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Gravity
Newton and Gravity
As the story goes,
one day in 1665, a
young man was
sitting under a tree
when, all of a
sudden, he saw an
apple fall from
above. And with the
fall of that apple,
Isaac Newton
revolutionized our
picture of the
universe.
Newton and Gravity
In an audacious proposal
for his time, Newton
proclaimed that the force
pulling apples to the
ground and the force
keeping the moon in
orbit around the earth
were actually one and
the same.
In one fell swoop,
Newton unified the
heavens and the earth in
a single theory he called
gravity.
Newton and Gravity
Newton set out to
describe why planets
moved in circular paths.
He knew that a force of
some kind was causing
the motion.
After extensive
experimentation and
mathematical work he
found a relationship
Newton and Gravity
“Pick a flower on
Earth and you move
the farthest star” –
Paul Dirac 1933
Newton realized that
all objects in the
universe attract
each other.
Gravity’s not just a good idea,
it’s the Law
Newton discovered
the relationship
between the force
exerted by two
masses on each
other.
We call this
relationship the Law
of Universal
Gravitation
The Universal Law of
Gravitation
m1 is the mass of one of
the objects.
m2 is the mass of the
other object.
r is the radius of
separation between the
center of masses of
each object.
FG is the force of
attraction between the
two objects.
G is the gravitational
constant
Inverse Square
Relationship
Newton’s Law shows
that gravity is effected
greatly by distance.
If two objects are
exerting a force of
1000N on each other
are moved 5 times
further away, then the
force of gravity will be
25 times less or 40 N.
F
1
d
2
Newton and Gravity
The unification of the
celestial with the
terrestrial—that the
same laws that
govern the planets in
their motions govern
the tides and the
falling of fruit here
on earth—it was a
fantastic unification
of our picture of
nature.
Newton and Gravity
Gravity was the first
force to be understood
scientifically.
Although Newton
discovered his law of
gravity more than 300
years ago, his equations
describing this force
make such accurate
predictions that we still
make use of them today.
In fact scientists needed
nothing more than
Newton's equations to
plot the course of a
rocket that landed men
on the moon.
Newton and Gravity
Yet there was a
problem. While his
laws described the
strength of gravity
with great accuracy,
Newton was
harboring an
embarrassing secret:
he had no idea how
gravity actually
works.
A New Idea
For nearly 250
years, scientists
were content to look
the other way when
confronted with this
mystery.
But in the early
1900s, an unknown
clerk working in the
Swiss patent office
would change all
that.
Einstein and Gravity
While reviewing
patent applications,
Albert Einstein was
also pondering the
behavior of light.
And little did Einstein
know that his
musings on light
would lead him to
solve Newton's
mystery of what
gravity is.
Einstein and Gravity
At the age of 26, Einstein
made a startling
discovery: that the
velocity of light is a kind
of cosmic speed limit, a
speed that nothing in the
universe can exceed. But
no sooner had the young
Einstein published this
idea than he found
himself squaring off with
the father of gravity.
Einstein and Gravity
The trouble was, the idea
that nothing can go
faster than the speed of
light flew in the face of
Newton's picture of
gravity.
Imagine that all of a
sudden, and without any
warning, the sun
vaporizes and completely
disappears. Now, let's
replay that catastrophe
and see what effect it
would have on the
planets according to
Newton.
Newton’s Dilemma
Newton's theory predicts
that with the destruction
of the sun, the planets
would immediately fly
out of their orbits
careening off into space.
In other words, Newton
thought that gravity was
a force that acts
instantaneously across
any distance. And so we
would immediately feel
the effect of the sun's
destruction.
Newton’s Dilemma
But Einstein saw a big problem with Newton's
theory, a problem that arose from his work
with light.
Einstein knew light doesn't travel
instantaneously. In fact, it takes eight minutes
for the sun's rays to travel the 93 million miles
to the earth.
And since he had shown that nothing, not
even gravity, can travel faster than light, how
could the earth be released from orbit before
the sun's disappearance reached our eyes?
Space-Time
Einstein came to think of
the three dimensions of
space and the single
dimension of time as
bound together in a
single fabric of "spacetime."
It was his hope that by
understanding the
geometry of this fourdimensional fabric of
space-time, that he could
simply talk about things
moving along surfaces in
this space-time fabric.
Space-Time
Like the surface of a
trampoline, this
unified fabric is
warped and
stretched by the
mass of heavy
objects like planets
and stars.
It's this warping or
curving of spacetime that creates
what we feel as
gravity.
Space-Time
A planet like the earth is
kept in orbit, not because
the sun reaches out and
instantaneously grabs
hold of it, as in Newton's
theory, but simply
because it follows curves
in the spatial fabric
caused by the sun's
presence.
So, with this new
understanding of gravity,
let's rerun the cosmic
catastrophe. Let's see
what happens now if the
sun disappears
Gravity Waves
The gravitational
disturbance that results
will form a wave that
travels across the spatial
fabric in much the same
way that a pebble
dropped into a pond
makes ripples that travel
across the surface of the
water.
So we wouldn't feel a
change in our orbit
around the sun until this
wave reached the earth.
Curved Space-Time
What's more, Einstein
calculated that these
ripples of gravity travel
at exactly the speed of
light.
And so, with this new
approach, Einstein
resolved the conflict with
Newton over how fast
gravity travels.
And more than that,
Einstein gave the world a
new picture for what the
force of gravity actually
is: it's warps and curves
in the fabric of space and
time
Einstein’s Fame
Einstein called
this new picture
of gravity
"General
Relativity," and
within a few short
years Albert
Einstein became a
household name.
RELATIVITY
If you are a fan of science fiction, then
you know that "relativity" is a fairly
common part of the genre..
For example, people on Star Trek
are always talking about the
space-time continuum, worm
holes, time dilations and all sorts
of other things that are based on
the principle of relativity in one
way or another. If you are a fan of
science you know that relativity
plays a big part there as well,
especially when talking about
things like black holes and
astrophysics.
WORMHOLES
A hypothetical "tunnel" connecting two
different points in spacetime in such a
way that a trip through the wormhole
could take much less time than a
journey between the same starting and
ending points in normal space.
The ends of a
wormhole could, in
theory, be intrauniverse (i.e. both exist
in the same universe)
or inter-universe (exist
in different universes,
and thus serve as a
connecting passage
between the two).
BLACKHOLES
A black hole is an object or region of
space where the pull of gravity is so
strong that nothing can escape from it,
i.e., the escape velocity exceeds the
speed of light
TIME TRAVEL
Time travel is the concept of moving
between different points in time in a
manner analogous to moving between
different points in space.
Time travel could hypothetically
involve moving backward in time
to a moment earlier than the
starting point, or forward to the
future of that point without the
need for the traveler to experience
the intervening period (at least not
at the normal rate). Any
technological device – whether
fictional or hypothetical – that
would be used to achieve time
travel is commonly known as a
time machine.
Although time travel has been a
common plot device in science
fiction since the late 19th century,
and the theories of special and
general relativity suggest methods
for forms of one-way travel into the
future via time dilation, it is
currently unknown whether the
laws of physics would allow time
travel into the past. Such backward
time travel would have the potential
to introduce paradoxes related to
causality, and a variety of
hypotheses have been proposed to
resolve them.
SPECIAL RELATIVITY
The two-postulate basis for special
relativity is the one historically used by
Einstein, and it remains the starting
point today.
First postulate
There is no preferred inertial frame of
reference.
Second postulate
As measured in any inertial frame of
reference, light is always propagated in
empty space with a definite velocity c
that is independent of the state of
motion of the emitting body.