If you’ve been keeping up with the news lately, you may have come across the following image, which represents a hugely historic moment for our understanding and study of space.
Image of the supermassive black hole at the center of the Messier 87 galaxy as taken by the Event Horizon Telescope (EHT). This planet-scale radio telescope array was made possible by international collaboration and the heroic efforts of scientists at almost 60 affiliated institutes. Photo credit: EHT
Yes, this is the first image of a black hole ever produced… and yes, it is blurry. But I can assure you, the technology required to take this photo is much more impressive than your iPhone’s portrait mode. In fact, the cost of this photo was about the same as 60,000 iPhone X’s. The reason? Because what you see in that image is 1) 55 million light years away from where you are sitting, 2) 6.5 billion times larger than our sun, and 3) cannot be seen with human eyes. Even seeing it in a photograph is a near-impossible feat.
Why was this such a big deal? And what really is a black hole anyway? To answer these questions about such huge universal entities, we have to start a little closer to our level.
Let’s say you want to meet a friend for coffee. You tell them to meet you tomorrow morning at 10am at the Starbucks on 1st Ave and Chesterfield. Whether or not you are aware of it, you are giving your friend 4 pieces of information: three of them are where exactly you will be in 3-dimensional space (coordinates for up/down, left/right and forward/backward), and one of them is at what time. Without any one of these coordinates, I have bad news. Your friend is going to miss your meeting.
These four dimensions comprise spacetime, a term we use to describe the 3 dimensions of space we move around in plus the fourth dimension of time. We therefore, knowingly or not, need four coordinates to describe where and when we are at any position in the universe, at any time.
Objects with increasing mass press down on the plane of spacetime, and this dimpling affects the objects around it. Image credit: Caltech LIGO lab
Okay. So we live in a four-dimensional universe we refer to as spacetime. Now picture this. You and 3 friends are holding a blanket out flat at all four corners, suspended a few feet in the air. This blanket will be spacetime, which, for all we know, is a flat plane like this.
Imagine I throw a baseball into the middle of this blanket. That baseball is the Earth. And the dent it makes in the blanket is exactly what Earth’s mass does. It presses on spacetime and creates a dimple. That dimple is the Earth’s gravity.
Now imagine we throw a basketball onto the blanket, creating a larger dimple. Let’s call this the sun. The baseball, being the smaller (less massive) object, ends up rolling into the deeper dimple formed by the basketball. The baseball (earth) spins around the sun like a tetherball, stuck in the dimple formed by its gravity. Trapped in this dimple, the earth will never “fly away,” to escape the sun’s gravitational pull, because it cannot travel uphill (out of the dimple) without enough force pushing it there.
Summary so far:
Spacetime is the arena in which all interactions between things we know and observe - aka mass - happen, and the result of mass's influence on spacetime is gravity.
Everything in the universe that we experience has mass, and all mass exerts some amount of gravity, causing these dimples in spacetime. For us, our mass is so insignificant that this dimple essentially doesn’t matter, but for planets, suns, and galaxies, it’s a different story. The moon is tethered to the earth because it’s stuck in our gravitational dimple, while the earth rotates around the sun stuck inside its dimple, while the sun rotates around the Milky Way galaxy, stuck inside the dimple of the massive black hole in the center of the galaxy. Ah! I know. Gravity-ception.
In a black hole, a massive object presses down on spacetime until nothing, not even light, can escape. Credit: Istock
However, as your object gets bigger and heavier, it starts to push down on the blanket more and more. As the object’s mass increases, it becomes harder for things to escape. The energy required to push you out of the steep hole is just too great.
At some point, when the object has become massive enough and extremely dense (now there is an almost infinitely deep hole in the blanket), it disappears. In a hole this deep, even light becomes unable to escape the gravitational hole produced by the massive object. Voila. We now have a black hole.
So why can’t we see these “gravity-wells” in space? Light is made up of photons, which do not have mass - this is why they can move at such incredible speeds. Light is therefore not directly affected by the gravitational pull of black holes. But as mass changes the curvature of spacetime, light is affected because the path on which it moves changes. With no mass, light travels across spacetime in “straight lines” along the fabric of spacetime, unimpeded. Larger and larger amounts of mass alter the curvature of spacetime, which change the path that light travels, making it longer. When a massive object presses down far enough on spacetime, the object disappears, because the path that light follows no longer leads out of the hole; it just seems to double back on itself.
