If there’s such a thing as a celebrity theoretical physicist, Brian Greene is it, and he packed the city’s premiere lecture hall at Town Hall Seattle Wednesday evening for a talk titled, “Is Ours the Only Universe?” The talk helped sell many copies of Greene’s latest book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos, and likely also led to a spike in aspirin sales as many of us, whose ideas of parallel universes are limited to evil Spock with a beard, tried to recover from getting our heads around quantum mechanics, general relativity, string theory, and the notion of exact copies of ourselves a cajillion light years away pondering the same concepts.
Greene noted that parallel universes are something of a logical progression of the Copernican revolution that started when Copernicus suggested that the Earth was not at the center of the universe.
“The ideas that we’re thinking about today suggest that even the universe is not the center of the universe,” Greene said. “The universe is not the center of a wider cosmic landscape that we call the multiverse.
Greene says a big part of the puzzle is that the Big Bang theory leaves out a key piece of information: Just what was the bang?
“The Big Bang theory doesn’t tell us what banged or how it banged or if it even banged,” he explained. “It only tells us that a split second after something occurred, the universe continued to expand and cool down allowing structures like stars and galaxies ultimately to coalesce. We’ve been trying to fill in what happened at the beginning; what the bang was.”
One theory posits that powerful repulsive gravity many have triggered many outbursts, so that the Big Bang was not a unique event.
“Our universe would be the aftermath of one of those big bangs, but other universes could be the outcomes of their big bangs,” Greene said. “This is known as the inflationary multiverse and it’s one of the ways in which our universe could possibly be one of many.”
Another fascinating theory is that there may be infinite numbers of universes with exact copies of us out there. It’s called the Quilted Multiverse.
“In any finite region of space there can only be a finite amount of matter,” Greene explained. “In fact, by a little bit of quantum mechanics, there only are finitely many distinct configurations that that matter can assume.”
He used a deck of cards as an example. There are only so many cards, so if you keep shuffling long enough, the same arrangement of the deck is bound to repeat. Apply the same logic to the universe.
“If space goes on infinitely far, then the configurations of matter have to repeat too, because there are only finitely many different configurations that are possible,” Greene argued.
“If the configurations of particles of matter agree here and in some distant region, what would that mean? Well, we here are just a configuration of particles. We just are particles that make up you and me and everyone else in this room. So, if the particle configuration repeats out there, then we are out there. Copies of us are out there. In fact, if space goes on infinitely farther, infinitely many copies of us are out there in rooms like this having this conversation.”
It all depends on the assumption that space is infinite, and that quantum mechanics applies in the same way in the far reaches.
String theory started to gain momentum during the 1990s.
“The big puzzle for many, many years was that gravity as described by Einstein’s general relativity proves incompatible with the mathematics of quantum physics,” Greene said. “It’s as if you have one set of laws that work for stars and galaxies, another set of laws that do work well for molecules, atoms, and subatomic particles, but any time these two laws come together, the math just falls apart and becomes inconsistent.”
Many scientists assail the concept as unprovable. Greene says the math works.
“There’s no experimental evidence for string theory at all,” he said. “Zero. The reason we have some confidence that this is an interesting idea to pursue is because we believe in quantum mechanics; that is unassailable. We believe in the general theory of relativity; that is unassailable. We believe that the universe has to be governed by a consistent set of laws. Without string theory, quantum mechanics and general relativity are inconsistent. For us, that’s a very powerful reason for believing string theory may be going in the right direction, because it makes them compatible. That is no small feat.”
It was an interesting and thought-provoking evening with a celebrity theoretical physicist.