Notes

-A Brief History of Time NOTES-

General relativity is the solution to the paradox of relative motion and constant speed of light. It states that space and time are not absolute, and that mass can curve 4-dimensional spacetime so that moving forward through time will cause things to move together through space. This is perceived as gravity. Being closer toward a body of mass will cause you to move slower through time. Moving at a higher velocity will also cause you to move slower through time. This is because having more momentum means light bouncing off should move faster to other observers, but this is avoided by time slowing down. Being closer to a body of mass will also slow down time because the way mass curves spacetime causes the path through time to be longer in order to travel the same amount of space.

The expansion of the universe was concluded through several observations, particularly with the distance between stars. Change in position against the background of further away stars combined with the fact the path of light is altered by the curvature of spacetime (gravity) helped identify the position of stars relative to the earth using a method called parallax, essentially how much a star moved at different points of time. When painstakingly plotting the position of thousands of stars, you can find that galaxies move further away from the Milky Way, and galaxies further away move proportionally faster. Galaxies were also moving away from each other. We can tell stars are moving away because as light is emitted, it gets redshifted because the wavelength spreads out as the star moves away. Redshift is an important principle for identifying the motion of stars.

Several observations show that the universe must've begun at some point. Firstly, the universe looks pretty much the same from anywhere - on a large scale, that is. There are galaxies distributed pretty evenly about. Second, background microwave radiation can be detected everywhere, and that suggests that some sort of radiation was everywhere in the past and got stretched out. If you look at the oldest stars and reverse the expansion of the universe, taking into consideration the universal background radiation, you can assume the universe must've begun expanding as high energy singularity. This is now known as the Big Bang. We do not know what happened before the big bang, but since physics breaks down at the time of the big bang, it doesn't matter what came before, because it cannot be used to predict what happens next.

Electromagnetic radiation is released from stars constantly, and since it is constant, how come stars haven't released an arbitrarily high, or infinite, amount of energy? The theory to solve this is the beginning of Quantum Theory. Since higher frequency wavelengths obviously can't be released at an infinitely high rate, they must be sent out in packets that require a certain amount of energy to be released. Thus, you cannot send out high-frequency waves without high energy. However, that means that to observe the position of a particle, you have to use a certain amount of energy, which will change its position. This means the position and velocity of any particle will always be uncertain. This is called the uncertainty principle. It is important to mention here that particles and waves aren't too different, but for the sake of experiment, one performs certain functions different than the other. When shot randomly through two slits, electrons (essential waves) will interfere with each other, causing wave cancellation in some places more than others. The result is that there will be more electrons hitting in some places than others because in some places they interfered. If we tried to predict their trajectories at any point, we'd have to shoot waves at them, which will ultimately change their trajectory. Thus, the trajectory of electrons shot is representable only by probabilities, not any definite positions for each electron.

Quantum theory states that energy is released in packets. In some theories, these can be imagined as particles with a certain mass and spin. Light is released as a particle called a photon. Gravity is released as a particle called a graviton. These virtual particles (they cannot be directly detected) are released from matter particles, and this release will cause a recoil against the matter particle. The virtual particles do not have to obey the exclusion principle. These particles released from matter particles explain the 4 forces of the universe: gravity, electromagnetic, weak nuclear, and strong nuiclear. These each have their own particles released from real particles to cause interactions.