by Erika, astrophysicist — 20 August 2026
Quantum physics is the framework we use to describe nature at its most fundamental scales: atoms, electrons, photons, atomic nuclei and elementary particles. It emerged in the early twentieth century because classical physics, extraordinarily successful for planets, projectiles and machines, simply could not explain several observations made in the microscopic world. The spectrum of radiation emitted by hot objects, the photoelectric effect and the stability and structure of atoms all pointed towards something deeper. Energy and matter did not always behave as continuous, predictable entities. At sufficiently small scales, nature followed a different set of rules.
One of the central ideas is quantisation. Certain physical quantities, such as the energy of an electron bound inside an atom, can only take particular allowed values rather than any value whatsoever. An electron can move between these energy states by absorbing or emitting a photon whose energy corresponds to the difference between them. This apparently simple fact is enormously important. It explains why atoms emit and absorb light at specific wavelengths and why every chemical element has its own characteristic spectral fingerprint.
Quantum objects also possess properties that do not fit comfortably into our everyday intuition. Electrons and photons can display both particle-like and wave-like behaviour. Their state is represented mathematically by a wavefunction, which allows us to calculate probabilities for different measurement outcomes. Before measurement, a quantum system can exist in a superposition of possible states. There is also the uncertainty principle: certain pairs of physical quantities, such as position and momentum, cannot simultaneously possess arbitrarily precise values. This is not simply the result of imperfect instruments; it is built into the mathematical structure of quantum mechanics itself.
Another striking feature is entanglement. Two quantum systems can share a joint state in such a way that measurements made on them show correlations that cannot be reproduced by ordinary classical models based on local hidden properties. Experiments have repeatedly confirmed these predictions. Entanglement does not allow usable information to travel faster than light, but it has become one of the foundations of quantum information science, including quantum computing, quantum communication and new forms of precision sensing.
Despite its reputation for being mysterious, quantum physics is not an exotic theory that matters only inside laboratories. Much of modern civilisation depends on it. Semiconductor physics and therefore transistors, microprocessors and smartphones require quantum mechanics. Lasers depend on quantum transitions and stimulated emission. LEDs, solar cells, electron microscopes, superconducting devices, atomic clocks and many medical technologies ultimately rely on quantum phenomena. Current research is extending this into quantum computers, networks and exceptionally sensitive sensors based on atoms, photons and superconducting systems.
In astrophysics, quantum physics becomes particularly interesting because phenomena occurring on scales far smaller than an atom can determine the behaviour of entire stars.
One of the clearest examples is spectroscopy. When we observe a star, nebula or planetary atmosphere, we usually cannot travel there and collect a sample. Instead, we analyse its light. Because the allowed energy states of atoms and molecules are quantised, different substances absorb and emit photons at characteristic wavelengths. Those spectral lines allow us to identify hydrogen, helium, oxygen, carbon, iron and complex molecules across enormous distances. They also provide information about temperature, density, ionisation and motion. Much of what we know about the chemical composition of stars and the atmospheres of exoplanets is therefore possible because atomic quantum mechanics leaves identifiable patterns in the light reaching our telescopes.
Quantum mechanics is also one of the reasons stars can shine. In the core of the Sun, positively charged nuclei repel one another electrically. Classical physics alone would make it extremely difficult for most protons at solar-core temperatures to get close enough for the strong nuclear interaction to bind them. Quantum tunnelling changes the situation. A quantum particle has a finite probability of crossing an energy barrier that it could not overcome classically. This allows nuclear reactions to occur at astrophysically relevant rates inside stars. Without quantum tunnelling, stellar fusion, and therefore the long-lived energy production of stars like the Sun, would be radically different.
Quantum physics becomes even more dramatic when stars die. A white dwarf contains matter compressed to extraordinary densities. Gravity tries to squeeze the object further, but electrons obey the Pauli exclusion principle: identical fermions cannot all occupy the same quantum state. As the electrons are compressed into an increasingly restricted volume, they must occupy progressively higher momentum states, producing what we call electron degeneracy pressure. This quantum-mechanical pressure can support a white dwarf against gravitational collapse up to the Chandrasekhar mass limit. An object roughly the size of Earth can therefore contain a mass comparable to that of the Sun because its structure is controlled by quantum statistics.
Neutron stars push this physics still further. Their densities approach those found inside atomic nuclei, and quantum statistics, nuclear interactions, particle physics and general relativity all become essential simultaneously. Understanding the exact equation of state of neutron-star matter remains an active research problem. Observations of neutron-star masses, radii, pulsations and mergers consequently provide laboratories for forms of matter that cannot be reproduced easily on Earth. Neutron-star mergers are also important sites of nucleosynthesis, helping create some of the heaviest elements in the Universe.
Quantum physics may even be connected to the origin of cosmic structure itself. In the standard inflationary picture of the early Universe, microscopic quantum fluctuations were enormously amplified during cosmic inflation. These tiny primordial variations became variations in matter density, which gravity subsequently amplified over billions of years. They ultimately provided the seeds from which galaxies, galaxy clusters and the cosmic web developed. The tiny temperature anisotropies observed in the cosmic microwave background preserve information about these primordial fluctuations. In this sense, according to our current cosmological model, structures spanning millions of light-years may ultimately trace their history back to fluctuations that began on quantum scales.
Black holes reveal where our present understanding begins to encounter its limits. General relativity describes gravity and spacetime with extraordinary success, while quantum theory describes matter and the other fundamental interactions. But we still do not possess an experimentally established theory that completely unifies gravity with quantum mechanics. Quantum field theory in curved spacetime leads to phenomena such as Hawking radiation, according to which black holes should emit thermal radiation, but the deeper quantum description of spacetime and black-hole interiors remains unresolved. Developing a consistent theory of quantum gravity is consequently one of the major outstanding problems in fundamental physics.
Quantum physics is also beginning to influence the instruments with which we study the Universe. NASA and other research organisations are developing superconducting photon detectors, atomic sensors, quantum-enhanced interferometry and extremely precise clocks. Superconducting detectors can register extraordinarily weak astronomical signals, in some cases approaching single-photon sensitivity, while future quantum sensors may improve measurements of gravity, magnetic fields, navigation, gravitational waves and perhaps even searches for new fundamental physics. NASA's current technology programmes explicitly include quantum and superconducting detectors for future astrophysical observations.
So what is quantum physics “for”? At the most fundamental level, it is not merely a technology that we invented for a particular purpose. It is our best tested framework for describing how matter and radiation behave at microscopic scales. Its practical applications came because nature itself operates according to those rules. Once we learned to understand and manipulate them, we obtained transistors, lasers, atomic clocks and modern detectors. In astrophysics, those same rules allow us to decode the light of distant worlds, understand how stars generate energy, explain why white dwarfs exist, model neutron stars and nuclear reactions, investigate the early Universe and explore the boundary between gravity and quantum theory.
Perhaps the most remarkable aspect of quantum physics is therefore not that the microscopic world behaves strangely. It is that these microscopic rules are inseparable from the macroscopic Universe. The light from a galaxy billions of light-years away carries information encoded in quantum transitions inside atoms. The Sun shines partly because nuclei can tunnel through barriers. A dead star can resist gravitational collapse because electrons obey quantum statistics. And, if inflationary cosmology is correct, the distribution of galaxies across the observable Universe ultimately has roots in primordial quantum fluctuations. Quantum physics describes the very small, but its consequences are written across the entire cosmos.
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