*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*
{ Sila lawat Laman Hizbi-Net - http://www.hizbi.net }
{ Hantarkan mesej anda ke: [EMAIL PROTECTED] }
{ Iklan barangan? Hantarkan ke [EMAIL PROTECTED] }
*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*
Undilah PAS : MENENTANG KEZALIMAN & MENEGAKKAN KEADILAN
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A Brief History of Relativity
What is it? How does it work? Why does it change
everything? An easy primer by the world's most famous
living physicist
BY STEPHEN HAWKING
Toward the end of the 19th century scientists believed
they were close to a complete description of the
universe. They imagined that space was filled
everywhere by a continuous medium called the ether.
Light rays and radio signals were waves in this ether
just as sound is pressure waves in air. All that was
needed to complete the theory was careful measurements
of the elastic properties of the ether; once they had
those nailed down, everything else would fall into
place.
Soon, however, discrepancies with the idea of an
all-pervading ether began to appear. You would expect
light to travel at a fixed speed through the ether. So
if you were traveling in the same direction as the
light, you would expect that its speed would appear to
be lower, and if you were traveling in the opposite
direction to the light, that its speed would appear to
be higher. Yet a series of experiments failed to find
any evidence for differences in speed due to motion
through the ether.
The most careful and accurate of these experiments was
carried out by Albert Michelson and Edward Morley at
the Case Institute in Cleveland, Ohio, in 1887. They
compared the speed of light in two beams at right
angles to each other. As the earth rotates on its axis
and orbits the sun, they reasoned, it will move
through the ether, and the speed of light in these two
beams should diverge. But Michelson and Morley found
no daily or yearly differences between the two beams
of light. It was as if light always traveled at the
same speed relative to you, no matter how you were
moving.
The Irish physicist George FitzGerald and the Dutch
physicist Hendrik Lorentz were the first to suggest
that bodies moving through the ether would contract
and that clocks would slow. This shrinking and slowing
would be such that everyone would measure the same
speed for light no matter how they were moving with
respect to the ether, which FitzGerald and Lorentz
regarded as a real substance.
But it was a young clerk named Albert Einstein,
working in the Swiss Patent Office in Bern, who cut
through the ether and solved the speed-of-light
problem once and for all. In June 1905 he wrote one of
three papers that would establish him as one of the
world's leading scientists--and in the process start
two conceptual revolutions that changed our
understanding of time, space and reality.
In that 1905 paper, Einstein pointed out that because
you could not detect whether or not you were moving
through the ether, the whole notion of an ether was
redundant. Instead, Einstein started from the
postulate that the laws of science should appear the
same to all freely moving observers. In particular,
observers should all measure the same speed for light,
no matter how they were moving.
This required abandoning the idea that there is a
universal quantity called time that all clocks
measure. Instead, everyone would have his own personal
time. The clocks of two people would agree if they
were at rest with respect to each other but not if
they were moving. This has been confirmed by a number
of experiments, including one in which an extremely
accurate timepiece was flown around the world and then
compared with one that had stayed in place. If you
wanted to live longer, you could keep flying to the
east so the speed of the plane added to the earth's
rotation. However, the tiny fraction of a second you
gained would be more than offset by eating airline
meals
A Brief History of Relativity PAGE 1 | 2 | 3 | 4 | 5
Einstein's postulate that the laws of nature should
appear the same to all freely moving observers was the
foundation of the theory of relativity, so called
because it implies that only relative motion is
important. Its beauty and simplicity were convincing
to many scientists and philosophers. But there
remained a lot of opposition. Einstein had overthrown
two of the Absolutes (with a capital A) of 19th
century science: Absolute Rest as represented by the
ether, and Absolute or Universal Time that all clocks
would measure. Did this imply, people asked, that
there were no absolute moral standards, that
everything was relative? This unease continued
through the 1920s and '30s. When Einstein was awarded
the Nobel Prize in 1921, the citation was for
important--but by Einstein's standards comparatively
minor--work also carried out in 1905. There was no
mention of relativity, which was considered too
controversial. I still get two or three letters a week
telling me Einstein was wrong. Nevertheless, the
theory of relativity is now completely accepted by the
scientific community, and its predictions have been
verified in countless applications. A very important
consequence of relativity is the relation between mass
and energy. Einstein's postulate that the speed of
light should appear the same to everyone implied that
nothing could be moving faster than light. What
happens is that as energy is used to accelerate a
particle or a spaceship, the object's mass increases,
making it harder to accelerate any more. To accelerate
the particle to the speed of light is impossible
because it would take an infinite amount of energy.
The equivalence of mass and energy is summed up in
Einstein's famous equation E=mc2, probably the only
physics equation to have recognition on the street.
