Question: is time travel possible and if yes how?

Susan Cartwright answered on 16 Jun 2015:

One-way time travel is certainly possible, and has been demonstrated. This is because general relativity predicts – and experiments have shown – that time appears to flow more slowly if you are travelling at high speed, or if you are in a stronger gravitational field. Therefore, if you were to travel out from the Earth in a spacecraft at close to the speed of light, and then come back again, you would find that more time had passed on Earth than had passed for you, so you would effectively have travelled into the future (but with no way to return). The same thing would happen if you spent some time orbiting very close to a dense object like a neutron star or black hole (though this would be rather dangerous – you might not survive to visit your future).

Proper time travel, Dr Who style, may or may not be possible. There are certain geometries which according to General Relativity might generate “closed timelike curves” (i.e. you can return to the same point in space AND TIME, which is true time travel), but it is not completely obvious that these geometries can be realised in our universe. In most such cases, I don’t think you can travel back beyond the construction of your time machine – so I’m afraid visiting the dinosaurs is probably out.