Research with the World’s Largest Telescopes
Date: 10th December 2025 Speaker: Dr Katharine Johnston
In 2019, humanity glimpsed the unseeable: the shadow of a black hole. The remarkable image of M87’s supermassive black hole captured headlines worldwide, and the answer to how we achieved it lies in a simple but profound concept—building a telescope the size of Earth itself.
Why Size Matters in Astronomy
The drive to build ever-larger telescopes is governed by a beautiful relationship: resolution = λ/D. Here, λ represents the wavelength of light, and D is the telescope’s diameter. This equation tells us that bigger telescopes capture finer detail, which is why we engineer these massive structures despite the considerable technical challenges. But size matters for another reason too: telescopes are essentially giant light buckets. The larger the bucket, the more photons we collect, dramatically increasing our sensitivity to faint cosmic signals.
Radio telescopes take this principle to extremes, with some reaching 500 meters in diameter—dwarfing their optical cousins. Why so much larger? Again, that resolution equation holds the answer: longer wavelengths demand bigger telescopes to achieve comparable detail. But radio astronomy offers unique advantages that justify the scale.
Seeing Through the Cosmic Fog
Interstellar dust presents a formidable obstacle for optical astronomers. Like fog blocking a distant view, these microscopic particles—typically smaller than one micron—scatter and absorb visible light, obscuring the universe beyond. But longer radio wavelengths, much like Wi-Fi signals penetrating walls, pass straight through. As Dr Johnston notes in her lecture, “at wavelengths longer than a micron, you actually don’t have any more trouble with dust.”
This transparency transforms dust-shrouded regions into treasure troves of information. The submillimeter sky reveals not darkness but glowing clouds of gas and dust—material that emits thermally at these wavelengths, allowing us to map its mass and physical properties with precision.
Interferometry: Combining Telescopes
ALMA—the Atacama Large Millimeter/submillimeter Array—exemplifies the power of interferometry. Perched at 5,000 meters elevation in Chile’s Atacama Desert, 66 antennas work in concert, their combined signals creating a virtual telescope far larger than any single dish. The key insight: resolution depends not on dish size but on the baseline—the separation between antennas.
Earth’s rotation becomes an asset, allowing these arrays to fill an effective aperture as the planet turns. By recording signals with atomic-clock precision and later correlating them, interferometers synthesize observations from a telescope the size of their maximum baseline.
The Event Horizon Telescope: A Planetary Instrument
Taking this concept to its logical extreme, the Event Horizon Telescope (EHT) links radio observatories across the globe, achieving baselines up to 12,000 kilometers. This Earth-sized virtual instrument achieves angular resolution of approximately 20 microarcseconds—analogous to spotting a bottle cap on the Moon. According to the European Space Agency, such precision represents “a resolution sharp enough to read a newspaper in New York from a café in Paris.”
The EHT’s 2019 image of M87’s supermassive black hole and subsequent 2022 image of Sagittarius A* at our galaxy’s center represent triumphs not just of engineering but of fundamental physics. These observations test Einstein’s general relativity in its most extreme regime, where the black hole’s shadow matches theoretical predictions with stunning accuracy.
The Bigger Picture
These massive telescopes serve society beyond pure curiosity. Understanding cosmic origins, probing star formation regions, and testing fundamental physics all flow from these observations. The technology developed for interferometry—from data processing to atomic clocks—finds applications far beyond astronomy.
In the research context, these instruments open new windows on the universe. Dr Johnston’s own work uses ALMA to study massive star formation, revealing rotating disks of material around nascent stars through Doppler-shifted molecular emission lines. Each new capability raises fresh questions about planet formation, galaxy evolution, and the nature of spacetime itself.
The achievement encapsulated by the EHT exemplifies what becomes possible when we think big—literally. As the Event Horizon Telescope Collaboration stated: “We have shown that direct studies of the event horizon shadow of supermassive black hole candidates are now possible, thus transforming this elusive boundary from a mathematical concept to a physical entity that can be studied and tested.”
In the quest to understand the cosmos, sometimes the only solution is to build a telescope the size of a planet.
References:
- European Space Agency. “Event Horizon Telescope: How to Measure a Black Hole.” Available at: https://www.esa.int/Science_Exploration/Space_Science/Event_Horizon_Telescope_How_to_measure_a_black_hole
- Event Horizon Telescope Collaboration et al. (2019). “First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole.” The Astrophysical Journal Letters, 875(1), L1.