April 24, 2024

Eclipses make visible the bending of the Sun’s gravitational light

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An image of GAL-CLUS-022058s — the largest and one of the most complete Einstein rings ever discovered. Credit: ESA/Hubble & NASA, S. Jha

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An image of GAL-CLUS-022058s — the largest and one of the most complete Einstein rings ever discovered. Credit: ESA/Hubble & NASA, S. Jha

During the nighttime conditions created during the totality of a solar eclipse, such as the one on April 8, planets and stars are visible. Venus and Jupiter, surrounding the Sun, will be very visible, while Mercury will be quite faint.

There will be no bright stars near the Sun during this eclipse, but surprisingly, faint stars near it will appear displaced by a small amount due to its gravity. This displacement and the movement of Mercury were the first pieces of evidence in the early 20th century that confirmed Einstein’s new theory of gravity. These observations also led directly to the prediction of black holes.

With the incredible power of modern telescopes, our “best” astronomy sites have abundant evidence that gravity bends light, acting like a lens. If a background object’s alignment with a gravitational lens is nearly perfect, an “Einstein ring” of light will appear as a halo around it.

Curved light

The first modern studies of light were published by Sir Isaac Newton in the early 18th century. Although some of his discoveries are now strong evidence that light was a wave, he concluded at the time that light was made of particles and would in fact be affected by gravity.

French mathematician Pierre-Simon Laplace even proposed, in 1795, that gravity could be strong enough to attract light to a body, an early concept of black holes. However, in the late 19th century, Newton’s ideas about light were discarded and it was thought to be waves and therefore unaffected by gravity.

We now know that light has two aspects, waves and particles combined, but it took the genius of Einstein to realize that this doesn’t even matter: it was our understanding of gravity that needed to change, and he proposed the general theory of relativity.


A graph showing the bending of starlight by the Sun, as observed in Australia during an eclipse in 1922. The arrows are on a scale about 2,500 times larger than the circle representing the Sun; the small effect makes them appear further from the sun than they actually are. Credit: WW Campbell and RJ Trumpler/Lick Observatory Bulletin

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A graph showing the bending of starlight by the Sun, as observed in Australia during an eclipse in 1922. The arrows are on a scale about 2,500 times larger than the circle representing the Sun; the small effect makes them appear further from the sun than they actually are. Credit: WW Campbell and RJ Trumpler/Lick Observatory Bulletin

Although it was published in full in 1915, as early as 1911, Einstein predicted that light would be deflected by gravity. Einstein’s complete theory immediately resolved a long-standing problem that Mercury’s position did not agree with predictions made based on Newton’s theory of gravity, a great triumph.

Observing the bending of light seemed like a good second test of the revolutionary new concept of gravity as “curved spacetime,” but only the Sun, about 330,000 times more massive than Earth, was strong enough to bend light slightly. As the source of the light would be the stars, the effect could only be observed during an eclipse, when they could be seen close to the sun.

The effect is very small, less than a thousandth of the angle that the Sun’s – or Moon’s – disk forms in the sky.

New equipment, new observations

Astronomers began carrying tons of equipment, including telescopes up to five meters long, to eclipse trajectories and make high-precision measurements. The stars where the eclipse would occur had to be photographed months in advance at night and then photographed with the same large telescope during the eclipse.

The famous English astronomer Sir Arthur Eddington made the first conclusive observations in 1919 at observation sites in South America and Africa. This small effect is imperceptible to casual eclipse observers, but it had major implications, resulting in an entirely different field of study than star classification.

It was noticed in 1910 that there was a strange star called 40 Eridani that was much fainter than it should be, considering its high temperature. It seemed that some stars might have the mass of the Sun, but only the size of a planet.


Sagittarius A*, the black hole at the center of the Milky Way, is about 5 million times the mass of the Sun. Credit: EHT Collaboration

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Sagittarius A*, the black hole at the center of the Milky Way, is about 5 million times the mass of the Sun. Credit: EHT Collaboration

These were soon nicknamed “white dwarfs”, and in 1930 the young Indian astrophysicist Subrahmanyan Chandrasekhar discovered that they must be less than one and a half times the mass of the Sun, otherwise they would collapse. The discovery of the neutron in 1932 led to the idea of ​​neutron stars, more compact than white dwarfs, but even they have a mass limit.

In 1939, Robert Oppenheimer and colleagues modernized Laplace’s idea of ​​black hole collapse using Einstein’s theory, but that year a war broke out, diverting their attention.

Black holes seemed of little interest and even less reality until the topic was revived in 1968 by physicist John Wheeler, who had some trouble publishing the name “black hole” as it was considered risqué.

Soon, some binary stars were found that appeared to have very massive invisible companions. It was also realized that enigmatic and very distant quasars could be explained through black holes. It now appears that most large galaxies, including our own, have black holes at their centers.

Bending power

A few years ago, the Event Horizon Telescope radio telescope consortium took images of our galaxy’s black hole, which bends light and radio waves in a characteristic way so that its central region appears dark. Although black holes have the greatest bending power, clumps of mass in deep space – including mysterious dark matter – also bend light. Because the light from the distant objects they magnify to us took a long time to get here, it began its journey when the universe was young. This allows us to look back in time.

During the April 8 solar eclipse, other stars may be visible, but without first observing and measuring their positions, observers may not be able to tell that they are not where they should be. But it’s a good time to remember that the path to black holes began about a century ago, with that faintly visible Mercury – and its starlight bent by the Sun.

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