, 10 tweets, 3 min read Read on Twitter
As we celebrate the 100th anniversary of the first test of #Einstein's theory of General Relativity by deflection of light on the sun, let us not forget that 200 years ago the effect was thought to be unobservably small.

This estimate was possible because a similar effect should also take place in Newtonian gravity, just smaller by a factor of two. This was derived in 1801 by the German astronomer Johann Georg von Soldner.

Soldner was well aware that the effect he calculated was miniscule, but refused to apologize. He wrote:

"Incidentally I do not think it should be necessary for me to apologize that I publish this article even though the result indicates that the deviation is unobservably small."
and continued

"We must pay as much attention to knowing what theoretically exists but has no influence in practice, as we are interested in that what really affects practice. Our insights are equally increased by both."
Sure enough, 120 years later, technology had advanced sufficiently for Eddington to be able to measure the effect. At least in principle. In practice, the error on his measurement was so large it was strictly speaking inconclusive.
By the 1990s, one didn’t have to wait for solar eclipses any more. Data from radio sources, such as distant pulsars, measured by very long baseline interferometry (VLBI) could now be analyzed for the effect of light deflection.
In 1991, Robertson, Carter & Dillinger confirmed to high accuracy the light deflection predicted by General Relativity by analyzing data from VLBI accumulated over 10 years.

A wonderful update on this measurement comes from two Australian astronomers who analyzed the VLBI data from some selected radio sources over a period of 10 years. Paper is here:

In the image below, you can see how the apparent position of the blazar (1606+106) moves around over the course of the year due to light deflection on the sun. Each dot is one measurement point; the “real” position is marked zero on the axes.
How is this for an effect that was thought to be unobservable 200 years ago?

I think this holds a lesson for quantum gravity. Those saying now the effects are unobservably small will be proved wrong 200 years from now.
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