by Michio Kaku
As MIT physicist Alan Guth has said, “There is a universe where Elvis is still alive, and Al Gore is President.” Nobel laureate Frank Wilczek says, “We are haunted by the awareness that infinitely many slightly variant copies of ourselves are living out their parallel lives and that every moment more duplicates spring into existence and take up our many alternative futures.”
One point of view that is gaining in popularity among physicists is something called “decoherence.” This theory states that all these parallel universes are possibilities, but our wave function has decohered from them (i.e., it no longer vibrates in unison with them) and hence no longer interacts with them. This means that inside your living room you coexist simultaneously with the wave function of dinosaurs, aliens, pirates, unicorns, all of them believing firmly that their universe is the “real” one, but we are no longer “in tune” with them.
According to Nobel laureate Steve Weinberg, this is like tuning into a radio station in your living room. You know that your living room is flooded with signals from scores of radio stations from around the country and the world. But your radio tunes into only one station. It has “decohered” from all the other stations. (In summing up, Weinberg notes that the “many worlds” idea is “a miserable idea, except for all the other ideas.”)
So does there exist the wave function of an evil Federation of Planets that plunders weaker planets and slaughters its enemies? Perhaps, but if so, we have decohered from that universe.
QUANTUM UNIVERSES
When Hugh Everett discussed his “many worlds” theory with other physicists, he received puzzled or indifferent reactions. One physicist, Bryce DeWitt of the University of Texas, objected to the theory because “I just can’t feel myself split.” But this, Everett said, is similar to the way Galileo answered his critics who said that they could not feel the Earth moving. (Eventually DeWitt was won over to Everett’s side and became a leading proponent of the theory.)
For decades the “many worlds” theory languished in obscurity. It was simply too fantastic to be true. John Wheeler, Everett’s adviser at Princeton, finally concluded that there was too much “excess baggage” associated with the theory. But one reason that Everett’s theory is suddenly in vogue right now is because physicists are attempting to apply the quantum theory to the last domain that has resisted being quantized: the universe itself. Applying the uncertainty principle to the entire universe naturally leads to a multiverse.
The concept of “quantum cosmology” at first seems like a contradiction in terms: the quantum theory refers to the infinitesimally tiny world of the atom, while cosmology refers to the entire universe. But consider this: at the instant of the big bang, the universe was much smaller than an electron. Every physicist agrees that electrons must be quantized; that is, they are described by a probabilistic wave equation (the Dirac equation) and can exist in parallel states. Hence if electrons must be quantized, and if the universe was once smaller than an electron, then the universe must also exist in parallel states—a theory that naturally leads to a “many worlds” approach.
The Copenhagen interpretation of Niels Bohr, however, encounters problems when applied to the entire universe. The Copenhagen interpretation, although it is taught in every Ph.D.-level quantum mechanics course on Earth, depends on an “observer” making an observation and collapsing the wave function. The observation process is absolutely essential in defining the macroscopic world. But how can one be “outside” the universe while observing the entire universe? If a wave function describes the universe, then how can an “outside” observer collapse the wave function of the universe? In fact, some see the inability to observe the universe from “outside” the universe as a fatal flaw of the Copenhagen interpretation.
In the “many worlds” approach the solution to this problem is simple: the universe simply exists in many parallel states, all defined by a master wave function, called the “wave function of the universe.” In quantum cosmology the universe started out as a quantum fluctuation of the vacuum, that is, as a tiny bubble in the space-time foam. Most baby universes in the space-time foam have a big bang and then immediately have a Big Crunch afterward. That is why we never see them, because they are extremely small and short-lived, dancing in and out of the vacuum. This means that even “nothing” is boiling with baby universes popping in and out of existence, but on a scale that is too small to detect with our instruments. But for some reason, one of the bubbles in the space-time foam did not recollapse into a Big Crunch, but kept on expanding. This is our universe. According to Alan Guth, this means that the entire universe is a free lunch.
In quantum cosmology, physicists start with an analogue of the Schrödinger equation, which governs the wave function of electrons and atoms. They use the DeWitt-Wheeler equation, which acts on the “wave function of the universe.” Usually the Schrödinger wave function is defined at every point in space and time, and hence you can calculate the chances of finding an electron at that point in space and time. But the “wave function of the universe” is defined over all possible universes. If the wave function of the universe happens to be large when defined for a specific universe, it means that there is a good chance that the universe will be in that particular state.
Hawking has been pushing this point of view. Our universe, he claims, is special among other universes. The wave function of the universe is large for our universe and is nearly zero for most other universes. Thus there is a small but finite probability that other universes can exist in the multiverse, but ours has the largest probability. Hawking, in fact, tries to derive inflation in this way. In this picture a universe that inflates is simply more likely than a universe that does not, and hence our universe has inflated.
