Science |
The awesome power of the atomic
bomb Einstein's theory of relativity led to
the creation of incredibly powerful nuclear weapons, the destructive capabilities
of which have changed the face of world politics. But it also gave us nuclear
power, which now accounts for a fifth of all the world's electricity. |
Riding
a light beam back in time Einstein loved to devise "thought experiments" to help him understand the consequences of his ideas, and it was such an experiment that triggered his insights into space and time. He is standing next to a clock, and just at the moment its hands show 12 he is whisked off on a beam of light at 300,000 kilometres a second for 300,000 kilometres. According to common sense, his journey will have taken one second. And if Einstein looks at a watch in his hand, it will indeed say one second past 12. But if he could take a look over his shoulder, the clock back at base would still say precisely 12. The reason is simple: as Einstein is travelling at the speed of light, the light rays transmitting the image of the clock with the new time of one second past haven't been able to catch up with him - while he travels at the speed of light, it will always seem to be 12 precisely. Who is right? The answer is "both" - everything, including time, is relative, and what you measure as time depends on how fast you are moving. Time "slows down" the faster you go. |
When the young Einstein imagined what it would be like to ride on a light beam, he took us all on a fascinating tour .But how much does his thinking explain about the universe now? |
The theory of relativity: dreamt up by
Einstein, and comprehensible only to geniuses.
That's the perception But it's a myth. Anyone can understand the basics of
the theory it lust took a genius to invent it in the first place.
Today, scientists such as Professor Stephen Hawking regularly use Einstein's
theory in their search for the key to the origin
of the universe. But Einstein's discoveries have a direct impact on all
our lives. The post Cold War world is being shaped by those who have nuclear
weapons - whose origin and power lie in that most famous equation of relativity
E = mc2.
Nuclear power, another spin- off from Einstein's theory, now accounts
for a fifth of the world's electricity. Scientists are trying to go one better
and bring nuclear fusion, the power source of the sun and stars, down to
earth. Able to turn a kilogram of hydrogen isotope "fuel" from sea-water
into the energy equivalent of 7000 tonnes of oil, fusion could solve our
energy problems - courtesy of E = mc2.
Mapping the brain
Positron emission tomography (PET) scans locate brain tumours without surgery. The positron was only discover thanks to relativity. |
Even hospital patients benefit from relativity, in the form of positron emission
tomography (PET) scanners, which are used to perform brain scans. The existence
of positrons -
the "antimatter" versions of electrons
- was completely unknown until the 4920s when Paul Dirac combined
Einstein's theory with ideas about electromagnetic waves such as light.
Relativity may now be one of the foundations of modern physics, but it took
years for mainstream scientists to take seriously relativity's implications
- such as the ability of time to slow down, mass to increase and objects
to shrink the faster theytravel. Some scientists never could, or would, accept
them.
Yet for Einstein these implications were just the beginning. They became
part of the theory of "special relativity", which deals with objects
moving at constant speed relative to one another. That was in 1905. Then
in 1915 Einstein published his theory of "general relativity" (GR),
which deals with the more widespread or general case of objects that are
changing speed relative to one another.
GR led Einstein to an astonishing new conception of gravity. According to
his theory, this familiar force is actually the result of the curving of
the very fabric of space and time by mass - a little as a large sheet of
rubber would curve if a cannonball were placed upon it.
General relativity, and its application to the entire universe, made Einstein
famous; but, 90 years on, cracks are starting to appear. Observations of
distant galaxies seem to suggest that the universe is younger than some of
the stars in it. This implies that something is missing from general
relativity.
A vital clue comes from the fact that Einstein's view of gravity fails to
incorporate anything from quantum theory, the laws
of the sub-atomic world which are better supported by scientific evidence
even than relativity. To date, nobody has managed to successfully combine
Einstein's vision of gravity with quantum theory. Who will make the next
breakthrough? Budding Einsteins should start here.
Special theory of relativity
E = mc2 it is the most famous equation in the whole of science.
Its symbols imply that there's enough energy (F) locked up in the mass (m)
of a kilogram of matter to power a house for a million years. It underpins
the power of the sun and stars - and nuclear weapons.
