Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
Beyond the Horizon?
When it comes to black holes, the event horizon is the final boundary between the known and the unknown of black hole mechanics. We have a (somewhat) clear understanding of everything that goes on around one, but past the event horizon is anyone’s guess. This is because of the immense gravitational pull of the black hole preventing light from escaping past this boundary. Some people have dedicated their lives to finding out the truth of the inner designs of the black hole, and here is but a sampling of some possibilities.
The Area Around the Event Horizon
According to theory, a black hole is surrounded by plasma that arises from colliding and infalling matter. This ionized gas not only interacts with the event horizon but also with the magnetic fields around a black hole. If the orientation and charge is right (and one is a distance of 5-10 Schwarzchild radii from the event horizon), some of the infalling matter gets trapped and goes round and round, slowly losing energy as it slowly spirals in towards the black hole. More focused collisions occur now, and lots of energy is released each time. Radio waves are released but are hard to see because they emanate when matter is the densest around the black hole and where the magnetic field is strongest. Other waves are released as well but are nearly impossible to discern. But if we rotate among the wavelengths, we will find different frequencies as well, and transparency through the material can grow depending on the matter that is around (Fulvio 132-3).
So what is a potential deviation from the standard model? Alexander Hamilton, from the University of Colorado in Boulder, used computers to find his theory. But he did not initially study black holes. In fact, his area of expertise was in early cosmology. In 1996, he was teaching astronomy at his university and had his students work on a project on black holes. One of them included a clip from Stargate. While Hamilton knew that it was just fiction, it did get the wheels in his head spinning as to what was really happening past the event horizon. He began to see some parallels to the Big Bang (which would be the basis for the hologram theory below), including that both have a singularity at their centers. Therefore, black holes may reveal some aspects of the Big Bang, and possibly lie in a reversal of it by drawing matter in instead of expelling it out. Besides, black holes are where the micro meets the macro. How does it work? (Nadis 30-1)
Hamilton decided to go all in and program a computer to simulate the conditions of a black hole. He plugged in as many parameters as he could find and imputed them along with relativity equations to help describe how light and matter behave. He tried several simulations, tweaking some variables to test different types of black holes. In 2001, his simulations gained the attention of the Denver Museum of Nature and Science, which wanted his work for their new program. Hamilton agreed and took a year-long sabbatical to improve upon his work with better graphics and new solutions to Einstein’s field equations. He also added new parameters such as the size of the black hole, what fell into it, and the angle that it entered the vicinity of the black hole. Altogether, it was over 100,000 lines of code! (31-2)
News of his simulations eventually reached NOVA, who in 2002 asked him to be a consultant on a program of theirs. Specifically, they wanted his simulation to show the journey that matter undergoes as it falls into a supermassive black hole. Hamilton had to make some adjustments to the space-time curvature portion of his program, imagining the event horizon like it was a waterfall to a fish. But he worked in steps (32-4).
First, he tried a Schwarzschild black hole, which has no charge or spin. Then he added charge, but no spin. This was still a step in the right direction despite black holes not processing a charge, for a charged black hole behaves similar to a rotating one and is easier to program. And once he did this, his program gave a result never before seen: an inner horizon beyond the event horizon (similar to the one found when Hawking looked at grey holes, as explored below). This inner horizon acts like an accumulator, gathering all the matter and energy that falls into the black hole. Hamilton’s simulations showed that it is a violent place, a region of “inflationary instability” as put by Eric Poisson (University of Gnelph in Ontario) and Werner Israel (University of Victoria in British Columbia). Simply put, the chaos of mass, energy, and pressure grows exponentially to the point where the inner horizon will collapse (34)
Of course, this was for a charged black hole that acts similar but is not a rotating object. So Hamilton covered his bases and instead got to the spinning black hole, a tough task. And guess what, the inner horizon returned! He found that something falling into the event horizon can go down two possible paths with wild endings. If the object enters in the opposite direction of the black hole’s spin then it will fall into an incoming beam of positive energy around the inner horizon and progress forward in time, as expected. However, if the object enters in the same direction as the black hole’s spin, then it will fall into an outgoing beam of negative energy and move backward in time. This inner horizon is like a particle accelerator with incoming and outgoing beams of energy whizzing by each other at nearly the speed of light (34).
If that weren’t weird enough, the simulation shows what a person would experience. If you were on the outgoing beam of energy, then you would see yourself moving away from the black hole, but to an observer on the outside, they would be moving towards it. This is because of the extreme curvature of space-time around these objects. And those beams of energy never stop, for as the beam’s velocity increases, so does energy and with increasing gravity conditions the velocity increases and etc., until more energy than was released in the Big Bang is present (34-5).
And as if that wasn’t bizarre enough, further implications of the program include miniature black holes inside a black hole. Each one would be smaller than an atom initially but then would combine with one another until the black hole collapses, possibly creating a new universe. Is this how a potential multiverse exists? Do they bubble off inner horizons? The simulation shows they do and that they break away via a short-lived wormhole. But don’t try to get to it. Remember all that energy? Good luck with that (35).
Black Hole Shadows
In 1973, James Bardeen predicted what has been verified by many computer simulations since then: black hole shadows. He looked at the event horizon (EH), or the point of no return from escaping the gravitational pull of a black hole and the photons surrounding it. Some lucky little particles will get so close to the EH that they will constantly be in a state of free-fall, aka orbiting the black hole. But if a stray photon’s trajectory puts it between this orbit and the EH, it will spiral into the black hole. But James realized that if a photon were generated between these two zones instead of going through it, it could escape but only if it left the area on a path orthogonal to the EH. This outer boundary is called the photon orbit (Psaltis 76).
