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from the Wheeler conference, with striking Greek features and long, shiny black hair. She seemed younger, too. She had a decade on me, but I’m not sure anyone would know it, and in her early thirties she was practically a newborn in her field. All said, she was not how you might imagine a physicist. When I told people I was into physics, they always seemed a little too surprised, and I imagined that Markopoulou knew what that was like. I smiled to myself, knowing that anyone glancing over at the two of us would assume that we were talking about boys or fashion, not the microscopic structure of spacetime. Not that I didn’t enjoy talking about boys and fashion. But today it was loop quantum gravity.
    I stood up to greet Markopoulou, shook her hand, and told her how great it was to finally meet in person. If she was thrown by my age, she didn’t show it. She slid into the banquette alongside me and we ordered some cold drinks. After some obligatory small talk, I launched into a barrage of questions. I was sure that she would be able to tell what a rookie I was, but I didn’t care. I was too excited by the prospect of learning physics straight from the mouth of a physicist. Who knew if I’d ever get the chance again?
    Markopoulou explained to me the notorious obstacles involved in uniting general relativity with quantum mechanics. It was Wheeler who first took seriously the need for such unification and made the bold leap of applying quantum theory to the universe as a whole. You would think there would be no need for such a feat, since quantum theory is about tiny things, not about universes. But, as even Bohr himself acknowledged, there’s no clear boundary separating the quantum world from the classical world, no state line marked by a billboardthat reads “Welcome to the non-quantum realm.” Yes, quantum mechanics requires a separation between the quantum system and its environment, observed and observer, inside and out. But the theory never tells us where to place the dividing line. The line is a moving target; it can be drawn anywhere and shifted to ever-bigger scales. If reality is quantum, then reality is quantum. It doesn’t reach some scale and stop—it’s quantum mechanics all the way up.
    Of course, in ordinary quantum mechanics you could at least
pretend
to draw a distinction between observer and observed, arbitrarily slicing the universe in two, calling one side the classical measuring device and the other the quantum system. But when it came to the universe as a whole, you couldn’t even fake the procedure. The universe, by definition, is the whole of spacetime, the complete set of everything that exists. It has no outside. No outside, no observers.
    Quantum cosmology was born when Wheeler had to kill some time between flights. It was 1965 and he had a layover in North Carolina. He asked his friend and fellow physicist Bryce DeWitt, who happened to live nearby, to keep him company for a few hours at the airport. It was there that they wrote down an equation, which Wheeler called the Einstein-Schrödinger equation, everyone else came to call the Wheeler-DeWitt equation, and DeWitt himself eventually called “that damned equation.”
    That damned equation was meant to solve a problem that had plagued earlier attempts to quantize general relativity. In quantum mechanics, time is always external to the system; clocks live in that murky classical realm—the “environment”—where observers reside. Wavefunctions describe the physical system at an instant of time; the wavefunction then evolves
in
time according to the Schrödinger equation. When it comes to spacetime, though, there’s no such thing as spacetime
at an instant
, because spacetime contains
all
instants. And you can’t have spacetime evolve
in
time, because it
is
time. The only way forward seemed to be this: break four-dimensional spacetime into three dimensions of space and