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String theory vs Loop quantum gravity: Wild hunt for Quantum Gravity

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The gauge bosons of the standard model of particle physics are responsible for 3 of the 4 known forces in the universe. A force is conferred is through the exchange of virtual bosons. So for example in electromagnetism, an exchange of virtual photons results in an exchange of momentum which results in two like charges repelling each other.

Gravity is missing from this picture because in General relativity, gravity is not a force, but is a curvature of space-time. The problem is that stars and planets are made of molecules, atoms and radiation. And the forces that hold the atoms together are due to discrete units of virtual particles. It is the exchange or swapping of these virtual bosons that holds or breaks up atoms and molecules.

Quantum mechanics conflicts with general relativity, because QM treats every thing as being discrete, and GR treats everything as being continuous. We need a theory that combines the two because we live in one reality, not two different realities.

This is why most physicists believe General relativity is incomplete. Why can’t quantum mechanics be the one that is incomplete?
Of the 4 fundamental forces, 3 have very robust quantum mechanical theories. Only gravity lacks a quantum description. Quantum mechanics also has almost all of classical physics within in its limits. Classical physics like general relativity, does not have quantum effects. We have learned is that Quantum physics is the fundamental language of reality.

One way to quantize gravity is to quantize space-time itself. This is what loop quantum gravity or LQG does. It shows that the fabric of space-time is not continuous, but is made up of discrete quanta, like the pixels on a TV screen. This is different than string theory, because in string theory, space is the background or the canvas, on which strings vibrate.

Continue reading “String theory vs Loop quantum gravity: Wild hunt for Quantum Gravity” | >

When the Environment Writes the Rules of Quantum Dynamics

The transitions of hydrogen molecules embedded in a crystal depend on the surroundings—a behavior that could be used to tailor molecular quantum dynamics.

In quantum physics, we often learn that the rules governing a system are set by its symmetry. These rules—known as selection rules—determine which transitions between quantum states are allowed and which are forbidden. For example, rotational symmetry constrains how an atom’s angular momentum can change. But what if those rules are not fixed? A recent study of hydrogen (H2)—one of the simplest molecules in nature—showed that the allowed pathways between quantum states are determined not solely by the molecule’s internal symmetry but also by its surroundings. By embedding hydrogen molecules in different crystalline environments, Nathan McLane and colleagues from the University of Maryland, College Park, have demonstrated that the symmetry of the host material can selectively enable or suppress nuclear-spin transitions [1]. In doing so, the team revealed that quantum dynamics is not just an intrinsic property—it can be shaped by the environment.

H2 is one of the simplest systems for exploring quantum behavior. Its two identical protons can align their spins in two different ways: In so-called orthohydrogen the nuclear spins are parallel, whereas in parahydrogen they are antiparallel. Although this difference is subtle, it leads to markedly different physical properties for the two forms. Crucially, transitions between them are highly constrained: In an isolated hydrogen molecule, the overall wave function is symmetric under exchange of the two protons, and this exchange symmetry forbids direct conversion between ortho and para states [2]. This restriction makes H2 a textbook example of how symmetry governs quantum dynamics.

A flower-like pattern exposes chiral superconductivity’s long-sought fingerprint

With a carefully designed experiment and a handful of tin atoms, University of Tennessee, Knoxville’s physicists have found a long-sought form of superconductivity, taking one more step toward creating custom quantum materials.

Scientists have known about superconductivity for more than a century. At low temperatures, resistance in certain materials vanishes and they carry electrical current without losing any energy. Superconductors are part of particle accelerators and magnetic resonance imaging machines. While they need extremely cool environments to work, the mechanism that drives them is quite well understood: electrons, which normally repel each other, form pairs and carry the current.

A new era for ultrafast photonics: 2D mercury-acetylide frameworks for near-infrared nonlinear optics

In the increasingly digital world, the demand for faster, more efficient and miniaturized optical devices is ever-growing. From high-speed internet and secure quantum communications to advanced medical imaging and precision manufacturing, the backbone of these technologies is light, specifically how we can control and manipulate it at the nanoscale.

Two-dimensional (2D) materials have emerged as a game-changer in this arena, offering unique properties that can be harnessed for ultrafast photonics and nonlinear optical applications.

