Chapter 13

Black Holes

What do you think?

• Are black holes just holes in space?

• What is at the surface of a black hole?

• What power or force enables black holes to draw things in?

• Do black holes last forever?

Einstein’s Special Theory of Relativity• Your description of physical reality is the same regardless of

the velocity at which you move.

• Regardless of your speed or direction, you always measure the speed of light to be the same.

• So what?– The length of an object decreases as its speed increases– Clocks passing by you run more slowly than do clocks at rest– An object approaching the speed of light becomes infinitely massive.

• Concept of Spacetime

Einstein’s General Theory of Relativity predicts black holes

• Mass warps space resulting in light traveling in curved paths

Is it General Relativity right?

• The orbit of Mercury is explained by Relativity better than Kepler’s laws

• Light is measurably deflected by the Sun’s gravitational curving of spacetime.

• Extremely accurate clocks run more slowly when being flown in airplanes

• Some stars have spectra that have been gravitationally redshifted.

If we apply General Relativity to a collapsing stellar core, we find that it can be sufficiently

dense to trap light in its gravity.

Several binary star systems contain black holes as evidenced by X-rays emitted

Cygnus X-1

must have a mass of about 7

times that of the Sun

Other black hole candidates include:

• LMC X-3 in the Large Magallenic Cloud orbits its companion every 1.7 days and might be about 6 solar masses

• Monoceros A0620-00 orbits an X-ray source every 7 hours and 45 minutes and might be more than 9 solar masses.

• V404 Cygnus has an orbital period of 6.47 days which causes Doppler shifts to vary more than 400 km/s. It is at least 6 solar masses.

Supermassive black holes exist at the centers of most galaxies

Supermassive black holes exist at the centers of most galaxies

Primordial black holes may have formed in the early universe

• The Big Bang from which the universe emerged might have been chaotic and powerful enough to have compressed tiny knots of matter into primordial black holes

• Their masses could range from a few grams to more massive than planet Earth

• These have never been observed

• Mathematical models suggest that these might evaporate over time.

How big is a black hole?

Matter in a black hole becomes much simpler than elsewhere in the universe

• No electrons, protons, or neutrons• Event horizon

– the shell from within light cannot escape

• Schwarzschild radius (RSch)

– the distance from the center to the event horizon

• gravitational waves– ripples in spacetime which carry energy away from the black hole

• The only three properties of a black hole– mass, angular momentum, and electrical charge

Structure of a Kerr (Rotating)

Black Hole

In the Erogoregion, nothing can remain at rest as spacetime here is being pulled around the black hole

Falling into a black hole is an infinite voyage as gravitational tidal forces pull spacetime in such a way

that time becomes infinitely long

Black holes evaporate

Virtual particles that appear in pairs near a event horizon may not be able to mutually annihilate each other if only one manages to survive a trip along the event horizon.

What did you think?

• Are black holes just holes in space?No, black holes contain highly compressed matter-they are not empty.

• What is at the surface of a black hole?The surface of a black hole, called the event horizon, is empty space-there

is no stationary matter there.

• What power or force enables black holes to draw things in?The only force that pulls things in is the gravitational attraction of the

matter in the black hole.

• Do black holes last forever?No, black holes evaporate.

Self-Check1: Provide brief descriptions of the special and general theories of

relativity.

2: List the principal features used to describe black holes and explain the significance of each.

3: List the three properties that completely characterize a black hole.

4: Describe how energy can be extracted from certain types of black holes and indicate what property a black hole must have to allow such extraction.

5: Discuss the evidence that suggest that black holes exist in binary systems and briefly describe how searches for such objects are conducted.