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Showing posts with label elementary particles. Show all posts
Showing posts with label elementary particles. Show all posts
The quantum mechanical entanglement of particles plays an important role in many technical applications. To date, however, the effect has been difficult to measure experimentally.
Physicists from the Technical University of Munich (TUM), the University of Innsbruck and the Institute of Photonic Sciences (ICFO) in Barcelona have now developed a new protocol to detect entanglement of many-particle quantum states using established measuring methods.
In quantum theory, interactions between particles create fascinating correlations known as entanglement. They cannot be explained by any means known to the classical world.
Entanglement is a consequence of the probabilistic rules of quantum mechanics and seems to permit a peculiar instantaneous connection between particles over long distances that defies the laws of our macroscopic world – a phenomenon that Einstein referred to as “spooky action at a distance.”
Developing protocols to detect and quantify entanglement of many-particle quantum states is a key challenge for current experiments because entanglement becomes very difficult to study when many particles are involved. “We are able to control smaller particle ensembles well, where we can measure entanglement in a relatively straight forward way,” says quantum physicist Philipp Hauke. However, “when we are dealing with a large system of entangled particles, this measurement is extremely complex or rather impossible because the resources required scale exponentially with the system size.
”Markus Heyl from the Technical University of Munich, Philipp Hauke and Peter Zoller from the Department of Theoretical Physics at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences in collaboration with Luca Tagliacozzo from the Institute of Photonic Sciences in Barcelona (Spain) have found a new way to detect certain properties of many-particle entanglement independent of the size of the system and by using standard measurement tools.
Entanglement measurable via susceptibility
“When dealing with more complex systems, scientists had to carry out a large number of measurements to detect and quantify entanglement between many particles,” says Philipp Hauke. “Our protocol avoids this problem and can also be used for determining entanglement in macroscopic objects, which was nearly impossible until now.”
Using this new method, physicists can employ tools already well established experimentally. In their study published in Nature Physics the team of researchers gives explicit examples to demonstrate its framework: The entanglement of many-particle systems trapped in optical lattices can be determined using laser spectroscopy while the well-established technique of neutron scattering is utilized for measuring entanglement in solid-state systems.
The physicists successfully demonstrated that the quantum Fisher information, which can provide reliable proof for genuine multipartite entanglement, is in fact measurable. The researchers emphasize that entanglement can be detected by measuring the dynamic response of a system to a perturbation, which can be determined by comparing individual measurements.
“For example, when we move a sample through a time-dependent magnetic field, we can determine the system’s susceptibility towards the magnetic field through the measurement data and thereby detect and quantify internal entanglement,” explains Hauke.
Manifold applications
Quantum metrology, i.e. measurement techniques with increased precision exploiting quantum mechanics, is not the only important field of application of this protocol. It will also provide new perspectives for quantum simulations, where quantum entanglement is used as a resource for studying properties of quantum systems.
In solid-state physics, the protocol may be employed to investigate the role of entanglement in many-body systems, thereby providing a deeper understanding of quantum matter. The research work was supported by the European Community, the European Research Council (ERC), the Austrian Science Fund, the Spanish Government and the German National Academy of Sciences Leopoldina.
Publication:
Measuring multipartite entanglement via dynamic susceptibilities. Philipp Hauke, Markus Heyl, Luca Tagliacozzo, Peter Zoller. Advanced Online Publication, Nature Physics, on 21 March 2016. - DOI: 10.1038/nphys3700
The new particle is the first tetraquark to contain four quarks of different "flavors." | Photo by Fermilab
Research led by Indiana University physicist Daria Zieminska has resulted in the first detection of a new form of elementary particle: the "four-flavored" tetraquark.
Zieminska, a senior scientist in the IU Bloomington College of Arts and Sciences' Department of Physics, is a lead member of the team responsible for the particle's detection by the DZero Collaboration at the U.S. Department of Energy's Fermi National Laboratory, which announced the discovery Feb. 25.
She also delivered the first scientific seminar on the particle and is an author on a paper submitted to Physics Review Letters, the premier journal in physics, describing the tetraquarks' observation.
"For most of the history of quarks, it's seemed that all particles were made of either a quark and an antiquark, or three quarks; this new particle is unique -- a strange, charged beauty," said Zieminska, who has been a member of the DZero experiment since the project's establishment in 1985. "It's the birth of a new paradigm. Particles made of four quarks -- specifically, two quarks and two antiquarks -- is a big change in our view of elementary particles."
