A Brief History Of Time by Stephen Hawking

Today we have successfully developed financial markets, healthcare systems, insurance providers, economies, and so much more. But what if we want to understand where all this comes from? Or where we, the developers of such systems, come from? Or even, where earth, the universe, and the concept of time comes from? Does time have a beginning, and even an end? And with what we know today, is there a single unified theory to describe it all? Stephen Hawking dives into these concepts in his book, “A Brief History Of Time”.

??? Stephen Hawking (1942–2018) was a renowned British theoretical physicist, cosmologist, and author known for his groundbreaking work on black holes, general relativity, and the nature of the universe. Despite being diagnosed with ALS at the age of 21, which left him almost completely paralyzed, Hawking made significant contributions to science, including the theoretical prediction that black holes emit radiation, now known as "Hawking radiation".

My ?? takeaways:

Astronomy goes as far back as Aristotle (340 BC) who argued that the earth was a round sphere rather than a flat plate. A couple of hundred years later, Ptolemy (200 AD) constructed a complete cosmological model with the earth at its center. Later however, Copernicus (1473) argued that it was the sun, not earth, at the center. Galilei (1564) later backed this up by finding that Jupiter is orbited by several moons and thus not everything orbits around earth. Kepler later adjusted the model by stating that planets move in ellipses rather than circles. The shape of these movements were later explained by Newton’s law of universal gravitation.

So now that a model of the universe has been created, is it fixed? Before the twentieth century everyone assumed the universe to be giant, eternal, and unchanging. If Newton’s law holds, it is impossible to have an infinite static model of the universe where gravity is always attractive, as all stars will fall into each other. So does the universe have a finite beginning? And if so, does the universe have a finite ending? Hubble (1889) made the incredible observation that wherever you look, distant galaxies are moving away from us. Also, around ten or twenty thousand million years ago, they were all in the same place, with an infinite density of the universe. The concept of a “Big Bang” was established.

Now to truly figure out whether the universe has a beginning or an end, a general theory needs to be found. One that contains all that we know, and that describes all that is to happen. As Hawking describes it: “A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations.”. Also any physical theory is always provisional, you can never truly prove it.

Today scientists describe the universe in two partial theories. One describing the force of gravity and structure of the universe, the general theory of relativity. The other describing phenomena on extremely small scales, quantum mechanics. Thus a theory needs to be found that joins the two together, which means that it contains a range from a millionth of a millionth of an inch to as large as a 1 with twenty-four zeros after it. This theory will be the quantum theory of gravity.

First Hawking states that one must not see time as fixed, but that it is dependent on space. “In relativity, …, all observers must agree on how fast light travels. They still, however, do not agree on the distance the light has traveled, so they must therefore now also disagree over the time it has taken. (The time taken is the distance the light has traveled – which the observers do not agree on – divided by the light’s speed – which they do agree on.) In other words, the theory of relativity put an end to the idea of absolute time!”. Hence events in space can be best described in a four-dimensional space called space-time (3D + time).

Through this concept, Hawking explains: “the light spreading out from an event forms a (three-dimensional) cone in (the four-dimensional) space-time. This cone is called the future light cone of the event. In the same way we can draw another cone, called the past light cone, which is the set of events from which a pulse of light is able to reach the given event”. Einstein (1879) finally made a revolutionary suggestion that space-time is ‘warped’ and not flat. Hence in space-time, earth moves in a straight path, but for us it appears to move in a circular orbit in 3D space. Also according to general relativity, light should be bent by gravitational fields and that time moves slower near a large body of mass. Hence Newton’s laws put an end to absolute position in space and the theory of relativity puts an end to absolute time.

Now with these concepts in place, we may wonder whether time has a beginning, and/or an ending. Hubble (1889) made the discovery that most galaxies, viewed from earth, are red-shifted. Meaning that the wavelengths of received waves are longer and hence the matter is moving away from us. Thus is earth at the center of everything, or is everything moving away from each other? As in, the universe is continuously expanding. Hence there must have been a point at which the distance between neighboring galaxies must have been zero. This point in the general theory of relativity is called a singularity, a point at which the theory itself breaks down. Such singularities also occur within regions of space-time, called black holes. Penrose and Hawking wrote a paper stating: “there must have been a big bang singularity provided only that general relativity is correct and the universe contains as much matter as we observe.”.

The theory of general relativity remains robust, except in extreme cases like black holes and the universe's origin. Meanwhile, quantum mechanics introduces an element of randomness and significantly influences chemistry, biology, and modern technology. Despite its widespread application, quantum mechanics has not yet been fully integrated into our understanding of gravity and the large-scale structure of the universe. But does this even matter? As among the four fundamental forces: gravity, electromagnetic (involving electrons and quarks), weak nuclear (responsible for radioactivity), and strong nuclear (which binds quarks within protons and neutrons and also holds these nucleons together in the atomic nucleus) – gravity is the weakest.

But as Hawking writes: “Grand unified theories do not include the force of gravity. This does not matter too much, because gravity is such a weak force that its effects can usually be neglected when we are dealing with elementary particles or atoms. However, the fact that it is both long range and always attractive means that its effects all add up. So for a sufficiently large number of matter particles, gravitational forces can dominate over all other forces. This is why it is gravity that determines the evolution of the universe. Even for objects the size of stars, the attractive force of gravity can win over all the other forces and cause the star to collapse.”.

If gravity governs the evolution of the universe, does it also dictate its beginning and end? Stephen Hawking further elaborates on the behavior of matter (or information) when it enters a black hole, suggesting that black holes may not be as completely black as once thought. He discovered that black holes emit particles and radiation originating just outside the event horizon, a phenomenon now known as “Hawking radiation.” This raises the question: Is the edge of a black hole, known as the event horizon, a point in space where time effectively stops? Is this the end of time, and does this mean that there is a beginning of time?

Since black holes represent singularities in general relativity, Hawking and Penrose proposed that the universe also had a beginning—a singularity in time. However, at this origin point, the theorems of general relativity break down, leaving open the question of whether time truly starts there and precisely how it is formed, as Hawking describes:

“The idea that space and time may form a closed surface without boundary also has profound implications for the role of God in the affairs of the universe. With the success of scientific theories in describing events, most people have come to believe that God allows the universe to evolve according to a set of laws and does not intervene in the universe to break these laws. However, the laws do not tell us what the universe should have looked like when it started – it would still be up to God to wind up the clockwork and choose how to start if off. So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be. What place, then, for a creator?”.

Therefore, the ultimate goal is to discover a complete, consistent, and unified theory that encompasses all partial theories as approximations, without the need to adjust the theory by choosing arbitrary values for certain constants. This pursuit is known as “the unification of physics.”. However, it remains uncertain whether such a theory even exists. And if we do eventually discover a unified theory, will it truly provide us with a complete understanding of our own existence?

?? Given that everything in the universe functions so precisely, the question arises: is there a creator, or is this simply explained by the “weak anthropic principle”, or the observation selection effect? This principle suggests that the universe appears as it does because, if it were different, we would not be here to observe it. As Hawking writes: “Why is the universe the way we see it?". The answer is then simple: if it had been different, we would not be here!”.

The book covers a really wide range of topics, from the beginning of astronomy, to black holes, quantum physics, the expanding universe, and much more. I hope to have given you a great first impression. For those truly interested I recommend reading the book, as so many concepts are written out in such detail, that it is truly a masterpiece for anyone interested in astronomy and our own existence. I also recommend this video by Veritasium: https://youtu.be/6akmv1bsz1M?si=S1H5p5dAlrL8qqNz, who truly explains the concept a black hole well for the more visual learner.