The Shapiro delay

2.9.2 The Shapiro delay

More recent verifi cations of Einstein’s theories relating to the curvature of space– time due to the Sun’s mass have been made with planetary radar observations of the planet Mars. In the 1960s, Irwin A. Shapiro realised that there was another, and potentially far more accurate, way of testing Einstein’s theory. Shapiro was a pioneer of radar astronomy and realised that the time that a radar pulse would take to travel to and from a planet would be affected if the pulse passed close to the Sun. Figure 2.18a fi rst shows the direct path that a radar pulse would take to and from Mars if we could imagine that the Sun was not present and that, as a consequence, the radar pulse travelled in a direct straight line. Figure 2.18b then shows that, due to the curvature of space by the Sun, a radar pulse sent along this precise path would not reach Mars, but curve away to the left. The pulse that would reach Mars,

shown in Figure 2.18c, has to take a path slightly to the right of its true position so the curvature of space near the Sun would defl ect it towards Mars. The echo would follow exactly the same path in reverse. As the pulse has had to follow a longer route to Mars and back it will obviously take longer than if the Sun was not present. The radar pulse will thus be delayed. The Shapiro delay, as it is called, can reach up to 200 µs and has provided an excellent test of Einstein’s theory.

Further tests, of even higher accuracy, using the Shapiro delay have been made by monitoring the signals from spacecraft as the path of the signals passed close

72 Introduction to Astronomy and Cosmology

Figure 2.18 The Shapiro delay.

to the Sun. In 1979, the Shapiro delay was measured to an accuracy of one part in a thousand using observations of signals transmitted to and from the Viking spacecraft on Mars. More recently, observations made by Italian scientists using data from NASA’s Cassini spacecraft, whilst en route to Saturn in 2002, confi rmed Einstein’s General Theory of Relativity with a precision 50 times greater than previous measurements. At that time the spacecraft and Earth were on opposite sides of the Sun separated by a distance of more than 1 billion km (approximately 621 million miles). They precisely measured the change in the round-trip travel time of the radio signal as it travelled close to the Sun. A signal was transmit- ted from the Deep Space Network station in Goldstone, California which travelled to the spacecraft on the far side of the Sun and there triggered a transmission which returned back to Goldstone. The use of two frequencies to make simultane- ous measurements enabled the effects of the solar atmosphere on the signal to be eliminated so giving a very precise round-trip travel time. The Cassini experiment confi rmed Einstein’s theory to an accuracy of 20 parts per million.