The Astronomy Book

152

moves through spacetime, the
time dimension dilates, and
the space dimensions contract.
From the point of view of Pat back
at the station, the speeding train’s
length is compressed, making it
look very squashed and stubby.
However, it is all normal to Bob;
anything he measures on board
will have the same length as
when the train was stationary.
This is because his means of
measurement, such as a ruler,
has contracted along with space.

Warping spacetime
In Einstein’s universe, gravity
is recast not as a force but rather
the effect of warps in the geometry
of spacetime caused by the
presence of mass. A large mass,
such as a planet, bends space,
and so a smaller object, such as
a meteor, moving in a straight line
through space nearby, will curve
toward the planet. The meteor
has not changed course—it is
still moving along the same line
in space; it is just that the planet
has bent that line into a curve.
Warps in spacetime can be
visualized as balls deforming a
rubber sheet, making depressions
or “gravity wells.” A large “planet”
ball makes a well, and a smaller
“meteor” ball will roll into the

well. Depending on its trajectory,

speed, and mass, the meteor might

collide with the planet or roll back

up the other side of the well and

escape. If the trajectory is just right,

the meteor will circle around the

planet in an orbit.

The warps created by matter

also bend time. Two distant

objects—for this explanation,

a red star and a blue star—are not

moving in relation to one another.

They are in different points of space,

but at the same point in time, the

same “now.” However, if the red

star moves directly away from the

blue, its passage through time

slows compared to the blue star’s.

That means the red star shares

a “now” with the blue star in the

past. If the red star travels directly

toward the blue one, its “now”

is angled toward the blue star’s

future. Consequently, events that

are observed simultaneously from

one reference frame may appear to

occur at different times in another.

THE THEORY OF RELATIVITY

Proof of relativity

Einstein’s physics were initially

met with bafflement from most of

the scientific community. However,

in 1919, the English astronomer

Arthur Eddington demonstrated

that this new way of describing

the universe was indeed accurate.

He traveled to the Atlantic island

of Principe to observe a full solar

eclipse and specifically to look

at the background of stars near

to the sun. Light from stars travels

to Earth along the most direct

route, known as the geodesic. In

Euclidean geometry (the geometry

of Newtonian physics), that is a

straight line, but in the geometry

Everything must be made

as simple as possible.

But not simpler.

Albert Einstein

A light beam shines into an elevator from an observer with a flashlight

standing outside. The paths of the light beam are shown as they will be

observed from inside the elevator. If the elevator is accelerating, the beam

will curve downward. Light is similarly curved toward a source of gravity.

Zero motion Constant velocity Acceleration

Lift

Relativity solved the puzzle of

perturbations in the orbit of the planet

Mercury (pictured) that could not be

explained by Newtonian physics,

which had first been noticed in 1859.