Word: radar
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Dates: during 1960-1969
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Street is one of the developers of radio navigation and group and ship radar. He has done important research on cosmic rays and on devices for detecting atomic particles. He is co-author of the a college text, General Physics. Since 1966, he has been assistant dean of the Faculty...
Last year, during two intervals when Mercury and Earth were on opposite sides of the sun, a team led by Physicist Irwin Shapiro bounced high-frequency signals from M.I.T.'s exceptionally precise Haystack radar antenna off the planet Mercury. On their way to and from Mercury, the signals, which travel at the speed of light, had to pass close to the sun. During these passages, according to the Einstein equations, solar gravity should have actually slowed them down, lengthening their 23-minute round-trip time to Mercury by one five-thousandth of a second...
Eight Gigahertz. The M.I.T. team also had to design a new radar transmitter that would operate at eight gigahertz (pronounced with hard gs), which is 8 billion cycles per second. Radar beams of lower frequency would be significantly slowed down by electrons in the solar corona, making it difficult to separate out the delay actually caused by the sun's gravity. Corrections for Mercury's surface irregularity had to be calculated; round-trip time to a Mercurial valley would be longer than to a mountaintop. It was also essential for the researchers to screen out any extraneous radio...
Painstaking preparations paid off. As Mercury began to move behind the sun, M.I.T. computers detected increasing delays in the return of radar signals slowed by the sun's gravitational field. Plotted against the theoretical delays predicted by the Einstein equations, the actual delay time formed a remarkably similar curve, increasing to approximately one five-thousandth of a second just before Mercury passed behind...
Test results, which Shapiro regards as only preliminary, could be inaccurate by as much as 20%, and still leave some room for doubt about relativity. But refinements in the radar technique could soon reduce the uncertainty to less than 1%, he says, and further confirm or definitely overthrow Einstein's general relativity...