All About Space
These portals through space and time might be real after all, but how would we go about detecting one?
Usually confined to the pages of science fiction, astronomers are starting to think these portals through space-time might be real after all
It was the plot for an epic 2014 Hollywood blockbuster. In Interstellar, a crew of astronauts travel across space on the hunt for an alternative home for humanity. Yet they don’t leave our Solar System by the conventional route; instead they head into a wormhole in the vicinity of Saturn and emerge almost immediately in a distant galaxy. These wormholes – shortcuts in space and time – have long been a staple of science fiction.
But some scientists believe we may soon be able to prove that they are a real part of the universe – as real as the Sun and the stars or you and I. The scientific term for this exotic object is an Einstein-Rosen bridge, which is a clue as to where the idea came from. Wormholes are rooted in Albert Einstein’s general theory of relativity – his groundbreaking masterpiece that turned our ideas about gravity on their head. For centuries we thought we knew how gravity worked, thanks to Isaac Newton. Apples fell to the ground and the Earth stayed in orbit around the Sun because of a gravitational pull between the objects. Yet Einstein saw it differently, suggesting that what we experience as gravity is simply a bending of space and time. Under this radical new regime, the Earth orbits the Sun because our star’s mass warps the space around it, much like a bowling ball would warp a bed sheet if it were placed in the centre of it. Our planet is simply following the local curvature of this fabric, which Einstein called ‘space-time’.
Such a crazy idea was in dire need of experimental evidence to back it up. Crucially, a solar eclipse in 1919 offered just such an opportunity. When the Moon blocked out the Sun, it was dark enough to see stars close by. Yet we don’t see these stars where they really
“With this new set of rules it would be possible for an observer to go through a wormhole and cross over to another region of the universe”
are because the Sun’s gravity bends their light on its way to us. Newton and Einstein’s competing pictures of gravity predicted different amounts of bending, allowing us to see who was right. Einstein came out on top: massive objects do indeed bend the space-time around them.
Imagine space as a vast sheet of paper. You live at one end and you want to travel to the other end. Ordinarily you’d have to trudge across the entire length of the page to get there. But what if you folded the paper in half instead? Suddenly where you are and where you want to be are right next to each other. You simply have to jump that tiny gap. We call these objects wormholes because it is like a worm trying to navigate its way around an apple. To get from the top to the bottom it has two choices: crawl around the outside or chew a shortcut through the middle.
Until recently our chances of finding these objects – if they even exist – were slim at best.
But that changed in February 2016 when the scientists behind the LIGO (Laser Interferometer Gravitational-Wave Observatory) experiment, based in the US, announced the first-ever detection of gravitational waves. These are tiny ripples in the fabric of space-time, predicted by general relativity, which spread out through the universe much like ripples on a pond. “It was a game-changer,” says Vitor Cardoso, a physicist at the University of Lisbon in Portugal. Two black holes – each about 30 times more massive than the Sun – had rammed into each other 1.3 billion years ago. Their violent crash sent a tsunami of gravitational waves roaring out through space-time, eventually reaching the LIGO instrument in September 2015.
Cardoso’s research suggests that two colliding wormholes would produce a similar burst of gravitational waves. Excitingly, however, he says the resulting waves would be slightly different, allowing us to distinguish between black holes and wormholes. The key here is what’s known as the ‘ringdown’ – the way in which the gravitational waves die away after the initial collision. It’s similar to the way the sound of a ringing bell fades over time. “With two colliding wormholes you would see the ringdown – just like you see for black holes – but if your detector is very sensitive then seconds, or tens of seconds, after the main burst you would see something different,” he says. This is due to the
nature of black holes – gravitational Goliaths that swallow anything that gets too close. The ringdown of colliding black holes always gets quieter, quickly fading away to silence. But with colliding wormholes, after the silence you get an echo – a sudden, late signal as the gravitational waves bounce off the wormholes’ surface. You can’t get that with black holes as they swallow everything.
Unfortunately LIGO currently isn’t sensitive enough to pick up these late changes. However, researchers are upgrading LIGO’s instruments, and it could be possible in “ten years from now or so,” Cardoso says. Another exciting project on the horizon is the European Space Agency’s (ESA’s) Laser Interferometer Space Antenna (LISA). This is a gravitational-wave observatory in space that has a tentative launch date of 2034. However, in 2015 the ESA launched LISA Pathfinder – a test mission to develop certain key technologies that are vital for LISA’s success. In April 2016 the ESA announced that LISA Pathfinder had indeed shown that LISA was feasible.
But ringdowns of collisions might not be the only route to finding a wormhole. Diego Rubiera-Garcia, Cardoso’s former colleague at the University of Lisbon, has another idea. He’s been studying what goes on deep inside a black hole. The conventional picture of black holes, as described by general relativity, has all the infalling mass squeezed down into an infinitely small, infinitely dense point – a singularity. “Any observer who approaches this point is destroyed,” says Rubiera-Garcia. “After that you will disappear from space-time… there is nowhere else for you to go.” It is at this singularity that general relativity breaks down – its equations stop making sense. This leaves many physicists confident that we need a new set of rules to replace general relativity in such an extreme environment.
And that’s where wormholes come in. When Rubiera-Garcia applied one of the alternative sets of rules to the physics of black holes, the singularity disappeared, and the mathematics yielded a wormhole in its place. “Then it would be possible for an observer to go through this wormhole and cross to another region of the universe,” he says.
The trouble is that this shortcut through the cosmos might just be a phantom of the mathematics: the alternative to general relativity that Rubiera-Garcia
“To even create a wormhole requires exotic matter that we have never seen here on Earth”
used to find it might not be how our universe really works. As with all good scientific theories, it needs to be tested, just as Einstein’s was in 1919. That’s where gravitational waves come back in.
Once we have built up a significant library of gravitational-wave detections, we can trawl through the data looking for departures from what general relativity predicts we should see. If these departures are found – and they match what the alternative theory predicts – it could signify that wormholes do indeed lurk inside black holes. The first detection of gravitational waves has ushered in a new era, one in which we may well find out that wormholes aren’t just science fiction after all.
If they exist, could we travel through a wormhole?
“The possibility of using wormholes to travel is not completely excluded at a theoretical level. However, to even create a wormhole requires exotic matter that we have never seen here on Earth.” Vitor Cardoso University of Lisbon, Portugal
“The problem is that usually these wormholes are very small, and when I say small, I mean really, really small, so it is not possible for a real observer to pass through that wormhole.” Diego Rubiera-Garcia Complutense University of Madrid, Spain