A Side of Science: Testing Einstein

In my latest video, I talk about the 100th anniversary of the solar eclipse that propelled Einstein into the public’s eye by providing a test for his general theory of relativity.

However, that solar eclipse wasn’t actually the first test of general relativity…nor, of course, was it the last. Sometimes it feels like everyone wants to take a shot at Einstein to prove him wrong, but they have to take a number, because scientists have been doing that for the last century.

Tests of general relativity from the past century.
Credit: NASA/GSFC

Why? Because that’s what you do in science. You can never fully prove a theory, but you can disprove it, so you test it in as many different ways as possible to know if it holds up and under what situations it holds up.

Here are just a few astronomical tests that have been done in the last century:

This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER’s primary mission.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The 1919 solar eclipse wasn’t actually the first test of general relativity. In the mid-1800s, astronomers had noticed that Mercury’s orbit didn’t behave as they expected using Newtonian gravity.

In both descriptions of gravity, the perihelion of Mercury’s orbit – the point in the orbit when the planet is closest to the Sun – precesses around the Sun. In other words, that point where Mercury is closest to the Sun moves in the orbit. However, it moves more than Newtonian gravity predicts.

The answer to why Mercury’s orbit changes falls right out of general relativity. Essentially it is because Mercury’s orbit is affected by the increased curvature of space-time due to its proximity to the Sun.

In the center of this image, taken with the NASA/ESA Hubble Space Telescope, are two faint galaxies that seem to be smiling. The two orange eyes are the galaxies SDSSCGB 8842.3 and SDSSCGB 8842.4 and the misleading smile lines are actually arcs caused by an effect known as strong gravitational lensing. 
Credit: NASA & ESA

The solar eclipse test was dependent on the Sun creating a “dent” in space-time that causes the path of light to bend as it passes near the Sun. But it’s not just the Sun that deflects light — all mass does. In fact, with enough mass in the right alignment nature can become a kind of telescope, with light from a distant object getting magnified and distorted by that intervening mass.

The first “gravitational lens” was discovered in 1979, when astronomers observed a double image of a type of galaxy called a quasar. Today we have seen many examples, including the image above where a group of galaxies observed by the Hubble Space Telescope appears to be smiling at us!

As neutron stars collide, some of the debris blasts away in particle jets moving at nearly the speed of light, producing a brief burst of gamma rays.
Credit: NASA’s Goddard Space Flight Center/CI Lab

Another prediction of general relativity is that when a mass changes speed or direction it produces wiggles in space-time that we call gravitational waves. Einstein actually waffled on whether or not he believed that gravitational waves were real, but ultimately, after some input from research partners, he landed on them existing (Einstein, A.; Rosen, N. “On gravitational waves.” J. Frank. Inst. 1937, 223, 43–54).

If you follow science at all, you’ve probably heard about gravitational waves in the past several years. In 2016, the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) reported the very first detection of these waves passing by Earth. And the latest run of LIGO is currently detecting them every week or so.

However, it’s interesting to note that we had already observed the effects of gravitational waves before they were directly detected passing through our solar system by LIGO. That first observation came in the 1970s when a pair of astronomers – Hulse and Taylor – found a pulsar in a binary orbit with another star. Pulsars are the crushed core of a dead star that spin and emit light that appear to us as pulses of light. Those pulses are extremely regular and can keep time even better than an atomic clock!

Using the exquisite timing that the pulsar in this binary system offered, Hulse and Taylor discovered that the orbit of these two stars was shrinking by exactly the amount that would be predicted if the system was losing energy by emitting gravitational radiation. They even won a Nobel Prize for this discovery!

The first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87.
Credit: Event Horizon Telescope Collaboration

General relativity predicts that there are places where space-time is so distorted not even light can escape — we call these places black holes. The first candidate, an X-ray source called Cygnus X-1, was observed in 1965.

We’ve come a long way since then. Just last month the NSF-supported Event Horizon Telescope released the very first image of the shadow of a black hole.

And these are just a few tests of general relativity. There are other places we see it in astronomy and in physics. In fact, you can even find general relativity in your pocket – the GPS on your phone depends on general relativity to pinpoint your position on Earth!

And, in case you missed my 1919 solar eclipse video, be sure to check it out:

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