Aug 25th 2021

A WISE PROVERB suggests not putting all your eggs in one basket. Over recent decades, however, physicists have failed to follow that wisdom. The 20th centuryand, indeed, the 19th before itwere periods of triumph for them. They transformed understanding of the material universe and thus peoples ability to manipulate the world around them. Modernity could not exist without the knowledge won by physicists over those two centuries.

Your browser does not support the

Get The Economist app and play articles, wherever you are

In exchange, the world has given them expensive toys to play with. The most recent of these, the Large Hadron Collider (LHC), which occupies a 27km-circumference tunnel near Geneva and cost $6bn, opened for business in 2008. It quickly found a long-predicted elementary particle, the Higgs boson, that was a hangover from calculations done in the 1960s. It then embarked on its real purpose, to search for a phenomenon called Supersymmetry.

This theory, devised in the 1970s and known as Susy for short, is the all-containing basket into which particle physicss eggs have until recently been placed. Of itself, it would eliminate many arbitrary mathematical assumptions needed for the proper working of what is known as the Standard Model of particle physics. But it is also the vanguard of a deeper hypothesis, string theory, which is intended to synthesise the Standard Model with Einsteins general theory of relativity. Einsteins theory explains gravity. The Standard Model explains the other three fundamental forceselectromagnetism and the weak and strong nuclear forcesand their associated particles. Both describe their particular provinces of reality well. But they do not connect together. String theory would connect them, and thus provide a so-called theory of everything.

String theory proposes that the universe is composed of minuscule objects which vibrate in the manner of the strings of a musical instrument. Like such strings, they have resonant frequencies and harmonics. These various vibrational modes, string theorists contend, correspond to various fundamental particles. Such particles include all of those already observed as part of the Standard Model, the further particles predicted by Susy, which posits that the Standard Models mathematical fragility will go away if each of that models particles has a heavier supersymmetric partner particle, or sparticle, and also particles called gravitons, which are needed to tie the force of gravity into any unified theory, but are not predicted by relativity.

But, no Susy, no string theory. And, 13 years after the LHC opened, no sparticles have shown up. Even two as-yet-unexplained results announced earlier this year (one from the LHC and one from a smaller machine) offer no evidence directly supporting Susy. Many physicists thus worry they have been on a wild-goose chase.

They have good reason to be nervous. String theory already comes with a disturbing conceptual price tagthat of adding six (or in one version seven) extra dimensions to the universe, over and above the four familiar ones (three of space and one of time). It also describes about 10500 possible universes, only one of which matches the universe in which human beings live. Accepting all that is challenging enough. Without Susy, though, string theory goes bananas. The number of dimensions balloons to 26. The theory also loses the ability to describe most of the Standard Models particles. And it implies the existence of weird stuff such as particles called tachyons that move faster than light and are thus incompatible with the theory of relativity. Without Susy, string theory thus looks pretty-much dead as a theory of everything. Which, if true, clears the field for non-string theories of everything.

The names of many of these do, it must be conceded, torture the English language. They include causal dynamical triangulation, asymptotically safe gravity, loop quantum gravity and the amplituhedron formulation of quantum theory. But at the moment the bookies favourite for unifying relativity and the Standard Model is something called entropic gravity.

Entropy is a measure of a systems disorder. Famously, the second law of thermodynamics asserts that it increases with time (ie, things have a tendency to get messier as they get older). What that has to do with a theory of gravity, let alone of everything, is not, perhaps, immediately obvious. But the link is black holes. These are objects which have such strong gravitational fields that even light cannot escape from them. They are predicted by the mathematics of general relativity. And even though Einstein remained sceptical about their actual existence until the day he died in 1955, subsequent observations have shown that they are indeed real. But they are not black.

