People really want to know if time travel's possible.
As far as we know,
according to laws of physics, travelling
into the future is indeed possible.
We're always travelling forward in time
at the rate of 1 second
per second per second.
Always forward.
How would you go forward?
There's a number of ways of doing it.
You can travel really, really,
really fast, or travel really, really close to a high gravity source.
The faster you go, the slower time appears to travel.
If you were to go on a spaceship,
travelling close to the speed of light, and I was to stay here on Earth,
and you were to go around for 10 years and do a loop and come back to Earth,
when you come back, you would be younger than I am.
Because in travelling close to light
speed, you've experienced time at a different rate.
This actually happens every single day.
If you ever use your phone and pushed the button and dropped a pin to get your
location, you're interacting with at least four satellites in space.
These satellites whiz around the earth at great speed, and they also sit next
to a large gravitational mass,
which is the Earth.
Now, both of these things have an effect
on how these satellites experience time.
Because they're travelling fast around
the Earth, it means that time slows down for the satellites.
We can actually measure this.
We actually need to correct for it; otherwise, GPS won't work.
At the same time, because they're sitting next to the large gravitational mass
of the Earth, that means time speeds up for them.
We actually have to correct for that.
With the GPS satellites,
they're experiencing time dilation, and we actually have to put corrections
because they are actually doing time travel.
Travelling into the past, as far as we know, is fraught with difficulty.
We don't know if that really is possible to engineer.
Even if it were, it seems that it creates paradoxes.
You could make a time machine, however that works,
go back in time and stop yourself from making the time machine.
So that's a paradox.
The universe doesn't like nonsense.
It seems to protect itself from that.
It could be that the universe forbids time
travel entirely, or there are ways around that we have not yet thought of.
I don't think we can go backwards in time.
I think if we could go backwards in time,
then the world would be full of people from the future.
I don't think they would all hide.
The world would be full of people from the future in the way that the world
is full of people from Birmingham or from France.
If the future was accessible to the past,
then you would expect to see these people all the time.
One of the most interesting applications
for time travel that I can think of is in computers.
If you imagine a loop of string, and you loop it round, and that is 1-second loop.
This is called a closed timelike infinity loop.
You have your single loop that is
1 second, and you put a calculation in one end, and it takes 1 second to get out.
But within the loop, you could go around for 1 second,
or 10 seconds, or a million years, or 100 trillion years.
If you had a really complicated equation or calculation that needed to be
performed, you could put it in this closed timelike infinity loop,
and it would go round and round and round and round and round for the trillion years
until it's ready, and then it will pop up the other end.
But for you on the outside, 1 second has passed.
So there's a concept there of
being able to use loops of time in order to perform really complex calculations,
so using time travel to build supercomputers.