Book Review: A Brief History of Time‎‎‎‎‎‎‎‎ by Stephen Hawking

About the Book:

A Brief History of Time by Stephen Hawking was originally published in 1988 but has been updated and revised by Hawking several times since. The most recent revision was released in 2011, 8 years before Hawking passed away. This is the revision I read and just wow! I’m definitely a nerd for space and this book really hit the spot. Hawking wrote this book in a way that makes its content easily understandable to the masses, which I love. One problem I think we have as a society today, as well as in the scientific community, is that most newer developments and areas of ongoing study are written for scientists (naturally!) which leads your average person to ignore it. This makes understanding the theories and topics very difficult. Hawking’s goal with A Brief History of Time was to explain everything in a very simplified manner. I am no scientist or mathematician, so getting the background on the big bang and black holes without having to see all the proofs and formulas was very refreshing. Hawking covers everything from the big bang (the beginning of time as is measurable by humans thus far) to the concept of time travel and wormholes. Each chapter in this book covers a new topic in depth in a way that is easy to understand. So lets get started!

Summary:

Chapter 1 (Our Picture of the Universe) begins with how our picture of the universe has evolved over time. He goes into the history of different models of the universe and how those models were supported and eventually disproved.

Aristotle concluded the Earth was spherical and that the Sun and other planets rotated around us.
Copernicus concluded that the Earth and other planets actually orbit the Sun.
Kepler concluded the Earth and planets orbit was elliptical and not circular.
Newtons publications confirmed Kepler’s findings through mathematics.

Throughout the book, Hawking stresses the point that despite coming closer and closer to a more accurate picture of the universe we have still been wrong in one way or another. Science today, although more accurate than 100 years ago, is still probably wrong about a lot of things! But through continued work and persistence we get closer and closer to having a completed picture of the universe.

Chapter 2 (Space and Time) really starts to mess with our common sense beliefs. Hawking describes how our previous belief that time was absolute (meaning an event would always take the same amount of time, no matter your position when measuring it) is actually not true! We find throughout this book that the physics we know and love don’t work the same at really really fast speeds (the speed of light) or on a very very small scale (quantum mechanics). Through experiments by Michelson-Morley on the speed of light and Einstein’s special theory of relativity, we come to the conclusion that space and time are actually linked (spacetime) and that the motion of an object through space will impact how it experiences time. This is about the point where I had to start watching YouTube videos to help me understand exactly what was going on. I found a really great channel that summarizes Hawking’s book chapter by chapter which I will link at the end of this post!

Chapter 3 (The Expanding Universe) covers how we discovered that our universe is expanding. When we look at distant stars in the sky we are seeing them as they were in the past, because light takes so long to travel across space for us to see. Hubble discovered that light from distant galaxies was shifted to the red. Redshift is when the wavelength of visible light is stretched and appears different to the observer depending on where they are located. In our case, because the universe is expanding away from us, it makes the light appear red. The red shift in galaxies Hubble observed was directly proportional to their distance away from Earth. In other words- because far away galaxies have this redshift, it means they are getting further and further away from us, and that the universe is expanding!

Einstein actually found this to be the case when formulating general relativity, but was so convinced that the universe must be static (not expanding) that he added a “cosmological constant” to counteract or balance out the effects of expansion! That just goes to show us that what we think must be true might not actually be true! Never just assume something to be true because that’s what we’ve always believed!

In addition to Hubble finding out the universe is expanding, Friedmann made two assumption about the universe we find to be accurate: that the universe is identical wherever you happen to be located (homogeneity) and that it looks identical in every direction (isotropy). Simply put, our location in the universe is nothing incredibly special. The same stuff (galaxy clusters and stars) can be found in any direction and at roughly the same frequency.

