Newton
Newton's Laws were a breakthrough in physics and helped explain the movement of objects on Earth. He also created the equation describing gravity between two objects (F = Gm₁m₂/r²) that describes gravitation as acting instantaneously over a large space, a concept that deeply troubled Newton. There is also the idea of "absolute time" in all of Newton's equations. Time is the same no matter where you are.
His Laws:
- Any object in motion will stay in motion unless acted on by another force (object act rest will stay at rest)
- Acceleration of an object is directly proportional to net force and inversely proportional to its mass (F = ma)
- For every action there is an equal an opposite reaction
Special Relativity
Einstein's first major breakthrough. Special Relativity holds that no matter what speed you are traveling, the laws of science remain the same. It also extends space time to 4-dimensions. X, y, z, and time.
The reason this is so profound is because we found that the speed of light is the same no matter where you are pointed. If you are moving away from a light emitting source you would assume that the light coming towards you is moving slower than if you were stationary. Conversely, if you were moving towards it, you would assume it was moving faster. However, this was not the case. No matter which direction you face, the speed of light is the same. Why is this?
The solution to this is to understand that speed = distance / time. In order for this equation to be balanced and for our observations to give us the same speed no matter what direction we are facing, the time factor must be a malleable variable as well.
This also allows for time travel into the future. If time is malleable and all parties at all speeds have to agree on the speed of light, then people moving faster must experience time at a slower rate relative to those moving slower for the speed of light to remain the same. This means that if you wanted to go to the future, all you would have to do is move really fast (approaching the speed of light) and then come back. Time would have been moving a lot slower on the spaceship and a lot faster on Earth and you will be in the future. I will not pretend that I fully understand this concept.
Einstein also created the equation E = mc². This states that energy = mass times the speed of light squared. This creates an equivalence of mass and energy. Mass and energy are the same thing!
As mass increases, the energy required to move it increases as well. However, because of the c² multiplication, energy increases at a faster rate than mass. This means that it requires increasingly more energy to move something the faster you want to go. If you want to move at the speed of light, you need infinite energy, which is impossible. Thus, the equation states it is impossible for an object with mass to move the speed of light.
General Relativity
This redefines what gravity is. Don't think of it as a force. It is actually a result of the curvature of space-time. The orbits of the planets are actually due to the warping of space time by the sun. An ellipse is the shortest path in space-time. Imagine a plane's shortest path on the earth. It is not a straight line on a map, but a curved one. This is the same idea for orbits of the planets in space-time. Look up what a geodesic is if you want to learn more.
Light is also curved by the warping of space time. When light from a star gets near the sun and then to Earth, our perception of the star's location is slightly off because the sun warped the light as it was moving to Earth and thus our perception of the star's location.
Time should also run slower near a massive body. This was derived using the principle of equivalence:
- Laws of science the same for all moving observers at all speeds
- It is impossible to tell if you are in rest in a gravitational field or uniformly accelerating in empty space (imagine being in a moving elevator in space vs. standing on land). Gravity on Earth has an acceleration of -9.8 m/sec squared but thats no different than if you were accelerating at 9.8 m/sec squared through space.
Thus, the stronger the gravitational field, the stronger the acceleration you would have if you were instead moving in empty space, and thus the greater the acceleration the closer you would be moving to the speed of light, and as established in special relativity, the closer you are moving to the speed of light the slower time will move in order to keep the laws of science the same.
Black Holes
Black Holes occur when a dead star has such dense mass and such a large gravitational field that not even light itself has enough velocity to escape the gravitational pull of the dead star.
Black holes release radiation. This is the antiparticles moving backwards in time. Their correlating particle is in the hole. I need to expand more on this section.
The Big Bang
The rules of Relativity break down at the Big Bang. There is infinite density, temperature, and curvature at The Big Bang.
The Big Bang occurred at the start of the universe when there was no mass or space, but infinite energy. I need to expand more on this section.
Uncertainty Principle/Quantum Theory
The uncertainty principle says that you cannot perfectly determine the path of a moving particle at the quantum level. Once you are at a small enough level, you can only determine probabilities for where a particle could end up in its path of motion.
This is because the simple act of spreading light on a particle in order to observe it will change its trajectory. Thus we are left with the interesting conclusion that the higher precision with which we observe a particle's position the less accurately we can observe its velocity. The more accurately observed a particle's velocity, the less accurately you will observe its position.
Quantum theory also states that particles are both particles and waves at the same time.
Wormholes
Wormholes are the idea that there is a way to warp the curvature of space-time in order to traverse large swaths of space in far less of a time. General relativity allows for the existence of wormholes. However, many think that they will not be possible if we establish a theory of quantum gravity.
String Theory
String theory states that particles in the world are not actually little dots, but really tiny strings. It aims to bridge the gap between quantum theory and general relativity. The math is all screwy but string theory tries to make the math work between the two.
It requires 10 dimensions to work. However, many people think that the other 6 dimensions are curled into our 4 dimensional world.
Writing Style
The writing style is very simple and easy to understand, while the concepts are very challenging to wrap your head around. It sounds like he is having a conversation at a dinner table.
Strengths
The pictures in this book are really good at helping explain the concepts that are being described. Hawking is also very good at giving real-life commonplace analogies for the various concepts described. By comparing things to everyday life, the abstract concepts can be better understood.
He also does a great job at giving a broad overview of the world of physics for the layperson. From the start of physics to the potential futures, I feel like I know a lot more in many different areas than I knew before.
Weaknesses
I wish some of the mathematical equations were actually written out at times. At times, Hawking gives relationships between variables in certain equations and how when certain ones increase others decrease. While it is for the sake of ease of reading that these equations are not explicitly written, simply giving the relationship without showing the equation makes it harder to deeply understand what is going on. Actually seeing even a little bit of the formulas would solidify my understanding more.
In one sentence
The universe is a wacky incomprehensible place and physics seeks to understand the laws that govern it.
Score
8/10