Dancing with the stars

Charlotte Tomlinson delves into space by discussing recent research that proves Einstein's theory of relativity.

Scientists have observed a star circling a supermassive black hole, yet more proof for Einstein's theory of relativity. This breakthrough was outlined in a paper published this April from the Max Planck institute for Extraterrestrial Physics, using observations from European Southern Observatory's Very Large Telescope. This star had been observed for 27 years, and was described as sensing the effects of general relativity.

What is Einstein's theory of relativity?

General relativity is a theory about gravity. Initially, it was thought that gravity is a force between two objects. According to Newton, gravity is generated by the mass between two objects, causing them to be attracted to each other. This was one of the first explanations for why the planets orbit the sun, because the sun was the most massive object observed.

Einstein developed a theory of gravity with spacetime curvature as the key ingredient. What he refers to as ‘spacetime’ is the three dimensions of space and one of time woven together like a fabric.

Planets and other massive bodies, or any bodies with mass in this context, can stretch and curve spacetime creating a dip. This allows planets to remain in orbit as the sun’s large mass allows other celestial bodies to follow the curve generated in spacetime.

Scientists have observed the motion of star S2 orbiting the compact radio source Sagittarius A* at the centre of the Milky Way. Sagittarius A* is a compact, astronomical radio source 26,000 light years away from the sun, right at the centre of our galaxy. The fact that it is a radio source means that it emits radio waves, making it one of the most curious objects in our universe. Its incredible mass governs the orbits of stars in its vicinity. The 2020 Nobel Prize winners for Physics, Reinhard Genzel and Andrea Ghez, labeled Sagittarius A* as a supermassive black hole. A supermassive blackhole is the largest type of black hole, with a mass billions of times the mass of the sun. These commonly exist in the centre of galaxies, such as our Milky Way.

S2 is one of the closest stars ever found in orbit of this super massive black hole at less than 20 billion kilometres. S2's orbit constructs a rosette shape, almost like it is dancing! Most orbits from stars and planets are elliptical, meaning they move closer to and further away from the object they are orbiting. S2’s orbits precesses. This implies that the location of its closest point to the supermassive black hole changes with each turn. It is hurtling through space at almost 3% of the speed of light, completing an orbit once every 16 years. It is this change in location at each turn that generates the rosette pattern.

How does this prove General relativity?

General relativity provides a precise prediction of how much S2’s orbit changes, with the latest measurements from this research exactly matching the theory. Under Newton’s laws, S2 should have continued along exactly the same path as its previous orbit. However, the peculiar pattern of its orbit is further proof that Einstein's theory is correct.

This is an important breakthrough not only because it backs Einstein’s theory of relativity even further, but also because it helps us learn more about supermassive black holes. Now that we know that S2 measurements follow general relativity precisely, we can set stringent limits on how much invisible material, like dark matter or possibly even smaller black holes, are around Sagittarius A*.