Chapter 4 (The Uncertainty Principle) deals with Heisenberg’s uncertainty principle. The principle states that both the speed and the position of a particle can not be known for certain. It’s worth watching some extra videos on this topic, but basically if we spend the energy trying to determine the exact speed of a particle, its position will end up changing and vice versa, if we try to determine the exact position of a particle its speed will change. This is because what we are calling “particles” act both like a particle and a wave simultaneously (wave-particle duality). The implications of the uncertainty principle are that we will never be able to use scientific laws to predict with 100% certainty the future of the universe.

The distinction is then made between Einstein’s theory of relativity (does not include the uncertainty principle) and quantum theory (does include the uncertainty principle). Hawking believes that to understand and predict what happens at a singularity (a situation where gravity is incredibly strong, like a black hole) we must find a way to reconcile both theories, or combine them to form a new more comprehensive theory.

Chapter 5 (Elementary Particles and Forces of Nature) deals with the smallest indivisible particle, or the building blocks of all matter. It was once believed that everything was made up of the four elements (earth, water, fire, or air) until we discovered the atom, which can be broken down into protons, neutrons, and electrons. Now we know that there are particles even smaller called quarks which make up the protons and neutrons. What you need to know about quarks:

There are six different “flavors” of quarks (up, down, strange, charm, bottom, and top). Don’t ask me how they were named because I have no idea!
There are three different “colors” of quarks (red, green, and blue).
There are anti-quarks for every “normal” quark.

Quarks are very interesting but also very confusing, so I’m just going to leave it at that!

The four base forces that control the universe are:

Gravitational forces (the weakest of the four, acting between objects with mass)
Weak forces (which governs particle decay, acting on quarks)
Electromagnetic forces (acts on electrically charged particles)
Strong forces (the strongest of the four, binds quarks together so they can create protons and neutrons)

Chapter 6 (Black Holes) and Chapter 7 (Black Holes Ain’t So Black) deal with Hawking’s research on black holes. Black holes are basically units so massive that light can not escape its gravitational pull, meaning we can’t see what it looks like because its own light keeps getting sucked back into it! But there are other ways to observe black holes; visible light from the opposite side of the singularity will get distorted as its light waves are redirected around the black hole. Objects being sucked into a black hole would also give off X-rays that we could (and have!) detected.

Because a black hole has entropy, it must have a temperature, and must emit radiation. Hawking theorized that when a pair of particles (a particle and an antiparticle) approach a black hole, one of two things can happen: Some particle pairs will borrow energy from the vacuum of space and annihilate each other, thus returning the energy, while other particle pairs will have either the particle or the antiparticle absorbed by the black hole while the other particle goes free. Since there is no annihilation of the particle pair and release of energy, the free particle is essentially taking energy from the black hole. Because of this, Hawking believes that black holes must shrink and eventually evaporate over time.

Since this book was published this phenomenon has been widely accepted and is now known as Hawking Radiation!

Chapter 8 (The Origin and Fate of the Universe) is probably my favorite chapter because it describes exactly what the universe was like from the big bang until now! Based on all the theories discussed previously in the book, we can determine what the state of the universe was all the way back to the big bang (its temperature, what particles were present, their density, and the timing it took for them to form other particles and atoms).

I watched many many lecture videos on this topic afterwards to get a better understanding of the timeline:

Within the first seconds after the big bang the universe was so hot that quarks and photons were zooming around and colliding with one another, and that’s all they were able to do! It was too hot for them to start combining to form larger particles.
As the universe cooled (around 3 minutes after the big bang) quarks began to combine into protons and neutrons.
500,000 years after the big bang (500,000 years of nothing but particles bouncing around the universe!) Hydrogen was formed and then nuclear fusion occurred to form Helium and other basic elements.
After the first billion years, galaxies started to form from matter that began to gravitate towards one another. This is how we got to the composition of space as we know it today- as time elapsed and the universe expanded, matter condensed into galaxies leaving vast stretches of space empty.

