by Marcus Chown
If Van Raamsdonk is correct, space-time might actually emerge out of ‘quantum information’. But the result applies only to a simplified toy model and no one has been able to prove that it applies in the real Universe. Nevertheless, the idea that entanglement is essential for space-time to exist appears to be supported by another line of reasoning.
In 2013, Maldacena and Susskind drew people’s attention to two papers by Einstein, both coincidentally published in 1935. On the surface, they address subjects that could not be more different. But Maldacena and Susskind conjectured that they might actually be intimately related.
In the first paper, Einstein, Boris Podolsky and Nathan Rosen highlighted the quantum phenomenon of entanglement and pointed out (incorrectly) that such ‘spooky action at a distance’ is so ridiculous that it can mean only that quantum theory is flawed and incomplete.32 In the second paper, Einstein and Rosen showed that short-cuts through space-time are permitted to exist by the general theory of relativity.33 Today, we know them as ‘wormholes’, a term coined by the American physicist John Wheeler, who also coined the term ‘black hole’. Just as a wormhole through the centre of an apple enables a worm to take a shortcut to the far side rather than crawl around the surface, a space-time wormhole would enable a space traveller to take a shortcut across the Universe. After entering one mouth, they might need to crawl only a few metres before being spat out of the other mouth on the far side of the Galaxy.
According to Maldacena and Susskind, the connection that physicists call a wormhole is equivalent to quantum entanglement. In other words, if two particles are connected by entanglement, they are effectively joined by a submicroscopic wormhole. Remarkably, wormholes in space-time and quantum entanglement may merely be different ways of describing the same underlying reality.
If entanglement occurs because of the existence of microscopic wormholes in space-time – and such wormholes are essential for the very existence of space-time — then reducing entanglement would be expected to damage the fabric of space-time, just as found by Van Raamsdonk. The answer to the question ‘What is space made of?’ may well be quantum entanglement/wormholes. Take your pick. According to Maldacena and Susskind they are the same basic phenomenon.
Dazzled by dualities
Maldacena’s demonstration that ‘quantum field theory’ on the horizon of a 5D Universe manifests itself as general relativity in the space inside the horizon is an example of the existence of superficially very different depictions of the same physical situation. The existence of such ‘dualities’ often enables a problem that is impossible to solve from one point of view to be easily soluble from another point of view. And string theory, it turns out, is awash with dualities.
A very typical duality of string theory is that the physics at ultra-small scales looks exactly like the physics at ultra-large scales. The origin of ‘T-duality’ is in the fact that strings can both move around or wrap around an extra space dimension, so momentum can be exchanged with winding. This takes the physics from the small to the large and vice versa.
A key consequence of this particular duality is that, on the smallest scales, the parameters of physics such as the strength of gravity do not skyrocket to infinity as predicted by Einstein’s theory of gravity. Instead, they remain well-behaved as they would on the largest scales. Intuitively, this makes sense since strings have a finite size. Since they cannot be squeezed into zero volume, this neatly avoids the catastrophic singularity that general relativity predicts for the beginning of the Universe.
Dualities are by no means unique to string theory. They are also found in other fields of physics such as quantum theory, which is of course famous for its wave-particle duality. In truth, though, the separate wave and particle ways of looking at the microscopic building blocks of matter were the currency of discussion only while quantum theory was in the process of development – and continue to be currency only in popular-science books like this one! With the discovery of a self-consistent framework of quantum theory in the mid-1920s, wave-particle duality went away. The quantum machinery of Schrödinger and Heisenberg manipulates mathematical entities such as ‘wave functions’ that are neither wave nor particle but something for which we have no word in our vocabulary nor any analogue in the everyday world.
In much the same way that wave-particle duality was an indication that an adequate theory of the quantum still eluded physicists, the dualities in string theory indicate that string theory is not complete. ‘We are not quite there yet,’ says Berman. ‘The true, deeper theory will have no dualities.’
