You can find lockers everywhere in Japan, so I was naturally expecting to find some at Tokyo's Hiroo Station. It was something of a shock when there weren't any. Not only did their non-existence mean I had to lug my suitcase through the rain, but it seemed an inauspicious omen given that I was en route to a meeting about a hundred-million-dollar attempt to detect something that physicists are sure must exist, but haven't found.
One hundred years ago, Albert Einstein predicted that the fabric of space should be rippling with waves. His image of the universe is one of a taut rubber sheet on which massive objects create curved indentations. Gravity is the result these curves, forcing lighter objects to move towards the more deeply embedded heavier ones. As objects move, the sheet flexes to reflect their new positions, creating oscillations in spacetime that travel outwards as a gravitational wave. These waves are a probe to the Universe’s most secret events, from the interiors of collapsing stars to black holes…or they would be if only we could detect them.
Among of the strongest gravitational wave sources imaginable is the merger of two black holes. Yet even this rare event would distort spacetime only enough to briefly change the distance between the Earth and the sun by the width of a hydrogen atom. To put it mildly, this makes detection a serious challenge.
One of the best hopes for achieving the necessary sensitivity is light. In a technique known as interferometry, a laser beam is split to send two light waves down perpendicular tunnels several kilometers in length. The waves reflect off mirrors to return to the same position and recombine. The intensity of the newly recombined beam depends on the alignment (or phase) of the peaks and troughs in the two waves. This makes it incredibly sensitive to how far each wave has traveled before it recombines.
For gravitational wave detection, the tunnel lengths ensure that the peaks from one wave meet the troughs from the second wave. This is known as destructive interference and it cancels the light of the recombined wave entirely. But when a gravitational wave passes, it distorts the lengths of the tunnels, changing how far each wave travels. The peaks and troughs of the two light waves then no longer perfectly align; the beams no longer cancel each other, and a signal is produced.
This technique is behind the US-based detector, LIGO. During 3.5 years of operation between 2005 - 2010, LIGO saw no gravitational waves. However, its sensitivity was only enough to detect the very strongest sources and these are rare occurrences in our Galaxy. Now one century after Einstein’s prediction, LIGO has gotten an upgrade (it's now called Advanced LIGO) and Japan is opening a new detector. It is time for business.
Buried 200 m below the Ikenoyama mountain in the Gifu prefecture of Japan, the Kamioka Gravitational Wave Detector (KAGRA) is about to turn on its lasers for the first time. Led by the 2015 Physics Nobel Laurette, Takaaki Kajita, KAGRA’s sensitivity should allow it to detect gravitational waves up to 700 million light years away, ten times further than the previous generation of detectors. Its main target will be binary neutron stars; incredibly dense stellar corpses that emit energy in the form of gravitational waves as their orbits around each other decay. Its enormous range means it's not restricted to events only in our own galaxy, so KAGRA should detect multiple gravitational wave events per year.
KAGRA’s underground location minimizes seismic noise from the ever-rumbling Japan. The surrounding gneiss rock is famous for its hardness, causing the mountain to act as a single entity that is resistant to shakes. KAGRA’s second new feature is cryogenic cooling, bringing the system down to an frosty -253° Celsius (20 K), to stifle any thermal vibrations.
Such a large-scale interferometer is a new venture for Japan and the next two years will be devoted to testing before full operation begins in 2018. While a positive detection would be an incredible achievement, a single detector cannot determine where the gravitational wave originated. For this, KAGRA will join with Advanced LIGO and two other detectors in Europe to form a global network.
But what if KAGRA opens its eyes to nothing? Like the train station lockers, could gravitational waves be expected but non-existent? This could be the most exciting result, since it would imply new physics. While it is unlikely gravitational waves do not exist at all, their strength, frequency or waveform might differ from predictions. Alternatively, the events that produce strong gravitational waves could be less energetic than we suspect.
One final note: A rumor is circulating on Twitter that Advanced LIGO has seen some sort of signal. The LIGO team hasn't confirmed it, and even if there really is a signal, it could be an artificial one inserted into the detectors to make sure the data analysis pipeline is working properly.
Either way, we're entering a new era in the search for gravitational waves, which is going to give us a new view on the universe.