Trespassing on Einstein's Lawn (32 page)

Of course, Hawking and Hertog weren't the first to claim that observations made today could determine the past. Wheeler was. I
thought back to his delayed-choice experiment, that twisted take on the double slit in which observers choose whether a photon takes two paths or just one, even if the choice is made billions of years
after
the photon begins its journey.
“The past has no existence except as it is recorded in the present,” Wheeler wrote. “By deciding what questions our quantum registering equipment shall put in the present we have an undeniable choice in what we have the right to say about the past. What we call reality consists of a few iron posts of observation between which we fill in by an elaborate papier-mâché construction of imagination and theory.”

Now Hawking and Hertog were applying the delayed choice not to a handful of photons but to the entire universe. The big bang, inflation, 13.7 billion years of cosmic evolution … it's not that these things didn't happen; it's that they happened
right now.
The universe's past is out there for us to peer into with our telescopes—but it starts with us. Like a choose-your-own-adventure story in reverse, we choose the history of the universe.

As a teenager, I had once wondered, What if I was just born, right now, only my brain came complete with a full set of false memories, so that in reality this is the very first moment of my life and yet I feel as though I have been alive for fifteen years? Of course, I couldn't remember all the way back to my birth. The first several years were more or less blank, the occasional fragments provided by photographs and home videos. As far as I could tell, I had woken up in the middle of my life and was stuck trying to make sense of a story without a beginning or an end. But the possibility that this very moment contained within it all other moments meant, for me, that the existence of the past would never be so puzzling as the existence of the present, which itself was a kind of origin, slippery and mysterious and unaccounted for.

Still, when it came down to it, I had always conceded that time was at least approximately real and linear, a kind of tether, an umbilical cord, binding us to our own births and before them to cosmic history, stretching back through empty, interstellar space, past the stars where the atoms in our bodies were forged, through webs of galaxies, tracing the slopes and swells of space until it reached a beginning. A big bang. But the no-boundary proposal told a dramatically different story:
there's no beginning to anchor the cord. As it reaches back toward the big bang, time becomes space and the cord turns a corner, following the curve of the new spatial dimension and looping back on itself, then continuing on its way and ending up back where it started. According to the no-boundary proposal, time is an umbilical cord that tethers us to ourselves.

And really, wasn't that what Wheeler's U-diagram was all about? That giant eyeball was ogling the past. The universe gives rise to the observer, then the observer looks back in time and gives rise to the universe. A self-excited circuit that seems to turn that relentless chronological march on its head: after, then before.

Was all of this top-down business a flagrant violation of causality? Of the inviolable laws of physics? I was dying to ask Hawking. Hertog agreed to forward an email to him on my behalf.
He might respond
, Hertog told me,
but don't get your hopes up.

A few days later, Hawking responded.

“Is there really a kind of backward causation taking place?” I had asked him.

“Observation of final states … determine[s] different histories of the universe,” Hawking replied. “However, this backward causality is an angel's-eye view from outside the universe. A worm's-eye view from inside the universe would have the normal causality.”

At first glance, that made sense. From outside the universe, where you could see the tangled superposition of possible histories, you could watch an observer in the present select a single past. To the observer here on the inside, though, the past just seems to be sitting there, as if it always had been there.

But the more I thought about it, the weirder that was. Why would the laws of physics break down in the God's-eye (angel's-eye) view? If anything, you'd expect it to work the other way around—that from a God's-eye view, where you could take in the whole of nature, everything would finally make sense, all the disparate pieces of the puzzle snapping into place, leaving the laws of physics pristine and complete. Break that symmetry with a single observer's (worm's) limited viewpoint
and you'd expect to see some kind of violations, the broken puzzle pieces' jagged edges. I scribbled my confusion in my notebook.
Laws of physics intact only when viewed inside a single light cone?

If nature's laws hold up only within a given observer's light cone, then somehow physics really is tied to observers, as Wheeler had envisioned. Did that mean that an observer really is a radioactive jewel, “the flashing purpose that lights up the whole dark universe”? I was already aware of the many disturbing questions the whole jewel thing raised.

Of course, Hawking and Hertog weren't actually singling out individual observers. They weren't suggesting that the history of the universe was different for me than it was for my father. But that's only because the measurements my father and I would use as input at step two—measurements of the universe's geometry or the expansion rate—would be exactly the same, given how close we were to each other, astronomically speaking. But if there were some observers off in a distant galaxy whose light cones barely overlapped with ours, their measurements could feasibly be quite different. If so, their whole cosmic history could be different. It's not merely that they would calculate a different history; they would literally live in a universe with an objectively different past.

Did that mean they would live in a different
universe
? That the universe itself wasn't invariant? Wasn't real?

I wasn't sure. Hawking seemed to be leaning in that direction. Still, the no-boundary proposal was just that: a proposal. It left all singular cosmologies out of the sum of possible histories out of conviction that the universe had to contain its own explanation on the inside. It seemed like solid reasoning to me—what exactly was the other option? Still, in physics, assumptions are never enough. You have to derive them from something deeper.

In the meantime, Hawking and Hertog looked at the cosmic history they had spun out from the top down, at the wavefunction of the universe that had emerged from the sum of all possible histories that began with no boundary and ended with the universe we see today, a wavefunction containing probabilities for any measurement we want to make. Interestingly, the most probable history was one in which the universe underwent a brief period of early inflation. This top-down
inflation didn't require the usual fine-tuning, nor did it go eternal and produce anything beyond our observable universe.
History of the universe begins right now
, I wrote in my notebook,
and ends at the cosmic horizon. Nonetheless, it
looks
like it began 13.7 billion years ago and underwent a brief period of inflation. Observer looks back in time and gives rise to the history of the universe, sees exactly the kind of history needed for the observer to exist in the first place.