In 1974 Stephen Hawking, of Cambridge University, showed that quantum effects at a black holes boundary allow it to radiate particlesespecially photons, which are the particles of electromagnetic radiation, including light. This has peculiar consequences. Photons carry radiant heat, so something which emits them has a temperature. And, from its temperature and mass, it is possible to calculate a black holes entropy. This matters because, when all these variables are plugged into the first law of thermodynamics, which states that energy can be neither created nor destroyed, only transformed from one form (say, heat) into another (say, mechanical work), what pops out are Einsteins equations of general relativity.

That relationship was discovered in 2010 by Erik Verlinde of Amsterdam University. It has serious implications. The laws of thermodynamics rely on statistical mechanics. They involve properties (temperature, entropy and so on) which emerge from probabilistic descriptions of the behaviour of the underlying particles involved. These are also the particles described by quantum mechanics, the mathematical theory which underpins the Standard Model. That Einsteins equations can be rewritten thermodynamically implies that space and time are also emergent properties of this deeper microscopic picture. The existing forms of quantum mechanics and relativity thus do indeed both seem derivable in principle from some deeper theory that describes the underlying fabric of the universe.

String theory is not so derivable. Strings are not fundamental enough entities. But entropic gravity claims to describe the very nature of space and timeor, to use Einsteinian terminology, spacetime. It asserts this is woven from filaments of quantum entanglement linking every particle in the cosmos.

The idea of quantum entanglement, another phenomenon pooh-poohed by Einstein that turned out to be true, goes back to 1935. It is that the properties of two or more objects can be correlated (entangled) in a way which means they cannot be described independently. This leads to weird effects. In particular, it means that two entangled particles can appear to influence each others behaviour instantaneously even when they are far apart. Einstein dubbed this spooky action at a distance, because it seems to violate the premise of relativity theory that, in the speed of light, the universe has a speed limit.

As with black holes, Einstein did not live long enough to see himself proved wrong. Experiments have nevertheless shown he was. Entanglement is real, and does not violate relativity because although the influence of one particle on another can be instantaneous there is no way to use the effect to pass information faster than light-speed. And, in the past five years, Brian Swingle of Harvard University and Sean Carroll of the California Institute of Technology have begun building models of what Dr Verlindes ideas might mean in practice, using ideas from quantum information theory. Their approach employs bits of quantum information (so-called qubits) to stand in for the entangled particles. The result is a simple but informative analogue of spacetime.

Qubits, the quantum equivalent of classical bitsthe ones and zeros on which regular computing is builtwill be familiar to those who follow the field of quantum computing. They are the basis of quantum information theory. Two properties distinguish qubits from the regular sort. First, they can be placed in a state of superposition, representing both a one and a zero at the same time. Second, several qubits can become entangled. Together, these properties let quantum computers accomplish feats such as performing multiple calculations at once, or completing certain classes of calculation in a sensible amount of time, that are difficult or impossible for a regular computer.

And because of their entanglement qubits can also, according to Dr Swingle and Dr Carroll, be used as stand-ins for how reality works. More closely entangled qubits represent particles at points in spacetime that are closer together. So far, quantum computers being a work in progress, this modelling can be done only with mathematical representations of qubits. These do, though, seem to obey the equations of general relativity. That supports entropic-gravity-theorys claims.

All of this modelling puts entropic gravity in pole position to replace strings as the long-sought theory of everything. But the idea that spacetime is an emergent property of the universe rather than being fundamental to it has a disturbing consequence. It blurs the nature of causality.

In the picture built by entropic gravity, spacetime is a superposition of multiple states. It is this which muddies causality. The branch of maths that best describes spacetime is a form of geometry that has four axes at right angles to each other instead of the more familiar three. The fourth represents time, so, like the position of objects, the order of events in spacetime is determined geometrically. If different geometric arrangements are superposed, as entropic gravity requires, it can therefore sometimes happen that the statements A causes B and B causes A are both true.

This is not mere speculation. In 2016 Giulia Rubino of the University of Bristol, in England, constructed an experiment involving polarised photons and prisms which achieved exactly that. This spells trouble for those who have old-fashioned notions about causalitys nature.

