A Brief History Of Time

The division of the universe into either matter or force originated with Aristotle, who believed all matter is continuous; nothing was too small to subdivide. Another idea, proposed by Democritus, was that matter is made up of clumps of tiny bits called atoms. Without any data to support either side, the debate between these theories raged on for centuries until chemist and physicist John Dalton observed atoms clumping together in molecules. Almost 100 years later, Einstein provided physical evidence for the atomist school of thought. In 1905, Einstein noted that Brownian motion—the “random motion of small particles of dust suspended in liquid” (65)—could be the result of the atoms comprising the liquid bumping into the specks of dust. Near the end of the 19th century, J. J. Thomson showed show the existence of even smaller particles called electrons, and in 1911, Ernest Rutherford proved that atoms have internal structures. Today, scientists know atoms comprise negatively charged electrons that orbit much heavier, positively charged protons and their partners, neutrons, which were discovered in 1932 and have no charge. In the mid-1960s, physicists realized that protons and neutrons were also made up of particles, which physicist Murray Gell-Mann named quarks. Quarks come in six varieties called “flavors,” referred to as up, down, bottom, top, strange, and charmed. Each flavor comes in one of three different “colors”: red, blue, or green. They’re too small to have actual color or flavor; these are simply naming conventions. Three quarks, one of each color, make up a single proton or neutron.

Since scientists have divided atoms into two smaller portions beyond what Democritus postulated, it is unclear just how small the basic building blocks of the universe are. As quantum mechanics states, particles must be measured by something with a wavelength smaller than they are, meaning the quantum used to measure will have enormous amounts of energy, so scientists are limited by their ability to harness enough energy to view something on such a small scale. To examine sub-atomic particles, scientists first used electric fields to charge electrons with enough energy for measurement. The energy is called an “electron volt.” Initially, these fields were powered by chemical reactions and provided only low levels of energy. By the time Hawking wrote this chapter, scientists could charge electrons to the thousands of millions of electron volts. Hawking states that, while even smaller particles are not impossible, current theoretical reasons indicate the smallest unit of particles has already been found.  

Everything in the universe—both matter and forces—can be understood as a particle. All sub-atomic particles have characteristic “spins.” A particle of spin 0 looks the same from any direction. A particle of spin 1 is like an arrow: It looks different from different angles and will only look the same if turned in one complete 360-degree revolution. A particle of spin 2 is like a two-headed arrow and looks the same every 180 degrees of spin. Higher-spin particles look the same at smaller degrees of spin, while a particle of spin 1/2 only looks the same when turned around two times. Every known particle in the universe can be divided into either matter or forces; spin 1/2 particles make up the matter in the universe while spins 0, 1, and 2 particles are the source of forces that act on the matter in the universe.

Wolfgang Pauli won the 1945 Nobel Prize for his 1925 discovery that all  spin-1/2 particles obey an exclusion principle that prevents two matter particles from existing in the same state—they cannot have the same velocity and position. This explains why the particles that make up matter don’t collapse under the influence of the forces created by the spin 0, 1, and 2 particles. Otherwise, everything would collapse into a dense blob of matter instead of atoms with distinct protons and neutrons.

In 1928, Paul Dirac married the ideas in quantum mechanics with general relativity in a new theory that predicted the positron, a kind of “antielectron.” Dirac won the 1933 Nobel prize for figuring out that every particle has an opposite anti-particle. If a particle touches its antiparticle, they are annihilated.  

When they touch, matter particles emit force-carrying particles and the recoil from this event causes the matter particle’s velocity to change. The emitted force-carrying particle is absorbed by another matter particle, whose velocity is also changed. This results in the effect of a force between the two matter particles. Currently, scientists divide all force-carrying particles into four distinct categories, but one major goal of researchers is to develop a unified theory that shows each force is one aspect of a single force behaving in different ways. Three of these forces—weak nuclear force, strong nuclear force, and electromagnetic force—have been unified, but the fourth—gravitational force—Hawking discusses in a later chapter.

Though it is the weakest of all the forces, gravity is universal and acts over large distances. In large collections of mass particles, such as stars, this has a big effect. Gravitons can travel long distances; those that travel between Earth and the sun cause the Earth to orbit the sun.

Electromagnetic force only interacts with particles carrying an electrical charge, which can be either positive or negative. Like charges repel while different charges attract. Though the electromagnetic force is significantly stronger than gravity, the electrical charges across large bodies are roughly equal, which causes the force to have a negligible effect. Electromagnetic particles are called photons. When an electron orbits closer to the nucleus of an atom, it emits a photon. Likewise, a photon that collides with an electron adds its energy to the electron and causes it to jump into a higher orbit.

The weak nuclear force responsible for radioactivity is made of particles with spin 1 that also have mass. There are three types: W-, W+, and Z° (“zee naught”). Together, they’re called “massive vector bosons” (74), and they interact only with mass particles of spin 1/2. At very high energies, these particles behave the same way as photons; thus, they’re all considered different versions of each other.

The fourth force particle is called the gluon. It conducts the strong nuclear force that binds quarks together into protons and neutrons, and binds all of them together into the nucleus of an atom. At super-high energies, these particles become similar to massive vector bosons and photons. Physics, therefore, is partway toward a complete Grand Unified Theory of everything. The equipment required to prove that all the basic particles behave the same way at super-high energies would be as large as the solar system, but certain low-energy experiments hint that the theory is correct.

