söndag 25 september 2011

Bending of Light by Gravity?

Eddinton's plate superimposed on a comparison plate showing outward displacements of stars near the sun during an eclipse.

Einstein's general theory of relativity is believed to be confirmed by an observed apparent shift of position of a star appearing in the sky close to the Sun, claimed to coincide with a bending of light ray by the gravitation from the Sun predicted by relativity theory of size:
  • 1.75" x R/D
where R is the radius of the Sun and D the minimal distance of the light ray from the center of the Sun. For a light ray passing close to the Sun the bending would thus be 1.75".

Since it is difficult to observe a star close to the Sun in the sky, because the Sun light is very much stronger than the light from the star, observations have been made during a full eclipse when the Sun light is shaded by the Moon. The first such observation was made in 1919 by Eddington, who claimed to observe precisely the predicted 1.75" and thereby rocketed Einstein to world fame. Later eclipse observations are claimed to confirm Eddington's results.

Let us now subject the eclipse observations as confirmation of general relativity, to scientific scrutiny. The following questions present themselves:
  1. Why use only eclipse observations with R = D, when the bending effect should present also for say D = 10 R in which case the star may be visible without eclipse shading?
  2. Are there confirming such observations without eclipse?
  3. Is it possible that light rays passing close to the Sun are subject to aberration from other effects than gravitation, like strong magnetic fields?
  4. Is it possible that during an eclipse the phenomenon of stellar aberration varying between 0 and 20" (as discussed in a previous post), disappears because the motion of the Earth with respect to the Sun in the case of no Sun light has no effect?
To use eclipse observations as confirmation of general relativity, it seems as if the answers to the above questions would have to be: 2 Yes, 3 No, 4 No. Is this so?

5 kommentarer:

  1. All 4 of your questions were asked during the 1920s when Einstein's prediction was being assessed. In #1, the displacements of many stars with impact parameters (b)several times R were measured; a plot of displacement vs impact parameter allowed extrapolation to b=R; #3 was argued about for years.

    I recommend reading the interesting book "Einstein's Jury" by Jeffrey Crellinstein.

  2. I am skeptical to anything assessed hundred years ago, and also to many of the all too perfect matches between relativity and observations, like time dilation by GPS and the possibly related bending of light by gravitation. But of course you may be right that everything Einstein said is correct.

  3. I don't know if everything Einstein said was correct, but it has passed all experimental tests so far. The bending of starlight is a simple test, but there are other tests that are cleaner and allow for better accuracy, such as the Shapiro time delay and energy dissipation from a binary pulsar due to gravitational wave emission. There are even better tests of the bending prediction, such as using VLBI to detect light deflection by Jupiter, that give an agreement with GR to 0.02%. Clifford Will has written much on this, including a 2006 paper available on the arXiv and a really good book, "Was Einstein Right?"

  4. The image at the top of the post is not one of Eddington's plates: it's a frame from the BBC television drama "Einstein and Eddington" (2008). This (fake) plate exaggerates (for dramatic purposes) both the number of stars visible (Eddington's plates typically had only a handful of visible stars) and the size of the displacement (by a factor of more than fifty).

  5. If this is a fake plate, there should be more recent plates made by modern techniques!!