However, Lucien Hardy of the Perimeter Institute, in Canada, has discovered a way to reformulate the laws of quantum mechanics to get around this. In his view, causality as commonly perceived is like data compression in computing: it is a concept that gives you more bang for your buck. With a little bit of information about the present, causality can infer a lot about the futurecompressing the amount of information needed to capture the details of a physical system in time.

But causality, Dr Hardy thinks, may not be the only way to describe such correlations. Instead, he has invented a general method for building descriptions of the patterns in correlations from scratch. This method, which he calls the causaloid framework, tends to reproduce causality but it does not assume it, and he has used it to reformulate both quantum theory (in 2005) and general relativity (in 2016). Causaloid maths is not a theory of everything. But there is a good chance that if and when such a theory is found, causaloid principles will be needed to describe it, just as general relativity needed a geometry of four dimensions to describe spacetime.

Entropic gravity has, then, a lot of heavy-duty conceptual work to back it up. But it is not the only candidate to replace string theory. Others jostling for attention include an old competitor called loop quantum gravity, originally proposed in 1994 by Carlo Rovelli, then at the University of Pittsburgh, and Lee Smolin, of the Perimeter Institute. This, and causal dynamical triangulation, a more recent but similar idea, suggest that spacetime is not the smooth fabric asserted by general relativity, but, rather, has a structureeither elementary loops or triangles, according to which of the two theories you support.

A third option, asymptotically safe gravity, goes back still further, to 1976. It was suggested by Steven Weinberg, one of the Standard Models chief architects. A natural way to develop a theory of quantum gravity is to add gravitons to the model. Unfortunately, this approach got nowhere, because when the interactions of these putative particles were calculated at higher energies, the maths seemed to become nonsensical. However, Weinberg, who died in July, argued that this apparent breakdown would go away (in maths speak, the calculations would be asymptotically safe) if sufficiently powerful machines were used to do the calculating. And, with the recent advent of supercomputers of such power, it looks, from early results, as if he might have been right.

One of the most intriguing competitors of entropic gravity, though, is the amplituhedron formulation of quantum theory. This was introduced in 2013 by Nima Arkani-Hamed of the Institute of Advanced Study at Princeton and Jaroslav Trnka of the University of California, Davis. They have found a class of geometric structures dubbed amplituhedrons, each of which encodes the details of a possible quantum interaction. These, in turn, are facets of a master amplituhedron that encodes every possible type of physical process. It is thus possible to reformulate all of quantum theory in terms of the amplituhedron.

Most attempts at a theory of everything try to fit gravity, which Einstein describes geometrically, into quantum theory, which does not rely on geometry in this way. The amplituhedron approach does the opposite, by suggesting that quantum theory is actually deeply geometric after all. Better yet, the amplituhedron is not founded on notions of spacetime, or even statistical mechanics. Instead, these ideas emerge naturally from it. So, while the amplituhedron approach does not as yet offer a full theory of quantum gravity, it has opened up an intriguing path that may lead to one.

That space, time and even causality are emergent rather than fundamental properties of the cosmos are radical ideas. But this is the point. General relativity and quantum mechanics, the physics revolutions of the 20th century, were viewed as profound precisely because they overthrew common sense. To accept relativity meant abandoning a universal notion of time and space. To take quantum mechanics seriously meant getting comfortable with ideas like entanglement and superposition. Embracing entropic gravity or its alternatives will require similar feats of the imagination.

No theory, though, is worth a damn without data. That, after all, is the problem with Supersymmetry. Work like Dr Rubinos points the way. But something out of a particle-physics laboratory would also be welcome. And, though their meaning is obscure, the past few months have indeed seen two experimentally induced cracks in the Standard Model.

