[thedoc (OP)]

Gregg asked the Naked Scientists:
   
What is the Planck length and its connection to string theory? (this is one of my favorite subjects besides computer science).

What are gravitational waves? How is the Earth affected by them?
 
Have a very happy holidays,
 
Gregg (from Montreal, Canada).
What do you think?

[Mike Gale]

Wikipedia says, "There is currently no proven physical significance of the Planck length. However, it is theoretically considered to be the quantization of space which makes up the fabric of the universe, also referred to as quantum foam." The idea is that classical laws of physics (e.g. Newton, SR, GR) break down if you zoom in too far. The strings in String Theory are supposedly of the order of the Planck length, which is why they can't be observed directly.
Gravitational waves are the way in which changes in a gravitational field (e.g. due to the motion of a gravitating mass) propagate through space and time. Newton thought that changes would propagate to all points in space instantaneously. Einstein begged to differ. LIGO detects them by comparing the apparent speed of light in the parallel and tangent arms of an interferometer.

[jeffreyH]

The Planck length is 1/2 the radius of the event horizon of a Planck mass black hole. Therefore the smallest possible Compton wavelength should be 4 Planck lengths. So to probe the scales at which string theory begins to operate the wavelength has to approach this size. This requires an astronomical amount of energy. Which is why string theory won't be tested anytime soon.