There is an exact 50-50 chance that the cat is alive, or is dead. Until we look at the cat, we do not know. Can we know? Can the state of the cat be described before we look at it, other than by a quantum superposition of “alive” and “dead”?
It must rank as one of the most famous cases of animal abuse in history. Thankfully, though, it is an entirely fictional thought experiment. Called “Schrödinger’s cat,” the idea is that a cat is put in a sealed box with a device containing a radioactive atom that has a 50-50 chance of decaying within an hour. If the atom decays, a flask of poison is broken and the cat dies. The Austrian physicist Erwin Schrödinger constructed this macabre “experiment” in 1935 to make a point: he thought that although the newly developed quantum theory could predict the behavior of particles, it could not be a true description of reality because it led to bizarre consequences. Specifically, quantum theory allowed particles to possess contradictory properties until they were measured. To highlight what he saw as an insoluble problem he asked how the contents of the box could be described at the end of the hour before somebody peeped inside.
The condition of the cat obviously depends upon whether the radioactive atom has decayed or not, and this decay depends on probability. The physics of probable states is the basis of much of quantum theory (see What is a Black Hole?). In Newtonian physics, everyday objects interacted in ways that were rigidly repeatable and as predictable as clockwork. When someone threw a ball in the air, there was no doubt that it would fall back to the ground; the only question was how long it would remain in the air and Newton showed that this could be precisely calculated. On the scale of individual atoms, however, probability enters the mix and physicists have to use quantum theory. This shows us that events are not rigidly repeatable in the subatomic realm, so probabilities need to be ascribed to outcomes. Hence, in radioactive decay, there is a calculable chance that an atomic nucleus will decay within a given time, but there is not a guarantee. What makes the nucleus “decide” to decay at a particular instant is completely unknown to us; we simply have to accept that probability is hard-wired into the Universe. Anything that relies on the inherent uncertainty of subatomic processes is said to be a quantum system. Returning to Schrödinger’s cat, quantum theory can provide an equation that perfectly describes the radioactive atom in a state that represents both decayed and yet-to-decay possibilities. But what does this mean for the cat? Must the cat be thought of as simultaneously alive and dead until the box is opened? Schrödinger considered this ludicrous.
“Anyone not shocked by quantum mechanics has not yet understood it.”
NIELS BOHR 20TH CENTURY PHYSICIST
Once the box is opened, the mystery is solved. The cat will be either alive or dead, depending on whether the radioactive atom has decayed or not. But this leaves us with another dilemma: what happens to the “unused” state of the atom, the alternative possibility that was not observed? Did it simply cease to exist when the box was opened?
Danish physicist Niels Bohr did indeed think the alternative state just vanished. Puzzled by how to interpret the mathematics of quantum theory, he and colleague Werner Heisenberg had decided in 1927 that the act of observation forces the quantum system to “make up its mind,” and become one thing or the other. Prior to the observation, the quantum system was in a mixed state, a “superposition” as physicists call it, of all possible outcomes. This is known today as the “Copenhagen interpretation.”
At its heart, the Copenhagen interpretation says something exceptionally profound—that measurement creates reality. The common-sense problem with this view is, as Schrödinger pointed out, that we have a zombie cat for an hour, half-alive half-dead, until the box is opened and somehow the act of observation makes it either dead or alive. Instinctively this sounds wrong—why should the act of observation be essential in creating reality? Surely we have to believe that the Universe is “solid,” even when we turn our backs on it. For the first time, physicists were faced with a fundamental problem that verged on philosophy: does quantum theory describe reality or is it just a mathematical trick that gives the right answer?
Trying to square the mathematics with reality has perplexed physicists for the better part of a century. In 1957, American physicist Hugh Everett proposed that we should take quantum theory at face value and believe that its mathematics does describe reality. Therefore, when the equations show different possible outcomes, all of them must be played out somewhere. Everett had no idea where these alternative realities might be located, but in one “universe” the cat would live; in another it would die. Our own Universe meanders from one quantum decision to the next, tracing just one of a multitude of paths through reality.
