90 PART 1^ |^ EXPLORING THE SKY
Special Relativity
Einstein began by thinking about how moving observers see
events around them. His analysis led him to the fi rst postulate of
relativity, also known as the principle of relativity:First postulate (the principle of relativity): Observers
can never detect their uniform motion except relative to
other objects.
You may have experienced the fi rst postulate while sitting on a train
in a station. You suddenly notice that the train on the next track has
begun to creep out of the station. However, after several moments
you realize that it is your own train that is moving and that the other
train is still motionless on its track. You can’t tell which train is mov-
ing until you look at external objects such as the station platform.
Consider another example. Suppose you are fl oating in a
spaceship in interstellar space, and another spaceship comes
coasting by (■ Figure 5-11a). You might conclude that it is mov-
ing and you are not, but someone in the other ship might be
equally sure that you are moving and it is not. Of course, you
could just look out a window and compare the motion of your
spaceship with a nearby star, but that just expands the problem.
Which is moving, your spaceship or the star? Th e principle of
relativity says that there is no experiment you can perform to
decide which ship is moving and which is not. Th is means that
there is no such thing as absolute rest—all motion is relative.
Because neither you nor the people in the other spaceship
could perform any experiment to detect your absolute motion
through space, the laws of physics must have the same form in
both spaceships. Otherwise, experiments would produce diff er-
ent results in the two ships, and you could decide who was mov-
ing. So, a more general way of stating the fi rst postulate refers to
these laws of physics:First postulate (alternate version): Th e laws of physics
are the same for all observers, no matter what their
motion, so long as they are not accelerated.of particles. In Newton’s view, if he knew the location and
motion of every particle in the universe, he could, in principle,
derive the past and future of the universe in every detail. Th is
mechanical determinism has been undermined by modern quan-
tum mechanics, but it dominated science for more than two
centuries during which scientists thought of nature as a beautiful
clockwork that would be perfectly predictable if they knew how
all the gears meshed.
Most of all, Newton’s work broke the last bonds between
science and formal philosophy. Newton did not speculate on the
good or evil of gravity. He did not debate its meaning. Not more
than a hundred years before, scientists would have argued over
the “reality” of gravity. Newton didn’t care for these debates. He
wrote, “It is enough that gravity exists and suffi ces to explain the
phenomena of the heavens.”
Newton’s laws dominated astronomy for two centuries.
Th en, early in the 20th century, Albert Einstein proposed a new
way to describe gravity. Th e new theory did not replace Newton’s
laws but rather showed that they were only approximately correct
and could be seriously in error under special circumstances.
Einstein’s theories further extend the scientifi c understanding of
the nature of gravity. Just as Newton had stood on the shoulders
of Galileo, Einstein stood on the shoulders of Newton.
Einstein and Relativity
In the early years of the last century, Albert Einstein
(1879–1955) (■ Figure 5-10) began thinking about how
motion and gravity interact. He soon gained international fame
by showing that Newton’s laws of motion and gravity were only
partially correct. Th e revised theory became known as the the-
ory of relativity. As you will see, there are really two theories of
relativity.
5-3
SCIENTIFIC ARGUMENT
How do Newton’s laws of motion explain the orbital motion of
the moon?
The key here is to build your argument step by step. If Earth and
the moon did not attract each other, the moon would move in a
straight line in accord with Newton’s fi rst law of motion and van-
ish into deep space. Instead, gravity pulls the moon toward Earth’s
center, and the moon accelerates toward Earth. This acceleration
is just enough to pull the moon away from its straight-line motion
and cause it to follow a curve around Earth. In fact, it is correct
to say that the moon is falling, but because of its lateral motion it
continuously misses Earth.
Every orbiting object is falling toward the center of its orbit but
is also moving laterally fast enough to compensate for the inward
motion, and it follows a curved orbit. That is an elegant argument,
but it raises a question: How can astronauts fl oat inside space-
craft in a “weightless” state? Why might “free fall” be a more
accurate term?■ Figure 5-10
Einstein has become a symbol of
the brilliant scientist. His fame be-
gan when he was a young man and
thought deeply about the nature
of motion. That led him to revolu-
tionary insights into the meaning
of space and time and a new un-
derstanding of gravity.