摘要：Get to grips with the deepest layer of reality we know of with our inventory of the subatomic realm, from known particles like quarks and the Higgs boson to hypotheticals including the fifth force and strings in 11 dimensions

Get to grips with the deepest layer of reality we know of with our inventory of the subatomic realm, from known particles like quarks and the Higgs boson to hypotheticals including the fifth force and strings in 11 dimensions

The ancient Greeks speculated that it might be air, fire or water. A century ago, physicists felt sure it was the atom. Today, we believe that the deepest layer of reality is populated by a diverse cast of elementary particles, all governed by quantum theory. From this invisible, infinitesimal realm, everything we see and experience emerges. It is a world full of wonder, yet it can be mystifying in its weirdness. Or at least it can often feel that way.

What you’ll find below is a concise, clear-eyed guide to the known particles and forces – from electrons, quarks, and neutrinos to photons and the Higgs boson – as well as the quantum laws and phenomena that give quantum physics its reputation for strangeness, including wave-particle duality, entanglement, and the uncertainty principle. You will also discover the hypothetical particles that could make sense of cosmological conundrums such as dark matter and dark energy, and the stranger things that might lurk beneath the quantum realm. Finally, you will have many of your questions answered, not least what is a theory of everything anyway?

MATTER

We start with what we pretty much know for sure. Visible matter consists of atoms, and at the centre of atoms are protons and neutrons. But even these aren’t elementary particles, as detailed by the current “standard model” of particle physics, our leading description of reality on the tiniest scales. So we begin, deep down, with what matter is really made of.

Electrons

Weighing in around 1800 times lighter than protons or neutrons, electrons add very little to the overall mass of atoms. Without the electron, however, we would scarcely be able to feel matter at all. That is because electrons have a negative charge and exist in an “orbit”, or cloud, surrounding atomic nuclei. When you touch something, the atoms in your fingertip aren’t directly butting up against the ones in an object. Instead, what you are feeling is the mutual repulsion between the negative electrons surrounding the atomic nuclei in your finger and those in the object, via the force of electromagnetism (see “Photons: Electromagnetism”).

The electron plays the lead role in almost all other aspects of everyday life, too. By and large, when atoms bind in solids, liquids and gases, it is through the transfer or sharing of electrons, to balance charge and make things stable. All chemical reactions – from photosynthesis to combustion, from decomposition to the subtle reactions involved in our sense of taste and smell – similarly boil down to electron rearrangement. They are also the vehicles of electricity: their fine manipulation in transistors, which control the flow of electrical current, is what makes computers and many other modern technologies possible.

Quarks (up and down)

“Three quarks for Muster Mark!” heckles a character in Finnegans Wake, James Joyce’s famously indecipherable novel. Joyce apparently intended “quark” to mean the caw of a crow. For physicist Murray Gell-Mann, however, the term was suitably bizarre to describe the elementary particles he successfully predicted in the 1960s and that combine to constitute protons and neutrons.

Here, quarks come in two types, up and down – but don’t take these labels literally. The difference is that the up quark has a fractional charge (that is, a fraction of the electron’s charge) of +⅔, as opposed to the down quark’s –⅓, and is less massive. The neutron contains two down quarks and an up; the proton, two up quarks and a down, all stuck together with another type of elementary particle – the gluon (see “Gluons: The strong force”). Do the maths and you see that this is why the proton is slightly lighter than the neutron and has an overall positive charge. It is this that ropes negative electrons into orbits around atomic nuclei.

Neutrinos

In the standard model of particle physics, electrons and neutrinos are classed as leptons, a word derived from the Greek for “small”. And while the electron’s mass is small relative to that of protons, neutrons and quarks, it is a beast compared with the neutrino’s, which is believed to be a million times less. What’s more, being electrically neutral and therefore heedless of the electromagnetic force, the neutrino is the wispiest of matter particles, zipping through objects almost completely undetected. Did you notice that, while reading this, billions of neutrinos – produced via nuclear fusion in the sun – were coursing through your eyeballs? Us neither.

Still, neutrinos do interact with matter very occasionally, via the weak force (see “W & Z bosons: The weak force” ) that is involved in various types of nuclear decay. But we don’t always see what we expect in these events. That is because quantum laws allow neutrinos to oscillate between different flavours: an “electron neutrino” can transform into a “muon neutrino” or a “tau neutrino”. In this way, neutrinos are a window into the strange fact that matter comes in what particle physicists refer to as three generations.

