Get used to the idea of an infinite number of universes, says superstring theorist Brian Greene in his new book, The Hidden Reality. Alexander Fabry presents a handy guide to the mind-bending new science.
Extra dimensions are old news. The newest mind-bending descriptions of reality dreamed up by the world’s smartest physicists, and explained by superstar superstring theorist Brian Greene in his latest book The Hidden Reality, include untold numbers of extra universes. A million universes isn’t cool. You know what’s cool? Ten to the 500th power universes.
Greene was already an important string theorist when his enormously successful first book— The Elegant Universe—catapulted him into the firmament of popular physics with the other stars of the genre, Carl Sagan and Stephen Hawking. Selling more than a million copies, the book was for many people a first close encounter with the deeply weird world of superstring theory. In that volume, and in his later gig hosting a PBS miniseries based on the bestseller, Greene introduced quantum mechanics and Einstein’s theories of special and general relativity before heading off into the multi-dimensional realms of Calabi-Yau manifolds and super-symmetric string theory.
In his new book, Greene takes us down the rabbit hole yet again, this time setting a course for the terra incognita of parallel universes, hidden worlds, alternate realities, holographic projections, and multiverse simulations. Greene likes to drop you into the middle of the action first and then explain the backstory (and sometimes it does feel like a full-scale intellectual invasion is happening), but he has an elegant knack for anticipating questions and immediately dealing with any confusion or objections.
Greene describes nine different theories which imply that we are living in a vast multiverse. He’s pretty confident that some of them are true, but less sanguine about others (chances are small that we’re living in a vast simulation with other universes running as parallel processes on some higher-order cosmic computer). One important takeaway, though, is what he calls the “Copernican pattern”: over the past 500 years, each new cosmological discovery has removed us from being in a privileged position in the universe. First, we realized that the sun, not the earth, was the center of the solar system; then we discovered that our sun is just an ordinary star in the Milky Way, and that our galaxy is just an ordinary galaxy among about 80 billion or so that are visible. Greene takes this further: what we call our universe is only one piece of an unbelievably vast multiverse.
So Just What Is a Multiverse?
The simplest type of parallel world Greene describes is called the quilted multiverse. Simply put, our cosmic horizon is about 14 billion light-years away. Even the Hubble Space Telescope can’t see beyond that, because light from further away hasn’t yet had time to reach us yet. But just because that’s the horizon of our vision doesn’t mean that that’s the edge of the whole universe: ships don’t fall off earth when they sail out of view. In fact, Greene argues, there are very good reasons to think that the big bang created an infinite universe. If matter is more or less evenly distributed through the whole thing, then there must be other pieces that look just like ours, other islands in the cosmic sea, each with a radius of 14 billion light years, that are simply beyond our limited horizon. These islands make up the patchwork of a quilted multiverse.
There are something like 10 to the 500th power different possible flavors of string theory, each of which would result in a different set of observed particles and physical constants.
Sort of like how monkeys typing for an infinite amount of time would eventually produce Shakespeare, an infinite universe would mean by statistical laws that there must exist other patches that look exactly like our own. That means somewhere there is an island universe containing a bizarro earth filled with doppelgangers of you and me—in fact, infinitely many of them, though they’re far too distant for us to reach.
That may seem utterly strange, but it’s just math following from a few basic assumptions (read Greene to be convinced). For the more exotic multiverses, Greene has to get into some serious physics.
Turning on the Microwave
It turns out that space is suffused with a low-level background radiation, a sea of microwave photons which are the result of the primordial cooling of the universe after the big bang. This so-called cosmic microwave background is actually one of the contributors to television static, and it appears almost entirely uniformly distributed across the sky (measured as a temperature, it’s 2.7 degrees above absolute zero). That’s puzzling: from our perspective, two opposite points on our cosmic horizon are 28 billion light-years apart, too far to have ever communicated. So how did they get to be exactly the same temperature? A neat trick is to introduce a period of rapid inflation just after the big bang. This allows the tiny primordial universe enough time to get a nicely uniform temperature distribution before blowing it apart.
This inflation is possible because of something called the cosmological constant, introduced by Einstein into his equations of general relativity. He called it his “greatest mistake” because he used it to get rid of a pesky term in his equations, but it now seems like one of his greatest predictions. Essentially, the cosmological constant is the same thing as dark energy, a force that’s pushing the universe to expand. And it fits in precisely with contemporary experiments. As Greene tells it, the cosmological constant powered the early period of cosmic inflation which gives us the uniform cosmic microwave background.
But what does this mean for multiverses? Confusingly, the cosmological constant isn’t actually a constant: its changing value is why that period of unbelievably rapid inflation has slowed to our current gradual cosmic expansion with a near-zero cosmological constant. Greene posits that what we call our universe is simply a part of an even vaster entity which is still undergoing incredible expansion, i.e., where the cosmological constant is still high. The part that we call our universe condensed out of that inflationary chaos like a water droplet forming out of a cloud of steam. That droplet, furthermore, looks infinite from the outside, and therefore contains within it the whole quilted multiverse talked about before. But floating out of reach are infinitely many other droplet universes as well.
One of the embarrassments of string theory has been its inability to tell us why exactly our universe is the way it is. In fact, there are something like 10 to the 500th power different possible flavors of string theory, each of which would result in a different set of observed particles and physical constants. By combining string theory and the inflationary multiverse, Greene posits that these different droplet universes might each embody a different flavor of string theory. The entirety of string theory is the hidden reality, but we are able to experience only one of its possible configurations.
To See and Be Seen
If multiverses within multiverses are making your head hurt, buckle down because Greene has six other possibilities for parallel worlds. He also has one big philosophical point, though, that could change the way we view the very goals of science. We will never see these other universes, but we might find evidence that they do exist. Though Greene is optimistic about the possibility of confirming some of the predictions of string theory and multiverse physics in the near future, as the Large Hadron Collider at CERN comes online and as the cosmic microwave background is mapped even more accurately, certain questions don’t seem immediately resolvable. Is there a particular reason why an electron weighs as much as it does, or that gravity is as strong as it is? Maybe not, Greene suggests, except that these values are one of the enormous number of possibilities given by string theory and that if these values were even slightly different, life as we know it would be impossible.
We see what we see not because there is some mathematical imperative that things must be this way, but rather (in a bit of anthropic reasoning that strikes some as circular, but others as deeply profound) because of the very fact that we are here and able to marvel at our universe in the first place.
Alexander Fabry is a writer based in New York City. He studied the history of science at Harvard and at Cambridge University.