Passage V

Albert Einstein predicted that light passing by a massive object would be deflected from its original direction of motion (Angle θ in Figure 1).

Einstein also predicted that light's travel time between certain planets would be increased by the Sun's gravity. Experiments 1 and 2 tested Einstein's predictions.

Experiment 1

Einstein predicted that for light of any wavelength skimming the visible edge of the Sun, θ = 1.75 arcsec. To measure θ in visible light (wavelength = 0.00005.cm),scientists observed the stars appearing at the Sun's edge during a total solar eclipse, and they plotted the stars' positions in the sky. Next, they plotted the positions of the same stars when the stars did not appear close to the Sun; θ equaled the angle between the 2 positions of each star. An average of θ was found for all the stars tested. Then, scientists measured and found average values of θ for light at radio wavelengths (3.1 cm and 11.6 cm) skimming the Sun's visible edge.

The results are given in Table 1. (Note: ± signifies the uncertainty in a measurement. For example, at 0.00005 cm, the true value of θ was most likely between 1.57 and 1.87 arcsec.)

Experiment 2

Scientists bounced radio waves off the surface of Venus as Venus moved around the Sun. Figure 2 shows the approximate positions of Venus (V) relative to the Sun(S) and Earth (E) on Days 0, 150, 300, and 450.

The delay measured by the scientists equaled the extra time the radio waves took to make the round trip between Earth and Venus because of the Sun's gravity (see Figure 3).