Universe
Not so much a theory of the universe as a
simple picture of the planet we call
home, the flat-earth model proposed that
Earth’s surface was level. Although
everyday experience makes this seem a
reasonable assumption, direct observation
of nature shows the real world
isn’t that simple. For instance, when a sailing
ship heads into port, the
first part that becomes visible is the crow’s-nest,
followed by the sails,
and then the bow of the ship. If the Earth were flat, the
entire ship would
come into view at once as soon as it came close enough to
shore. The Greek
philosopher Aristotle provided two more reasons why the Earth
was round.
First, he noted that Earth’s shadow always took a circular bite out
of the
moon during a lunar eclipse, which would only be possible with a
spherical
Earth. (If the Earth were a disk, its shadow would appear as an
elongated
ellipse at least during part of the eclipse.) Second, Aristotle knew
that
people who journeyed north saw the North Star ascend higher in the sky,
while
those heading south saw the North Star sink. On a flat Earth, the
positions
of the stars wouldn’t vary with a person’s location. Despite these
arguments,
which won over most of the world’s educated citizens, belief in a
flat Earth
persisted among many others. Not until explorers first
circumnavigated the
globe in the 16th century did those beliefs begin to die
out. Ptolemy, the
last of the great Greek astronomers of antiquity, developed an
effective
system for mapping the universe. Basing much of his theory on the work
of his
predecessor, Hipparchus, Ptolemy designed a geocentric,
or
Earth-centered, model that held sway for 1400 years. That Ptolemy
could place
Earth at the center of the universe and still predict the
planets’ positions
adequately was a testament to his ability as a
mathematician. That he could do
so while maintaining the Greek belief that
the heavens were perfect—and thus
that each planet moved along a circular
orbit at a constant speed—is nothing
short of remarkable. Copernicus made a
great leap forward by realizing that the
motions of the planets could be
explained by placing the Sun at the center of
the universe instead of Earth.
In his view, Earth was simply one of many planets
orbiting the Sun, and the
daily motion of the stars and planets were just a
reflection of Earth
spinning on its axis. Although the Greek astronomer
Aristarchus developed
the same hypothesis more than 1500 years earlier,
Copernicus was the
first person to argue its merits in modern times. Despite the
basic truth of
his model, Copernicus did not prove that Earth moved around the
Sun. That
was left for later astronomers. The first direct evidence came
from
Newton’s laws of motion, which say that when objects orbit one
another, the
lighter object moves more than the heavier one. Because the Sun
has about
330,000 times more mass than Earth, our planet must be doing
almost all the
moving. A direct observation of Earth’s motion came in 1838
when the German
astronomer Friedrich Bessel measured the tiny displacement,
or parallax, of a
nearby star relative to the more distant stars. This
minuscule displacement
reflects our planet’s changing vantage point as we
orbit the Sun during the
year. How did the universe really begin? Most
astronomers would say that the
debate is now over: The universe started with
a giant explosion, called the Big
Bang. The big-bang theory got its start
with the observations by Edwin Hubble
that showed the universe to be
expanding. If you imagine the history of the
universe as a long-running
movie, what happens when you show the movie in
reverse? All the galaxies
would move closer and closer together, until
eventually they all get crushed
together into one massive yet tiny sphere. It
was just this sort of thinking
that led to the concept of the Big Bang. The Big
Bang marks the instant
at which the universe began, when space and time came
into existence and all
the matter in the cosmos started to expand. Amazingly,
theorists have deduced
the history of the universe dating back to just 1043
second (10 million
trillion trillion trillionths of a second) after the Big
Bang. Before
this time all four fundamental forces—gravity, electromagnetism,
and the
strong and weak nuclear forces—were unified, but physicists have yet
to
develop a workable theory that can describe these conditions. During
the
first second or so of the universe, protons, neutrons, and
electrons—the
building blocks of atoms—formed when photons collided and
converted their
energy into mass, and the four forces split into their
separate identities. The
temperature of the universe also cooled during this
time, from about 1032 (100
million trillion trillion) degrees to 10 billion
degrees. Approximately three
minutes after the Big Bang, when the temperature
fell to a cool one billion
degrees, protons and neutrons combined to form the
nuclei of a few heavier
elements, most notably helium. The next major step
didn’t take place until
roughly 300,000 years after the Big Bang, when the
universe had cooled to a
not-quite comfortable 3000 degrees. At this
temperature, electrons could combine
with atomic nuclei to form neutral
atoms. With no free electrons left to scatter
photons of light, the universe
became transparent to radiation. (It is this
light that we see today as the
cosmic background radiation.) Stars and galaxies
began to form about one
billion years following the Big Bang, and since then the
universe has simply
continued to grow larger and cooler, creating conditions
conducive to life.
