The problem was self-collision, and Spock had solved it in nine days, and the solution was standing in the room and did not know it yet.
"Every one of us was aboard," he said. "During the relevant six hours, every one of us who stood on that plain was at a duty station on that ship. Mister Scott in engineering. Doctor McCoy in sickbay. Lieutenant Galloway on deck five. Ensign Brandt in the security office on deck three. Lieutenant Uhura and myself on the bridge.
"A reading cannot contain a reader who was written. If any one of us enters that layer at that hour, we collide with ourselves. The collision is not survivable, and it is not survivable for the layer either. It would tear. And a torn layer is a wound, and it would bleed into the one above it forever, and I have modeled the consequences and I am not going to describe them."
"Then nobody can go," said Scotty.
"Somebody can go." He turned. "The prohibition is spatial and temporal, not personal. What cannot happen is that a person occupy a location in which that person is already recorded. What can happen is that a person occupy a location in which they are not. Lieutenant Uhura was on the bridge, at the communications station, for the entire watch. There is a record of her there, and she cannot go to the bridge. She was not in the port auxiliary junction on deck seven."
Nobody spoke.
"The console that overloads is the helm station. It overloads because a relay in the port auxiliary junction fails under the temporal surge and dumps the entire forward load through the helm board. That junction is nineteen meters from the bridge and one deck down, in a corridor that was empty, at that hour, for forty-one minutes. I have checked it against the duty roster, the sensor log, and the maintenance schedule, eleven times. It is the only place on that ship where a living person may stand in that hour without being a paradox. And the relay can be pulled by hand."
McCoy said, "Then send me."
"You were in sickbay, Doctor. Eleven meters from that junction, and moving, and your movements are not fully recorded, and I will not put you in a corridor you may walk down."
"Then send Galloway."
"Lieutenant Galloway was on deck five, with a patrol route that passes within nine meters of that junction at the relevant minute."
"Then send Scotty."
"Mister Scott is the only living human being who can hold this vessel inside the Guardian's field for the eleven minutes required, and if he is in a corridor in 2267 then all of us die in 2269, and I would rather not."
And Nyota Uhura, who had understood four sentences ago, said: "It's me."
"Yes," said Spock.
"Because the only place on that ship I wasn't is a corridor on deck seven."
"That is the operation."
* *
He told her the second reason an hour later, alone, in the observation lounge, with the door locked.
"There is a second reason it must be you. I did not give it in front of the others. I am going to give it to you now, and then you are going to have an opportunity to refuse, and nobody will ever know you were asked."
Uhura sat down.
"The dead layer is not a place. It is a signal. A coherent record of a lived state, held in a medium we do not understand, underneath a louder and more recent record. When you step into it, you will be inside both. You will be receiving two histories at once, at full strength, and they will be equally real, and one of them is false, and there is no instrument in the universe that will tell you which."
"And I'll have to know."
"For eleven minutes. While the ship you are standing in tells you, with every wall and every sound and every face, that you are wrong."
He folded his hands. "Vulcan discipline cannot do this. I spent nine days establishing it, and it was not a pleasant nine days. Vulcan discipline functions by suppression. It resolves conflicting input by silencing one channel. If I go into that layer, I will silence one of them, and I will not be able to control which, and I will perform the operation perfectly. And I may perform it in the wrong universe.
"You do not suppress. You separate. You have spent your entire professional life holding two signals at once and refusing to lose either of them, and you are the only person aboard who has ever been trained to do it. And it is not a metaphor, Lieutenant. It is the literal content of your qualification."
Nyota Uhura sat very still. "You're telling me I'm the only one who can hear him."
Spock did not answer for a moment. "I am telling you that you are the only one who will still know which voice is real."
* *
She sat in the observation lounge a long time after he had gone. Then she took out the padd, and unlocked it, and opened a new entry.
ENTRY 6,012. I am going to see him tomorrow. I have wanted this for two years, and I have just been told I am going to be within ninety seconds of him and am not permitted to go and look. I want to write down, before it happens, what I intend to do. I intend to walk down that corridor and pull that relay and come home. I intend to not go up to the bridge. I am writing this because I do not entirely trust myself, and because if I am reading it afterward then it means I did the right thing.
She looked at it. Then she wrote:
If I do not come back, somebody give this to him. He will not know what it is. Give it to him anyway.