LATE last year, two Japanese researchers made the startling claim that when a gyroscope spins in one sense, it loses weight, but that its weight remains unaltered when it spins in the opposite sense. Other physicists are attempting to repeat the experiment, and, already, the first results are in.

The original experimenters, Hideo Hayasaka and Sakae Takeuchi of Tokohu University, placed a gyroscope on one arm of a chemical balance and spun the rotor by electrical means. They then measured the speed of its rotation.

Hayasaka and Takeuchi found that when a gyroscope spins in a clockwise sense – looking down on it from above – it loses weight. The amount it loses is only about five-thousandths of one per cent of its resting weight. The researchers also found that the faster the gyroscope spins, the more weight it loses (see Figure). They published their work in Physical Review Letters (vol 63, p 2701).

The Japanese results received so much publicity in the US that Robert Park of the American Physical Society (which publishes Physical Review Letters) said: ‘We have been flooded with calls from around the world. Most are from people claiming to have had the idea first; some say they have a patent on it; several have pointed out that flying saucers work that way.’

The Japanese claim is not the first of its kind. for example, Eric Laithwaite, the inventor of the linear motor, made a number of observations about the peculiar properties of gyroscopes when he was at Imperial College, London, in the 1970s. And Hayasaka himself has worked on the problem for at least 10 years. He published his basic results on gyroscopes in 1978 in his university’s technology reports.

Several pieces of evidence suggest that Hayasaka and Takeuchi had to counter a series of critical reports by referees before their paper was accepted. Physical Review Letters published their paper 18 months after receiving it, which is an exceptional delay for a journal of scientific letters. And the two researchers devote the second half of their paper to anticipating and answering possible criticisms.

Most scientists are sceptical of the claims. They cannot explain the size of the observed weight loss by any of the corrections to Newton’s theory of gravity that they normally apply. And there is no other physical effect that they know of that depends on the direction of spin of a gyroscope.

Researchers pay close attention to two aspects of controversial and delicate experiments such as this one. First, they look for so-called ‘systematic effects’. These can produce results similar to the phenomenon the researchers intend to measure. Secondly, they ask whether the conclusion drawn by the experimenters is justified by the quality of their data.

In the case of Hayasaka and Takeuchi’s experiment, there are two quite obvious alternative causes of their results. First, the various components of the experiment could interact by electrical or magnetic means. Secondly, there could be mechanical effects in the balance.

The experimenters attempted to head off both these problems. They reduced the possibility of magnetic effects by repeating the experiment under differing conditions: for instance, they turned the gyroscope upside down and placed the whole apparatus inside a room that was shielded from magnetic fields.

Despite these steps, other researchers have still speculated that there may have been interaction between the electrical components used to spin the rotor, or the device used to measure the spin, and the apparatus. They have also suggested that an electric charge could have been deposited on the rotor.

Hayasaka and Takeuchi attempted to counter speculation about the mechanical behaviour of the apparatus by repeating the experiment using an electronic balance. But there is still a problem. In both cases, the gyroscope transfers angular momentum to the balance as it slows down. If it were not fixed down in some way, the balance would start to spin around itself.

Some physicists speculate that the balance provides the forces needed to cope with the spin-down of the gyroscope in such a way that it incorrectly registers the weight with one direction of spin, but not the other. For the chemical balance, this might happen if the knife-edge pivot was not perfectly made.

A second step in criticising an experiment is to suppose that the experimenters have indeed measured what they intend to measure. The question then is: Do the accuracy of the measurements justify the claimed conclusion? In the present case, the data of Hayasaka and Takeuchi looks too good – their measured points lie too close to the straight line they have drawn through them, when compared with the errors in each measurement. Statistically, more of them – about one in three – should lie away from the line.

The real test of the experiment will of course come when other groups repeat it. The first such reports are now coming in. A team at the highly-respected joint Institute for Laboratory Astro physics and the National Institute of Standards and Technology at Boulder, Colorado, has repeated the experiment. Jim Faller and his colleagues report no anomalous reduction in the weight of their gyroscopes.

If the results from Japan are true, the consequences would be profound. An explanation for flying saucers remains unlikely, though, because the gyroscope would need to spin at 200 million revolu tions per minute to counterbalance all its weight! The main consequence would be the overthrow of our present understanding of the force of gravity. Effects that are based on spin are not new; Einstein himself foresaw their possibility.

The embarrassing part about the present claims would be that the effect is much larger than existing theories with spin effects could plausibly accommodate. Indeed, if the same effect applied to spinning elementary particles, there might be an observable contribution in experiments testing Einstein’s ‘equivalence principle’: that all bodies experience the same gravitation acceleration. Similarly, there could be effects in atomic spectra.

