Exactly 100 years ago this month, Albert Einstein first proposed his theory of general relativity. Just over a week ago physicists announced that the theory has finally been confirmed.
Einstein’s ground-breaking theory predicted, amongst other things, that the acceleration of massive objects would cause ripples, called gravitational waves, which move through the fundamental backdrop of the universe, much in the same way that regular water waves may ripple in a cup of coffee. Whereas the ripples in your drink may be caused by the act of dropping a cube of sugar into your mug, these gravitational waves were successfully detected from the merging of two black holes over a billion light-years away.
Physicist David Reitze began the announcement bluntly, “Ladies and gentlemen, we have detected gravitational waves.” His words were met with jubilance at the Washington D.C. press conference, as he paused to let the significance of a century’s worth of painstaking effort to resonate among the audience.
At the University of Toronto’s Canadian Institute for Theoretical Astrophysics’ (CITA) webcast viewing event, which took place at the Burton Tower of the McLennan Physical Laboratories, the applause was only outdone by the radiant smiles that were shared among members of our own physics department.
“It’s momentous,” said Luis Lehner of the Perimeter Institute for Theoretical Physics in Waterloo. “It marks the beginning of our ability to peek at the universe through a completely new window.”
Gravitational waves are created by powerful events, like a binary black hole merging. Black holes are some of the densest and heaviest objects in the universe, with some having masses four million times greater than our sun. When these massive objects collide, they release a large burst of energy in a short amount of time. This energy is dispersed via ripples that travel throughout the entire universe. As gravitational waves travel, they compress space in one direction while stretching it in the other, a phenomena that scientists believed they could identify.
Over large distances however, gravitational waves usually fade as their energy dissipates, turning into no more than a whisper, leading scientists to fear that they would never be able to detect their existence. In this case, however, the energy released by the black hole collision was so great that its gravitational waves were able to remain detectable after travelling over a billion light-years to Earth.
Determined to detect gravitational waves, the U.S. National Science Foundation (NSF) invested more than $1.1 billion (US) into the construction of the Laser Interferometer Gravitational-wave Observatory (LIGO), which is described as “the most precise measuring device ever built.”
LIGO is comprised of two separate detectors, one in Livingston, Louisiana and one in Hanford, Washington. The detectors were designed around the concept of gravitational waves compressing space in one direction and expanding it in the other.
LIGO’s first observational run began in 2002 and ended in 2010 without having detected any gravitational waves. The NSF remained confident however, and a major upgrade was made to the detectors, making LIGO more sensitive. As it turns out this was a brilliant decision because the signal was only just quiet enough to have evaded detection before LIGO’s recent upgrade.
Each detector is made of two four-kilometre long perpendicular arms that have ultrapure glass mirrors at their ends. A beam of light is split into two and shot down both tubes, bouncing off the mirrors and returning to the starting point. LIGO is able to detect gravitational waves by measuring miniscule differences between the journeys of the two beams — if nothing interferes with the beams, their recombining will cancel each other out.
A light sensor is waiting in case something changes. Because of the perpendicular arms, the single dimension compression and stretching caused by gravitational waves will compress only one arm and stretch the other. So if a gravitational wave warps the path of one of the lasers, the two beams will be marginally misaligned, and the laser will hit the photodetector, alerting scientists to the deformity.
1.3 billion years ago, in a galaxy far far away, a pair of black holes were circling each other, slowly spiraling inwards, until they merged into one massive black hole. The two black holes had the equivalent weight of 36 and 29 times that of our sun respectively — much larger than most black holes, which typically have a mass equal to about ten times our sun. At the time of the collision, scientists estimate that they orbited each other at an astounding rate of 75 orbits per second.
The resulting black hole, however, was not the 65 solar masses one would expect from addition, but rather 62. This collision resulted in the mass of about three suns being converted to energy and released in a fraction of a second, which gave rise to particularly turbulent gravitational waves.
That wave first reached the LIGO facility in Louisiana, followed by the one in Washington state just seven milliseconds later. This allowed physicists to locate the black-hole collision as having occurred somewhere in the southern sky. The tiny time delay in itself proved that gravitational waves move at the speed of light.
As well as confirming a century-old theory, the detection of gravity waves may also have a practical application that can help us uncover more secrets of the universe. Until now scientists have relied on light to observe the cosmos, but if we can find a way to design telescopes that use gravitational waves, we may be able to probe into parts of the universe where even light cannot reach and drastically increase our observational field.
Such ‘Einstein telescopes’ could potentially track black-hole mergers, identify the collisions of ultra-dense neutron stars, investigate exploding stars and unearth theoretical “cosmic strings” left over from the big bang. Gravitational waves will give scientists an identifiable marker for when objects don’t emit visible light.
Scientists from the California Institute of Technology and the Massachusetts Institute of Technology have led the project, supported by a variety of international scientists and institutions. In fact, the scientific paper published names 1,004 individual authors.
The members of the LIGO Scientific Collaboration based at the University of Toronto include Harald Pfeiffer, Prayush Kumar, Kipp Cannon and Heather Fong, a physics graduate student. CITA researchers contributed to the search pipelines that identified the black hole merging and the theoretical waveforms that established the black hole masses and spins.
With this new discovery, we are one step closer to peering further into the final frontier and understanding where our universe came from.