Thinking about “multiverses” takes us beyond our universe to a realm of parallel dimensions. We will not just explore today’s popular multiverses —theories that suggest ours is one marble in an infinite bag; we’ll see why some physicists regard them as a threat to science.
Few scientists would dispute that there’s lots of stuff over the cosmic horizon, beyond what we can ever see. Light from the farthest visible objects traveled for more than 13 billion years, from galaxies currently located some 40 billion light-years away. There’s no reason for galaxies to simply stop existing at this visible boundary. Moreover, the strong evidence that space is flat on large scales means that the vast bulk of the universe must lie over the horizon, beyond where objects recede at light-speed. In fact, many cosmologists think the overarching universe may be infinite in size and thus infinite in its inventory of galaxies. (We will return to this “infinite” business a bit later.)
Since we’ll be forever blind to and ignorant of objects whose light can never reach Earth, we could call all that stuff a separate universe. Logically, it has the same physical laws as ours. This is the most straightforward type of multiverse. As renowned Massachusetts Institute of Technology physicist Max Tegmark notes, “If you define our universe as everything we can observe, then I’d bet good money on there being parallel universes.”
After we accept the frustration of never knowing anything about those gazillion galaxies, stars, and planets, nothing’s particularly weird about the situation. Some simply label that vast realm the “rest-o-universe.” It’s simple: There’s them and us. Almost all astronomers are on board that ship.
The modern multiverse
Things get controversial and downright bizarre when we examine the main multiverse hypotheses, the ones that have intrigued the public for over a decade. Their champions are well-known theorists and science popularizers like Stephen Hawking, Michio Kaku, Brian Greene, and Tegmark.
MIT’s Alan Guth created the original inflation theory in 1979 to describe our universe’s rapid expansion in its first few fractions of an instant. He supports a revised version originated by Stanford theorist Andrei Linde called chaotic inflation.
This says that as the cosmos wildly and forever increases in size, various inflating parts of space form. Each belongs to the same universe but grows so large that inhabitants will never know what happens in any other parts of the universe. In some of these parts, laws of physics may be realized in different ways,” says Linde. “Thus, one may say that our universe in effect becomes a multi-verse, a huge eternally growing fractal consisting of many different ‘universes’ with different properties.”
No one can ever traverse the space between these bubbles, one of which is our cosmos. Thus we have an infinitude of separate universes.
Guth and Linde shared the $1 million 2014 Kavli Prize in Astrophysics with fellow founder Alexei Starobinsky for their inflation work. They even met President Obama.
A different multiverse involves string theory. Originally, theorists postulated tiny strings along with many hypothesized extra dimensions as a hopeful way to explain the four fundamental forces—gravity, electromagnetism, and the strong and weak interactions—and the standard model of particle physics.
It has not succeeded. Beyond the utter lack of evidence for extra dimensions, it does not predict our universe, nor can it explain the accelerating expansion of the cosmos discovered in 1998. So it’s increasingly viewed as a failed theory. But it does “predict” 10000 different ways reality can materialize. This itself is vague to the point of useless.
A very different multiverse is based on quantum theory’s many worlds interpretation (MWI).
We’ve known for over 80 years that the act of observation can alter experimental results. But why? Moreover, why does an electron, when it comes to a fork in the road, go one way and not the other?
The MWI, a mainstream if minority interpretation of reality, claims that every choice in nature creates a separate universe that then breaks off from the others and continues on. If you turn left at an intersection instead of turning right, another “you” materializes who does indeed turn right. An entire new universe continues with that reality permanently enshrined. You are never aware of any other universe but the one you know, and the same is true of all the other yous that made other choices.
Separate universes even sprout up when a leaf fails here and not there, a few inches away. Many worlds’ multi-verses thus keep increasing their numbers wildly.
Math makes it real
Then you have the mathematical universe hypothesis, also called the ultimate ensemble. This multiverse contains everything that is mathematically possible. It’s championed by those, like Tegmark, who think mathematics is the basic reality of the cosmos as opposed to being a human conceptual tool. This view says that everything that can happen mathematically does happen in its own separate universe.