For each of the four original, or “first-generation”, elementary matter particles, physicists have also found not one, but two duplicates, differing only from the originals in their greater mass. For the up quark, the heavier duplicates are the charm quark and top quark; for the down quark, there is the strange quark and bottom quark; for the electron, we have the muon and tau; and for the electron neutrino, the muon neutrino and tau neutrino.

“Who ordered that?” asked Nobel physics laureate Isidor Rabi when the muon was discovered in 1936. Physicists haven’t lost his incredulity. Some of these second and third-generation particles are heavy indeed – the top quark dwarfs even the mighty Higgs boson – yet, as far as we know, they are identical to their forebears in all other respects. No one knows why they exist. In fact, they don’t seem to have a very active role in the universe at all.

What we can say is that they make things more complicated. With a total of six different quarks, for instance, nature is known to permit more than 150 composite particles, in addition to the familiar proton and neutron. We have discovered many two and three-quark composites and are now finding four and five-quark ones. All of which might sound like nothing more than stamp collecting. But the hope is that small variations in the rates of the events producing these composite particles will explain why nature seems to prefer matter over antimatter (see “Where has all the antimatter gone?”).

FORCES

Matter would be incomparably boring on its own: forces make the universe tick. But our modern understanding of force is nothing like the classical notion of one lump of stuff exerting itself on another. Instead, we know that there are four fundamental forces and that they themselves consist of particles that are being constantly emitted and absorbed by matter.

https://solarsystem.nasa.gov/resources/392/solar-fury/?category=solar-system_sun

Source: NASA's Goddard Space Flight Center Published: November 30, 2017 On Jan. 22, 2012, the Sun erupted with a solar flare, a coronal mass ejection, and a burst of highly energetic protons known as solar energetic particles. The solar flare was only medium in size. But the other two events packed quite a punch creating the most intense solar radiation storm since 2003. Within minutes of the eruption, solar particles swirled into the Earth's magnetosphere?the protective envelope that shields our planet from the sun's powerful rays. Dazzling auroras electrified the night sky as the coronal mass ejection raced behind the flare at almost 1,400 miles per second and hit Earth within 36 hours. For three days the storm degraded radio transmissions at high latitudes, forcing some airplanes flying polar routes?where pilots rely exclusively on radio navigation?to be rerouted. Watch the video below for multiple views of the eruption as captured by sun-observing satellites.

The electron, a matter particle of everyday physics, works in partnership with the carrier of the electromagnetic force, the photon. When two electrons repel each other, they recoil in opposite directions as one of them emits a photon and the other absorbs it. The arrangement of atomic electrons gives materials their colour, but it is the absorption and re-emission of photons by a material that conveys its colour to the retinas of our eyes. Photons also make the rearrangement of electrons in chemical reactions possible, by putting energy in or taking it away.

Sometimes, we see these photons as visible light, such as the flame produced in a combustion reaction. More often, we don’t. A flame also produces photons in the form of heat (infrared radiation). These are invisible because their energy is too low for our eyes to detect. Photons that are even less energetic make up radio waves, while very high-energy photons constitute the equally invisible but dangerous ultraviolet rays, X-rays and gamma radiation.

Gluons: The strong force

The strong nuclear force, which holds together quarks inside protons and neutrons, is delivered by the gluon – no prizes for guessing where that gets its name. Gluons operate through something that is unique to quarks, known as colour, a special quantum property that has nothing to do with colour as we know it. Quarks and gluons come in three colours, red, green and blue, which mix to make white. In an interaction, a gluon can change the colour of a quark, but if it does, another gluon will change the colour of a neighbouring quark, so that white remains their overall mix. For instance, a proton’s three quarks might at one moment be red, blue and green; at another, they might be green, blue and red. No one said particle physics was simple.

Gluons have such a strong grip that quarks are never observed in isolation. This also means gluons harbour a lot of energy, which translates, via Einstein’s mass–energy equivalence, to a lot of mass. In fact, the vast majority of the mass of the atom comes not from quarks and electrons, but from gluons.

