By doing so, many telescopes located on different continents can form one virtual Earth-sized telescope. ![]() To achieve the highest angular resolutions possible from the surface of Earth, the EHT exploits a technique known as very long baseline interferometry (VLBI), in which astronomers at radio dishes across the globe observe the same target simultaneously, record the data they collect on hard drives, and then later combine all those data using a supercomputer to form a single image. The Event Horizon Telescope (EHT) project is an international effort to overcome these hurdles and perform detailed observations of a black hole. Consequently, there are only a few wavelengths of light that can escape from the black hole’s edge to be observed by us on Earth. Second, even material that emits the light we want to detect-that glowing whirlpool of crushed matter spiraling in toward the horizon-is itself opaque to most wavelengths of light. First, they occur at the very centers of galaxies, deep within dense clouds of gas and dust that block most of the electromagnetic spectrum. What is more, such black holes are obscured from our view in two ways. To resolve an object so small, a telescope must have an angular resolution more than 2,000 times finer than that achieved by the Hubble Space Telescope. The nearest example is Sagittarius A*, the four-million-solar-mass black hole at the center of the Milky Way its event horizon would appear to be only 50 microarcseconds across, or roughly the size of a DVD seen on the moon. Even the supermassive black holes now thought to inhabit the centers of most galaxies, which weigh in at millions or billions of our sun’s mass and in some cases have diameters larger than our solar system, are so far away from Earth that they subtend incredibly tiny angles on the sky. Notably we have to contend with the black hole’s tiny size when viewed from Earth. There is, of course, a catch: developing a telescope that can resolve a black hole horizon poses several challenges. If we could observe a black hole with a telescope with enough magnifying power to resolve the event horizon, we could follow matter as it spirals down toward the point of no return and see whether it behaves as general relativity says it should. This causes the infalling matter to reach temperatures of billions of degrees-which, ironically, makes the vicinity immediately surrounding a black hole one of the brightest spots in the cosmos. Near the black hole, the crushing force of gravity compresses inflowing matter, known as the accretion flow, into ever smaller volumes. The interior of a black hole is unobservable, but the gravitational field surrounding these objects causes matter close to the horizon to produce huge amounts of electromagnetic radiation that telescopes can detect. And nowhere in the universe today is gravity stronger than at the edge of a black hole-at the event horizon, the boundary beyond which gravity is so overwhelming that light and matter that pass through can never escape. To put general relativity to its greatest test, we need to see whether it holds up where gravity is extremely strong. Every assessment to date has been conducted in rather weak gravitational fields. ![]() So far, however, Einstein’s theory has had it easy. Scientists have been trying unsuccessfully to poke holes in Albert Einstein’s general theory of relativity for a full century.
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