JON CARTWRIGHT

Finding out whether gravity – and therefore space-time itself – is quantum in nature has long been thought impossible. But innovative new ideas might be about to help answer this crucial question

Physics is tough. Want to spot a ripple in space-time? You just need a detector capable of seeing a length change less than one-millionth the size of an atom. Want to find a Higgs boson? No problem – so long as you have $7 billion, 14 years and 6000 scientists to hand. Still, one experiment is so hard as to make even the cheeriest physicist gulp: testing the idea that gravity is quantum.

A theory of quantum gravity is the outstanding goal of modern physics. It would reconcile two currently incompatible pillars of our description of the universe: general relativity, our large-scale theory of gravity; and quantum mechanics, our microcopic account of nature’s other fundamental forces. Individually, these have been thoroughly tested, always passing with flying colours. Yet try to combine them, and things fall apart. If we could show that gravity is quantum in nature, perhaps by finding a quantum particle of it, the problem would be all but solved. However, even our most powerful detectors don’t come close to the extraordinarily high energies thought to be needed to find these so-called gravitons.

Not long ago, the late theorist Freeman Dyson echoed the mood among many physicists when he argued that quantum gravity might simply be untestable. But recently, some have begun to claim that it may not be so. If true, we could soon see the first hints of how the two most fundamental theories of nature relate to each other. “It seems to me that, technologically speaking, the time is opportune,” says Vlatko Vedral, a theorist at the University of Oxford. […]

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String theory may be our best attempt at a theory of everything, except that it can’t describe an expanding universe like ours. Now a radical new twist on the idea could finally fix that – but it requires us to completely reimagine reality

String theory is the best candidate we have for a theory of everything. Bend to its rule and the various tangled theories of conventional physics emerge as part of a sublime, higher-dimensional tapestry. It can unify all four of nature’s forces, including the most troublesome of all, gravity. With any luck, it can also tame big bangs and black holes without losing the thread.

There’s just one catch: string theory can’t explain a universe like ours. Its maths can describe gazillions of different possible universes, just not one expanding at an accelerating rate, which is precisely what we see ours doing. To be sure, no one knows what is driving this acceleration – a mysterious “dark energy” is the usual placeholder. According to theory, though, it probably shouldn’t be happening at all.

For 25 years, this has been a big problem, but now we may have found a way past it. Superficially, the answer won’t shock anyone used to the extravagance of modern physics: we just have to rethink our universe as part of a much grander enterprise. Do this, and it can balloon to its heart’s content – indeed, accelerated expansion seems to come naturally. But this new scheme may be the wildest yet, one in which our familiar space is delicately poised between high-dimensional hyperspace and complete nothingness. “In our proposal, our existence is like a shadow – a projection onto a wall at the end of the world,” says Antonio Padilla, a physicist at the University of Nottingham in the UK. […]

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The International Space Station will burn up and splash down into the Pacific sometime around 2030. What could possibly go wrong? And will we ever see anything like the ISS again?

The International Space Station (ISS), as well as being the most expensive object ever made, can also lay claim to being one of the most cooperative endeavours in scientific history. Since the beginning of the century, it has been continuously inhabited by a total of 280 crew members – and counting – from 23 countries. While leaders on the ground have been squabbling or even threatening war, astronauts and cosmonauts have been circling Earth unconstrained by geopolitical borders, floating in serene microgravity.

But nothing lasts forever. Sometime around 2030, the ISS project will come to an end. From its orbit about 400 kilometres above Earth, the space station will fall through the atmosphere, burning up and splintering into a thousand pieces before crashing into the Pacific Ocean. It is unlikely that any of it will ever be seen again.

Artificial satellites reenter the atmosphere all the time – almost every day, in fact. But the $150 billion ISS is no ordinary satellite. More than 100 metres long, and with the mass of a fully loaded jumbo jet, it is by far the largest and most complicated one ever built.

Managing the end of the ISS’s life is far from straightforward. How can such a cumbersome object, all 420,000 kilograms of it, be brought down and destroyed safely? Should it be destroyed at all? And will we ever see its ilk again? […]

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For the first time, we are able to predict exotic new states of matter. It could lead to a tech revolution, says Jon Cartwright

THE tenets of physics can seem carved in stone. The speed of light is a constant. There are four fundamental forces. Theoretically, rules like these are open to revision. But new contenders had better come with a chisel and a very big hammer.

You would be forgiven for thinking this confidence also applies to something as fundamental as the different states of matter. As we learned in school, there are three of them: solid, liquid, gas. Right?

Actually, these are only the start. We now know of all sorts of exotic states, from superconductors to Bose-Einstein condensates, quantum spin liquids to topological insulators. The sheer number is as bewildering as their names. Strangely, no one can give you a definitive list: there could be as few as four of them or perhaps thousands.

Sorting this mess out isn’t just a matter of satisfying our curiosity. If we can pin down exactly what constitutes a state of matter, we should be better able to predict and discover new ones. That would not only have great technological benefits, but it could also give us fresh ways to probe the nature of reality.

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