The ‘liquid’ nature of spacetime in quantum gravity models

An illustration of the liquid spacetime concept. Credit: Jason Ralston/Flickr

Se lo spaziotempo fosse un liquido, avrebbe una viscosità bassissima, come i “superfluidi“. Un lavoro che ha visto collaborare la Scuola Internazionale Superiore di Studi Avanzati (SISSA) di Trieste con l’Università Ludwig Maximilian di Monaco ha mostrato come dovrebbero comportarsi gli “atomi” che compongono il fluido dello spaziotempo, secondo alcuni modelli di gravità quantistica. Le considerazioni proposte in questo lavoro impongono vincoli molto stretti al verificarsi di effetti legati a questa eventuale natura “fluida” dello spaziotempo, mostrando che è possibile discriminare tra i modelli di gravità quantistica finora sviluppati al fine di superare la Relatività Generale.

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Using the Universe as ‘tool’ to detect gravitons

La gravità è l’unica tra le quattro forze fondamentali per la quale gli scienziati non hanno ancora rivelato la sua unità fondamentale. Infatti, secondo il modello standard delle particelle elementari si ritiene che l’interazione gravitazionale venga trasmessa attraverso il gravitone, allo stesso modo con cui l’interazione elettromagnetica viene trasmessa dai fotoni. Oggi, nonostante esistano delle basi teoriche a favore dell’esistenza dei gravitoni rimane, però, il problema di rivelarli, almeno sulla Terra dove le possibilità sono estremamente basse se non quasi nulle.

For example, the conventional way of measuring gravitational forces, by bouncing light off a set of mirrors to measure tiny shifts in their separation, would be impossible in the case of gravitons. According to physicist Freeman Dyson, the sensitivity required to detect such a miniscule distance change caused by a graviton requires the mirrors to be so massive and heavy that they’d collapse and form a black hole. Because of this, some have claimed that measuring a single graviton is hopeless. But what if you used the largest entity you know of, in this case the Universe, to search for the telltale effects of gravitons. That is what two physicists are proposing. In the paper, “Using cosmology to establish the quantization of gravity”, published in Physical Review D (Feb. 20, 2014), Lawrence Krauss, a cosmologist at Arizona State University, and Frank Wilczek, a Nobel-prize winning physicist with MIT and ASU, have proposed that measuring minute changes in the cosmic background radiation of the Universe could be a pathway of detecting the telltale effects of gravitons.

Krauss and Wilczek suggest that the existence of gravitons, and the quantum nature of gravity, could be proved through some yet-to-be-detected feature of the early Universe.

“This may provide, if Freeman Dyson is correct about the fact that terrestrial detectors cannot detect gravitons, the only direct empirical verification of the existence of gravitons”, Krauss said. “Moreover, what we find most remarkable is that the Universe acts like a detector that is precisely the type that is impossible or impractical to build on Earth”. It is generally believed that in the first fraction of a second after the Big Bang, the Universe underwent rapid and dramatic growth during a period called “inflation.” If gravitons exist, they would be generated as “quantum fluctuations” during inflation. Ultimately, these would evolve, as the Universe expanded, into classically observable gravitational waves, which stretch space-time along one direction while contracting it along the other direction. This would affect how electromagnetic radiation in the cosmic microwave background (CMB) radiation left behind by the Big Bang is produced, causing it to become polarized. Researchers analyzing results from the European Space Agency’s Planck satellite are searching for this “imprint” of inflation in the polarization of the CMB.

Krauss said his and Wilczek’s paper combines what already is known with some new wrinkles.

