The hallmark of a scientific theory is that it makes predictions that can be verified or refuted. A theory which does not make such predictions is not of much use to science. The hallmark of a good scientific theory is that it makes predictions that turn out to be true. By this criterion Einstein’s theories of relativity (specific and general) are good – very good.
The general theory of relativity has recently had yet another prediction validated. The general theory of relativity deals primarily with gravity, showing the equivalence of gravity and acceleration and treating gravity as a curvature of space-time. According to general relativity, for example, when a comet swings around the sun it is actually traveling in a straight line but through space curved by the mass of the sun.
The general theory, published by Einstein in 1915, was not just a set of ideas but a complex set of mathematical equations. The validation of general relativity rested not in simple observations but rather in careful measurements showing that Einstein equations were accurate – more accurate than competing theories.
The first test of general relativity came in 1919 when a solar eclipse offered the opportunity to observe the degree to which the mass of the sun would bend light from stars behind the sun. According to Newton’s classical theory of mechanics light would be bent by gravity – the photons would be attracted by the gravitational force of a large mass and would bend in their path. The general theory of relativity also predicts that light would bend, but because it is traveling through curved space. So both theories predict the bending of starlight by the sun visible during a solar eclipse – the difference was in the amount of bending. Newtons equations give about 0.8 second of arc as the predicted degree of bending, while Einstein’s equations gave 1.75 seconds of arc.
In 1919 British astronomer Arthur Eddington made an expedition to photograph the solar eclipse for the purpose of measure the deflection of starlight to test Einstein’s general theory. His results confirmed Einstein – getting about 1.61 seconds of arc as the measured degree of bending. Ironically, later review of Eddington’s results indicate that his measurements were probably not precise enough to make this determination and he was likely being favorably biased toward Einstein. But of course later measurements have validated Einsteins equations for the bending of light.
The general theory of relativity has recently been tested again – this time also involving an eclipse and the test of the theory also resting not in the presence or absence of a phenomenon but in the precise measurements of the amount of an effect. Astronomers have discovered a system including two massive pulsars in orbit around each other. In addition, the plane of their orbit almost exactly lines up with the view from earth. This means that as one pulsar passes behind the other it will be eclipsed. This is a rare configuration and provides the first opportunity to test the accuracy of Einstein’s equations regarding a specific consequence of this massive system.
According to general relativity the effect of the large mass of one pulsar should cause the spin-axis of the other to precess (like the circular rotation of the axis of a spinning top). Astronomers already know that such precession occurs, but they have not been able to measure it precisely enough to test th predictive power of Einstein’s equations.
Well, now they have and Einstein’s equations precisely match the observed precession of these pulsars. You win again, Dr. Einstein.
In response, study author Rene Breton is quoted as saying:
“It’s not quite right to say that we have now ‘proven’ General Relativity. However, so far, Einstein’s theory has passed all the tests that have been conducted, including ours.”
Einstein’s theory of general relativity has proven to have powerful and precise predictive power. This is good and bad, depending on your perspective. At this point I think it is safe to say that Albert was likely onto something. His special and general theories of relativity bear some meaningful relationship to how the universe actual works and his equations are mathematically valid. It’s possible that these theories will never be overturned.
However, it is often the case in science that a theory is not overturned but yet a deeper theory is discovered, making the original theory still true but now limited in its scope. This is what Einstein did to Newton. Newtons laws of mechanics are still true, as far as they go, but they are now understood to be a limited case of the deeper understanding provided by Einstein.
The question is, will a cosmologist one day do to Einstein what Einstein did to Newton – discover a deeper description of the universe that relegates relativity theory to a limited scope? Physicists had some clue that there were problems with Newton’s theory because there were anomalies that could not be explained within Newton’s classical world. For example, the precession of the perihelion of Mercury could not be make to fit with Newton’s equations. General relativity, however, matched the measurements of Mercury’s orbit precisely.
The point is that unexplained anomalies – observations that did not fit predictions – led to better theories with more accurate predictions. So while it is nice to confirm Einstein’s theories, cosmologists are still hunting for a true anomaly that does not accord with Einstein. Such anomalies would be the needed clue to a still deeper theory of the universe. That would be exciting and interesting, but it remains to be seen if such anomalies are there to be found, or if the good Dr. Einstein is the final word.