3.Black Holes

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The Very End of Space and Time

Just as white dwarfs can get too massive for their electrons to support them, so too can neutron

stars become too heavy for their neutrons. When gravity wins in a neutron star, however, Nature has

no more tricks left. All of the matter is sucked down into one single point. With all this density, space

and time warp so severely that the very fabric of spacetime is torn. The gravity is so extreme that not

even light can escape, and time itself comes to an end. A black hole

has formed.

We call a black hole a star, even though it is completely unlike what we normally think of when we

think of a star. To see how, take a disc reaching through the middle of this star, !ust as we did before.

The hill feature we saw in the neutron star has turned into something much worse here. We can"t

show all of it in this picture# it won"t fit into our space and time. $f we try to draw a circle around this

hill, we find that no rope from the outside can possibly reach down to its center without snapping.

What"s worse, the relative rate of time"s flow slows to nothing at the edge.

Part of the geometry around a black hole. Time's relative rate of flow around the black hole is shown on thecolor scale. Time appears to slow significantly and even stop at the edge of the hole.What does this mean?

This hole in the middle of the hill has a name. $t represents a sphere in three%dimensional space

called the event horizon

. This uni&ue surface is the place where everything inside is cut off from the rest of the 'niverse. $t

turns out that anything that moves to the inside of this sphere can never get back out. (ven light,

travelling as fast as anything possibly can, cannot get out. This is why it"s called a blackhole.

The si)e of this surface grows with the mass of the black hole. $f our *un turned into a black hole,

the event hori)on could be surrounded by a circle !ust + kilometersabout - milesaround. The

*un as it is today is over four million kilometers/ million milesaround.0 A black hole which is

twice as heavy would be twice as big around.
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charge, and spin will look exactly the same in every other respect. A lone black hole is not expected

to have much of a charge, so astrophysicists usually !ust talk about the mass and spin of a black

hole.

The Evidence for Black Holes

! pair of &globular star clusters& viewed by the ubble $pace Telescope. (y studying the motions of stars in

these pictures astronomers have deduced that something very massive yet very small lies at the center ofeach cluster. (lack oles are the only known candidate for what these massive ob)ects may be.

6ne of the problems with black holes is that they arenaturallyvery hard to see. 7ortunately,

black holes affect things around them through their intense, unmatchable gravity. These nearby

things fre&uently give off light, allowing us to watch as the black hole pulls them around. 7or

example, we might see a star floating in what appears to be empty space. 3ut if this star is moving in

a circle, we would know that there must be some massive ob!ect warping the spacetime near the

star. 3y measuring how &uickly the star is orbiting, astronomers can deduce how heavy the central

ob!ect is. $f there is enough mass packed into a small enough volume, with no light coming out, we

would likely have a black hole. *tars orbiting like this around unseen dark ob!ects have actually been

found. 3y watching stars like the ones in the pictures, astronomers can measure how &uickly theyare orbiting. These particular clusters of stars have been measured, and are suspected of harboring

black holes.

The center of our own *ilky Way gala+y as seen by the ,handra -%ay Telescope.

The most likely place we would expect to find a large concentration of mass is right among lots of

other massat the center of a galaxy, for instance. $f a black hole took up residence in the middle of


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a galaxy, it could eat up many stars and grow into a huge black hole. $ndeed, astronomers have

found stars in the center of our own 8ilky Way galaxy orbiting with astonishing speeds. *ome stars

orbit the center every 9: years, at speeds of over /,;;; miles per hour. That means that whatever is

warping spacetime in the center of our galaxy is several million times as massive as our *un. All that

mass, however, is packed into a region !ust a few times bigger than our *un is now. With so much

mass packed into such a small region, physicists can"t imagine anything but a black hole.

$n fact, it seems likely that most galaxies have a supermassive black hole at their centers. *ome of

these black holes are swallowing matter so &uickly that the matter falling in bumps into other matter.

All this bumping, at the extreme speeds around a black hole, makes the matter heat up

tremendously. All that heating makes the matter glow, as in the picture below on the left, which

shows a galaxy with a supermassive central black hole at its center.

