Tick... tick. So, that's like the pendulum, the beam of light
bouncing between the mirrors, and you could use that actually
to build a very accurate clock. Then Einstein imagined
what that clock would look like if it were moving relative to us. So, what I'm going to have happen is,
Jim is going to be moved along the stage. Keep moving the clock.
And then we can dim the light. And we can see what that looks like
from our perspective. We're stationary relative to Jim.
And we've also got... There's a little box there you can see.
That's Jim's head camera. So Jim is seeing, of course, the clock
in exactly the way we pictured it, when it was stationary relative to us. The light beam is bouncing up and down
between the mirrors, but if you look... We've got little video effects on there,
so you can see the trail. You can see that the beam of light
that we see is tracing out a triangular pattern across the stage. Beautiful. Thank you. -Can I get off now?
-You can, yes. -Thank you, Jim.
-It was a bit fast. Thank you What a great use
of that wonderful intellect... But it was being beautifully demonstrated. What we saw there was,
if I sketch it out again, from our perspective now,
from the audience's perspective. Here are all those mirrors, so, this is
the light clock that Jim was carrying, but you saw that from your perspective,
watching Jim move, the light took a kind of triangular path, as it bounced across the stage
between the mirrors. Here is what Einstein's postulate,
if you like, Einstein's suggestion
that the speed of light, is constant for all observers, implies... See, this path is obviously longer
than this path. So, if we all agree on the speed of light, then it is obvious that it must take
the light longer to tick for the moving clock than it does
for the stationary clock. Moving clocks run slowly. This is true. Time really did pass
at a different rate for Jim. It passed at a different rate for him
than it did for you in the audience,
watching Jim move. There's no sleight of hand here.
Jim really is a time traveller. Our time is personal to us. This is what Einstein had discovered.
There's no such thing as absolute time. Now, why we don't notice this
in everyday life is because the amount by which time slowed down
for Jim was minuscule, because the speed
he was traveling was so small compared to the speed of light. But if we'd have sent Jim off in a
rocket... Would you like that? A rocket flying out into space. Let's say that we catapulted Jim off at 99.94% the speed of light,
for five years, according to his watch. Then we tell Jim to turn around
and come back. It takes another five years
to get back to the Earth, so, for him, the journey would take ten years, but for us, with our watches
ticking faster than Jim's, 29 years would have passed. Jim would return in 2042,
having aged only ten years. It's a real effect.
He'd be a time traveller. Time travel into the future is possible.