However, the search for materials that combine stability, tunability and high performance in the near-infrared (NIR) region, a crucial window for telecommunications and sensing, remains a significant challenge.

Levitated nano-ferromagnet confirms a 160-year-old physical prediction

Ferromagnets, such as iron, cobalt, and nickel, are materials with a strong, spontaneous, and permanent magnetic field. Over 150 years ago, the physicist and mathematician James Clerk Maxwell speculated that under specific conditions, non-spinning ferromagnets or electromagnets would behave as gyroscopes, objects that maintain their orientation, typically due to the angular momentum arising from spinning.

Maxwell hypothesized that this unique gyroscopic behavior would arise from the relationship between a ferromagnet’s magnetism and its angular momentum within a specific set-up. While numerous studies tested this prediction, so far it had never been proven experimentally.

Researchers at the Institute of Photonics and Nanotechnology IFN-CNR and the Bruno Kessler Foundation recently observed the effect predicted by Maxwell in a non-spinning and levitated ferromagnetic sphere. Their observations, presented in a paper published in Physical Review Letters, could open new exciting possibilities for the development of quantum technologies and for the collection of highly precise measurements.

Observing exotic quasiparticle states in kagome superconductor CsV₃Sb₅

A research team led by Prof. Hao Ning of the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, in collaboration with Anhui University and the University of Science and Technology of China, has identified two distinct types of unusual low-energy quasiparticle states in the kagome superconductor CsV3Sb5 using single-atom impurities as local “quantum probes” combined with scanning tunneling spectroscopy.

The study was recently published in Nature Physics.

CsV3Sb5 has attracted growing interest for its unusual crystal structure and complex quantum phenomena. Evidence for time-reversal symmetry breaking remains under debate, and the mechanism of its superconductivity is still not fully understood. Studying its response to single-atom impurities provides a promising way to address these questions.

Physicists reveal universal speed limit on quantum information scrambling

Theoretical physicists in the US have discovered a “speed limit” on the time taken for quantum information to spread through larger systems. Publishing their results in Physical Review Letters, Amit Vikram and colleagues at the University of Maryland have proved for the first time that this minimum time is closely linked with a system’s entropy and temperature, perhaps paving the way for a deeper understanding of quantum information across a wide range of physical settings.

In 1974, Stephen Hawking proposed for the first time that black holes aren’t entirely black. As well as emitting thermal radiation (now known as “Hawking radiation”), they also exhibit thermodynamic properties including temperature and an entropy proportional to their surface area.

Since entropy is a measure of the information carried by a system, this means a black hole’s surface effectively stores a finite number of “qubits”: the quantum equivalent of classical bits, each capable of storing quantum information as a superposition of two states simultaneously. In this way, the black hole’s temperature as described by Hawking governs how these qubits interact and evolve over time.

What is quantum gravity? Scientists think it could explain the beginning of our universe

“General relativity works extraordinarily well in many settings, but when we run it back to the Big Bang, and apply it to the inside of black holes, it predicts a singularity: a moment where density, curvature and temperature formally become infinite. That is usually a sign that the theory is being pushed beyond where it can be trusted,” Afshordi told Space.com. “In other words, general relativity is likely incomplete for describing the very first moments of the universe, when quantum effects should also matter.”

Afshordi explained that in the standard picture of the Big Bang, scientists usually start with Einstein’s theory of gravity, then add extra ingredients to explain the earliest moments of the universe, most notably a hypothetical “inflation field” to account for the initial rapid expansion of the cosmos.”

‘Poor man’s Majoranas’ can be used as quantum spin probes

A Majorana fermion is a particle that would be identical to its antiparticle. Such an object has not yet been found. However, certain solid materials exhibit analogous behavior as if Majorana fermions were present through collective excitations of the system called quasiparticles.

In addition to generating interest in basic science as key components for understanding the material world, Majorana fermions have primarily been studied due to their potential technological applications in areas such as fault-tolerant quantum computing.

The main theoretical model used in this study is the Kitaev wire. It is a one-dimensional superconducting chain formed by electrons or collective excitations. Under certain conditions, it generates an isolated Majorana fermion at each end without altering the total energy of the system.

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