I knew that proponents of eternal inflation were not going to be fans of Hawking's new theory. After all, it denied eternal inflation's origin in a well-defined inflaton field and stymied its attempts to create anything beyond our horizon, let alone an infinite multiverse. What's more, it turned inflation itself into a kind of observer-dependent illusion. But just for kicks, I picked up the phone and called Andrei Linde, the inflationary evangelist who had yelled at me back in Davis when I had suggested that the mysterious low quadrupole might cause some physicists to abandon inflation. I asked him what he thought of Hawking's theory and braced myself for an impassioned tirade. But this time his answer was short and sweet: “I don't buy it.”

Hartle, understandably, was more sympathetic to Hawking's cause. As strange as their theory sounds, he said on the phone, it's really the only way forward given that we are stuck inside the universe. “It's a different viewpoint, but it's sort of inevitable. Cosmologists certainly should be paying attention to this work.”

The no-boundary proposal offered to slay the quantum dragon, removing the singularity from the universe's origin and allowing us to explain the universe from the inside. But there was another reason for cosmologists to pay attention to Hawking and Hertog's work. Top-down cosmology provided a possible answer to Wheeler's question, one that had been resounding in my head for years:
If an anthropic principle, why an anthropic principle?

The absurd value of the observed dark energy wasn't the only thing that seemed inexplicably fine-tuned for the existence of biological life, although it was clearly the worst offender. Fine-tuning is rampant—change one or two physical values by even the slightest amount and our existence is rendered impossible. Had the distribution of matter in the early universe been just a little lumpier, black holes would have
formed in place of stars and galaxies. Had it been just a little smoother, we'd see no structure at all. If the weak nuclear force had been a little less weak, the only element around today would be hydrogen; a little weaker and we'd have nothing but helium. Either way, no stars could form. Without stars there's no carbon; without carbon, no life. The strength of gravity, too, was just right—a little stronger and our Sun would have burned out after some ten thousand years, far too quickly to sustain biological evolution. Had the mass difference between the proton and neutron been slightly off, atoms themselves would be unstable. And then there was the cosmological constant. That fine-tuned son of a bitch.

Many physicists were following Susskind's lead in explaining those coincidences with the infinitely varied landscape of string vacua, physically realized in the infinitely large multiverse produced by eternal inflation. It was the argument that fanned the flames in David Gross's eyes.

Hawking and Hertog bought into the string landscape—but they didn't accept eternal inflation as the means for populating it with physical universes. Instead, they saw the varied worlds described by string theory as possible histories that exist not in physical space but in the mathematical superposition from which our universe's history is derived. You can still use anthropic arguments to explain all the apparent fine-tuning without ever having to reference anything outside our own cosmic horizon. Hawking and Hertog had turned the landscape on its head: instead of multiple universes with a single history, you have a single universe with multiple histories.

When you think about it, all that fine-tuning makes sense in a top-down universe. If the history of the universe, along with all of its physical properties, is determined by our observations, then of course it will be a universe perfectly suited for us—how else would we be here to make those observations? Anthropic coincidences are problematic for bottom-up cosmology because you are starting with some initial state that's completely independent of observers; the universe evolves forward in time until observers like us just happen to arise, a fluky byproduct of physics and happenstance. Given random initial conditions some 14 billion years ago, of course we're scratching our heads and
asking, what were the odds that the universe would just happen to have every minute ingredient to cook up the fragile stew of life? Top-down cosmology, on the other hand, doesn't raise the question. Instead of starting with a cosmic history and evolving observers, top-down cosmology starts with observers and evolves a history. And if you start with life, you're bound to end up with a life-friendly universe.

Why an anthropic principle?
I wrote in my notebook.
Because the universe is observer-dependent?

Such jewel-toned thoughts about life made me nervous—any theory that relied on humans or consciousness as being some kind of “special” ingredient instantly struck me as crackpot. But Hawking and Hertog weren't suggesting that observers' consciousness magically collapses the wavefunction of the universe. In their model, there's no collapse, just the interference of the universe's many paths through history. It wasn't like you
needed
life to create a universe; it was simply that there
is
life in this one, therefore the only histories relevant to the quantum sum will inevitably be life-supporting ones. Which of course was all a bit circular in its logic. Like a self-excited circuit.

I opened up a blank document on my computer, rolled up my sleeves, and prepared to write my article. It was quite a story. Here was Stephen Hawking telling us to give up all our old ideas about cosmology—the ones that told us there was some origin of time independent of us, some 14-billion-year evolution that had nothing to do with our observations, some independent reality, some way the past “really was.” By combining Wheeler's self-excited circuit with the landscape of string theory, Hawking had me thinking that it might not be long before we crossed the universe off the ultimate reality list.

When I finished my article, I sent it to Hertog to check for inaccuracies and told him to feel free to pass it along to Hawking. A few days later, an email popped up in my inbox. I opened it, and there inside were the best words a reality-hunting pseudo-journalist could imagine:
The article is remarkably good and clear. Stephen.

On a brisk April morning in 2008 I wandered happily into the
New Scientist
office, ready to kick off another day of tracking down reality,
but a fellow editor greeted me with a somber expression. My smile dropped. “What's wrong?”

“John Wheeler died.”

I was stunned. Speechless. It's not that I couldn't have seen it coming—he was ninety-six years old. But it felt like a blow all the same.