Free Worlds Enough and Time by Joe Haldeman

that we understand each other. That you understand exactly what you are asking.”
    “Fair enough.”
    He slid a keyboard out from under the table and tapped a few keys. A complex exploded diagram of ’Home’s interior appeared on the screen. A list unrolled in the upper left-hand corner, titled “Location Referents,” giving numbers from (1.) Agriculture to (47.) Workshops: General. He highlighted (5.) Education and (6.) Entertainment with blue and green; patches of those colors appeared all over the ship.
    “These are the spaces that you and Mr. Smith control. So to speak.”
    “That’s interesting. You can see how spread out they are.”
    “Yes. Not for no reason, of course. Your proposal had to do with storage space.”
    “Office space, too. Smith and I are practically at opposite ends of the ship. Yet Education and Entertainment share many of the same supplies. Our people are always running back and forth unnecessarily.”
    “Perhaps not unnecessarily.”
    “I’d be willing to give up my place in Uchūden and move back here with Tom, with Smith.”
    “Very nice of you.” He leaned back in his chair and swiveled around, looking at me over steepled fingers. His face screwed up into a wrinkled prunish mask of concentration. He really could have used eyebrows. “Dr. O’Hara, do you know what moment of inertia is?”
    “No. Never heard of it.”
    “It has to do with the way things spin. Like an ice skater, you know? She goes around with her arms out, spinning at a certain rate.” He held his arms out and pulled them in slowly. “Then she brings them in and spins much faster.”
    “Conservation of angular momentum,” I said, not completely helpless.
    “Very good. Another way to look at it is that she has changed … she has changed the distribution of mass in her body, relative to the axis of spin. That is what moment of inertia is. The same amount of energy is tied up in her spinning, but because the mass is in different places, she spins faster.”
    “I think I see what you’re getting at. You can’t just move weight around ’Home arbitrarily.”
    “Indeed we cannot. But it isn’t a matter of simply changing the
rate
of spin. It is a matter of making sure the axis of spin remains the same as the ship’s geometrical axis. That is not clear.”
    “Uh … not really.”
    “We cannot … let me see.” He made vague circular gestures. “We can’t allow a large lump of mass on one side, not balanced by something on the other side. The ship would wobble.”
    “Which would throw us off course?”
    “Worse. We might begin to tumble. That would destroy the ship in seconds. We would break apart.”
    I remembered seeing an old film from the early days of space flight, a rocket rising from the pad and then suddenly spinning off in crazy cartwheels and exploding. Our explosion would be more spectacular, with all that antimatter. Would they see us from New New, a brief bright star?
    I’ve lived in rotating vessels about 98 percent of my life. Suddenly I felt dizzy. “What about now? Everybody walking back and forth?”
    He flapped a hand. “It’s trivial, and it averages out. All the biomass in the ship isn’t a hundredth of one percent of the total. If everybody were packed into one room on Level One, and stayed there for weeks, the effect might be measurable.
    “But you see, that is the problem: the effect is cumulative. You move a grand piano from one room to another, it will probably stay there for most of a century. Each thirty-three seconds it will pull the ship slightly out of line in the same direction.”
    “Unless we put another piano in the opposite direction, or something.”
    “Yes.” He turned and stared at the screen, leaning forward on both elbows. “I don’t want to exaggerate this problem to you. There is no real danger so long as we are reasonably careful. My personal problem, since I am in charge of this aspect of civil engineering, is that the complexity of shifting a set