A supernova remnant of a star in the constellation Taurus, resulting in the Crab Nebula. Photo credit: NASA
Let’s look at a real-world example. Black holes are usually formed when stars die. A large, dying star first experiences a supernova, a massive explosion that sends much of its mass into space. When the star was alive and burning, the inward contraction of all its mass (that dimpling of the blanket of spacetime caused by gravity) was counteracted by the fact that it was on fire, burning and constantly pushing mass away from itself. That balance between its intense gravity and being lit by the fires of nuclear fusion kept the star stable.
However, after the star explodes into a supernova and the fires of nuclear fusion go out, all of the star’s leftover matter collapses in on itself under the weight of its own gravity.
As more and more star stuff packs into a smaller and smaller space, the dying star’s gravity pushes down harder and harder on the blanket of spacetime. To give you a sense of the mass we’re working with here, try to imagine an object ten times bigger than our sun packed into an area the size of Chicago (that’s a lot of rock). This scenario often happens when a large star dies, and creates a big enough hole in spacetime that even light cannot escape. We have ourselves another black hole.
Considering the size of a star required to create a black hole is not that much bigger than our own (about 3 times larger), it is likely that the formation of black holes happens pretty often. We have identified many of these stellar mass (star-born) black holes when stars or clouds of gas get close enough to be sucked into the black hole’s gravity, emitting light that we can see. But most of these black holes will remain invisible and undiscovered. Scientists estimate that there could be as many as a billion stellar mass black holes in our galaxy alone!
Now, light is the fastest thing we know of, and it’s what allows us to see everything in the universe. But because no light escapes from a black hole, and they are essentially invisible against the blackness of space (they don’t emit or reflect light), we are clueless as to what is inside.
A black hole can siphon off material from stars and nebulae when it comes too close. Image credit: Chandra X-Ray Observatory
To see them, we have to look at the effects they have on the objects around them. If something gets close to the event horizon of a black hole – or the border before which nothing, not even light, can escape – it will be slowly pulled into the gravitational hole created by the object like water swirling down a drain.
When a black hole comes across a neighboring star, for example, it can pull the star’s material into it, tearing the star apart. The light emitted on the horizons of black holes as they swallow the matter of stars, a process called accretion, can create some of the brightest displays we have ever seen in the Universe.
Because of these limits and the fundamentally elusive nature of black holes, capturing a photo of one took some serious thinking outside the box. The Event Horizon Telescope project was an undertaking by several nations to link their telescopes together to form an “Earth-sized” telescopic lens and ultimately capture the image of the supermassive black hole called Messier 87, a whopping 55 million light-years away (for reference, a light-year is the distance that light travels in a year and this distance is more than 300,000,000,000,000,000,000 miles from us). *Insert Death Star reference here*
The Submillimeter Telescope in Tucson, AZ is one of eight radio telescopes used in the EHT. Photo credit: David Harvey
The South Pole Telescope in Antarctica is another player in the EHT array, and the most remote. Photo credit: Junhan Kim
These eight radio telescopes were found in places like Hawaii, Spain, Chile, Mexico, Arizona and Antarctica, and for the “photo capture” to work, the weather had to be perfectly clear in all of these places at the same time. This wasn’t the first time a little bit of luck was needed to make good science happen, and it won’t be the last.
For this project, unlike my camping trip last weekend, the weather gods were on our side. The result? Our first real visualization of a black hole, a feat that was once thought to be impossible. The orange halo you see in the photo is the swirling, hot gas surrounding the black hole; somewhere in that endless void, a supermassive object has pushed the fabric of spacetime to the brink.
So what could be inside a black hole? We will likely never know. Humans, equipment, or any matter wouldn’t survive the trip across the horizon, at least in the same shape or composition. We do have some theories, though. An object dense enough to create a supermassive black hole may very well tear a hole in the blanket of spacetime, creating a wormhole to another part of the universe, or to somewhere far weirder. For all we know, in a black hole the laws of physics we are familiar with might not apply, and things like the speed of light, the passage of time, and the fabric of the dimensional world could be entirely different.
These computer-generated images created for the movie Interstellar were surprisingly accurate renditions of what we now know black holes to look like. Credit: Interstellar
In these illustrations, the event horizon surrounding the “singularity,” or object with infinite mass, represents the boundary beyond which light cannot escape the singularity’s gravity.
One thing we do know: anywhere near a black hole, finding your friend for coffee using the traditional coordinates of spacetime would be a challenge you don’t want to undertake. For now, we keep to our immaculately defined, beautifully unsurprising little corner of the Universe, protected by the unbreakable laws of physics as we tend to our gardens, and take photos of the distant sky.
-Jason
