Cosmo 101 (a bit of math included)

Well…I was gonna talk about tidal effects and the Schwarzschild metric but I think I would like to talk about our homework more. I know that sounds weird but…it is, my homework, I mean. We are currently working with special relativity and it is just really weird. I’ve always known, well since 1687 (Newton’s year), which feels like forever, I’ve always known that gravity is a force and that v=d/t and that t is the same for everybody that hasn’t even been a concern, but now it is and I don’t really know how to handle it. Now all of the equations that I’ve always fallen back on are not valid anymore and I don’t know which ones are and aren’t. For example there are two (or possible more) types of time in relativity. There is measured time which we represent with a normal t and there is wristwatch (or proper) time, which is designated with the greek letter tau. Now which of those do you use if you want to find the velocity? And for example if you have people traveling at different speeds they will see different times for different events. In other words not everyone agrees on the time separation, delta t, between two events. And not everyone agrees on the spacing either. But there is hope (haha I sound like a public service announcement) there are some values that are the same for everybody, tau is the same no matter who you are or how fast you are going. The equation to calculate tau is tao^2=t^2-s^2. Here s is the displacement in space. It’s the same as the r we used a couple weeks ago.