Einstein's Monsters by Chris Impey

Author:Chris Impey
Language: eng
Format: epub
Tags: Epub3
Publisher: W. W. Norton & Company

FIGURE 47. The first experimental test of general relativity in 1959 was the most precise physics experiment ever attempted at the time. Harvard physicists Robert Pound and Glen Rebka measured the energy of gamma rays from an iron-57 radioactive decay traveling upward and downward over a distance of 22.6 meters. The photon traveling downward was blueshifted and the photon traveling upward was redshifted by exactly the amount predicted by general relativity. The experimental precision required for this test was a few parts in 1015. R. Nave/Hyperphysics

So far we have described the “classical tests” of general relativity. They use situations where gravity is so weak that space-time curvature and distortion is slight and extremely accurate measurements are needed. Nearly fifty years ago, Irwin Shapiro, the longtime director of the Harvard–Smithsonian Center for Astrophysics, proposed an ingenious weak gravity test of the theory. He realized there would be a slight delay in the round-trip travel time of radar signals bouncing off other planets if the path of the photon took it near the Sun. Using measurements of radar bounced off Mercury and Venus, before and after they were eclipsed by the Sun, he confirmed general relativity at a 5% level.27 This test was repeated in the outer Solar System by NASA’s Cassini spacecraft to give agreement at the 0.002% level.28

These tests affirm general relativity and its superiority over Newton’s theory. But there’s something vaguely unsatisfying about testing relativity in places where space is as flat as an Iowa cornfield. It’s like test-driving a Lamborghini in a parking lot. Sure, it performs better than your old Ford Taurus, but that’s setting the bar low. It’s much better to drive both cars fast in the mountains, where the Lamborghini powers up the hills and hugs the curves while the Taurus overheats and careens off the road. Astronomers look forward to eventually testing the theory with black holes, where the effects on radiation should be spectacular. As we’ll see in the next section, large gravitational redshifts have been detected from black holes using spectroscopy of the accretion disk.

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