Ever since a trip through a wormhole was first portrayed in 2001: A Space Odyssey 50 years ago, the idea of them has captured the public imagination. And small wonder: they’re the ultimate form of cosmic travel: a way of zipping across galaxies in an instant.
But while wormholes have become a staple of science fiction, among scientists they’ve been a source of endless frustration. Not because the idea is ridiculous, but because it isn’t. The astonishing fact is that wormholes are a natural consequence of current theories of gravity, and were investigated by Einstein himself over 80 years ago. Ever since, researchers have been trying to find out if such a bizarre theoretical possibility could be a reality.
And now they have made a major breakthrough – one which exploits deep connections between the nature of space and time and the laws of the subatomic world. The result is a new understanding of exactly what’s required to make a real-life wormhole.
Einstein first investigated the properties of wormholes with his colleague Nathan Rosen in 1935, using his theory of gravity known as General Relativity. They found that what we now call a black hole could be connected to another via a tube-like ‘throat’. Now called the Einstein-Rosen bridge, this seemed to open the way to taking shortcuts through space and time, entering a black hole in one part of the Universe and emerging from another perhaps millions of light-years away, but without taking millions of years to do so – thus effectively travelling faster than the speed of light.
It was a stunning idea, but in the early 1960s it was dealt a severe blow by John Wheeler, the brilliant US physicist who first coined the terms ‘black hole’ and ‘wormhole’. Together with fellow theorist Robert Fuller, he showed that the Einstein-Rosen bridge would collapse almost as soon as it formed. As Dr Daniel Jafferis, associate professor of physics at Harvard University explains: “We could jump in from opposite sides and meet in the connected interior, but then we would both be doomed.”
Jafferis is one of an elite group of theorists around the world searching for ways to dodge this problem. For years, the most promising idea has been to support the bridge using a type of ‘exotic matter’ with negative energy. As its name suggests, this is pretty weird stuff – so weird it’s capable of bending the normal rules of gravity. While ordinary matter always generates a gravitational pull, the negative energy produced by this exotic matter generates an antigravitational repulsion. Amazingly, such energy is known to exist. In the 1990s, astronomers discovered that the whole Universe is expanding under the antigravitational effect of so-called ‘dark energy’. There’s just one problem – the exact origins of dark energy are as yet unknown. The same goes for the exotic matter – no one has any idea how to create the stuff, let alone use it keep a wormhole open long enough to fly through.
The worm has turned
But now the debate over such so-called traversable wormholes has taken a radical new turn. It follows the discovery of a new way of keeping the bridge intact based on a surprising link between wormholes and quantum theory (the laws of the subatomic world). It emerged during attempts to solve a problem that has obsessed some of the greatest theorists of our time, including the late Stephen Hawking: what happens to objects that fall into a black hole?
Everyone knows there’s no escaping a black hole once inside it: the pull of gravity is too strong even for light to evade its clutches. Yet Hawking famously showed that a black hole doesn’t last forever, but eventually explodes in a burst of intense radiation, leaving no trace of whatever fell into it.
The trouble is, this contradicts one of the key principles of quantum theory, which states that information can never be destroyed. Black holes, however, seem quite capable of utterly destroying information about what they’ve consumed. This is the notorious ‘black hole information paradox’, and it hints at a big gap in our understanding of how the Universe works.
For decades, Hawking and many others tried to resolve the paradox without success. But now there’s growing excitement that the answer has been found. And it lies in the ability of wormholes to provide a way out of black holes. Put simply, theorists think the supposedly inescapable boundary of a black hole – the so-called event horizon – is riddled with tiny wormholes that allow information to seep out, along with the radiation which Hawking showed destroys black holes. This, in turn, has led to new insights into the nature of wormholes, and whether they can be traversed.
Until now, the only known way to traverse a wormhole was to stop the Einstein-Rosen bridge collapsing using the negative energy of exotic matter. “Quantum effects allow some negative energy,” explains Jafferis. “But it was long suspected that what is required for a traversable wormhole is physically impossible.”
