Black holes remain one of the biggest mysteries of the cosmos even 103 years after physicist Karl Schwarzschild first predicted their existence. Black holes cannot be seen or measured in conventional methods, and until recent developments, remained a hypothetical scenario. In 2015, scientists detected the side effect of two black holes merging together, which seemingly proved right one of the brightest minds of the 20th century – Albert Einstein. Black holes locked into an orbit around one another – binary black holes – will eventually fall towards each other and merge.
The tremendous force of impact radiates so-called gravitational waves, which according to the European Space Agency (ESA) are “fluctuations in the fabric of spacetime”.
Both black holes and gravitational waves were resolved from Einstein’s theory of general relativity, which the German-born physicist presented in 1915.
General relativity remains to date the best theory explaining the inner-machinations of the universe.
ESA said: “Karl Schwarzschild derived the equations for black holes in 1916, but they remained rather a theoretical curiosity for several decades, until X-ray observations performed with space telescopes could finally probe the highly energetic emission from matter in the vicinity of these extreme objects.
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“As for gravitational waves, it was Einstein himself who predicted their existence from his theory, also in 1916, but it would take another century to finally observe these fluctuations.”
Another of Einstein’s theories, however, was proven only a few years after general relativity saw the light of day.
In 1919, a total eclipse of the Sun produced evidence of the gravitational bending of light or gravitational lensing.
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The theory proposes light will curve and bend when passing near a large source of mass or gravity, such as a planet or black hole.
In practice, this means the light from a distant star, for instance, will arrive on Earth at an angle to its point of origin.
ESA said: “In the framework of general relativity, any object with mass bends the fabric of spacetime, deflecting the path of anything that passes nearby – including light.
“An artistic view of this distortion, also known as gravitational lensing, is depicted in this representation of two merging black holes.
“One hundred years ago, astronomers set out to test general relativity, observing whether and by how much the mass of the Sun deflects the light of distant stars.
“This experiment could only be performed by obscuring the Sun’s light to reveal the stars around it, something that is possible during a total solar eclipse.”
Astronomers are yet to observe a black hole merger directly but experiments such as ESA’s LISA experiment hope to detect gravitational waves from orbit.
Another ESA project, the X-ray observatory Athena, will launch alongside LISA in the 2030s.
ESA said: “We don’t know what happens during such a cosmic clash so this experiment, much like the eclipse of 1919 that first proved Einstein’s theory, is set to shake our understanding of gravity and the universe.”