Now We See . . . the Black Hole at the Center of the Milky Way
I have always wondered why black holes arouse so much fascination among folks that barely understand what they are. I asked that question to my adult learners to whom I recently taught a 3-week course on black holes. My students struggled with their answer and after some pause pointed to one of the strange properties of these objects they had heard about. I realized that there is no correct answer; it’s the mystique that has grown around black holes that draws people in!
This fascination is undoubtedly exploited by science writers and journalists, but it ultimately stems from the physicists and philosophers themselves, who see in black holes the possibility of advancing the boundary between knowledge and mystery. Progress in fundamental physics has essentially been stalled over the last 50 years after the standard model of particle physics was established. It is a great model, and no observation has unambiguously contradicted it yet, but we also know that it cannot be the final answer. It offers no good explanation of dark matter or dark energy which make up 95% of the mass of our universe, and even more importantly it cannot consistently predict what will happen when gravity becomes exceedingly strong, where the laws of the standard model break down. There are two instances for sure where this does happen. At the very early stages of the big bang, when all of the matter in the universe was bound inside a tiny fireball smaller than an atom; and at the center of every black hole where all the matter that falls into it resides in a tiny clump.
There are many candidate theories that hold promise to solve the gordian knot physicists have been stuck with for half a century. These are collectively called theories of quantum gravity. These theories are complex and are by no means completely worked out, but it is guidance from empirical observations that will help us focus our energies and budget to make faster progress. One hope is to look for gravitational waves generated during the big bang, a sort of cosmic gravitational wave background like the cosmic microwave background. This is difficult but a space based gravitational wave detector called LISA is being designed for launch during the next decade.
The other big hope for making progress is study and observation of black holes. While it is impossible to get any information from the interior of a black hole, what happens at the center can affect the life cycle and dynamics of a black hole. Studying black holes with greater accuracy could serve as a testbed for verifying theories of quantum gravity. The photo that was published on May 12, 2022, is a remarkable step towards this goal. It was obtained by harnessing the combined power of 8 radio telescopes around the world to achieve an image resolution that would have been impossible to even imagine 10 years ago. It is like being able to take a photo from Earth of a donut lying flat on the Moon’s surface.
The first photo taken in 2019 was of a gargantuan black hole at the center of M87 galaxy, 55 million light years away. The second photo released this month is of the smaller black hole (4 million solar masses instead of 65 billion solar masses) at the center of our own galaxy, a mere 26,000 light years away. Smaller holes are more dynamic and much more difficult to photograph. Getting the accuracy, we need for testing new theories will take time, but this achievement is indeed a big deal. And how exciting to finally be able to see the “unseeable” at the center of our own galaxy.
Supermassive black holes hide many other secrets that astronomers have been dying to get their hands on. They represent a missing link in the long hierarchy of gravitational motions responsible for creating the structure of our large-scale universe. Moons circle around planets to make planetary systems, planets circle around stars to make solar systems, stars circle around supermassive black holes to make up galactic centers. Small galaxies circle around larger galaxies to make clusters and galactic clusters gravitate towards superclusters. There is strong suspicion that supermassive black holes played a big role in the formation of the first galaxies. But we don’t know how! These black hole photos, along with the images of the earliest galaxies we hope to get very soon from the James Webb Space Telescope, hold promise to crack this long-standing mystery.
There is some indication that supermassive black holes help create a zone of stability in their galaxy. This stability could be essential for life to evolve. If that is true, then black holes are our best friends and do not deserve their monstrous reputation. It may not be a coincidence that our sun is situated in the Goldilocks zone between our galaxy’s center and its edge. We may owe our life to not just our star but also our black hole.
Black holes could help us understand the nature of time. If the big bang marks the creation of time, then the center of a black hole marks its end. Time does not freeze at the edge of a black hole as some may have heard but Einstein’s theory suggests that time comes to an end at its center. If this is really true, then the question of what happens to matter after it reaches the center of a black hole may be as meaningless as the question of what was there before the big bang. Some physicists believe quantum effects at the center can prevent time from ending there, but we will only know for sure when we have a consistent theory of quantum gravity. So, philosophers too have a stake in black hole physics.
The 100-year history of coming to terms with the existence of black holes makes one wonder what other bizarre objects are waiting to be discovered out there. That black holes are a legitimate prediction of Einstein’s equations was first demonstrated by a young German astrophysicist, Karl Schwarzschild, just a few days after Einstein published his work in 1915. He carried out his amazing calculations while serving in the German army at the Russian front in World War I and unfortunately died 6 months later. Einstein admired his work but quickly concluded that what he had discovered was a mathematical oddity. He reasoned that such bizarre objects could never exist in the real world. For the next 50 years the topic of black holes was largely ignored. The few papers that were published on the topic were to prove that the pressure required to squeeze a star into a black hole was theoretically impossible to achieve.
What changed in 1967 was not a great new scientific insight but a decision by a Princeton physicist, Archibald Wheeler, to rename these objects as black holes instead of Schwarzschild singularities which is how they were referred to until then. Changing names for astrophysical objects can be as significant as it is for Hollywood celebrities. While a singularity is a mathematical infinity that deters physicists from exploration, a hole in space-time sounds mysterious and invites further research. Within a decade physicists convinced themselves that black holes could not only form but that they were the inevitable result of a large star’s death. And within a few years of searching, astronomers found many black hole candidates carrying the correct signature. It was a glorious time for Sci-Fi writers.
Worm holes are also legitimate solutions to Einstein’s equations. These objects can provide a conduit between two points in space billions of light years away making interstellar space travel much easier. They can also transport an object backward or forward in time. Most scientists today believe that such oddities are physically impossible but what if worm holes are still going through their “incredulity” phase. A better understanding of black holes could fuel more investigation into worm holes and suggest ways in which we could look for them. What a feast that would be for Sci-Fi writers and imagine the new conundrums it will create for philosophers.
In short, black hole hype is well deserved and the mystique around them has a solid scientific foundation. Keep your ears and eyes open for the next big milestone: a video movie of matter and gas swirling around a black hole.