By Emily Levesque, University of Washington
At first, physicists and astronomers were a bit more cautious than the general public in embracing Albert Einstein’s theory of general relativity. It was a thorny and difficult theory to understand mathematically, and it was put to test again and again for decades afterward. Today, however, general relativity has become a core tenet of physics with effects that one can see all around, once we know where to look.
Some applications of general relativity are surprisingly practical—the GPS navigation that we use in our car or on our smartphone only works thanks to Einstein’s special and general relativity! Keeping track of time down to a fraction of a second and pinpointing a location on our planet with an accuracy of just a few feet is an incredibly complicated task. To do it correctly, GPS technology has to account for the relative motion of satellites, the Earth, and even our car.
It also has to tweak the clocks it uses to match times across different frames of reference, and even accommodate changes in the curvature of spacetime caused by gravitational effects that vary as we move away from Earth’s surface.
Other general relativity effects are still best observed with powerful telescopes. The bending of light, predicted by Einstein, can also take a more extreme form if we look further out into the cosmos. Another prediction of general relativity notes that a very massive object can warp spacetime significantly, so much so that it begins to act like a lens that substantially distorts and magnifies background light. Other physicists recognized this possibility in the general relativity equations, and Einstein himself published a paper on it in 1936. And yet, it was astronomer, Fritz Zwicky, who first proposed that this effect should be particularly obvious by using massive clusters of galaxies.
When it comes to finding lots of mass concentrated into one place, galaxies are perfect candidates. Our own Milky Way has an estimated mass of 1.5 trillion suns—including stars, gas, dust, and dark matter—and in galactic terms we’re pretty average. Zwicky realized that using a massive galaxy as a spacetime-warping lens would produce immensely strange and distorted images of other, more distant galaxies whose light passed close to the lens.
Alignment of Lens and Background Object
In addition, the math of gravitational lenses predicted some spectacular sights, training a kind of gravitational funhouse mirror on far away galaxies. Background galaxies distorted by a massive lens would look brighter than expected and would be bent into curved slivers and offset from where they would appear in the night sky without the effects of general relativity.
The whole problem comes down to geometry, and a sufficiently perfect alignment of a lens and a background object could produce multiple images or even a perfectly-bent ring arcing around the lens.
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A Single Background Quasar
In 1979, a team of astronomers in Arizona discovered what looked like two quasars, brilliantly luminous cores of distant galaxies with supermassive black holes at their centers. More observations ultimately revealed that these two quasars were, in fact, a single background quasar, multiply-imaged and distorted by an enormous galaxy lying between the quasar and us—one could see it as the fuzzy blob between the two bright points from the quasar.
The match between the two quasar images wasn’t perfect—the light from the two lensed images traveled along slightly different paths and arrived at Earth at slightly different times with uniquely distorted appearances—but it was close enough to prove that the quasar was being lensed, and that the lensing was indeed happening according to Einstein’s general theory of relativity.
Today we see incredible and dramatic examples of gravitational lenses in observations of distant galaxies and galaxy clusters. One may recognize images like this from the Hubble Space Telescope, showing a host of distorted and curved background galaxies bent by massive objects sitting between us and them. Astronomers have observed perfect rings, weirdly-shaped galaxies malformed by the effects of multiple lenses, and even a supernova that appeared in the sky four times thanks to being multiply-lensed by a huge galaxy.
Enormous galaxy-sized gravitational lenses and the images they produce may be dramatic, and yet, even low-mass objects can warp spacetime and lens light. General relativity predicts lensing effects so exactly that any deviation from the expected behavior could indicate that there’s a bit of mass where we don’t expect and for a small star like our Sun, that extra mass sometimes comes from an orbiting planet.
The Cosmological Constant
Einstein’s work however, wasn’t infallible, and there are some details of general relativity that are still being explored and tested. Dark energy is one example. Einstein’s theory of general relativity originally predicted that the universe must either expand or contract.
Einstein recognized this in his equations but saw it as an error rather than a potent prediction. After all, at the beginning of the 20th century astronomers seemed to clearly see a static and unchanging universe, and the idea of expansion seemed absurd.
Einstein’s fix was to simply tack a number onto his equations, known as the cosmological constant, to allay these concerns. After work by Edwin Hubble and others revealed an expanding universe, he took the number back out, but it’s been re-inserted today. What Einstein called the cosmological constant is what astronomers today know as dark energy, the mysterious energy that helps to drive the expansion of the universe.
Evidence of Einstein’s Theory
Still, even with tweaks to accommodate the shape of our universe, Einstein’s general theory of relativity has proved to be an exceptional guide for how space, time, and gravity work, and we’re still finding new evidence of just how well this theory describes physics.
Needless to say, the implications of Einstein’s general theory of relativity are numerous and immensely far-reaching. From Arthur Eddington’s 1919 eclipse expedition, to the spectacular gravitational lenses and accurate GPS trackers we see today, evidence of Einstein’s incredible and successful theory is all around us.
Common Questions about the Implications of Einstein’s General Theory of Relativity
A prediction of general relativity notes that a very massive object can warp spacetime significantly, so much so that it begins to act like a lens that substantially distorts and magnifies background light.
Astronomers have observed perfect rings, weirdly shaped galaxies malformed by the effects of multiple lenses, and even a supernova that appeared in the sky four times thanks to being multiply-lensed by a huge galaxy.
What Albert Einstein called the cosmological constant is what astronomers today know as dark energy, the mysterious energy that helps to drive the expansion of the universe.