At the heart of the Milky Way lurks an object so extreme it defies description. For good reason – no one can see it.
We’re pretty sure something truly monstrous is there by the way nearby stars crazily whirl around, as if orbiting an object 4 million times the mass of our sun. For most physicists, that screams “black hole”. “We have a spectacularly good case for this, considering that we can’t see it directly,” says Daniel Marrone of the University of Arizona.
Black holes are objects so dense and so massive that nothing – not even the light by which we might see them – can ever escape their gravitational pull after straying too close. Predicted by Einstein’s general relativity, black holes are now thought to be ubiquitous in the cosmos. Overweight stars probably collapse into them at the end of their lives. Most, if not all, fully fledged galaxies revolve around supermassive examples of the genre. One essential question is whether these black holes were there first, as extreme scrunches in space-time that seeded the growth of galaxies, or whether they formed later as the dense cores of galaxies collapsed under their own weight.
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The detection last month of a gravitational wave apparently rippling out from the merger of two relatively small black holes is the latest bit of circumstantial evidence for black holes’ reality. “The LIGO signal looks like a very important test – maybe our best so far – that black holes, or something very similar to them, exist in nature,” says of the University of California, Santa Barbara.
But that only deepens the enigma. In theory at least, black holes throw up glaring paradoxes between general relativity and quantum theory, the two bedrocks on which our picture of physical reality perches. Models indicate that black holes must slowly evaporate to nothing over time, bleeding out an emission known as Hawking radiation. But ask what happens to the matter and light they swallow, and particularly any information encoded in it, and you won’t get a straight answer. “There doesn’t seem to be a fully consistent story of how they can do that without modifying some of our most basic physical principles,” says Giddings.
To begin to clear things up, we need to look at a black hole directly. That’s not as implausible as it might sound. The project Marrone is involved in, the , is a network of telescopes across the globe gearing up for full service in 2017. Its aim is to provide a shadowy image of the surface of no return surrounding the supposed black hole at the Milky Way’s heart. To do this, it will look for the glow given out by hot electrons spiralling in its intense magnetic fields.
That should prove whether the object has a mass and size consistent with that of a black hole. If it does, says Giddings, features such as the light distortion around it will give us clues to its internal workings – and so whether we are staring at the dark destroyers of theory.
Read more: 11 scientific wonders we know exist – but we’ve never seen