Everett’s idea has become known and respected as the “many-worlds” interpretation, but it was slow to catch on, mainly because Bohr refused to take it seriously. It was in the 1970s that physicists became interested in the hypothesis, because they were starting to use higher dimensions in their calculations, and they realized that these might provide locations for parallel universes, coincident with our own but shifted through a dimension we cannot perceive. Perhaps, they wondered, it was in these parallel universes that Everett’s “many worlds” could exist. It was the equivalent of letting the genie out of the bottle. Physics is now alive with the idea of parallel universes; there is even evidence pointing to a variety of different types. This panoply is known as the “multiverse.”
THE “MANY-WORLDS” INTERPRETATION: EVERY TIME A QUANTUM DECISION IS MADE THE UNIVERSE SPLITS, CREATING MANY PARALLEL UNIVERSES
In 2003, American physicist Max Tegmark classified the possible alternative universes into four different types. The first and simplest case comes about because our Universe may be infinite in size. Measuring the size of the Universe has been a preoccupation for astronomers since the 16th century. Every time they have devised a method to measure more distant celestial objects, they have been astounded by just how far away they are. In other words, the Universe has continually surprised us with how big it is, and this has led to the suspicion that it is in fact infinite. We cannot possibly see all of it, because, in the 13.7 billion years since the Big Bang, light can only have arrived from regions that were once no further than 13.7 billion light years away. Anything beyond this is impossible for us to observe at present.
Astronomers have been able to test whether the Universe is smaller than 13.7 billion light years, by looking for repeating patterns in the ripples of the cosmic microwave background radiation. To see why this would reveal a “small” Universe, think of the Earth’s surface. Forget for a moment that we perceive three dimensions, just take it for granted that the ground is flat. Start walking north; the ground continues to appear flat around you, and you keep walking. Ignore the obvious hindrance of the polar ice caps, oceans and mountains; just keep walking. Eventually, you will circle the Earth and arrive back where you started. Such a shape, in this case a sphere, is known as an unbounded but finite shape because you do not come across any boundaries but the surface of the Earth is finite in area. The Universe might be similar, the key being that space may be curved through a higher dimension than we can perceive. In our example of the Earth’s surface, it was curved through the third dimension, which we were pretending we could not sense. In the case of the Universe, it is curved through the fourth dimension (which Einstein introduced to explain gravity).
If the Universe curved completely back on itself in the fourth dimension, then a powerful telescope on Earth could, in principle, see the Milky Way Galaxy apparently very far off in space. But of course the light would have been traveling for billions of years and so the Milky Way would appear much younger than it is today. It would be like seeing all the way around the curve of the Earth and observing the back of your head far off in the distance—but you yourself would be a baby.
Looking for repeating patterns in the cosmic microwave background radiation is the somewhat more practical equivalent of searching the distant galaxies for a young Milky Way. To date, no such repeats have been found and this is taken as evidence that the Universe extends further than we can see—beyond 13.7 billion light years. The theory of inflation, that the Universe underwent a sudden period of exponential expansion just after the Big Bang (see How Did the Universe Form?), if proven, would mean that the Universe must be vastly bigger than this; in fact most cosmologists believe that inflation leads to an infinite Universe. Even if inflation is proved wrong, an infinite Universe is still a possibility.
If the Universe is infinite then every outcome—no matter how unlikely—is played out somewhere. Somewhere in the Universe there is an alternative Earth where the alternative “you” wrote this book and the alternative “me” is reading it. Think of any possibility that does not contravene the laws of physics and it will have happened. Tegmark calls these level I parallel universes. They possess the same laws of physics but started from different initial conditions, hence they are not quite the same. As time goes by, light will arrive from further and further away and these remote regions will come into view, gradually revealing these alternative universes to us.
