A black hole is a source of gravity so strong that nothing, not even light, the fastest thing in the universe, can escape. More formally, a black hole is a singularity (or “tear”) in the fabric of spacetime itself. They are some of the most mysterious and paradoxical phenomena in the universe. To this day, there is a great deal that scientists do not know about black holes.
What is the history of black holes?
Black holes originated from the field equations of Einstein’s theory of general relativity in the early 1900’s. Initially, they were just a mathematical phenomenon (solutions that cause a zero denominator) in the equations. However, in the years that followed, scientists began to theorize that if a star with a sufficiently large mass died, it would crush its entire mass down into a singularity.
A singularity is the beating heart of a black hole. It is a point in space as small as an atom that contains an extreme amount of mass; some black holes are many times the mass of the Sun. The implications of an object with this property were mind-boggling. Not only would it be an object of immense gravitational pull focused at a small point in space, but it would also be powerful enough to prevent light from escaping it. Due to this, it would be completely invisible to any observer, hence its name.
Scientists also began to discover some important properties of black holes, such as their size and the mass of a star required to create one. The former prediction implied that there would be an event horizon, a spherical boundary around the singularity through which light cannot escape. The latter determined that only the most massive stars could collapse into a black hole.
Another key discovery about black holes was made in the 1970’s, by the late theoretical physicist, Stephen Hawking. He claimed that black holes were not immortal, as they were originally thought to be. Hawking showed that black holes slowly leaked radiation off of their event horizon that caused them to lose mass at an increasing rate over time. This quantum phenomenon became known as Hawking radiation and is now a crucial factor in determining the future of the universe.
In the 1960’s, astronomers detected a strong source of x-rays from a star in the constellation Cygnus. This source was eventually determined to be the first black hole discovered by humans. The black hole was named Cygnus X-1 and is a black hole of about 15 times the mass of the Sun. The system it belongs to is about 6000 light years away from Earth.
Even more recently, in 2019, astronomers working at the Event Horizon Telescope (EHT) announced that they had successfully photographed a supermassive black hole in the center of a large elliptical galaxy over 55 million light years away. This black hole is much larger than Cygnus X-1, with an estimated mass of about 6.5 billion times the mass of the Sun. Although the image without context is unimpressive and blurry, it is the first of its kind and provides visual evidence that black holes actually exist.
What are some properties of black holes?
Black holes are the remnants of once bright and massive stars. As these stars begin to run out of fuel, they collapse in on themselves until the element iron is fused. At this point, the science behind nuclear fusion (the smashing of atoms together) dictates that the star begins to lose energy. In other words, up until iron, for every element fused, the star gains energy. But, as soon as iron is reached, the star no longer makes profit on the energy required to fuse it together. So, the star which has been burning for millions of years, quickly loses the battle against gravity. The outer layers collapse in on the dead core of the star at such a rapid rate that a singularity, and thus a black hole, is formed.
One of the prominent properties of black holes that scientists have predicted and observed is the way black holes interact with other matter. Specifically, when black holes “eat” matter, some of it does not fall into the hole and instead orbits around it. Eventually, this extra highly energetic matter forms what is called an accretion disk around the black hole. Scientists predict that this kind of disk is common in systems containing a black hole and one or more stars. The black hole slowly feeds off of the matter in the outer layers of the companion stars, creating an accretion disk of hot gas and dust.
Black holes come in all different sizes. Their size depends heavily on the mass of the star that collapsed to create it. However, scientists have discovered black holes with masses much greater than any one star. They have concluded that black holes can grow if they encounter one another. This type of interaction is predicted to be part of galaxy formation, as the center of galaxies is the perfect place for black holes to run into each other. Black holes of this type are called massive and supermassive black holes, depending on their mass.
Supermassive black holes at the center of galaxies seems to be an ordinary thing in the universe. Scientists now believe that there is one in the center of every galaxy; they may be a requirement for galaxies to form. The Milky Way is no exception. Astronomers have detected radio signals coming from the center of our galaxy in the constellation Sagittarius, which is an indicator of black hole activity. This, in addition to the sporadic and rapid movement of stars in the nucleus of the Milky Way, seems to point towards the existence of a supermassive black hole in the center.
What would happen if you fell into a black hole?
One of the most important consequences of Einstein’s theory of general relativity is the effect gravity has on the passage of time. General relativity says that the presence of mass or energy causes spacetime to curve, which in turn tells other masses how to move. The key takeaway from this definition is the idea that gravity not only manipulates space, but time too. Essentially, the more powerful the gravitational force on an object, the slower it’s time moves relative to some other object that’s at rest (or close to rest, like the Earth).
Does this mean that a person near a black hole moves in slow motion? Well, close, but not quite. You see, for that person, a second feels like a normal second, but compared to an observer on Earth, they would find that for each one of those seconds near the black hole, much more time on Earth has passed. Can you use black holes to travel through time? The quick answer is sort of. One of the fundamental principles of Einstein’s theory is that while time can be slowed and sped up, it cannot be reversed. In other words, while you could travel to the future by slowing down your time relative to Earth, you cannot go to the past. Believe it or not, this is not even the strangest aspect of a journey to a black hole.
If you find yourself passing through the event horizon of the black hole, it will appear to anyone watching your perilous adventure that you are frozen on the boundary of the hole. Perhaps even crazier, in your point of view, you will watch the entire lifetime of the universe play out in front of you in a single instant. You will see everything happen that will ever happen, all at once. As far as what happens inside the black hole, there is no way to know for sure, at least not yet. Since we know of nothing that can travel faster than light, there is currently no way for scientists to probe inside of a black hole and look at the singularity.
Another consequence of the immense gravitational pull of black holes is that any object close to the event horizon will stretch out to unimaginable lengths. This process is often referred to as “spaghettification”. Each unit of distance closer to the event horizon corresponds to an inordinate change in the strength of the local gravity. It would be like if the gravity on the second floor of an office building was half of the gravity on the first floor.
Due to this stretching and the science behind general relativity, the stretched object would also appear to get redder in color. The source of this color change is the Doppler effect, where the wavelengths of light reflecting off the object change after they pass through the gravitational field of the black hole. Unfortunately (or maybe fortunately), any human astronaut caught up in the pull of a black hole would almost certainly pass out and die before they even got close to the event horizon. The g-forces one would experience as they got close would be thousands to millions of times the livable amount. Thankfully, there are no black holes near Earth, so we are safe for now.
What do black holes tell us about the universe?
I mentioned earlier that Hawking radiation, which causes black holes to die slowly, helps scientists determine the future of the universe. While there are an enormous amount of basic factors to take into account, physicists predict that the universe will eventually be entirely composed of black holes and stray particles of light. In fact, the universe will likely live most of its lifetime without any matter; all the stars, planets, and nebulae in the universe will live and die in only a small fraction of the entire lifetime of the universe.
Perhaps even scarier, eventually these black holes will die and the universe will slowly become a great void of nothingness. However, the fate of the universe could end up being drastically different than this if even a few of the assumptions scientists make are wrong.
Some physicists predict that black holes are just passageways to other parts of the universe and that all of the matter that they eat is tossed out the other end at some “white hole.” There has been no evidence to support this, but our knowledge of black holes is so limited, so we cannot discount it. Almost more fantastical is the claim by some that black holes lead to other universes entirely. One thing is certain: understanding the paradoxical nature of black holes could answer important questions we have about the fate of the universe and our existence in it.