Now, the contrast between the photon orbit and the event horizon actually causes a shadow, for the event horizon is dark by its nature and the photon radius is bright because of the photons escaping the area. We can see it as a bright area to the side of the black hole, and with the generous effects of gravitational lensing magnifying the shadow, it is larger than the photon orbit. But, the nature of a black hole will impact how that shadow appears, and the big debate here is if black holes are cloaked or naked singularities (77).
Naked Singularities and No Hair
Einstein’s general relativity hints at many amazing things, including singularities. Black holes are just one type that the theory predicts. In fact, relativity projects an infinite number of possible types (according to the math). Black holes are, in fact, cloaked singularities, for they are hidden behind their EH. But black hole behavior can also be explained by a naked singularity, which has no EH. The trouble is that we don’t know a way for naked singularities to form, which is the reason why the cosmic censorship hypothesis was created by Roger Penrose in 1969. In this, physics simply doesn’t allow for anything besides a cloaked singularity. This seems highly likely from what we observe but the why part is what troubles scientists to the point that it borders on being a non-scientific conclusion. In fact, September of 1991 saw John Preskill and Kip Thorne make a bet with Stephen Hawking that the hypothesis is false and that naked singularities do exist (Ibid).
Interestingly, another black hole axiom that can be challenged is the no-hair theorem, or that a black hole can be described using only three values: its mass, its spin, and its charge. If two black holes have the same three values, then they are 100% identical. Even geometrically, they would be the same. If it turns out that naked singularities are a thing, then relativity would only need a slight modification unless the no-hair theorem was wrong. Depending on the truthfulness of no-hair, the shadow of a black hole will be a certain shape. If we see a circular shadow, then we know relativity is good, but if the shadow is elliptical then we know it needs a modification (77-8).
Looking at M87's Black Hole
Enter the Event Horizon Telescope (EHT), a planet-wide effort to observe our local SMBH. It makes use of very long baseline imaging, which takes many telescopes around the world and has them image an object. All those pictures are then superimposed upon each other to increase the resolution and achieve the desired angular distance we need. On top of that, the EHT will look at supermassive black holes in the 1-millimeter portion of the spectrum. This is critical, for most of the Milky Way is transparent (does not radiate) this except for our SMBH, making data collection easy (Psaltis 76).
Near the end of April 2019, it finally happened: The first picture of a black hole was released by the EHT team, with the lucky object being the supermassive black hole of M87, located 55 million light-years away. Some concern was raised when Taken in the radio spectrum, it matched the predictions that relativity put out tremendously well, with the shadow and brighter regions as expected. In fact, the orientation of these features tells us the black hole spins clockwise. Based on the diameter of the EH and luminosity readings, M87's black hole clocks ion at 6.5 billion solar masses. And the total amount of data collected to achieve this image? Only 5 petabytes, or 5,000 terabytes! Yikes! (Lovett, Timmer, Parks)
Looking at Sagittarius A*
For many years we did not know if Sagittarius A*, our local supermassive black hole, was truly its namesake or if it was a naked singularity. Imaging the conditions around A* to see if we have this naked singularity was quite challenging. Around the EH, material gets hot as tidal forces pull and tug on it while also causing impacts between objects. Also, galactic centers have lots of dust and gas, which obscure light information, and areas around SMBHs tend to radiate non-visible light. To even look at A*’s EH, you would need a telescope the size of Earth, for it is a total of 50 microseconds of arc or 1/200 of a second of arc. The full moon as viewed from Earth is 1800 arc seconds, so appreciate how small this is! We would also need 2000 times the resolution of the Hubble Space Telescope. The challenges presented here seemed insurmountable (Psaltis 76, Crane).
The EHT looked not only for a black hole shadow but also for hotspots around A*. Around black holes are intense magnetic fields which propel matter up in jets perpendicular to the rotation plane of the black hole. Sometimes these magnetic fields can get jumbled up into what we call a hotspot, and visually it would appear as a spike in brightness. And the best part is that they are close to A*, orbiting at near the speed of light and completing an orbit in 30 minutes. Using gravitational lensing, a consequence of relativity, we will be able to compare with theory how they should look, providing us with another chance to explore black hole theory (Psaltis 79, Crane).
Crane, Leah. "First image of our galaxy's black hole." New Scientist. New Scientist, 21 May 2022. Print. 8.
Fulvio, Melia. The Black Hole at the Center of Our Galaxy. New Jersey: Princeton Press. 2003. Print. 132-3.
Lovett, Richard A. "Revealed: A black hole the size of the solar system." cosmosmagazine.com. Cosmos, Web. 06 May 2019.
Nadis, Steve. “Beyond the Even Horizon.” Discover Jun. 2011: 30-5. Print.
Parks, Jake. "The nature of M87: EHT's look at a supermassive black hole." astronomy.com. Kalmbach Publishing Co. 10 Apr. 2019. Web. 06 May 2019.
Psaltis, Dimitrios and Sheperd S. Doelman. “The Black Hole Test.” Scientific American Sept. 2015: 76-79. Print.
Timmer, John. "We now have images of the environment at a black hole's event horizon." arstechnica.com. Conte Nast., 10 Apr. 2019. Web. 06 May 2019.
© 2016 Leonard Kelley