But how do we find the deeper theory?
Finding Neverland
Arkani-Hamed thinks that, in searching for a deeper, more fundamental, more true theory of physics there are several possible strategies that physicists might employ. The obvious one is to make a list of all the assumptions that have gone into obtaining the current picture, then cross them out one at a time in the hope that, gradually, the current best theory will morph into the much sought-after deeper theory. ‘Historically, however, this strategy has never worked,’ says Arkani-Hamed.
For reasons nobody knows, theories of physics are like Russian nesting dolls. Inside each perfect doll is another perfect doll. Similarly, behind each perfect and self-contained theory of the world, physicists find a deeper theory that is also perfect and self-contained. There is no gentle morphing of one theory into a deeper theory. Nature is simply not like that. ‘The laws of physics at one level are perfect,’ says Arkani-Hamed. ‘And, at the deeper level, they change into laws that are even more perfect.’ And the only way to get from one to the other is to make a heart-stopping leap in the dark. As Newton said: ‘No great discovery was ever made without a bold guess.’
Classical physics and quantum theory are a prime example of nature’s nesting-doll tendency. In the late nineteenth century, classical physics appeared perfect and self-contained. It had an apparently minor shortcoming in the guise of the ultraviolet catastrophe, considered important by Planck, and to a greater extent, by Einstein. But the deeper theory that fixed the ultraviolet catastrophe did not in any sense grow out of classical physics. The discovery of quantum theory instead essentially involved plucking out of thin air new principles and new equations, such as the Schrödinger equation, which were utterly incompatible with classical physics and in no sense could be deduced from it.
The way in which the laws of physics appear not to morph smoothly from one level to a deeper level but rather to change abruptly, even seismically, leaves physicists with only one option, says Arkani-Hamed. ‘Hold onto the physics we know for as long as possible, then jump!’
The physics we know is special relativity and quantum theory, and the only framework we have so far found that unites them is string theory. Arkani-Hamed advocates pushing such physics to breaking point. Then, like a man who has reached a cliff edge in the dark, taking a leap into the void in the hope of parachuting down onto a new island of physics off the coast. ‘Physics makes progress in a very discontinuous way,’ says Arkani-Hamed. ‘It is very important to be in the vicinity of the answer and so to leap from the right spot.’
The deeper theory will supplant Einstein’s theory of gravity, which of course breaks down at the singularities at the heart of black holes and at the beginning of time. ‘But it may also require an extension of quantum theory,’ says Arkani-Hamed.
‘Most theories signal their own demise – the theory of electromagnetism with the ultraviolet catastrophe, the general theory of relativity with its singularities – but quantum theory doesn’t seem to,’ says Berman. ‘With quantum theory we have seen something very deep.’
Although quantum theory is currently ‘fit for purpose’ – in that it predicts perfectly the outcome of all experiments – it assumes the existence of a universal clock marking time. ‘If the notion of time breaks down close to singularities, however, it is not clear that quantum theory can provide any guide for us,’ says Arkani-Hamed. ‘It’s only in the doma
in of cosmology – which deals with the origin, evolution and fate of the Universe — that quantum theory is potentially in trouble.’
‘The deeper theory will be neither general relativity nor quantum theory but a third theory,’ says Lee Smolin of the Perimeter Institute in Waterloo, Canada.
The problem in taking the next step is that all the fragmentary theories and insights gained from toy models of reality need to be put together. But nobody knows which ones are correct. And they may all be wrong. ‘String theory is part of the deeper theory,’ says Arkani-Hamed. ‘But it may not even be a central part.’
Up is the new down
When Arkani-Hamed proposes finding the deeper theory by stretching known physics to breaking point then making a leap into the unknown, he is implicitly assuming that we are in possession of all the observational data necessary to answer those big questions. Currently, we know of twelve building blocks of matter – six quarks and six leptons – and of four fundamental forces. But the familiar atomic matter that makes up the stars and galaxies and you and me is outweighed by a factor of about six by the mysterious dark matter. ‘Dark matter may be absolutely critical,’ says Arkani-Hamed. ‘There could be a property of the Universe that is a game-changer – that shows us that string theory is wrong.’
