A NEW computer is always welcome, isn鈥檛 it? It鈥檚 always faster than your old one, and it always does more stuff. An upgrade, the latest model with all the bells and whistles is an exciting prospect.
And when it comes to the kind of machine physicists are hoping for, you really are looking at something special. No ordinary upgrade for them: this will be the ultimate computer, and radically different from anything we have ever seen. Not only might it be supremely powerful, defying the logic of cause and effect to give instantaneous answers, it might also tell us exactly how the universe works. It might even tell us how our minds produce the phenomenon we call consciousness. Clear a space on your desk, then, for the quantum gravity computer.
Of course, there鈥檚 a chance it may not fit on your desktop because we don鈥檛 yet know what the machine will look like. Neither do we know how to build it, or even whether it will do all that its proponents hope. Nevertheless, just thinking about how this processor works could improve our understanding of the universe. 鈥淭he power of quantum gravity computers is one of the deepest problems in physics,鈥� says Scott Aaronson, a mathematician based at the University of Waterloo in Ontario, Canada.
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Quantum gravity is the name given to the ultimate quest in physics. Over the last hundred years, physicists have probed the workings of the universe at ever-smaller scales, right down to some of the smallest particles in nature. Our accelerators deliver higher and higher energies so we can smash particles into one another, breaking them to bits to discover how the building blocks of matter are held together.
Yet this microscopic realm, where quantum theory rules, fails to tell us anything about gravity and the structure of the universe itself. Though Einstein鈥檚 general theory of relativity portrays space and time as a stretchy fabric that bends to produce gravity, we know this is not the final answer.
Despite decades of research, so far no one has managed to make it all fit together 鈥� relativity cannot help us to explain what fundamental particles do, and quantum theory does not account for gravity.
Causal confusion
Our best guess is that space and time must be quantum in some way, and therefore that Einstein鈥檚 fabric is actually a patchwork quilt, made up of discrete elements of 鈥渟pace-time鈥�. The trouble is, it really is just a guess: it would take an atom-smasher the size of our solar system to investigate the ultimate nature of space and time experimentally.
Stabs at theories of quantum gravity abound, of course. Most of them try to describe what time and space will look like at scales of 10-35 metres, or how particles and fields arise 鈥� all, it has to be said, with little success. 鈥淚 don鈥檛 believe any of the current formulations,鈥� says Roger Penrose, a mathematician based at the University of Oxford. And this is why the quantum gravity computer is causing such a stir: it provides a whole new angle on the problem.
Its origins lie in the work of British mathematician Alan Turing. In the 1930s, Turing pioneered the field of information processing: he described how any mathematical operation 鈥� and hence any computation 鈥� is a sequence of steps involving an input and doing something to it to produce an output.
Every computer in existence works in the way Turing described. But that doesn鈥檛 mean that every conceivable machine will. Physicists began to realise they weren鈥檛 necessarily bound by Turing鈥檚 rules in 1985 when University of Oxford physicist David Deutsch made what seemed like a very simple statement. Deutsch pointed out that, whether it is the amount of electrical charge held on a circuit鈥檚 capacitor, or simply the number of beads in a particular position on an abacus, the information that a computer works with is always physical in some way. 鈥淚f you want to do any sort of computation you are necessarily using a physical device,鈥� says Lucien Hardy of the Perimeter Institute for Theoretical Physics, also in Waterloo.
鈥淎 quantum gravity computer would defy cause and effect to give instant answers and tell us how the universe works鈥�
That means the workings of the computer will always obey the physical laws that govern the system. For 鈥渃lassical鈥� systems such as a standard computer or an abacus, Deutsch鈥檚 realisation didn鈥檛 change anything much: the laws of computing were still those worked out by Turing. But, as Deutsch pointed out, classical systems aren鈥檛 the only physical systems that can hold information. Quantum systems can too.
That鈥檚 how Deutsch came up with the idea of a super-fast quantum computer. Information represented by the quantum state of an atom 鈥� whether it is spinning clockwise or anticlockwise, for example 鈥� is not bound by the same rules as classical computation. A quantum system can be in a 鈥渟uperposition鈥� of two or more states at the same time, so an atom can be said to be spinning clockwise and anticlockwise simultaneously. This means that its inputs are not as clearly defined as a classical system. Where each bit of information in a classical digital computer is either 0 or 1, on a quantum computer it can be both 0 and 1.
Quantum computers are radically different from Turing鈥檚 approach. That鈥檚 because a collection of quantum bits can be linked together so that they encode many different numbers at once. So while a classical computer deals with one number at a time, a quantum computer processes many inputs simultaneously. In 1994 Peter Shor showed that this type of processing could be used to factorise large numbers much faster than is possible on a classical computer.
The quantum computer may now have been surpassed, however. If information is always physical in some way, all physical processes can be seen as a form of information processing. At the smallest scales in the universe, physical processes will be governed by the rules of quantum gravity. Just as the rules for processing information in a quantum system are fundamentally different from those in a classical system, so the quantum gravity processing rules are different again. And this time, the changes are 鈥渆ven more radical鈥�, says Hardy, who has drawn up the first blueprint for a quantum gravity computer ().
