Another total eclipse of the sun is almost upon us, in 2006. Perhaps it
is timely to remind you of a prediction by Einstein, made ninety years
He predicted that the sun would bend light.
It had long been known that matter (or material) has mass (or weight).
What he had found that that pure energy - even in the absence of metter
- also carried mass.
Light is often called "electromagnetic", but this is misleading.
Electrons are matter - and there is no matter in light. Electromagnetic
radiation is really just magnetic radiation which has the power to
influence electricity. So we would expect light to be mass-free.
What happens when energy collides with matter? We know that when matter
collides with matter, the speed of motion is shared by the two bodies
involved. However, if energy - as light - collides with matter, it
cannot slow down. Light travels either at the speed of light, or does
So light will either bounce off the material, or be absorbed by it.
If a particle of material absorbs light, it should do nothing at all.
Only if the light has mass can it have momentum, and only if it has
momentum can it pass that on to the particle. The fact that particles
move faster when warmed up (Einstein's first study - Brownian Motion)
means that the light that warms them must have mass.
That the mass of matter can attract the mass of matter was known. The
force is proportional to the first mass times the second mass divided
by the distance between them squared. This is gravity.
The mass of energy was something new. Would the mass of energy attract
the mass of other energy? Passing beams of light through a tiny pinhole
showed that the same rule applied - mass one times mass two divided by
distance squared. This interaction between light and light is called
In 1916 Einstein was asked whether this new mass that he had found -
the mass of energy - would also interact with the mass of matter.
He said that as the mass of the energy was tiny, you would need a huge
piece of matter to influence it convincingly. Why not use the SUN?
Three years later, there was an eclipse of the sun - just like the one
tomorrow. Scientists followed the paths of bodies, such as stars, that
were disappearing in an almost straight line behind the sun.
During the eclipse, it could be seen that these stars rose up in a
curve, and did not disappear behind the edge of the sun at all. Could
this be Einstein's hybrid of gravity and diffraction? Could this be the
pull of the sun on the light?
There is, however, just one other possibility that springs to mind. If
the gravitational pull of the sun has drawn a "mist" of gas towards the
sun, or if the eruption of sunspots has splashed gas into the
environment, the light might be bent by REfraction, rather than
The density of the gas in the vicinity of the sun, and its refractive
properties needs to be known, before the bending of light rays by
gravitational diffraction can be confirmed. Nevertheless, during the
1919 eclipse, the scientists were adequately persuaded of the accuracy
of Einstein's prediction, and the mass of energy became an established
I have written a GO-article "Graviffraction - Einstein's Dream" for
those who are interested in such matters. It is available on the Web.
So tomorrow there may well be people tracing the paths of celestial
bodies across the edge of the sun, and testing again Einstein's
Charles Douglas Wehner