A Brief History of Time

A Brief History of Time by Stephen Hawking

I’ve never been big into physics. To me, physics is the most difficult topic of all and also the most ambitious because it literally is the study of how the world works. Reading this book made me realize how little I knew – even with two mandatory semesters of physics in college. I’ve heard all these terms before: space time, relativity, quantum mechanics, but wow did I know nothing about them. I can tell that Hawking tried very hard to dumb down the material for the laypeople, but I definitely started to have trouble following about a third of the way in. There are so many interwoven concepts, and sometimes the puzzle pieces turn out to be wrong, and our understanding of the universe(s) would have to be reconstructed from scratch. Between this book, watching meteor showers, and the 3BP series, I suddenly find myself very interested in space again – like when I was little. I still remember when I bought books about all the planets (RIP Pluto), and I actually still have planet wall decorations in my room. What happened?

1) Galileo knew that not everything orbited around the earth when he saw moons orbiting around Jupiter.

Copernicus had proposed the heliocentric model years before, but Aristotle’s geocentric model had been accepted for a long time. I appreciated how this book talked about the wrong conclusions that famous scientists had made throughout their lives. Hawking was (or seemed to be) very transparent about his own mistakes too. Being wrong and revising is the core of science.

2) Modern physics boils down to two main theories: general theory of relativity and quantum mechanics.

This was the first time I’d built a mental image of relativity and quantum mechanics. General theory of relativity is about gravity and speed of light, while quantum mechanics is about phenomena at an extremely small scale – including concepts like the uncertainty principle. The key challenge now is the reconciliation of these two seemingly conflicting theories.

3) Space time is warped by the mass and energy in it.

Earth moves in a curved orbit because this orbit is actually the “straight” path in curved space.

4) We discovered that the universe is expanding because the waves we detected were red-shifted.

If the universe were expanding, the waves from the stars would be red-shifted. If the universe were contracting, they would be blue-shifted.

5) By itself, the general theory of relativity predicts that all physical theories break down at the beginning of the universe.

This is the singularity. I found it interesting how much Hawking talks about the role of God. According to Wikipedia, Hawking was an atheist, so he was probably posing the hypothetical of a God to prove his point by contradiction.

6) A key part of quantum mechanics is the duality of particles and waves.

This is where I started to lose the plot a bit, so I’m not sure what to say about this supposedly crucial concept.

7) Any theory that obeys both relativity and quantum mechanics must obey the combined symmetry CPT.

This means that the universe must behave the same if we 1) swapped particles with anti-particles or 2) took a mirror image or 3) reversed the direction of time. Quite mindblowing. This is not like assuming independence.

8) Black holes look like they produce radiation, but the particles are actually coming from the event horizon around the edge of the black hole.

I don’t completely get it, but the black hole sucks in the particle while the anti-particle escapes, which makes it look like the black hole is emitting radiation.

9) Microwave background radiation is believed to be the remnants of the Big Bang.

The fact that the radiation is almost exactly the same in all directions suggests the universe overall is very smooth. Galaxies and stars are the result of very minor differences at the beginning of the universe.

10) The anthropic principle basically says that we see the universe as it is because we exist.

Sounds like the physics version of my claim that everything is subjective.

It’s amazing how recent so many of the discoveries are. Almost all the theories in this book were posited within the last century. Sometimes it feels like humanity isn’t making any progress, but I can’t imagine living even a century ago. How different will this world be in 100 years?

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