By JoEllen McBride, PhD
Light can’t escape it but Matthew McConaughey can use it to ‘solve gravity’. They’re the most massive things in our universe but we can’t actually see them. Black holes were theorized by Einstein in the early 1900s and have intrigued both scientists and the public for over a century. Up until recently, we could only see their effects on visible matter that gets too close but an Earth-sized telescope is about to change all that.
The term black hole sounds silly but it’s pretty descriptive of this invisible phenomenon. Astronomers call things black or dark because we can’t actually see them with current technology. Black holes form when a star is so massive that its own force of gravity pushes in harder than the molecules and atoms that make it up can push out. The star collapses; decreasing its size to almost nothing. But matter can’t just disappear so this incredibly small object still has mass which can exert a gravitational influence on stars or gases that get too close. If our Sun became a black hole out of nowhere (don’t worry, this can’t happen), the Earth and other planets would not notice a difference gravitationally. We’d all continue orbiting as before; things would just get a lot colder. I guess that’s one way to wash away the rain.
So that’s how a single black hole forms but you’ve probably heard references to ‘supermassive’ black holes before. These black holes have masses of many millions or billions of suns. So what died and made that massive of a ‘hole’? Supermassive black holes are not the product of a single object but are most likely formed by the merging together of many smaller black holes. We recently found evidence of this process from the ground based gravitational wave detector, LIGO, which can detect the waves that are produced when two smaller black holes merge. We also know supermassive black holes exist because we have seen their influence on other luminous objects such as stars and gas that’s been heated. We see jets of gas being shot out of the centers of galaxies at close to light speed. There is something incredibly massive at the center of our own galaxy that causes stars nearby to orbit at incredible speeds. The simplest explanation for these observations is that galaxies have supermassive black holes at their centers.
But there is another way we could ‘see’ a black hole which was impossible before this year. As stated before, light cannot escape a black hole but anything that becomes trapped in the gravitational well has to orbit for some time before it disappears. So there must be a point where we can still see material just before it’s lost forever; like an object that swirls around the edge of a whirlpool just before falling down the drain. This region is known as the event horizon and it’s basically the closest we can get to seeing a black hole. Currently, the supermassive black hole at the center of our galaxy, named Sagittarius A*, isn’t taking any material in but that doesn’t mean the event horizon is empty. Luminous material can orbit in the event horizon for a very long time, we just need to look at the right wavelength with a big enough camera.
The center of our galaxy is 8 kiloparsecs or 1,50,000,000,000,000,000 miles away. To put that in perspective, that’s about 1014 times larger than the distance between the U.S. coasts, 1011 times larger than the Earth-Moon distance and 100,000 times larger than the distance to the next closest star, Alpha Centauri. It’s really far away. The width of Sagittarius A*’s event horizon is estimated to be between the width of Pluto and Mercury’s orbit around our Sun. At its widest estimate, the event horizon of Sagittarius A* would span one-millionth of a degree on the sky. For comparison, the full moon spans about half a degree. So we’re gonna need a bigger telescope– an Earth-sized one.
Enter the Event Horizon Telescope (EHT). This network of telescopes operates at radio wavelengths and uses a technique that increases the size of a telescope without having to build a huge dish. The EHT combines telescopes in Arizona, Hawaii, Mexico, Chile, Spain and the South Pole to create an Earth-sized radio dish. A good analogy I’ve found is to picture you and five other friends are standing at various locations at the edge of a pond. You all know where you are located with respect to each other and the pond surface. Each of you also has a stopwatch and placed a bobber in the water directly in front of you. If a pebble gets dropped somewhere in the middle of the pond each of you will wait until you see the bobber start moving and begin recording the time and the up and down motions the bobber makes as the peaks and troughs of the wave passes by. After you’ve recorded enough bobs, you can meet back up with your friends to determine where the pebble was dropped and its size based on the ripples and when they reached each of your respective locations. The EHT will work similarly except the friends are telescopes pointed at Sagittarius A* and the water ripples are light waves.
Over 10 days at the beginning of April, these telescopes were in constant contact, monitoring the weather at each site, to coordinate their observations as best they could. Radio waves can usually penetrate everything but the wavelengths that these telescopes were looking at are blocked by water vapor, so clouds and rain mean no observing. On April 15th, they finished their run by successfully obtaining 5 days worth of observations. Now each site has to mail hard drives with their data to a central location, where the images can be properly aligned. The South Pole Telescope can only send out packages after their winter season ends in October, but data is already coming in from the other sites.
If everything went as planned, the images should add up to the highest resolution images ever taken of a black hole. This arrangement allows them to measure objects as small as a billionth of a degree. The estimated size of Sagittarius A*’s event horizon is larger than this, so a faint ring surrounding darkness should be visible in the final images. Hopefully, Sagittarius A* was ready for its close-up because humans are eager to see how their own depictions of black holes match up.