Now that we’ve seen our first one, we want more, and we want them better. Here’s how to get there.
To resolve any astronomical object, you must achieve resolutions superior to the apparent size of your target.
Shredded material accretes onto a black hole, gets absorbed or kicked out, and can re-form into planet-mass objects relatively quickly. In order to resolve the ‘hole’ in the center of this gas, the number of wavelengths that can fit across your telescope diameter must correspond to a sharper resolution than the apparent angular size of the ‘hole’ itself. (B. SAXTON (NRAO/AUI/NSF)/G. TREMBLAY ET AL./NASA/ESA HUBBLE/ALMA (ESO/NAOJ/NRAO))
The largest black holes, as viewed from Earth, possess event horizons merely tens of microarcseconds (μas) in angular size.
The Event Horizon Telescope’s first released image achieved resolutions of 22.5 microarcseconds, enabling the array to resolve the event horizon of the black hole at the center of M87. A single-dish telescope would have to be 12,000 km in diameter to achieve this same sharpness. (EVENT HORIZON TELESCOPE COLLABORATION)
A telescope’s resolution, meanwhile, is fundamentally determined by how many wavelengths of light fit across its physical diameter.
This composite image of a region of the distant Universe (upper left) uses optical (upper right) and near-infrared (lower left) data from Hubble, along with far-infrared (lower right) data from Spitzer. The Spitzer Space Telescope is nearly as large as Hubble: more than a third of its diameter, but the wavelengths it probes are so much longer that its resolution is far worse. The number of wavelengths that fit across the diameter of the primary mirror is what determines the resolution.(NASA/JPL-CALTECH/ESA)
We can surpass that limit by leveraging an array of telescopes,
using the technique of very-long-baseline interferometry.
The Atacama Large Millimetre/submillimetre Array, as photographed with the Magellanic clouds overhead. A large number of dishes close together, as part of ALMA, helps bring out many of the faintest details at lower resolutions, while a smaller number of more distant dishes helps resolve the details from the most luminous locations. The addition of ALMA to the Event Horizon Telescope was what made constructing an image of the event horizon possible. (ESO/C. MALIN)
By properly equipping and calibrating each participating telescope, the resolution sharpens, replacing an individual telescope’s diameter with the array’s maximum separation distance.
This diagram shows the location of all of the telescopes and telescope arrays used in the 2017 Event Horizon Telescope observations of M87. Only the South Pole Telescope was unable to image M87, as it is located on the wrong part of the Earth to ever view that galaxy’s center. Every one of these locations is outfitted with an atomic clock, among other pieces of equipment. (NRAO)
At the Event Horizon Telescope’s
maximum baseline and wavelength capabilities, it will attain resolutions of ~15 μas: a 33% improvement over the first observations.
All of these images of the same target were taken with the same telescope (Hubble), but are at increasing wavelengths as you go from left to right. That is the reason why they have higher, sharper resolutions on the left. The leftmost images also have a higher frequency as well as a shorter wavelength; in the radio portion of the spectrum, we often talk about frequency instead of wavelength, for mostly historical reasons. (NASA, ESA, AND D. MAOZ (TEL-AVIV UNIVERSITY AND COLUMBIA UNIVERSITY))
Currently limited to
345 GHz, we could strive for higher radio frequencies like 1-to-1.6 THz, progressing our resolution to just ~3-to-5 μas.
This photograph shows the Russian Spektr-R (RadioAstron) space-born radio telescope at the integration and test complex of Launch Pad №31 at the Baikonur Space Center. This is presently our largest, most powerful radio telescope in space. If we outfitted an array of telescopes like this with the equipment necessary to sync them up with the rest of the Event Horizon Telescope, we could extend our baseline to hundreds of thousands of kilometers. (RIA NOVOSTI ARCHIVE, IMAGE #930415 / OLEG URUSOV / CC-BY-SA 3.0)
But the greatest enhancement would come from extending our radio telescope array into space.
The Earth-Moon distances as shown, to scale, relative to the sizes of the Earth and Moon. This is what it looks like to have the Moon be approximately 60 Earth radii away: the first ‘astronomical’ distance ever determined, more than 2000 years ago. Note how much longer a baseline the Earth-Moon distance would give us compared to simply the diameter of Earth. (NICKSHANKS OF WIKIMEDIA COMMONS)
Outfitting them with atomic clocks and rapid data downlinks could extend our baseline to the size of the Moon’s orbit.
When material is devoured by a black hole, it will heat up and emit radiation in a variety of wavelengths. While our first image of a black hole’s event horizon came from observing at a frequency of 230 GHz and with a baseline of around 12,000 km, higher frequencies and longer baselines could potentially lead to images as sharp as this artist’s illustration shown here. (NASA/JPL-CALTECH)
With both frequency and baseline improvements, we could reach ~0.05 μas resolution: 440 times sharper than our first event horizon image.
In April of 2017, all 8 of the telescopes/telescope arrays associated with the Event Horizon Telescope pointed at Messier 87. This is what a supermassive black hole looks like, where the event horizon is clearly visible. Only through VLBI could we achieve the resolution necessary to construct an image like this, but the potential exists to someday improve it to be hundreds of times as sharp. (EVENT HORIZON TELESCOPE COLLABORATION ET AL.)
Mostly Mute Monday tells a scientific story in images, visuals, and no more than 200 words. Talk less; smile more. Ethan Siegel is the author of
Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.