DOES SPACE HAVE A FINAL DIMENSION?
There could be more to the universe than meets the eye with extra facets beyond space and time
Curiouser and curiouser!” cried Alice. In Lewis Carroll’s surreal Victorian story the eponymous character goes on adventures in Wonderland. There she meets a cast of quirky characters in a world that’s so alien it defies belief and common sense. To reach this otherworldly place she disappears down a rabbit hole and into another dimension.
As wonderful and wacky as Alice in Wonderland is, some physicists believe something similar might be at play in our universe. “It all starts with the hierarchy problem,” says Kris Pardo from the Department of Astrophysical Sciences at Princeton University. Gravity, it seems, doesn’t play by the rules, particularly when you compare it to the other major forces. “It’s just so much weaker,” says Pardo. It’s ten thousand trillion trillion trillion-times feebler than the strong nuclear force that helps bind atomic nuclei together, for example. To see just how weak gravity really is, remember that you can jump into the air and temporarily overcome the collective gravitational pull of six trillion trillion kilograms of the Earth beneath your feet.
The puzzle of why gravity is so puny compared to its sibling forces is one of the greatest mysteries in physics. It has led some researchers to suggest that gravity must do the astronomical equivalent of Alice and disappear down a celestial rabbit hole. What if gravity isn’t really weaker – we only perceive it that way because it leaks into additional dimensions? If you could be an all-seeing eye, capable of observing every dimension at once, you wouldn’t encounter a hierarchy problem at all. Such an idea might sound far-fetched but, thanks to recent breakthroughs, Pardo has been able to test it.
In 2015, physicists detected gravitational waves for the first time. These ripples in the very fabric of space were predicted by Albert Einstein 100 years earlier. Calamitous events in the universe send out rolling waves, much like those created when a stone is dropped into a pond. When the waves pass through the Earth they can be detected by experiments like the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US.
Its four-kilometre (2.5-mile) arms are sensitive to changes in space equivalent to one-ten-thousandth the width of a proton. The discovery was so monumental that the Nobel Prize in Physics was awarded to the founders of the facility in 2017.
The first gravitational waves detected by LIGO came from the collision of two black holes about
1.3 billion light years away. But in August 2017 another type of collision was picked up: two neutron stars smashing together 130 million light years away. This event – known as GW170817 after the date it was first detected – presented a unique opportunity to test the idea of gravity leaking into extra dimensions. That’s because a neutron star merger produces a searing flash of light in the form of gamma rays along with the gravitational waves. Black holes, on the other hand, famously gobble up light. So GW170817 became the first event ever detected to emit both light and gravitational waves.
Pardo and his colleagues were able to compare the flash of gamma rays with the gravitational waves. “We think we know how much energy is released in the form of gravitational waves after an event like the one that we saw,” Pardo says. “And we
Gravity may disappear into extra dimensions, just as Alice disappeared down the rabbit hole in Alice in Wonderland