At the beginning of the universe, there might have been as many particles as anti-particles, but there’s no sign in the present universe of the radiation that should be emitted as these opposites annihilate each other. Instead, a small asymmetry in the laws of physics allows for slightly more quarks and electrons to be produced than anti-quarks and anti-electrons. Most would have been annihilated quickly, and what’s left is the matter we see in the universe. 

In 1783, Cambridge theorist John Michell proposed that some stars are so heavy that the light they emit gets dragged back into them by their massive gravity. This would only happen if light were made of particles; in the 1800s, that theory was eclipsed by the theory that light is made of waves. In the 1900s, scientists realized that atom-sized things are both particles and waves, so the giant-star gravity theory came back into vogue. Such stars are now called black holes because no light can escape them, and they appear dark.

Stars form when huge gas clouds coalesce. As the ball of gas grows larger and denser, it heats up as its atoms bounce off each other more and more. When dense enough, the cloud’s atoms jam together into larger atoms and give off extra light and heat. This “controlled hydrogen bomb explosion” keeps the star from contracting even more from its own gravity (85). Very large stars burn especially hot and use up their fuel in millions of years; Earth’s sun is smaller and will last for billions of years.

When a star’s fuel runs out, it collapses into a very dense, hot object much smaller than the original star. Smaller stars become 1,000-mile-wide white dwarfs, whose collapse is halted by the electrons in their closely packed atoms. Stars about 1.5 times larger than Earth’s sun—a size called the Chandrasekhar limit in honor of the Cambridge student, Subrahmanyan Chandrasekhar, who figured it out in 1928—will collapse further until all the protons and neutrons are packed together into what’s called a neutron star about 10 miles across with “a density of hundreds of millions of tons per cubic inch” (86).

Robert Oppenheimer—who later directed the US atomic bomb project—proved in 1939 that even more massive stars contract so much that everything—even light—inside a spherical border around the star, called the event horizon, cannot escape the star’s gravity. The region within is called a black hole. Since time slows down near a massive gravity field, an astronaut traveling into the black hole and sending signals back to his spaceship would appear, to the ship’s crew, to slow down his transmissions in the final second. The last signal he sent before disappearing into the black hole would literally take forever to get to the spaceship. Hawking and Roger Penrose showed, in the late 1960s, that such stars must collapse into an infinitely dense point where the laws of science also collapse. Such an event is essentially the Big Bang in reverse. Things outside the black hole, though, are protected from the star’s internal violation of physics by its event horizon. No star is perfectly identical to any other, but the laws of Relativity mean that all black holes end up identical except for size, which depends on their mass, and rotation rate. Scientists like to say, “A black hole has no hair” (95): They basically all look the same.

The first evidence of black holes was found in 1963 by Maarten Schmidt at the Mt Palomar Observatory in California. He discovered very faraway objects that gave off enormous amounts of energy. These “quasi-stellar objects” (96)—quasars—are black holes that contain the collapsed mass of entire galactic centers. Further evidence for black holes comes from stars that rotate around something invisible that pulls gas from the visible star and accelerates it until the gas gives off x-rays. The visible star’s orbital period shows that the invisible companion is too large to be a neutron star. Black holes thus reveal themselves by their effects on nearby objects.

It’s possible that there are more black holes than stars, which may explain the extra mass needed to make galaxies rotate as fast as they do. The centers of galaxies seem to contain very large black holes: Even larger ones power the bright centers of quasars. Small black holes may have formed in the high pressures of the very early universe. A super-large hydrogen bomb might also cause a black hole to form.

If two black holes merge, the area of their event horizon is the sum of their two separate ones. The size of this new event horizon also is the size of the black hole’s total entropy, or state of disorder. This is similar to two boxes, one with oxygen and one with nitrogen, which are joined, and the wall removed between them, so that the oxygen and nitrogen mix together and thereby become less organized. Figure 7.3 shows two black holes merging. They travel upward through time, their paths represented by small tubes that combine to form a single, larger tube; the resulting diagram looks something like a pair of pants, with the two small tubes as the legs. The volumes of the two black holes thus combine into one larger black hole.

Black holes leak when virtual particles—created out of nothing by the uncertainty principle—form next to the event horizon. These manifest as pairs of opposites, a particle with positive energy and an anti-particle with negative energy. Sometimes both particles fall into the black hole, but sometimes the positive-energy particle escapes, while the weak negative-energy particle drops into the black hole. Negative energy means negative mass, so the negative-energy particle reduces the total mass-energy of the black hole. To outsiders, the positive-energy particle seems to escape from the black hole. In this way, black holes very slowly evaporate. A small black hole will leak faster and faster and get hotter and hotter until its remaining mass escapes the hole nearly all at once, “in a tremendous final burst of emission, equivalent to the explosion of millions of H-bombs” (111). Black holes with the masses of large stars leak much less energy than they absorb from the cosmic microwave background radiation. Even when the universe becomes cooler than such black holes, it will take “about a million million million million million million million million million million million years (1 with sixty-six zeros after it)” for them to disappear (112).

Small black holes may have formed during the Big Bang. These “primordial” black holes of perhaps a billion tons each are much warmer and leak faster; they can evaporate in 10 to 20 billion years, and some may already be gone. Black holes produce gamma rays, but the total cosmic background gamma radiation suggests that all primordial black holes together can’t be more than a millionth of the universe’s total matter.

Nuclear test-ban satellites use gamma-ray detectors to note any nuclear-bomb tests, and they detect about 16 bursts per month, but they appear from every direction and therefore arrive from outside the solar system. Some of these may be evaporating primordial black holes, but they’re mainly associated with other unusual events, like collisions of neutron stars.