It’s the technique, from the Event Horizon Telescope, that brought us a black hole’s image. Here’s how it works.
The Event Horizon Telescope has accomplished what no other telescope or telescope array has ever done: imaged the event horizon of a black hole directly. A team of more than 200 scientists using data from eight independent telescope facilities across five continents all joined together to achieve this monumental triumph. While there are many contributions and contributors that are well-deserving of being highlighted, there’s a fundamental physics technique that it all depended on: Very-Long-Baseline Interferometry, or VLBI. Patreon supporter Ken Blackman wants to know how that works, and how it enabled this remarkable feat, asking:
You’re on; let’s do it.
For a single telescope, everything is relatively simple. Light comes in as a series of parallel rays, all originating from the same distant source. The light strikes the telescope’s primary mirror, and gets focused to a single point. If you put an additional mirror (or set of mirrors) along the light’s path, they don’t change that story; they simply change where that light winds up converging to a point.
All of those light rays arrive to that final point at the same time, where they can then be either combined into an image or saved as raw data, to be processed into an image at a later time. That’s the ultra-basic version of a telescope: light arrives from a source, gets focused into a small region, and recorded.
But what if you don’t have a single telescope, but multiple telescopes that are networked together in some sort of array? You might think that you could just go about the problem in a similar way, and focus the light from each telescope the way you’d do it for a single-dish telescope. The light would still arrive in parallel rays; each primary mirror would still focus that light down to a single point; each telescope’s light rays arrive at the final point at the same time; all that data can then be collected and stored.
You could do that, of course. But that would only give you two independent images. You could combine them, but that would only average the data out. It would be as though you observed your target with a single telescope at two different times, and added the data together.
That doesn’t help you with your big problem, which is that you need the critical enhanced resolution that comes with using a network of telescopes linked together with VLBI. When you successfully link multiple telescopes together with the VLBI technique, it can give you an image that has the light-gathering power of the individual telescope dishes added together, but (optimally) with the resolution of the distance between the telescope dishes.
This technique has been famously used many times, not merely for imaging a black hole and not even with radio telescopes alone. In fact, perhaps the most spectacular example of VLBI was used by the Large Binocular Telescope, which has two 8-meter telescopes that are mounted together, behaving with the resolution of a ~23-meter telescope. As a result, it can resolve features that no single 8-meter dish can, like erupting volcanoes on Io while it experiences an eclipse from another of Jupiter’s moons.
The key to unlocking this type of power is that you need to be able to put your observations together at the same moments in time. The light signals that are arriving at the telescopes are arriving after slightly different light-travel-times, owing to the varying distance, at the speed of light, that it takes the signal to travel from the source object to the varying detectors/telescopes on Earth.
You must know the arrival time of the signals at the different telescope locations across the world in order to be able to combine them together into a single image. Only by combining data that corresponds to viewing the same source simultaneously can we achieve the maximum resolution that a network of telescopes is capable of offering.
The way we do this, practically, is by making use of atomic clocks. At every one of the 8 locations across the globe where the Event Horizon Telescope takes data is an atomic clock, which enables us to keep time to precisions of a few attoseconds (10-18 s). There was also the need to install specialized computational equipment (both hardware and software) to enable the observations to be correlated and synced up between the different stations around the world.
You have to observe the same object at the same time with the same frequency, all while correcting for things like atmospheric noise with a properly-calibrated telescope. It’s a labor-intensive task that requires enormous precision. But when you get there, the payoff is astounding.
The above image might look like it has nothing to do with a black hole, but it’s actually one of the most famous images from the most powerful single array of radio telescopes out there: ALMA. ALMA stands for the Atacama Large Millimetre/Submillimetre Array, and is composed of 66 independent radio dishes that can be adjusted to be spaced apart from 150 meters all the way up to 16 kilometers.
