Why does light bend away from mass?

Reading Big Bang by Simon Singh and it describes proof of Einstein’s General Theory of Relativity. The proof by Eddington was observation of a star behind the sun during an eclipse. Illustrations in the book show the often used plane distorted by a sphere. But that illustration only confused me. Shouldn’t the star to be observed be near the sun but appear closer to it because of the distortion of spacetime? Instead the observation was of a star that was behind the sun but could be seen because of the distortion.

The Sun is going to bend the light of the star towards itself. This will have the same effect as moving the star away from the Sun. Draw a line from the observer to the apparent position of the star, and then draw another line from the point of closest approach to the Sun to the actual position of the star.

So the fact that you can see the star proves that the light was bent?

I was a little too quick in my last response. The book has a diagram of the star’s light being bent around the Sun.

More or less, except it is likely the star was not actually behind the sun. But it will appear further from the sun than it actually is. Draw a diagram. If you like put a star just behind a limb of the sun as seen from where you are. Draw a straight line from the sun to the limb and beyond. It will miss you. Now curve that line towards the sun a bit. Now it will hit you. So what it appears to you is that the star has moved further from the sun than it really is. The appearance is the mirror image of the reality.

The book has two illustrations.
One illustration has the star behind the sun. The light is bent around the sun not towards it.
The other illustration is a graph of the 1922 Australian eclipse data. All stars have an observed position away from the sun from their actual position.

Sorry, I can’t find an illustration on the 'net.

“Bending around” = “towards”. Planets are bent towards the sun, and their slow speed means they end up going round and round and round. Slower objects, like the occasional comet coming in from the outer solar system moves slower and gravity bends the path so much it intersects with the sun.

Light, which moves very fast, and does not actually have mass, is bent very little by gravity, but rays that would otherwise have missed us completely can be bent sufficiently to hit us.

Here’s Wikipedia’s illustration, but it’s probably similar to the one in your book.


To be more precise about the whole things, the light isn’t bent at all. It travels in a perfectly straight line. It is the space, itself, that is bent - or “warped”, more people say.

And this is really the most important point here. What is so exciting about Einsteinian relativity is that the framework of space and time warps, such that objects that stay a constant distance away from a mass are accelerating, which is why they require a force pushing them away from the mass to stay where they are. So gravity isn’t a force unto itself at all, it’s just the effect of accelerating your mass away from another big mass to keep their distance constant in warped spacetime.

As a follow up to Napier’s post there is this little mental experiment that also demonstrated that gravitation isn’t really a force in the Newtonian sense. If it were, light wouldn’t bend under the influence of gravity because it is massless in the Newtonian world.

It has been established by theory and experiment that, given a gravitational field that is essentially uniform over the size of a laboratory it is impossible to distingush between being in the field and being in an accelerating laboratrory with no field.

This drawing shows an accelerating laboratory with a ray of light coming in a hole at the right and passing out a hole at the left. There is no gravitation at all in the setup. A scientist inside the laboratory feels the floor pressing on his feet with a force which he ascribes to the gravitation of a planet in accordance with Newton’s law.

If he measures the distance that the light ray departs from a straight line across his laboratory it is exactly the same departure that would occur if his assumption about being in a gravitational field were correct. And so he interprets his result as showing that gravity bends light.

Notice that there is no force on the light at all. The bending that the scientist measures is an artifact of the acceleration and the fact that over the size of laboratory space is flat so there is a straight line, in the Euclidian sense, to which he can compare the path of the light beam.

Oh yes. I neglected to say that the idea for this little thought experiment wasn’t in any way mine. It came to me from a book. I’ve forgotten what book, it might have been one by Asimov.

I’ve marked it up to just bad illustrations which show light being bent away from the sun.

Thanks all for the input.

Remember, the star is giving off light in all directions. It’s just that only those rays of light which happen to reach us (most don’t) are of interest to us. So if you took away the Sun from that diagram, the ray of light that got bent would still be there, and it’d still leave the original star in the same direction, but it’d miss us by a fair margin. Instead, we’d see a completely different ray of light that came “straight” to us. Put the Sun back in, and that ray that was “straight to us” without the Sun is still there, too, but now it doesn’t hit us because the Sun is in the way.

Based on this quote from Chronos post above I get the uneasy feeling that you think that a curved path around the sun means that gravity isn’t attracting the light toward the sun.

That’s reinforced by this statement. If the illustrations show the light wrapping around the sun they are not bad illustrations.

Study this picture and explanation. If you have further questions ask them.