As points of contact between mathematics and reality, particles straddle both worlds with an uncertain footing. When I recently asked a dozen particle physicists what a particle is, they gave remarkably diverse descriptions. They also described two major research thrusts in fundamental physics today that are pursuing a more satisfying, all-encompassing picture of particles.
Two millennia later, Isaac Newton and Christiaan Huygens debated whether light is made of particles or waves. The discovery of quantum mechanics some years after that proved both luminaries right: Light comes in individual packets of energy known as photons, which behave as both particles and waves.
Wave-particle duality turned out to be a symptom of a deep strangeness. The wave function representing an electron, say, is spatially spread out, so that the electron has possible locations rather than a definite one.
A particle is thus a collapsed wave function. But what in the world does that mean? Why does observation cause a distended mathematical function to collapse and a concrete particle to appear?
Nearly a century later, physicists have no idea. The picture soon got even stranger. In the s, physicists realized that the wave functions of many individual photons collectively behave like a single wave propagating through conjoined electric and magnetic fields — exactly the classical picture of light discovered in the 19th century by James Clerk Maxwell.
In addition to photons — the quanta of light — Paul Dirac and others discovered that the idea could be extrapolated to electrons and everything else: According to quantum field theory, particles are excitations of quantum fields that fill all of space. In positing the existence of these more fundamental fields, quantum field theory stripped particles of status, characterizing them as mere bits of energy that set fields sloshing. Yet despite the ontological baggage of omnipresent fields, quantum field theory became the lingua franca of particle physics because it allows researchers to calculate with extreme precision what happens when particles interact — particle interactions being, at base level, the way the world is put together.
The properties of these particles and fields appeared to follow numerical patterns. By extending these patterns, physicists were able to predict the existence of more particles. The patterns also suggested a more abstract and potentially deeper perspective on what particles actually are.
Mark Van Raamsdonk remembers the beginning of the first class he took on quantum field theory as a Princeton University graduate student. Taking the apparently correct definition to be general knowledge, the professor skipped any explanation and launched into an inscrutable series of lectures. Take, for example, an equilateral triangle. Rotating it by or degrees, or reflecting it across the line from each corner to the midpoint of the opposite side, or doing nothing, all leave the triangle looking the same as before.
These six symmetries form a group. The group can be expressed as a set of mathematical matrices — arrays of numbers that, when multiplied by coordinates of an equilateral triangle, return the same coordinates.
We're moving to ukri. Some links may take you there. If you can't find what you're looking for, try ukri. Matter is everything that exists in the Universe — it is all the stuff that was created in the Big Bang.
Particle physicists believe that matter is built of twelve types of fundamental particle — the building blocks of the universe. These fundamental particles cannot be broken down any further. Diagram of the Standard Model. Neutrinos , electrons, muons and taus make up a category of fundamental particles called leptons.
Quarks, which make up protons and neutrons, are another type of fundamental particle. Together with the leptons, quarks make up the stuff we think of as matter. Once upon a time, scientists believed that atoms were the smallest possible objects ; the word comes from the Greek "atomos," meaning "indivisible. Then, throughout the s and '60s, particle accelerators kept revealing a bevy of exotic subatomic particles, such as pions and kaons.
In , physicists Murray Gell-Mann and George Zweig independently proposed a model that could explain the inner workings of protons, neutrons and the rest of the particle zoo, according to a historical report from SLAC National Accelerator Laboratory in California. Residing inside protons and neutrons are tiny particles called quarks, which come in six possible types or flavors : up, down, strange, charm, bottom and top.
Protons are made from two up quarks and a down quark, while neutrons are composed of two downs and an up. The up and down quarks are the lightest varieties.
Because more-massive particles tend to decay into less massive ones, the up and down quarks are also the most common in the universe; therefore, protons and neutrons make up most of the matter we know. By , physicists had isolated five of the six quarks in the lab — up, down, strange, charm and bottom — but it wasn't until that researchers at Fermilab National Accelerator Laboratory in Illinois found the final quark, the top quark.
Gravity is the weakest but it has an infinite range. The electromagnetic force also has infinite range but it is many times stronger than gravity. The weak and strong forces are effective only over a very short range and dominate only at the level of subatomic particles. Despite its name, the weak force is much stronger than gravity but it is indeed the weakest of the other three.
The strong force, as the name suggests, is the strongest of all four fundamental interactions. Particles of matter transfer discrete amounts of energy by exchanging bosons with each other. The Standard Model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains well how these forces act on all of the matter particles.
However, the most familiar force in our everyday lives, gravity, is not part of the Standard Model, as fitting gravity comfortably into this framework has proved to be a difficult challenge. The quantum theory used to describe the micro world, and the general theory of relativity used to describe the macro world, are difficult to fit into a single framework.
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