The results could also affect scientists' understanding of "quark matter," the hot, dense material that existed moments after the Big Bang, and which may still exist in the super-dense interior of neutron stars.
Quarks are the building blocks that form subatomic particles, the most familiar of which are protons and neutrons, each composed of three quarks. There are six types, or "flavors," of quarks: up, down, strange, charm, bottom and top. Each of these also has an antimatter counterpart.
A tetraquark is a group of four quarks, the first evidence for which was recorded by scientists on the Belle experiment in Japan in 2008. But the new tetraquark is the first quark quartet to contain four different quark flavors: up, down, strange and bottom.
Currently, Zieminska leads the "heavy flavor" group of the DZero experiment, which encompasses the study of all particles containing one or more "heavy quarks," including the new tetraquark, dubbed X(5568) for its mass of 5568 Megaelectronvolts, roughly 5.5 times the mass of a proton. The DZero experiment is led by Dmitri Denisov, a staff scientist at the U.S. Department of Energy's Fermilab.
"Daria was the lead person on the tetraquark observation and performed calculations, cross-checking and other work required to answer the hundreds of questions of the rest of the team," said Denisov, co-spokesman for the DZero experiment. "She was an active participant in the design and construction of the experiment and in the collection of the data."
A chart compares mesons, composed of two quarks; baryons, composed of three quarks; and the lesser understood tetraquark, composed of four quarks. | Photo by Fermilab
The DZero experiment is also responsible for other fundamental physics discoveries, including the first observation, with the Collider-Detector at Fermilab experiment, of the elusive Higgs boson particle decaying into bottom quarks.
Other IU scientists engaged in the DZero project include the late Andrzej Zieminski, former professor of physics at IU Bloomington, who also joined the project in 1985, and Rick Van Kooten, IU vice provost for research, who joined in 2002 during "phase 2" of the project, which involved upgrades to the detector partially constructed at IU. Hal Evans, professor, and Sabine Lammers, associate professor, both at IU, also contributed to the upgraded detector.
DZero is one of two experiments collecting data from Fermilab's Tevatron proton-antiproton collider, once the most powerful particle accelerator in the world, officially retired in 2011. Zieminska and colleagues uncovered the existence of X(5568) based on analysis of billions of previously recorded events from these collisions.
As with other discoveries in physics, Zieminska said the new tetraquark’s discovery was a surprise. Alexey Drutskoy, a colleague at Russia's National Research Nuclear University, spotted indications of the tetraquark signal in summer 2015, after which Zieminska joined him in the hunt. Only after performing multiple cross-checks, in collaboration with Alexey Popov, another Russian colleague, did the team confirm they were observing evidence for a new particle.
Although nothing in nature forbids the formation of a tetraquark, four-quark states are rare and not nearly as well understood as two- and three-quark states. Zieminska and colleagues plan to deepen their understanding of the tetraquark by measuring various properties of the particle, such as the ways it decays or how much it spins on its axis.
The discovery of the tetraquark also comes on the heels of the first observation of a pentaquark -- a five-quark particle -- announced last year by CERN's LHCb experiment at the Large Hadron Collider.
Zieminska is also a member of the ATLAS Experiment at CERN, the European Organization for Nuclear Research.
A total of 75 institutions from 18 countries are members of the DZero Collaboration.
Researchers have now created and photographed synthetic magnetic monopoles under laboratory conditions. These observations lay the foundation for the underlying structure of the natural magnetic monopole – the detection of which would be a revolutionary event comparable to the discovery of the electron. The results were recently published in Nature magazine.
Although predicted over 80 years ago, the fundamentally quantum-mechanical configuration of the monopoles has not previously been observed in any physical system. The reported results demonstrate the structure in an ultracold atomic gas.
“Our achievement opens up amazing avenues for quantum research. It feels incredible to have been a part of such a major breakthrough,” says a delighted Dr. Mikko Möttönen from Aalto University, Finland.
Evidence for magnetic monopoles has been sought in sources as diverse as lunar samples and ancient micas. The multibillion-euro LHC particle accelerator at CERN has also been used in the search – but no magnetic monopoles have been convincingly identified. The discovery of the synthetic monopole provides a stronger foundation for these efforts.
