Another Check for Einstein

According to Einstein's theory of general relativity, a gravitational field has three effects on light: 1) it deflects it to one side, 2) it changes its frequency, and 3) it slows it down. All these effects are slight, and although the first two have been detected already, the third was until recently beyond the range of observation. But now Dr. Irwin I. Shapiro of M.l.T.'s Lincoln Laboratory proposes to check on Einstein by using the solar system itself as his laboratory.

With the new Haystack antenna that can project a narrow beam of 8,000-megacycle, 1.5-inch microwaves that behave just like light. Dr. Shapiro plans to follow the planet Venus around its orbit, accurately measuring the time that the microwaves take to reach their target and bounce back. While Venus is well away from the sun, that time can be translated into the planet's calculated distance on its well-known orbit. But as Venus begins to swing behind the sun, the microwaves will pass through the strongest part of the sun's gravitational field. If Einstein was right, they will be slowed down ever so slightly. Dr. Shapiro figures that during their 25-minute round trip of about 300 million miles, the gravity-hampered waves will lag by two ten-thousandths of a second, making Venus appear about 40 miles farther away than it really is.

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