Now, Jafferis and his colleagues Dr Ping Gao and Dr Aron Wall think they’ve discovered another source. “What we found is that a direct interaction between the [black holes at the] two ends of a non-traversable wormhole can lead to negative energy,” says Jafferis. The resulting antigravitational effect then stops the Einstein-Rosen bridge from collapsing, therefore making the wormhole traversable.
When Jafferis and his colleagues say “direct interaction”, they mean that the two black holes forming the mouths of the wormhole are affecting each other across real, ordinary space. “Binary black hole systems consuming each other’s Hawking radiation is a good example,” says Jafferis. “The consuming of the radiation is the direct connection.”
In a tangle
So, the good news is that traversable wormholes really can exist. Better still, according to Jafferis there’s no problem sending a human through one of them, at least in principle. But, perhaps unsurprisingly, there are some major problems to overcome.
First, the black holes can’t just be the standard type formed from the collapsed remnants of huge stars; they have to be ‘maximally entangled’. This refers to a strange quantum connection that can exist between two objects, so that anything done to one affects the other instantly – no matter how far apart they are.
Like negative energy, the bizarre phenomenon of quantum entanglement really exists. It was first detected in lab experiments nearly 40 years ago, and it’s now being investigated by companies like Google for creating ultra-fast quantum computers. Yet while subatomic particles can be entangled relatively easily in the lab, no one has any idea how to do the same with black holes. “We can’t even make unentangled black holes, let alone precisely quantum entangled ones,” explains Jafferis.
Yet direct interaction between two black holes comes with a catch: it forbids any amazing time travel trickery. But could it still allow faster-than-light travel? That’s a tricky question, says Jafferis. Gravity, space and time are all intimately linked, and that messes with the very notion of speed. According to Jafferis, calculations based on the wormhole types studied so far suggest that using them would actually be slower than simply travelling directly through space. He admits, though, that the details have yet to be fully worked out. So, it seems that science fact is still running a little behind science fiction. The laws of nature seem to insist that wormholes can either perform amazing feats but collapse in an instant, or be traversable but useless.
Yet time and again, nature has sprung big surprises on theorists. The mere possibility of black holes was disputed for decades, and Einstein himself refused to believe in quantum entanglement. Could it be that somewhere in the Universe lie natural wormholes performing their miracles?
Science or sci-fi?
The possibility of observing a real-life wormhole is now the focus of research by theorists using a mix of mathematics and computer models. The challenge is spotting the difference between normal black holes and those that are the portals of wormholes. According to Rajibul Shaikh, a gravity theorist at the Tata Institute of Fundamental Research in Mumbai, India, the answer may lie in subtle differences in the way they affect their surroundings – and in particular the behavior of light. “As predicted by Einstein’s General Relativity, photons undergo bending in a gravitational field,” he explains.
The intense gravity of black holes creates incredibly hot, bright accretion discs around them, formed of matter spiralling down to its doom. The otherwise invisible hosts of these discs then reveal their presence as a pitch-black shadow cast on them. It’s the shape of this shadow that could reveal when a black hole is actually something even more bizarre. According to Shaikh, the telltale signs of a wormhole come from the gravitational effect of its throat on the resulting shadow.
“What I found is that the shape of the shadow of a slowly rotating wormhole would be very similar to the almost perfectly disc-like shadow cast by a slowly rotating black hole,” he explains. “But a faster spinning wormhole would cast a shadow which is more distorted than that of a black hole with the same spin.”
He stresses that research is still in progress, and the results so far are based on specific types of black holes and wormholes. “There’s no guarantee the type of rotating wormholes I considered are the most common.”
But Shaikh points out that astronomers already have the means to detect the effects predicted to exist around wormholes. Known as the Event Horizon Telescope (EHT), it consists of a global network of radio antennas able to make studies of black holes and wormholes. “And it has already started taking data,” says Shaikh. It could just be that, half a century after it made its debut on movie screens, the space-time wormhole is about to become more than just science fiction.