That鈥檚 because physicists believe the universe has a peculiar characteristic at the quantum gravity scale, one that might be more important than anyone has ever realised. Quantum gravity, as we understand it so far, seems to do away with the notion of cause and effect.
A fuzzy causality is almost inevitable in quantum gravity, says Hardy. After all, even the theories that it will replace show hints of causal confusion. According to Einstein鈥檚 theories of relativity, if two people are moving relative to one another, it is sometimes impossible for them to tell whether one event happens before another. Einstein鈥檚 universe has no universal past, present and future.
Cosmic anarchy
In quantum theory there are many things that are impossible to measure precisely, such as a particle鈥檚 position and momentum. Put the two theories together to make a quantum theory of gravity and it is almost inevitable that we are going to have trouble with notions of cause and effect: the logic of tock following tick or output following input just won鈥檛 apply in the quantum-gravity universe.
鈥淭he logic of tock following tick just doesn鈥檛 apply at the smallest scales in the universe鈥�
Aaronson agrees with Hardy. 鈥淕eneral relativity says that the causal structure can vary, and quantum mechanics says that anything that can vary can be in superposition,鈥� he says. 鈥淪o to me, an indefinite causal structure seems like the main new conceptual feature.鈥�
It is this feature that might unlock the ultimate secrets of our universe when exploited by a quantum gravity computer. In standard computing, one thing happens after another; if you鈥檝e ever done even the very minimum of programming, then you鈥檒l know that 鈥� as in this sentence 鈥� 鈥渋f鈥� is usually followed by 鈥渢hen鈥�. Even quantum computing has this notion of input followed by output. Quantum gravity computing, however, will not be inconvenienced by such temporal concerns.
Though nobody knows exactly how space and time operate in quantum gravity, there will certainly be no such thing as a fixed sequence of processing steps. There might even be influences that run backward in time, Hardy says: a quantum gravity computer may have some 鈥渋nsight鈥� into the results of its computation without even running it.
Hardy isn鈥檛 the first to come up with the idea of a radically different kind of computation based on quantum gravity; Penrose is credited with that achievement 鈥� though he says he was inspired by Deutsch 鈥� and has speculated that it might also shed light on the phenomenon of consciousness (see 鈥淪pace, time and the quantum mind鈥�). Hardy is, however, the first to describe how a quantum gravity computer might work (see 鈥淗ow to build a quantum gravity computer鈥�). The big question is how powerful it could be: will it be the ultimate processor?
It turns out this is a hard question to answer. Traditionally, a computer鈥檚 power is rated by the number of computations it can do in a given time. IBM鈥檚 Blue Gene computer currently tops the world rankings for classical computers: it can do 280 trillion calculations per second. In theory, a quantum computer can do even better. It will be able to crack the world鈥檚 toughest codes in the blink of an eye.
The quantum gravity computer, on the other hand, can鈥檛 compete under these rules because 鈥渜uickly鈥� doesn鈥檛 mean anything in a scheme where space and time can鈥檛 be separated. Or, as Aaronson puts it: 鈥淚t would be nice if the quantum gravity theorists could at least tell us what they mean by 鈥榯ime鈥�.鈥�
Nevertheless, Hardy thinks there is good reason to suppose the quantum gravity computer would indeed be a more powerful machine than anything we have so far envisioned. The fact that it might glimpse its results without running a computation hints at this, he says 鈥� though he admits this is just speculation.
What鈥檚 more convincing, he says, is the difficulty of simulating a quantum gravity computer on a quantum computer. The fact that we have no algorithm for simulating quantum systems on classical computers highlights the gulf between a classical computer and a quantum computer. If a quantum computer cannot simulate a quantum gravity computer, then that implies there might be another huge leap in computing power waiting to be exploited.
There are two reasons why Hardy believes this to be the case. The first is practical: if a quantum gravity computer can defy the conventions of time, then simulating it would require an absurdly enormous quantum computer. The second is what Hardy calls a lack of 鈥渟cale invariance鈥�. The gates, or processing elements, of a quantum gravity computer would work at a scale where there is no causality. Any attempt to simulate them on a bigger machine 鈥� which has to follow sequential, causal rules 鈥� will necessarily fail. 鈥淎t larger scales there will be definite causal structure,鈥� Hardy says. 鈥淪o it won鈥檛 have the same causal connections, and so won鈥檛 be able to simulate the quantum gravity computer.鈥� If the difficulty of simulation is any guide, the quantum gravity computer reigns supreme.
It is a controversial conclusion, though. Seth Lloyd of the Massachusetts Institute of Technology thinks there is no reason to invoke a discontinuity that separates quantum gravity from more familiar processes. For a start, he says, it鈥檚 not clear that the uncertainty in causal relations would be restricted to the smallest scales: it may also apply at the scales on which quantum computers operate. When signals travel through quantum computers they exhibit quantum fluctuations 鈥� in the exact route they take, for example. This can result in a fuzzy causality in the machines, Lloyd says.