Of all of Einstein's achievements, this equation is the most impressive.
Yet its origins lie in a simple question which the 16- year-old Einstein
asked himself: what would it be like to ride on a light beam? Einstein had
a knack of setting himself riddles that went right to the heart of problems.
The fantastical ride on a light beam is no exception: it led him to develop
relativity.
Weird things clearly happen when you travel at the speed of light - things
that defy everyday experience, like time standing still. But,
most importantly, Einstein's riddle shows that the speed of light has a very
special significance: it is the fundamental speed at which information travels
around the universe.
This special status for the speed of light had already been suspected by
the Scottish scientist James Clerk Maxwell. During the 1860s, Maxwell
wrote down mathematical equations for the behaviour of electricity and magnetism.
One of these equations suggested that the speed of electromagnetic waves
- including light - was a fundamental constant, a law of physics which has
the same value for everyone, all the time.
Fanning the
fires of nuclear fusion Static electricity weaves a crackling web over the coolant of a particle beam fusion accelerator. The beams hammer a pellet made from deuterium and tritium with five trillion watts per square centimetre, collapsing it in a nuclear fusion reaction. The accelerator, at Sandia Laboratories, New Mexico, is the only one in the world. |
This sounds innocuous enough until you think about it. Then it hits you:
how can the speed of light be the same for everyone? Obviously, the relative
speed of a train depends on how fast you are moving yourself. If you are
on the platform, the speed may be 200 km/h, but to a car travelling on a
road alongside the track, its speed might appear to be just 90 km/h.
The puzzle over the speed of light deepened during the 1880s, following
experiments by the American Albert Michelson, who tried to measure
how much the earth's own speed affected the speed of light. Investigating
a now-discarded theory of how light travels, Michelson thought a light beam
fired at a mirror in the direction of the earth's orbit should be affected
by the earth's movement. A beam fired at right angles to the earth's motion,
however, would have a different relative speed. After many attempts neither
Michelson nor anyone else found any difference: the speed of light seemed
to he constant, the same whether with or against the earth's motion.
Around the turn of the century, scientists came up with all manner of theories
to explain this "crazy" answer. Two professors, Hendrik Lorentz of Holland
and Ireland's George FitzGerald, even claimed that the cause was the result
of half of the Michelson apparatus shrinking relative to the other.
Incredibly, Einstein showed that they were right - but for the wrong reasons.
FitzGerald and Lorentz pinned the blame on molecular change inside Michelson's
apparatus. Einstein showed that the "shrinkage" stems from an apparent need
to keep the laws of physics - including the speed of light the same for all
observers.
But how to explain this "need"? Put simply, time itself is relative
and depends on how one is moving: moving clocks run slower than stationary
ones (see box above). Similarly, relative
lengths depend on speed too: moving rulers shrink relative to stationary
ones.
The clever thing is that these two effects of speed - on time and on length
- together cancel out any difference in the speed of light. So no matter
how fast you travel the speed of light remains the same. Similar arguments
lead to the conclusion that relative mass also increases with speed. But
isn't this just some sort of trick? Are the shrinkage and the time-stretching
real? Yes, and they have even been measured.
At ordinary speeds, these strange effects are not noticeable, and "common-sense"
results prevail. At earth's speed round the sun (110,000 km/h) the difference
amounts to just five parts in a billion - which would have been just detectable
using optical instruments available to Michelson.
However, scientists have long had access to objects that routinely travel
at very close to the speed of light: sub-atomic
particles. Just three years after Einstein put forward his theory, the
German physicist Alfred Bucherer showed that electrons really do gain mass
as their speed increases.
More sophisticated experiments in the 1960s and 1970s using giant "atom smashers"
like the machines at CERN in Geneva, confirmed
Einstein's other predictions. These showed that fast-moving protons behave
like flattened discs rather than tiny spheres when they smash into stationary
protons, demonstrating the shrinkage effect of speed. Unstable particles
known as muons survive longer when moving near the speed of light - an effect
that explains how these particles, which have a half-life of just
two-millionths of a second, can penetrate further into our atmosphere than
the 600 metres predicted by "common
sense".
