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Gravity
What is gravity?
BY ROBERT LAMB
The Road Runner cartoons play fast and loose
with gravity.
Given the opportunity, Wile E. Coyote usually
chases his quick-footed foe off the edge of a cliff -- but only plummets when
he realizes he's running on thin air.
Needless to say, gravity doesn't work like this
in real life.
But what's going on when boulders or Acme anvils
plummet to the Earth?
Let's start with your feet and work our way up
to, oh, the entire universe. Grab a chair, because this is going to take an
entire page to explain.
Gravity holds your feet down to the ground
because the mass of the planet exerts a gravitational pull on the mass of your
body.
In fact, gravity stirs up an attraction between
any two objects in the universe: moons,
dust motes, coyotes -- you name it.
Wherever you find matter, you'll find gravity.
You could never travel to a gravity-free planet, only one with greater or
lesser mass resulting in greater or lesser gravity.
On a larger scale, gravity arranges cosmic
bodies into orbits and even causes drifting space particles to pull together
slowly into larger and larger clumps that eventually become planets, stars and galaxies.
Back in the 1600s, Isaac Newton defined gravity
as a universal force acting on all matter.
According to his theory, the exact expression of
gravity came down to mass and distance. The farther apart two particles are and
the less massive they are, the less the gravitational force.
That's Newton's law of universal gravitation in
a nutshell, and it stood unchallenged for three centuries.
Then, in the 1900s, a wild-haired physicist by
the name of Albert Einstein stepped in the ring and let fly with his general
theory of relativity.
Einstein argued that gravity was far more than
just a force; it was a curve in the fourth dimension of space and time.
Given sufficient mass, an object can cause an
otherwise straight beam of light to curve.
Astronomers call this effect gravitational
lensing, and it's one of the primary methods of detecting unobservable
cosmic phenomena such as black holes.
Similarly, the less gravity there is, the faster
time passes, a phenomenon known as gravitational time dilation.
For instance, a clock aboard an orbiting
satellite advances slightly faster than a counterpart on Earth's surface.
While Einstein's theory brought gravity up to
speed with modern science, we still don't know everything about gravity.
Some scientists attribute gravity to
hypothetical particles called gravitons, which -- in theory --
cause objects to be attracted to one another.
Finally, there's the field of quantum
gravity, in which scientists attempt to reconcile general relativity with
quantum theory.
Quantum theory addresses how the universe works
at the smallest subatomic levels.
The field has helped scientists develop the
standard model of particle physics, which details most of the inner workings of
the universe -- with one notable exception. The standard model doesn't explain
gravity.
So while quantum theory and relativity together
explain most of the observable universe, they also contradict each other at
times, such as in the study of black holes or the early universe.
Not surprisingly, numerous scientists continue
to work toward a unified theory.
Whatever theories we ultimately adopt, it's
difficult to overstate the importance of gravity. It's the glue that holds the
cosmos together, even if it still stirs up unanswered questions about the
universe.
About Robert
Lamb
As a
child, Robert Lamb dreamed of becoming a mad scientist when he grew up. As this
profession proved to be largely fictional, however, he swallowed his heartbreak
and turned his attention to the written word instead. He earned his bachelor's
degree in creative writing from the University of Tennessee in Knoxville, which
launched him on a career path through high school English classrooms,
small-town newsrooms and finally into the offices of HowStuffWorks.
As a
senior writer and podcaster for Stuff to Blow Your Mind, Robert
now spends his days sifting through all the scientific wonders that make the
world so mad and amazing. He currently lives in Atlanta with his lovely wife
and their beautiful one-eyed cat. When he's not researching the apocalypse or
the miracle we call a space toilet, he enjoys listening to electronic music,
painting the odd miniature and writing fiction.
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