Physics: Probing Einstein with Radar

In the 53 years since Albert Einstein published his general theory of relativity, it has withstood determined attacks and ingenious experiments by other scientists anxious to test its validity. Although no experimental results have contradicted the theory, they have not been precise enough to rule out opposing theories that differ in small but significant details. Now a new technique has been used to check out Einstein: interplanetary radar. Preliminary radar tests also have failed to find a flaw in general relativity, a scientist from Massachusetts Institute of Technology’s Lincoln Laboratory announced last week, and radar soon should provide results accurate enough to help confirm the theory—or to seriously undermine it.

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.

Detecting so minute a change was no easy task. Using data gathered by the Haystack antenna and by other observatories, the researchers plotted both Earth’s and Mercury’s orbits to a degree of accuracy never before obtained; it was essential to know Mercury’s exact distance at the time of the test to calculate the difference in round-trip time caused by solar gravity.

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 noise that might interfere with the attenuated, incredibly weak return signals, which, Shapiro says, had “less than a thousandth of the power that is expended by a housefly walking up a wall at a speed of one millimeter a century.”

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 the sun.

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.