Shedding Light on Black Holes

July, 2009

By Tom Koonce

Antelope Valley Astronomy Club

Lancaster, California

Black Holes…  Just their name sounds like something out of science fiction.  Maybe this is one reason why they have been the focus of misconceptions and misguided theories.  This month, the theme of the International Year of Astronomy is centered on the objects that weigh heavily (pun intended) on the minds of theoretical physicists and leading astronomers… Black Holes.

First a bit of background on the subject.

The gravitational force exhibited by a celestial body is directly related to its mass and inversely proportional to the square of the distance which the object is away from that mass.  So how does a black hole generate its enormous gravity even though its mass is reduced to an infinitesimal point?

Consider a star with the mass and radius of the red supergiant Betelgeuse.  Under normal circumstances, an object could orbit the star at a distance outside of Betelgeuse’s stellar atmosphere.  But if the entire mass of Betelgeuse was compressed down to become a black hole and in the absence of Betelgeuse’s stellar atmosphere, the object could pass much closer to the black hole’s center of mass… so close, in fact, that the gravitational force it could experience would be incredibly high.

Another concept to realize is that if the Sun were to suddenly be replaced with a black hole of equal mass, the Earth would continue to orbit it in the exact same manner as it does today, except that the lack of sunlight would render the Earth incapable of sustaining life.

A common question that comes up during casual conversation about this subject is, “If I went through a black hole, where would I go?”  The straight-forward blunt answer?  “To your death!”   You literally would be torn to pieces by the gravitational tidal forces during your approach to the event horizon and then, with unerring certainty, what gelatinous mess remained would be squashed much, much flatter than a pancake as your remains fell deeper into the gravity well.  Black holes are not a mode of transportation to another universe, but they are efficient “matter compactors,” sweeping up all mass that passes too near.  Of course they can’t draw in matter from light years away, but as matter falls into a black hole it becomes (perhaps) infinitely compressed by its overwhelming gravitational force.

Imagine what a black hole looks like and you probably picture the graphic popularized by the media; a two dimensional plane with a funnel-shaped hole descending towards the black hole’s singularity.  This stylized perception of the three dimensional nature of the object has misled many people to think of a black hole as a hole in space, like a hole in the backyard, or perhaps a tunnel in space-time leading to other parts of our own universe.  The event horizon is a spherical region around the black hole, inside of which the black hole’s gravity is so strong that nothing can achieve escape velocity – nothing, not even light.  Because light can’t escape, space artists have envisioned the object as a black blob against a field of distant stars.  This black blob is surrounded by a fairly bright disk of material caught in the gravitational field.  Why is it bright?  As all of the dust and matter spirals in closer to the black hole it is rubbing against other matter, heating it up by friction until it gets to millions of degrees.  It is this dust outside of the event horizon that is radiating light.

What would a glimpse below the event horizon look like?  How important would it be to you to find out?  It would be a one-way trip to find out.  Nothing, not even light, can escape from below the event horizon… but photons of light could orbit the black hole.  Since there is an equivalent mass for the energy of a photon (E = mc2), light is affected by gravitational forces.  Photons can orbit a black hole if conditions are right.  Since there are photons continuously falling into black holes, many must get trapped in this manner.  We can’t see the photons because they are orbiting and not radiating outward and striking our retinas.  If we were somehow able to glimpse just below the event horizon, on that one way trip into gravitational flatness, I believe you would see bright light surrounding you; you would see photons instead of blackness.  Your final view would be of all of the light shed upon the black hole.