The anthropic principle provides more multiverse rationales. This is the theory that while our universe is simple in many ways, it contains dozens of physical constants whose values are Goldilocks-perfect for life to exist. If the fine-structure constant that governs the strength of the electromagnetic force was barely different, or the power of the strong force or the strength of gravity were slightly tweaked, there could be no atoms, no stars, and no chance for life. Ours is a universe so precisely fine-tuned for life, science must ask why.
By arguing for the existence of countless other random universes, the vast majority of which would not have the physical properties that permit life, multiverse advocates can say, “See? Taken as an aggregate, nothing special is going on. The multiverse landscape contains every sort of universe, and we just happen to live within one of those that supports life.” It is a way to make our exceptional-seeming cosmos less extraordinary.
A few other multiverse candidates exist, but you get the idea. The whole thing is exciting because it unveils breathtaking new possibilities that paint the overall universe to be even vaster than we’d assumed.
The infinite yous
Speaking of “vast,” let’s go back to that infinity business, which figures prominently in multiverse models. If true, there’s definitely another “you” out there on another Earth in a nearly identical universe to ours. After all, infinity means no limits. Guth maintains that a multiverse leads to the conclusion that “anything that can happen will happen—and it will happen an infinite number of times.”
So another “you” exists who wore the same socks to the prom with the little hole in one. This other you even has your cat, with identical markings. Infinity means there wouldn’t be just one exact copy of you with matching dental fillings. No, there are infinite duplicate yous. There also must be an endless army of yous with one opposite personality trait. None of them would ever leave a dirty glass in the sink.
Doesn’t smell right? Happily, there’s been a recent anti-infinity trend among physicists. Last year , a prominent pair of theorists, mathematician George Ellis and physicist Joseph Silk, wrote an editorial in the journal Nature encouraging cosmologists to heed the warnings of mathematician David Hilbert, who died in 1943. “Although infinity is needed in mathematics, it occurs nowhere in the physical universe,” the pair opined.
In any case, internal mathematical consistency is not enough to adequately support a multiverse theory, whether it involves strings, inflation, or hypothesized higher-dimensional colliding membranes that could smack our universe into another. That’s because math can diverge from physical reality, as Zeno showed in 450 B.C.E. with his famous Achilles and the tortoise thought experiment.
But multiverse hypotheses generate deep objections that go beyond their underlying math. Critics say they are non-falsifiable and thus indistinguishable from philosophy and no more useful than saying, “God did it.” Last December, Ellis and Silk wrote, “The imprimatur of science should be awarded only to a theory that is testable.”
Astronomers thought this swirl pattern in the cosmic microwave background was caused by gravitational waves from the early uni-verse’s rapid expansion due to Inflation. But follow-up observations showed it was mostly from galactic dust.
Going further, last year, writing in Edge, Paul Steinhardt, Princeton University’s Albert Einstein Professor in Science and an early inflation advocate, wrote, “The notion that we live in a multiverse in which the laws of physics and the properties of the cosmos vary randomly from one patch of space to another [should be retired].”
Strong words. Well, should we really ax the whole multiverse business? A big problem many critics cite is that multiverse models don’t predict anything, and thus they allow everything. Moreover, Steinhardt is convinced multiverse theories are put forward mainly to try to salvage failed hypotheses like string theory.
“Science is useful insofar as it explains and predicts why things are the way they are and not some other way,” he wrote. “… A Theory of Anything is useless because it does not rule out any possibility and worthless because it submits to no do-or-die tests.”
Still, “parallel universes” are such a sci-fi staple, they’re more familiar to the public than most other aspects of theoretical physics and astronomy. TV science specials remain in love with the multiverse idea and usually let its champions make on-screen claims of putative evidence.
In 2014, the most convincing ever astrophysical support emerged for the multiverse and made front pages around the world.