“While the realization that gravitational waves are produced by inflation is not new, and the fact that we can calculate their intensity and that this background might be measured in future polarization measurements of the microwave background is not new, an explicit argument that such a measurement will provide, in principle, an unambiguous and direct confirmation that the gravitational field is quantized is new”, he said. “Indeed, it is perhaps the only empirical verification of this very important assumption that we might get in the foreseeable future”. Using a standard analytical tool called dimensional analysis, Wilczek and Krauss show how the generation of gravitational waves during inflation is proportional to the square of Planck’s constant, a numerical factor that only arises in quantum theory. That means that the gravitational process that results in the production of these waves is an inherently quantum-mechanical phenomenon. This implies that finding the fingerprint of gravitational waves in the polarization of CMB will provide evidence that gravitons exist, and it is just a matter of time (and instrument sensitivity) to finding their imprint. “I’m delighted that dimensional analysis, a simple but profound technique whose virtues I preach to students, supplies clear, clean insight into a subject notorious for its difficulty and obscurity”, said Wilczek. “It is quite possible that the next generation of experiments, in the coming decade or maybe even the Planck satellite, may see this background”, Krauss added.

ASU: Researchers propose a new way to detect the elusive graviton

arXiv: Using Cosmology to Establish the Quantization of Gravity

A Planck star instead of singularity inside black holes

E’ una idea proposta da due astrofisici, Carlo Rovelli e Francesca Vidotto, che in un articolo suggeriscono che un oggetto, noto come stella di Planck, possa esistere al centro dei buchi neri, una proposta che eliminerebbe perciò il concetto di singolarità facendo sì che l’informazione possa riemergere in qualche punto dello spazio nel nostro Universo.

The current thinking regarding black holes is that they have two very simple parts, an event horizon and a singularity. Because a probe cannot be sent inside a black hole to see what is truly going on, researchers have to rely on theories. The singularity theory suffers from what has come to be known as the “information paradox“, black holes appear to destroy information, which would seem to violate the rules of general relativity, because they follow rules of quantum mechanics instead. This paradox has left deep thinking physicists such as Stephen Hawking uneasy, so much so that he and others have begun offering alternatives or amendments to existing theories. In this new effort, a pair of physicists suggest the idea of a Planck star. The idea of a Planck star has its origins with an argument to the Big Bang theory, this other idea holds that when the inevitable Big Crunch comes, instead of forming a singularity, something just a little more tangible will result, something on the Planck scale. And when that happens, a bounce will occur, causing the Universe to expand again, and then to collapse again and so on forever back and forth.

Rovelli and Vidotto wonder why this couldn’t be the case with black holes as well, instead of a singularity at its center, there could be a Planck structure, a star, which would allow for general relativity to come back into play.

If this were the case, then a black hole could slowly over time lose mass due to Hawking Radiation, as the black hole contracted, the Planck star inside would grow bigger as information was absorbed. Eventually, the star would meet the event horizon and the black hole would dematerialize in an instant as all the information it had ever sucked in was cast out into the Universe. This new idea by Rovelli and Vidotto will undoubtedly undergo close scrutiny in the astrophysicist community likely culminating in debate amongst those who find the idea of a Planck star an answer to the information paradox and those who find the entire idea implausible.

arXiv: Planck stars

Massive or massless neutrinos?

Gli scienziati avrebbero risolto uno dei problemi aperti dell’attuale modello cosmologico standard combinando i dati del satellite Planck e quelli ottenuti grazie al fenomeno della lente gravitazionale allo scopo di determinare la massa dei neutrini.

The team, from the universities of Nottingham and Manchester, used observations of the Big Bang and the curvature of spacetime to accurately measure the mass of these elementary particles for the first time. The recent Planck spacecraft observations of the Cosmic Microwave Background (CMB), the fading glow of the Big Bang, highlighted a discrepancy between these cosmological results and the predictions from other types of observations. The CMB is the oldest light in the Universe, and its study has allowed scientists to accurately measure cosmological parameters, such as the amount of matter in the Universe and its age. But an inconsistency arises when large-scale structures of the Universe, such as the distribution of galaxies, are observed. Dr Adam Moss, from The University of Nottingham’s School of Physics and Astronomy said: “We observe fewer galaxy clusters than we would expect from the Planck results and there is a weaker signal from gravitational lensing of galaxies than the CMB would suggest. A possible way of resolving this discrepancy is for neutrinos to have mass. The effect of these massive neutrinos would be to suppress the growth of dense structures that lead to the formation of clusters of galaxies.” Neutrinos interact very weakly with matter and so are extremely hard to study. They were originally thought to be massless but particle physics experiments have shown that neutrinos do indeed have mass and that there are several types, known as flavours by particle physicists. The sum of the masses of these different types has previously been suggested to lie above 0.06 eV (much less than a billionth of the mass of a proton). Dr Moss and Professor Richard Battye from The University of Manchester have combined the data from Planck with gravitational lensing observations in which images of galaxies are warped by the curvature of spacetime.

