The Astronomy Book

151

Therefore, according to special
relativity, when matter moves, it
becomes more massive. These
mass increases can be measured
on the everyday, human scale, but
are negligible. However, they have
a marked effect when objects are
moving very quickly. For example,
the protons accelerated by the
Large Hadron Collider (LHC)
particle accelerator travel very
close to the speed of light—within
99.999 percent. Additional energy
does very little to this speed, and
instead boosts mass. At full power,
the protons in the LHC are nearly
7,500 times more massive than
they were when stationary.

Speed limit
With the relationship between
speed and mass, relativity highlights
another basic principle: the speed
of light is the upper limit of motion
through space. It is impossible for an
object with mass—a nuclear particle,
spaceship, planet, or star—to travel
at the speed of light. As it approaches
light speed, its mass becomes almost
infinite, time slows nearly to a stop,
and it would take an infinite amount
of energy to push it to light speed.

To generalize his theory, Einstein

linked gravity to his ideas about

energy and motion. Taking an

object in space and removing all

reference points, it is not possible

to tell if it is moving. There is no

test that can be done to prove that

it is. Therefore, from the point of

view of any object, or reference

frame, it stays still while the rest

of the universe moves around it.

Einstein’s happiest thought

This is easiest to picture if

everything is moving at a constant

speed. According to Newton’s first

law of motion, an object maintains

its motion unless a force acts to

accelerate it (change its speed or

direction). When Einstein included

the effects of acceleration in his

theory, it led to an insight that he

called his “happiest thought”: it

was not possible to differentiate

why an object accelerated—it could

be because of gravity, or it could be

another force. The effect of both was

the same and could be described

by the way the rest of the universe

moved around the reference frame.

ATOMS, STARS, AND GALAXIES

The theory of relativity

cannot but be regarded as

a magnificent work of art.

Ernest Rutherford

New Zealand physicist

From inside an

elevator, a person

cannot tell whether

they are being

accelerated upward

by a force pushing

the elevator from below

or pulled downward

by the gravity of a

mass underneath the

elevator. Either way,

they feel a sense of

weight as the floor

pushes against them,

and objects dropped

from a height accelerate

down to the floor. This

is Einstein’s equivalence

principle, which he

described as his

“happiest thought.”

Gravity Acceleration

Lift

Mass underneath

pulls down

Force

pushes up

Einstein had described motion in

terms of the links between mass,

energy, and time. For a general

theory, he needed to add space.

It was not possible to understand

the path of an object through

space without considering its path

through time. The result was that

mass moves through spacetime,

which has a four-dimensional

geometry, as opposed to the

usual three dimensions (up, down,

and side to side) of the everyday

concept of space. When an object ❯❯

Each ray of light moves in the

coordinate system “at rest”

with a definite, constant

velocity independent of

whether this ray of light is

emitted by a body at rest

or a body in motion.

Albert Einstein