MORE than three decades after it was conceived, one of the strangest
satellites ever designed is slowly taking shape in a laboratory outside San
Francisco. Its mission is to carry four spheres the size of ping pong balls into
orbit, where it will set them spinning at a rate of 9000 revolutions per minute.
The spinning spheres will act as tiny gyroscopes, and their axes will point
towards IM Pegasus, a star in the constellation of Pegasus, a few degrees above
the celestial equator. But during the course of the two-year mission, scientists
expect the gyroscopes to be deflected by a force that has never been observed
before.
The mission is called Gravity Probe B and it is based on the strange ability
of a gyroscope to point always in the same direction (which is why they are used
as navigation aids for aircraft, submarines and even spacecraft). By this
principle, the axes of the spinning spheres should remain locked on IM Pegasus
as the satellite orbits the Earth. But physicists have other expectations. They
say that a force predicted by Einstein鈥檚 theory of relativity, will push the
axes away from the star by an amount so small that it is difficult to imagine,
let alone measure. The force is gravitomagnetism and, if it exists at all, is as
different from ordinary gravity as electricity is from magnetism. Failure to
find the force would be a devastating blow and could mean that scientists will
have to rewrite Einstein鈥檚 theory鈥攚hich has become one of the cornerstones
of modern physics.
When Isaac Newton first toyed with the notion of gravity he wasn鈥檛 bothered
by considerations such as the speed of light or the idea of space-time. As far
as he was concerned, gravity acted instantaneously between the Earth and an
apple and he had no reason to think otherwise.
Advertisement
All this changed in 1905 when Albert Einstein published his theory of special
relativity, which proclaimed that nothing can travel faster than the speed of
light. Newton鈥檚 theory of gravity troubled Einstein because it allowed
gravitational forces to act instantaneously, even across the vast distances
between galaxies that light would take hundreds of millions of years to
cross.
To resolve this conflict, in 1916 Einstein published his theory of general
relativity in which the gravity of the Sun, for example, is no longer a force
but a distortion in the very fabric of the Universe. A simple analogy is the way
in which a heavy ball sitting on a rubber sheet distorts it, making an
indentation, so that any small objects in the vicinity tend to roll towards it.
The Earth may seem to follow an elliptical orbit around the Earth, says Francis
Everitt, the British-born physicist who heads the Gravity Probe B project at
Stanford University in California, 鈥渂ut what it鈥檚 actually doing is following
straight lines in curved space-time鈥�. Gravity Probe B is now taking shape in
Everitt鈥檚 laboratory.
General relativity neatly avoids the problem of instantaneous transmission,
but is it true? Relativity makes several predictions that allow it to be tested.
Starlight, for example, should be influenced by a gravitational field and the
elliptical orbit of Mercury should gradually turn in its plane. Both of these
effects have been observed, but physicists want more.
A big drag
Relativity also predicts that when a massive object rotates it tends to drag
space and time with it. This effect is known as frame dragging and it should
manifest itself as a force that pushes a gyroscope鈥檚 axis out of alignment as it
orbits the Earth. Gravity Probe B will attempt to measure the force,
gravitomagnetism, giving scientists an important insight into how it might
affect objects that are much larger than ping pong balls, such as black
holes.
At the same time, the gyroscopes will experience a much bigger
force鈥攖he geodetic effect which is a result of the warping of space-time
predicted by Einstein (see
Diagram). This force will tend to push their axes in
a direction perpendicular to the frame-dragging effect which allow it to be
measured separately.. The geodetic effect is hundreds of times bigger than frame
dragging and the experiment should measure its size with an accuracy of 0.01 per
cent鈥� the most severe test of general relativity ever undertaken.
