In that elevator, you will experience your normal weight.

Because you're being accelerated by a spacecraft at that same acceleration as

you would be falling towards the Earth's surface if the Earth's surface

were not there.

And in Einstein's view, there's no way you could distinguish these two situations,

yet they are clearly quite different.

In one the gravity force is operating, and in the other,

a force caused by a spacecraft is accelerating you at the same rate.

This is the basis of his theory.

This is the equivalent principle.

The fact that you cannot distinguish these situations.

It's artificial and arbitrary that we just ascribe one to gravity and

we ascribe one to the other force.

Now let's think of another experiment.

We can use the same elevator as the idea.

Imagine you're in an elevator, stuck in the elevator, a windowless elevator and

you don't really know your situation and you have a flashlight.

If the elevator is not moving, you can shine the flashlight across

the elevator and it will hit the opposite wall at the same level as you aim it.

Imagine that the elevator is moving at a constant speed.

You can see the light is travelling across the elevator and so will hit a place on

the opposite wall slightly below the level where you aimed it.

A simple vector addition of the speed of light with the speed of the elevator.

If the elevator is accelerating, the same thing will happen.

Only the light will take a curved trajectory because

of the elevator's acceleration.

And once again, the light beam will hit a point on the opposite wall slightly below,

directly opposite from you.

Well, that's the situation where the elevator is accelerating upward.

The light beam is deflected, slightly.

Einstein's point of relativity was that the same thing applies when the elevator

is sitting stationary on the Earth and you are subject to the Earth's

gravity acceleration, 9.8 meters per second, per second.

And so he implies that the same phenomenon,

that slight curvature of the beam of light, should also happen to the man or

woman stationary in the elevator.

That is the gravitational deflection of light.

That is gravitational lensing.

And it is, of course, observed.

So we know that even the Earth, with its rather subtle gravity, is subtly bending

space and time around it, leading to this gravitational deflection of radiation.

And indeed, it can be measured.

General relativity was a profound re-conception of time and

space, from the hands of Einstein.

Instead of the linear time and

space of Newton that were independent of each other, we had coupled space time

that could be curved according to the mass energy density in any region of space.

This amazing conception of gravity has been validated by experiments over

the 100 years since general relativity was first published.