Last week’s plebeian scare-mongering about the world ending at the wraparound of the Mayan calendar did not distract sophisticated readers of gr-qc and quant-ph from a more arcane problem, the so-called Firewall Question. This concerns what happens to Alice when she falls through the event horizon of a large, mature black hole. Until recently it was thought that nothing special would happen to her other than losing her ability to communicate with the outside world, regardless of whether the black hole was old or young, provided it was large enough for space to be nearly flat at the horizon. But lately Almheiri, Marlof, Polchinski, and Sully argued (see also Preskill’s Quantum Frontiers post and especially the comments on it) that she instead would be vaporized instantly and painlessly as she crossed the horizon. From Alice’s point of view, hitting the firewall would be like dying in her sleep: her experience would simply end. Alice’s friends wouldn’t notice the firewall either, since they would either be outside the horizon where they couldn’t see her, or inside and also vaporized. So the firewall question, aside from being central to harmonizing no-cloning with black hole complementarity, has a delicious epistemological ambiguity.
Notwithstanding these conceptual attractions, firewalls are not a pressing practical problem, because the universe is far too young to contain any of the kind of black holes expected to have them (large black holes that have evaporated more than half their total mass).
A more worrisome kind of instant destruction, both practically and theoretically, is the possibility that the observable universe—the portion of the universe accessible to us—may be in a metastable state, and might decay catastrophically to a more stable ground state. Once nucleated, either spontaneously or through some ill-advised human activity, such a vacuum phase transition would propagate at the speed of light, annihilating the universe we know before we could realize—our universe would die in its sleep. Most scientists, even cosmologists, don’t worry much about this either, because our universe has been around so long that spontaneous nucleation appears less of a threat than other more localized disasters, such as a nearby supernova or collision with an asteroid. When some people, following the precautionary principle, tried to stop a proposed high-energy physics experiment at Brookhaven Lab because it might nucleate a vacuum phase transition or some other world-destroying disaster, prominent scientists argued that if so, naturally occurring cosmic-ray collisions would already have triggered the disaster long ago. They prevailed, the experiment was done, and nothing bad happened.
The confidence of most scientists, and laypeople, in the stability of the universe rests on gut-level inductive reasoning: the universe contains ample evidence (fossils, the cosmic microwave background, etc.) of having been around for a long time, and it hasn’t disappeared lately. Even my four year old granddaughter understands this. When she heard that some people thought the world would end on Dec 21, 2012, she said, “That’s silly. The world isn’t going to end.”
The observable universe is full of regularities, both obvious and hidden, that underlie the success of science, the human activity which the New York Times rightly called the best idea of the second millennium. Several months ago in this blog, in an effort to formalize the kind of organized complexity which science studies, I argued that a structure should be considered complex, or logically deep, to the extent that it contains internal evidence of a complicated causal history, one that would take a long time for a universal computer to simulate starting from an algorithmically random input.
Besides making science possible, the observable universe’s regularities give each of us our notion of “us”, of being one of several billion similar beings, instead of the universe’s sole sentient inhabitant. An extreme form of that lonely alternative, called a Boltzmann brain, is a hypothetical fluctuation arising within a large universe at thermal equilibrium, or in some other highly chaotic state, the fluctuation being just large enough to support a single momentarily functioning human brain, with illusory perceptions of an orderly outside world, memories of things that never happened, and expectations of a future that would never happen, because the brain would be quickly destroyed by the onslaught of its hostile actual environment. Most people don’t believe they are Boltzmann brains because in practice science works. If a Boltzmann brain observer lived long enough to explore some part of its environment not prerequisite to its own existence, it would find chaos there, not order, and yet we generally find order.
Over the last several decades, while minding their own business and applying the scientific method in a routine way, cosmologists stumbled into an uncomfortable situation: the otherwise successful theory of eternal inflation seemed to imply that tiny Boltzmann brain universes were more probable than big, real universes containing galaxies, stars, and people. More precisely, in these models, the observable universe is part of an infinite seething multiverse, within which real and fake universes each appear infinitely often, with no evident way of defining their relative probabilities—the so-called “measure problem”.
Cosmologists Rafael Bousso and Leonard Susskind and Yasunori Nomura (cf also a later paper) recently proposed a quantum solution to the measure problem, treating the inflationary multiverse as a superposition of terms, one for each universe, including all the real and fake universes that look more or less like ours, and many others whose physics is so different that nothing of interest happens there. Sean Carroll comments accessibly and with cautious approval on these and related attempts to identify the multiverse of inflation with that of many-worlds quantum mechanics.
Aside from the measure problem and the nature of the multiverse, it seems to me that in order to understand why the observed universe is complicated and orderly, we need to better characterize what a sentient observer is. For example, can there be a sentient observer who/which is not complex in the sense of logical depth? A Boltzmann brain would at first appear to be an example of this, because though (briefly) sentient it has by definition not had a long causal history. It is nevertheless logically deep, because despite its short actual history it has the same microanatomy as a real brain, which (most plausibly) has had a long causal history. The Boltzmann brain’s evidence of having had long history is thus deceptive, like the spurious evidence of meaning the protagonists in Borges’ Library of Babel find by sifting through mountains of chaotic books, until they find one with a few meaningful lines.
I am grateful to John Preskill and especially Alejandro Jenkins for helping me correct and improve early versions of this post, but of course take full responsibility for the errors and misconceptions it may yet contain.