![]() Instead, each atom existed in a superposition of two locations, separated by up to seven centimeters. During the atoms’ flight, the scientists put the atoms in a state called a quantum superposition, in which particles don’t have one definite location. Kasevich and colleagues studied the tiny particles using atom interferometry, which takes advantage of quantum mechanics to make extremely precise measurements. “When you do the test with atoms … you’re testing the equivalence principle and stressing it in new ways,” says physicist Mark Kasevich of Stanford University. But atoms, which are subject to the strange laws of quantum mechanics, could reveal its weak points. So far, the equivalence principle has withstood all tests. The upshot: An object’s acceleration under gravity doesn’t depend on its mass or composition. That principle states that an object’s inertial mass, which determines how much it accelerates when force is applied, is equivalent to its gravitational mass, which determines how strong a gravitational force it feels. In confirming Galileo’s gravity experiment yet again, the result upholds the equivalence principle, a foundation of Albert Einstein’s theory of gravity, general relativity. As the atoms rose and fell, both varieties accelerated at essentially the same rate, the researchers found. The team launched clouds of these atoms about 8.6 meters high in a tube under vacuum. Researchers compared rubidium atoms of two different isotopes, atoms that contain different numbers of neutrons in their nuclei. “It represents a leap forward,” says physicist Guglielmo Tino of the University of Florence, who was not involved with the new study. Two different types of atoms had the same acceleration within about a part per trillion, or 0.0000000001 percent, physicists report in a paper in press in Physical Review Letters.Ĭompared with a previous atom-drop test, the new research is a thousand times as sensitive. In recent years, researchers have taken to replicating this test in a way that the Italian scientist probably never envisioned - by dropping atoms.Ī new study describes the most sensitive atom-drop test so far and shows that Galileo’s gravity experiment still holds up - even for individual atoms. "As NASA moves forward with exploration endeavors, our lunar science missions will be the light buoy leading the path for future human activities," said Jim Green, director of the Planetary Division, Science Mission Directorate, Washington.According to legend, Galileo dropped weights off of the Leaning Tower of Pisa, showing that gravity causes objects of different masses to fall with the same acceleration. GRAIL’s high-resolution gravity measurements will complementLRO’s high-resolution observations of the lunar surface. Aftera 30-year hiatus, LRO represents NASA's first step toward returning humans tothe Moon. The orbiter also will look for potentiallunar resources and document aspects of the lunar radiation environment. In 2008, the agency will launch the Lunar Reconnaissance Orbiter,or LRO, to circle the Moon for at least a year and take measurements to identifyfuture robotic and human landing sites. GRAIL will support NASA's exploration goals as the agency plans to returns humansto the Moon. Each GRAIL spacecraft will carry the cameras todocument their views from lunar orbits. Up to five cameras aboardeach spacecraft will allow students and the public to participate in GRAIL’smission of lunar exploration. Zuber's teamof expert scientists and engineers includes former NASA astronaut Sally Ride,who will lead the mission's public outreach efforts. GRAIL's principal investigator is MariaZuber of the Massachusetts Institute of Technology in Cambridge. As with GRACE, both GRAIL spacecraft will be launched on asingle launch vehicle. The GRACE satellites measure gravity changes related to the movementof mass within Earth, such as the melting of ice at the poles and changes inocean circulation. The measurement technique that GRAIL will use was pioneered by the joint U.S.-GermanEarth observing Gravity Recovery and Climate Experiment, or GRACE, mission launchedin 2002. Scientists will use the gravity field information from the two satellites to X-ray the moon from crust to core to reveal the moon's subsurface structures and, indirectly, its thermal history. ![]() The mission also will answer longstanding questions about Earth's moon and provide scientists a better understanding of how Earth and other rocky planets in the solar system formed. GRAIL will fly twin spacecraft in tandem orbits around the moon for several months to measure its gravity field in unprecedented detail. The Gravity Recovery and Interior Laboratory, or GRAIL, mission is a part of NASA's Discovery Program.
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