But we can go farther and imagine
an object so massive and compact that if we turn on a laser beam on its
surface gravity's pull will bend it back towards the surface. Think
what this means: since no light can leave this object it will appear
perfectly black, this is a *black hole*.
An object which comes sufficiently close to a black hole
will also disappear into it (since nothing moves faster than light if
an object traps light it will also trap everything else).

The effect of a black holes, like all gravitational effects, decreases
with distance. This means that there will be a ``boundary'' surrounding
the black hole such that anything crossing it will be unable to leave
the region near the black hole; this boundary is called the *
black-hole horizon* see Fig. 7.10
Anything crossing the horizon is permanently trapped. Black holes are
prefect roach motels: once you check in (by crossing the horizon), you
never check out.

The distance from the black hole to the horizon is determined by the
mass of the black hole: the larger the mass the mode distant is the
horizon from the center. For a black hole with the same mass as out sun
the horizon is about 3 km from the center; for black holes with a
billion solar masses (yes there *are* such things)
this is increased to *3×10 ^{9}* km, about
the distance from the sun to Uranus. For very massive black holes the
horizon is so far away from the center that an
observer crossing it might not realize what has just happened, only
later, when all efforts to leave the area prove futile, the dreadful
realization of what happened will set in.

Imagine a brave (dumb?) astronaut who decides to through the horizon and into the nearest black hole and let us follow his observations. The first effects that becomes noticeable as he approaches the event horizon is that his clock ticks slower and slower with respect to the clocks on his spaceship very far from the black hole (see Sect. 7.4) to the point that it will take infinite spaceship time for him to cross the horizon. In contrast it will take a finite amount of astronaut time to cross the horizon, an extreme case of the relativity of time.

As the astronaut approaches the horizon the light he emits will be more and more shifted towards the red (see Sect. 7.4) eventually reaching the infrared, then microwaves, then radio, etc. In order to see him the spaceship will eventually have to detect first infrared light, then radio waves, then microwaves, etc.

After crossing the horizon the astronaut stays inside. Even though the crossing of the horizon might not be a traumatic experience the same cannot be said for his ultimate fate. Suppose he decides to fall feet first, then, when sufficiently close to the black hole, the gravitational pull on his feet will be much larger than that on his head and he will be literally ripped to pieces.

So far black holes appear an unfalsifiable conclusion of the General Theory of Relativity: their properties are such that no radiation comes out of them so they cannot be detected from a distance, and if you should decide to go, you cannot come back to tell your pals whether it really was a black hole or whether you died in a freak accident. Doesn't this contradict the basic requirement that a scientific theory be falsifiable (Sect. 1.2.1)?

Well, no, General Theory of Relativity even in this one of its most extreme
predictions *is* falsifiable. The saving circumstance
is provided by the matter surrounding the black hole. All
such stuff is continuously being dragged into the hole
(see Fig. 7.11) and devoured, but in the
process it gets extremely hot and radiates light,
ultraviolet radiation and X rays. Moreover, this cosmic
Maelstrom is so chaotic that the radiation changes
very rapidly, sometimes very intense, sometimes much
weaker, and these changes come very rapidly (see Fig.
7.12). From this changes one can estimate the size
of the object generating the radiation.

On the other hand astronomers can see the gravitational effects on near-by stars of whatever is making the radiation. And from these effects they can estimate the mass of the beast. Knowing then the size, the manner in which matter radiates when it comes near, and the mass one can compare this to the predictions of General Theory of Relativity and decide whether this is a black hole or not. The best candidate for a black hole found in this way is called Cygnus X1 (the first observed X ray source in the constellation Cygnus, the swan).

All the ways we have of detecting black holes depend on the manner
in which they affect the matter surrounding them. The most
striking example is provided by some observation of very
distant X-ray sources which are known to be relatively compact
(galaxy size) and very far away. Then the very fact that we
can see them implies that they are extremely bright objects,
so bright that we know of only one source that can fuel
them: the radiation given off by matter while being swallowed
by a black hole ^{}. So the picture we have of
these objects, generically called active galactic nuclei,
is that of a supermassive (a billion solar masses or so)
black hole assimilating many stars per second, and in disappearing
these stars give off the energy that announces their demise.

All this from the (apparently) innocent principle of equivalence.