They conclude that the current discrepancies can be resolved if massive neutrinos are included in the standard cosmological model.

They estimate that the sum of masses of neutrinos is 0.320 +/- 0.081 eV (assuming active neutrinos with three flavours). Professor Battye added: “If this result is borne out by further analysis, it not only adds significantly to our understanding of the sub-atomic world studied by particle physicists, but it would also be an important extension to the standard model of cosmology which has been developed over the last decade”.

Nottingham University: Massive neutrinos solve a cosmological conundrum

arXiv: Evidence for massive neutrinos from CMB and lensing observations

How to ‘listen to’ the birth of black holes?

La partecipazione dell’Australia alla potenziale scoperta delle onde gravitazionali, e quindi alla capacità di “ascoltare” la nascita di un buco nero, riceverà oggi una accelerata. Questo è il giorno in cui i fisici di tutto il continenti australiano si incontreranno all’Australian International Gravitational Research Centre presso Gingin, quasi 100 Km da Perth. L’obiettivo del meeting è quello di lanciare una missione a livello nazionale che abbia lo scopo di espandere la partecipazione dell’Australia ai progetti americani ed europei unendosi così alla ricerca delle elusive perturbazioni dello spaziotempo.

Gravitational waves are ripples in the curvature of spacetime. They are thought to mark the beginning of time at the Big Bang and the end of time as black holes are born. They are generated by extreme cosmic events such as colliding stars and supernova explosions. Theory predicts that they carry vast amounts of energy at the speed of light. While their power can exceed the power of all the stars in the Universe, their effects are miniscule and difficult to detect. Centre Director, The University of Western Australia’s Winthrop Professor David Blair, said 1000 physicists around the world are currently involved in the search which is focused on the commissioning of three enormous supersensitive detectors that will start operating within the next few years in the USA and Europe, with another under construction in Japan. “The expected step in sensitivity will extend their reach tenfold and increase the number of expected signals 1000-fold“, he said. Professor Peter Veitch, Chair of the Australian Consortium for Gravitational Astronomy, said: “The new advanced detectors change the whole game. For the first time we have firm predictions: both the strength and the number of signals. No longer are we hoping for rare and unknown events. We will be monitoring a significant volume of the Universe and for the first time we can be confident that we will ‘listen’ to the coalescence of binary neutron star systems and the formation of black holes. Once these detectors reach full sensitivity we should hear signals almost once a week“. Data from the detectors will be used in conjunction with optical telescopes that will search the sky for visible signs of the catastrophic events signaled by the gravitational waves. Australia is contributing two telescopes to the search: the Zadko telescope at Gingin and the Skymapper telescope at Coonabarrabran in New South Wales. The data from the detectors will be distributed to data analysis teams in many countries. The Australian data analysis team has developed special techniques for digging signals out of the unavoidable noise in the detectors, plus special techniques that use graphics processing units for detecting signals the instant they occur (instead of traditional techniques which can take minutes or hours to identify signals). This fast detection method is especially important if optical telescopes are going to be able to locate distant explosions the moment they occur.

One of the most exciting sources is expected to be the coalescence of pairs of neutron stars to form a black hole, giving out a burst of gamma rays and a flash of light that astronomers call a kilonova.

