THE Universe is full of things that are difficult to locate: car keys, umbrellas, matching socks. Some eventually turn up unaided, others have a way of resisting even the most determined search. But everything we lose track of in everyday life is united by one solid physical fact: wherever it is, it can only be in one place at a time.
Until you bring quantum mechanics into the picture, that is. Weird as it seems, your car keys could be hiding in your coat pocket and under a pile of newspapers at the same time, while not fully existing in either place. It may run counter to intuition, but that鈥檚 what standard quantum theory says. And because the theory does such a good job of describing how the Universe鈥檚 most fundamental building blocks behave, most physicists feel they have no choice but to go along with it.
鈥淚t鈥檚 one of these things you learn when you鈥檙e an undergraduate,鈥� says Oxford University鈥檚 Roger Penrose. 鈥淚nitially students will say 鈥榃hat the hell鈥檚 going on?鈥�. Then they find that the lecturer seems to know what he鈥檚 talking about and so, after a while, they get used to it.鈥�
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But some people, he says, still worry about it鈥攁nd he counts himself among them. Penrose accepts that electrons, atoms and other particles on that tiny scale can be in more than one place at a time; there鈥檚 simply too much evidence to believe otherwise. But he won鈥檛 accept that this means a bunch of keys behaves the same way.
This unwillingness to follow the party line has led Penrose into proposing some new physics that will, he hopes, make our Universe a saner place. Penrose鈥檚 theory imposes a cutoff on quantum weirdness at a certain size, and he鈥檚 even helped to devise an experiment to test the idea. If it works, it could pave the way for a unification of quantum mechanics with Einstein鈥檚 theory of relativity, something physicists are desperate to achieve. A more immediate payoff could be the resolution of a well-known paradox of quantum theory. Thanks to Penrose, Schr枚dinger鈥檚 cat might finally get out of its box.
Erwin Schr枚dinger鈥檚 famous feline, the innocent victim of a strange quantum thought experiment, has been alive and dead for nearly 70 years now. Its problems begin when it climbs into a box where a loaded gun is pointed at its head. A photon of light is then fired at a half-reflecting mirror called a beam splitter. If the photon passes through the mirror, it automatically triggers a light-sensing device, which fires the gun and shoots the cat dead. If, on the other hand, the mirror reflects the photon away, the cat lives on.
The trouble is, in the quantum world both possibilities occur simultaneously. Physicists refer to this as a 鈥渟uperposition of states鈥�. According to orthodox quantum theory, the superposition endures until someone looks into the box to see what has happened. The observation doesn鈥檛 have to be direct. The photon might be seen to have arrived at one place or another, for example, allowing someone to infer the state of the cat. But whatever form it takes, this act of observation plays a critical role: it is the physical transmission of information during an observation that causes the superposition to collapse and seal the cat鈥檚 fate. Physicists call this leakage of information 鈥渄ecoherence鈥�.
But what if there is no decoherence? What if the photon and the mirror and the gun and the cat remain completely isolated from the outside world? According to quantum mechanics, that can only mean that the cat is in a superposition of states. It is both alive and dead, and neither alive nor dead, and it remains in this peculiar condition indefinitely, until someone opens the box.
You may find all this hard to swallow鈥攂ut then it鈥檚 meant to be. Schr枚dinger only came up with it as a way of demonstrating that there must be something wrong with quantum theory. But instead of trying to pin down where the flaw might lie, physicists have developed a number of interpretations of quantum mechanics to explain how superpositions and their consequences might work.
The most popular amounts to little more than blind faith: an acceptance that this is just the way it is. Penrose dismisses this view. 鈥淚t鈥檚 a kind of defeatist attitude because then we don鈥檛 ask questions about what鈥檚 really there,鈥� he says. Another long-standing approach is the 鈥渕any-worlds鈥� interpretation of quantum mechanics, in which the entire Universe splits in two at the instant a photon hits a half-reflecting mirror. In one universe the photon passes through and the cat dies; in the other the photon is reflected and the cat is reprieved.
Penrose balks at the idea that the Universe has to cleave in two every time a particle is observed to be in one place or another. 鈥淵ou鈥檙e led to this absolutely enormous, horrendous superposition of all these alternatives,鈥� he says. It would mean a staggering array of multiple universes springing into being at every moment. Does it really take the creation of an entire universe to kill one cat?
Yet, whatever the explanation, it鈥檚 an experimental fact that these superpositions do indeed occur鈥攁t least at the quantum scale. 鈥淭here鈥檚 no doubt in my mind that superpositions are real,鈥� says physicist Anton Zeilinger of the University of Vienna. 鈥淭his is one of the essential properties of matter.鈥� And Zeilinger is equally convinced that they can exist鈥攊n principle, at least鈥攔ight up to the scale of car keys. His research group has managed to fire a fullerene molecule, which contains sixty carbon atoms, through two separate apertures at once, demonstrating quantum superposition at a surprisingly large scale. Zeilinger is confident that as his experimental skills improve he鈥檒l be able to put even larger things鈥攍ike bacteria鈥攊nto superpositions. And if it can happen to bacteria, why should cats be any different?
