Seeing Relativity: No Bungees Attached!
Elevendy-one years ago (that's a hundred and eleven to the non-Shireborne), Chabot Observatory Director Charles Burckhalter set forth on an expedition across the plains of India, risking bandits, tigers, famine, and plague, on a hunt for big game: a rare meeting of the Sun and the migrating Moon in a total solar eclipse. Little could he know, years later his then-world-class astrophotographs of the event would be used in an effort to prove or disprove the General Theory of Relativity published by Albert Einstein in 1916.
Einstein's General Theory of Relativity is the one that describes gravity, that attractive force between objects, not as some kind of invisible bungee cord but as a distortion, or warping, of space (and time) by matter. Other objects within the warped, "curved" space accelerate "downhill" along the curve, toward the mass producing the distortion. Likewise, objects moving through the gravity field are deflected, their trajectories curved toward the mass.
The trick was how to prove this theory with observational evidence. That gravity is a fact is easily demonstrated by holding up an object and then letting go of it. Gravity, whatever gravity is, accelerates it "downward"—in our case, toward the center of Earth's mass.
But how to demonstrate that gravity, as Einstein theorized, is the effect of space-time warped by matter causing objects to slide down the "uneven ground," (yeah, using a lot of quotation marks, I know) was a bit like trying to prove that gnomes are responsible for missing keys (as we all know they are, but just can't prove!)
So, an experiment was proposed to try to sort out the culprit of gravity as curved space-time, or "merely" an invisible bungee cord. If space-time is in fact warped around a massive object, then not only other material objects, but non-material forms of energy should follow along the contours of the warp and also be deflected. Light was the handiest observable, non-material form of energy available.
As the theory went, the greater the mass of an object, the greater the deflection, and the Sun was by far the most massive nearby object available. If the stars behind the Sun's vicinity could be observed to shift their apparent position—as a consequence of their light rays being deflected as they flew past the Sun on their way to Earth—then General Relativity could be observationally confirmed. And there were astronomers in both camps—pro-Relativity and pro-Invisible-Bungee-Cords—trying to observe the effect, or the lack thereof.
A total solar eclipse in 1919 made itself available only three years after Einstein published, and so became a very important eclipse to science. (And, coincidentally, occurred on almost the same day of the year, May 29, as the May 28, 1900 eclipse captured by Burckhalter–which meant that the stars whose positions were being measured, the Hyades cluster in Taurus, were the same in both cases.) In the early 20th century, before space-based satellite observatories existed, stars could not be observed close to the Sun's disk, the Sun being so bright as to drown them out completely.
But during a total solar eclipse, the Moon temporarily blocks the Sun's bright disk, briefly giving astronomers a glimpse of the starry background surrounding the Sun. Photographs of the stars' positions taken during the eclipse could be compared to those of the same stars taken at a different time of the year, when the Sun wasn't present in that location, and so the space-time distortions of gravity (if there were any) might be observed in the deflection of the stars' apparent positions.
Charles Burckhalter's expedition photos from the 1900 eclipse were accessed as part of the observational experiment—including by those on the bungee-advocacy side of the aisle. His innovations in a technique for photographing solar eclipses made his image plates quite valuable among the 1900 sets.
In the end, General Relativity was, in fact, demonstrated observationally, by Arthur Eddington and others, which opened up a whole new universe of astounding possibilities, including the origin of the universe in the Big Bang, and the existence of the mind-bending, light-devouring objects called Black Holes.
But, back in the day, Burckhalter was probably more concerned by the sounds of tigers creeping through the bush and reports of the surrounding plague epidemic as he snapped his shots of the darkened Sun above….