On March 23rd a team from CERN, the organisation that runs the LHC, reported an unexpected difference in behaviour between electrons and their heavier cousins, muons. These particles differ from one another in no known properties but their masses, so the Standard Model predicts that when other particles decay into them, the two should each be produced in equal numbers. But this appears not to be true. Interim results from the LHC suggest that a type of particle called a B-meson is more likely to decay into an electron than a muon. That suggests an as-yet-undescribed fundamental force is missing from the Standard Model. Then, on April 7th, Fermilab, Americas biggest particle-physics facility, announced the interim results of its own muon experiment, Muon g-2.

In the quantum world, there is no such thing as a perfect vacuum. Instead, a froth of particles constantly pops in and out of existence everywhere in spacetime. These are virtual rather than real particlesthat is, they are transient fluctuations which emerge straight out of quantum uncertainty. But, although they are short-lived, during the brief periods of their existence they still have time to interact with more permanent sorts of matter. They are, for example, the source of the black-hole radiation predicted by Hawking.

The strengths of their interactions with types of matter more conventional than black holes are predicted by the Standard Model, and to test these predictions, Muon g-2 shoots muons in circles around a powerful superconducting magnetic-storage ring. The quantum froth changes the way the muons wobble, which detectors can pick up with incredible precision. The Muon g-2 experiment suggests that the interactions causing these wobbles are slightly stronger than the Standard Model predicts. If confirmed, this would mean the model is missing one or more elementary particles.

There is a slim chance that these are the absent sparticles. If so, it is the supporters of supersymmetry who will have the last laugh. But nothing points in this direction and, having failed thus far to stand their ideas up, they are keeping sensibly quiet.

Whatever the causes of these two results, they do show that there is something out there which established explanations cannot account for. Similarly unexplained anomalies were starting points for both quantum theory and relativity. It looks possible, therefore, that what has seemed one of physicss darkest periods is about to brighten into a new morning.

This article appeared in the Science & technology section of the print edition under the headline "Bye, bye, little Susy"

Read the original:

