Matter

• 1. Introduction

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  Matter is everything you can touch, and almost none of what fills the universe. A car has mass and volume, so we call it matter. Yet when scientists measured the visible cosmos, only about 4.6% of it turned out to be the stuff atoms are made of. The rest is dark matter and dark energy, whose composition no one in a laboratory has ever observed. Matter sounds like the most basic word in science. It may be one of the slipperiest.

  In physical science, matter is any substance that has mass and takes up space by having volume. Every object you can hold is built from atoms, and atoms from interacting subatomic particles. But that tidy picture hides a problem. In the Standard Model of particle physics, matter is not even a fundamental concept, because the particles inside atoms have no inherent size or volume at all. So why does a chair feel solid? Why does it occupy a place that nothing else can occupy at the same moment?

  This is a story about a question people have asked since antiquity, and answered in wildly different ways. It runs from an Indian philosopher writing around the 6th century BCE, to white dwarf stars, to the strange admission that most of the mass in your own body comes from something other than the particles inside it.

• 2. The Solidity Puzzle

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  A chair holds your weight, yet the electrons and quarks inside it have no well-defined sizes or positions. Subatomic particles are governed by their quantum nature. They can act like waves as well as particles. They do not behave the way everyday objects appear to behave. So the familiar image of an atom, a nucleus of protons and neutrons wrapped in a cloud of orbiting electrons that take up space, is only somewhat correct.

  The Pauli exclusion principle is what makes matter occupy space. The principle applies to particles called fermions, which include quarks and leptons. It forces some point particles, and the composites and atoms they build, to keep their distance from one another. Two particles cannot be in the same place at the same time, in the same state. That refusal to overlap is the property we feel as solidity.

  The observation that matter occupies space goes back to antiquity. The explanation for why it occupies space is recent. Two extreme examples make the principle vivid: white dwarf stars and neutron stars, where the exclusion principle holds collapsing matter against the crush of gravity.

• 3. Where The Mass Hides

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  Most of what composes the mass of ordinary matter is not the particles you would name first. To a great extent, the mass of an atom is the sum of the masses of its protons, neutrons, and electrons. Dig deeper, and the picture inverts. Protons and neutrons are made of quarks bound together by gluon fields, and those fields contribute significantly to the mass of hadrons.

  The numbers are stark. The sum of the masses of the three quarks in a nucleon is approximately 12.5, low compared to the mass of a nucleon itself, which is approximately 938. Most of the mass of everyday objects comes from the interaction energy of its elementary components. The binding energy of quarks inside protons and neutrons does the heavy lifting.

  This also explains why a nuclear bomb does not destroy matter. None of the baryons, the protons and neutrons inside atomic nuclei, are destroyed in the reaction. There are as many baryons after as before. What gets released is nuclear binding energy, as baryons settle into mid-size nuclei carrying less energy per nucleon than the small and large nuclei they came from.

• 4. Quarks And Leptons

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  Carithers and Grannis put it plainly: ordinary matter is composed entirely of first-generation particles, namely the up and down quarks, plus the electron and its neutrino. Everything you touch reduces to that short list. Higher-generation particles quickly decay into first-generation particles, so they are not commonly encountered.

  The Standard Model groups matter particles into three generations, each with two quarks and two leptons. The second generation adds the charm and strange quarks, the muon and the muon neutrino. The third generation brings the top and bottom quarks and the tau and tau neutrino. One natural explanation is that the heavier generations are excited states of the first. If that is true, quarks and leptons would be composite particles rather than elementary ones.

  Quarks carry colour charge, the strong-interaction equivalent of electric charge, and they undergo radioactive decay through the weak interaction. Leptons, the most famous being the electron, carry no colour charge and feel no strong interaction. Not everything with mass counts as ordinary matter. The W and Z bosons that mediate the weak force are not made of quarks or leptons, so they are not ordinary matter even though they have mass.

