[Physics] Physics Digest, Vol 4, Issue 1

[carmam at tiscali.co.uk]

Carl, I like the way you think. Gravitational lensing is very difficult to reconcile with the observed effect(s) of the Einstein Cross and others. This is a passage from http://www.alternativephysics.org/book/Abell1689.htm . Bernard is talking specifically about Abell 1689, but the same reasoning applies to Einstein's Cross.>>Next, how strong is this lens? We see a high degree of light-bending. According to General Relativity this requires a lot of gravity.Galaxies are massive objects and galaxy clusters even more so. Our own galaxy for example, the Milky Way, is 200 billion times heavier than our Sun. So clusters of galaxies would be many times heavier and must therefore have huge gravitational fields, right?

Not quite. As large as our galaxy is, its gravitational field is actually quite weak. So weak in fact that it takes 200 million years for our Sun to complete an orbit. The reason is that distances between stars are quite large, and distance rapidly reduces gravity by the inverse square. Based on how our Sun moves through the galaxy we can estimate it experiences a gravitational pull of 2x10-7 m/s2. That's 50 million times weaker than Earth's surface gravity!

So here's the problem. We know from observation that the strength of gravity near our Sun is only sufficient to bend light by a barely perceptible amount. Yet the Abell 1689 cluster, whose gravity must be far weaker, is able to bend light by an enormous degree.

If Abell 1689 did have the amount of gravity necessary to bend light like this, its forces would rapidly pull the cluster together. But Abell couldn't have such gravity because, due to increased distances, the gravity outside a galaxy is far weaker again than the already weak gravity within it. That would explain why the cluster barely moves.<< The one thing that I was not sure about was the actual amount of bending. Using a right angle triangle calculator, the degree of bending caused by Abell 1689 is a half angle of 0.026 degrees. Below is also from Wikipedia :- >>Albert Einstein predicted in 1936 that rays of light from the same direction that skirt the edges of the Sun would converge to a focal point approximately 542 AUs from the Sun.<<The Sun's surface gravity is 274m/s^2, which gives an angle half angle of 0.106 degrees, and this angle is 4.1 times greater than Abell 1689 .You are correct Carl, the gravity around Einstein's Cross will be nowhere near as strong as around Abell 1689, and so is just not sufficient to bend light by the amount needed to form the four images.For the record, I do not think that the arcs around Abell 1689 are caused by gravitational lensing either.
Tom Hollings.


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