A physics experiment on the drawing board
for the International Space Station could help find the grand unifying
"Theory of Everything" suggested by Albert Einstein
by Patrick L Barry
Sooner or later, the
reign of Albert Einstein, like the reign of Isaac Newton before
him, will come to an end. An upheaval in the world of physics that
will overthrow our notions of basic reality is inevitable, most
scientists believe, and currently a horse race is underway between
a handful of theories competing to be the successor to the throne.
In the running are
such mind-bending ideas as an 11-dimensional universe, universal
"constants" (such as the strength of gravity) that vary over space
and time and only remain truly fixed in an unseen 5th dimension,
infinitesimal vibrating strings as the fundamental constituents
of reality, and a fabric of space and time that's not smooth and
continuous, as Albert Einstein believed, but divided into discrete,
indivisible chunks of vanishingly small size. Experiment will ultimately
determine which triumphs.
A new concept for an
experiment to test the predictions of Albert Einstein's relativity
more precisely than ever before is being developed by scientists
at NASA's Jet Propulsion Laboratory (JPL). Their mission, which
effectively uses our solar system as a giant laboratory, would help
narrow the field of vying theories and bring us one step closer
to the next revolution in physics.
It may not weigh heavily
on most people's minds, but a great schism has long plagued our
fundamental understanding of the universe. Two ways of explaining
the nature and behavior of space, time, matter, and energy currently
exist: Albert Einstein's relativity and the "standard model" of
quantum mechanics. Both are extremely successful. The Global Positioning
System (GPS), for instance, wouldn't be possible without the theory
of relativity. Computers, telecommunications, the Internet, automobiles
(and lets try to forget the auto
insurance quotes you need!) meanwhile, are all spin-offs of
But the two theories are like different languages, and no one is
yet sure how to translate between them. Relativity explains gravity
and motion by uniting space and time into a 4-dimensional, dynamic,
elastic fabric of reality called space-time, which is bent and warped
by the energy it contains. (Mass is one form of energy, so it creates
gravity by warping space-time.) Quantum mechanics, on the other
hand, assumes that space and time form a flat, immutable "stage"
on which the drama of several families of particles unfolds. These
particles can move both forward and backward in time (something
relativity doesn't allow), and the interactions between these particles
explain the basic forces of nature - with the glaring exception
The stalemate between
these two theories has gone on for decades. Most scientists assume
that somehow, eventually, a unifying theory will be developed that
subsumes the two, showing how the truths they each contain can fit
neatly within a single, all-encompassing framework of reality. Such
a "Theory of Everything" would profoundly affect our knowledge of
the birth, evolution, and eventual fate of the universe.
According to Einstein's theory of general relativity, the
sun's gravity causes starlight to bend, shifting the apparent
position of stars in the sky.
Slava Turyshev, a scientist
at JPL, and his colleagues have thought of a way to use the International
Space Station (ISS) and two mini-satellites orbiting on the far
side of the sun to test the theory of relativity with unprecedented
accuracy. Their concept, developed in part through funding from
NASA's Office of Biological and Physical Research, would be so sensitive
that it could reveal flaws in Einstein's theory, thus providing
the first hard data needed to distinguish which of the competing
Theories of Everything agree with reality and which are merely fancy
The experiment, called Laser Astrometric Test Of Relativity (LATOR),
would look at how the sun's gravity deflects beams of laser light
emitted by the two mini-satellites. Gravity bends the path of light
because it warps the space through which the light is passing. The
standard analogy for this warping of space-time by gravity is to
imagine space as a flat sheet of rubber that stretches under the
weight of objects like the sun. The depression in the sheet would
cause an object (even a no-mass particle of light) passing nearby
the sun to turn slightly as it went by.
In fact, it was by
measuring the bending of starlight by the sun during a solar eclipse
in 1919 that Sir Arthur Eddington first tested Einstein's theory
of general relativity. In cosmic terms, the sun's gravity is fairly
weak; the path of a beam of light skimming the edge of the sun would
only be bent by about 1.75 arcseconds (an arcsecond is 1/3600 of
a degree). Within the limits of accuracy of his measuring equipment,
Eddington showed that starlight did indeed bend by this amount -
and in doing so effectively impeached Newton.
LATOR would measure this deflection with a billion (109) times the precision
of Eddington's experiment and 30,000 times the precision of the
current record-holder: a serendipitous measurement using signals
from the Cassini spacecraft on its way to explore Saturn.
The curvature of space due to the sun's mass caused signals
from the Cassini probe to curve on their way to Earth.
"I think [LATOR] would
be quite an important advance for fundamental physics," says Clifford
Will, a professor of physics at Washington University who has made
major contributions to post-Newtonian physics and is not directly
involved with LATOR. "We should continue to try to press for more
accuracy in testing general relativity, simply because any kind
of deviation would mean that there's new physics that we were not
aware of before."
The experiment would
work like this: Two small satellites, each about one meter wide,
would be launched into an orbit circling the sun at roughly the
same distance as Earth. This pair of mini-satellites would orbit
more slowly than Earth does, so about 17 months after launch, the
mini-satellites and Earth would be on opposite sides of the sun.
Even though the two satellites would be about 5 million km apart,
the angle between them as viewed from Earth would be tiny, only
about 1 degree. Together, the two satellites and Earth would form
a skinny triangle, with laser beams along its sides, and one of
those beams passing close to the sun.
Turyshev plans to measure
the angle between the two satellites using an interferometer mounted
on the ISS. An interferometer is a device that catches and combines
beams of light. By measuring how waves of light from the two mini-satellites
"interfere" with each other, the interferometer can measure the
angle between the satellites with extraordinary precision: about
10 billionths of an arcsecond, or 0.01 µas
(micro-arcseconds). When the precision of the other parts of the
LATOR design are considered, this gives an overall accuracy for
measuring how much gravity bends the laser beam of about 0.02 µas
for a single measurement.
"Using the ISS gives
us a few advantages," Turyshev explains. "For one, it's above the
distortions of Earth's atmosphere, and it's also large enough to
let us place the two lenses of the interferometer far apart (one
lens on each end of the solar panel truss), which improves the resolution
and accuracy of the results."
Image courtesy Slava Turyshev
interferometer will be mounted to the solar-panel truss
of the ISS, which automatically rotates to continuously
face the sun.
The 0.02 µas accuracy
of LATOR is good enough to reveal deviations from Einstein's relativity
predicted by the aspiring Theories of Everything, which range from
roughly 0.5 to 35 µas. Agreement with LATOR's measurements would
be a major boost for any of these theories. But if no deviation
from Einstein is found even by LATOR, most of the current contenders - along
with their 11 dimensions, pixellated space, and inconstant constants - will
suffer a fatal blow and "pass on" to that great dusty library stack
in the sky.
Because the mission
requires only existing technologies, Turyshev says LATOR could be
ready to fly as soon as 2009 or 2010. So it may not be too long
before the stalemate in physics is broken and a new theory of gravity,
space, and time takes the throne.