The Astronomy Book

149

At the end of the 19th century,
many believed that physicists
had fully figured out the laws of the
universe. All that was now needed
were more precise measurements.
However, even as a child, Einstein
was not convinced that physics had
been solved. At the age of 16, he
asked himself a question: “What
would I see if I were sitting on a
beam of light?” In the Newtonian
context, young Albert would be
traveling at the speed of light. Light
coming from in front would reach
his eyes at twice the speed of light.
When looking back, he would see
nothing at all. Even though light from
behind was traveling at the speed
of light, it could never catch up.

Annus mirabilis
Einstein’s first job was working as a
patent clerk in Bern, Switzerland. It
afforded him a lot of spare time to
devote to private study. The fruit of
this solitary work was the Annus

Mirabilis (miracle year) of 1905, when

he presented four papers. These

included two linked discoveries:

special relativity and the equivalence

of mass and energy, summed up

by the equation E=mc^2 ( p.150).

Special relativity

Einstein used thought experiments

to develop his ideas, the most

significant of which involved two

men—one on a speeding train and

the other standing on the platform.

In one version (below), inside the

train, Bob shines a flashlight at a

mirror directly above him on the

ceiling. He measures the time the

light takes to travel to the mirror and

back. At the same time, the train is

passing the platform at close to the

speed of light. From the platform,

the stationary observer Pat sees

the light beam shine to the mirror

and back, but in the time it takes

for the beam to travel, the train has

moved, meaning that, rather than

See also: Gravitational theory 66–73 ■ Curves in spacetime 154–55 ■ The birth of the universe ■
Dark energy 298–303 ■ Gravitational waves 328–31

ATOMS, STARS, AND GALAXIES

Inside the speeding
train, Bob shines a
light beam directly
up and down. Bob
measures the time it
takes for the light to be
reflected back to him as
the distance straight up
and down divided by c
(the speed of light).

traveling straight up and down, the

beam travels diagonally. To Pat on

the platform the light beam has

traveled farther, so, since light

always travels at the same speed,

more time must have passed.

Einstein’s explanation for

this took an enormous leap of

imagination, which became

the basis of special relativity.

Speed is a measure of units ❯❯

On the platform,
Pat observes the beam
traveling diagonally.
It is still traveling at
the same speed c, so
more time must have
passed than for Bob as
the light has traveled
a longer distance.

Pat, stationary observer

If you can’t explain it to

a six year old, you don’t

understand it yourself.

Albert Einstein