Lost in a Parallel Universe, or Why I Fell In Love With Giovannina
By Davide Castelvecchi
Some time last spring I was on the phone with physicist Carlo Rovelli, interviewing him for a New Scientist article, when our conversation started veering into the philosophical. I asked him, Why do so many physicists say that the laws of nature appear to be exquisitely fine-tuned? Slightly tweak nature’s constants, they say — constants such as the mass of the electron, or the strength of gravity — and the universe would have stayed a chaotic broth of elementary particles, forming no stars, no planets, no chemistry, and no life.
To my surprise, I found out that Rovelli, a theoretical physicist at the University of the Mediterranean in Marseille, shared my incredulity. “I don’t find these arguments too convincing,” he said in his Italian with a northern accent. “I often think it’s just our lack of fantasy. If the fundamental constants were a bit different, maybe another world would come out, one that’s just as complicated. […] There would be different physics, and perhaps there would be beings wondering why the constants have those values.”
He proceeded to illustrate. “Say I moved to this town, and fell in love with Giovannina. It’s happened, and it’s beautiful and wonderful and extraordinary — I don’t want to spoil the poetry of it. But had I moved to a different town, I could have fallen in love with Martuccia, and I would be just as happy.”
Only a few months after our phone call, it seems like the universe — whether it’s finely tuned or not — really listened to us: A team of physicists produced a detailed picture of just such an alternative town. Armed with some of the fantasy others had lacked, Roni Harnik, of the Stanford Linear Accelerator Center, and his collaborators conjured up a plausible universe with different laws, but still able to form stars and planets, complex chemistry, and perhaps intelligent life. The physical laws in their universe are not just a little bit different: One of nature’s fundamental forces, the weak nuclear force, is entirely missing. They called it the weakless universe.
The weakless universe came out with perfect timing. Throughout the year, the issue of fine tuning has been at the center of an ongoing controversy, which my colleague and fellow blogger Buzz Skyline named Physics Catfight of the Year. Several books appeared dealing with this subject, and even Stephen Hawking weighed in.
The idea of a universe with different laws may be less academic than it sounds. As Stanford cosmologist Andrei Linde first suggested in the 1980s, the big bang that created our universe could be just one of many big bangs, arising from a primordial vacuum like blossoms from a blooming tree; each of these coexisting universes could in principle have different laws.
Most physicists believe that our universe, finely tuned as it is, would essentially be the only interesting one. But is that really so, Harnik and his collaborators wondered. So they came up with the idea of a universe with no weak force. The paper they wrote (also available here) is a quick trip through the entire natural history of the weakless universe, starting with the big bang, and the results make for a gripping read — think something half-way between Bill Bryson’s A Short History of Nearly Everything and Lewis Carrol’s Through the Looking Glass, but with equations.
To demonstrate that they could still get an interesting universe without the weak force, perhaps the trickiest part was to make protons and neutrons combine to form atomic nuclei. In our universe, the weak nuclear force is responsible for one of the most important kinds of nuclear reaction, known as beta decay. Together with nuclear fusion, beta decay is a crucial step for creating elements heavier than hydrogen in the big bang and in stars and supernova explosions.
So it wasn’t clear that weakless stars would end up producing a variety of elements. But the right combination of small changes in the physical laws, the team showed, would still lead to the creation of all elements up to iron, i.e., of the interesting half of the periodic table.
The weakless universe is a little strange in other ways. With no elements heavier than iron producing heat by radioactivity, planetary cores would quickly cool down. As a result, weakless planets would have no continental drift, no volcanoes, no earthquakes. But weakless planets might still be hospitable to life. “Of course it is not obvious that a molten core or plate tectonics are necessary for life-as-we-know-it,” Graham Kribs, one of the paper’s co-authors, told me in an email. On the other hand, their paper says, chemistry in the weakless universe is virtually indistinguishable from that in our Universe. Presumably, this complex chemistry could be the premise for the evolution of life.
Potentially the weakest part of the weakless universe is the fudging required to make antimatter disappear. In our universe, antimatter should have come out of the big bang just as copious as matter. Nobody knows for sure where the antimatter went, but some of the leading attempts to answer that question rely on the weak force (and in particular, on its handedness). Harnik et al. posited other mechanisms for the disappearance of antimatter.
Another question: if it’s true, as some chemists have proposed this year, that life as we know it was deeply influenced by the weak nuclear force (keep reading this Top 10), what would life look like in a weakless universe?
In principle, many more interesting universes could exist, even more exotic than the weakless universe. Harnik & Co. stress that they tried to be conservative and concoct a universe as similar to ours as possible, to keep calculations doable with familiar methods. But this was a practical matter, they write, not one of principle. “There is probably a wide range of habitable universes with parameters and structures that look nothing like our Universe.”