Expert: Jayendra Upadhye - 12/5/2008

Question
Hi This problem has been bugging me since I saw the BBC drama about Eddington and Einstein.  This is to do with Alfred Eddington proving Einstein’s theory of relativity by using the eclipse of 1919 to do it; specifically his theory that a large mass (the sun) would bend light [there was a BBC drama about this on TV recently]. Eddington set about proving this by taking pictures during the eclipse of the Hyades star cluster (not sure if this name is right?) which appeared right next to the sun during this eclipse. Einstein’s theory stated that if this picture, taken during the eclipse, was compared with a picture taken in exactly the same place at night ( i.e. Without the suns gravitational mass interfering with the stars light) then there would be a slight discrepancy as to this actual clusters position when the photographic plates were overlaid with one another – the stars actual position on these two plates would be different proving that the sun does indeed bend light. The problem is this; I can understand the theory behind this but how did Eddington take a picture, the base plate picture, against which the picture taken during the eclipse was compared? How did he know that the base plate picture he had taken was in exactly the same place, allowing for all the possible discrepancies (angle of telescope, rotation of the earth, actual location the picture was taken on the earth)? thanks

Answer
Hi,
I understand.
However, the stars being what they are, in terms of immovability over relatively short span of time such as a year, it is not at all difficult to imagine how it could have been done.
I am NOT basing any of my premises on what actually WAS done, but one can often be accurate if one "sticks to common sense".

1 - He could have just taken a potshot of the area in question (hence forward to be called AIQ!!), when that area was part of the night sky (sun having moved into "diametrically opposite" side), and stored it away for later comparison at 'time of eclipse'.

2 - He could take a snap shot well before occultation by the sun, of our AIQ and then another with AIQ just around the solar limb.

Due to the space warp caused by the sun,
a - occultation would be delayed,
b - The AIQ would appear to be stretched out as the space warp would force individually oculted stars to "remain visible" over the solar limb! This distortion would appear best in a time lapse series of pics had Arthur thought of doing so.

Our brains (like in music), are meant to make sense out of a succession of patterns that cross our visual field (hence movies work and computer screens can simulate movies too). The viewer of such a movie would "percieve" a distortion / ripple, in the image very clearly.

A single sine wave changing pitch in quick succession may appear as music to our brains (example:- whistling of a tune), but play the notes at one minute intervals and we may not make head or tail of it.

Scientists being what they are (cool - and cold too - guys), always do things methodically and dispassionately. Their personal prestige, respectability and success depends on doing things as prosiacally and meticulously as possible. The self / ego is almost always out of the equation in such publicised events as the 1919 sighting.

Too much is at stake.

So Eddington may have used something much more ingenious than what poor me could come up with. And even my methods would give good results! What to say about him?

As an aside, note that the size of betelguese (the red giant) predicated by Eddington based on his mass luminosity relations and understanding of stellar nucleo-physics, was corroborated by michelson-morely independently when the turned their 20 foot interferometer towards it. (Poor guys were searching for the elusive ether!)

here is a link on eddington.
http://en.wikipedia.org/wiki/Arthur_Eddington

regards
Jayen