By now, I'm sure everyone has seen the picture of a black hole so let's look at what is really interesting about it.
First, it isn't really a picture in the traditional sense. All the data was taken from radio telescopes, not optical. So those aren't even the real colors. Radio doesn't have color. It is a color code to illuminate some specific aspect of the radio received -- maybe wavelength, maybe intensity, probably temperature, but it isn't really clear.
The actual accomplishment here, and it is a stunning one, is the technique. It is called very long baseline interferometry.
Interferometry has been around for a long time. Especially in the radio spectrum, resolution (the ability to see neighboring objects as two distinct objects rather than an amorphous blob) depends is proportional to the wavelength of the radiation and inversely proportional to the diameter of the telescope objective. My former astronomy pals should remember this.
Bigger is worse. And radio has a very long wavelength so its resolution is spectacularly bad. You can beat that down by making the diameter of the radio telescope bigger, but that has practical limits. At some point, you just can't maintain the shape of the dish accurately enough because of gravity. It sags. So you either have a limit on the size of the dish, or you give up the idea of being able to steer it so you can put supports under the dish. The 300 meter telescope at Arecibo, Puerto Rico is one of these. It can only see whatever the Earth happens to be pointed toward at that moment.
Interferometry is a way of faking a big dish. Let's say you have two telescopes 1000 miles apart and you have both of them look at the same thing at the same time. You bring the two sets of data together, match the times, and add. Since they are looking at the thing from different directions they get slightly different readings and that allows you to see variations in intensity that one telescope alone would miss.
But to get an image of the sort you've seen, you can't just stare into space. You have to simultaneously scan across a tiny region of the sky. So you have to coordinate the movements of two dishes as they shift tiny fractions of a degree and you need a mechanism capable of moving that big thing a tiny fraction of a degree. The coordination becomes more difficult the further the telescopes are from each other. You can't just send them both a signal saying "Move now." That's limited by the speed of light on a dedicated line, so your command center would have to be located exactly the same distance from each, to a precision of millimeters or better. If you try to use something like the Internet you might as well not bother. This is obviously a significant engineering challenge.
Then there is the challenge of having a mechanical system capable of changing the direction of a 300 meter dish that weights hundreds of tons by a tiny fraction of a degree. I'd be very surprised if this black hole was more than an arc second across. Break out your old high school protractor. Look at how far apart the degree marks are. Now imagine dividing that into 60 equal size pieces. That's an arc minute. So now take one of those 60 pieces and divide it into 60 pieces. That's an arc second. Now you have to scan across that arc second in even smaller steps to generate the image we saw. That's obviously a significant engineering challenge.
Then there is time stamping the data. You can't just do that with your watch, or even a stopwatch. You have to do it with an atomic clock. Those are cheaper than they used to be thanks to the military. Our need to drop precision guided munitions on various people's heads has led to the development of an atomic clock on a chip for about a thousand dollars. GPS works by comparing the time where you are to the time broadcast by a satellite. The bigger the difference the further you are from that satellite. Do that with several satellites and you pinpoint your location. You really need a better clock than one of those but they're still not prohibitively expensive.
The hard part comes in synchronizing the clocks so that they give the same time readings. You might think well, just take them to the same place, synchronize them there, then carry them to where they need to be.
That won't work. They have to be moving to do that, and the relativistic time dilation effect is sufficient to make the readings change. That's how accurate atomic clocks are. They were used to experimentally verify that time dilation is real. So clock synchronization is obviously a significant engineering challenge.
What is impressive about this image is not so much the image itself, but the fact that the team of astronomers was able to solve this challenges sufficiently well with telescopes so far apart that they faked a telescope the size of the whole Earth (about 5000 miles across). Eight telescopes, in fact. And some of those eight telescopes are themselves an array of telescopes creating a smaller interferometer. You can't get bigger than that without going to space, where there are other engineering challenges.
It took 20 years to get there. The Event Horizon Telescope Team is led by Shep Doeleman, an astrophysicist now based at Harvard. In an interview a few years ago, he said. “You have to synchronize all the measurements to within a microsecond, and save the signals so that you can compare them and combine them. You also need to know the geometry of the Earth and the locations of the telescopes to within a centimeter.”
And you haven't contended with the weather yet.
Pictured below: one of the telescopes involved, the Atacama Large Millimeter Array (ALMA) in Chile. Also the locations of all the telescopes involved.
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