Among the consequences of this law is that if the
nucleus of a uranium atom fissions (splits) into two
nuclei with slightly less total mass, a tremendous
amount of energy is released. In 1939, with World War
II looming, a group of scientists who realized the
implications of this persuaded Einstein to overcome
his pacifist scruples and write a letter to President
Roosevelt urging the U.S. to start a program of
nuclear research. This led to the Manhattan Project
and the atom bomb that exploded over Hiroshima in
1945. Some people blame the atom bomb on Einstein
because he discovered the relation between mass and
energy. But that's like blaming Newton for the gravity
that causes airplanes to crash. Einstein took no part
in the Manhattan Project and was horrified by the
explosion. Although the theory of relativity fit well
with the laws that govern electricity and magnetism,
it wasn't compatible with Newton's law of gravity.
This law said that if you changed the distribution of
matter in one region of space, the change in the
gravitational field would be felt instantaneously
everywhere else in the universe. Not only would this
mean you could send signals faster than light
(something that was forbidden by relativity), but it
also required the Absolute or Universal Time that
relativity had abolished in favor of personal or
relativistic time. Einstein was aware of this
difficulty in 1907, while he was still at the patent
office in Bern, but didn't begin to think seriously
about the problem until he was at the German
University in Prague in 1911. He realized that there
is a close relationship between acceleration and a
gravitational field. Someone in a closed box cannot
tell whether he is sitting at rest in the earth's
gravitational field or being accelerated by a rocket
in free space. (This being before the age of Star
Trek, Einstein thought of people in elevators rather
than spaceships. But you cannot accelerate or fall
freely very far in an elevator before disaster
strikes.) If the earth were flat, one could equally
well say that the apple fell on Newton's head because
of gravity or that Newton's head hit the apple because
he and the surface of the earth were accelerating
upward. This equivalence between acceleration and
gravity didn't seem to work for a round earth,
however; people on the other side of the world would
have to be accelerating in the opposite direction but
staying at a constant distance from us.
On his return to Zurich in 1912 Einstein had a
brainstorm. He realized that the equivalence of
gravity and acceleration could work if there was some
give-and-take in the geometry of reality. What if
space-time--an entity Einstein invented to incorporate
the three familiar dimensions of space with a fourth
dimension, time--was curved, and not flat, as had been
assumed? His idea was that mass and energy would warp
space-time in some manner yet to be determined.
Objects like apples or planets would try to move in
straight lines through space-time, but their paths
would appear to be bent by a gravitational field
because space-time is curved. With the help of his
friend Marcel Grossmann, Einstein studied the theory
of curved spaces and surfaces that had been developed
by Bernhard Riemann as a piece of abstract
mathematics, without any thought that it would be
relevant to the real world. In 1913, Einstein and
Grossmann wrote a paper in which they put forward the
idea that what we think of as gravitational forces are
just an expression of the fact that space-time is
curved. However, because of a mistake by Einstein (who
was quite human and fallible), they weren't able to
find the equations that related the curvature of
space-time to the mass and energy in it. Einstein
continued to work on the problem in Berlin,
undisturbed by domestic matters and largely unaffected
by the war, until he finally found the right
equations, in November 1915. Einstein had discussed
his ideas with the mathematician David Hilbert during
a visit to the University of Gottingen in the summer
of 1915, and Hilbert independently found the same
equations a few days before Einstein. Nevertheless, as
Hilbert admitted, the credit for the new theory
belonged to Einstein. It was his idea to relate
gravity to the warping of space-time. It is a tribute
to the civilized state of Germany in this period that
such scientific discussions and exchanges could go on
undisturbed even in wartime. What a contrast to 20
years later! The new theory of curved space-time was
called general relativity to distinguish it from the
original theory without gravity, which was now known
as special relativity. It was confirmed in spectacular
fashion in 1919, when a British expedition to West
Africa observed a slight shift in the position of
stars near the sun during an eclipse. Their light, as
Einstein had predicted, was bent as it passed the sun.
Here was direct evidence that space and time are
warped, the greatest change in our perception of the
arena in which we live since Euclid wrote his Elements
about 300 B.C. Einstein's general theory of
relativity transformed space and time from a passive
background in which events take place to active
participants in the dynamics of the cosmos. This led
to a great problem that is still at the forefront of
physics at the end of the 20th century. The universe
is full of matter, and matter warps space-time so that
bodies fall together. Einstein found that his
equations didn't have a solution that described a
universe that was unchanging in time. Rather than give
up a static and everlasting universe, which he and
most other people believed in at that time, he fudged
the equations by adding a term called the cosmological
constant, which warped space-time the other way so
that bodies move apart. The repulsive effect of the
cosmological constant would balance the attractive
effect of matter and allow for a universe that lasts
for all time. This turned out to be one of the great
missed opportunities of theoretical physics. If
Einstein had stuck with his original equations, he
could have predicted that the universe must be either
expanding or contracting. As it was, the possibility
of a time-dependent universe wasn't taken seriously
until observations were made in the 1920s with the
100-in. telescope on Mount Wilson. These revealed that
the farther other galaxies are from us, the faster
they are moving away. In other words, the universe is
expanding and the distance between any two galaxies is
steadily increasing with time. Einstein later called
the cosmological constant the greatest mistake of his
life. PAGE 1 | 2 | 3 | 4 | 5
General relativity completely changed the discussion
of the origin and fate of the universe. A static
universe could have existed forever or could have been
created in its present form at some time in the past.