But Penrose thinks Zeilinger is wrong. He reckons Schr枚dinger鈥檚 cat is safe鈥攁nd that what comes to its rescue is plain old gravity. 鈥淚f you don鈥檛 accept that the Schr枚dinger scenario holds at all levels, then there must be a stage at which it changes,鈥� says Penrose. 鈥淎nd the best place to expect that is when gravitational effects become important.鈥�
Einstein described what we call gravity as curves in space and time, created by matter and energy. The Earth鈥檚 mass, for example, creates a bowl-like depression in space-time which forces the Moon to roll around us like a racing car on a banked track (see Graphic). On the quantum scale these effects are too tiny to be observed directly. The tiny bowl in space-time created by a single atom has no measurable effect on other atoms whizzing by, but the effect is there nonetheless. Even a massless photon can distort space-time a tiny bit thanks to the pure energy it carries.
Penrose suggests that whenever a particle is in a superposition of states, then space-time must also be in a superposition. 鈥淵ou have two separate distortions of space-time,鈥� he says. But the particle would have slightly different amounts of gravitational energy, with respect to the objects around it, in each of these two states. So the superposition introduces an uncertainty鈥攁 kind of cosmic ambiguity鈥攊n the energy of the whole system. 鈥淵ou can鈥檛 make a consistent notion of what its energy is,鈥� Penrose says.
This, he says, creates an 鈥渦ncertainty鈥� in the energy. That鈥檚 not a big problem in itself: Heisenberg鈥檚 uncertainty principle is now a familiar and proven cornerstone of physics. And one of its consequences is that if you want to allow a large uncertainty in a system鈥檚 energy, then that energy uncertainty can only exist for a very short time.
Which is exactly Penrose鈥檚 point. He says the energy uncertainty between two superposed states means that all superpositions are inherently unstable and limited to a certain lifetime. The bigger the object in superposition, the more energy difference between the individual states that make up the superposition, and thus the bigger its energy uncertainty. And the bigger the uncertainty, the faster it decays into one state or the other.
Penrose estimates that for an object the size and mass of a proton, a superposition of states can last for millions of years. Atoms and molecules would not last as long in superposition, but still far too long for the collapse to be measured. So far this agrees with Zeilinger鈥檚 experimental observations.
But long before an object contains enough atoms to span the thickness of a cat鈥檚 whisker, its large gravitational energy prevents a superposition from lasting longer than a millionth of a second. And so gravity explains why, much as you鈥檇 sometimes like to, you鈥檝e never found yourself in two states or places at once鈥攁nd neither would Schr枚dinger鈥檚 cat.
Penrose calls this process 鈥渙bjective state reduction鈥�, meaning that the superposition collapse happens on its own schedule whether we observe it or not. Such objectivity is anathema to traditional quantum physicists so, not surprisingly, the idea has earned some sceptical reactions. Zeilinger rejects it utterly. 鈥淚 think there will not be a breakdown of quantum superposition anywhere,鈥� he says. 鈥淣ot at the scale Roger predicts and not at other scales.鈥�
But Penrose is quick to point out that he hasn鈥檛 just pulled his idea out of a hat. The Nobel laureate Richard Feynman once made a tentative suggestion that a theory uniting quantum mechanics and relativity might lead to an objective state reduction, and others have taken up and built on this idea. Penrose says he is simply continuing the work of this free-thinking minority.
And he has the support of some other leading theoretical physicists, including Lee Smolin of the Perimeter Institute in Waterloo, Ontario. 鈥淚t rings true to me that quantum mechanics is an approximation to a more fundamental theory,鈥� he says. 鈥淎nd it rings true to me that this might have something to do with gravity.鈥� As yet, physicists can鈥檛 say much more. As Penrose readily admits, his approach is not a fully developed mathematical theory, more a rough guess as to where and how quantum mechanics breaks down. 鈥淚t just deals with general principles,鈥� he says. It doesn鈥檛 provide an equation for how a superposition evolves into one or other of its possible states.
Penrose knows the surest way to gain acceptance for his idea is to confirm it with an experiment. To that end Penrose has enlisted the help of Dik Bouwmeester and William Marshall at Oxford鈥檚 Centre for Quantum Computation (CQC). Bouwmeester heads the optics group at the centre, and together with Marshall he has begun to develop an apparatus that will reproduce the key elements of the Schr枚dinger thought experiment and look for signs of Penrose鈥檚 objective state reduction in action.