W & Z bosons: The weak force

The carriers of the weak nuclear force, which governs some types of radioactive decay, are friendly giants. Unlike the photon and gluon, which are massless and travel at light speed, the W and Z bosons are slow and could tip the scales even against atoms of iron. Yet, compared with the photon and gluon, the effect that the W and Z bosons have on other particles is hundreds of billions of times feebler. That is because, in quantum physics terms, mass doesn’t equate to strength, but reach, or rather lack of it: the weak force has a range of less than a quadrillionth of a millimetre (10-15mm).

Still, they can have powerful consequences. In the fusion of hydrogen into helium in the sun, one of the up quarks in the hydrogen’s proton changes into a down quark, transforming that proton into a neutron – a necessary step as, unlike hydrogen, helium requires neutrons as well as protons. To make this happen, the former proton’s positive charge must be carried away with the emission of a W+ particle. In this way, the humble W boson keeps the sun shining – and makes life on Earth, and you, possible.

The Higgs boson

Mass isn’t an easy concept. We tend to equate it with weight, but that is really a measure of gravitational attraction, even though the terms weight and mass are often used interchangeably. A proper definition of mass relates to inertia, in the sense of how much an object resists acceleration when a given force is applied to it. A bicycle has a certain inertial mass; a 50-car freight train, much more so.

Named after theorist Peter Higgs, one of the people who predicted its existence, the Higgs field is what gives particles inertial mass. Neither force nor matter, the Higgs field is unique among quantum fields in having a finite intensity at all points in space, even in a vacuum when there isn’t enough energy for its particle manifestation, the Higgs boson, to be present. The matter particles, as well as the W and Z bosons of the weak interaction, are effectively bogged down by the Higgs field to varying degrees, thereby acquiring inertial mass. Otherwise, they would be massless and move, like photons and gluons, at the speed of light.

Many questions surround the Higgs. Can it interact with itself, to acquire its own mass? Is there just one type of Higgs or are there more, perhaps ones that constitute dark matter, the mysterious stuff that keeps galaxies from flying apart? Maybe the biggest mystery, though, concerns the measured value of the Higgs boson mass and the intensity of the underlying field. At 125 gigaelectronvolts, it is hefty – equivalent to the mass of over 100 protons or that of a moderately sized molecule – yet also seemingly precisely tuned to make life possible. If it were just a few times more massive, atomic nuclei would be unbalanced, hydrogen would be the only stable element and the universe would be very bland.

Worse still, according to theoretical calculations, the Higgs field isn’t necessarily in its most favourable state. At any moment, it could slip into a more stable configuration that isn’t just a few, but billions of times more intense, instantly turning our orderly universe into chaos. This would be a purely random event, based on a phenomenon known as quantum-mechanical tunnelling, in which a particle can surmount an energy barrier that appears insurmountable. Fortunately, such an event is predicted to occur just once every googol (10100) years or so.

UNDISCOVERED PARTICLES

The list of known particles comes to about 30, depending on how you count them. But now we must enter the unknown – the thicket of hypothetical particles that have been proposed to solve the things we don’t yet fully understand, from gravity and the big bang to the mysterious presences of dark matter and dark energy.

https://www.nasa.gov/image-feature/goddard/2022/hubble-studies-a-spectacular-spiral

A Hubble space telescope image of spiral galaxy NGC 1961

Gravitons

Gravity itself isn’t in question: it is the fourth fundamental force and the only one to have achieved household-name status. But its force carrier, the graviton, remains hypothetical. That is partly because no one is sure that gravity can be quantised (see “Quantisation”) and partly because, even if it can, its quantum particle would be incredibly difficult to detect.

This is because gravity is so excruciatingly weak. Earth produces a gravitational field strong enough to keep our feet on the ground most of the time, yet even children can momentarily escape its pull every time they jump. The field would have to be a whole lot stronger to manifest a single graviton. How strong? Well, physicists have imagined placing a graviton detector with the mass of Jupiter in orbit around a neutron star, which are notoriously dense and, as such, perhaps the biggest single sources of gravity besides black holes. They predicted that the detector could record one graviton every 10 years. If the detector was 100 per cent efficient. Maybe.

Inflatons

The universe we observe is surprisingly uniform. On the grandest scales, one patch of sky looks much like another: a continuous pattern of stars and galaxies, almost as though they evened themselves out during the expansion from the big bang. But that is impossible, according to Einstein’s special theory of relativity, as it would require information to travel faster than light.