It is not possible, for instance, to rule out the existence of dark particles and dark forces that might change profoundly our understanding of physics. ‘There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy,’ warned Shakespeare’s Hamlet.
It is a remarkable fact that only about 4.9 per cent of the mass-energy of the Universe is normal matter – the stuff of the Standard Model – and, of that, we have so far spotted only about half with our telescopes. The remainder is suspected to be hydrogen gas floating between the galaxies, which is either too cold or too hot to give out detectable light.34 By comparison, the dark matter accounts for about 26.8 per cent of the mass-energy of the Universe and the ‘dark energy’ about 68.3 per cent.
As mentioned before, the dark energy — despite being the major mass component of the Universe – was discovered only in 1998. It is invisible, fills all of space and has repulsive gravity. In fact, its repulsive gravity is what is speeding up the expansion of the Universe and what led to its discovery.35
If schools are still teaching children that gravity is an attractive force, they are behind the times. More than two-thirds of the stuff in the Universe — the stuff causing its expansion to accelerate – has repulsive gravity. ‘We know there is gravity because apples fall from trees. We can observe gravity in daily life,’ says dark energy researcher Adam Riess of Johns Hopkins University in Baltimore. ‘But if we could throw an apple to the edge of the Universe, we would observe it accelerating.’
Dark energy probably does not have the potential to throw a spanner in the works as damaging as does dark matter, since general relativity and quantum theory predict the existence of vacuum energy – though nobody knows how the two dovetail together.36
So much for missing observational data about our Universe -are we also missing a big idea? ‘Our framework is spectacularly right in any ways,’ says Arkani-Hamed. ‘But it is also obvious that something big is wrong. The next step will require some revolutionary new ideas.’ As John Wheeler once said: ‘Behind it all is surely an idea so simple, so beautiful, that when we grasp it – in a decade, a century, or a millennium – we will all say to each other, how could it have been otherwise?’
Berman points out that while the anomalous motion of Uranus was explained by Le Verrier’s prediction of Neptune, the anomalous motion of Mercury was not explained by his prediction of Vulcan. It required a new idea: a fundamental change to gravity. ‘Dark matter might be there and responsible for the anomalous motions of stars and galaxies,’ says Berman. ‘Or we’ve got to change gravity.’37
Is a big idea missing?
At this very moment there could be another Einstein out there who is in possession of the missing idea which will pull everything together and single-handedly create a new revolution in physics. But history suggests that a lone genius may be insufficient.
Einstein’s theory of relativity was certainly the product of a lone genius – although Einstein himself remarked: ‘I’m no Einstein.’ But Arkani-Hamed points out that other revolutions in physics have taken more than one person. Quantum theory, for instance, was the work of about twenty physicists over a span of about twenty-five years. The Standard Model of particle physics required a similar number of people over a similar time span. The odds are therefore that the deeper theory than general relativity will be more like these revolutions than the Einsteinian revolution, and future historians of science will not talk of Newton, Einstein and a third name.
Arkani-Hamed is expecting a revolution in our picture of the world more profound than the quantum revolution of the 1920s. In fact, he draws a parallel with the birth, development and crystallisation of quantum theory. The first hint of the new worldview came with Planck’s discovery of the quantum in 1900. Later, in 1913, the Danish physicist Niels Bohr used the quantum to explain the atom in an ad hoc manner. Finally, by 1927, came the creation of a self-consistent quantum theory built on solid foundational principles. ‘At this moment I think we are about halfway to our ultimate destination,’ says Arkani-Hamed. ‘In quantum terms, we have reached around 1917 to 1918.’
