⏳ Is time travel possible?

Forward: yes

Travelling into the future is just time dilation: move fast or sit low in gravity and less time passes for you. Astronauts return from the ISS a few milliseconds 'in the future'; a ship at a steady 1 g could reach the galaxy's centre in 20 of its crew's years while 26,000 pass on Earth. Established

Backward: the equations allow it

Einstein's equations have exact solutions containing closed timelike curves — paths that loop back to their own past: Gödel's rotating universe (1949), an infinitely long rotating cylinder (Tipler, 1974), wormholes turned into time machines (Morris, Thorne and Yurtsever, 1988), and pairs of fast-moving cosmic strings (Gott, 1991). Established as mathematics.

The catch

Every recipe needs something missing from our universe: a universe that rotates (ours doesn't measurably), something infinitely long, or large amounts of 'exotic' matter with negative energy. Established (what the recipes need) · Contested (whether any could ever be met).

The paradoxes

What stops you preventing your own birth? Novikov's self-consistency principle says only consistent histories happen — you might try, but something always intervenes. Some quantum versions let the traveller arrive in a different branch. Speculative

Does nature forbid it?

Hawking's chronology protection conjecture (1992): quantum effects build up and destroy any time machine as it forms, 'keeping the world safe for historians'. Unproven either way. Contested

Hawking also held a party for time travellers in 2009 and sent the invitations only afterwards. Nobody came. Analogy — a joke, not evidence, but a good one.

The verdict

Forward: real and routine. Backward: allowed by the maths in special universes, impossible by every known means in ours, and open in principle. The honest answer is 'we don't know — but don't hold your breath'.