Quarantine

trappings with which I’ve embellished the Ensemble don’t come from the loyalty mod itself, and the zombie boy scout has no need for them.

In any case, I have no choice. Po-kwai says firmly, ‘Just listen. The technicalities are messy, but the essentials are simple. Have you heard of the quantum measurement problem?’

‘No.’

‘What about Schrodinger’s Cat?’

‘Of course.’

‘Well, Schrodinger’s Cat is an illustration of the quantum measurement problem. Quantum mechanics describes microscopic systems—subatomic particles, atoms, molecules—with a mathematical formalism called the wave function. From the wave function, you can predict the probabilities of getting various results when you make measurements on the system.

‘For example: suppose you have a silver ion, prepared in a certain way, passing through a magnetic field and then striking a fluorescent screen. Quantum mechanics predicts that half the time, you’ll see a flash on the screen as if the ion veered upwards in the magnetic field, and half the time you’ll see a flash as if it veered downwards. That can be explained by the ion having a spin, which makes it interact with the field; it gets pushed either up or down, depending on the way its spin is pointing, relative to the field. So by observing the flashes on the screen, you’re measuring the ion’s spin.

Or suppose you have a radioactive atom with a half-life of one hour. Point a particle detector at it which is wired up to a device which breaks a bottle of poison gas and kills a cat, if the atom decays. Enclose the whole set- up in an opaque box; wait an hour, and then look inside. If you do the experiment again and again—with a fresh atom and a fresh cat each time—quantum mechanics predicts that half the time, you’ll find the cat dead, and half the time you’ll find it alive. By seeing which it is, you’ll have measured whether or not the atom has decayed.’

‘So… where’s the problem?’

‘The problem is: before you make a measurement in either of these cases, the wave function doesn’t tell you what the outcome is going to be; it just tells you that there’s a fifty-fifty chance either way. But once you’ve made the measurement, a second measurement on the same system will always give the same result; if the cat was dead the first time you looked, it will still be dead if you look again. In terms of the wave function, the act of making the measurement has, somehow, changed it from a mixture of two waves, representing the two possibilities, to a “pure” wave—called an eigenstate—representing just one. That’s what’s called “the collapse of the wave function”.

‘But why should a measurement be special? Why should it collapse the wave function? Why should some measuring device—itself made up of individual atoms, all of which are presumably obeying the very same quantum mechanical laws as the system being measured—cause a mixture of possibilities to collapse into one? If you treat the measuring device as just another part of the system, Schrodinger’s equation predicts that the device itself should end up in a mixture of states—and so should anything that interacts with it. The bottle of poison gas should end up described by a wave function which is a mixture of a broken state and an unbroken state—and the cat should end up as a mixture of a dead state and a living state. So why do we always see the cat in one pure state, dead or alive?’

‘Maybe the whole theory’s simply wrong.’

‘No, it’s not as easy as that. Quantum mechanics is the most successful scientific theory ever— if you take for granted the collapse of the wave function. If the entire theory was wrong, there’d be no such thing as microelectronics, lasers, Optronics, nanomachines, ninety per cent of the chemicals and pharmaceuticals industry… Quantum mechanics meets every experimental test that anyone’s ever performed—so long as you assume that there’s this special process called “measurement”—which obeys totally different laws from the ones that operate the rest of the time.

‘So, the aim of studying the quantum measurement problem is to pin down exactly what a “measurement” is, and why it’s special. When does the wave function collapse? When the particle detector is triggered? When the bottle is broken? When the cat dies? When someone looks in the box?

‘One view is to shrug and say: quantum mechanics correctly predicts the probabilities of the final, visible results—and what more can you ask for? Atoms are only revealed through their effects on scientific instruments, so if quantum mechanics lets you calculate, precisely, what percentage of the time you’ll get various instrument readings—or positions of flashes of light, or cat mortalities—you have a complete theory.

‘Other people have tried to show that the wave function ought to collapse when the system reaches a critical size—or a critical energy, or a critical degree of complexity—and that any useful measuring device would be well over the threshold. People have invoked thermodynamic effects, quantum gravity, hypothetical nonlinearities in the equations… all kinds of things. None of which has ever quite explained the facts.

‘Then there’s the many-worlds theory—’

‘Alternative histories, parallel universes…’

‘Exactly. In the many-worlds theory, the wave function doesn’t collapse. The entire universe splits into different versions, one for every possible measurement. One universe has a dead cat, and an experimenter who saw that it was dead; another universe has a live cat, and an experimenter who saw that it was alive. The trouble is, the theory doesn’t say why any of this should happen—or even at what point the universe splits. Detector? Bottle? Cat? Human? It doesn’t really answer anything.’

‘Maybe there are no answers; maybe it’s all just a metaphysical quibble—’

She shakes her head. ‘Metaphysics has been an experimental science since the nineteen eighties. Although, personally, I’d like to think that the field really began in earnest from today.’ She glances at her watch. ‘Sorry, yesterday. Tuesday, the twenty-fourth of July, two thousand and sixty-eight.’

She waits patiently—with a faintly smug grin—until it hits me:

‘In the brain? Somehow, you’ve shown that the collapse of the wave function happens in the brain?’

‘Yes.’

‘But… how? What’s any of this got to do with influencing the ions, making them all go one way? Aren’t you using some kind of electromagnetic effect—’

‘Wow! No biological field could be strong enough—’

‘That’s what I thought. But—how, then?’

‘The mod does two things. The first one is, it stops me collapsing the wave function; it disables the parts of the brain that normally do so. But if that was all it did, the ions would still be random, fifty-fifty… it’s just that it would be you, Leung, Tse and Lui who’d be collapsing the system, instead of me.

‘But the mod also allows me to manipulate the eigen-states—now that I no longer clumsily, randomly, destroy all but one of them. It lets me change their relative strengths—and hence change the probabilities of the experiment’s possible outcomes.

‘In theory, I suppose I could then collapse the wave function myself—but it’d make the experiment less elegant to have the same person do both. So, the people in the control room collapse the whole system—which includes the silver ion, the fluorescent screen, and me—but only after I’ve changed the odds so they’re no longer fifty-fifty.’

‘So… everyone in the control room is part of the experiment? That’s why the histograms don’t change until after you’ve spoken the ion’s direction—because if we knew the results before you’d had a chance to influence the probabilities, we’d collapse the ions randomly?’

‘That’s right.’

I think it over for a moment. ‘You say we collapse “the whole system”. So you exist as a mixture, until we hear your voice?’

‘Yes.’

‘And what does that… feel like?’

She laughs. ‘That’s the most frustrating thing of all: I don’t know! I literally don’t remember. Once I’m collapsed, I end up with only one set of memories; I only recall seeing one flash of light on the screen. I don’t even remember what it’s like to operate