A Brief History of Time: Chapter 5 Summary & Analysis

Aristotle thought the universe was made up of earth and water, which tended to sink (gravity), and wind and fire, which tended to rise (levity). He thought matter was continuous, meaning matter could be divided and divided again infinitely. Fellow Greek philosopher Democritus disagreed and believed in atoms.

The argument wasn’t settled until 1803, when John Dalton discovered the existence of molecules. Einstein provided important evidence when he explained the random movement of dust in liquid was caused by the dust and liquid atoms colliding. J. J. Thompson at Cambridge had already proven the existence of electrons, and later Ernest Rutherford showed the atom had a nucleus, around which the electrons orbit.

James Chadwick discovered the neutron (which has no charge) made up the nucleus of an atom along with the previously discovered proton, and later won the Nobel Prize for his discovery.

In the mid 1900s, Murray Gell-Mann discovered quarks and won the Nobel Prize for his work on them. There are six “flavors” of quark: up, down, strange, charmed, bottom, and top, which were found in succession. Each comes in three “colors”: red, green, and blue. These names are just labels. Quarks form the proton, electron, and neutron. Scientists can create other particles from quarks, but these are unstable.

The question remains as to what the truly indivisible particle is. The smallest wavelength of light we can see is larger than these particles, so we cannot “look” at them. But if particles are also waves, and higher-energy particles have smaller wavelengths, scientists can aim to harness ever-higher energies to look at ever-smaller particles. As technology advances, scientists might even be able to achieve higher energies, but scientist think they already have found the smallest particles in existence.

Particles have a property called spin, which reveals what a particle looks like from different directions. A particle with spin 0 is like a dot—it looks the same when viewed from any angle. Spin 1 means it is like an arrow—it must turn one time to look the same from the same viewpoint. Spin 2 means the particle can turn halfway and look the same, and so on. There is even a spin ½, which means a particle must turn twice before it looks the same. Particles of spin ½ are all the particles that take form as matter. Particles of spins 0, 1, and 2 create forces between matter particles.

Wolfgang Pauli won the Nobel Prize for discovering that two matter particles cannot exist in exactly the same space going at the same velocity. This is called the Pauli exclusion principle. The way particles inherently repel each other and spread out is what gives the universe its structure and stops it from collapsing and becoming “soup.” 

Paul Dirac was the first to propose a theory consistent with both the general theory of relativity and quantum mechanics. He showed mathematically how spin ½ works and predicted that electrons should have partners, antielectrons or positrons. This later lead to his Nobel Prize. Indeed, every particle has an anti-particle, it is now known, and the two can cancel each other out.

Forces acting between matter particles are carried by the force particles—that is, those particles of spin 0, 1, and 2. Matter particles emit the force-carrying particles, which then change the particle’s course from the recoil resulting from the emission. The force-carrying particle is then absorbed by another matter particle, also effecting the velocity of this second matter particle. These force carrying particles do not obey the Pauli exclusion principle, so they can build up to become bigger forces. Their range depends on their mass, and they are considered virtual particles as they cannot be detected directly.

Still, scientists can see the effect such force particles have. They’re observable in the form of “classical” waves, such as light or gravity. Scientists have created four classes of forces for these types of particles, which they hope one day to unite as four types of one single force.

The first class is gravity. Every particle feels gravity, according to its mass and energy. It is also the weakest of the four forces. It always attracts and acts over long distances, unlike other forces. Big things, like the earth, can create a large overall gravitational force. A particle of spin 2 called the graviton carries this force. Because it has no mass, the graviton can travel great distances. Although a virtual particle, gravitons are the reason the earth orbits the sun.

Another class is the electromagnetic force. This affects only electrically-charged particles like electrons, but not gravitons. It is much stronger than gravity, and comes in the form of positive or negative charge. If two charges match, such as positive and positive, they repel each other, while opposite charges attract each other. On the small, atomic scale, electromagnetic forces dominate all activity. This force arises from the exchange of photons. Real photons are made when electrons move along the set orbits of an atom. We see photons as light, and they can be captured in photographs.