Bouwmeester and Marshall plan to put a microscopic crystal, less than a micrometre across and containing about 1015 atoms, into a superposition of being in two different places at once. Tiny though this crystal is, by quantum standards it is huge鈥攚ell within the realm of the macroscopic world. Penrose estimates the crystal鈥檚 mass will force any superposition to decay within 1/100th to 1/10th of a second. That鈥檚 slow enough to measure the change, and fast enough to distinguish the result from standard quantum mechanics, which decrees the superposition should last indefinitely.
The experiment will begin with a laser firing a photon at a beam splitter (see Graphic), which sends the photon along two paths at once in classic quantum fashion. Along one path the photon heads towards a mirror, which reflects it back to the beam splitter. On the other path, the photon hits a crystal suspended on a carbon filament in such a way that it will recoil slightly when hit by the photon. The superposition of the two photon paths forces the crystal into its own superposition of two locations at once: one where it has recoiled and the other where it remained stationary because the photon took the other path.
After bouncing off the face of the crystal, the photon then heads towards a second mirror. This portion of the photon鈥檚 flight is timed precisely so that the photon is reflected back to arrive at the crystal at the very moment this has returned to its original position. The crystal then reflects the photon back to the beam splitter, where it recombines with its parallel state as if they had never been apart.
鈥淚f quantum mechanics with all of its superpositions remains true, then it will always go back out the way it came,鈥� says Penrose. In this case, everything about the original photon is preserved. It passes through the beam splitter the way it came, and goes straight back into the laser. The photon detector placed in the alternate path from the beam splitter will detect nothing. If the detector does register nothing every time the experiment is run, it will confirm that each photon is indeed 鈥渁ware鈥� of both paths.
But the researchers will arrange the experiment so that the photon鈥檚 flight through the apparatus takes more than one-tenth of a second. In this case, Penrose says the crystal鈥檚 superposition should decay while the photon is still in the apparatus. That means the two possible paths will have reverted to one, and the photon鈥檚 superposition will also have to decay. In this scenario, the photon comes to the beam splitter having been forced to choose one path or another. At the beam splitter it might be sent back into the laser, but half of the time it will be reflected another way and end up in the waiting detector.
As elegant as it looks on paper, the experiment contains a formidable array of technical challenges. Everything will have to be arranged to ensure that the two parallel states of the photon are undisturbed by the world outside the experiment. But there are many ways to upset things and bring about decoherence, such as thermal energy jiggling the atoms in the crystal. The key to the experiment will be minimising such thermal noise and then seeing if the results change when parameters such as the size of the crystal are varied slightly.
The challenge in designing the experiment is compounded by the fact that only an X-ray photon has enough punch to give the crystal the required kick, and X-rays are notoriously difficult to reflect. And the fact that the photon must be in flight for at least 1/10th of a second means the experiment has to take place over distances on a par with the diameter of the Earth. One solution to this problem will be to place the experiment in space, on two co-orbiting platforms. In deference to Schr枚dinger and his cat, Penrose has contrived to call this arrangement of his experiment the Free-orbit Experiment with Laser-Interferometry X-rays, or FELIX.
It is too early to know if the experiment will work with available technology, but no one doubts the possibility is worth investigating. 鈥淎nything that tests the foundations of quantum mechanics experimentally is an important thing to do and they should be encouraged to do it,鈥� says Smolin. Artur Ekert, head of the CQC, agrees that this kind of experiment should be performed, regardless of the arguments surrounding the idea. 鈥淚t鈥檚 likely quantum mechanics as we know it today is not the ultimate theory of quantum reality,鈥� he says. 鈥淢any physicists are satisfied with producing beautiful experiments that are just bound to confirm things. What Roger鈥檚 trying to do is really push the limits. It鈥檚 really exciting.鈥�
Bouwmeester is equally enthusiastic but, ironically, he thinks the experiment will ultimately prove Penrose wrong. 鈥淧ersonally, I think the effect will not be there,鈥� he says. 鈥淨uantum mechanics is so simple and beautiful鈥攁lthough bizarre to the human mind鈥擨 think that what we will learn is that we should accept it as it is.鈥�
But Penrose is confident he鈥檚 onto something. It may or may not lead to a theory of quantum gravity, but he is convinced that this will expose the incompleteness of quantum theory as it stands today. Schr枚dinger鈥檚 cat might finally escape its box.
But a stranger mystery remains. Why, asks Penrose, are so many of his fellow physicists, including his chief collaborator, so willing to sit back and accept the strange implications of quantum mechanics? 鈥淲hat puzzles me isn鈥檛 the mystery of Schr枚dinger鈥檚 cat, but the mystery of human beings.鈥