Today is the 100th anniversary of the publication of Albert Einstein’s General Theory of Relativity.
The field equations contain all the information needed to define general relativity, describe its key properties, and address a question of crucial importance in physics, namely how the theory can be used for model-building. Not that WC or all of his readers are going to grasp all of those derivations in one go.
But the test of any theory is to see what it predicts and then determine if those predictions are borne out by experimental observations. Anything else is just the science equivalent of Donald Trump’s racist cant or Ben Carson’s dubious personal history. So on the occasion of this 100th anniversary, let’s have a look at some of the predictions and see if they were proven true by experimental observations over the last century. Which are slightly more accessible to WC and his readers than the equations themselves.
General relativity predicts that gravity would influence the passage of time: the greater the gravitational field, the slower time will appear to pass to an outside observer. This has been repeatedly confirmed in the laboratory, in field observations, and as a side effect of the Global Positioning System. Tests in much stronger gravitational fields are provided by the observation of binary pulsars. Confirmed.
General relativity predicts that the path of light is bent in a gravitational field; light passing a massive body is deflected towards that body. Astronomical observations can detect, for example, light from a distant star or galaxy or quasar being bent as it approaches conjunction with the Sun. Ditto for the closely related effect of gravitational time delay. Confirmed.
General relativity predicts the apsides of any orbit (the point of the orbiting body’s closest approach to the system’s center of mass) will precess—the orbit is not an ellipse, but akin to an ellipse that rotates on its focus, resulting in a rose curve-like shape.This effect was actually observed by Urbain Le Verrier in 1859 in the orbit of Mercury, but unexplained by Newtonian mechanics. Confirmed.
General relativity predicts a binary system will emit gravitational waves, thereby losing energy. Due to this loss, the distance between the two orbiting bodies will decrease, as will their orbital period. The first observation of a decrease in orbital period due to the emission of gravitational waves was made by Hulse and Taylor, using the binary pulsar PSR1913+16 they had discovered in 1974. Since then, several other binary pulsars have been found, in particular the double pulsar PSR J0737-3039, in which both stars are pulsars. Confirmed.
General relativity predicts something called gravitational lensing. If a massive object is situated between the astronomer and a distant target object with appropriate mass and relative distances, the astronomer will see multiple distorted images of the target. Depending on the configuration, scale, and mass distribution, there can be two or more images, a bright ring known as an Einstein ring, or partial rings called arcs. The earliest example was discovered in 1979; since then, more than a hundred gravitational lenses have been observed. The effect has even developed into an astronomical tool for indirect detection of dark matter. Confirmed.
General relativity predicts the formation of a black hole, a region of space from which nothing, not even light, can escape whenever the ratio of an object’s mass to its radius becomes sufficiently large, In the currently accepted models of stellar evolution, collapsed stars of around 1.4 solar masses, and stellar black holes with a few to a few dozen solar masses, are thought to be the final state for the evolution of massive stars. Usually a galaxy has one supermassive black hole with a few million to a few billion solar masses in its center. By definition, science can never directly observe a black hole; only indirect evidence that they exist. General relativity has played a central role in predicting that indirect evidence, and observations confirm those predictions. Confirmed indirectly.
General relativity predicts the existence of something called gravitational waves, analogous to electromagnetic waves. Gravitational waves are ripples in the metric of spacetime that propagate at the speed of light. The existence of gravitational waves have been indirectly observed in effects like orbital decay, described above. There are earth-based and solar-system based experiments under way to attempt to confirm the existence of gravitational waves; at this point there isn’t much data. Inconclusive.
It’s impossible for WC to summarize a century of research and development of physics in a blog post. Let alone understand that science. But general relativity has emerged as a highly successful model of gravitation and cosmology, which has so far passed many unambiguous observational and experimental tests. However, there are strong indications the theory is incomplete. Difficulties arise when scientists try to apply general relativity at the quantum level. The increasing evidence for the existence of dark matter and the even weirder dark energy suggest general relativity may be an incomplete model, much as Newtonian mechanics were when Albert Einstein published his seminal paper 100 years ago today.
[WC gives credit to Wikipedia for having the nerve to try and explain general relativity and its implications in lay terms. It fails in that attempt, but props for the attempt.]