“The creation of a synthetic magnetic monopole should provide us with unprecedented insight into aspects of the natural monopole,” says Prof. David S. Hall from Amherst College, USA. “It's not every day that you get to poke and prod the analogue of an elusive fundamental particle under highly controlled conditions in the laboratory.”, he continues.
“Synthesis of the monopole is the starting point for many new breakthroughs in quantum physics research. In the future, we want to get even a more complete correspondence with the natural magnetic monopole.”, says Dr. Möttönen.
A magnetic monopole is a particle just like an electron, but with a magnetic rather than an electric charge. Some 80 years ago Paul A. M. Dirac, one of the founders of quantum physics, discovered a quantum-mechanical structure allowing the existence of magnetic monopoles. Dirac’s original framework has now been experimentally realized for the first time.
Figure caption. Artistic illustration of the synthetic magnetic monopole (Heikka Valja)
Further information
Mikko Möttönen, docent, Dr. Tech. Aalto University Department of Applied Physics and O. V. Lounasmaa Laboratory QCD Labs mikko.mottonen*at*aalto.fi tel. +358 50 594 0950 http://physics.aalto.fi/qcd//
Mikko Möttönen is the leader of the theoretical and computational part of the research. Theoretical ideas and computational modelling was very important for the success of the creation of the monopole. The modelling was carried out using the supercomputers at CSC — IT Center for Science Ltd.
David S. Hall is the leader of the experimental part of the research. The synthetic magnetic monopoles were created in the Physics Laboratories at Amherst College, United States of America.
“A magnetic monopole is an isolated magnetic pole, magnetic charge, and a point-like source of magnetic field.”
An electron is a point-like particle that carries a so-called elementary electric charge. This means that an electron is an isolated source of an electric field.
Can a magnetic field have a similar point-like source?
Every one of us has likely held two bar magnets and noticed that their ends either attract or repel one another. The ends of the magnet are referred to as poles and every magnet has one end that is a north pole and one that is a south pole. A magnetic north pole attracts a magnetic south pole, but repels another north pole. In general, opposite poles attract, and identical poles repel. In this respect, magnetism is very much like electricity, which exhibits the same attractive and repulsive behavior involving positive and negative electric charges.
When a bar magnet breaks, two smaller bar magnets are created, each with its own north and south pole. You can break each of these smaller magnets in two, and so on, and every resulting magnet has a north pole and a south pole. Even at the atomic level, north and south poles always appear together. One cannot produce in this way a solitary pole, or monopole, that acts as a single point source of the magnetic field.
Are there other ways to find magnetic monopoles?
As yet, not a single natural magnetic monopole has been verifiably observed. This was initially considered to be a problem, because theoretical models that described the post-Big-Bang period predicted that they should be quite common. However, a special model for the expansion of the universe was developed that can explain the extreme rarity of these particles.
According to some theories, the energy content (mass) of a single magnetic monopole is so large that if it were completely used to recharge the battery of an electric car, this vehicle would be able to travel for kilometres with the energy. This explains why magnetic monopoles are probably not likely to occur in a particle accelerator. If the mass of a magnetic monopole really is that large, the energy released from the collision of a negatively and positively charged monopole would be as much as the energy released in the explosion of a kilogram of dynamite!
Dirac monopole
“A Dirac monopole is a point-like source of a possibly artificial magnetic field that forms at the endpoint of a quantum whirlpool.”
In quantum mechanics, an electron is described by a diffuse wave-like object rather than a point-like particle. Paul Dirac was the first person to understand the importance of studying the end points of quantum-mechanical whirlpools within these electron waves. He noticed that when an electron has such a terminating vortex, a magnetic monopole inevitably forms at the end point. A terminating vortex is the defining characteristic of the Dirac monopole.
Dirac also noticed that if the universe contains even a single magnetic monopole, it specifies the smallest possible value for an electric charge. All observed charges must be integer multiples of this minimum value; in other words, charge must be quantized. The existence of a monopole would therefore explain the experimental observation that electric charge is quantized.
Dirac monopoles are generally analyzed in a fairly simple quantum-mechanical model. Magnetic monopoles have since been studied in more general, so-called unified field theories, in which they could exist in the absence of a terminating vortex.
Synthetic magnetic field
“A synthetic magnetic field is an artificial field that leads to particle dynamics equivalent to those of an electric charge in a corresponding natural magnetic field.”
