Redefining Reality - The Intellectual Implications of Modern Science

If you woke up tomorrow in a small metal room with no windows
and nothing but a bathroom scale, could you tell whether you
were on Earth in an elevator or in space in a rocket? If you
stepped on the scale and it read your normal weight, you might
be at rest in the Earth’s gravitation or you might be accelerating
in empty space. If it read 0, you might be accelerating in the
no gravity. There is no way to tell the difference.
Here, Einstein borrows a principle from the 17 th -century German
thinker Gottfried Leibniz: the identity of indiscernibles. If there is
no difference between two things, they are, in fact, the same thing.
If there is no difference between the elevator and rocket ship cases,
gravitation and acceleration are just different ways of describing the
same thing. Einstein would take this as the basis of his new theory.
If we add more data, there seems to be a way to tell the difference
between the elevator and the rocket. If you poke a hole in one wall
of the metal room, any light from outside will come in through the
hole and travel across the room, and a bright spot will appear on the
opposite wall. With a ruler, you can measure the height of that light
spot and the height of the hole.
Suppose you step on the scale and it reads your normal weight.
You are trying to determine if you are at rest in a normal
gravitational context or accelerating upward in a region of no
gravity. You measure the height of the hole and the height of
the light spot on the wall.
Light always takes the shortest route between two points, which
is a straight line. If you are at rest, you would expect the light
spot to be directly across from you. But if you are accelerating,
you would expect the spot to be lower than the hole because
in the time it took the light to move across the room, the room
itself would have moved up. From the vantage point of someone
in the room, it would look as if the light rays curved down.
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