Maybe the universe we see is merely a speck of a primordial super-universe, a speck that, for a brief moment, inflated much faster than the general expansion. Rather like blowing up a single pixel from a digital image, our cosmos was filled with a space that was comparably bland and featureless. In this idea, this cosmic inflation was driven by a particle known as the inflaton, which has some of the same characteristics as the Higgs boson. In fact, some theorists think the Higgs could be the inflaton, although it would have had to behave very differently in the early universe.

Quintessence

We have come a long way since the “steady state” theory, popular until the middle of the 20th century, which held that space is essentially unchanging. All observations suggest that there was once a big bang and that space has been expanding ever since. Many physicists believe there was an early burst of particularly rapid expansion called cosmic inflation. Now, it seems that the expansion of space is again accelerating, though far more slowly than in the early universe, due to some mysterious entity known only as dark energy.

Dark energy has a strong claim to being the most mysterious thing out there. No one knows what it is and physicists are struggling to figure out precisely how it behaves, given that its effects take place over billions of years. Still, there are a few candidates. One is that, in addition to the four known fundamental forces, there is a fifth force, or “quintessence”. The related particle doesn’t have a name. What we do know is that, by necessity, the quintessence must be weaker than gravity, so it is difficult see how we would ever detect one of its particles.

https://wwwmpa.mpa-garching.mpg.de/galform/cr/images.shtml

A simulation of the distribution of dark matter in the local universeCopyright: Max Planck Institute

Weakly interacting massive particles

People, planets, stars: these are just a tiny fraction of the universe’s stuff. In fact, there must be at least five times more matter than we can see, otherwise galaxies wouldn’t generate enough gravity to stop themselves flying apart. The extra stuff is called dark matter, but what exactly is it?

Dark matter is invisible, in the sense that it doesn’t absorb or reflect light. Yet we can infer a few things about its properties. It must interact with visible matter only weakly, otherwise we would have detected it already. And it probably consists of massive particles – up to 10 times that of the Higgs – otherwise it would be created too easily in today’s universe and make things unstable. Weakly interacting massive particles, or WIMPs, seem to tick all of the boxes. These have long been the favoured dark matter candidates, but despite the odd teasing hint, they have never been discovered.

Axions

Perhaps dark matter isn’t made up of matter particles at all, but force carriers. If so, the particles could be so weakly interacting that, unlike WIMPs, they form a stable population in today’s universe whatever their mass. Based on these criteria, the bookies’ favourite is the axion. Named after a brand of washing detergent, the axion was originally introduced to “clean up” a different problem: why the strong force affects quarks and antiquarks in precisely the same way, when the basic theory permits wild discrimination. The axion field would enforce equilibrium in the strong force, like a finger on the balance. If the particle solves dark matter too, all the better.

Sterile neutrinos

Arguably the simplest candidate for dark matter is one that does nothing but boost the gravity of galaxies and other large structures. Unlike its known counterparts – the electron, muon and tau neutrinos – the sterile neutrino would be indifferent, or “sterile”, to the weak force. It would interact via gravity, and gravity alone, quietly yet firmly keeping galaxies intact. A preponderance of sterile neutrinos could easily be dark matter, then, and maybe solve some other problems. Their fields could mix with those of the known neutrinos, explaining why these have a very small but finite mass. And because sterile neutrinos would decay to produce more matter than antimatter, they could explain why our universe is dominated by matter (see “Where has all the antimatter gone?”).

Dark photons

What if the dark universe is as complex as our own? Could there be dark stars? Dark planets? Dark civilisations? There is no evidence for any of this, but it is still tempting to think that dark matter isn’t just some idle blob. Perhaps, like visible matter, it has at least one force of nature it can call its own. Enter the dark force and its carrier: the dark photon. This would make dark matter particles interact with one another, in a closed community. Should it exist, the consequences of a dark photon are tantalising, given the incredible diversity of phenomena for which the normal photon is responsible. It could bind dark matter particles into atoms, for instance, or form the basis of a dark chemistry.

DEEPER LAYERS OF REALITY

There are at least four fundamental forces, but why four? In fact, physicists hope that they will turn out to be facets of one single, unified force – a universal set of equations that describe the behaviour of everything in the universe. Success depends on reconciling the two greatest physical theories of the 20th century: quantum theory and Einstein’s theory of gravity. Time to venture into the deepest unknown.