A slightly different version of inflation, known as “chaotic inflation,” makes it possible that new universes sprouted away from our own because of the way that quantum theory works. If this happened, it would have set into motion a chain reaction that continues somewhere in the multiverse today—in short, a never-ending sequence of other universes being born. These comprise Tegmark’s level II parallel universes. Unlike the level I alternative universes, they do not just lie a long way away, but inhabit entirely different dimensions of space.
In one of these level II parallel universes, the way the forces of nature evolved may be different from the way this happened in our Universe, and so the strength of these forces could be different. This would be reflected in the constants of nature taking on different values from the ones in our Universe. Perhaps gravity is a bit stronger and so stars are pulled together more quickly, burn more brightly and hence live shorter lives. Or the strong nuclear force is a little weaker, making more atoms radioactive; this would generate more heat inside planets, creating more volcanic activity.
Some of the parallel universes may be “flat,” with only two extended dimensions, whereas some may have four spatial directions, or six, or none. There is no fundamental difference between the other parallel universes and our own; all have the same laws of physics. We believe that a universe transforms itself, from the high-energy state just after the Big Bang to the low-energy state of today, in an essentially random process. The strengths of the forces, the number of dimensions, even the variety of particles, are all fixed by this unpredictable process. Think of it as a ball whizzing around the top of a roulette wheel. Each ball that the croupier spins begins in a high-energy state indistinguishable from any other spin of the wheel. When it loses energy and falls down into the wheel it eventually ends up in one of the numbered slots, each one an equally valid outcome. In the case of a roulette wheel there are 37 or 38 different pockets, but for a collapsing universe there are an infinite number of possibilities. So some level II universes will be similar, even identical to ours, while others will be vastly different.
In his next category, the level III parallel universes, Tegmark turned his attention to Schrödinger’s cat and Everett’s original suggestion of “alternative realities” in which every possibility is played out. Tegmark found a subtle but important difference between these parallel universes and the previous two types. It is to do with how they are created.
According to Everett’s many-worlds interpretation of quantum theory, the Universe splits when a quantum decision is revealed—such as when the box in Schrödinger’s cat experiment is opened. This whittling of possibilities into a certainty is known as “decoherence” and, until the mid-1990s, physicists did not know how it happened. In refuting Everett’s ideas, Bohr had talked cryptically about the act of observation being needed to force the quantum system to make its decision, but he failed to say what defined an observer. Was the cat in Schrödinger’s thought experiment an observer of its own condition, or was human self-awareness required? Could particles themselves be “observers” by dint of their physical interactions?
An experiment conducted by Serge Haroche and colleagues in 1996 using rubidium atoms and microwaves provided the answer by showing that decoherence occurs as atoms interact with their surroundings. No intelligent observers were needed, just the random interaction of other particles. The conclusion therefore is that particles are indeed Bohr’s “observers” and that the act of observation is equivalent to a physical interaction between particles. This solves the worst aspect of Schrödinger’s cat experiment, namely the period in which the animal is half-dead, half-alive. This scenario never happens because the interaction of the particles inside the box—the atoms in the cat, the radioactive particle, the molecules in the air and in the poison—will mean that if the bottle breaks and releases the poison, the cat will be killed instantaneously as our common sense would suggest.
The many-worlds interpretation can be thought of as offering a “life after death” for the timelines that are rejected in our Universe. As one possibility comes to an end for us, so a new universe springs into existence to play it out. But there is a remarkable coincidence here: the possibilities played out in the level III parallel universes will be identical to those played out in level I examples. It is just that level I universes are separated from us by vast tracts of space. Level III universes supposedly pop into existence, presumably in some other dimension, as our reality unfolds. As yet, no one can reconcile these two similar yet different concepts.
