"Hi, my name is Becky and I study black holes."
Without fail, this is a great conversation starter, and inevitably I get inundated with questions almost immediately. Black holes are interesting, black holes are scary, and above all, black holes are hard to understand.
So in this post, I'd like to share a little of what I know about black holes and how they behave; my hope is that this will clear up some common misconceptions about black holes and provide everyone with their own interesting conversation points.
We can’t know for certain, but astronomers estimate that there are between 100 to 200 BILLION galaxies in the universe. (A German supercomputer just predicted that there may be as many as 500 billion.) Most, if not all, of these galaxies host what’s called a supermassive black hole. Just to be explicit, a supermassive black hole is probably best known as a song by Muse, but astronomers formally define a SMBH as the largest class of black hole. Weighing in at greater than 100,000 times the mass of the Sun, these are the sumo wrestlers of the gravity well world and could pack a mean punch in the galactic dynamics ring.
So why haven’t we been warned of these alleged supermassive black holes? Can't they suck everything in and KILL US ALL?
It’s time to clear up some common misconceptions. Before I began my career in astrophysics, I believed that black holes only formed from extremely massive stars, that they suck anything and everything in like a vacuum, that they’re black, that they’re holes, that they are sometimes tunnels to another dimension or alternate universe, and that you can actually fall into one. None of these statements are true, and here’s why:
1. They only form from massive stars.
Stellar mass black holes do indeed form from stars, but black holes form whenever the density of the mass/energy passes a threshold to trigger formation. If the Earth were the size of a nickel, it would form a black hole of the same mass.
2. Black holes suck anything and everything in like a vacuum.
Black holes behave themselves gravitationally – if the Sun were a black hole of the same mass, the Earth’s orbit would be unchanged, we would just be rather chilly without the Sun’s radiative energy.
3. They are black.
The term “black hole” is a misnomer since “black” indicates an absence of radiation, yet Stephen Hawking proved that energy leaks from a black hole in the form of Hawking radiation (check Wikipedia for more information on this vast subject). This results in radiation and mass loss from the black hole.
4. Black holes are holes.
Hole is a two-dimensional term that doesn’t fit a three-dimensional universe. Black holes would be more adequately described as spheres.
5. They are tunnels to another dimension or alternate reality.
This was a convenient solution to the Romulan problem in Star Trek, but has never been proven and may be impossible to prove outside of the science fiction universe.
6. You can fall into a black hole.
The technicality here is that you fall for an infinitely long amount of time as viewed from outside the black hole, so you never completely fall to the “bottom.”
Come again? If you can’t actually fall to the bottom of a black hole, what would it feel like to fall into a supermassive black hole or just a black hole in general? Believe it or not, professional theoretical astrophysicists are paid to study this exact question. They push the boundaries of quantum field theory, Einstein’s general relativity, Einstein’s special relativity, classical mechanics, particle physics, and thermodynamics to derive a comprehensive theory of death by black hole. I will now compile their conclusions to describe death via SMBH.
As the black hole approaches, you observe a spherical bending of light around it. This happens because the black hole is dense enough to put a dent in space-time itself. This means that light follows a curved path around the black hole to reach your eyes. You are also able to see an inner black region, the boundary of which marks the Schwarzschild radius, or the event horizon.
At first, you maintain a stable orbit around the SMBH, but once you pass the event horizon, you have passed a point of no return. This event horizon marks the point at which not even light (which matches the speed limit of the universe at just over a billion kilometers per hour) can escape the gravity of the black hole. Past this point, physics starts to get weird. You notice that time, space, and light itself behave in strange, relativistic ways. I should probably mention at this point that you would not live long enough to pass the event horizon – black holes are energetic beasts so they are always surrounded by a radius of plasma and extremely hot particles known as the “firewall” – passing through the firewall would mean sure death. So, already dead, you fall through the horizon and you notice the universe becomes brighter and brighter as space falls faster than light. At this point, you would die a second time via spaghettification – the tidal gravitational forces within a black hole exert more force on your feet than on your head. Neil DeGrasse Tyson once described the process of spaghetiffication:
As you’re being snapped in half, and then halved again, you could look around yourself and notice that the universe has become two-dimensional. As you fall faster than light, the light above your head is stretched towards the red end of the spectrum (this effect is called Doppler Shifting and happens to the frequency of an ambulance siren as it passes by) and the light by your feet is blue-shifted as you rush towards it.
You are traveling so fast, that the light is shifted beyond the visible part of the electromagnetic spectrum, and you see only a narrow band of the universe around you – so in your last moments of life the world would be two-dimensional. Time is also behaving extremely strange at this point in your journey. Although it takes you a couple of hours to fall into a supermassive black hole (as opposed to a fraction of a second with a stellar mass black hole), to an outside observer you will never pass the event horizon. Since you are falling infinitely fast, time will pass infinitely slowly for you from your friend’s point of view. This is strange and unexpected, but black holes are the most gravitationally dense objects known in the universe, so they push physics of strangeness. So, in the end, from your own point of view, if you don’t die from the firewall you will most certainly fall prey to the extremely painful process of spaghetiffication.
But don’t worry; it’s space so nobody can hear you scream.