First Lawrence Krauss came out with a new book, A Universe From Nothing: Why There Is Something Rather Than Nothing (based in part on a popular YouTube lecture), which addresses this question from the point of view of a modern cosmologist. Then David Albert, speaking as a modern philosopher of science, came out with quite a negative review of the book in the New York Times. And discussion has gone back and forth since then: here’s Jerry Coyne (mostly siding with Albert), the Rutgers Philosophy of Cosmology blog (with interesting voices in the comments), a long interview with Krauss in the Atlantic, comments by Massimo Pigliucci, and another response by Krauss on the Scientific American site.
Executive summary
This is going to be kind of long, so here’s the upshot. Very roughly, there are two different kinds of questions lurking around the issue of “Why is there something rather than nothing?” One question is, within some framework of physical laws that is flexible enough to allow for the possible existence of either “stuff” or “no stuff” (where “stuff” might include space and time itself), why does the actual manifestation of reality seem to feature all this stuff? The other is, why do we have this particular framework of physical law, or even something called “physical law” at all? Lawrence (again, roughly) addresses the first question, and David cares about the second, and both sides expend a lot of energy insisting that their question is the “right” one rather than just admitting they are different questions. Nothing about modern physics explains why we have these laws rather than some totally different laws, although physicists sometimes talk that way — a mistake they might be able to avoid if they took philosophers more seriously. Then the discussion quickly degrades into name-calling and point-missing, which is unfortunate because these are smart people who agree about 95% of the interesting issues, and the chance for productive engagement diminishes considerably with each installment.
How the universe works
Let’s talk about the actual way physics works, as we understand it. Ever since Newton, the paradigm for fundamental physics has been the same, and includes three pieces. First, there is the “space of states”: basically, a list of all the possible configurations the universe could conceivably be in. Second, there is some particular state representing the universe at some time, typically taken to be the present. Third, there is some rule for saying how the universe evolves with time. You give me the universe now, the laws of physics say what it will become in the future. This way of thinking is just as true for quantum mechanics or general relativity or quantum field theory as it was for Newtonian mechanics or Maxwell’s electrodynamics.
Quantum mechanics, in particular, is a specific yet very versatile implementation of this scheme. (And quantum field theory is just a particular example of quantum mechanics, not an entirely new way of thinking.) The states are “wave functions,” and the collection of every possible wave function for some given system is “Hilbert space.” The nice thing about Hilbert space is that it’s a very restrictive set of possibilities (because it’s a vector space, for you experts); once you tell me how big it is (how many dimensions), you’ve specified your Hilbert space completely. This is in stark contrast with classical mechanics, where the space of states can get extraordinarily complicated. And then there is a little machine — “the Hamiltonian” — that tells you how to evolve from one state to another as time passes. Again, there aren’t really that many kinds of Hamiltonians you can have; once you write down a certain list of numbers (the energy eigenvalues, for you pesky experts) you are completely done.
We should be open-minded about what form the ultimate laws of physics will take, but almost all modern attempts to get at them take quantum mechanics for granted. That’s true for string theory and other approaches to quantum gravity — they might take very different views of what constitutes “spacetime” or “matter,” but very rarely do they muck about with the essentials of quantum mechanics. It’s certainly the case for all of the scenarios Lawrence considers in his book. Within this framework, specifying “the laws of physics” is just a matter of picking a Hilbert space (which is just a matter of specifying how big it is) and picking a Hamiltonian. One of the great things about quantum mechanics is how extremely restrictive it is; we don’t have a lot of room for creativity in choosing what kinds of laws of physics might exist. It seems like there’s a lot of creativity, because Hilbert space can be extremely big and the underlying simplicity of the Hamiltonian can be obscured by our (as subsets of the universe) complicated interactions with the rest of the world, but it’s always the same basic recipe.
So within that framework, what does it mean to talk about “a universe from nothing”? We still have to distinguish between two possibilities, but at least this two-element list exhausts all of them.
by Sean Carroll, Discover Magazine | Read more:
