## Saturday, 27 June 2020

### Time travel inside a black hole

We can now use the geodesic that we have found and tested to plot a spacetime diagram all the way to the centre of a black hole. The graph shows the result for our famous beacon, or even for a foolish astronaut.

As we saw before they dawdle at the event horizon for ever ($t\rightarrow\infty$ as $r\rightarrow2GM$). But we know you can cross an event horizon and when that happens in the distant future they hurry backwards in time and soon get to a reasonable $r,t$!

Meanwhile the astronaut looking at a wristwatch sees proper time, $\tau$, ticking by steadily and reaches the centre in finite time.

Luckily the astronauts time travel cannot be observed from the outside. Our red faces are saved.

Given everything we have done before the calculations are simple. It's a one pager in Commentary 5.6 Time Travel.pdf.