What lurks at the centre of our galaxy, the Milky Way? It’s a big question with an even bigger answer. Supermassive, in fact, as identified by Prof. Andrea Ghez and Prof. Reinhard Genzel. The two astronomers were key players in making the discovery: That a supermassive object lurks at the centre of our galaxy, likely a black hole. Ultimately, this was the result of a vast collaborative project, as both academics would attest to. This is demonstrated by the following two papers, which are just two fragments of a vast foundation of academic literature that made the discovery possible: The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy’s Central Black Hole: A Paradox of Youth and Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole. It might seem obvious that something big lies at the heart of the galaxy, but understanding its exact nature is incredibly difficult. That’s science. And it’s not just any science, it’s the type of science that wins you a Nobel prize.
So what exactly did Ghez and Genzel discover that earnt them the Nobel prize in Physics (shared with Penrose’s theory), and what are the interesting bits we can pluck out from this sprawling research project? Prof. Ulf Danielsson from the Nobel Committee explains in an interview immediately following the prize’s announcement that Ghez and Genzel monitored the stars orbiting the centre of our galaxy in order to prove that a supermassive object lurks there. This monitoring required some very interesting physics: The second paper linked above discusses post-Newtonian dynamics, what does this mean and why is it relevant?
In the 17th Century, Sir Isaac Newton derived the universal law of gravitation, which states that objects with mass are attracted to one another. Basically, Newton’s theory explained how different things fall. On Earth, we can watch the rain falling to the ground whilst the Moon seemingly remains held above the sky. However, this is a deceptive illusion, the Moon is falling - it’s just falling in a circle around us, this is what we call an orbit. Miraculously, Newtonian dynamics describe how all of these things occur. With this mathematical toolkit, we can describe the sort of physics that most people are familiar with: the movement of tennis balls, planets, and rockets.
Building on Newton’s work over 200 years later, Albert Einstein turned everything on its head with his Theory of General Relativity which revealed that space can be curved and twisted by massive objects (objects with mass). Now, let’s turn our heads towards the centre of the galaxy, which lies between the constellations of Sagittarius and Scorpio behind a mass of dark nebulae known as the Great Rift. If the Rift were not there, our nights would be illuminated by the countless colossal stars that encircle the galaxy’s core. All that mass means lots of curved space, which in turn results in confusing behaviour that Newtonian dynamics is simply unable describe. With post-Newtonian dynamics, which incorporates general relativity, we can measure and study these stars accurately.
But what lies beyond these giant stars at the very centre? Is it a black hole, a celestial object of incredible mass? As Ghez explains in her 2004 TED talk, there are only 3 characteristics of a black hole that we can identify: mass, charge, and spin. Despite being a vicious entity that can twist the fabric of the universe, these supermassive objects are elusive. Ghez goes on to explain that she instead uses the behaviour of the stars in the galaxy’s core, behaving in a post-Newtonian way, to identify and understand the object at the centre. In particular, a star called SO-2 completes its orbit of the centre of the galaxy every 15 years. This allowed the astronomers to work out that something is sitting in the galactic core that’s smaller than our Solar System and 4 million times heavier. If it is a black hole, as the results suggest, then it has huge implications for the future, and the past, of our galaxy.
This year’s Nobel Prize in Physics is a celebration of our progress in understanding the primal forces that created us through chance and shall consume us through destiny. Pretty cool.