Bending Light

This image has nothing to do with this blog!

Imagine you go to bed, but when you wake up, you are no longer in bed. You are floating inside of a square room. The room has no windows and no door, you cannot see outside the room.

Getting over the initial shock of finding yourself in such a situation, you try to figure out what is going on. You see an envelope stuck to the wall and you can just about reach out and open it. The envelope has some questions:

Are you floating in space? or are you in an elevator in free fall? How would you know the difference?

You start to ponder this and realise that this is a tricky one. If you are in free fall, just like an astronaut in orbit you will not feel gravity. But if you were in deep space far from any object you could be stationary and not feel any gravity. So you wonder, am I moving or am I stationary.

Then a thought enters your head.

"Hold on one minute. If I had a laser that could fire a beam from one side of the room to the other then I would know if I was in free fall."

Your thinking goes like this.

"if I am in space and not moving and I fire a laser from one side of the room to the other then it will follow a straight line, but, if I am in free fall then by the time the laser reaches the other side of the room the lift will be accelerating due to gravity and will have dropped ever so slightly, so it will actually seem as if the light as moved up the wall."

So you look around and bingo, you spot a laser, a spirit level and an extra sensitive detector capable of detecting movements in the Angstrom range (1 Angstrom is 0.1 nm which is 1/10 billionth of a meter - very small) next to the envelop.

You get the laser horizonal and fix the detector to the wall opposite. You fire the laser and discover that the beam travels straight and true. So you repeat the experiment on each of the other two pair of walls (if you are in a room there are 6 walls - floor and ceiling count as two). Each time the beam goes straight across.

So you smile to yourself safe in the knowledge that you must be floating in space either stationary, or at a constant velocity. Clever eh?

Just then you find yourself falling to the floor as gravity returns. Ah, you think, I must be accelerating I am on the move. But then one wall opens and you find that you are at the bottom of the elevator and you are stationary.

This can't be right you think. If the light travelled to a point directly opposite then it must actually have been bent down slightly to compensate for the downward drop of the elevator. It is at that point you realise that you were actually accelerating all the time! You also realise that you can't tell the difference between floating out in space and being in free fall in a gravitational field, without looking out the window!

That doesn't make sense you think light doesn't bend it goes in a straight line! ... and then you remember reading that light is actually bent in a gravitational field!

This is known as the Equivalence principle and was used by Einstein to help develop his theory of General Relativity. Cool eh?