Scientists Use Warped Light to Test Einstein’s Theory and Weigh Stars

When Einstein developed his general theory of relativity, commercial radio didn’t even exist yet. He could not possibly have imagined all of the fancy, high-tech equipment that scientists would use over the next 100 years to test—and verify—his predictions. In fact, he wasn’t even sure that all of his predictions could be tested experimentally because they resulted in such tiny, hard-to-measure effects.

Just last week, an international team of researchers confirmed one of Einstein’s predictions when they showed how a nearby star warps light that comes a little too close. Led by Kailash Sahu from the Space Telescope Science Institute, the measurements reveal the mass of the nearby star, providing a new type of scale that astronomers can use to better understand stars and galaxies. The team presented these results at the 230th Meeting of the American Astronomical Society and in a paper in the journal Science.

This illustration reveals how the gravity of a white dwarf star warps space and bends the light of a distant star behind it.
Image Credit: NASA, ESA, and A. Feild (STScI).

At the core of Einstein’s theory of general relativity is the idea that objects with mass warp space-time. Like a bowling ball placed on a trampoline, heavier objects distort space-time (the surface of the trampoline) more than light objects. What we call gravity is the result of these distortions. In addition to providing a theoretical framework, Einstein suggested a few ways to experimentally test his theory. Sir Arthur Eddington, an English scientist and supporter of general relativity, led one of these tests during a solar eclipse on May 29, 1919.

According to Einstein’s predictions, light from a distant star should bend as it passes by the sun because of the way the sun curves space-time. The distortion should cause the starlight to appear as though it came from a slightly different place than the star’s true location. Newton’s theories of gravity predicted that the light would bend too, but not by as much.

Prior to the 1919 eclipse, Eddington measured the true location of stars in a bright star cluster that he knew the sun would cross during the solar eclipse. Then, during the eclipse he photographed the sun and surrounding stars. Normally the light from the sun would d the surrounding stars, but stars from the cluster were just visible during the eclipse. By comparing the true location of the stars to where they appeared to be during the eclipse, Eddington confirmed Einstein’s general theory of relativity.

In this recent work, Sahu’s team made an analogous and much more sensitive measurement. They realized that by using the Hubble Space Telescope, it might be possible to measure the expected change in position of a distant star when a nearby star other than the sun crossed its path. This has never been done before, in large part because the change in position of the distant star is predicted to be tiny, up to 1000 times smaller than what Eddington observed during the eclipse. The size of the change depends on how the background star and nearby object align, the mass of the nearby object, and the relative distance between each object and the observer.

After starting with a catalog of more than 5,000 nearby stars that appear to move quickly across the sky relative to most other stars, the team set its sights on Stein 2051 B, a white dwarf star about 18 light years away (yep, that’s still considered “nearby”). White dwarfs are the burned-out cores of stars that were once up to eight times the mass of our sun. Along with most stars in the galaxy, our sun will one day be a white dwarf.

Like Eddington but with more advanced technology and greater precision, the team mapped out the location of Stein 2051 B relative to background stars. Then they used a camera on Hubble to observe the star several times over two years, as it passed between us and a distant background star. Stein 2051 B is 400 times brighter than this distant star, so measuring the apparent location of the star when it was eclipsed by Stein 2051 B was a challenge. One team member compared it to picking out a firefly hovering next to a light bulb, but they did it—and the results agreed with Einstein’s theory.

This research didn’t just test the general theory of relativity, though, it also provided important information on white dwarfs. Since the shift in the apparent position of a background star depends on the mass of the nearby object, the researchers were able to determine the mass of the Stein 2051 B from their data (0.675 times the mass of our sun). Einstein predicted that it would be theoretically possible to determine a star’s mass this way, but thought it would be nearly impossible in reality.

This mass determination is especially interesting for Stein 2051 B. Its mass had been estimated previously using a less reliable method and the outcome was pretty wacky and hotly debated. This research puts those questions to rest and shows that Stein 2051 B is just a normal, average-mass white dwarf. The mass determination also supports a key prediction in the theory that describes how the mass and radius of white dwarf stars are related.

This summer, much of the United States will be treated to a total solar eclipse on Monday, August 21st. Physics Buzz readers in the path of the eclipse are invited to join astronomy enthusiasts in repeating Eddington’s experiment. For details, check out NASA’s activity Testing General Relativity. We’d love to see your results! If you live in another part of the word, check out this list to see if a total solar eclipse is heading your way soon.

Kendra Redmond

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