Parallel Worlds (40 page)

Although
Einstein later found an exact solution of his equations that allowed for
gravity waves, he despaired of ever seeing his prediction verified in his
lifetime. Gravity waves are extremely weak. Even the shock waves of colliding
stars are not strong enough to be measured by current experiments.

At present,
gravity waves have only been detected indirectly. Two physicists, Russell Hulse
and Joseph Taylor, Jr., conjectured that if you analyze circling binary neutron
stars that chase each other in space, then each star would emit a stream of
gravity waves, similar to the wake created by stirring molasses, as their orbit
slowly decays. They analyzed the death spiral of two neutron stars as they
slowly spiraled toward each other. The focus of their investigation was the
double neutron star PSR 1913+16, located about 16,000 light-years from Earth,
which orbit around each other every 7 hours, 45 minutes, in the process
emitting gravity waves into outer space.

Using Einstein's
theory, they found that the two stars should come closer by a millimeter every
revolution. Although this is a fantastically small distance, it increases to a
yard over a year, as the orbit of 435,000 miles slowly decreases in size.
Their pioneering work showed that the orbit decayed precisely as Einstein's
theory predicted on the basis of gravity waves. (Einstein's equations, in
fact, predict that the stars will eventually plunge into each other within 240
million years, due to the loss of energy radiated into space in the form of
gravity waves.) For their work, they won the Nobel Prize in physics in 1993.

We can also go
backward and use this precision experiment to measure the accuracy of general
relativity itself. When the calculations are done backward, we find that
general relativity is at least 99.7 percent accurate.

LIGO GRAVITY WAVE DETECTOR

But to extract
usable information about the early universe, one must observe gravity waves
directly, not indirectly. In 2003, the first operational gravity wave detector,
LIGO (Laser Interferometer Gravitational-Wave Observatory), finally came
online, realizing a decades-old dream of probing the mysteries of the universe
with gravity waves. The goal of LIGO is to detect cosmic events that are too
distant or tiny to be observed by Earth telescopes, such as colliding black
holes or neutron stars.

LIGO consists of
two gigantic laser facilities, one in Hanford, Washington, and the other in
Livingston Parish, Louisiana. Each facility has two pipes, each 2.5 miles
long, creating a gigantic L-shaped tubing. Within each tube a laser is fired.
At the joint of the L, both laser beams collide, and their waves interfere with
each other. Normally, if there are no disturbances, then the two waves are synchronized
so that they cancel each other out. But when even the tiniest gravity wave
emitted from colliding black holes or neutron stars hits the apparatus, it
causes one arm to contract and expand differently than the other arm. This
disturbance is sufficient to disrupt the delicate cancellation of the two
laser beams. As a result, the two beams, instead of canceling each other out,
create a characteristic wavelike interference pattern that can be
computer-analyzed in detail. The larger the gravity wave, the greater the
mismatch between the two laser beams, and the larger the interference pattern.

LIGO is an
engineering marvel. Since air molecules may absorb the laser light, the tube
containing the light has to be evacuated down to a trillionth of atmospheric
pressure. Each detector takes up 300,000 cubic feet of space, meaning that LIGO
has the largest artificial vacuum in the world. What gives LIGO such
sensitivity, in part, is the design of the mirrors, which are controlled by
tiny magnets, six in all, each the size of an ant. The mirrors are so polished
that they are accurate to one part in 30 billionths of an inch. "Imagine
the earth were that smooth. Then the average mountain wouldn't rise more than
an inch," says GariLynn Billingsley, who monitors the mirrors. They are so
delicate that they can be moved by less than a millionth of a meter, which
makes the LIGO mirrors perhaps the most sensitive in the world. "Most
control systems engineers' jaws drop when they hear what we're trying to
do," says LIGO scientist Michael Zucker.

Because LIGO is
so exquisitely balanced, it is sometimes plagued by slight, unwanted vibrations
from the most unlikely sources. The detector in Louisiana, for example, cannot
be run during the day because of loggers who are cutting trees 1,500 feet from
the site. (LIGO is so sensitive that even if the logging were to take place a
mile away, it still could not be run during the daytime.) Even at night, vibrations
from passing freight trains at midnight and 6 a.m. bracket how much continuous
time the LIGO can operate.