While the geodetic effect was first detected in 1988, gravitomagnetism has
remained hidden because it is extremely weak. To get some idea of how weak it
is, imagine that the axes of the spinning spheres are a kilometre long. In the
course of a year, this force would move the ends of the axes by the width of a
human hair, an angle of only 40 milliarcseconds. Gravity Probe B is designed to
measure this effect with an accuracy of 1 per cent but it will be no easy
task鈥攖he slightest interference from unwanted forces will overwhelm the
results.
Silence please
鈥淚t鈥檚 extremely difficult to test,鈥� says Everitt. That鈥檚 why the experiment
will be carried out in space where the rumblings of passing cars and noisy
laboratories are absent. Even so, on the scale of gravitomagnetism, space is
filled with a cacophony of unwanted noise. For example, the probe will sweep
repeatedly through the Earth鈥檚 magnetic field, generating currents and forces
that could overwhelm the results. Somehow the spheres must be shielded from this
field. And then there is the measurement itself. How can the orientation of the
spheres be monitored with such precision without deflecting them in any way?
Everitt and his team have spent more than two decades solving these problems.
The best way to measure gravitomagnetism, Everitt says, is to use
superconductivity鈥攖he phenomenon of zero resistance that occurs when
certain materials are cooled below a critical temperature. When superconductors
rotate, strange things begin to happen. In an ordinary conductor, the electrons
in the material are dragged around as it rotates. In a superconductor, however,
they get left behind, creating a current that generates a magnetic field. This
magnetic field鈥攌nown as the London moment after the German physicist who
predicted it, Fritz London鈥攊s precisely aligned with the spin axis.
Creating a London moment on each sphere is the key to the Gravity Probe B
experiment.
The gyroscopes will spin in a vacuum inside sealed chambers at the heart of
the spacecraft. The gap between the spheres and the walls of the chambers will
be only a few thousandths of a centimetre, so any expansion could ruin the
results. The chambers will be made from fused quartz that expands very little
when heated. 鈥淚t鈥檚 a very stable material,鈥� says Sasha Buchman, who is in charge
of the gyroscope system.
The spheres will have to be almost flawless since any imperfections would
create a torque that mimics the effect the researchers are trying to measure.
Variations in density within each sphere must be kept below a few parts per
million, thousands of times better than a typical ball bearing. Each must be
electrically neutral so that no charge can build up which might swamp the
results. And their surfaces must be polished to within 40 atomic layers of a
perfect sphere.
The ideal material for the spheres is quartz, which can be polished with
immense accuracy. The spheres will be so perfect that if they were the size of
the Earth, the highest mountain would be only two metres tall. To create a
London moment, the spheres will be coated with a layer of the metal niobium only
1000 atoms thick which will become superconducting when cooled below 4
kelvin.
While the spheres are shielded from the outside world, the spacecraft will be
subject to drag from the upper reaches of the Earth鈥檚 atmosphere. At an altitude
of 600 kilometres this is a small effect but one that would eventually cause the
spacecraft to collide with the spheres within it. To prevent this, the
spacecraft鈥檚 position must be continuously measured relative to its cargo, and
corrected.
This will be done by monitoring the position of a test mass鈥攁 quartz
sphere identical to the ones used as gyroscopes鈥攖hat will sit at the
spacecraft鈥檚 centre of mass. This is the point at which the centrifugal force
resulting from the probe鈥檚 circular motion precisely balances the Earth鈥檚
gravitational field and so the test mass should follow a perfect orbit. By
centring itself on the test mass the spacecraft will follow the same orbit. 鈥淚n
essence we鈥檙e getting the spacecraft to chase the test mass,鈥� says Mac Keiser, a
physicist at Stanford.
Then there is the measurement itself. During the course of the experiment,
the changes in the London moment will be tiny but well within the measuring
capability of superconducting measuring devices known as SQUIDs which will sit
within the quartz housing. SQUIDs (superconducting quantum interference devices)
are ideal for measuring small signals because they do not create the 鈥渘oise鈥�
that is usually associated with electronic components and which could swamp the
signal. 鈥淎 SQUID has the best signal-to-noise ratio of any detector,鈥� says Jim
Lockhart, co-director of the team at Stanford that designed the measuring
equipment. The SQUIDs are capable of measuring a change in the magnetic field
corresponding to a movement of only 0.1 milliarcsecond.