Gene?ve / Switzerland - April 2010 : CERN the European Organization for Nuclear Research where the Higgs boson was detected in 2012 in the ATLAS and CMS experiments, conducted with the LHC accelerator; Shutterstock ID 1287557629; purchase_order: -; job: -; client: -; other: -

The CMS detector at the Large Hadron Collider, near Geneva

X bosons and leptoquarks

Combining the electromagnetic and weak forces provided the theoretical basis from which we predicted the existence of the Higgs boson (see “What is a theory of everything?”), so it follows that incorporating the strong force will also entail new particles. One of the possibilities for such a “grand unified theory” is that quarks, governed by the strong force, can convert into electrons, neutrinos and other leptons governed by the electromagnetic and weak forces. Such conversions would have to be mediated by new force-carrying particles, known as X bosons and leptoquarks.

Sparticles

In particle physics terms, matter and force are different only in as much as matter particles take up space, whereas force particles don’t (see “The exclusion principle”). For decades, however, physicists have entertained a deeper, “supersymmetric” theory that makes matter and force two sides of the same coin. It holds that every known matter particle has a corresponding, heavier force particle, and vice versa, designated by an “s” prefix or “ino” suffix. The force partner of a quark is a heavy “squark”, for instance, while the matter partner of a gluon is a heavy “gluino”, and so on.

Supersymmetry has long been invoked as a potential grand unified theory. It also provides a neat way of explaining why the Higgs boson is much, much lighter than theoretical predictions would otherwise imply: the extra, heavy particles balance the equations. The problem is that the world’s biggest particle smasher, the Large Hadron Collider at CERN near Geneva, has found no evidence of supersymmetry.

Magnetic monopoles

Every magnet has two poles: north and south. Which is strange, when you think about it. Electrical terminals also often come in pairs – positive and negative – yet it is possible to create a single negative or positive electric pole on its own, by concentrating electrons in one spot or taking them away. Magnetism is related to electricity – they are both governed by electromagnetism. So why can’t we produce magnetic monopoles?

Physicist Paul Dirac was one of the first to suggest that magnetic monopoles could exist, in the early 1930s. Only decades later did physicists revisit them with any conviction, driven by mathematics suggesting that they have to exist if we are to bring all four of the forces of nature within a single theory of everything. We are yet to find them.

Strings

The most famous contender for a theory of everything, known as string theory, says that the most elementary things of nature aren’t point-like particles, but vibrating strings. All these strings are made of the same stuff, but the pattern in which a string vibrates determines what kind of particle it manifests as. Like a musical string, which can express different tones, a particle string can “sound” like a quark, or an electron, or a photon, or any other particle in the current standard model. It can even vibrate like that most elusive of force carriers, the graviton (see “Gravitons”). The catch is that strings need to vibrate in 11 dimensions.

Proponents say this isn’t as fantastical as it sounds, as the extra dimensions might be curled up around the strings so tightly as to be beyond our abilities to detect. Still, string theory has taken a knocking in recent years because it acts as a master theory for supersymmetry and its multitudinous offspring, none of which have been found.

Loops

The starting point of another theory, known as loop quantum gravity, is that the fourth force can never be treated like the other three. It is, in a sense, more fundamental. Accordingly, this theory focuses purely on reconciling quantum physics and gravity by proposing that space-time itself has something like an atomic structure. Specifically, it says space-time comes in finite loops, which cling together to create a new stage for reality to play out on called a spinfoam. Loop quantum gravity is a long way from incorporating all of what we know and describe in the standard model of particle physics, and it is no better than string theory at making testable predictions.

Twistors

Dating back to the 1960s, twistor theory is very much the underdog among theories of everything. The brainchild of Nobel laureate Roger Penrose, it says that space-time and its force manifestation, gravity – not to mention other fields and particles – are merely the gloss of a deeper reality that consists of chains of cause and effect. Although the chains themselves can never be broken, the apparent location and timing of events within them is variable, so that their full description is something broad and contorted: a twistor.

If that sounds confusing, you are in good company. Twistor theory is so conceptually difficult that even Penrose shies away from explaining it in interviews. But although it has been unpopular for decades, it has experienced a renaissance in recent years, with theorists finding intriguing connections to string theory.