It might be thought on reading this that every time we make a conscious decision, other universes are conjured up, but such an extrapolation would be wrong. This behavior is restricted to quantum processes. The only way our conscious decisions could create alternative realities is if somewhere deep inside the brain a decision is based upon a single quantum particle that spontaneously changes its state. This tiny happening would then need to be amplified in our consciousness to become a decision. Intuitively, this feels wrong, since decisions seem like a much higher processing of information. Each of us makes decisions by weighing evidence and past experiences and “computing” what we hope will be the best course of action, not by a random alteration of a particle from one quantum state to another.
“If we look at the way the Universe behaves, quantum mechanics gives us fundamental, unavoidable indeterminacy, so that alternative histories of the Universe can be assigned probability.”
MURRAY GELL-MANN CONTEMPORARY PHYSICIST
Flipping a coin to provide an outcome is not a quantum process either. A coin flip is governed by factors that are hard to predict, but not because of quantum uncertainty. It takes place on a scale much larger than the scales at which quantum effects apply. So, sadly, deciding whether or not to finish this chapter is not going to cause an alternative universe to spring into reality—you may as well read on.
Suppose that physicists succeed in finding a theory of everything that describes the “superforce” that controls the Universe and gave rise to the physics of today. We would be forgiven for thinking that their job was done. Far from it—the hard work could just be starting because they would then have to set about answering the really important question: “Why should the laws of physics be as they are?” The final level of parallel universes rests on the answer to this question and is perhaps the most philosophically influenced.
It was originally hoped that string theory (see Was Einstein Right?) would answer the question and tell us why the Universe behaves the way it does; that it would provide some deep reason why the constants have the values that they have and the laws are as they are. That hope faded because there appeared to be many different versions of string theory, all of which seem equally plausible. By 1995, physicists had developed between them five distinct string theories, each one using ten dimensions in its calculations. In trying to decide which of these competing theories was correct, they found something remarkable. The various ten-dimensional string theories could be shown to be different manifestations of the same, much broader theory, if they simply added another dimension to the mathematics. They called this eleven-dimensional model “M-theory,” perhaps for Mother theory although no one seems to recall the reason; some have even suggested Magical.
Physicists now envisage the many possible string theories that can come out of M-theory as a landscape, where valleys contain self-consistent universes and mountains represent energy barriers between them. Our Universe sits in one of the valleys, but as yet we do not know which string theory describes it, or indeed whether string theory is correct at all. The other valleys in the M-theory landscape all have different laws of physics and different constants of nature, and are the level IV universes. Some of them will be similar to our Universe, others will be vastly different, and many physicists now suggest that every combination of laws is tried out in one of the parallel universes. Some scientists even argue that if you reject this interpretation of the multiverse then your only recourse is to believe that God created our Universe (see Is There Cosmological Evidence for God?).
A strategy for how to search for level III and level IV universes currently eludes physicists, but there is a way to verify that level I and level II universes exist. Astronomers are currently searching for proof that inflation happened. If they find it, they will also have proof that parallel universes exist, because any form of inflation is thought to create level I parallel universes, which would be very far away in space and have the same values for their physical constants. If the variation known as chaotic inflation is confirmed, there will also be level II universes, “budding” off from our Universe and having different values of the physical constants.
Inflation leaves its mark on the cosmic microwave background radiation in the form of fluctuations in the density of the cosmic matter and in the orientation of the cosmic microwave background rays. Many observations of the microwave background have been broadly consistent with inflation, although some discrepancies have been uncovered. The recently launched European spacecraft Planck will investigate further by taking highly accurate pictures of the microwave background. The orientation of the microwave radiation is known as “polarization” and should have been imprinted on the microwaves during inflation by gravitational waves coursing through the Universe in the split second after the Big Bang. The gravitational waves moved through the fabric of space–time like ripples on a pond; as they passed, they would have alternately squeezed and then stretched matter. The different versions of inflation theory impose different patterns on the microwave background.
If inflation of one type or another is proven to be true, then physicists will have to accept the fact that parallel universes do exist. And all of us will have to come to terms with the idea that there are many different versions of each of us out there somewhere.