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11 March 2015updated 01 Jul 2021 11:43am

Dead stars and deep secrets: we are still in the dark when it comes to black holes

Despite Einstein and Hawking, we still know very little about black holes.

By Michael Brooks

Black holes have long given physicists and astronomers a reason to get up in the morning. Despite there being one at the heart of every galaxy, nobody really understands much about these cosmic monsters. And now it turns out we understand less about them than we thought.

Black holes, in effect, are punctures in the fabric of the universe. Einstein showed that gravity is a distortion of space; think how pulling on a small part of a balloon deforms its smooth curves. When a particularly heavy star runs out of fuel, the pull of its remaining material causes the star to collapse. This creates an ever-stronger gravitational field at its centre; so strong, the distortion of space turns into a rip.

When this idea was first suggested in 1935 experts dismissed it as “stellar buffoonery”. Now, though, we’re sure that space is riddled with black holes – still making fools of our best thinkers.

The most recent humiliation was the discovery of a black hole that is, quite literally, a bright young thing. Black holes can become a strong source of light because the material being sucked into them glows as it accelerates towards its doom. The newest black hole on the block creates a light show equivalent to 420 trillion suns. The real surprise, however, is its youth. It formed just 900 million years after the Big Bang. According to standard theories, that doesn’t give enough time for the kind of cosmic cataclysm required for black-hole formation. So it shouldn’t exist.

The discovery, made by Chinese and American astronomers and published in the journal Nature, is just the latest in a series of mysteries. For one thing, we still haven’t resolved what is known as the “firewall paradox” – what happens to information that falls into a black hole?

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To physicists, everything physical contains information – the information needed to describe an object, or construct a replica with all its atoms in the right arrangement. Some suggest that such information is not a by-product of physical existence but the root cause of it, something that might even extend to the universe as a whole. Stephen Hawking showed in the 1970s that black holes eventually evaporate, vanishing from the universe. A fundamental law of physics says that information can’t be destroyed; so where does the information end up after the black hole disappears?

We don’t know. There are suggestions that it never really falls into the black hole – that it can’t get beyond the cosmic firewall. Alternatively, it does fall in but becomes preserved in the structure of space surrounding the black hole’s surface – the area known as the event horizon. Yet still, no solution holds together completely: it’s a mystery.

Also mysterious is the precise nature of that rip in space at the heart of a black hole. Known as the singularity, it is essentially a blank page in physics. Einstein’s relativity governs very big things and only very big things – you need quantum physics to tell you about the very small things, such as puncture holes in space. Unfortunately, as currently formulated, relativity and quantum theory are mathematically incompatible. There is nothing we can say about the heart of a black hole.

This is also why the study of black holes has become central to the effort to develop a final theory of physics. The aim is to combine relativity and quantum physics, our two best theories, to explain everything in the universe. Black holes are the most obvious places where these two theories are at work together, which is what makes every new discovery about them so important.

However, it seems unlikely, given their inaccessibility, that these dead stars will ever give up their deepest secrets. When it comes to black holes we may be for ever in the dark. 

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