The theory that our universe came from the “nothingness” of the space-time foam might seem to be totally untestable, but it is consistent with several simple observations. First, many physicists have pointed out that it is astonishing that the total amount of positive charges and negative charges in our universe comes out to exactly zero, at least to within experimental accuracy. We take it for granted that in outer space gravity is the dominant force, yet this is only because the positive and negative charges cancel out precisely. If there was the slightest imbalance between positive and negative charges on the Earth, it might be sufficient to rip the Earth apart, overcoming the gravitational force that holds the Earth together. One simple way to explain why there is this balance between positive and negative charges is to assume that our universe came from “nothing,” and “nothing” has zero charge.
Second, our universe has zero spin. Although for years Kurt Gödel tried to show that the universe was spinning by adding up the spins of the various galaxies, astronomers today believe that the total spin of the universe is zero. The phenomenon would be easily explained if the universe came from “nothing,” since “nothing” has zero spin.
Third, our universe’s coming from nothing would help to explain why the total matter-energy content of the universe is so small, perhaps even zero. When we add up the positive energy of matter and the negative energy associated with gravity, the two seem to cancel each other out. According to general relativity, if the universe is closed and finite, then the total amount of matter-energy in the universe should be exactly zero. (If our universe is open and infinite, this does not have to be true, but inflationary theory does seem to indicate that the total amount of matter-energy in our universe is remarkably small.)
CONTACT BETWEEN UNIVERSES?
This leaves open some tantalizing questions: If physicists can’t rule out the possibility of several types of parallel universes, would it be possible to make contact with them? To visit them? Or is it possible that perhaps beings from other universes have visited us?
Contact with other quantum universes that have decohered from us seems highly unlikely. The reason that we have decohered from these other universes is that our atoms have bumped into countless other atoms in the surrounding environment.
Each time a collision occurs, the wave function of that atom appears to “collapse” a bit; that is, the number of parallel universes decreases. Each collision narrows the number of possibilities. The sum total of all these trillions of atomic “mini-collapses” gives the illusion that the atoms of our body are totally collapsed in a definite state. The “objective reality” of Einstein is an illusion created by the fact that we have so many atoms in our body, each one bumping into others, each time narrowing the number of possible universes.
It’s like looking at an out-of-focus image through a camera. This would correspond to the microworld, where everything seems fuzzy and indefinite. But each time you adjust the focus of the camera, the image gets sharper and sharper. This corresponds to trillions of tiny collisions with neighboring atoms, each of which reduces the number of possible universes. In this way, we smoothly make the transition from the fuzzy microworld to the macroworld.
So the probability of interacting with another quantum universe similar to ours is not zero, but it decreases rapidly with the number of atoms in your body. Since there are trillions upon trillions of atoms in your body, the chance that you will interact with another universe consisting of dinosaurs or aliens is infinitesimally small. You can calculate that you would have to wait much longer than the lifetime of the universe for such an event to happen.
So contact with a quantum parallel universe cannot be ruled out, but it would be an exceedingly rare event since we have decohered from them. But in cosmology, we encounter a different type of parallel universe: a multiverse of universes that coexist with each other, like soap bubbles floating in a bubble bath. Contact with another universe in the multiverse is a different question. It would undoubtedly be a difficult feat, but one that might be possible for a Type III civilization.
As we discussed before, the energy necessary to open a hole in space or to magnify the space-time foam is on the order of the Planck energy, where all known physics breaks down. Space and time are not stable at that energy, and this opens the possibility of leaving our universe (assuming that other universes exist and we are not killed in the process).
This is not a purely academic question, since all intelligent life in the universe will one day have to confront the end of the universe. Ultimately, the theory of the multiverse may be the salvation for all intelligent life in our universe. Recent data from the WMAP satellite currently orbiting the Earth confirms that the universe is expanding at an accelerating rate. One day we may all perish in what physicists refer to as the Big Freeze. Eventually, the entire universe will go black; all the stars in the heavens will blink out and the universe will consist of dead stars, neutron stars, and black holes. Even the very atoms of their bodies may begin to decay. Temperatures may plunge to near absolute zero, making life impossible.
As the universe approaches that point, an advanced civilization facing the ultimate death of the universe could contemplate taking the ultimate journey to another universe. For these beings the choice would be to freeze to death or leave. The laws of physics are a death warrant for all intelligent life, but there is an escape clause in those laws.
Such a civilization would have to harness the power of huge atom smashers and laser beams as large as a solar system or star cluster to concentrate enormous power at a single point in order to attain the fabled Planck energy. It is possible that doing so would be sufficient to open up a wormhole or gateway to another universe. A Type III civilization may use the colossal energy at their disposal to open a wormhole as it makes a journey to another universe, leaving our dying universe and starting over again.
A BABY UNIVERSE IN THE LABORATORY?