It was a South Pole experiment called BICEP2. Yet dramatically and amazingly, within a few short months, the find was discredited. Turned out, it wasn’t the experiment but the interpretation that proved faulty. Globally headlined as the first-ever verification of gravity waves that are the supposed signature of inflation—the wild expansion of the newborn universe—the observations are now thought to be at least mostly, if not completely, due to the existence of cosmic dust particles in our Milky Way Galaxy.
Before the BICEP2 results were discredited, they were cited by some multiverse cheerleaders as support for the chaotic inflation model, which was in turn offered as evidence for parallel universes. Critics howled at what they characterized as chains of unwarranted assumptions. Some continue to ask why the multiverse idea always seems exempt from science’s normally strict standards and is allowed to tiptoe along the shorelines of mere conjecture.
Actually, as long as speculative cosmological models are allowed into the party, it’s also possible that reality lies in the opposite direction from the multiverse. We’re talking about the notion that even our own cosmos is simple rather than multifarious.
Ancient Greek thinkers Iike Parmenides and Zeno were among the first to argue that the cosmos (or “Being”) is a single undifferentiated entity. By this account, there are no separate events happening around us. Rather, the cosmos is a single event. This view of fundamental oneness, which has some modern quantum support, has appeared in nearly all cultures through the centuries and is antithetical to multiverses.
The bottom line seems to be that while “multiverse” is mind-stretching and certainly possible, it may be pointless and even counterproductive as an explanation for our visible universe’s properties. Meanwhile, debate rages over whether any observation could ever disprove multiverse models—which pushes them outside the realm of science.
In a Newsweek cover story in 2012, Columbia University cosmologist Brian Greene wrote: “Evidence for the multiverse might come from potential collisions between our expanding universe and its neighbors. Such a cosmic fender bender would generate an additional pattern of temperature variations in the microwave background radiation [the remnant radiation of the Big Bang] that sophisticated telescopes might one day detect. Many consider this the most promising possibility for finding evidence in support of the multiverse. That there are ways, long shots to be sure, to test the multiverse proposal reflects its origin in careful mathematical analysis. Nevertheless, because the proposal is unquestionably tentative, we must approach it with healthy skepticism and invoke its explanatory framework judiciously.”
Tegmark agrees: “Parallel universes are not a theory, but [they’re] predictions of certain theories which are arguably testable, for example cosmological inflation. This is why they’re discussed not only in science fiction, but increasingly also at science conferences.”
Not even wrong?
It’s noteworthy that highly credentialed individuals still use equivocal language as they describe even the most plausible multiverse models out there. Evidence might one day be detected for certain theories which are “arguably testable?”
Is such fuzziness good enough to qualify as science?
Given the current multiverse infatuation, it may be fairest to give the last word to a prominent skeptic. Columbia University mathematical physicist Peter Woit, who maintains the popular multiverse-critical blog Not Even Wrong, pulls no punches.
“Physicists had huge success in coming up with powerful compelling fundamental theories during the 20th century,” he explains, “but the last 40 years or so have been difficult, with little progress. Unfortunately, some prominent theorists have now basically given up and decided to take an easy way out. The multiverse is invoked as an all-purpose, untestable excuse. They allow theoretical ideas like string theory that have turned out to be empty and consistent with anything to be kept alive instead of abandoned. It’s a depressing possibility that this is where physics ends up. But I still hope this is a fad that will soon die out. Finding a better, deeper understanding of the laws of physics is incredibly challenging, but it’s within our capability as humans, as long as the effort is not overwhelmed by those selling a non-answer to the problem.”
Whoa, intense. We’ve got to toss the multiverse if we care about physics!
Of course, if an infinite multiverse does exist, some other Woit is out there saying the exact opposite.’’
 See Bob Berman, “Multiverses: Science or Science Fiction?” Astronomy (43, 9, 2015, pp. 28-33). Bob Berman is Astronomy’s Strange Universe columnist. His latest book is ZOOM: How Everything Moves (Little, Brown and Co., 2014).