“Action? Like what? A sellout to NASA?”

“Maybe. And you have to cut your links with this character.”

Cornelius Taine smiled coldly.

Malenfant’s hands, clasped behind his back, showed white knuckles.

The meeting broke up without agreement on a way forward. And on the way out, Maura whispered to Emma, “Carter? Who the hell is Carter?”

Emma didn’t get to her apartment until midnight that night. When she walked in the door she told the TV to turn itself on. And there, on every news channel, was Cornelius Taine.

Cornelius Taine:

So, Dr. Taine, you’resaying that these people from the future — the ones you call downstreamers — have reached into the past, to us. To send a message.

Yes. We believe so.

But if the downstreamers exist — or will exist — they survived this catastrophe of yours. Will survive. Whatever. Right? So why did they need to send a message?

You’re asking me about causal paradoxes. The downstreamers are saving their grandmothers, us, from drowning. But if she had drowned they wouldn’t even exist, so how can they save her? Right?

Umm. . . yeah. I guess —

There’s a lot we don’t understand about time. What happens if you try to change the past is at the top of the list. Let me try to explain. It is a question of transactions, back and forth in time.

The Feynman radio works on the notion of photons — electromagnetic wave packets — traveling back in time. Fine.

But photons aren’t the only waves.

Waves lie at the basis of our best description of reality. I mean, of course, the waves of quantum mechanics. These waves represent flows of — what? Energy? Information? Certainly they crisscross space, spreading out from every quantum event like ripples.

We have good equations to tell us how they propagate. And if we know the structure of the waves we can tell a great deal about the macroscopic reality they represent. A clumping of the waves here means this is the most likely place to find that traveling electron emitted from over there.

But, like electromagnetic waves, quantum wave packets emitted from some event travel both forward and backward in time. And these backward waves are vital to the structure of the universe.

Suppose you have an object of some kind that changes the state of another: a source and a detector, maybe of photons. The source changes state and sends quantum waves off into future and past. The future-traveling wave reaches the detector. In turn this emits waves traveling into both future and past, like echoes.

Here’s the catch. The quantum echoes cancel out the source waves, both future and past, everywhere — except along the path taken by the ordinary retarded waves. It’s like a standing wave set up between source and receiver. Because no time passes for a wave traveling at light speed, all of this is timeless too, set up in an instant.

It’s called a transaction, as if source and detector are handshaking. “Hi, I’m here.” “Yes, I can confirm you are.”

So there really are waves traveling back in time?

So it seems. But you don’t have to worry about them.

I don’t?

No. There are no back-in-time paradoxes, you see, because the backward waves only work to set up the transaction; you can’t detect them otherwise.

And that’s how our reality works. As the effects of some change propagate through space and time, the universe knits itself into a new form, transaction by transaction, handshake by handshake.

Umm. And this is quantum mechanics, you say? So what happened to all that quantum funny stuff? The collapsing wave function, and Schrodinger’s cat, and the Many Worlds Interpretation, and —

Oh, you can forget all that. We study that today the way we study Roman numerals. Now that we know what quantum mechanics is really all about, it’s hard to imagine how people in those days thought like that. Do you follow?

Umm. . . Madeleine?

Let me get this straight. If I go back and change the past, I create a new universe that branches off at that point… right? If I kill my grandmother, I get two universes, one where she lived and I was born, one where she dies and I was never born —

No. Perhaps you haven’t heard me. It just doesn’t work like that.

There is only one universe at a time. New universes may bud off from others, but they are not “parallel” in the way you say. They are separate and entire, with their own self-consistent causalities.

So what happens if I go back in time and do something impossible, like kill my granny? Because if she dies, I could never be born, and could never have killed her.

Each quantum event emerges into reality as the result of a feedback loop between past and future. Handshakes across time. The story of the universe is like a tapestry, stitched together by uncountable trillions of such tiny handshakes. If you create an artificial timelike loop to some point in spacetime within the

negative light cone of the present—

Whoa. In English.

If you were to go back in time and try to change the past, you would nullify all those transactions, the handshakes between future and past. You would damage the universe, erasing a whole series of events within the time loop.

So the universe starts over, from the first point where the forbidden loop would have begun to exist. The universe, wounded, heals itself with a new set of handshakes, working forward in time, until it is complete and self-consistent once more.

Then changing the past is possible.

Oh, yes.

Tell me this, Dr. Taine. According to this view, even if you do go back and change the past, how do you know you succeeded? Won ‘tyou change along with the past you altered?

We don’t know. How could we? We’ve never tried this before. But we think it’s possible a conscious mind would know.

How?

Because consciousness, like life itself, is structure. And structure persists as the cosmic tapestry changes.

Think about a DNA molecule. Some of the genes are important for the body’s structure; some are just junk. If you could perturb reality, consider possible alternate destinies for that molecule, you could see a lot of variation in the junk without affecting the operation of the molecule in any significant way. But if there’s a change in the key structural components, those that contain information, the molecule may be rendered useless. Therefore, the key structure must be stable in the face of small reality changes.

So if in some way our minds span reality changes…

Then maybe we’ll be able to perceive a change, an adjustment of the past. Of course this is speculative.

And what about free will, Dr. Taine? Where does that fit into your grand plan ?

Free will is a second-order effect. Even life is a second-order effect. Light dancing from the rippled surface of time’s river. It is not the cause even of the ripples, let alone the great majestic flow itself.

That’s onegosh-darnedgloomy view.

Realistic, however.

You know, our time is just a bubble far upstream that must seem utterly insignificant compared to the great enterprises of the future. But it isn’t insignificant, because it’s the first bubble. And if we don’t survive the Carter catastrophe, we lose everything — eternity itself.

Emma Stoney:

The media types had it all: the Carter prediction, the message from the future, the real reason for the redirection of the Nautilus. All of it.

Emma was convinced it was Cornelius himself who had leaked the Carter stuff. It increased the pressure on Bootstrap hugely, but that only seemed to reinforce Malenfant’s determination to fight his way through this: to maintain his links with Cornelius, continue on to Cruithne, and launch again.