The undiscovered country
‘This is the most exciting time to be doing physics since the 1920s,’ says Arkani-Hamed. ‘Every generation since the ancient Greeks could have asked: “Where did the Universe come from?” and “What are space and time?” But all previous generations had a lot of other questions to answer before they could get to address these big ones. We’ve answered them. Now the big questions are the next questions.’
According to Arkani-Hamed, this is a singular moment in the history of fundamental physics. For the first time in history, we have a framework which allows us to ask the big questions and fantastic experimental probes such as the LHC to help us answer them. ‘We’ve got to base camp at Everest,’ says Arkani-Hamed. ‘Before us we can see the beast.’
How long until we get to our destination? ‘Maybe we need the results from five experiments, that’s all,’ says Arkani-Hamed. ‘On the other hand, it could take us 500 years. But I don’t think so. I’m more optimistic than that.’
The deeper theory will tell us about the birth of the Universe. It will tell us about where space and time and everything came from, and most importantly why they exist. But it will also tell us, in the words of Einstein, ‘whether God had any choice in the creation of the World’.
But such a theory, as well as telling us profound things about the world we live in, may also give us technological mastery over that world. Maxwell’s unification of electricity and magnetism in 1863 led ultimately to both special relativity and quantum theory. Quantum theory can be said to have created the modern world, giving us lasers and computers, iPhones and nuclear reactors. Inventions which exploit quantum theory are estimated to account for about 30 per cent of the GDP of the United States.
Maxwell’s theory also predicted the existence of radio waves and led directly to our connected world, in which data and moving pictures and the invisible chatter of billions continually course through the air all around us. Neither Maxwell nor any of his contemporaries predicted any of this. If people in the nineteenth century could have seen television or the Internet or mobile phones, they would probably have considered them less technological artefacts than devilish manifestations of the supernatural.
Who knows what the deeper theory than Einstein’s will give us? ‘I defy gravity,’ said Marilyn Monroe. And maybe we will too. Perhaps we will gain mastery over space and time, the ability to create wormholes, to build star ships or to fabricate time machines. ‘We might learn how to create universes in the lab,’ says Arkani-Hamed.
‘Nothing’, as Michael Faraday remarked, ‘is too wonderful to be true.’<
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‘Whether you can go back in time is held in the grip of the law of quantum gravity,’ says Kip Thorne. ‘We are several decades away from a definitive understanding, 20 or 30 years, but it could be sooner than that.’38
‘The rapid progress true science now makes, occasions my regretting sometimes that 1 was born so soon,’ wrote Benjamin Franklin. ‘It is impossible to imagine the height to which may be carried in a thousand years the power of man over matter. We may perhaps learn to deprive large masses of their gravity and give them absolute levity, for the sake of easy transport.’39
As the example of Maxwell’s theory shows, the spin-offs of the deeper theory are likely to be as stupendous as they are unguessable. Science-fiction writer Arthur C. Clarke put it best when he said: ‘Any sufficiently advanced technology will be indistinguishable from magic.’40
Brace yourself for the magical world just over the horizon. Who knows what we will find in the undiscovered country?
Notes
Chapter 1
1 1714, ‘Portsmouth Collection’ of Newton’s papers, 1714.
2 Elizabeth Knox, The Vintner’s Luck, Vintage, London, 2000.
3 William Stukeley, Memoirs of Sir Isaac Newton’s Life, 1752, pp. 46-9.
4 Fouad Ajami, "The Arab World’s Unknown Son’, Wall Street Journal, 12 October 2011.
5 Daniel Defoe, Journal of the Plague Year, 1722.
6 ‘He died on March the 20th, 1727, after more than eighty-four years of more than average bodily health and vigour; it is a proper pendant to the story of the quart mug to state that he never lost more than one of his second teeth’ – Augustus De Morgan, Essays on the Life and Work of Newton, 1914.
7 Stukeley, Memoirs of Sir Isaac Newton’s Life, pp. 46-9.
