Hi. so, i like time travel. I like it a lot.
today’s information is on the topic of how we can use einstein’s theory of gravity to generate time machines.
this is a real thing, you guys
the word “causal” refers to chains of cause-and-effect, and “causal structure” refers to how different points in a spacetime can are related…
“Subluminal” means “slower than the speed of light”. One way to imagine a lightcone is to imagine that at point P, we set off a flashbulb. The trajectories of the photons that emerge from the lightbulb divide the spacetime into two parts: the parts that can be affected by what happens at point P, and the parts that cannot. Also, if you start at point P, you’ll always be stuck inside this sphere of expanding photons. So the lightcone gives you a boundary in the spacetime between places you can and cannot go.
In Einstein’s theory of gravity, the spacetime pushes the matter around according to how its curved; and the matter distribution causes the spacetime to warp and curve. One way of picturing this curvature is by looking at how the light-cones are warped and twisted and tilted in different places throughout the spacetime.
the illustrative thing about a lightcone is that it tells you which direction “forward in time”. If you can tilt them sideways and have time flow “sideways” if you want :p
In other words, we can engineer a spacetime where the light-cones all twist around on themselves, allowing us to move backwards in time. The only consequence of this engineering procedure is that dictates the type of matter required to generate it. Sometimes you end up with some pretty crazy matter.
Lets look at a couple of FANTASTIC time-machine spacetime structures!
Sorry i’m not a good artist.
This “time machine” is called a Tipler cylinder. It’s an infinitely long rotating cylinder. the rotation of the massive cylinder causes the light-cones to tip over in the vicinity of the cylinder.
Okay, so another type of time machines involves wormholes, where if you pass through it, you’ll end up in the past. This time machine was popular as a sand-box for time-travel thought experiments performed by Morris, Thorne and Yurtsever.
There are a lot of other fun spacetimes where time travel is possible.
For more information on time travel and “closed timelike curves”, please consult your local friendly neighbourhood internet.