Electrons are not the only physical systems that can exhibit terminating vortices. Thus a Dirac monopole can also appear in other systems, such as the Bose-Einstein condensate. Rather than being related to the natural magnetic field, this monopole can be associated with a synthetic magnetic field. Importantly, the structure of the monopole is identical to that of a Dirac magnetic monopole. This is why the Dirac monopole observed in the synthetic magnetic field is closer to a natural magnetic monopole than any earlier observation.
Spin
“Roughly speaking, spin indicates how fast a particle is spinning around its own axis, and the orientation of that axis.”
Spin is a magnetic property of many particles, including electrons, protons, neutrons, and even many types of atoms. For example, the electron spin is composed of two basis states: up or down. This describes whether the electron is spinning around its axis in a clockwise or counter-clockwise direction.
A particle with a non-zero spin creates a magnetic field around it. However, this is not a monopole field – it is a so-called dipole field with both north and south magnetic poles, just like a bar magnet. Even this smallest of bar magnets cannot be broken into two separate magnetic monopoles.
In fact, bar magnets are composed of countless numbers of small spin dipoles, nearly all of which point in the same direction. Overlapping poles of different sign cancel out the field of each other, and thus the field of an ideal bar magnet looks as if it has magnetic poles only at its ends.
Spins tend to align along an externally applied magnetic field, which is the key to the creation of the synthetic magnetic monopole.
Synthesis of a monopole
“A monopole is created in a Bose–Einstein condensate by using an external magnetic field to guide the spins of the atoms forming the condensate.”
In 2009, Aalto University researchers Ville Pietilä and Mikko Möttönen published theoretical results demonstrating a method to create Dirac monopoles in a Bose–Einstein condensate. The idea involves using external magnetic fields to rotate the atomic spins. A Dirac monopole forms in the condensate as a result of the spin rotation. This method was adopted by the researchers in creating the synthetic magnetic monopole.
The Dirac monopole forms in the artificial magnetic field of the condensate, not in the physical magnetic field which steers the spin degree of freedom. Thus, a natural magnetic monopole is not needed to create the synthetic monopole.
Caption: Synthesis of a monopole in time, starting from panel a and ending with panel c. The arrows show the direction of the physical magnetic field produced in the laboratory. This magnetic field also directs the internal spin degree of freedom of the Bose–Einstein condensate in the direction of the arrows. The end result is that the condensate begins to move as if it were electrically charged and affected by a magnetic monopole in the position marked by the black circle in the image. Click for the full-resolution image.
The Bose–Einstein condensate
“A Bose–Einstein condensate behaves like a single giant atom, even though it can contain millions.”
A Bose–Einstein condensate is sometimes considered to be the fifth state of matter, in addition to solid, liquid, gas, and plasma. In the condensate, the importance and location of individual atoms becomes vague and the system behaves as if it were a single large atom. The first Bose–Einstein condensates were achieved in 1995, and this work received the Nobel Prize in 2001.
“Bose–Einstein condensates provide a window from our world into the quantum wonderland. The more often I peek at it, the more I want to stay there,” says enchanted Dr. Möttönen.
Since Bose–Einstein condensates contain many atoms, photographs of them can be taken using technology that is in part similar to that used in ordinary digital cameras. In addition, the condensates can be forced into the desired shape by means of external magnetic fields and laser beams. These properties make condensates a unique tool for developing new phenomena and quantum technologies. In addition to being used with magnetic monopoles, condensates can simulate the properties of various useful materials to the accuracy of a single atom. One of the daydreams of condensate researchers involves finding a solution for the development of superconducting materials that function at room temperature.
What in the world is quantum physics?
“Quantum physics describes natural phenomena most accurately.”
Quantum physics (also quantum mechanics) is a theory developed over the past 100 years that has been observed to describe the reality in more detail than any other model. It is particularly useful for explaining atomic-level phenomena, which is impossible using classical physics. On the other hand, quantum physics reproduces the same results as classical physics on the large scale.
In quantum mechanics, an electron can take on wave-like properties, sometimes appearing as an extended object rather than a point particle. It is this property of extension, which is shared with Bose-Einstein condensates, that permits the observation of the quantum whirlpools essential to detecting the effect of the magnetic monopole.
