PHYS 43, SRJC, Spring ’11: May 12, 2011 Joshua Chen, David Tran, and Eileen O’Byrne- Hudson

TOWARDS THE EVENT HORIZON

The event horizon is the radius around a black hole within which no matter can escape the gravitational attraction of the black hole. Not even light. WHAT IS AN EVENT HORIZON?

CAUSE OF THE EVENT HORIZON In general relativity, gravity is manifested as bends in spacetime. Since a black hole has so much mass, the gravitation pull does not allow light to escape when it is close by. As shown in the first diagram, the pit in spacetime is so deep that nothing can get out.

SPACE TIME DIAGRAMS This diagram shows the behavior of light cones near the event horizon, rs. Notice that at the event horizon, the light cannot escape, it can only stay still or go further toward the center of the black hole.

MORE SPACE TIME DIAGRAMS These are simplified, two dimensional space time diagrams of light near the event horizon of a black hole.

CROSSING THE HORIZON One does not feel or even see anything special as they cross the horizon. There is no one way to affect someone who crosses the horizon. An observer on the outside will simply see the traveller slow down until he stops at the event horizon. The traveller also appears more and more redshifted over time.

AROUND THE EVENT HORIZON There are virtual particles that are created from the event horizon. Normally in space particles annihilate but because of the strong field of the black hole, this doesn’t always happen. The virtual pair, particle and anti-particle separate, with one going into the horizon and another escaping

ENTROPY In the universe entropy is always increasing. Notice that black hole absorbs mass. The virtual particles exist because they compensate for the “order” created from black holes which are vacuuming up mass. Their radiation is called Hawking Radiation, discovered by Stephen Hawkings. This radiation satisfies the quantum effects that require the event horizons to have a temperature and emit radiation.

THE SCHWARZSCHILD RADIUS The Schwarzschild radius is the distance from the center of an object such that if all of the object’s mass were compressed to that radius, the gravitational pull would prevent light from escaping. In other words, it’s how small you have to squish something down to so it becomes a black hole The formula assumes, however, that the mass in question isn’t rotating

INTERESTING SCHWARZSCHILD RADII The Sun’s Schwarzschild radius is about 3 km The Earth’s Schwarzschild radius is only 9 mm, or about the size of a bean The universe’s Schwarzschild radius is 10 billion light years, according to the mass we’ve calculated

SOURCES wongturn.wordpress.com