Light in an Accelerating Room
Brian Greene illustrates the science behind Einstein's Theory of Relativity using an accelerating room model.
When Albert Einstein has a new idea --a good idea-- he does not let it go. He
shakes it, he twists it, he caresses it in search of a jewel that may be hidden
inside. And that's what happens with this newfound link between
accelerated motion and gravity. Einstein's mind returns again and again
to images of people falling from rooms-- rooms accelerating an empty space are
falling down toward the earth. And in the process he realizes something profound.
Imagine that I am in one of those rooms freely falling toward the earth and as
I'm floating around inside I click on a laser. What will I see? Well this is not a
trick question. I'll see if a laser light goes straight
across the room and hits a corresponding point on the opposite wall-- that red dot
over there. But what would you all see watching this from the surface of the
earth? Well you will surely agree that the light begins at the laser and ends
at that red dot, but because from your perspective gravity is causing the room
to accelerate downward you will see the light go in a curved trajectory after
all that is the only way that the light can begin at the laser and end at the
red dot given that from your perspective the room is falling ever more quickly
while the light is in transit. What's the conclusion? In a gravitational field
light falls. As light sweeps along the contours of space imbued with gravity
and traces out curved trajectories; a direct link between gravity and
curvature. And with this vital insight Einstein, can begin to see the glimmers
of a new understanding of gravity taking shape. The idea is that if there's no
matter, no gravity, then space is not curved. But if we bring in a massive body
like the sun, then the environment, it responds. The
environment warps. The environment curves in response to the presence of the sun,
in response to the motion of the sun, and if we were to take a two-dimensional
slice through this environment we see that the earth itself is kept in orbit
because it's rolling along a valley in a curved environment that the sun creates.
This is Einstein's proposal for how gravity works: warps and curves in the
fabric of space and time. It's not a proposal that's meant to only work out
there in outer space, it's meant to work everywhere even here.