What ARE Black Holes?

JS Communications Skill

Seán Hogan

Black holes are a common concept in Astrophysics, and are one of the celestial bodies (an object in space) that nearly all people on Earth are familiar with. A large planet that sucks in all the surroundings in an inescapable vortex. Well, that is where the science-fiction description of black holes overpowers the physically real one. In this blog post, I am going to describe black holes as they are, to the best of the scientific community’s knowledge. As well as tackle some common misconceptions about black holes. So hopefully by the end of this, you will be able to help your friends from getting sucked into an inescapable vortex of misinformation.

So… what IS a black hole?

A black hole is a celestial body where its escape velocity is greater than the speed of light. Which might seem like I have just used another bit of physics jargon to explain black holes, but I promise, this is pretty easy to understand.

Escape velocity is defined by the formula below, where G in Newton’s Gravitational Constant, a value which is the same for every planet, M is the mass of the planet and r the radius of the planet. Escape velocity is the minimum speed an object must constantly travel at to escape the influence of gravity of an object. You can see that the mass of the object trying to escape the gravitational pull of a planet or celestial body is not used in the formula, only properties of the planet itself.

The Formula for Escape Velocity

So in order for the escape velocity to greater than the speed of life, the ratio between the mass of a celestial body and its radius needs to me 6.74 x 10²⁶kg for every metre of radius. Numbers at this scale can be hard to comprehend, so hopefully to give context; Earth has this ratio at 9.37 x 10¹⁷ and our Sun has this ratio at 2.86 x 10²¹. Don’t leave the fact that 21 is only 5 less than 26 make you think those two values are close, the Sun would need 100,000 times the amount of mass. To put it into better perspective, in order for Earth to be a black hole, it would need to be a sphere of radius 8mm.

Could a black hole appear right by the Earth and suck us all in and destroy life as we know it?

No. That will not happen. If humanity is wiped out from something from the stars, it will not be a random black hoe appearing. In order for something of such an incredible density to be created, random chance is not enough. Most black holes form with the death of stars. As a star dies, its own gravitational force has the star collapse in on itself, called a supernova. So a star’s death is needed to form a black hole, the circle of life. The only star of notable proximity to us is, well the Sun. Our Sun is not approaching the end of its lifetime and when it does, it will not have enough mass to turn into a black hole.

An Artist’s Depiction of a Supernova

Okay, but they do suck you in, right?

Well, yes and no? All celestial bodies have some mass, meaning they have some gravity, which means they “suck” you in (in physics we would generally say “attracts you”). This idea that black holes are dragging everything towards them to absorb and destroy them is plain incorrect. In order to get to the point where you get taken by the black hole, you would be at a distance that means you would be brought down to the surface of any planet of equivalent mass. To give perspective, at the centre of the Milky Way there exists a supermassive (meaning at least 100,000 times the mass of the sun) black hole that the rest of the galaxy orbits. Or if you prefer, if I was to replace every planet, every moon and the Sun in our solar system with black holes of similar masses, Earth’s trajectory would remain exactly the same. So be very careful when travelling near a black hole, as crossing the event horizon, the point where light can no longer escape might not even be noticed by you, looking out of a black hole looks completely normal. But then again if you decide to play with fire and go anywhere near a black hole, it is kind of your own fault.

Fair enough, what happens in a Black hole?

Well, that is where the knowledge of astrophysicists ends unfortunately. Black holes are incredibly complex objects. Since no light can escape after it passes the Schwarzschild radius (the point where even light cannot escape), no information about what is inside can be transferred back, it is physically impossible for us to know what happens in black hole. Some physicists use the term “hairless” to describe that very little information we can gleam from a black hole. The main way we can even figure out they are there are from their gravitational effect on other celestial bodies. However, black holes are theorised to slowly lose mass via a very complicated theory called Hawking Radiation. So given enough time, we should see what is left after a black hole, but for black holes of a size large enough they could be observed by humans, that timeframe is too long. I am talking about a number with over 60 zeros, for the smaller black holes. That is even larger than physicists believe the universe has existed for, by nearly by a factor of a number with 50 zeroes! Hopefully some time far, far, far in the future, some black hole will fully evaporate, and humanity (or what comes after us) can see the traces of those dark voids.

I saw that film Interstellar, doesn’t time go slower near a black hole?

That is a really, REALLY difficult question. But that’s good! Asking difficult questions are exactly what fields like astrophysics are all about. Regardless, I doubt I will be able to give you a satisfactory answer but it is to do with the general relativity model of time, or spacetime rather. Trying my absolute best here to explain what I know briefly and clearly, in most of our models regarding time, space and time are intrinsically linked. Objects will larger masses are able to “warp” space around them, like placing a heavy object on a trampoline (this is also how gravity is somewhat explained). Since space gets warped severely by this massive objects, so will time. That is brushing over a lot of  details so I highly recommend going out and doing your own bit of reading and research on the topic.

That was a lot, is there any fun quick facts I could tell friends at parties about Black holes?

Okay, since you were so good to read the entire way through my blog. I will tell you what people theorise happens if you fall in a black hole. It is called Spaghettification. Simply put, if you were to go in feet first, like a slide, you would be stretched out thin like a piece of spaghetti. Your feet would be experiencing a stronger gravity force than you head, so will be pulled faster and will stretch out your body. Cool but a little body-horror-esque if you try and work out the details of the process.

Spaghettification Pictured. It probably would feel amazing for a brief period

Thank you very much for reading, I hope you go further and do further research on space. There truly is no limit to the cosmos, and our Earth is only one tiny part of it.

References for Further Reading

[1] H. R. Smith, “What Is a Black Hole?” Accessible from https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html Aug 2018

[2] L. Lerner, “Black holes, explained” Accessible from https://news.uchicago.edu/explainer/black-holes-explained#:~:text=Black%20holes%20are%20regions%20in,not%20even%20light%20can%20escape. Oct 2022

[3] D.C. Gibson, “Black holes” TERRESTRIAL AND EXTRATERRESTRIAL SPACE DANGERS: OUTER SPACE PERILS, ROCKET RISKS AND THE HEALTH CONSEQUENCES OF THE SPACE ENVIRONMENT, pg. 73-82, 2015