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Everything We Know and Don't Know About Black Holes

March 26, 2018


26,000 light years away, a supermassive black hole is spinning at the center of our galaxy. A black hole as wide as the orbit of Mercury. A black hole more than 4,000,000 times as massive as the sun. And everything in the galaxy is swirling around it.


So, does that mean that a black hole is dragging everything into it until there is nothing left, like a cosmic drain centered in the great space bathtub that we call the Milky Way?


In short, no. But black holes have gotten a bad rep, so lets set some things straight.


A black hole is just an object so dense that as you approach it, even light will contort and bend until eventually, even it can't escape. And since nothing travels faster than the speed of light, this just means absolutely nothing can escape. While that might sound a little bit terrifying to you, the first thing you should know is that there is a black hole sitting at the center of the galaxy we live in (the Milky Way). It is named Sagittarius A, and is actually not so terribly big... for its kind.


Black holes were first proposed by natural philosopher John Michell in 1783, and generally divided into two classes that are hugely different in size. First, there are stellar-mass black holes, which have the mass of several to maybe a few dozen suns. These are formed from supernova explosions at the end of giant star lifetimes.


Then, there are supermassive black holes, and I don't mean the song written by Muse. I'm talking quite literally super massive black holes, like Sagittarius A. Astronomers don't know for sure how supermassive black holes form or why they shrink, but ours is actually pretty small, with just four million solar masses; many are more than 100 million solar masses.


But the much more important fact is that a black hole's gravity doesn't just suck things in like some sort of cosmic vacuum cleaner. Most things in our galaxy are so far away from it, that they barely even feel the black hole's gravitation.


In fact, here on Earth, the gravitational pull between a person standing 30 feet away from you is 10,000 times stronger than the gravitational pull of our galaxy's distant supermassive black hole.


And even objects that are a lot closer to it will most often just fall into an elliptical orbit around it. The fact is: our galaxy is orbiting Sagittarius A just like we are orbiting the Sun, but we won't get drawn into it.


Now, I'm not saying that hanging out near a black hole would be pleasant or even comfortable... In fact, I don't recommend it at all.


This is partly because of the effect created by clouds of gas that form around black holes, called accretion disks. These gasses become superheated, producing strong emissions at x-ray and radio wavelengths. So in the immediate area of a typical supermassive black hole, there is a lot of rather unfriendly high-energy radiation flying around.


Another concern are tidal forces. These forces are exerted because every planetary body has one side of it closer to the black hole than its other side. Take for example your own body and the ground. Unless you're doing a handstand, your feet are closer to the ground than the top of your head, right? Well, that closer side feels a stronger gravitational pull than the far side does, and the closer that body gets to the massive object, the bigger the difference becomes.


Eventually, this difference becomes so great that it causes the body to stretch out.


The gravitational pull of the Sun and Moon on Earth are also what cause waves in the ocean. These waves are tides, or in other words: tidal waves.



Now, the boundary that stakes out the maximum danger zone around any black hole is its event horizon. This is the distance from the center where the escape velocity equals the speed of light.


The side that's closer to the black hole would begin to accelerate faster than the farther side, creating an inescapable stretching force that physicists rather delightfully refer to as spaghettification - yes, that's an actual word.


Another misconception about black holes is whether or not they are actually black. Now, this entire post I have been calling them black holes. And yes, it is an educated guess to say they are black because light cannot escape them, right? But one of Stephen Hawking's most important contributions to space was his application of quantum mechanics.


As noted in my previous post: Black holes actually emit radiation, which is weird, right? An object with gravity so strong even light can't escape it, yet it releases radiation. This energy today has become known as Hawking radiation.


But with the application of E = MC^2, the energy going away from the black hole must be coming from whatever content lies inside the black hole. For all we know, black holes can be made of anything: stars, light, dust, even unicorns. The only way we would know for sure is if we took a look at the information within the radiation, and the problem scientists are facing now, is interpreting this information.


During a Caltech Q&A panel I participated in with postdoctoral black holes and general relativity researcher, Leo Stein, I asked: What is the current status on what we can gather from Hawking radiation, and what are we able to do with our progress?


His response: "A lot of people have worked on this. It's possible that the radiation that's coming out is actually what's called quantum entangled with other radiation. So if you could gather all of the information and sort of reassemble it in a certain way, then you would have the information that fell into it.


Unfortunately, the current understanding is that it can't just be entangled with other radiation. There's not enough entanglement possible. And if you did map out how much entanglement is possible, you'd find there's way more entropy. So there's still an information problem, but we're working on it." 


Nonetheless, we don't have to worry about any of that because our galaxy is orbiting Sagittarius A. Even if we replaced the Sun with a black hole, Earth would not be swooped away like some noodle into some cosmic vacuum cleaner.


All we can do for now is look up, question, and strive to learn more about these "spooky things," as Albert Einstein so called them.


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"Black Holes." NASA. Web.

Green, Hank. "The Black Hole at the Center of Our Galaxy." SciShow Space, YouTube. 25 March, 2018. Web video.

Stein, Leo. "The Truth About Black Holes." Caltech. 23 March, 2018. Lecture.

"Why Does the Ocean Have Waves?" National Ocean Service. Web.

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