Three excellent reasons exist for believing in the big-bang
theory. First,
and most obvious, the universe is expanding. Second, the theory
predicts that
25 percent of the total mass of the universe should be the helium
that formed
during the first few minutes, an amount that agrees with
observations.
Finally, and most convincing, is the presence of the cosmic
background
radiation. The big-bang theory predicted this remnant radiation,
which now
glows at a temperature just 3 degrees above absolute zero, well before
radio
astronomers chanced upon it. Friedmann made two simple assumptions about
the
universe: that when viewed at large enough scales, it appears the same
both
in every direction and from every location. From these assumptions
(called the
cosmological principle) and Einstein’s equations, he developed
the first model
of a universe in motion. The Friedmann universe begins with a
Big Bang and
continues expanding for untold billions of years—that’s the
stage we’re in
now. But after a long enough period of time, the mutual
gravitational attraction
of all the matter slows the expansion to a stop. The
universe then starts to
fall in on itself, replaying the expansion in
reverse. Eventually all the matter
collapses back into a singularity, in what
physicist John Wheeler likes to call
the "Big Crunch." Gravitational
attraction is a fundamental property of
matter that exists throughout the
known universe. Physicists identify gravity as
one of the four types of
forces in the universe. The others are the strong and
weak nuclear forces and
the electromagnetic force. More than 300 years ago, the
great English
scientist Sir Isaac Newton published the important generalization
that
mathematically describes this universal force of gravity. Newton was
the
first to realize that gravity extends well beyond the boundaries of
Earth.
Newton's realization was based on the first of three laws he had
formulated to
describe the motion of objects. Part of Newton's first law, the
Law of Inertia,
states that objects in motion travel in a straight line at a
constant velocity
unless they are acted upon by a net force. According to
this law, the planets in
space should travel in straight lines. However, as
early as the time of
Aristotle, the planets were known to travel on
curved paths. Newton reasoned
that the circular motions of the planets are
the result of a net force acting
upon each of them. That force, he concluded,
is the same force that causes an
apple to fall to the ground--gravity.
Newton's experimental research into the
force of gravity resulted in his
elegant mathematical statement that is known
today as the Law of Universal
Gravitation. According to Newton, every mass in
the universe attracts every
other mass. The attractive force between any two
objects is directly
proportional to the product of the two masses being measured
and inversely
proportional to the square of the distance separating them. If we
let F
represent this force, r the distance between the centers of the masses,
and
m1 and m2 the magnitude of the two masses, the relationship stated can
be
written symbolically as: is defined mathematically to mean "is
proportional
to.") From this relationship, we can see that the greater the
masses of the
attracting objects, the greater the force of attraction between
them. We can
also see that the farther apart the objects are from each other,
the less the
attraction. It is important to note the inverse square
relationship with respect
to distance. In other words, if the distance
between the objects is doubled, the
attraction between them is diminished by
a factor of four, and if the distance
is tripled, the attraction is only
one-ninth as much. Newton's Law of Universal
Gravitation was later
quantified by eighteenth-century English physicist Henry
Cavendish who
actually measured the gravitational force between two one-kilogram
masses
separated by a distance of one meter. This attraction was an extremely
weak
force, but its determination permitted the proportional relationship
of
Newton's law to be converted into an equation. This measurement
yielded the
universal gravitational constant or G.