The majority of physics experiments take place in a small lab found in the basement of a university science building. However, for a grand hypothesis, one needs a grand experiment. The year was 1919 and a nervous Albert Einstein awaited the results of an experiment on the scale never before seen nor bested up to the present day. Four years earlier, Einstein published a paper outlining his Theory of General Relativity, the bold claims of this paper brought on skepticism at first, but soon he would win over the hearts and minds of every scientist and civilian alive. Einstein postulated that the immense mass of large bodies, such as the Sun, would bend the fabric of space-time and thus, light waves would be bent as they travelled near these gravity wells. Such a proposition goes against the days accepted theory put forth by Newton 200 years earlier: that all light travels in straight lines. So how could Einstein prove that light can be bent by gravity?
The answer would come courtesy of one of nature’s most awe inspiring events, a total solar eclipse. A solar eclipse occurs when the Moon passes directly in front of the Sun, and thus blocks the sunlight from reaching Earth. These eclipses can only occur when the Moon is new in its phase. Not because the Moon is biggest at that time, it is always the same size no matter the phase! But rather because that is the only phase in which the Moon can possibly be in line with the Earth and Sun. We have a new Moon 13 times per year but we don’t experience 13 solar eclipses a year due to the Moon’s 5 degree orbital tilt. The total blockage of the sun during a solar eclipse only lasts about 7.5 minutes, but for that time the sky darkens and the stars come out. Einstein’s theory postulated that the light from distant stars which were hidden behind the Sun would be bent around the Sun and therefore be observable. Normally, this theory would be impossible to test since the Sun’s light drowns out any observable stars. But the uniqueness of the eclipse allowed just enough time for Sir Arthur Eddington, who took on the challenge of capturing the picture on Einstein’s behalf, to take a photograph of the sky, including stars. The idea was, if we know the exact celestial coordinates that the Sun blocks, we will know the exact stars that lay hidden behind the Sun. When the lights go out during the eclipse, if those stars are visible at the fringe of the eclipse, then we know their light has been bent, thus proving the theory.
Once the picture (see the original negative here!) was developed and scrutinized, it was determined that Einstein was correct. He became an overnight celebrity not only in the science community, but among everyday people too. His theory still stands as accepted in the physics community to this day and has been verified by the eclipse test many more times, and by other tests designed in years to come, but none used tools as grand as a total solar eclipse!
Want to learn more about the factors that go into how and when a solar eclipse can occur? Check out this awesome video!
