Quantum & Fundamental Physics for Astronomy

The process by which a quantum system loses its delicate superposition through unavoidable interaction with its environment, giving rise to the definite, classical world we observe. Decoherence explains why quantum weirdness is hard to see at everyday scales and is a central challenge for building quantum computers.

A deep correlation between quantum particles such that measuring one instantly fixes the state of the other, however far apart they are — what Einstein called 'spooky action at a distance.' Confirmed in ever-more-stringent experiments, it does not allow faster-than-light signalling and is a foundation of quantum information science.

An intrinsic form of angular momentum carried by particles, quantised in units of the reduced Planck constant and unrelated to any literal spinning. Spin divides particles into fermions and bosons and, through the Pauli exclusion principle, into the electron shells of atoms; the spin-flip of hydrogen produces the 21-centimetre radio line that maps neutral gas across the Galaxy.

The principle that a quantum system can exist in a combination of states at once until it is measured, when a single outcome is realised. It is the heart of quantum weirdness and of quantum computing, and it is the reason atomic transitions and particle interactions must be described in probabilities rather than certainties.

The quantum ability of a particle to cross an energy barrier it classically could not surmount. Tunnelling lets protons in the Sun's core fuse despite their electrical repulsion — without it the Sun could not shine — and it is central to the theory of how radiation escapes a black hole.

The fundamental limit that a particle's position and momentum cannot both be known with arbitrary precision — a property of quantum nature itself, not of imperfect instruments. Together with the Pauli exclusion principle it sets the scale of the degeneracy pressure — the resistance of fermions such as electrons and neutrons to being crowded together — that holds up white dwarfs and neutron stars against gravity.

The mathematical object that captures everything knowable about a quantum system; its squared magnitude gives the probability of finding a particle in a given state. Wave functions underlie the discrete energy levels of atoms — and therefore the spectral lines that let astronomers read the composition, temperature, and motion of distant stars and gas.

The finding that light and matter each behave as both waves and particles depending on how they are observed — light as photons in the photoelectric effect, electrons as interfering waves in a double slit. It reconciled centuries of debate about the nature of light and underlies how detectors count the photons that carry astronomy's information.

The lowest possible energy of a quantum system, which is never zero — even empty space seethes with quantum fluctuations. This vacuum energy is a real, measured effect (the Casimir force), and its possible connection to the accelerating expansion of the Universe is one of the deepest open problems in physics.

Matter's mirror image — particles with the same mass but opposite charge, such as the positron discovered in cosmic rays in 1932. When matter and antimatter meet they annihilate into pure energy. Antiparticles are produced in high-energy astrophysical environments and are detected in cosmic rays reaching the Earth.

The fundamental constituents of matter and force in the Standard Model — the quarks and leptons that make up matter, and the force-carrying bosons. Everything from a star to a galaxy is built from a handful of these particles, and understanding them connects the physics of the very small to the structure of the whole Universe.

The unexplained fact that the Universe is made almost entirely of matter, even though the Big Bang should have produced matter and antimatter in equal amounts, which would have annihilated completely. Why a tiny excess of matter survived — the problem of baryogenesis — is one of the great open questions in physics and cosmology.

The quantum phenomenon in which a neutrino changes flavour — electron, muon, or tau — as it travels. Its discovery proved that neutrinos have a small mass, resolved the decades-old 'solar neutrino problem' of the Sun apparently producing too few, and was recognised with the 2015 Nobel Prize in Physics.

The four basic interactions of nature — gravity, electromagnetism, and the strong and weak nuclear forces. Together they govern everything from the fusion that powers stars to the collapse of a massive core, and unifying them, especially gravity with the quantum forces, remains a central goal of physics.

The particle associated with the Higgs field, which gives many elementary particles their mass. Predicted in the 1960s and discovered at CERN's Large Hadron Collider in 2012, it completed the Standard Model; the origin of mass it explains is fundamental to why matter — and therefore stars and galaxies — exists as it does.

A tiny, electrically neutral particle that interacts only very weakly and so streams almost unimpeded through matter — trillions pass through you each second. Neutrinos are produced in the Sun's core, in supernovae, and by cosmic rays, and detecting them opens a window onto processes hidden from light, a pillar of multi-messenger astronomy.

The relativistic shortening of an object along its direction of motion as seen by an observer it moves past, becoming dramatic near the speed of light. It is the companion of time dilation, and it explains how short-lived muons created high in the atmosphere by cosmic rays still reach the ground.

Einstein's result that mass and energy are two forms of the same thing, related by the speed of light squared. It is the accounting behind starlight: fusing hydrogen into helium in the Sun's core converts about four million tonnes of mass into energy every second, and it governs the annihilation of matter with antimatter.

The principle that gravitational and inertial mass are identical, so that being in free fall is locally indistinguishable from floating in empty space. This insight led Einstein from special to general relativity, recasting gravity as the curvature of spacetime; it is tested to extraordinary precision by lunar laser ranging and dedicated satellites.

The relativistic effect by which time runs slower for fast-moving observers and deeper in a gravitational field. It is measured every day — GPS satellites must correct for it — and it steepens near neutron stars and black holes, where clocks near the horizon crawl relative to distant ones. Even distant supernovae are seen to fade in slow motion as the Universe expands.

Fleeting, unavoidable variations in the energy of a field, permitted by the uncertainty principle. Stretched to astronomical size during cosmic inflation, tiny quantum fluctuations are believed to have seeded the density variations we now see imprinted on the cosmic microwave background and grown into galaxies and clusters.

A sea of relic neutrinos released about one second after the Big Bang, when the young Universe became transparent to them — the neutrino analogue of the cosmic microwave background. Its existence is strongly supported by its imprint on primordial element abundances and the microwave background, though the neutrinos themselves are far too low in energy to have been detected directly.

The vast mismatch between the tiny dark-energy density measured in the Universe and the enormous vacuum energy that naive quantum theory predicts — a discrepancy of many tens of orders of magnitude. Often called the worst prediction in physics, it remains one of the deepest unsolved problems where quantum theory meets gravity.

A theoretical ceiling on the energy of cosmic-ray protons that reach us from far away: above roughly 5×10¹⁹ electronvolts they collide with cosmic-microwave-background photons and lose energy, so the highest-energy cosmic rays must originate relatively nearby. Observatories studying the ultra-high-energy sky test this predicted cutoff.

The energy that fills empty space even in the absence of matter or radiation, arising from quantum fields in their lowest state. Vacuum energy is a leading candidate for the dark energy driving cosmic acceleration and is closely tied to the cosmological constant in Einstein's equations.