Life, the universe and everything Physics seeks the future - The Economist

- Quantum Computing in Silicon Breaks a Crucial Threshold for the First Time - Singularity Hub - January 24th, 2022
- Microsoft Quantum Computing Executive Sees Progress After Lagging Rivals - The Information - January 24th, 2022
- US Army visits Brazil in search of technological partnership for Security and Defense areas - Dialogo-Americas.com - January 24th, 2022
- LG Electronics Joins the IBM Quantum Network - Database Trends and Applications - January 24th, 2022
- Hyperion Research Says 2021 Will Be an Exceptional Growth Year for the Global HPC Market - HPCwire - January 24th, 2022
- MSU Forms Quantum Alliance with Purdue and University of Michigan - HPCwire - January 8th, 2022
- EPFL engineers find promising way to boost computing power of quantum computers. - Science Business - January 8th, 2022
- Power Moves: Meet the newest faces on the DMV's quantum computing scene - Technical.ly DC - January 8th, 2022
- 2022 will be the year of deeptech say investors - Sifted - January 8th, 2022
- Super-Resolution Imaging of a Single Cold Atom on a Nanosecond Timescale - SciTechDaily - January 8th, 2022
- Will Innovation Make Us Better Off? - Forbes - January 8th, 2022
- The tech that will change the dialogue n 2022 - Mint - December 29th, 2021
- Is Taiwan's Five-year Quantum Computing and Talent Initiative the Wrong Strategy for the Island Nation? - OODA Loop - December 22nd, 2021
- 10 technology trends that could prove to be real game-changers - Mint - December 22nd, 2021
- Deep tech in 2022: the future is looking artificially intelligent - Information Age - December 22nd, 2021
- ixFintech Group Limited Announces Launch of ixWallet 2.0 and Plans to Launch New Asset-backed TeaCoin - Business Wire - December 22nd, 2021
- 15 Truly Unbelievable Ways Science Changed the World in 2021 - Fatherly - December 22nd, 2021
- What Is Quantum Computing? | NVIDIA Blog - December 20th, 2021
- What is Quantum Computing? | IBM - December 20th, 2021
- Quantum computing: Now Rigetti explores qutrits as well as qubits - ZDNet - December 20th, 2021
- Research Team Reaches Milestone in Quantum Computing with Error Correction - HPCwire - December 20th, 2021
- Spain-based startup Multiverse Computing receives 12.5M from EIC to bring quantum computing to finance companies - Silicon Canals - December 20th, 2021
- Smart Internet Lab will deliver Quantum Data Centre of the Future - ITP.net - December 20th, 2021
- Which emerging technologies present the greatest opportunities for business? - The Globe and Mail - December 20th, 2021
- The Quantum Moments: Top Quantum Computing Things to Recall from 2021 - Analytics Insight - December 12th, 2021
- Research Fellow, Quantum Hardware Engineer, NQCH, Centre for Quantum Technologies job with NATIONAL UNIVERSITY OF SINGAPORE | 274414 - Times Higher... - December 12th, 2021
- IOG Has Already Started Process of Making Cardano Resistant Against Quantum Attacks - CryptoGlobe - December 12th, 2021
- Microsoft and KPMG will try out quantum algorithms on real-world problems - GeekWire - December 3rd, 2021
- Honeywell Superpositions Itself in the Quantum Computing Industry With New Company Quantinuum - TECHdotMN - December 3rd, 2021
- Where does EU stand in the quantum computing race with China and US? - TechHQ - December 3rd, 2021
- Pistoia Alliance predicts a focus on the fight against antimicrobial resistance and a surge in quantum computing research for 2022 - Bio-IT World - November 25th, 2021
- Atom Computing: A Quantum Computing Startup That Believes It Can Ultimately Win The Qubit Race - Forbes - November 22nd, 2021
- Creating Dynamic Symmetry in Diamond Crystals To Improve Qubits for Quantum Computing - SciTechDaily - November 22nd, 2021
- Don Kahle: Quantum quandaries are emerging - The Register-Guard - November 22nd, 2021
- 4 key threats to the new central bank digital currencies - World Economic Forum - November 22nd, 2021
- IBM creates largest ever superconducting quantum computer - New Scientist - November 15th, 2021
- Quantum computing breakthrough may help us learn about the earliest moments of the universe - TechRadar - November 15th, 2021
- Atos and NVIDIA to Advance Climate and Healthcare Research With Exascale Computing - HPCwire - November 15th, 2021
- IBM Launches Its First Quantum Computing Certification | The Info-Tech Brief - Oakland News Now - November 15th, 2021
- Intel Marks 50th Anniversary of the Intel 4004 - HPCwire - November 15th, 2021
- QCI Qatalyst Selected by BMW Group and Amazon Web Services as a Finalist in the Quantum Computing Challenge - HPCwire - November 11th, 2021
- Will Quantum Computers Burst The Bitcoin Boom? - Forbes - November 10th, 2021
