her existence. There was no solution, only an inescapable ending.
The TARDIS shuddered before settling with a gentle thrum. The Doctor checked the monitors, feeling the emptiness around him.
The glint of distant star light, resonating ice crystals, a smile that reverberated in his very core, all one and the same for the awe they left behind. A fleeting touch, the unheard boom of planets colliding, a single glance, gamma bursts and the x-ray afterglows as a star explodes, altering the fabric of space, who's to say which leaves the greater effect.
He gave up his mindless fidgeting and leant against the console, knowing he didn't have the strength to open the doors and set foot outside. Not this time, nor any of the others he had come back.
They had been two objects in opposite orbits, chasing, following, waiting for the inevitable collision.
It was a perfectly ordinary planet, though for him it was a gravity well that consumed him. The Library, where the remnant of her lay, which one day would turn to dust. He remained silent, hidden in the confines of the TARDIS, drawn here yet immobilised.
Her timeline twisted through his thoughts, paradoxically starting where it finished. He couldn't bring himself to step outside into the abandoned Library, full of guilt and grief he could never vent, prohibited from unravelling her timeline to save her, to hold her again.
He closed his eyes. He could feel the echo, the vibration of her still. All that she was, is, will be, drifting in the vastness of the ever changing configuration of atoms that was this universe. The thought brought t little solace.
The Doctor opened his eyes and looked across the console to the doors, knowing what was outside. So close to her, yet impossibly distant. He wanted to reach into the computer core holding her consciousness, to see her laugh, run his fingers through her wild hair. Yet he didn't, not when he couldn't stay, couldn't hold her, feel her warmth.
He breathed out slowly, painfully, caught in memories of their time together. Wrapped in grief he refused to share, that time had yet to temper. He somehow found the strength to go on, to put one foot in front of the other and continue his story. At least until the bonds that held together all that he was fell apart, until he could drift with her, with all of them.
He set the controls and left the library, though never truly leaving River behind. He could never change her ending. Instead he searched the dark recesses of the universe for reminders, holding the echo of River inside him, for as long as he could. Never enough, but all that he had left.
Atoms and half forgotten echoes are all that there is in the end. It could all mean nothing, everything, or perhaps a bit of both. The Doctor smiled sadly as the TARDIS dematerialised, imagining what her reaction would have been to his morose thoughts.
He stubbornly held onto that smile, because it is what she would have wanted. Though it was bleak comfort, fitting of the emptiness her absence left in him.