e-Pulsar - Dec 97

e-Pulsar Dec 97 - Jan 98

Astronomical Feature of the Month
EVENT HORIZONS

An event horizon is the defining edge of the most mysterious and fantastic astronomical object known to humankind - a black hole. The interesting thing is, an event horizon isn’t a physical object. It’s actually a distance. To be only slightly less confusing, an event horizon is the definition of the distance from the center of a black hole at which the acceleration due to gravity is exactly the speed of light. There is no "boundary," no "edge." The mass of the black hole curves space-time so much that any object whose orbital altitude above the black hole falls below the horizon distance can never increase its altitude and escape, not even if it could travel as fast as light. Event horizons are one-way signs on the highway of the universe. Indeed, light itself cannot escape, hence the nick- name "black" hole. So, if light can’t escape, what is an artist supposed to paint? Here’s a few thoughts.... As an object falls towards a black hole, light reflecting off of it towards a distant observer must "climb" out of the steep gravity well. In the process, the light loses energy and is red-shifted. The observer would see the falling object turn redder and redder as it got closer to the event horizon. The opposite is almost true from the object’s viewpoint. Light from distant objects would gain energy from the pull of gravity and would blue-shift as it got closer to the event horizon. The difference is what happens to the view of the universe from the falling object. The intense gravity acts like a giant fish-eye lense and all of the in-falling light is concentrated into a circle overhead. The closer you get to the event horizon, the smaller the circle becomes. At 1.01 times the event horizon distance, the entire universe would be squished in to a circle covering about 30 degrees of the sky (picture). At the instant before crossing the event horizon, the universe would shrink to a brilliant point overhead. Here’s more - as objects get close to the horizon, they are stretched by the difference in gravitational pull between the top and bottom of the object. This tidal effect can shear a star to peices - which could make for some pretty interesting art… interested?

Event Horizon plot

Plot showing shrinkage of visible universe while approaching an event horizon. Source: Black Holes: A Traveller’s Guide, by Clifford A. Pickover, a book I highly recommend.