Time travel can be complicated - and complicating, so let's ignore all the paradoxical stuff
and talk about a few simple ways that you can time travel without leaving your home.
First,
1) Do nothing - you're already traveling through time! I mean, here you are - fifteen seconds
into the future since the start of this video! Easy, huh? The point is, we're all always
traveling forwards through time. But that's boring time travel. What's interesting is
time travel relative to other people. To do that...
2) Start walking, and you'll travel through time relative to someone standing still! We've
known for over a century that time and space are really just two components of a single
"spacetime", and the faster you move, the slower time will pass for you. If you take
a walk around the block, you'll be 3 femtoseconds younger than your friend who stayed home.
Except, in order to walk around the block, you had to...
3) Stand up. You're now further from the Earth, and so gravity is a tiny bit weaker for you.
Which means you've traveled through time relative to your friend who's sitting down. That's
right, more gravity makes time slow down, too. If you stand up for a minute your feet
will have aged 10 femtoseconds less than your head. On the other hand, GPS satellites high
in orbit experience less of earth's gravity and thus travel noticeably faster through
time than we do, which is why their clocks are calibrated to run slow.
But maybe you want to time travel more than a few femtoseconds. Get ready for your head
to start spinning - I mean, the universe.
4) Because if the whole universe were spinning really fast, general relativity predicts there
would be time-loops all over the place. Moving along one of these loops, you'd always feel
like you were moving forwards in time, but overall you'd loop around and travel back
to a time and place in your own past. It's a little like how you can keep moving forwards
on the Earth, but Earth's curvature brings you back to where you started.
Unfortunately, our universe isn't spinning. Maybe it would be easier to build...
5) An infinitely long, super-dense spinning cylinder, which would also curve space-time
enough to create a time-loop. The problem, of course, is how do you build something that's
infinite in size? Maybe you could just make it really really really big? MmmNo. If you
tried to squeeze this time machine into finite space, you'd need negative energy - something
nobody knows how to create - to make it work. Otherwise you'd end up with a black hole.
Wait, but what if instead of a black hole, we...
6) Built a wormhole? Wormholes are hypothetical (but not physically impossible) bridges through
space-time, shortcuts that can instantaneously connect two different places and times in
our universe. If you had a wormhole you might be able to use it to travel into the past
or the future.
The problem is that no-one knows how to build a wormhole. Or, once you've built it, how
to keep it from collapsing: as Sean Carroll has eloquently written, "keeping wormholes
open requires a form of negative energies. Nobody knows how to make negative energies,
although they occasionally slap the name "exotic matter" on the concept and pretend it might
exist."
Well that's too bad... but as consolation, welcome to the future! Almost three minutes
have passed since the beginning of this video, and we have personal jet-packs now!