• 5. A Word With Many Edges

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  There is no broad consensus in physics for a general definition of matter. The term almost always travels with a specifying modifier. Physicists speak of condensed matter, partonic matter, dark matter, anti-matter, strange matter, and nuclear matter. The physicist Alfven even coined koinomatter, from the Greek for common matter, to mark ordinary matter apart from antimatter.

  The history of the concept is a history of fundamental length scales. Define matter at the atomic level and it means atoms. Define it deeper and it means hadrons, or leptons and quarks. Each scale produces a different, internally consistent definition. A definition based on protons, neutrons, and electrons even reaches things that are neither atoms nor molecules, such as the electron beams in an old cathode ray tube television.

  General relativity offers yet another edge. There, mass is not additive, and one cannot simply add the rest masses of particles to get a system's total. The energy-momentum tensor quantifies the amount of matter instead. In cosmology, matter is sometimes taken to be anything that is not purely gravity, which folds light and other massless fields into matter after all.

• 6. Strange And Degenerate

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  Degenerate matter is the ground state of a gas of fermions at a temperature near absolute zero. The exclusion principle allows only two fermions per quantum state, one spin-up and one spin-down. So the fermions stack up through the available levels, and the pressure of the gas grows enormous, depending on the number of fermions rather than the temperature. Neutron stars and white dwarfs are made of this matter.

  Subrahmanyan Chandrasekhar showed that white dwarf stars have a maximum allowed mass because of the exclusion principle. That demonstration caused a revolution in the theory of star evolution. The same quantum rule that keeps a chair solid sets a ceiling on how massive a dead star can be before it collapses further.

  Strange matter pushes further still, a liquid of up, down, and strange quarks. The strange matter hypothesis of Bodmer and Witten proposes that this could be more stable than nuclear matter. In that version, the nuclei around us are merely metastable. Given enough time, or the right stimulus, they would decay into droplets of strange matter called strangelets. Such droplets are hypothesized to range from femtometers in size up to kilometers, in the form of quark stars.

• 7. The Antimatter Riddle

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  Antimatter is built from the antiparticles of ordinary matter, and when a particle meets its antiparticle, the two annihilate. Both may convert into other particles of equal energy, following Albert Einstein's equation E = mc2. The products might be high-energy gamma-ray photons, or other particle-antiparticle pairs, carrying kinetic energy equal to the mass difference of what went in and what came out.

  In the early universe, matter and antimatter are thought to have been equally represented. Their balanced production should have led them to completely annihilate each other, leaving a universe that does not exist. In October 2017, scientists reported further evidence that the two are identical, sharpening the paradox. Removing the antimatter requires a CP symmetry violation, and the lingering imbalance remains one of the great unsolved problems in physics.

• 8. From Kanada To Descartes

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  Long before particle physics, the idea that matter is built from discrete blocks appeared independently in ancient Greece and ancient India. The Indian philosopher Kanada, writing around the 6th century BCE, became the most followed voice of the Nyaya-Vaisheshika school. Jain philosophers gave each atom qualities such as taste, smell, touch, and color, and held that humid or dry atoms cling together to form matter, with the soul attaching and transmigrating at each rebirth.

  Aristotle, who lived from 384 BCE to 322 BCE, was the first to set the conception on a sound philosophical footing, in his Physics. His word for matter, hyle, literally means wood or timber, the raw material for building. For him matter was not atoms but whatever persists through a change of substance. When a horse eats grass, the grass does not survive in the horse, but some aspect of it, its matter, does.

  Rene Descartes, who lived from 1596 to 1650, originated the modern conception. A geometer at heart, he postulated matter as an abstract mathematical substance whose only property is extension in length, breadth, and depth. Isaac Newton, born in 1643 and dying in 1727, inherited that mechanical view, then restored intrinsic properties such as mass. His gravity acted at a distance, which quietly repudiated the Cartesian rule that bodies interact only by contact, and pointed the way toward the electron, the quark, and every redefinition that followed.