Can we detect wormholes?
New research suggests wormholes could be more than science fiction, and that we could detect them
We’ve all seen wormholes in sciencefiction: openings in the very fabric of space through which spaceships can pass, opening up a method of faster-than-light travel in a universe that otherwise wouldn’t allow it. These shortcuts in space-time can be natural or human-made, and their exits can send you into other realms of normal space-time or into hyperspace, a set of higher dimensions laying closely alongside our own, but through which we can travel more quickly than in normal space. Their profusion in fiction might suggest they’re common.
In the set of dimensions we call reality, however, it’s not so easy. Wormholes have not been observed, and remain speculative. They’re consistent with Einstein’s theory of general relativity, which allows space-time to be distorted by gravity. Black holes are one such special solution to the equations, and wormholes are another. As we’ve only just got our first good look at a black hole, it’s entirely possible that wormholes could be out there somewhere, just waiting to be discovered.
“Wormholes are allowed in theory, and they have been intensely studied theoretically,” says Dr Andreea Font of the Astrophysics Research Institute at Liverpool John Moores University. “They’re mostly regarded as theoretical curiosities, and we don’t actually have any tangible evidence of their existence.”
When Einstein and fellow physicist Nathan Rosen were working together at Princeton in the mid-1930s, they produced an idea of layers of folded space-time connected by ‘bridges’ we would today call wormholes. Another name for one type of wormhole is an Einstein-Rosen bridge in tribute to their work. All they required to create this idea was the complicated maths of general relativity and Maxwell’s equations, which cover electromagnetism. The pair would also go on to predict gravitational waves, but Einstein-Rosen bridges would be less of a success – they collapse so quickly nothing would ever be able to travel through them.
One problem with detecting a wormhole is telling it apart from a black hole. We just don’t know enough about either type to label one definitely as a wormhole, while the other is ‘merely’ a black hole. The existence of a one-way event horizon may be a clue that an object is a black hole, but again we’re in the world of pure theory. “We would like to know if there are any differences, but from the outside they may be indistinguishable,” says Font. “However, they both act on their surroundings through gravity, so there might be some telltale signs. Some scientists have suggested that a wormhole could perturb the orbits of stars nearby in a different way than a black hole would. Some gravity would leak out from the other side through the wormhole, changing the orbits of stars just enough to be noticeable.
“Obviously they don’t emit any light, but they could affect the light of the optical material that’s falling in or coming out. For example, there have been some suggestions that gamma rays might look different coming from a wormhole rather than a black hole, but we don’t have consistent studies of the observational signatures of wormholes the way we do of some black holes, and we can’t distinguish the two.”
What makes things harder is that there may be different types of wormhole. There are some
“Wormholes are allowed in theory, and they have been intensely studied theoretically”
Dr Andreea Font
that link different points in the same universe, possibly billions of light years apart. Some could link different points in time, as general relativity sees time as the fourth dimension of space-time. Others could punch through between different universes, particularly in brane cosmology, which sees universes form on four-dimensional sheets within 11-dimensional space.
“There could be microscopic, very tiny wormholes, and there could be very big ones,” says Font. “If there are holes that are very big, they could potentially be at the hearts of massive galaxies. Some have suggested that the centre of mass of galaxies could be supermassive black holes, but with a wormhole hidden in there too. But we would need an additional sign – an extra piece of evidence that is not explained by black holes – in order to reach the conclusion that it could be a wormhole.”
Even the black hole at the centre of our own galaxy, Sagittarius A* (Sgr A*), has been nominated as a wormhole candidate. Dejan Stojkovic, professor of physics at the University at Buffalo, has studied a star called S2 that orbits Sgr A*, looking for deviations in its orbit that could be ascribed to a wormhole: “If you have two stars, one on each side of the wormhole, the star on our side should feel the gravitational influence of the star on the other. The gravitational flux will go through the wormhole,” said Stojkovic.
“When we reach the precision needed in our observations, we may be able to say that a wormhole is the most likely explanation if we detect perturbations in the orbit of S2,” he continues. “But we cannot say ‘this is definitely a wormhole’. There could be some other explanation, something else on our side perturbing the motion of this star.”
The problem is still a lack of data. “If the wormhole is always connecting with, let’s say, another part of the universe where there’s some matter on the other side,” says Font, “that matter would exert some additional gravitational pull on the stars orbiting around the Milky Way, or the centre of the Milky Way, and that would change their orbits very slightly. But unfortunately at the moment our instrumentation is not good enough to detect this additional perturbation, so this is just speculation.”