The third force is the weak nuclear force. It creates radioactivity and acts on matter particles, but not force-carrying ones. Abdus Salam and Steven Weinberg both put forward ideas on this force that linked it with the electromagnetic force in 1967. They said three types of particle called massive vector bosons carried this weak nuclear force, and are spin-1 particles. These particles only seem different at low energies, and at high energies they all act the same, much like a roulette ball has 37 slots to fall into at low energies but flies round and round in a circle at high energies; at low energies, it looks like there are 37 types of ball. This is called spontaneous symmetry breaking, when at low energies these particles break their symmetry, take on higher masses, and travel shorter ranges.

The fourth type is the strong nuclear force. This holds the atom together. The gluon, a particle of spin 1, carries this strong nuclear force and interacts only with quarks and itself. This force exhibits confinement, meaning all types of particles that create a structure (for example quarks, which come in different “colors,” red, green, or blue) must add up to a white color, meaning one each of the three colors must be involved. This can also create unstable particles, such as mesons, which are made when quarks join with antiquarks. These fit the no color rule (they are white, e.g. because a red particle joins with an anti-red one), but the particle and anti-particle pair can annihilate each other, creating other particles in the process. Quarks and gluons can therefore not go about alone and unconnected, as they have color, and particles must be “white” to be stable. At high energies, the strong nuclear force becomes weaker, and quarks and gluons can start to pull free.

Grand unifying theories try to bring the last three forces together, though the name doesn’t quite fit as they don’t include gravity. The idea is that at some high energy, the strong nuclear force would be weakened, and the electromagnetic and weak nuclear force would strengthen, so that all three would be equal. In this theory, they could thus be different types of the same force.

The grand unification energy, as it is called, is not known. It cannot be tested, as a particle accelerator with enough power to do so would have to be the size of the universe. But scientists can test low-energy outcomes of the theory. For example, protons could spontaneously decay into smaller particles. But the probability of that is very low, and it has never been observed.

How humans beings came to be is possibly due to the reverse process—the production of quarks and protons. If there were regions of anti-matter (made of anti-particles), there would be a lot of radiation given off at the border with other regions, where particles would meet anti-particles and annihilate, giving off energy. The universe must therefore be mostly matter or anti-matter, or all the collisions and annihilations would leave very little matter behind. It is possible the early universe had an equal amount of each, but the laws of physics do not apply to matter and anti-matter in the same way, resulting in the imbalance we see today.

The laws of physics can obey certain kinds of symmetries. Symmetry C refers to the laws applying to particles and anti-particles in the same way. Symmetry P is the laws being the same in a mirror image situation, for example a particle spinning clockwise or anti clockwise. Symmetry T is laws having the same effect if the passage of time is reversed.

In 1956, Tsung-Dao Lee and Chen Ning Yang found the weak nuclear force does not obey symmetry P. This was proven true by Chien-Shiung Wu, who caused radioactive atoms to spin in a magnetic field, first one way then the other. More electrons were given off in one direction than the other. Lee and Yang won the Nobel Prize for their idea.

The weak nuclear force also does not obey the symmetry C, meaning it would cause a universe of anti-particles to not behave like our own. But it does obey the combined CP symmetry—meaning a mirror image antiparticle universe would develop in the same way as our own. Any theory that obeys the general theory of relativity and quantum mechanics must follow the symmetry of CPT. J. W. Cronin and Val Fitch proved the universe does look the same when following only the symmetry of CP, that is, swapping particles for anti-particles and taking the mirror image, but not when reversing time. Therefore, the laws of physics do not follow the symmetry of T, the reversal of time. 

As the universe expands and cools, forces that do not obey the symmetry of T cause more anti-electrons to become quarks than electrons to become anti-quarks, creating the matter we see today. Of course, they are only named that way because they are the majority. If it were the other way round, the names would be exchanged also.

Grand unified theories of these forces have not yet incorporated gravity. But gravity is a weak force and doesn’t factor much on the atomic scale. Yet, because its effects build up, for large structures, gravity wins out, which is why it creates the universe’s structure. Stars’ gravity eventually causes them to collapse, and it is what happens in that time, when they become a black hole, that draws general relativity and quantum mechanics together.