One of the hottest pieces of new work described at the meeting concerned
the behaviour of matter, or the ‘equation of state’, at very high densitites.
At present, we can observe the behaviour of large amounts of matter at very
high densities only indirectly, by monitoring the behaviour of neutron stars.
In such objects, more matter than there is in our Sun is packed together
at about the density of the nucleus of an atom. Rotating neutron stars,
which have magnetic fields, produce beams of radiation that flick through
space like the beam of a lighthouse. If such a beam passes across Earth
our instruments can detect it as a repeating pulse of radio waves, or pulsar.

Astrophysicists can calculate the structure of a pulsar by using general
relativity to describe the strong gravitational field together with an appropriate
equation of state to describe the way matter holds itself up against the
pull of gravity. John Friedman, of the University of Wisconsin, described
a general relativistic model of a recently discovered pulsar which spins
1968 times a second (the number is easy to remember because the first discovery
of a pulsar was reported in 1968). Friedman found that this neutron star
can be stable only if the equation of state is ‘soft’, so that pressure
builds up slowly inside the star as density increases.

But relativists already knew that some stiffness is needed to make other
pulsars, such as the famous binary pulsar, stable. The two requirements
set rather close limits on the range of stiffnesses allowed for neutron
stars. It turns out that none of the equations of state within the region
that works for rotating neutron stars (which are partly held up by angular
momentum) is stiff enough to allow the same stars to hold themselves up
once they stop spinning.

Physicists conclude that when a pulsar loses its spin by radiating gravitational
waves it must ultimately collapse into a black hole. This means that there
may be no non-rotating neutron stars at all.

One topic that has been hot for the past three years, though, now seems
to be cooling off. Eric Adelberger, from the University of Seattle, reviewed
the evidence for the ‘fifth force’ and concluded that, in the light of many
contradictory experimental results, the original fifth force idea was dead.
There is certainly not a single extra force (additional, that is, to gravity,
electromagnetism, and the strong and weak nuclear forces), and there is
no credible evidence for any effect at all. But there might be some subtle
modification to gravity that is worth investigating further because the
experiments are relatively cheap and the investigations have a bearing on
unified theories of physics that attempt to describe the four forces in
one set of mathematical equations.

Genuinely cosmic implications of general relativity were the concern
of Bernard Fort, of the University of Toulouse. The first identification
of a gravitational lens, in which the gravity from a galaxy along the line
of sight bends light from a more distant object to form a visible image,
was made only in 1979, and about 10 examples are now known. But Fort focused
on the more recent discovery of a different kind of image produced by the
effect of a whole cluster of galaxies on light from the depths of space.

These natural telescopes produce multiple images, usually elongated
into an arc of light around the lensing galaxies. The first such image was
identified just two years ago, but already there are as many known examples
as of the more simple kind of gravitational lens first noticed 10 years
ago.

Roger Blandford, from the California Institute of Technology, pointed
out that although the relativity involved in the effect is trivial (it was
one of the things Einstein himself worked out many years ago), as more observations
of this kind are made the statistics of the effect can be used to infer
how much matter there is in the Universe. The more dense the Universe is,
and the more matter clusters of galaxies contain, the more likely it is
that lensing will occur; so studies of this kind might one day tell us whether
the Universe will continue to expand forever or contain so much matter that
gravity will eventually cause it to recollapse.

If it recollapses, the future may hold a ‘big crunch’ in which all the
matter in the Universe is squeezed into an infinitesimally small volume
at infinite temperature. Otherwise, the Universe will fade away as the matter
spreads out infinitely thinly.

Einstein would have been familiar with all that. But there is plenty
going on in relativity today that he would find unfamiliar. Abhay Ashtekar,
from Syracuse University in New York State, has been developing a new way
to write the equations of general relativity – Einstein’s equations. This
turns out to be very effective in providing a simple form of the so-called
‘constraint’ equations, which give the gravitational potentials.

The key to the success of Ashtekar’s approach is that it specifies the
total effect as you move around a closed loop, not the value at a single
point. The ‘loop space’ approach, pioneered also by Carlo Rovelli, at the
University of Rome and Lee Smolin at Syracuse, still involves many technical
difficulties, and details remain to be worked out. But there are hints that
it may provide a way to bring together the two great theories of 20th-century
physics, relativity and quantum mechanics. Ashtekar and his colleagues have
already shown that the results for weak gravitational fields and for a simple
(but unrealistic) world where space has just two dimensions, not three,
are satisfactory.

More technical, or less novel, aspects of the meeting covered superstrings,
cosmic strings, colliding gravitational waves and quantum cosmology, as
well as new experiments such as the laser interferometers designed to detect
gravitational radiation. Such experiments may one day give us a new ‘window’
on the Universe to supplement the optical window. Already, relativists are
looking forward eagerly to the next gathering, in Argentina in 1992.