How to make a time machine

As we understand physics today, time machines might be possible.  If they are possible then they have a major limitation, which is that backwards-in-time travel is cannot be before the time machine was first switched on.  The reason is that the only way we know to make time machines is to distort space and time to allow 'Closed Timelike Curves' (CTCs), and these closed curves can't loop back to before the time machine was turned on as that's the earliest that one of these curves can start.  Creating a time machine is like creating a subway tunnel through time - you can enter and exit any time along its length but you can't get off before the start.

There are many ways that a time machine might be made.  There is one idea which requires an infinite spinning massive cylinder, which doesn't seem very practical.  Perhaps the most likely way is to use a wormhole.   A wormhole is a shortcut between two points.  One point might be on Earth, the other around Alpha Centauri, and it might take only seconds to travel between these points through the wormhole.

[Just a quick point - wormholes aren't anything like as shown in science fiction TV series: they don't look like funnels.  The entrances to a wormhole are spherical, not circular.]

To make a time machine, construct a wormhole, say on Earth, with two ends A and B close together.  Now, take end B on a trip to the stars (anywhere will do) travelling close to the speed of light for, say, 100 years, but with B only experiencing 10 years.  Then, bring end B back to Earth.   Time connects differently through the wormhole than outside it.  If you now enter end B, you will exit end A 90 years ago!

This might not be possible.  Some predict that quantum effects would result in destruction of a time machine just before it's possible to use it.  But it's a fun idea.