Yet these are not the most dramatic consequences of Einstein's theory; that
comes from the effect of relativity on energy. Einstein showed that if the
laws of physics are the same at all speeds, energy due to speed - kinetic
energy - is really only part of the "total energy" an object possesses.
According to relativity, all objects have an amount of energy that depends
only on their mass. To be precise, this energy (E) is given by the mass (in)
times the speed of light squared: E = mc2.
In theory, just one kilogram of material could be converted into as much
energy as a nuclear power station produces in three years. Unfortunately,
in reality there is no way to extract more than a tiny fraction of the total
energy locked in mass.
Not that this fraction is not dramatic. For example, when the nuclei of
lightweight atoms like hydrogen are fused together in intense heat - as in
the centre of the sun - a small proportion of the mass of the end-product
is converted in energy, via E = mc2. Because the speed of light
(c) is such a huge quantity -300 million metres per second - this means the
sun can emit energy equivalent to the detonation of 100 billion H-bombs every
second for ten billion years.
But the famous equation has its dark side: an H-bomb as small as a car can
produce the explosive equivalent of a million tonnes of TNT. However, weapons
experts are discussing the use of nuclear weapons as a means of protecting
the earth from asteroids. Only missiles carrying nuclear weapons could deflect
the impact of space debris. Perhaps, one day, E = mc2 and the
theory of relativity may literally save the planet.
Reflections on the speed of light |
Crucial confirmation that the speed of light is a constant
was provided by an earlier experiment designed to prove the opposite. In 1887 two American scientists, Albert Michelson (right) and James Morley, built a machine to test James Clerk Maxwell's theory that light propagates through the "aether" - an invisible medium permeating space. If this was the case, the speed of light should be affected by the speed of the Earth relative to the aether. In the experiment two beams of light were reflected back and forth between two pairs of mirrors, one pair parallel to the Earth's orbit, the other at right angles to it. The beams intersect to produce an interference pattern. When the entire apparatus was turned through 90 degrees, the pattern remained unchanged, showing that the speed of light remained constant. |
Interfering with the data The cross-shaped "interferometer" (right) had a massive base (below) to prevent the slightest distortion. Despite the results, some physicists suggested the two beams were going at different speeds but the equipment had been compressed by the aether - to the exact extent to make the speeds seem the same. |
General Theory and cosmology
Let go of this magazine, and it falls to the ground. But why? Just saying
"gravity" is no answer - what is gravity? Even the great Isaac Newton had
no clear answer, preferring instead to talk vaguely of some sort of "influence".
It is Einstein and his theory of relativity that provide us with our best
understanding of this most fundamental force in the universe.
Einstein reached his new concept of gravity by widening the scope of his
brilliant "special" theory of relativity. This applies only to things moving
at a constant speed relative to each other. But gravity is a force, and forces
change the velocity of objects. So to understand gravity Einstein needed
a "general" theory of relativity.
A cornerstone of general relativity (GR) is the same idea that led Einstein
to special relativity: the laws of physics must be the same for everyone.
But Einstein needed another brilliant insight - like his imaginary ride on
a light beam - to help him push beyond the limits of special relativity.
It came to him in November 1907 in the Berne Patent Office, where he was
an inventions examiner.
Trainee astronauts experience "zero-g" in an aircraft
flying so that its acceleration cancels the effect of gravity. |
He suddenly realised that someone free-falling is not aware of the force
of gravity. Imagine you're in a lift when the cable breaks: you're in free-
fall. Now imagine taking a set of keys out of your pocket and letting them
go: they will just stay where they are - relative to the free-faller (ignoring
air- resistance, of course). For the free-faller, it's as if the force of
gravity has been switched off. Einstein realised he could exploit this fact,
which had been known from the time of Galileo: objects, no matter what their
mass, accelerate at the same rate under gravity. This enabled him to cancel
out the local effect of gravity and use special relativity again. But it
had to work everywhere, not just locally.