Hawking also describes how the universe would be different if the rate of expansion was faster or slower. If it were slower, gravity would have caused the expansion to slow until it stopped and reversed (start contracting), eventually the universe would collapse into a single point (the opposite of the big bang). If it were faster, the universe would be almost completely empty. The anthropic principle can be used to answer some of our questions about how we see the universe. The anthropic principle basically states that there is no reason the universe is configured the way it is as opposed to some other equally possible configuration. But if it were configured a different way in which life could not have formed, we would not be here viewing it and asking the questions we are asking.

Chapter 9 (The Arrow of Time) discusses the three “arrows of time”:

The thermodynamic arrow of time is the direction which entropy (chaos and disorder) increases. If this arrow were reversed, the entropy of the universe would decrease.
The phycological arrow of time is how we perceive time (from past to future). If this arrow were reversed we would be able to remember events from the future.
The cosmological arrow of time is the direction of the universe, in our case it is expanding. If this arrow were reversed then the universe would be contracting.

Hawking goes on to use the “no boundary principle” to describe why, if the cosmological arrow of time reversed and the universe began to contract, the universe would not be able to support intelligent life. Therefore humans all experience the three arrows of time moving in the same direction.

This chapter deals with some information I still don’t entirely understand so please read it for yourself or watch some videos to clarify the concepts!

Chapter 10 (Wormholes and Time Travel) deals with the possibility of time travel. To travel back in time one would have to travel faster than the speed of light, which is not possible. Einstein-Rosen bridges, or wormholes, were thought to provide the solution to this problem by relocating matter to either a different position in space or back in time, but to function it would require matter with a negative energy density which is not currently known and violates the principles of general relativity.

Chapter 11 (The Unification of Physics) comes back to the idea of a unified theory of the universe (combining general relativity with quantum mechanics). Hawking is optimistic we will be able to achieve this in the not so distant future. At the time this book was published, String Theory was becoming popular as a way to achieve this, but there have been no major developments as of yet.

My Thoughts on the Book:

Space, time, and the history of the universe have always been fascinating to me. I personally love how random it all is, just particles zooming around and then getting bigger until BAM here we are today! Reminds me of a course I took in college on the origins of life (maybe I should write some about that!) on how life on Earth basically started the same way- tiny particles forming microorganisms which then come together and ending up reproducing and evolving into us! Granted that’s a very very oversimplified version of what happened but in any case it’s just crazy to think about! All that randomness makes me feel good knowing there is no rhyme or reason things ended up how they are, which means to me that there really is no purpose in life, and that’s freeing. It means I can do whatever, live however, and it doesn’t matter. I know that would scare a lot of people but I find it comforting.

Throughout his book Hawking mentions a Creator of the universe and mentions his visit with the popes quite often. I believe he does this, again, to welcome in the average person who might hold religious believes. He never outright says there is no God or Creator, even though his and others findings have implied that as a possibility. Or maybe he’s just too wise to give a definitive answer on the subject, because like we’ve seen, just because we believe something doesn’t mean it’s true and the opposite can also be true. Better not to limit ourselves by committing to a Creator or to there not being a Creator. For those of you wondering what my views are on the subject of religion, I’d say at this point in my life I am agnostic, which is pretty much the stance Hawking takes throughout his entire book. Whether there is or isn’t I don’t know and I don’t much care at the moment!

It is also really cool to see the recent scientific developments since this book was published in the 80’s. Like I mentioned in the summary, Hawking wrote about his theory of particle pairs at a black hole and now it is widely accepted as factual in the scientific community. It’s even named after Hawking! A lot of work has been done on the topic of black holes since the 80’s. The first picture ever captured of a black hole was just in 2019! Hawking missed seeing it by a year, I bet he would have been thrilled.

I highly recommend reading A Brief History of Time if you get the chance, and also watching some videos to get a visual on what’s being discussed. I will link all the videos I watched below! Thanks for reading!

-Atlas

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Parth G does a great job at explaining the concept of Spacetime!