Book Review: “A Brief History of Time” by Steven Hawking

This is the second time I read this book, first time being more than 20 years ago. This time I am motivated by the movie, “Martian.” The underlying thesis of the book appears to be still valid, although it’s now in its 10th anniversary edition from its original 1988 edition. Many of the theories are still difficult to understand for me even with my years of engineering/physics study. Some of the topics are a refresh for me like Uncertainty Principle.

My key takeaways are:

1. The universe is still expanding.
2. No absolute time and space.
3. Grand unified theory is very difficult to prove (takes as much energy as the solar system).
4. Black holes are not so black.
5. No single theory on how the universe began.

This is quite an interesting book for those with some background in physics and curious about the universe and how it all started.

A summary:

1. Our Picture of the Universe: Some history of how humans come to the understanding of the universe since the Aristotle time and the two theories that describe them: Newton’s Gravity (large scale) and Quantum Mechanics (small scale).

2. Space and Time: Nothing can travel faster than the speed of light if the source is traveling – can’t add to the speed of the source. Unfortunately, Newton’s gravity theory goes against the idea because the gravitational effects is instantaneous or infinite speed. General Theory of Relativity gets rid of absolute time and Newton’s Laws of Motion puts an end to the absolute position. Everyone carries on his/her own clock measuring time depends on where he is and how he’s moving.

3. The Expanding Universe: By looking at the stars’ spectra (red-shifted due to the Doppler Effect), astronomers can tell the stars from other galaxies are mostly moving away from us. Otherwise, the gravity would cause the universe to collapse. This may explain why the universe looks similar in all directions or receives the same amount of radiation from all directions. This leads to the possibility of a singularity or the Big Bang as the origin of time, when general relativity breaks down.

4. The Uncertainty Principle: Heisenbger’s theory that the more accurately you try to measure the position of a particle, the less accurate you can measure its speed, the product of the two are the Plank’s constant. The quantum state is a combination of position and velocity. Electrons orbiting around the nucleus like a wave, with a wavelength that depended on its velocity number.

5. Elementary Particles and the Forces of Nature: Four categories of force: 1) gravitational, 2) electromagnetic, 3) weak nuclear force (responsible for radioactivity), 4) strong nuclear force, which holds the quarks together in the proton and neutron. Lots of discussion about anti-matter or anti-quarks.

6. Black Holes: Lights have a duality of particles (bent by gravity) and waves of quantum mechanics. When a star loses its energy, it starts to cool off. But the Pauli exclusion principles keep particles away from one another, maintaining at a constant radius. If it’s less than Chandrasekhar limit, it would turn into a neutron star. If more, it may explode or break up to under the limit. Moving objects send out gravitational “waves” that eventually reaches a steady state as it loses energy like Earth around Sun and crushing into Sun in 10^27 years. Black holes can be deduced by observing a star orbit around on its own – most likely orbiting around a black hole.

7. Black Holes Ain’t So Black: Black holes can emit radiation because of conservation of energy. What goes in as particle mass due to gravitational force must come out as radiation or anti-particles.

8. The Origin and Fate of the Universe: The universe started out in a big bang and very hot. As it expanded, it cools. Lots of discussion about anthropic principle (we can explain it because we exist); it’s hard to explain why the universe is the way it is. The total energy of the universe is a big zero. Classic theory of gravity, the universe may have existed for an infinite time or begins in a singularity. By quantum mechanics, there may not be a boundary condition nor edge.

9. The Arrow of Time: The increase of disorder or entropy with time gives the direction of time – thermodynamic sense of time, different from cosmological sense of time (universe expanding) and psychological sense (we feels time passes).

10. Wormholes and Time Travel: Cosmic strings are discussed. Wormholes may be the way to warp space time to travel back in time. Lots of discussion about time traveling forward and backward using various theories. No conclusion.

11. The Unification of Physics: String theory can be used to unify the physics theories. Even we could find one, we may not be able to prove it.

The days after we discover the unify theory, we may be able to discuss why we and the universe exist – a true triumph to the human race.