Quantum technologies use the laws of quantum physics relieved from classical restrictions to produce practical applications. For example, development of a quantum computer – a potentially super-fast problem solver – is one of the key goals of quantum technologies. A quantum computer would be able to find a solution to certain problems very quickly by using methods that are impossible in the logical framework of a normal computer.
“The laws of quantum physics make it possible to take shortcuts. Among other things, this is the basis of the super-fast speed of a quantum computer,” explains Möttönen.
Future directions
In the future, the research groups will concentrate on more in-depth research into the structure of a synthetic magnetic monopole. They are also interested in the dynamics of monopoles and their interactions with other synthetic particles. One interesting idea involves trying to create a monopole that is not bound to a whirlpool in the same way as is the Dirac monopole. This type of structure could possibly describe a natural magnetic monopole in even more detail.
Scientists on the IceCube experiment discovered two extraterrestrial neutrinos with energies higher than any neutrino anyone had detected before.
Physicists on the IceCube experiment were in for a jolt. In processing data taken by their strings of more than 5000 light-sensitive detectors suspended under Antarctic ice, they discovered two particles called neutrinos with 1000 times more energy than the ones that regularly zip through IceCube’s detectors. They are the highest-energy neutrinos ever observed.
Nearly all of the neutrinos that IceCube sees are produced in Earth’s atmosphere. These atmospheric neutrinos tend to have energies somewhere between 1 and 10—and occasionally as high as 100—trillion electronvolts. The two unusual neutrinos appear to have come from far out in space and carried an impressive 1000 trillion electronvolts of energy. Each one lit up hundreds of IceCube’s detectors.
“These two neutrinos were discovered in an analysis that was optimized to see something else entirely—so they were quite the surprise,” says Kurt Woschnagg, an IceCube collaborator from the University of California, Berkeley.
Since these were the most exciting particles IceCube had ever observed, collaborators spent much time talking and writing about them.
“These events were originally named with long strings of numbers,” says graduate student Jakob van Santen. “But that was confusing—people would accidentally mix them up by transposing numbers.”
So van Santen named them Bert and Ernie, after the Sesame Street characters that their event displays somewhat resembled. He named Bert because it came from a more horizontal direction, whereas Ernie was more vertical and slightly larger, sort of like the heads of each Muppet.
“Unfortunately my memory of Sesame Street was a bit too dim, as was pointed out to me some time later: Bert is the one with the elongated head, and Ernie is the wide one,” van Santen says. “By that time it was too late; the names had stuck.”
Through careful analysis of Bert and Ernie, the collaboration confirmed that they had seen extremely high-energy neutrinos produced not in the Earth’s atmosphere, but somewhere beyond. Because neutrinos interact rarely with matter, these neutrinos may serve as messengers, carrying clear information about the most powerful cosmic events, including gamma-ray bursts, black holes and the formation of stars. The collaboration is now refining and expanding their analysis to learn more about these two zingers while also hunting for additional high-energy extraterrestrial neutrinos. Where there’s a Bert and an Ernie, there may also be a Mr. Snuffleupagus and an Oscar the Grouch.
A new approach to the unification of General Theory of Relativity and Quantum Theory
Present-day physics cannot describe what happened in the Big Bang. Quantum theory and the theory of relativity fail in this almost infinitely dense and hot primal state of the universe. Only an all-encompassing theory of quantum gravity which unifies these two fundamental pillars of physics could provide an insight into how the universe began. Scientists from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Golm/Potsdam and the Perimeter Institute in Canada have made an important discovery along this route. According to their theory, space consists of tiny “building blocks”. Taking this as their starting point, the scientists arrive at one of the most fundamental equations of cosmology, the Friedmann equation, which describes the universe. This shows that quantum mechanics and the theory of relativity really can be unified.
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.
Continuous space is broken down into elementary cells
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.
Quantum gravity could now answer questions regarding the Big Bang
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.
Many researchers believe that physics will not be complete until it can explain not just the behaviour of space and time, but where these entities come from.
“Imagine waking up one day and realizing that you actually live inside a computer game,” says Mark Van Raamsdonk, describing what sounds like a pitch for a science-fiction film. But for Van Raamsdonk, a physicist at the University of British Columbia in Vancouver, Canada, this scenario is a way to think about reality. If it is true, he says, “everything around us — the whole three-dimensional physical world — is an illusion born from information encoded elsewhere, on a two-dimensional chip”. That would make our Universe, with its three spatial dimensions, a kind of hologram, projected from a substrate that exists only in lower dimensions.