In this project the Pawsey Centre supercomputers will be equipped with ‘search pipelines’ developed at ANU, Melbourne and UWA. These are massive computer codes designed to separate signals from the noise. Each pipeline is optimised for a specific type of signal, such as the chirps expected as neutron stars spiral together and black holes form. Using these codes, Australian students will be able to play a major role in the first discovery of gravitational waves. The project will be launched at the Gravity Discovery Centre (GDC) by the Chair of GDC Fred Deshon, the Chair of the Gravitational Wave Observatory Development Committee Jens Balkau and the Chair of the Australian Consortium for Gravitational Astronomy Peter Veitch. The GDC which also includes the Gingin Observatory shares the Gingin site with the Australian International Gravitational Research Centre and provides public education on the big questions of the Universe.

UWA: Australian scientists to ‘listen’ to the formation of black holes

Is our Universe a hologram?

E’ circolata di recente nei media la notizia pubblicata da Nature secondo la quale un gruppo di fisici giapponesi avrebbero formulato una teoria che “potrebbe essere considerata l’evidenza più chiara sul fatto che il nostro Universo sarebbe una gigantesca proiezione“. Nei loro articoli, Yoshifumi Hyakutake e colleghi della Ibaraki University in Giappone spiegano  come la loro idea suggerisca che la realtà fisica, così come noi la concepiamo, potrebbe essere in definitiva un ologramma appartenente ad un altro spazio bidimensionale.

In 1997, theoretical physicist Juan Maldacena proposed that an audacious model of the Universe in which gravity arises from infinitesimally thin, vibrating strings could be reinterpreted in terms of well-established physics. The mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity. Maldacena’s idea thrilled physicists because it offered a way to put the popular but still unproven theory of strings on solid footing, and because it solved apparent inconsistencies between quantum physics and Einstein’s theory of gravity. It provided physicists with a mathematical “Rosetta stone”, a ‘duality’, that allowed them to translate back and forth between the two languages, and solve problems in one model that seemed intractable in the other and vice versa (see ‘Collaborative physics: String theory finds a bench mate‘). But although the validity of Maldacena’s ideas has pretty much been taken for granted ever since, a rigorous proof has been elusive.

In two papers posted on the arXiv repository, Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues now provide, if not an actual proof, at least compelling evidence that Maldacena’s conjecture is true.

In one paper, Hyakutake computes the internal energy of a black hole, the position of its event horizon (the boundary between the black hole and the rest of the Universe), its entropy and other properties based on the predictions of string theory as well as the effects of so-called virtual particles that continuously pop into and out of existence (see ‘Astrophysics: Fire in the Hole!‘). In the other, he and his collaborators calculate the internal energy of the corresponding lower-dimensional cosmos with no gravity. The two computer calculations match. “It seems to be a correct computation”, says Maldacena, who is now at the Institute for Advanced Study in Princeton, New Jersey and who did not contribute to the team’s work.

The findings “are an interesting way to test many ideas in quantum gravity and string theory”, Maldacena adds.

The two papers, he notes, are the culmination of a series of articles contributed by the Japanese team over the past few years. “The whole sequence of papers is very nice because it tests the dual [nature of the universes] in regimes where there are no analytic tests. They have numerically confirmed, perhaps for the first time, something we were fairly sure had to be true, but was still a conjecture, namely that the thermodynamics of certain black holes can be reproduced from a lower-dimensional Universe”, says Leonard Susskind, a theoretical physicist at Stanford University in California who was among the first theoreticians to explore the idea of holographic universes. Neither of the model universes explored by the Japanese team resembles our own, Maldacena notes. The cosmos with a black hole has ten dimensions, with eight of them forming an eight-dimensional sphere. The lower-dimensional, gravity-free one has but a single dimension, and its menagerie of quantum particles resembles a group of idealized springs, or harmonic oscillators, attached to one another. Nevertheless, says Maldacena, the numerical proof that these two seemingly disparate worlds are actually identical gives hope that the gravitational properties of our Universe can one day be explained by a simpler cosmos purely in terms of quantum theory.

