Circa 2006
We perceive space and time to be continuous, but if the amazing theory of loop quantum gravity is correct, they actually come in discrete pieces.
Category: particle physics – Page 380
When magnetic fields around a black hole reconnect, they can slow down plasma particles near the event horizon, which cause the black hole to lose energy when it swallows them.
A newly-designed atomic clock uses entangled atoms to keep time even more precisely than its state-of-the-art counterparts. The design could help scientists detect dark matter and study gravity’s effect on time.
Such primordial black holes (PBHs) could account for all or part of dark matter, be responsible for some of the observed gravitational waves signals, and seed supermassive black holes found in the center of our Galaxy and other galaxies. They could also play a role in the synthesis of heavy elements when they collide with neutron stars and destroy them, releasing neutron-rich material. In particular, there is an exciting possibility that the mysterious dark matter, which accounts for most of the matter in the universe, is composed of primordial black holes. The 2020 Nobel Prize in physics was awarded to a theorist, Roger Penrose, and two astronomers, Reinhard Genzel and Andrea Ghez, for their discoveries that confirmed the existence of black holes. Since black holes are known to exist in nature, they make a very appealing candidate for dark matter.
The recent progress in fundamental theory, astrophysics, and astronomical observations in search of PBHs has been made by an international team of particle physicists, cosmologists and astronomers, including Kavli IPMU members Alexander Kusenko, Misao Sasaki, Sunao Sugiyama, Masahiro Takada and Volodymyr Takhistov.
To learn more about primordial black holes, the research team looked at the early universe for clues. The early universe was so dense that any positive density fluctuation of more than 50 percent would create a black hole. However, cosmological perturbations that seeded galaxies are known to be much smaller. Nevertheless, a number of processes in the early universe could have created the right conditions for the black holes to form.
The Kondo effect influences the electrical resistance of metals at low temperatures and generates complex electronic and magnetic orders. Novel concepts for data storage and processing, such as using quantum dots, are based on this. In 1998, researchers from the United States published spectroscopic studies on the Kondo effect using scanning tunneling microscopy, which are considered ground-breaking and have triggered countless others of a similar kind. Many of these studies may have to be re-examined now that Jülich researchers have shown that the Kondo effect cannot be proven beyond doubt by this method. Instead, another phenomenon is creating precisely the spectroscopic ‘fingerprint’ that was previously attributed to the Kondo effect.
Normally the resistance of metals decreases as the temperature drops. The Kondo effect causes it to rise again below a threshold value typical to the material in question, the so-called Kondo temperature. This phenomenon occurs when magnetic foreign atoms, such as iron, contaminate non-magnetic host metals, such as copper. Simply put, when a current flows, the atomic nuclei are engulfed by electrons. The iron atoms have a quantum mechanical magnetic moment. This causes the electrons in the vicinity to align their spin antiparallel to the moment of the atom at low temperatures and to hang around the cobalt atom like a cloud on a mountaintop. This hinders the flow of the electrons—the electrical resistance then increases. In physics, this is known as entanglement, the strong coupling of the moment of the impurity with the spins of the surrounding electrons.
For mathematicians and computer scientists, 2020 was full of discipline-spanning discoveries and celebrations of creativity. We’d like to take a moment to recognize some of these achievements.
1. A landmark proof simply titled MIP = RE” establishes that quantum computers calculating with entangled qubits can theoretically verify the answers to an enormous set of problems. Along the way, the five computer scientists who authored the proof also answered two other major questions: Tsirelson’s problem in physics, about models of particle entanglement, and a problem in pure mathematics called the Connes embedding conjecture.
2. In February, graduate student Lisa Piccirillo dusted off some long-known but little-utilized mathematical tools to answer a decades-old question about knots. A particular knot named after the legendary mathematician John Conway had long evaded mathematical classification in terms of a higher-dimensional property known as sliceness. But by developing a version of the knot that yielded to traditional knot analysis, Piccirillo finally determined that the Conway knot is not slice.
3. For decades, mathematicians have used computer programs known as proof assistants to help them write proofs — but the humans have always guided the process, choosing the proof’s overall strategy and approach. That may soon change. Many mathematicians are excited about a proof assistant called Lean, an efficient and addictive proof assistant that could one day help tackle major problems. First, though, mathematicians must digitize thousands of years of mathematical knowledge, much of it unwritten, into a form Lean can process. Researchers have already encoded some of the most complicated mathematical ideas, proving in theory that the software can handle the hard stuff. Now it’s just a question of filling in the rest.
A multitasking nanomachine that can act as a heat engine and a refrigerator at the same time has been created by RIKEN engineers. The device is one of the first to test how quantum effects, which govern the behavior of particles on the smallest scale, might one day be exploited to enhance the performance of nanotechnologies.
Conventional heat engines and refrigerators work by connecting two pools of fluid. Compressing one pool causes its fluid to heat up, while rapidly expanding the other pool cools its fluid. If these operations are done in a periodic cycle, the pools will exchange energy and the system can be used as either a heat engine or a fridge.
It would be impossible to set up a macroscale machine that does both tasks simultaneously—nor would engineers want to, says Keiji Ono of the RIKEN Advanced Device Laboratory. “Combining a traditional heat engine with a refrigerator would make it a completely useless machine,” he says. “It wouldn’t know what to do.”
Antiferromagnetism is a type of magnetism in which parallel but opposing spins occur spontaneously within a material. Antiferromagnets, materials that exhibit antiferromagnetism, have advantageous characteristics that make them particularly promising for fabricating spintronic devices.
In contrast with conventional electronic devices, which use the electrical charge of electrons to encode information, spintronics process information leveraging the intrinsic angular momentum of electrons, a property known as “spin.” Due to their ultrafast nature, their insensitivity to external magnetic fields and their lack of magnetic stray fields, antiferromagnets could be particularly desirable for the development of spintronic devices.
Despite their advantages and their ability to store information, most simple antiferromagnets have weak readout magnetoresistivity signals. Moreover, so far physicists have been unable to change the magnetic order of antiferromagnets using optical techniques, which could ultimately allow device engineers to exploit these materials’ ultrafast nature.
Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, or CNMS, contributed to a groundbreaking experiment published in Science that tracks the real-time transport of individual molecules.
A team led by the University of Graz, Austria, used unique four-probe scanning tunneling microscopy, or STM, to move a single molecule between two independent probes and observe it disappear from one point and instantaneously reappear at the other.
The STM, made available via the CNMS user program, operates under an applied voltage, scanning material surfaces with a sharp probe that can move atoms and molecules by nudging them a few nanometers at a time. This instrument made it possible to send and receive dibromoterfluorene molecules 150 nanometers across a silver surface with unprecedented control.
“Klein tunnelling” has been observed directly for the first time.
A curious effect called “Klein tunnelling” has been observed for the first time in an experiment involving sound waves in a phononic crystal. As well as confirming the century-old prediction that relativistic particles (those travelling at speeds approaching the speed of light) can pass through an energy barrier with 100% transmission, the research done in China and the US could lead to better sonar and ultrasound imaging.
Quantum tunnelling refers to the ability of a particle to pass through a potential-energy barrier, despite having insufficient energy to cross if the system is described by classical physics. Tunnelling is a result of wave–particle duality in quantum mechanics, whereby the wave function of a particle extends into and beyond a barrier.
Normally, the probability that tunnelling will occur is less than 100% and decreases exponentially as the height and width of the barrier increase. However, in 1929 the Swedish physicist Oskar Klein calculated that an electron travelling at near the speed of light will tunnel through a barrier with 100% certainty – regardless of the height and width of the barrier.