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.
Hawking once again uses Aristotle to show, first, the errors of assuming without evidence, and second, how far the human race has come in understanding the universe, thanks to modern scientific approaches.
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.
Scientists achieved rapid progress once they began to actually look for such particles rather than debate them, and also due to the assistance newer technologies provided. Each discovery supported and spurred on the next, in the ongoing chain that is scientific progress.
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.
Chadwick’s Nobel Prize demonstrates that people welcomed these discoveries, seeing their worth in the mission to understand the universe and its workings.
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.
After finding the constituent parts of the atom, such as the proton and electron, and their functions, scientists soon set about determining what made up these particles, eventually finding quarks. Their curiosity still unquenched, scientists proceeded to determine the different kinds of quarks and how they relate and function.
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.
The obvious question is how long this process of discovering ever-smaller particles will continue. There is a current impediment, Hawking explains, in that scientists do not yet have the tools to look more closely, as the highest energy yet harnessed and applied in such experiments still cannot directly see the smallest particles thought to exist.
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.
Having found these infinitesimally small particles, scientists moved on to determining their nature and characteristics. Scientists devised a way to distinguish particles from one another that revolves around this idea of spin, referring to how many times a particle must rotate to still look the same.
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.”
If spin ½ particles, which make up matter, are in the same place, they will not have the same velocity, meaning they will soon move away from each other. The fact that matter particles inherently repel each other in this way gives the universe structure, because if they didn’t, there would be nothing to stop all matter from clumping up together, instead of forming into more comlex structures, something scientists have not taken for granted.
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.
Spin ½ particles must turn twice before they look the same, a phenomenon Dirac explained mathematically. His prediction that there ought to be antielectrons was later proven correct, earning him a Nobel Prize, a further sign that, provided one has the evidence to back it up, the scientific community would welcome new and challenging ideas.
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.
Just as a gun recoils from the effort of shooting a bullet, a particle will change its velocity when it emits a force-carrying particle. Another particle absorbs this energy, causing its velocity to also change. These force-carrying particles, however, cannot be detected directly. In this way, scientists have been able to study particles they cannot even see.
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.
Scientists have classified these force-carrying particles they cannot directly detect into four categories according to their effects, in their determination to understand everything.
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.
Gravity, the first of these unseen forces, is something humans come across in every moment of daily life. While this force is unseen, its effects are clear. The rationality of studying even unseen realities in the universe is therefore apparent.
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.
Hawking offers an everyday example of such particles when discussing photons, which are captured in photographs. Understanding these laws allows humans to create technologies that preserve precious memories and remember loved ones. Humanity’s curiosity about the workings of the universe prompts ingenuity, which is applied with sentimentality, a feeling which the curiosity perhaps originates from in the first place.
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.
Salam and Weinberg’s theory suggests how forces might act when they have higher energies than humans are currently able to produce. As such, it is currently unprovable. The idea is that the different types of forces are all ultimately the same, which can be seen when they have high energies. But when viewed at low energies, as they are today, they are stuck with one function due to their lack of energy. This theory of the different forces really being one kind of force echoes Hawking’s dream of a unified theory of physics, showing he is not alone in his quest.
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.
The strong nuclear force has a particular rule called confinement, which means that only certain types of quarks can form together to create a particle. Quarks each have a kind of color, which is simply a way of referring to their properties, rather than a specific color. When joining together, these colors must all be present to cancel out and be stable, called “white.” This can also be achieved by a quark of one color joining with an anti-quark of the same color, though this is still unstable because particles annhiliate when meeting their anti-particles. In short, after finding out about quarks and their different properties, scientists wanted then to find out how they all relate.
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.
This theory of finding the unifying properties of these forces again echoes Hawking’s mission to unify all of physics. Even the current road block is the same: integrating gravity. It seems, gradually, all theories are progressing down the same road. At some point, there ought to be a destination.
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.
For now, scientists can only theorize, rather than prove this idea of a grand unifying energy that will make all the energies act the same. Observing this effect is beyond their reach, but that has never stopped humans from theorizing before.
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.
Returning to the question of how humans came to be, Hawking highlights the fact that all these advances in science and technology essentially come from humans’ desire to understand their universe, in turn, because of the desire to understand their place in it. By applying the logic Hawking states here, humans can determine fundamental truths about the universe, which, crucially, is supported by the observable universe.
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.
The reason for the imbalance in particles and anti-particles in the universe relates to these symmetries that Hawking outlines. Having determined laws that describe the universe, scientists have also thoroughly quality checked them to see how they hold in all situations.
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.
Lee and Yang suggested the weak nuclear force does not have the same effect on particles moving in the opposite direction, which Wu proved by causing electrons to do exactly that—a direct link between curiosity, ingenuity and discovery. In contrast to the Penzias and Wilson example, Lee and Yang won the Nobel Prize for their suggestion, rather than Wu for her work in proving it.
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.
Uncontent with understanding the laws of physics in this universe, physicists analyzed whether those laws would hold in alternate scenarios, where the universe was comprised of anti-particles rather than particles, or developed in a mirror image of our own. One mathematical theory on the unified theory of physics says it ought to follow the symmetry of CPT, all three symmetries, but the laws of physics do not have the same effect when time is reversed, indicating there is work still to be done to understand this unifed theory.
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.
By expanding on these theories, Hawking argues that scientists should not focus only on the universe they can see, but should take a step back in order to look more objectively. In an anti-particle dominated universe, scientists would call the dominant anti-particles just regular particles. Looking at hypothetical universes thus aids in assessing this one without prejudice or assumptions.
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.
Gravity seems to be the problem child of physics—it has yet to be incorporated into grand unified theories of the forces, and is (so far) incompatible with quantum mechanics, making a unified theory of everything currently unreachable. Hawking links into the next chapter, promising to show more about this uncooperative force.