Nature: Simulations back up theory that Universe is a hologram

arXiv: Quantum Near Horizon Geometry of Black 0-Brane

arXiv: Holographic description of quantum black hole on a computer

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Towards understanding the Big Bang

Le leggi della fisica non sono in grado di descrivere cosa accadde durante il Big Bang. Infatti, sia la teoria dei quanti che la relatività generale non permettono di spiegare lo stato fisico singolare, infinitamente denso e caldo che caratterizzava le fasi iniziali della storia dell’Universo. Forse un giorno, la formulazione di una teoria che permetta di descrivere la gravità su scale quantistiche potrebbe fornirci una risposta (vedasi Idee sull’Universo). Oggi, alcuni scienziati del  Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) a Golm/Potsdam e del Perimeter Institute in Canada hanno fatto una scoperta importante in questo contesto. La loro idea è quella di assumere che lo spazio consista di piccolissime unità chiamate “mattoni fondamentali”. Partendo da questo concetto, gli scienziati arrivano alla formulazione delle equazioni più importanti della cosmologia, e cioè le equazioni di Friedmann, che permettono di descrivere l’Universo. Il risultato è che questo processo mostra, in definitiva, che la meccanica quantistica e la relatività possono essere effettivamente unificate.

For almost a century, the two major theories of physics have coexisted but have been irreconcilable: while Einstein’s General Theory of Relativity describes gravity and thus the world at large, quantum physics describes the world of atoms and elementary particles. Both theories work extremely well within their own boundaries; however, they break down, as currently formulated, in certain extreme regions, at extremely tiny distances, the so-called Planck scale, for example. Space and time thus have no meaning in black holes or, most notably, during the Big Bang. Daniele Oriti from the Albert Einstein Institute uses a fluid to illustrate this situation: “We can describe the behaviour of flowing water with the long-known classical theory of hydrodynamics. But if we advance to smaller and smaller scales and eventually come across individual atoms, it no longer applies. Then we need quantum physics“. Just as a liquid consists of atoms, Oriti imagines space to be made up of tiny cells or “atoms of space”, and a new theory is required to describe them: quantum gravity.

In Einstein’s relativity theory, space is a continuum. Oriti now breaks down this space into tiny elementary cells and applies the principles of quantum physics to them, thus to space itself and to the theory of relativity describing it. This is the unification idea.

A fundamental problem of all approaches to quantum gravity consists in bridging the huge dimensional scales from the space atoms to the dimensions of the Universe. This is where Oriti, his colleague Lorenzo Sindoni and Steffen Gielen, a former postdoc at the AEI who is now a researcher at the Perimeter Institute in Canada, have succeeded. Their approach is based on so-called group field theory. This is closely related to loop quantum gravity, which the AEI has been developing for some time. The task now consisted in describing how the space of the Universe evolves from the elementary cells. Staying with the idea of fluids: How can the hydrodynamics for the flowing water be derived from a theory for the atoms? This extremely demanding mathematical task recently led to a surprising success. “Under special assumptions, space is created from these building blocks, and evolves like an expanding Universe“, explains Oriti. “For the first time, we were thus able to derive the Friedmann equation directly as part of our complete theory of the structure of space“, he adds. This fundamental equation, which describes the expanding Universe, was derived by the Russian mathematician Alexander Friedmann in the 1920s on the basis of the General Theory of Relativity. The scientists have therefore succeeded in bridging the gap from the microworld to the macroworld, and thus from quantum mechanics to the General Theory of Relativity: they show that space emerges as the condensate of these elementary cells and evolves into a Universe which resembles our own. Oriti and his colleagues thus see themselves at the start of a difficult but promising journey. Their current solution is valid only for a homogeneous Universe, but our real world is much more complex. It contains inhomogeneities, such as planets, stars and galaxies. The physicists are currently working on including them in their theory. And they have planned something really big as their ultimate goal.

On the one hand, they want to investigate whether it is possible to describe space even during the Big Bang.

