Moon-based lasers could uncover exotic physics

 作者:亓浸     |      日期:2019-02-28 04:05:05
By David Shiga (Image: NASA) NASA is funding the development of lasers that could be placed on the Moon to check for subtle deviations from the standard theory of gravity. Lasers have been used to make very precise measurements of the Earth-Moon distance since the Apollo era, when astronauts left reflectors at three sites on the lunar surface. A fourth reflecting device is attached to a robotic lunar lander launched by the Soviet Union. To pin down the Moon’s distance, scientists bounce light from Earth-based lasers off of these reflectors and measure how long it takes to return. Because the Moon’s motion is governed by gravity, such studies can be used to test whether Einstein’s general theory of relativity gives an accurate description of this motion. Some speculative theories of cosmology, such as one inspired by string theory that involves exotic particles called dilatons, predict deviations from general relativity. Other theories predict that the gravitational constant (G), which measures the basic strength of gravity, is not constant at all, but varies with time. These deviations are predicted to be very subtle, so more precise measurements are needed to either detect them or rule them out. But making the measurements precise enough is a challenge. That is because the laser loses strength both on the way to the Moon and on the return journey, resulting in an extremely weak signal. Currently, only three observatories on Earth have equipment sensitive enough to detect these feeble pulses. The dearth of detectors makes it difficult to determine and account for complicating effects, such as slight fluctuations in the Earth’s shape, that influence the distance measurements. That is why Slava Turyshev of NASA’s Jet Propulsion Laboratory in Pasadena, California, US, and colleagues want to put a more sophisticated device on the Moon. They are one of seven teams that recently received NASA funding to study science that could be done on the Moon. Called a laser transponder, the device would detect an incoming laser pulse from Earth and then fire its own laser back in response. The resulting signal would be much stronger and could therefore be detected by dozens of different observatories around the world. This in turn would allow for more accurate measurements of the Moon’s distance. “By putting a transponder on the Moon, we open lunar ranging to a much broader community and get this global averaging effect that can be quite substantial and important,” says team member Tom Murphy of the University of California in San Diego, US. Turyshev’s team aims to develop a suitcase-sized laser transponder that astronauts could bring with them on future missions to the Moon. They also plan to develop new reflectors that would allow the three observatories that can currently detect the weak signals to achieve accuracies of about 1 millimetre in their distance measurements, compared to the current norm of about 20 mm. And new reflectors may eventually be needed to replace the current ones, even if NASA never upgrades to a laser transponder. Murphy says he has noticed a big decline – by more than a factor of 10 – in the brightness of the pulses returned by the reflectors. “I think they’ve suffered degradation in their 30 years on the Moon,” he says,