*ometimes this hot matter gets caught up in a magnetic field. 3ecause of the odd nature of the

magnetic force, such &uickly moving matter may actually curve away from the black hole, and be

shot out away from the center of the galaxy. A supermassive 3lack 5ole in the middle of so much

matter would actually be extraordinarily bright. Astronomers have identified many sources they

callactive galactic nuclei

or A


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The active galactic nucleus of *01 produced this 23333333333333333 mile%long )et as seen by the ubble$pace Telescope.

A #uasar

is a similar ob!ect seen in the far reaches of space. To the casual observer, &uasars appear !ust like

regular nearby stars. 5owever, careful measurements by astronomers show that they are

extraordinarily far away. This means that they must be extraordinarily bright, also. $n fact, the light

they give off is sometimes brighter than do)ens of normal galaxies. 7or something to be so

energetic, it must have plenty of matter to fuel itthat is, it must be fantastically massive. 6n the

other hand, the light coming from a &uasar changes greatly every few months, days, or hours. $f a

huge ob!ect were to change in brightness, we would expect that change to happen very slowly. $n

particular, scientists believe that an ob!ect can only change in brightness as &uickly as it takes light

to move from one side of the ob!ect to the other. This means that a typical &uasar might only be

comparable in si)e to our solar system. Again, all that matter packed into such a small region can

only be explained by a black holeas far as we know. All of this evidence is not rock%solid.

6bservations with gravitational waves have the potential to give us far more solid facts to prove the

existence of black holes.

Hawking Radiation

The black holes we've seen above were defined to be regions of spacetime from which nothing can escape.!ccording to general relativity such regions can e+ist and in fact must be the final state of any starweighing in at more than a few times as much as our $un. 4nfortunately that's not the whole story.

,urrent theories of physics are only appro+imately correct. 5ewton's classical theory of motion and gravitywas )ust like this. 6t worked very well in most cases but there were e+ceptions. With the 7eneral Theory ofelativity 8instein filled many of the gaps. e could not fill all of them however.

There is another modern theory of physics called #uantum mechanics. This theory deals with very small and very energetic particles. 9uantum mechanics is vital for e+amplewhen trying to describe electrons moving around an atom or inside a computer chip. Trying to describethese things with general relativity leads to complete failure. :n the other hand trying to describe gravitywith #uantum mechanics also leads to complete failure. 5either theory is always wrong but neither theory isalways right.

9uantum field theory in curved spacetimeis an attempt to bring #uantum mechanics into the curved spacetime of general relativity. This theory allowsus for instance to understand the behavior of tiny particles like electrons and photons in areas of e+treme
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curvature/around a black hole for e+ample.

:ne of the many surprising predictions of #uantum field theory is that the vacuum of space is actually aboiling soup of virtual particles. These are pairs of particles that flit into and out of e+istence very briefly. ;irtual particles will borrowenergy from the empty space around them leaving that space with negative energy. :rdinarily the emptyspace will then pull the virtual particles back to regain its energy. 6n an area of e+treme gravity however the

particles can gain enough gravitational energy that the are able to return the borrowed energy to the emptyspace and survive as real particles. :ne virtual particle falls into the hole while the other escapes. ! blackhole is constantly giving off this faint glow of escaping particles as awking radiation. 6t gives up its energyand mass to the particles steaming from it. :ver very long times this leakage drains mass from the blackhole so that it shrinks down. 6f nothing were to fall in to offset this loss the hole could evaporate intonothingness.

6n $tephen awking used the theory of #uantum fields in curved spacetime to prove that thisphenomenon takes place. 6t was a great surprise to everyone/especially because it seemed to contradictone of awking's own previous great successes in elativity theory and to go against intuition about what ablack hole is. :ver time more and more e+perts came to agree with Professor awking. 5ever one to shyfrom controversy awking has come to support another new idea about black holes. is new position mayyet rescue something of our notion about black holes as places where all is lost forever.

We can imagine sending something into a black hole and trying to see if the evaporation coming out was

affected by whatever we dumped in. awking's belief currently@ is that whatever has gone into the black holewill have absolutely no effect on what comes back out. What is surprising about this position is that it runscontrary to what awking believed for many years. is reversal is simply a demonstration of the difficultythat even the most respected e+perts have in understanding how black holes work. There is still much to bee+plored tested and understood about black holes and spacetime in general as well as #uantummechanics. ow these two theories/relativity and #uantum mechanics/fit together is perhaps the greatestand most promising mystery in all of physics.
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3.Black Holes

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