Surprising Physics: Acceleration Faster Than Freefall

[Creator]

there was a beautiful problem in Physics Teacher of May I guess in which a ball is lauched horizontally in between two vertical and parallel walls. The ball bounce in the walls while the gravity pull it down. After 1 second the ball will be 4,9 meter below. This is Ok. But now consider the walls are substituted by mirrors, with 100% reflectivity. Now, instead of a ball, we send a laser beam horizontally. Of course the beam will reflect back and forth, but the question is: what will happen with the light 1 second later? (Hint: GR)

The light beam will be 4.9 meters lower after 1 sec. (provided the mirrors are perfectly parallel!) Not likely, however, that you'll be able to do a table top experiment since even the best mirrors would not have sufficiently perfect reflectivity to bounce back and forth the required 3 x 10^8 times required for a mirror separation of 1 meter! Good gedanken experiment though.

Another one I like to do ( for H.S. demo) is to use a plastic ruler (bent backwards) to strike two marbles simultaneously, launching them from off the top of a table. Since each marble is struck with a different force one goes much farther horizontally than the other. The question to the class is: "Which one will hit the ground first?" Pretty simple, but it gets the gravitational point across.

 

[DaTario]

That's is it!

I also found it very interesting in deed.

 

[pallidin]

The light beam will be 4.9 meters lower after 1 sec. (provided the mirrors are perfectly parallel!) Not likely, however, that you'll be able to do a table top experiment since even the best mirrors would not have sufficiently perfect reflectivity to bounce back and forth the required 3 x 10^8 times required for a mirror separation of 1 meter! Good gedanken experiment though.

Another one I like to do ( for H.S. demo) is to use a plastic ruler (bent backwards) to strike two marbles simultaneously, launching them from off the top of a table. Since each marble is struck with a different force one goes much farther horizontally than the other. The question to the class is: "Which one will hit the ground first?" Pretty simple, but it gets the gravitational point across.

I would imagine that in far less than 1 second, the light will be completely absorbed by the less than perfect reflections.
Even if the mirrored walls were 1000 feet apart, in one second each photon would have "potentially" reflected about 1 MILLION times ! I don't think(could be wrong) that a photon can reflect that many times without total loss.

 

[DaTario]

I guess, the best superconducting cavities can have the photon bouncing inside for still less than 0.5 seconds. There are perspectives of arriving at 1 second.

The experimental group dealing with such cavities is the French, Ecole Normale Superieur at Paris, Mr. Serge Haroche may be the lead researcher.

 

[Lyuokdea]

Although everybody here has probably heard of this, I've always enjoyed the three polarizer experiment. Basically, you set up two polarizers at 90 degree angles to each other so that there is no transmitted light through both polarizers. Then you set up a third polarizer in between the first two polarizers and spin it around changing the transmittted light from 1/8 of the original intensity to zero intensity and everywhere in between.

Without the mathematical idea of projections, it doesn't make any sense whatsoever, but then with the idea of projections, it shows just how important mathematics is in the physical world.

~Lyuokdea