This 'holographic principle' is strange even by the usual standards of theoretical physics. But Van Raamsdonk is one of a small band of researchers who think that the usual ideas are not yet strange enough. If nothing else, they say, neither of the two great pillars of modern physics — general relativity, which describes gravity as a curvature of space and time, and quantum mechanics, which governs the atomic realm — gives any account for the existence of space and time. Neither does string theory, which describes elementary threads of energy.
Van Raamsdonk and his colleagues are convinced that physics will not be complete until it can explain how space and time emerge from something more fundamental — a project that will require concepts at least as audacious as holography. They argue that such a radical reconceptualization of reality is the only way to explain what happens when the infinitely dense 'singularity' at the core of a black hole distorts the fabric of space-time beyond all recognition, or how researchers can unify atomic-level quantum theory and planet-level general relativity — a project that has resisted theorists' efforts for generations.
“All our experiences tell us we shouldn't have two dramatically different conceptions of reality — there must be one huge overarching theory,” says Abhay Ashtekar, a physicist at Pennsylvania State University in University Park.
Finding that one huge theory is a daunting challenge. Here, Nature explores some promising lines of attack — as well as some of the emerging ideas about how to test these concepts (see 'The fabric of reality').
Gravity as thermodynamics
One of the most obvious questions to ask is whether this endeavour is a fool's errand. Where is the evidence that there actually is anything more fundamental than space and time?
A provocative hint comes from a series of startling discoveries made in the early 1970s, when it became clear that quantum mechanics and gravity were intimately intertwined with thermodynamics, the science of heat.
In 1974, most famously, Stephen Hawking of the University of Cambridge, UK, showed that quantum effects in the space around a black hole will cause it to spew out radiation as if it was hot. Other physicists quickly determined that this phenomenon was quite general. Even in completely empty space, they found, an astronaut undergoing acceleration would perceive that he or she was surrounded by a heat bath. The effect would be too small to be perceptible for any acceleration achievable by rockets, but it seemed to be fundamental. If quantum theory and general relativity are correct — and both have been abundantly corroborated by experiment — then the existence of Hawking radiation seemed inescapable.
A second key discovery was closely related. In standard thermodynamics, an object can radiate heat only by decreasing its entropy, a measure of the number of quantum states inside it. And so it is with black holes: even before Hawking's 1974 paper, Jacob Bekenstein, now at the Hebrew University of Jerusalem, had shown that black holes possess entropy. But there was a difference. In most objects, the entropy is proportional to the number of atoms the object contains, and thus to its volume. But a black hole's entropy turned out to be proportional to the surface area of its event horizon — the boundary out of which not even light can escape. It was as if that surface somehow encoded information about what was inside, just as a two-dimensional hologram encodes a three-dimensional image.
In 1995, Ted Jacobson, a physicist at the University of Maryland in College Park, combined these two findings, and postulated that every point in space lies on a tiny 'black-hole horizon' that also obeys the entropy–area relationship. From that, he found, the mathematics yielded Einstein's equations of general relativity — but using only thermodynamic concepts, not the idea of bending space-time1.
“This seemed to say something deep about the origins of gravity,” says Jacobson. In particular, the laws of thermodynamics are statistical in nature — a macroscopic average over the motions of myriad atoms and molecules — so his result suggested that gravity is also statistical, a macroscopic approximation to the unseen constituents of space and time.
In 2010, this idea was taken a step further by Erik Verlinde, a string theorist at the University of Amsterdam, who showed2 that the statistical thermodynamics of the space-time constituents — whatever they turned out to be — could automatically generate Newton's law of gravitational attraction.
And in separate work, Thanu Padmanabhan, a cosmologist at the Inter-University Centre for Astronomy and Astrophysics in Pune, India, showed3 that Einstein's equations can be rewritten in a form that makes them identical to the laws of thermodynamics — as can many alternative theories of gravity. Padmanabhan is currently extending the thermodynamic approach in an effort to explain the origin and magnitude of dark energy: a mysterious cosmic force that is accelerating the Universe's expansion.