As far-fetched as some of these ideas appear, they have been seriously considered by physicists. For example, when trying to understand how the big bang got started, we have to analyze the conditions that may have led to that original explosion. In other words, we have to ask: how do you make a baby universe in the laboratory? Andrei Linde of Stanford University, one of the cocreators of the inflationary universe idea, says that if we can create baby universes, then “maybe it’s time we redefine God as something more sophisticated than just the creator of the universe.”
The idea is not new. Years ago when physicists calculated the energy necessary to ignite the big bang “people immediately started to wonder what would happen if you put lots of energy in one space in the lab—shot lots of cannons together. Could you concentrate enough energy to set off a mini big bang?” asks Linde.
If you concentrated enough energy at a single point all you would get would be a collapse of space-time into a black hole, nothing more. But in 1981 Alan Guth of MIT and Linde proposed the “inflationary universe” theory, which has since generated enormous interest among cosmologists. According to this idea, the big bang started off with a turbocharged expansion, much faster than previously believed. (The inflationary universe idea solved many stubborn problems in cosmology, such as why the universe should be so uniform. Everywhere we look, from one part of the night sky to the opposite side, we see a uniform universe, even though there has not been enough time since the big bang for these vastly separated regions to be in contact. The answer to this puzzle, according to the inflationary universe theory, is that a tiny piece of space-time that was relatively uniform blew up to become the entire visible universe.) In order to jump-start inflation, Guth assumed that at the beginning of time there were tiny bubbles of space-time, one of which inflated enormously to become the universe of today.
In one swoop the inflationary universe theory answered a host of cosmological questions. Moreover, it is consistent with all the data pouring in today from outer space from the WMAP and COBE satellites. It is, in fact, unquestionably the leading candidate for a theory of the big bang.
Yet the inflationary universe theory raises a series of embarrassing questions. Why did this bubble start to inflate? What turned off the expansion, resulting in the present-day universe? If inflation happened once, could it happen again? Ironically, although the inflation scenario is the leading theory in cosmology, almost nothing is known about what set the inflation into motion and why it stopped.
In order to answer these nagging questions, in 1987 Alan Guth and Edward Fahri of MIT asked another hypothetical question: how might an advanced civilization inflate its own universe? They believed that if they could answer this question, they might be able to answer the deeper question of why the universe inflated to begin with.
They found that if you concentrated enough energy at a single point, tiny bubbles of space-time would form spontaneously. But if the bubbles were too small, they would disappear back into the space-time foam. Only if the bubbles were big enough could they expand into an entire universe.
On the outside the birth of this new universe would not look like much, perhaps no more than the detonation of a 500-kiloton nuclear bomb. It would appear as if a small bubble had disappeared from the universe, leaving a small nuclear explosion. But inside the bubble an entirely new universe might expand out. Think of a soap bubble that splits or buds a smaller bubble, creating a baby soap bubble. The tiny soap bubble might expand rapidly into an entirely new soap bubble. Likewise, inside the universe you would see an enormous explosion of space-time and the creation of an entire universe.
Since 1987 many theories have been proposed to see if the introduction of energy can make a large bubble expand into an entire universe. The most commonly accepted theory is that a new particle, called the “inflaton,” destabilized space-time, causing these bubbles to form and expand.
The latest controversy erupted in 2006 when physicists began to look seriously at a new proposal to ignite a baby universe with a monopole. Although monopoles—particles that carry only a single north or south pole—have never been seen, it is believed that they dominated the original early universe. They are so massive that they are extremely hard to create in the laboratory, but precisely because they are so massive, if we injected even more energy into a monopole we might be able to ignit
e a baby universe into expanding into a real universe.
Why would physicists want to create a universe? Linde says, “In this perspective, each of us can become a god.” But there is a more practical reason for wanting to create a new universe: ultimately, to escape the eventual death of our universe.
THE EVOLUTION OF UNIVERSES?
Some physicists have taken this idea even further, to the very limits of science fiction, in asking whether intelligence may have had a hand in designing our universe.
In the Guth/Fahri picture, an advanced civilization can create a baby universe, but the physical constants (e.g., the mass of the electron and proton and the strengths of the four forces) are the same. But what if an advanced civilization could create baby universes that differ slightly in their fundamental constants? Then the baby universes would be able to “evolve” with time, with each generation of baby universes being slightly different from the previous generation.
If we consider the fundamental constants to be the “DNA” of a universe, it means that intelligent life might be able to create baby universes with slightly different DNA. Eventually, universes would evolve, and the universes that proliferated would be those that had the best “DNA” that allow for the flourishing of intelligent life. Physicist Edward Harrison, building on a previous idea by Lee Smolin, has proposed a “natural selection” among universes. The universes that dominate the multiverse are precisely those that have the best DNA, which is compatible with creating advanced civilizations, which in turn create more baby universes. “Survival of the fittest” simply means survival of the universes that are most favorable to producing advanced civilizations.