Confirm General Relativity: Eddington's Starlight Experiment

[Bob R]

TL;DR Summary

If Earth's motion about the sun is described by General Relativity why was Eddington's experiment with the bending of starlight needed to confirm the theory?

If Earth's motion about the sun is described by General Relativity why was Eddington's experiment with the bending of starlight needed to confirm the theory? In other words, don't we see enough common phenomena in our every day experiences to confirm GR without verifying subtle phenomena such as the bending of starlight and the detailed orbit of Mercury?

 

[Ibix]

If Earth's motion about the sun is described by General Relativity why was Eddington's experiment with the bending of starlight needed to confirm the theory?

The orbit of the Earth is also well-described by Newton's theory of gravity. To see which one of GR and Newton is right you need to do an experiment where the two theories predict different results, different enough to be detectable with the kit you have. Hence Eddington going to South America.

I gather that modern measurements are precise enough to detect the difference between Earth's orbit as predicted by GR and Newton, and GR wins there too. But a hundred years ago we did not have equipment that good.

 

[Nugatory]

don't we see enough common phenomena in our every day experiences to confirm GR without verifying subtle phenomena such as the bending of starlight and the detailed orbit of Mercury?

The FIRST test of general relativity, so basic that we don’t usually think of it as a test, was to ask “Does GR agree, to within the limits of experimental accuracy, with Newton’s theory everywhere that Newton’s theory works (which is, pre-Eddington, everywhere except the orbit of Mercury)?” If it had failed that test, GR would have been rejected out of hand, falsified by experiments and observations that had already been made.

Thus all the “common phenomena” you mention will support Newtonian gravity and GR equally well; they tell us that both are viable theories but offer no help in choosing between them. To do that, we need to look at where they disagree enough that observation can tell us which one is more right.

It’s worth taking a moment to review just how small the differences between the predictions of Newtonian gravity and GR are when working with common phenomena. For example, the anomalous precession of Mercury was not recognized until 1859 (150 years after Newton) because it is so small, less than one degree over more than a century... And that’s the big readily visible one, the one that we didn’t have to go looking for after GR told us that it might be there.

Also worth reading: https://chem.tufts.edu/answersinscience/relativityofwrong.htm

 

[Teichii492]

If Earth's motion about the sun is described by General Relativity...

Although General Relativity has something to say about the motion of the Earth around the Sun, it was not the purpose of the Eddington experiment. The experiment was to show that the gravity of the sun would deflect the light from distant stars, the orbit of the Earth is only factored into this for the geometry of the calculations and any changes to the Earths orbit due to General Relativity are too small to matter for the calculations of this effect.

why was Eddington's experiment with the bending of starlight needed to confirm the theory?

As above, but also the Eddington experiment was to confirm one prediction of General Relativity, one of many predictions, the confirmation of which have lead to a consesus on the validity of the theory as a whole. Previous to the Eddington experiment there was an observation of a conflict between Newtonian predictions of the orbit of Mercury and the observed orbit. This difference was explained by General Relativity and contributed to building the consensus.

In other words, don't we see enough common phenomena in our every day experiences to confirm GR without verifying subtle phenomena such as the bending of starlight and the detailed orbit of Mercury?

The obsevable effects of General Relativity occur at non-intuitive scales and although they do affect 'common phenomena' the effective goes unnoticed by humans. However, as detection sensitivity has increased we can detect the effects of General Relativity at scales that are more intuitive. For example, the Pound-Rebka experiment showed that it is possible to detect how the gravitational field of the Earth affects the motion of gamma rays with a difference in height (of the source) of only 22.5m.

 

[FactChecker]

In today's world, probably the most generally appreciable confirmation of GR is the accuracy of GPS. It would be very inaccurate if it did not account for the effects of GR.