Sky and Telescope - February 2017 - 16

```Runhed for 2nd Spread

Anatomy of a

Black Hole

Artwork by Casey Ree
eed
d

What is a black hole?
A black hole is a pit in the fabric of spacetime. Space and
time, according to Einstein's theory of special relativity, are
interchangeable parts of a thing called spacetime: much as
width, height, and depth are dimensions of a box, so space and
time are dimensions of spacetime. Although the dimensions
of space and time are relative and can change, contracting
or dilating depending on your frame of reference - an effect
noticeable when dealing with strong gravity or relativistic
speeds - units of spacetime are absolute.

A Massive Object's
Effect on Spacetime

3D

In this wacky landscape, a black hole is a cosmic pothole.
Black holes are places where the density of matter grew
so high that the outward pressure of the matter's particles
couldn't withstand the inward pull of gravity, and gravity, like
a master poker player, gathered all the chips together and
crushed them into what's called a singularity. We don't
really understand what happens at the singularity; we
only know that there, classical physics breaks down.
A singularity and a black hole are not the same thing.
The singularity hides inside a black hole, screened
from view by the black hole's event horizon. The event
horizon is the so-called point of no return: the region
within which nothing, not even light, can escape the
black hole's gravitational pull.

2D

A Non-spinning Black Hole's
Effect on Spacetime

However, this all becomes far more complicated if the
black hole is spinning (see facing page).
What would happen if you fell into a black hole?
If it were a stellar-mass black hole, you'd be dead before
you passed the event horizon. That's because, if you
think of a black hole as a pit, a stellar-mass black hole
has steeper sides than a supermassive black hole. The
tidal forces become too strong too fast for you to survive
to the event horizon, resulting in your spaghettification (yes,
that's the technical term).
So let's travel into a supermassive black hole. Passing the
event horizon, you wouldn't notice much (except some fun light
effects and several extra g's of gravity). But as you drew closer
to the singularity, gravity would stretch and squeeze you like
you were dough in a bread machine. At this point you'd die.

As you approached the event horizon, a second person far,
far away would watch your image slow down and redden.
Theoretically, at the event horizon your image would freeze.
But in practice you would disappear: the photons lose energy
as it becomes harder for them to climb out of the black hole's
gravitational well, and their wavelength would increase until it
grew past the observer's detection capabilities - making the
image invisible. So your image would redden and dim with time,
16

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

Event horizon

Singularity

CREDIT X X X X X X X X X X X X X X X

What would someone watching see as you fell in?

```

Sky and Telescope - February 2017

Contents
Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
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Sky and Telescope - February 2017 - Cover3
Sky and Telescope - February 2017 - Cover4
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