- Quantum Computing Market Research Report by Technology, by Deployment, by Offering, by End-Use, by Application, by Region - Global Forecast to 2026 -... - November 10th, 2021
- Quantum Computing Inc. to Present at the ROTH 10th Annual - GlobeNewswire - November 10th, 2021
- QCI Qatalyst Selected by BMW Group and Amazon Web Services - GlobeNewswire - November 10th, 2021
- Nvidia Declares That It Is A Full-Stack Platform - The Next Platform - November 10th, 2021
- ANET: Add These 3 Soaring Computer Hardware Stocks to Your Watchlist - StockNews.com - November 10th, 2021
- Rigetti and Oxford Instruments Participate and Sponsor The City - Marketscreener.com - November 10th, 2021
- Lost in Space-Time newsletter: Will a twisted universe save cosmology? - New Scientist - November 10th, 2021
- IonQ Is First Quantum Startup to Go Public; Will It be First to Deliver Profits? - HPCwire - November 6th, 2021
- QUANTUM COMPUTING INC. Management's Discussion and Analysis of Financial Condition and Results of Operations, (form 10-Q) - marketscreener.com - November 6th, 2021
- Pasqal named startup of the year by L'Usine Nouvelle - EurekAlert - November 6th, 2021
- Quantum Blockchain Technologies Plc - Update on FPGA and ASIC Development - Yahoo Finance UK - November 6th, 2021
- Quantum Xchange Joins the Hudson Institute's Quantum Alliance Initiative - PRNewswire - November 6th, 2021
- Is This the Right Time for a Cryptography Risk Assessment? - Security Boulevard - November 6th, 2021
- IBM and Raytheon Collaborating on AI, Cryptography, and Quantum Computing - Datamation - October 30th, 2021
- AWS Announces Opening of the AWS Center for Quantum Computing - HPCwire - October 30th, 2021
- China makes a quantum computer streets ahead of the US - Fudzilla - October 30th, 2021
- CyberHive's Gareth Lockwood on how quantum computing changes the rules of threat protection - TechCentral.ie - October 30th, 2021
- Amazon partners with UCLA on science hub focusing on AI and its social impact - Yahoo Finance - October 30th, 2021
- Sumitomo Corporation Quantum Transformation (QX) Project - Quantum Computer Improves Performance of Traffic Control for Flying Cars, One Step Closer... - October 20th, 2021
- 3 CQE members Receive Awards from the American Physical Society - HPCwire - October 20th, 2021
- INSIDE QUANTUM TECHNOLOGY New York, The Largest Business Quantum Technology Conference and Exhibition, Announces Focus on Quantum Safe Initiatives and... - October 20th, 2021
- Incredible Growth of Quantum Computing in Health Care Market by 2028 | D-Wave Solutions, IBM, Google EcoChunk - EcoChunk - October 20th, 2021
- IonQ and University of Maryland Researchers Demonstrate Fault-Tolerant Error Correction, Critical for Unlocking the Full Potential of Quantum... - October 12th, 2021
- Quantum computing startups pull in millions as VCs rush to get ahead of the game - The Register - October 12th, 2021
- Zapata, University of Hull researchers take quantum computing to deep space - FierceElectronics - October 12th, 2021
- IBM and Raytheon Technologies collaborate on AI, cryptography and quantum technologies - Scientific Computing World - October 12th, 2021
- How science and diplomacy inform each other - SWI swissinfo.ch - swissinfo.ch - October 12th, 2021
- Digital Wealth Management Fees to Increase Threefold to $12.6 Billion By 2026 - Yahoo Finance - October 12th, 2021
- Is Neuromorphic Computing The Answer For Autonomous Driving And Personal Robotics? - Forbes - October 12th, 2021
- IonQ is set to make its public trading debut. Here's a look at the quantum computing company's 2021 highlights - Technical.ly DC - October 2nd, 2021
- Connecting the Dots Between Material Properties and Superconducting Qubit Performance - SciTechDaily - October 2nd, 2021
- Quantum Computing in Agriculture Market to Witness Stellar CAGR During the Forecast Period 2021 -2026 - Northwest Diamond Notes - October 2nd, 2021
- What is quantum computing? - September 21st, 2021
- Why quantum computing is a security threat and how to defend against it [Q&A] - BetaNews - September 21st, 2021
- 'This Is The Beginning Of A New Industry': College Park Looks To Quantum Computing To Spark Office Growth - Bisnow - September 21st, 2021
- Prepare for the next phase of digital transformation at The Quantum Computing Summit - UKTN - UKTN (UK Technology News - September 21st, 2021
- A Simple Equation Indicates Wormholes May Be the Key to Quantum Gravity - Interesting Engineering - September 21st, 2021
- Explore Trends and COVID-19 Impact on Quantum Computing Market 2021 Research Report and Industry Forecast till 2027 | Know More Stillwater Current -... - September 21st, 2021