Testing such ideas empirically will be extremely difficult. In the same way that water looks perfectly smooth and fluid until it is observed on the scale of its molecules — a fraction of a nanometre — estimates suggest that space-time will look continuous all the way down to the Planck scale: roughly 10−35 metres, or some 20 orders of magnitude smaller than a proton.
But it may not be impossible. One often-mentioned way to test whether space-time is made of discrete constituents is to look for delays as high-energy photons travel to Earth from distant cosmic events such as supernovae and γ-ray bursts. In effect, the shortest-wavelength photons would sense the discreteness as a subtle bumpiness in the road they had to travel, which would slow them down ever so slightly. Giovanni Amelino-Camelia, a quantum-gravity researcher at the University of Rome, and his colleagues have found4 hints of just such delays in the photons from a γ-ray burst recorded in April. The results are not definitive, says Amelino-Camelia, but the group plans to expand its search to look at the travel times of high-energy neutrinos produced by cosmic events. He says that if theories cannot be tested, “then to me, they are not science. They are just religious beliefs, and they hold no interest for me.”
Other physicists are looking at laboratory tests. In 2012, for example, researchers from the University of Vienna and Imperial College London proposed5 a tabletop experiment in which a microscopic mirror would be moved around with lasers. They argued that Planck-scale granularities in space-time would produce detectable changes in the light reflected from the mirror (see Naturehttp://doi.org/njf; 2012).
Loop quantum gravity
Even if it is correct, the thermodynamic approach says nothing about what the fundamental constituents of space and time might be. If space-time is a fabric, so to speak, then what are its threads?
One possible answer is quite literal. The theory of loop quantum gravity, which has been under development since the mid-1980s by Ashtekar and others, describes the fabric of space-time as an evolving spider's web of strands that carry information about the quantized areas and volumes of the regions they pass through6. The individual strands of the web must eventually join their ends to form loops — hence the theory's name — but have nothing to do with the much better-known strings of string theory. The latter move around in space-time, whereas strands actually are space-time: the information they carry defines the shape of the space-time fabric in their vicinity.
Because the loops are quantum objects, however, they also define a minimum unit of area in much the same way that ordinary quantum mechanics defines a minimum ground-state energy for an electron in a hydrogen atom. This quantum of area is a patch roughly one Planck scale on a side. Try to insert an extra strand that carries less area, and it will simply disconnect from the rest of the web. It will not be able to link to anything else, and will effectively drop out of space-time.
One welcome consequence of a minimum area is that loop quantum gravity cannot squeeze an infinite amount of curvature onto an infinitesimal point. This means that it cannot produce the kind of singularities that cause Einstein's equations of general relativity to break down at the instant of the Big Bang and at the centres of black holes.
In 2006, Ashtekar and his colleagues reported7 a series of simulations that took advantage of that fact, using the loop quantum gravity version of Einstein's equations to run the clock backwards and visualize what happened before the Big Bang. The reversed cosmos contracted towards the Big Bang, as expected. But as it approached the fundamental size limit dictated by loop quantum gravity, a repulsive force kicked in and kept the singularity open, turning it into a tunnel to a cosmos that preceded our own.
This year, physicists Rodolfo Gambini at the Uruguayan University of the Republic in Montevideo and Jorge Pullin at Louisiana State University in Baton Rouge reported8 a similar simulation for a black hole. They found that an observer travelling deep into the heart of a black hole would encounter not a singularity, but a thin space-time tunnel leading to another part of space. “Getting rid of the singularity problem is a significant achievement,” says Ashtekar, who is working with other researchers to identify signatures that would have been left by a bounce, rather than a bang, on the cosmic microwave background — the radiation left over from the Universe's massive expansion in its infant moments.
Loop quantum gravity is not a complete unified theory, because it does not include any other forces. Furthermore, physicists have yet to show how ordinary space-time would emerge from such a web of information. But Daniele Oriti, a physicist at the Max Planck Institute for Gravitational Physics in Golm, Germany, is hoping to find inspiration in the work of condensed-matter physicists, who have produced exotic phases of matter that undergo transitions described by quantum field theory. Oriti and his colleagues are searching for formulae to describe how the Universe might similarly change phase, transitioning from a set of discrete loops to a smooth and continuous space-time. “It is early days and our job is hard because we are fishes swimming in the fluid at the same time as trying to understand it,” says Oriti.