Even something
as faint as ocean waves striking the coastline miles away can affect the
results. Ocean waves breaking on North American beaches wash ashore every six
seconds, on average, and this creates a low growl that can actually be picked
up by the lasers. The noise is so low in frequency, in fact, that it actually
penetrates right through the earth. "It feels like a rumble," says
Zucker, commenting about this tidal noise. "It's a huge headache during
the Louisiana hurricane season." LIGO is also affected by the tides created
by the Moon's and Sun's gravity tugging on Earth, creating a disturbance of
several millionths of an inch.

In order to
eliminate these incredibly tiny disturbances, LIGO engineers have gone to
extraordinary lengths to isolate much of the apparatus. Each laser system rests
on top of four huge stainless steel platforms, each stacked on top of each
other; each level is separated by springs to damp any vibration. Sensitive
optical instruments each have their own seismic isolation system; the floor is
a slab of 30-inch- thick concrete that is not coupled to the walls.

LIGO is actually
part of an international consortium, including the French-Italian detector
called VIRGO in Pisa, Italy, a Japanese detector called TAMA outside Tokyo, and
a British-German detector called GEO600 in Hanover, Germany. Altogether, LIGO's
final construction cost will be $292 million (plus $80 million for commissioning
and upgrades), making it the most expensive project ever funded by the National
Science Foundation.

But even with
this sensitivity, many scientists concede that LIGO may not be sensitive enough
to detect truly interesting events in its lifetime. The next upgrade of the
facility, LIGO II, is scheduled to occur in 2007 if funding is granted. If LIGO
does not detect gravity waves, the betting is that LIGO II will. LIGO scientist
Kenneth Libbrecht claims that LIGO II will improve the sensitivity of the
equipment a thousandfold: "You go from [detecting] one event every 10
years, which is pretty painful, to an event every three days, which is very
nice."

For LIGO to
detect the collision of two black holes (within a distance of 300 million
light-years), a scientist could wait anywhere from a year to a thousand years.
Many astronomers may have second thoughts about investigating such an event
with LIGO if it means that their great-great-great . . . grandchildren will be
the ones to witness the event. But as LIGO scientist Peter Saulson has said,
"People take pleasure in solving these technical challenges, much the way
medieval cathedral builders continued working knowing they might not see the
finished church. But if there wasn't a fighting chance to see a gravity wave
during my life career, I wouldn't be in this field. It's not just Nobel fever .
. . The levels of precision we are striving for mark our business; if you do
this, you have 'the right stuff.' " With LIGO II, the chances are much
better of finding a truly interesting event in our lifetime. LIGO II might
detect colliding black holes within a much larger distance of 6 billion
light-years at a rate of ten per day to ten per year.

Even LIGO II,
however, will not be powerful enough to detect gravity waves emitted from the
instant of creation. For that, we must wait another fifteen to twenty years for
LISA.

LISA GRAVITY WAVE DETECTOR

LISA (Laser
Interferometry Space Antenna) represents the next generation in gravity wave
detectors. Unlike LIGO, it will be based in outer space. Around 2010, NASA and
the European Space Agency plan to launch three satellites into space; they will
orbit around the Sun at approximately 30 million miles from Earth. The three
laser detectors will form an equilateral triangle in space (5 million kilometers
on a side). Each satellite will have two lasers that allow it to be in
continual contact with the other two satellites. Although each laser will fire
a beam with only half a watt of power, the optics are so sensitive that they
will be able to detect vibrations coming from gravity waves with an accuracy of
one part in a billion trillion (corresponding to a shift that is one hundredth
the width of a single atom). LISA should be able to detect gravity waves from a
distance of 9 billion light-years, which cuts across most of the visible
universe.

LISA will be so
accurate that it might detect the original shock waves from the big bang
itself. This will give us by far the most accurate look at the instant of
creation. If all goes according to plan, LISA should be able to peer to within
the first trillionth of a second after the big bang, making it perhaps the most
powerful of all cos- mological tools. It is believed that LISA may be able to
find the first experimental data on the precise nature of the unified field
theory, the theory of everything.

