In 1803, John Dalton, an English chemist, made a landmark discovery. Matter, he declared, couldn’t be broken down into anything more elemental than atoms. They were the tiniest, indivisible units possible.
Today, of course, we know that they’re not the solid billiard balls they were once believed to be. They have complex interiors. Each atom has a nucleus, which, again, is made of neutrons and protons. Around this tight core, energetic electrons, one or more, zip around in concentric orbits.
The matryoshka doll-like nesting structure continues. Neutrons and protons, in turn, are made of still smaller particles.
The Standard Model of physics, which is a map of the subatomic space, describes the nature of these particles, and how they interact.
The cosmic equivalent of a single LEGO brick, a dozen of these, the fermions, are the building blocks of everything that we know in the observable universe—from a petal of hibiscus to a spoon of table salt to an asteroid to Europa to a white dwarf to the Sun’s rays.
Six of those are quarks. They come in six varieties, known by the curiously vanilla names of “up,” “down,” “charm,” “strange,” “bottom,” and “top.” A proton, for instance, is made of two “up quarks” and one “down quark.”
The other six are leptons. These include the electron, and its two heavier siblings—the muon and the tau—as well as three neutrinos.
These particles are anything, but inert. They socialize furiously through four pivotal forces. In order of diminishing strength, they are:
- Strong (that makes protons and neutrons stick.)
- Electromagnetic (which makes a nail cling to a magnet.)
- Weak (that turns a proton into a neutron or the other way round.)
- Gravity (which draws all bodies towards each other.)
These forces, themselves, piggyback on yet other particles.
The strong force is conveyed by massless, charge-free particles called gluons. It’s what binds quarks, and acts only at very, very close quarters—10-15 meters—but at that range it’s 1038 times stronger than the gravitational force.
Electromagnetic force, which comes into play between any two charged particles, is transmitted by photons, which too, have no mass and no charge. It’s these that keep the negatively-charged electrons spinning faithfully around a positively-charged nucleus.
Light, too, is a beam of photons. As they can be swapped between two particles over infinite distance, we’re able to see starlight from the edge of far-flung galaxies.
The weak force is mediated by three relatively enormous particles, known collectively, as bosons: the W+, the W-, and the Z.
While the Standard Model’s sets of elegant equations offer a flawless narrative of the dynamics of the universe on a Lilliputian scale, with its zoo of elementary particles, and their engagement—it drops the ball when it comes to tackling gravity, a force that operates on a Brobdingnagian scale (as compared to the other three, that is.) It’s the invisible tug that makes a rock roll down a hill or keeps the Moon in Earth’s thrall or curves space-time.
Gravity would have to be riding on gravitons, per the Standard Model. But are there any? None have been found. And that’s the rub. Coming up with a quantum answer to the question of gravity has been one of the monumental challenges of conventional physics.
But “string theory” has a solution that may explain the workings of all the four forces seamlessly. Because it tries to weave together quantum field theory—which explains the behavior of objects at the teeniest level—and general relativity—which explains the behavior of objects at the grandest level—it’s sometimes, flatteringly, dubbed as the “Theory of Everything.”
It posits that the myriad elementary particles that we see as different dots are, in reality, manifestations of one article: a string. When it vibrates one way, we see, in it, an electron. When it oscillates another way, we see, in it, a quark, and so on.