More than 26,000 light years away, deep within the heart of our galaxy, lies Sagittarius A* - four millions Suns' worth of unseen mass concentrated into an area just nine times the size.
Sheperd Doeleman, assistant director of the MIT Haystack Observatory, believes this mysterious entity is a supermassive black hole - and he's building a telescope the size of the Earth to prove it. "We have good evidence there are black holes at the centre of galaxies just from the orbits of stars around them, but we've never actually seen one," he explains. "The magnifying power required is equivalent to that needed to pick out a grapefruit on the surface of the Moon from Earth."
Read more: What is Einstein's theory of relativity?
This, combined with the wavelength required to pierce all the gas and dust between us and Sagittarius A*, means that making a direct observation requires an aperture of planetary proportions. But rather than adopting the Death Star approach of celestial construction, the Event Horizon Telescope (EHT) project, which Doeleman directs, plans to use what we already have - connecting together a globe-spanning network of nine radio telescopes to turn the Earth into one giant virtual lens.
"We can synchronise these telescopes to swing towards Sagittarius A* at the same time, record all the signals, then bring these all together in a supercomputer and play them back," Doeleman explains. "It works in the same way that the parabolic shape of an optical lens converges all the light rays to a focal point."
The final telescopes required to make the observation, including the Atacama Large Millimeter Array in Chile, are due to come online in spring 2017, but a partial EHT has already been used to assess the size of Sagittarius A* and to make the first ever measurements of a black hole's magnetic fields. All nine telescopes involved will still represent only small fragments of a virtual lens, but as the Earth rotates, so their movement will trace out a much fuller parabola - if a less than perfect one. To correct for the Earth's squashed shape, signals from some telescopes will be delayed relative to others. The difficulty, however, isn't just in the data processing, but in the weather. "Radio waves can go through walls - but at these frequencies, water vapour is a big problem," says Doeleman: "We need clear skies simultaneously at all our telescope locations."
Even with all of this magnification power, how exactly do you produce an image of something characterised by the complete absence of light? "By definition, black holes are unseeable," says Doeleman."Matter is at such densities that gravity compresses everything into a single point. Just outside of that is the event horizon, the point from which even light cannot escape. Yet that gravitational pull also makes black holes some of the brightest spots in the universe. They attract so much gas and dust into such a small volume and, as all this material starts rubbing against itself, the friction heats it up to hundreds of billions of degrees. So this black hole is sitting in the middle of a luminous soup."
Thus in the image produced by the radio telescopes' combined signals, the black hole should appear as a shadow, the exact size and shape as that described by Einstein's field equations. "If we can image this in enough detail we'll be able to test Einstein's theory of general relativity at the event horizon itself," Doeleman says.
Of course, Einstein's theory of general relativity has been repeatedly confirmed at every turn, most recently by the Advanced Laser Interferometer Gravitational-wave Observatory's measurement of the gravitational waves produced when two black holes merge. What new information could an image of the event horizon offer? One area of uncertainty is what's known as the information paradox. "The information swallowed by a black hole is seemingly lost completely," says Doeleman.
"If you burn an encyclopaedia, an advanced civilisation could still reconstitute it and the information it contained. But if you throw it into a black hole it seems it's just gone. Quantum-mechanically that doesn't make sense, so understanding where the information goes is a mystery. But there are ways that the quantum states inside the black hole might manifest outside of it, so it's a mystery we may be able to address."
Even the existence of black holes shouldn't be taken for granted. The evidence for them may be strong, but it's still indirect enough to allow for other possibilities, and nowhere do Einstein's theories produce stranger and more unintuitive results than at the event horizon.
"Einstein himself disavowed black holes for many years," Doeleman points out. "He thought that something would prevent the catastrophic gravitational collapse of matter. But if he's right, and if this object is a black hole, then the shape of its shadow should conform to the no-hair theorem, which says that you can characterise a black hole by just its mass and its spin. If what we actually see is an irregular shape, then we'd have to look at scenarios in which it's either not a black hole, or that Einstein was wrong. But I always say: you never bet against Einstein."
Kathryn Nave is a regular contributor to WIRED.
The WIRED World in 2017 is WIRED's fifth annual trends briefing, predicting what's coming next in the worlds of technology, science and design
This article was originally published by WIRED UK