On the other hand, if galaxies are moving apart today,
they must have been closer together in the past. About
15 billion years ago, they would all have been on top
of one another and their density would have been
infinite. According to the general theory, this Big
Bang was the beginning of the universe and of time
itself. So maybe Einstein deserves to be the person of
a longer period than just the past 100 years. General
relativity also predicts that time comes to a stop
inside black holes, regions of space-time that are so
warped that light cannot escape them. But both the
beginning and the end of time are places where the
equations of general relativity fall apart. Thus the
theory cannot predict what should emerge from the Big
Bang. Some see this as an indication of God's freedom
to start the universe off any way God wanted. Others
(myself included) feel that the beginning of the
universe should be governed by the same laws that hold
at all other times. We have made some progress toward
this goal, but we don't yet have a complete
understanding of the origin of the universe. The
reason general relativity broke down at the Big Bang
was that it was not compatible with quantum theory,
the other great conceptual revolution of the early
20th century. The first step toward quantum theory
came in 1900, when Max Planck, working in Berlin,
discovered that the radiation from a body that was
glowing red hot could be explained if light came only
in packets of a certain size, called quanta. It was as
if radiation were packaged like sugar; you cannot buy
an arbitrary amount of loose sugar in a supermarket
but can only buy it in 1-lb. bags. In one of his
groundbreaking papers written in 1905, when he was
still at the patent office, Einstein showed that
Planck's quantum hypothesis could explain what is
called the photoelectric effect, the way certain
metals give off electrons when light falls on them.
This is the basis of modern light detectors and
television cameras, and it was for this work that
Einstein was awarded the 1921 Nobel Prize in Physics.
Einstein continued to work on the quantum idea into
the 1920s but was deeply disturbed by the work of
Werner Heisenberg in Copenhagen, Paul Dirac in
Cambridge and Erwin Schrodinger in Zurich, who
developed a new picture of reality called quantum
mechanics. No longer did tiny particles have a
definite position and speed. On the contrary, the more
accurately you determined the particle's position, the
less accurately you could determine its speed, and
vice versa. Einstein was horrified by this random,
unpredictable element in the basic laws and never
fully accepted quantum mechanics. His feelings were
expressed in his famous God-does-not-play-dice dictum.
Most other scientists, however, accepted the validity
of the new quantum laws because they showed excellent
agreement with observations and because they seemed to
explain a whole range of previously unaccounted-for
phenomena. They are the basis of modern developments
in chemistry, molecular biology and electronics and
the foundation of the technology that has transformed
the world in the past half-century. When the Nazis
came to power in Germany in 1933, Einstein left the
country and renounced his German citizenship. He spent
the last 22 years of his life at the Institute for
Advanced Study in Princeton, N.J. The Nazis launched a
campaign against "Jewish science" and the many German
scientists who were Jews (their exodus is part of the
reason Germany was not able to build an atom bomb).
Einstein and relativity were principal targets for
this campaign. When told of publication of the book
One Hundred Authors Against Einstein, he replied, Why
100? If I were wrong, one would have been enough.
A Brief History of Relativity PAGE 1 | 2 | 3 | 4 | 5
After World War II, he urged the Allies to set up a
world government to control the atom bomb. He was
offered the presidency of the new state of Israel in
1952 but turned it down. "Politics is for the moment,"
he once wrote, "while...an equation is for eternity."
The equations of general relativity are his best
epitaph and memorial. They should last as long as the
universe. The world has changed far more in the past
100 years than in any other century in history. The
reason is not political or economic but
technological--technologies that flowed directly from
advances in basic science. Clearly, no scientist
better represents those advances than Albert Einstein:
TIME's Person of the Century. Professor Hawking,
author of "A Brief History of Time," occupies the
Cambridge mathematics chair once held by Isaac Newton.
END
_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
( Melanggan ? To : [EMAIL PROTECTED] pada body : SUBSCRIBE HIZB)
( Berhenti ? To : [EMAIL PROTECTED] pada body: UNSUBSCRIBE HIZB)
( Segala pendapat yang dikemukakan tidak menggambarkan )
( pandangan rasmi & bukan tanggungjawab HIZBI-Net )
( Bermasalah? Sila hubungi [EMAIL PROTECTED] )
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Pengirim: art jeffries <[EMAIL PROTECTED]>