Einstein then discovered something surprising. The failure of special relativity
in the presence of gravity is the result of gravity curving
space and time. This led to a new concept
of gravity. Instead of a vague influence by which. for example, the sun holds
the earth in place, relativity led to the image of the sun sitting in space
and time and curving both, like a cannonball on a rubber sheet. The earth's
orbit follows the contours of this dip.
X marks the spot Gas and dust - the X shape in the middle - are dragged into a black hole in the Whirlpool galaxy. Space-time is so distorted not even light escapes |
It took Einstein eight years to reach his new theory of gravity. But the
rewards were vast. Newton's law emerged from GR as merely a special case,
true only for relatively feeble gravitational fields and low speeds.
In strong fields and at high speeds, GR starts to show its mettle - as Einstein
quickly showed. Since 1850, astronomers had been baffled by the behaviour
of the planet Mercury, whose orbit refused to obey Newton's law,changing
each year by a tiny amount. Observations of the planet's position were in
error by the equivalent of the width of a pin at 300 paces. Einstein showed
that GR could account for this discrepancy by the fact that Mercury is so
deep inside the intense gravitational field of the sun.
Using GR, Einstein predicted that the warping of space and time by the Sun's
gravity would bend the light of stars out of their predicted position, again
by a tiny amount (equivalent to the width of a pin 60 metres away). Newton
had predicted half the amount.
How the sun bends space,time and light |
|
Although our sun is an ordinary star, and not a massive
black hole, it distorts space-time into a "gravity well". As the sun moves
against the background of fixed stars, their positions seem to move slightly.
The effect is tiny - less than a thousandth of a degree - and is caused by
gravity bending light. Light from the sun appears to be coming from a slightly
different direction. It is bent because it follows a curving path across
the sun's space-time "dimple", instead of the straight line it would follow
in gravitationally undistorted space. |
In May 1949, two teams of British astronomers measured star positions during
a total eclipse of the Sun visible from Brazil. Einstein correctly predicted
five ten-thousandths of a degree of bending; again, Newton's theory predicted
only half that. This triumph marked the end of Newton's reign as principal
authority on gravity - and Einstein's elevation to Greatest Scientist of
the Century.
He was lucky, though: an arithmetical blunder years earlier had originally
led Einstein to reach the same light-bending value as Newton. A world war
and bad weather had stopped scientists checking out the faulty prediction.
The most impressive consequences of GR were yet to come, however. GR is our
best theory of gravity, and gravity is the force that rules the universe.
That makes GR the best theory we have for understanding the past, present
and future of the universe. Einstein's equations predicted the universe
expanding, but in 1916 he, like everyone else, believed the universe
to be infinite and unchanging. So he fiddled his equations to fit this
view.
By 1929, astronomers had proved Einstein wrong: the universe does
expand. He allegedly described the fiddle factor as the
biggest blunder of
his life. Yet, as Focus reported last December, astronomers are now
baffled by new observations of distant galaxies, which seem to show that
the universe is around seven billion years younger than some of the stars
it contains. Einstein's "fiddle factor" - which can be used as a cosmic force
countering gravity - can resolve this situation.
Hunting down gravity Professor Massimo Cerdonio (above) is building a gravity wave detector (left) to pick up gravitational echoes from supernova explosions. These waves should cause vibrations less than a billionth of a billionth of a metre in size. |
A race is now under way to find "gravitational waves", feeble ripples in
space and time which GR predicts are sent out by huge stellar explosions.
Some scientists hope to detect these ripples by picking up wave-like changes
in the length of huge metal bars. As gravity is a weak force, such changes
may be just one millionth of the diameter of an atomic nucleus. A Nobel prize
probably awaits the winner of this race.
There is, however, one outstanding problem with Einstein's theory. Scientists
now believe there are four fundamental forces at work in the universe: two
nuclear forces, electromagnetism and gravity. The first three forces come
under the banner of quantum theory, in which all forces are the result of
subatomic "exchange" particles jumping from place to place. But gravity does
not fit this scheme.
Attempts to marry Einstein's idea of gravity as space-time curvature with
quantum theory are unlikely to succeed until the next century. And no one
person is likely to achieve as much as Einstein did all those years ago as
a lowly patent clerk in Switzerland.
Robert
Matthews
Mar95 p30
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