Supermassive black holes certainly live up to their name, with one of the largest known specimens – which powers the TON 618 quasar
– a supercolossal 66 billion times the mass of the Sun. Sitting 10,400 million light years away, the quasar is the light emitted by the accretion disc of a black hole at the centre of a galaxy we can’t see. It shines with the brightness of
140 trillion Suns, and outshines the light from its entire home galaxy. Its event horizon – or Schwarzschild radius – is thought to be 40 times the distance from the Sun to Neptune. The
thing is big, but it’s still a black hole – there’s no evidence of anything else going on.
Another giant black hole resides at the centre of Messier 87, and this was recently imaged in polarised light by the Event Horizon Telescope to show the orientation of its magnetic field at its edge. The dark area of the hole itself is an oval rather than circular in the image, thanks to its rotation. Before scientists referred to an ‘event horizon’ or ‘Schwarzschild radius’, they sometimes used the phrase ‘Schwarzschild throat,’ which suggests a structure that could be passed through. The work of physicist Roy Kerr in the 1960s suggests that it would be impossible to stand still when around a spinning black hole, as space-time itself is literally dragged around by the spinning, which changes the shape we see to an oblate spheroid. “Some people have tried to apply their theories to the black hole at the centre of M87 and work out what would be the consequence of a wormhole there, but it has never happened the other way round, of actually observing something that could be an unmistakable sign of one,” says Font.
The hunt is on, not just for wormholes, but traversable wormholes too. Previous solutions to this problem have called for exotic matter with properties that resist the gravitational collapse – usually negative energy, which has yet to be discovered. Recent work from Jose BlázquezSalcedo from the Complutense University of Madrid and his colleagues, however, has discovered a way to bypass the inconvenient negative energy by assuming that matter interacts through electromagnetic fields, and is described as quantum wave functions. Satisfy
all these conditions, and a traversable wormhole becomes possible.
However, there’s another catch: the wormhole is traversable, but not by humans.
The researchers added Paul Dirac’s theory of quantum particles to general relativity and Maxwell’s equations, meaning anything entering the wormhole would need to be in a quantum state – effectively limiting it to microscopic groups of atoms.
Further work by Juan Maldacena and Alexey Milekhin out of Princeton suggests a different method of creating a human-traversable wormhole. It requires a model of physics beyond the Standard Model – the five-dimensional Randall-Sundrum model – but under these conditions a human could traverse a wormhole, travelling across the galaxy in under a second. To observers outside the wormhole, however, the journey would appear to take thousands of years. All of which is fine, as long as you can detect the wormhole in the first place, which is still proving difficult. Research from a team led by James B. Dent of Sam Houston State University, Texas, could lead to their detection, but only if a very specific thing happens.
It requires a black hole to pass through a wormhole. This may seem unlikely, but recent gravitational-wave detections have come from merging black holes, so it’s not impossible for a black hole and a wormhole to run into one another – if the latter exists, at least. The theoretical wormhole here is one between two universes, and as a black hole spiralled into it, it emitted the same gravitational waves as if two black holes had merged. What was different, however, was that when the black hole exited the wormhole on the other side, there was a cough, a distinct gravitational-wave emission that was different from that generated by a merger. We’ve only just started picking up gravitational waves, so it could be a while before we detect this rare event, if it ever happens.
It’s not just other universes that wormholes could connect to – what about other times? There’s nothing to stop this. Time travel is “accepted in special and general relativity. For example, when we travel near to the speed of light, we know that time slows down,” says Font.
The issue here is that while time travel to the future is fine, time travel to the past is fraught with problems. Stephen Hawking was adamant that the laws of physics would prevent backward time travel, and as Font points out, doing this could unleash a paradox: “If someone could travel into the past and emerge from the wormhole in the same spatial position, but at an earlier time, then they would be able to alter history, and they could even alter their own existence.” This is the famous ‘grandfather paradox’, but we still don’t know if the events of the past could be altered in this way.
Until the news breaks that scientists have detected a wormhole, seen that it is distinct in some way from a black hole and discovered that humans can travel through it, all of this lives in the realm of the highly theoretical. It’s the exciting consequences of their existence – time travel, faster-than-light travel and trips to different universes – that keeps them in our collective imagination, however, along with the inevitable Nobel prize for their discoverer.