Relativity and Bending Light

Albert Einstein’s Most Famous Prediction of General Relativity

Feb 6, 2008

While many physicists would have believed it to be impossible, Albert Einstein's prediction that light could be bent by gravity acted to change the course of physics.

In 1919 a team of researchers led by Sir Arthur Eddington conducted a famous experiment to measure the precise location of stars surrounding the sun during an eclipse (which, coincidentally, is the only time you can actually see the stars immediately adjacent to the sun). To the shock of scientists everywhere, Eddington’s results confirmed a phenomenon that Albert Einstein had predicted four years earlier – that the light from stars would “bend” as it passed by the sun, thus shifting the position of the stars ever so slightly.

Some have said that it was this confirmation (though some would argue that it was not entirely conclusive) which finally cemented Einstein’s place as a truly worldwide celebrity. Prior to this, his theory of General Relativity had been more or less untested – just an interesting, intelligent, idea. After Eddington, Einstein (and his science) could finally be taken seriously, and even the non-scientists of the world somehow found themselves standing in awe of the genius of this Swiss-German physicist.

Can Gravity Bend Light?

If light, as had been the opinion of physicists for some time, was inherently without mass, then how could it possibly be affected by gravity? After all, the force of gravity – so said Isaac Newton – is directly dependent upon the mass of two objects. Thus, light could not be bent by gravity.

Einstein’s General Theory of Relativity had a different approach to the subject.

Einstein used the analogy of an elevator to explain his thoughts on the equivalence of inertial mass and gravitational mass, saying that the sensation of being held to the Earth by gravity would be the same as being pulled through space while in an elevator with a constant acceleration of 9.8 m/s². In either case one would be held to the floor of the elevator with a seemingly identical force, and would not be able to tell the difference (thus, the two situations would be considered relative).

Providing that Einstein’s equivalence principle (the assertion that inertial mass and gravitational mass are equal and interchangeable) is correct, the elevator analogy can also be used to explain why gravity bends light.

How Einstein’s Principle Works

Consider an elevator motionless in space (so that there is no gravity inside and any occupants are in “freefall”). This elevator has a pin-sized hole in the wall, through which a tiny beam of light enters, creating a speck of light on the opposite wall, directly across from the hole (if one measured the distance from the floor of the elevator to the hole and to the speck of light, it would be equal).

Now, if this elevator began to be pulled forward through space, the inertial mass would pull the occupants to the floor of the elevator (mimicking the pull of gravity), and something peculiar would happen to the beam of light:

As the elevator’s acceleration increases, the prick of light will appear to move downward, for in the time it takes for the light to travel from the hole to the opposite wall, the elevator would already have moved forward slightly (though it would have to be moving rather quickly for this effect to be at all noticeable). In other words, because of the motion of the elevator, the beam of light would “bend” as it enters the elevator. Now, carrying this thought through to its conclusion – remember that the occupants of this elevator would have no way of knowing if the sensation they are feeling is caused by the elevator’s inertia or by some gravitational force (it could feel to them that they are on the surface of the Earth), so to these people, the bending of the beam of light appears to be caused by gravity.

According to Einstein and his General Theory of Relativity, these two situations are identical, and thus, gravity can indeed bend light (or at least create the appearance thereof).

The Verification and Vindication

In his theory he even calculated (using the dreaded Tensor Calculus of relativity) exactly how much the light from a distant star would appear to bend if it were to pass closely by an object as massive as, say, the sun, thus enabling Sir Arthur Eddington to mount his famous expedition and confirm Einstein’s theory to a very minute degree (more recent tests have been able to increase this accuracy quite dramatically).

This meant not only that Einstein was right in this particular prediction, but also that the rest of his General Theory of Relativity, including his “bizarre” views about what actually causes gravity, would have to be taken seriously.

Gribbin, J., & Gribbin, M. (2005). Annus Miribilis: 1905, Albert Einstein, and the Theory of Relativity. New York: Chamberlain Brothers.

Gardner, M. (1962). Relativity Simply Explained. Mineola, NY: Dover Publications, Inc.

Einstein, A. (1961). Relativity: The Special and the General Theory - A clear Explanation that Anyone can Understand. New York, NY: Random House.

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