1. 1-Sentence-Summary:
1.1. A Brief History Of Time is Stephen Hawking’s way of explaining the most complex concepts and ideas of physics, such as space, time, black holes, planets, stars and gravity to the average Joe, so that even you and I can better understand how our planet was created, where it came from, and where it’s going.
2. Favorite quote from the author:
2.1. "Intelligence is the ability to adapt to change." - Stephen Hawking
3. 3 lessons:
3.1. Theories can never be proven.
3.1.1. Theories are educated guesses.
3.1.2. Hawking says a theory is nothing more than a model, which correctly explains a big number of observations.
3.1.3. This has 2 great benefits:
3.1.3.1. You can make definite predictions about the future.
3.1.3.2. The theory can always be disproven, if evidence against it comes up.
3.1.4. An example:
3.1.5. Hawking loves the fact that theories can always be proven wrong in the future, and so should you. Stop assuming so much, and start finding evidence!
3.2. Time is not fixed, due to the speed of light.
3.2.1. The general statement of Einstein’s theory of relativity is that the laws of physics are the same for all freely moving observers and objects.
3.2.2. Because the speed of light constantly being 186,000 miles per second is one such law, it means that no matter where you are or where you’re going, the speed with which light reaches you is the same.
3.2.3. If one person travels 186,000 miles towards a ray of light and another travels 186,000 miles away from it, but the speed of light is constant, the light would reach the first person 2 seconds faster – that is at a different point in time.
3.2.4. That’s the reason why time is relative and why Mr. Einstein has become so famous.
3.3. There are 3 reasons why time can likely only move forward.
3.3.1. Time only moves forward thermodynamically.
3.3.1.1. Entropy is the tendency to increase disorder wherever possible.
3.3.1.1.1. For example your coffee mug only stays in mug form, because force is holding it together. As soon as you drop it, it’ll be happy to increase its entropy in the form of a whole bunch of shards.
3.3.1.1.2. However, it would never spontaneously reassemble itself on its own (and thus decrease entropy), so thermodynamically, time only moves forward.
3.3.2. Time only moves forward psychologically.
3.3.2.1. You can never “remember” the future.
3.3.2.1.1. For example after your mug breaks you can remember what it looked like before, but you can never know the exact position of the shards on the floor before you break it.
3.3.3. Time only moves forward cosmologically.
3.3.3.1. As the universe expands, its entropy increases. Since entropy constantly increasing also means that time moves forward, due to reason number 1, this adds to the point.
3.3.3.2. However, the universe could start contracting again, after reaching its maximum extension, thus reversing entropy and also time.
3.3.3.3. But Hawking says that we wouldn’t know: our bodies rely on entropy to break the food we eat down into its particles, which give us energy.
3.3.3.4. So if time were to ever start reversing, we’d have to die first.
3.3.3.5. But then again, who knows if that’ll ever happen, right? After all, it’s just a theory.