Causal sets
Such frustrations have led some investigators to pursue a minimalist programme known as causal set theory. Pioneered by Rafael Sorkin, a physicist at the Perimeter Institute in Waterloo, Canada, the theory postulates that the building blocks of space-time are simple mathematical points that are connected by links, with each link pointing from past to future. Such a link is a bare-bones representation of causality, meaning that an earlier point can affect a later one, but not vice versa. The resulting network is like a growing tree that gradually builds up into space-time. “You can think of space emerging from points in a similar way to temperature emerging from atoms,” says Sorkin. “It doesn't make sense to ask, 'What's the temperature of a single atom?' You need a collection for the concept to have meaning.”
In the late 1980s, Sorkin used this framework to estimate9 the number of points that the observable Universe should contain, and reasoned that they should give rise to a small intrinsic energy that causes the Universe to accelerate its expansion. A few years later, the discovery of dark energy confirmed his guess. “People often think that quantum gravity cannot make testable predictions, but here's a case where it did,” says Joe Henson, a quantum-gravity researcher at Imperial College London. “If the value of dark energy had been larger, or zero, causal set theory would have been ruled out.”
Causal dynamical triangulations
That hardly constituted proof, however, and causal set theory has offered few other predictions that could be tested. Some physicists have found it much more fruitful to use computer simulations. The idea, which dates back to the early 1990s, is to approximate the unknown fundamental constituents with tiny chunks of ordinary space-time caught up in a roiling sea of quantum fluctuations, and to follow how these chunks spontaneously glue themselves together into larger structures.
The earliest efforts were disappointing, says Renate Loll, a physicist now at Radboud University in Nijmegen, the Netherlands. The space-time building blocks were simple hyper-pyramids — four-dimensional counterparts to three-dimensional tetrahedrons — and the simulation's gluing rules allowed them to combine freely. The result was a series of bizarre 'universes' that had far too many dimensions (or too few), and that folded back on themselves or broke into pieces. “It was a free-for-all that gave back nothing that resembles what we see around us,” says Loll.
But, like Sorkin, Loll and her colleagues found that adding causality changed everything. After all, says Loll, the dimension of time is not quite like the three dimensions of space. “We cannot travel back and forth in time,” she says. So the team changed its simulations to ensure that effects could not come before their cause — and found that the space-time chunks started consistently assembling themselves into smooth four-dimensional universes with properties similar to our own10.
Intriguingly, the simulations also hint that soon after the Big Bang, the Universe went through an infant phase with only two dimensions — one of space and one of time. This prediction has also been made independently by others attempting to derive equations of quantum gravity, and even some who suggest that the appearance of dark energy is a sign that our Universe is now growing a fourth spatial dimension. Others have shown that a two-dimensional phase in the early Universe would create patterns similar to those already seen in the cosmic microwave background.
Holography
Meanwhile, Van Raamsdonk has proposed a very different idea about the emergence of space-time, based on the holographic principle. Inspired by the hologram-like way that black holes store all their entropy at the surface, this principle was first given an explicit mathematical form by Juan Maldacena, a string theorist at the Institute of Advanced Study in Princeton, New Jersey, who published11 his influential model of a holographic universe in 1998. In that model, the three-dimensional interior of the universe contains strings and black holes governed only by gravity, whereas its two-dimensional boundary contains elementary particles and fields that obey ordinary quantum laws without gravity.
Hypothetical residents of the three-dimensional space would never see this boundary, because it would be infinitely far away. But that does not affect the mathematics: anything happening in the three-dimensional universe can be described equally well by equations in the two-dimensional boundary, and vice versa.
In 2010, Van Raamsdonk studied what that means when quantum particles on the boundary are 'entangled' — meaning that measurements made on one inevitably affect the other12. He discovered that if every particle entanglement between two separate regions of the boundary is steadily reduced to zero, so that the quantum links between the two disappear, the three-dimensional space responds by gradually dividing itself like a splitting cell, until the last, thin connection between the two halves snaps. Repeating that process will subdivide the three-dimensional space again and again, while the two-dimensional boundary stays connected. So, in effect, Van Raamsdonk concluded, the three-dimensional universe is being held together by quantum entanglement on the boundary — which means that in some sense, quantum entanglement and space-time are the same thing.
Or, as Maldacena puts it: “This suggests that quantum is the most fundamental, and space-time emerges from it.”