One important
goal of LISA is to provide the "smoking gun" for the inflationary
theory. So far, inflation is consistent with all cos- mological data (flatness,
fluctuations in the cosmic background, and so forth). But that doesn't mean the
theory is correct. To clinch the theory, scientists want to examine the gravity
waves that were set off by the inflationary process itself. The
"fingerprint" of gravity waves created at the instant of the big bang
should tell the difference between inflation and any rival theory. Some, such
as Kip Thorne of Cal Tech, believe that LISA may be able to tell whether some
version of string theory is correct. As I explain in chapter 7, the
inflationary universe theory predicts that gravity waves emerging from the big
bang should be quite violent, corresponding to the rapid, exponential expansion
of the early universe, while the ekpy- rotic model predicts a much gentler
expansion, accompanied by much smoother gravity waves. LISA should be able to
rule out various rival theories of the big bang and make a crucial test of
string

EINSTEIN LENSES AND RINGS

Yet another
powerful tool in exploring the cosmos is the use of gravitational lenses and
"Einstein rings." As early as 1801, Berlin astronomer Johan Georg
von Soldner was able to calculate the possible deflection of starlight by the
Sun's gravity (although, because Soldner used strictly Newtonian arguments, he
was off by a crucial factor of 2. Einstein wrote, "Half of this deflection
is produced by the Newtonian field of attraction of the sun, the other half by
the geometrical modification ['curvature'] of space caused by the sun.")

In 1912, even
before he completed the final version of general relativity, Einstein
contemplated the possibility of using this deflection as a "lens," in
the same way that your glasses bend light before it reaches your eye. In 1936,
a Czech engineer, Rudi Mandl, wrote to Einstein asking whether a gravity lens
could magnify light from a nearby star. The answer was yes, but it would be
beyond their technology to detect this.

In particular,
Einstein realized that you would see optical illusions, such as double images
of the same object, or a ringlike distortion of light. Light from a very
distant galaxy passing near our Sun, for example, would travel both to the left
and right of our Sun before the beams rejoined and reached our eye. When we
gaze at the distant galaxy, we see a ringlike pattern, an optical illusion
caused by general relativity. Einstein concluded that there was "not much
hope of observing this phenomenon directly." In fact, he wrote that this
work "is of little value, but it makes the poor guy [Mandl] happy."

Over forty years
later, in i979, the first partial evidence of lensing was found by Dennis Walsh
of the Jordell Bank Observatory in England, who discovered the double quasar
Q0957+561. In 1988, the first Einstein ring was observed from the radio source
MG1131+0456. In 1997, the Hubble space telescope and the UK's MERLIN radio telescope
array caught the first completely circular Einstein ring by analyzing the
distant galaxy i938+666, vindicating Einstein's theory once again. (The ring is
tiny, only a second of arc, or roughly the size of a penny viewed from two
miles away.) The astronomers described the excitement they felt witnessing this
historic event: "At first sight, it looked artificial and we thought it
was some sort of defect in the image, but then we realized we were looking at a
perfect Einstein ring!" said Ian Brown of the University of Manchester.
Today, Einstein's rings are an essential weapon in the arsenal of astrophysicists.
About sixty-four double, triple, and multiple quasars (illusions caused by
Einstein lensing) have been seen in outer space, or roughly one in every five
hundred observed quasars.

Even invisible
forms of matter, like dark matter, can be "seen" by analyzing the
distortion of light waves they create. In this way, one can obtain
"maps" showing the distribution of dark matter in the universe. Since
Einstein lensing distorts galactic clusters by creating large arcs (rather than
rings), it is possible to estimate the concentration of dark matter in these
clusters. In i986, the first giant galactic arcs were discovered by
astronomers at the National Optical Astronomy Observatory, Stanford University,
and Midi-Pyrenees Observatory in France. Since then, about a hundred galactic
arcs have been discovered, the most dramatic in the galactic cluster Abell
2218.