But because SQUIDs are so sensitive and the effect they are measuring so
small, the Earth鈥檚 magnetic field must not be allowed to swamp the results.
Excluding this field was one of the major challenges that the team had to tackle
and, again, they turned to superconductors for help.
One of the properties of superconductors is that they are impervious to
magnetic fields and so can be used as magnetic shields. But creating a
superconducting box and cooling it simply traps whatever ambient fields there
are inside. The trick is to trap the field inside a superconducting 鈥渂alloon鈥�
and then dilute the field by inflating it. Inserting another balloon inside the
first, cooling and then inflating it, reduces the field even further. The
Gravity Probe B experiment will sit inside four lead balloons inflated in this
way that will help to reduce the ambient magnetic field by some 13 orders of
magnitude.
Of course, to make all this work the entire experiment must be cooled to
superconducting temperatures. The main bulk of the 3-tonne satellite is like a
huge Thermos flask filled with 2300 litres of superfluid helium at 2.3 kelvin.
This surrounds and cools the lead balloons, inside which the experiment sits in
a vacuum. Maintaining the flask and its contents at this temperature would be a
simple matter if it could be perfectly sealed. But the equipment must be
monitored and this information and the results of the experiment passed through
the walls of the flask so that they can be broadcast to Earth. The only way this
can be done is to use wires carrying a current. Inevitably this will create
heat, and even a small amount of heat energy could increase the temperature
significantly, says Richard Parmley, a physicist at Stanford. This would boil
away the helium dramatically reducing the lifetime of the experiment.
So reducing the heat transfer along these wires is yet another challenge for
the scientists. Ideally, the wires should be made of a material with a low
thermal conductivity but with sufficient electrical conductivity. According to
Parmley, the best material for this purpose is an alloy of copper which will
carry currents of up to 1.4 amps and yet be only half a millimetre thick or even
thinner. Each wire will be insulated so that the combined heat from the 700
wires entering the flask will boil off only 7 milligrams of helium per second.
This gas will then be used by the craft鈥檚 thrusters to control its attitude.
An even more serious heating problem could arise during the launch. Parmley
says the vibrations during liftoff will send the superfluid helium sloshing
around the flask, creating enough energy to significantly raise its temperature.
鈥淲hen you鈥檙e down near absolute zero it takes almost no energy to increase the
temperature dramatically,鈥� he explains. If the temperature of the lead balloons
rises above 7.2 kelvin, they loose their superconductivity and hence their
shielding ability. And if the Earth鈥檚 magnetic field penetrates the shielding it
cannot be removed, even if the temperature is lowered again later.
Baffles inside the flask should minimise sloshing, but whether this will be
enough to prevent excessive heating, Parmley cannot say. 鈥淚t鈥檚 such a difficult
calculation that we don鈥檛 have any confidence in it. I don鈥檛 even have a
number,鈥� he explains. So the flask is designed with an emergency cooling system.
Should the temperature of the lead balloons rise dramatically during the launch,
they will be flooded with liquid helium so that they are cooled both from the
inside and the outside. 鈥淚f the lead is encased in liquid helium we know it will
remain superconducting,鈥� says Parmley.
If all goes to plan, Gravity Probe B will be launched in 1999 at a cost of
some $500 million. Everitt says it is money well spent. He believes that
the results of the mission will have far-reaching implications for scientists
studying the nature of matter and the structure of the Universe. In the 80 years
since Einstein developed his ideas, several theorists have challenged his ideas
with competing theories. So far relativity has survived and few scientists
expect it to fall at this hurdle. Nevertheless, only Gravity Probe B will tell
us for sure.