鈥淚t is essentially no different to having a quantum computer strapped to a time machine鈥�
What鈥檚 more, Lloyd has developed a theory of quantum gravity based on the way information is processed as quantum systems interact (). This, he claims, demonstrates how quantum gravity can be modelled by a quantum computer. So quantum gravity computers may not be as powerful as Hardy believes.
For now, though, we just don鈥檛 know enough to say who is right. 鈥淯ntil we actually know the correct theory of quantum gravity, all bets are off,鈥� Lloyd says.
Aaronson鈥檚 money is on the Lloyd camp: quantum gravity computers can鈥檛 be more powerful than quantum computers, he says. In his view, it is a short step from ultra-powerful quantum gravity computers to total cosmic anarchy. If, as Hardy suggests, a quantum gravity computer might be able to see its result without having to run its algorithms, it is essentially no different to having a quantum computer strapped to a time machine. As we all know, time machines don鈥檛 make sense: they would enable us to do things like travel back in history to kill our grandparents and thereby cease to exist. 鈥淚t鈥檚 hard to come up with any plausible way to make quantum computers more powerful that wouldn鈥檛 make them absurdly more powerful,鈥� he says.
Penrose thinks it is impossible to judge until we know more about how things really work at the quantum gravity scale 鈥� it is a kind of chicken and egg situation. Also, he says, there is an important element missing from all the discussions: nobody is talking about the quantum measurement problem, famously embodied by Schr枚dinger鈥檚 cat. Locked in a box with a vial of poison, no one can tell whether the cat is alive or dead, so it can be in a superposition of both states at the same time. Only the action of making a measurement 鈥� opening the box 鈥� forces the cat to be in one state or the other.
Quantum theory can鈥檛 tell us what causes quantum systems to move from superposition states to 鈥渘ormal鈥� states during the process of observation and measurement. Penrose thinks that any quantum gravity theory should tell us, and so this ought to be part of any discussion of the quantum gravity computer鈥檚 properties. Perhaps, he suggests, the strange behaviour of quantum systems 鈥� and the fact that the strangeness disappears under certain circumstances 鈥� is related to the strange nature of cause and effect in quantum gravity.
Whatever the truth, this is why investigating the characteristics of the quantum gravity computer is so valuable. It ties theories to the real world, Aaronson says, and stops the important issues, such as a link with observable facts or staying within the bounds of what鈥檚 physically possible, from being swept under the carpet. After all, a computer has to produce an observable, measurable output based on an input and a known set of rules. 鈥淭he connection to observation is no longer a minor detail,鈥� Aaronson says. 鈥淚t鈥檚 the entire problem.鈥�
鈥淭he way quantum gravity handles information may be like the thought processes in the brain鈥�
Space, time and the quantum mind
A full and proper understanding of the way the universe works may be tied to how our minds produce conscious thought and action.
In his 1989 book The Emperor鈥檚 New Mind, University of Oxford mathematician Roger Penrose suggested that quantum gravitational processes must be uncomputable by all our current measures. Penrose did not leave it there, though. He then suggested the same uncomputable 鈥� but somehow quantum 鈥� processes might also lie behind human consciousness.
Lucien Hardy at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, lends support to this speculation in his latest paper (). The way quantum gravity handles information may indeed be like the thought processes in the brain, he says.
Hardy is reluctant to be drawn into making too much of the link, as there are far too many gaps in our knowledge of the mind and of quantum gravity to make any firm connection. Nevertheless, he says, the fact that quantum gravity would process information so differently to a classical or quantum machine makes the link more plausible and potentially fruitful. 鈥淚t鈥檚 consistent with the spirit of what Penrose is saying,鈥� he says. 鈥淚f the brain were a quantum gravity computer 鈥� and that鈥檚 a very big if 鈥� then I think the consequences of that would be an interesting thing to think about.鈥�
The physicist Freeman Dyson once said that mind and intelligence are 鈥渨oven into the fabric of the universe鈥�. He may have been more right than he ever imagined.
How to build a quantum gravity computer
What might a quantum gravity computer look like? Without a theory of quantum gravity it is hard to know, but Lucien Hardy of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, has come up with one possibility. 鈥淚t is all very tentative and speculative,鈥� he admits.
His blueprint imagines a system of probes weighing around 20 micrograms, massive enough to respond to gravitational forces and yet small enough to respond to quantum effects, such as the absorption of a photon.
To act as a computer, the probes would need to have inputs and outputs. Hardy supposes the probes take in photon signals from a network of GPS satellites positioned at different points in the causally fuzzy space-time. The photons would tell the probes what state the satellites鈥� internal clocks were in.
Although it would be impossible to tell whether one signal was received before another, the probes would accumulate information about each of the satellites. Each probe could then be programmed to emit a photon 鈥� or not 鈥� depending on this accumulated information. Thus there is input, processing and output.
The devil is in the detail, of course: a previous suggestion for probing the interaction of gravity and quantum mechanics involves building a massive space-based experiment. Hardy鈥檚 quantum gravity computer might be even more difficult to set up.
