In the previous part of what has now become an ongoing series on the causal structure of various general relativistic spacetimes, I discussed the causal structure of the flat, Minkowski spacetime of special relativity, and of the Schwarzschild vacuum outside a spherical, uncharged, non-rotating star which collapses to form a black hole. In this ( Read more... )

A commenter writing about my earlier post, "Relativity, FTL and Causality" said:

As far as FTL being equivalent to time travel, the above explanation is correct. As far as FTL being impossible at present, it is not quite. Richard's explanation lacks a mention of black holes. An object (with non-zero mass), falling into a black hole from rest, will ( Read more... )

<< Previous Article: "Spacetime and Coordinates"

We've disussed how an observer, Alice, can apply coordinates to a special-relativistic spacetime using just a clock and rays of light. Now we need to consider how things look to Bob if he's moving with respect to Alice at uniform speed. First let's consider two events, P and Q, that are simultaneous ( Read more... )

(This series begins with Maps of Physics)

In physics an event is something that takes place at a given location at a given time. This conforms closely to our everyday notion of an event. The collection of all events is called spacetime. In this series we'll often be concerned with sets of events and their relationships to each other (especially ( Read more... )