The First Ever Image of a Black Hole

Astronomers across the world are gearing up this week to make a bold attempt at taking the first ever image of a black hole. This daring idea will require the international cooperation of a telescope the size of the Earth and has the potential to turn everything we know about gravity on its head.

So what does a black hole actually look like? What telescope can make such an observation? And what could possibly go wrong?

What does a black hole look like?

Astronomers believe that the majority of galaxies like our Milky Way host supermassive black holes at their centers. Supermassive is really an official term and suggests masses from hundreds of thousands to a billion times the mass of our Sun. The supermassive black hole at the center of our galaxy, known as Sagittarius A* is 4 million times the mass of our Sun crammed into a space with a diameter less than roughly a third of the distance between the Earth and the Sun.

Despite their expected ubiquity and the fact that we have a supermassive black hole in our own galactic backyard, no one has ever imaged a black hole directly. That’s because, as the name implies, they are black meaning that they are so dense that even light cannot escape. Einstein’s equations of general relativity predict that the strong gravitational pull around such dense objects will warp the space around them, causing light to follow a curved path.

The German astronomer Karl Schwarzschild further determined that, as you get closer and closer to a black hole, this warping will eventually become so strong that light will bend back inward towards the black hole, unable to escape. This boundary that marks the point of no return is known as the event horizon or, in some cases, the Schwarzschild radius. Despite what Laurence Fishburne may tell you, there’s no coming back once you have passed beyond a black hole’s event horizon. Although you can check out my previous episode on what happens when you fall into a black hole.

Even without direct observations, we can observationally deduce the presence of a black hole in a few different ways. Tracking the motions of stars orbiting the center of our galaxy, for example, reveals that a massive yet optically invisible object must lurk there in order to explain how fast those stars are moving. Much of this work has been led by Astronomer Andrea Ghez and you can watch the animations of these stellar motions around our galaxy’s central supermassive black hole at her research group’s website.

We can also view supermassive black holes indirectly if they are actively accreting material. As matter falls into a black hole, it forms a very hot, swirling disk. Newly added material will emit high energy photons as it hits the disk, in the form of X-rays which can then be observed. And, of course, an entirely new method of observing black holes was introduced just last year when the first gravitational wave detection revealed two merging intermediate-mass black holes.

But all of these methods only offer an indirect look at the black hole. An experiment running from April 5th–April 14th will attempt to directly image the event horizon of our own black hole, Sagittarius A*.  As material like gas and dust approaches the black hole, it speeds up and emits energy which can be observed as a ring or crescent at the location of the event horizon. The black hole is expected to cast a shadow on a portion of this ring, and that shadow holds clues to the mass and size of the black hole that lurks within. The shape of that shadow is further predicted by Einstein’s theory of general relativity to be circular, a prediction that these observations can test directly.

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What telescope can observe a black hole?

So what telescope can even make this observation? Astronomers will require a telescope as large as the Earth, something they will achieve by observing simultaneously with multiple telescopes spanning the globe from the South Pole to Arizona and from Hawaii to Spain. This array of telescopes, which as of recently includes the Atacama Large Millimeter Array in the Chilean desert, is collectively known as the Event Horizon Telescope. All of these telescopes operate at 1.3 millimeters, or, in other words, will collect this data at radio wavelengths which are capable of penetrating the thick dust known to surround our central black hole.

Astronomers will require a telescope as large as the Earth.

Data collected at each telescope will be stored on hard drives which are then jetted to the Haystack Observatory near Boston. There the signals will then be combined via a supercomputer with the equivalent of ~800 CPUs. For more on how astronomers combine multiple smaller telescopes to act as one large telescope, check out my guest spot on the Titanium Physics podcast on interferometry.

The reason so many telescopes are required is because, given perfect observing conditions, the angular resolution achievable by a telescope is related to its size. The larger the telescope, or in this case, the larger the distance between individual telescopes in the telescope array, the finer detail that can be seen on the sky. The upcoming observation is expected to reach a resolution of <50 microarcseconds which is the equivalent of you being able to read the writing on a quarter sitting on a table in Los Angeles from an apartment in New York City. For comparison, that resolution is also about 2,000 times better than what the Hubble Space Telescope can see.

What could go wrong?

Although observations at radio wavelengths have the best chance of piercing through the dusty surroundings of the black hole, water vapour in the Earth’s atmosphere can still affect the clarity of the resulting images. The involvement of so many different telescopes thus requires that the weather be clear in every one of the locations.

Each data set is time stamped locally before being sent to the supercomputer where they will be processed together. Ultimately combining the information from the various telescopes requires extremely accurate atomic clocks, clocks that are custom built and rely on hydrogen masers. These clocks have to get the timing right down to a trillionth of a second per every second.

The amount of post-observing processing is so extensive that the final image is not expected until 2018. But it will have large implications for our understanding of something as fundamental as gravity.

Do event horizons actually exist? Are our theories on how light behaves in the presence of extreme gravity correct? How do black holes feed themselves through accretion? How are the large scale jets of collimated plasma that astronomers observe formed in the surroundings of the black hole?

If the experiment works works, the astronomers behind the Event Horizon Telescope project plan to move onto the supermassive black hole in the center of the giant elliptical galaxy M87. Although much farther away, this black hole is estimated to be 6 billion times the mass of our Sun, or 1,500 times the mass of Sag A*. So was Einstein right? Stay tuned …

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

Image courtesy of eventhorizontelescope.org