A few years ago, former AEI researcher Martin Bojowald found clues, as part of a simplified version of loop quantum gravity, that time and space can possibly be traced back through the Big Bang. With their theory, Oriti and his colleagues are hoping to confirm or improve this result. If it continues to prove successful, the researchers could perhaps use it to explain also the assumed inflationary expansion of the Universe shortly after the Big Bang as well, and the nature of the mysterious dark energy. This energy field causes the Universe to expand at an ever-increasing rate. Oriti’s colleague Lorenzo Sindoni therefore adds: “We will only be able to really understand the evolution of the Universe when we have a theory of quantum gravity“. The AEI researchers are in good company here: Einstein and his successors, who have been searching for this for almost one hundred years.

Max Planck Institute: Quantum steps towards the Big Bang

arXiv: Cosmology from Group Field Theory Formalism for Quantum Gravity

Understanding complexity in the early Universe with simpler models

Il comportamento statistico di alcuni sistemi complessi, come ad esempio l’Universo primordiale, può essere analizzato se viene ridotto ad un insieme di sistemi più semplici. Oggi due fisici, Petr Jizba della Czech Technical University a Praga e Fabio Scardigli ora alla Kyoto University in Giappone, hanno pubblicato i risultati di un lavoro di ricerca che riguarda le previsioni teoriche del comportamento dinamico di tali sistemi cosmologici.

Their work focuses on complex dynamical systems whose statistical behaviour can be explained in terms of a superposition of simpler underlying dynamics.

They found that the combination of two cornerstones of contemporary physics, namely Einstein’s special relativity and quantum-mechanical dynamics, is mathematically identical to a complex dynamical system described by two interlocked processes operating at different energy scales.

The combined dynamic obeys Einstein’s special relativity even though neither of the two underlying dynamics does. This implies that Einstein’s special relativity might well be an emergent concept and suggests that it would be worthwhile to further develop Einstein’s insights to take into account the quantum structure of space and time (post). To model the double process in question, the authors consider quantum mechanical dynamics in a background space consisting of a number of small crystal-like domains varying in size and composition, known as polycrystalline space. There, particles exhibit an analogous motion to pollen grains in water, referred to as Brownian motion. The observed relativistic dynamics then comes solely from a particular grain distribution in the polycrystalline space. In the cosmological context such distribution might form during the early Universe’s formation. Finally, the authors’ new interpretation focuses on the interaction of a quantum particle with gravity, that, according to Einstein’s general relativity, can be understood as propagation in curved space-time. The non-existence of the relativistic dynamics on the basic level of the description leads to a natural mechanism for the formation of asymmetry between particles and anti-particles. When coupled with an inflationary cosmology, the authors’ approach predicts that a charge asymmetry should have been produced at ultra-minute fractions of seconds after the Big Bang. This prediction is in agreement with constraints born out of recent cosmological observations.

EPJ: Removing complexity layers from the universe’s creation
EPJ C: Special relativity induced by granular space

20th International Conference on General Relativity and Gravitation (GR20) and the 10th Amaldi Conference on Gravitational Waves (Amaldi10)

The 20th  International Conference on General Relativity and Gravitation (GR20)  and the 10th Amaldi Conference on Gravitational Waves (Amaldi10) will  take place from 7th – 13th July 2013 at Uniwersytet Warszawski, Warsaw, Poland. GR20  is the latest in the series of triennial international conferences held under the auspices of the International Society on General Relativity and Gravitation. This conference series constitutes the principal international meetings for scientists working in all the areas of relativity and gravitation. The Amaldi conferences are held under the auspices of the Gravitational Wave International Committee. Since 1997, they have been held every two years and are regarded as the most important international conferences for the gravitational wave detection community. This time, in Warsaw,   GR20  and Amaldi10 are organized as a joint event.

The program of the conference, among many topics,  includes:  Planck Results,  Dark Energy,  Formation of the Trapped Surfaces, Dynamics of Asymptotically AdS spacetimes,  Gravity and Condensed Matter Correspondence,  Numerical Relativity and Its Applications to Astrophysics and High Energy Physics, Neutron Stars, Formation of Supermassive Black Holes, Modified Gravity as Alternatives to Dark Energy or Dark Matter,  Cold Atoms for Equivalence Principle Tests and GW Detection, Quantum Fields in Curved Space-time, Higher-Dimensional Spacetimes, Loop Quantum Gravity, Strings and Branes.  

