## Saturday, March 24, 2018

### Friday Science: Hawking 2 (Spacetime)

Last week I started Friday reviews of Stephen Hawking's A Brief History of Time.

Today I want to do a brief overview of the second chapter. The second chapter basically introduces spacetime, the theories of relativity. Here are some salient points:
• So Aristotle thought that you had to push a body in motion for it to stay in motion. Rest was the preferred state. This makes sense because friction slows down ordinary motion.
• But Galileo and Newton in the 1600s formulated a different law--a body in motion tends to stay in motion and a body at rest wants to stay at rest. This fact doesn't undermine God at all, but it does undermine one of Aquinas' arguments for God--God as the "prime mover," a concept Thomas Aquinas took from Aristotle.
• Another principle that Galileo and Newton formulated was the principle of relativity. Speeds are relative to each other. If I am going 30 mph on a train in relation to the ground and I throw a baseball forward at 30 mph, the baseball will be going 60 mph in relation to the ground.
• In the late 1600s, it was discovered that light moves very fast.
• In the 1800s, it was discovered that light moves at a fixed speed. At first they thought that everything was moving through some sort of ether, but Michelson-Morley showed apparently not.
• So there was a problem. A light shone from the front of a moving train proves not to move faster than a light shone from the ground or from a space ship. The principle of relativity was in danger.
• Einstein solved this with his special theory of relativity (1905). Space contracts as you approach the speed of light, and nothing can move faster than the speed of light.
• Einstein came up with spacetime diagrams--time on the y axis, one dimension of space on the x-axis.
• This suggests that cause-effect relationships can only take place within a "light-cone" that could emerge from an event at a point in time. Say light moves out from a point at a time. It spreads out in a sphere from the point. But if we diagram this light's progress with time as the z axis and two dimensions as space, the light spreads out in a cone on our diagram.
• If you take that same concept and play it backwards in time, there is a cone of light that could arrive at this location at the time of the event.
• These two light cones suggest the locations that could cause an event or be caused by an event. Because nothing can move faster than the speed of light, anything outside the cone in time or space could not have affected or be affected by the event at that point in spacetime.
• This theory messed up Newton's theory of gravity, because it connects force to distance. So if the distance changes, the gravity presumably would too.
• This problem eventually led to Einstein's general theory of relativity (1915). He supposed that gravity was not a force at all but rather a bending of space caused by mass. So what would normally be a straight line of motion looks like it curves because space itself curves. This made it possible to explain why Mercury's motion around the sun is a little different than Newton's theory would have predicted.
So this is all stuff I've read elsewhere.