ONE Hydrogen Atom, page-40

Some 20 years or so ago I read a well written and very interesting article by a well known Australian physicist who hypothesized that it was possible to go back in time, viz. build a time machines, if certain conditions could be attained. To date, those conditions, as far as I know, still cannot be achieved with our existing technical knowledge.

The physics of General Relativity easily permits going back in time but requires some pretty extreme conditions (space surrounding black holes etc). It's to do with the fact that rotating black holes drag space and time around with them, and this means that you can be locally always moving into your future, but due to the tilting of space and time, you end up moving globally into the past. I believe this feature of GR has been known since the 1930s.

This is where the idea of wormholes comes from. Mathematically they are quite valid. But we are a long, long way from being able to use them. They do happen all the time in the quantum realm though, where mini loops in spacetime pop into and out of existence constantly.