Why nature is quantum?

Sappiamo che gli scienziati sono come dei bambini perchè si pongono sempre la domanda sul ‘perché’ delle cose. Tra queste, una domanda a cui si deve ancora rispondere riguarda come mai la natura si comporta secondo le leggi della fisica quantistica. Oggi, due ricercatori, Corsin Pfister e Stephanie Wehner del Centre for Quantum Technologies presso la National University di Singapore stanno cercando di affrontare la questione in un articolo pubblicato su Nature Communications.

We know that things that follow quantum rules, such as atoms, electrons or the photons that make up light, are full of surprises. They can exist in more than one place at once, for instance, or exist in a shared state where the properties of two particles show what Einstein called “spooky action at a distance“, no matter what their physical separation. Because such things have been confirmed in experiments, researchers are confident the theory is right. But it would still be easier to swallow if it could be shown that quantum physics itself sprang from intuitive underlying principles. One way to approach this problem is to imagine all the theories one could possibly come up with to describe nature, and then work out what principles help to single out quantum physics. A good start is to assume that information follows Einstein’s special relativity and cannot travel faster than light. However, this alone isn’t enough to define quantum physics as the only way nature might behave. Corsin and Stephanie think they have come across a new useful principle. “We have found a principle that is very good at ruling out other theories“, says Corsin. In short, the principle to be assumed is that if a measurement yields no information, then the system being measured has not been disturbed. Quantum physicists accept that gaining information from quantum systems causes disturbance. Corsin and Stephanie suggest that in a sensible world the reverse should be true, too. If you learn nothing from measuring a system, then you can’t have disturbed it. Consider the famous Schrodinger’s cat paradox, a thought experiment in which a cat in a box simultaneously exists in two states (this is known as a ‘quantum superposition’). According to quantum theory it is possible that the cat is both dead and alive until the cat’s state of health is ‘measured’ by opening the box. When the box is opened, allowing the health of the cat to be measured, the superposition collapses and the cat ends up definitively dead or alive. The measurement has disturbed the cat. This is a property of quantum systems in general. Perform a measurement for which you can’t know the outcome in advance, and the system changes to match the outcome you get. What happens if you look a second time? The researchers assume the system is not evolving in time or affected by any outside influence, which means the quantum state stays collapsed. You would then expect the second measurement to yield the same result as the first. After all, “If you look into the box and find a dead cat, you don’t expect to look again later and find the cat has been resurrected“, says Stephanie. “You could say we’ve formalised the principle of accepting the facts“. Corsin and Stephanie show that this principle rules out various theories of nature. They note particularly that a class of theories they call ‘discrete’ are incompatible with the principle. These theories hold that quantum particles can take up only a finite number of states, rather than choose from an infinite, continuous range of possibilities. The possibility of such a discrete ‘state space’ has been linked to quantum gravitational theories proposing similar discreteness in spacetime, where the fabric of the Universe is made up of tiny brick-like elements rather than being a smooth, continuous sheet. As is often the case in research, Corsin and Stephanie reached this point having set out to solve an entirely different problem altogether. Corsin was trying to find a general way to describe the effects of measurements on states, a problem that he found impossible to solve. In an attempt to make progress, he wrote down features that a ‘sensible’ answer should have. This property of information gain versus disturbance was on the list. He then noticed that if he imposed the property as a principle, some theories would fail. Corsin and Stephanie are keen to point out it’s still not the whole answer to the big ‘why’ question: theories other than quantum physics, including classical physics, are compatible with the principle. But as researchers compile lists of principles that each rule out some theories to reach a set that singles out quantum physics, the principle of information gain versus disturbance seems like a good one to include.

See also: Is nature – deep down – actually discrete?

CQT: New principle may help explain why nature is quantum

arXiv: If no information gain implies no disturbance, then any discrete physical theory is classical