The world of physics is reeling from the aftershocks of the announcement of the discovery of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US. The observatory has confirmed that on 14 September last year it detected a signal caused by two black holes colliding and coalescing, squeezing and stretching the fabric of space-time.
Gravitational waves were predicted by Albert Einstein in his theory of relativity in 1915, and were the last of the theory’s predictions to be proved true. Einstein said that all massive bodies distort space-time (famously, like a heavy ball denting a rubber sheet), and that those distortions give rise to the phenomenon we experience as gravity. Events involving very large masses – such as colliding black holes – should ripple space-time, he said. The problem is that gravity is such a feeble force on the scale of the universe that such ripples would be tiny and difficult to detect against the ‘noise’ of all the other signals in the universe.
“This is a huge discovery in physics - up there with the Higgs boson, the mass of the neutrino, discovery of the electron, electromagnetism, the Copernican revolution and Newton's laws,” said Prof Andrew Coates, Professor of Physics at UCL. “It confirms what has only been theory before - that gravity affects space and time. The really new thing it will allow is to peer back towards the beginning of the Universe itself - up to now we have not been able to do that."
LIGO is two L-shaped instruments, with each arm measuring 4km in length. One of the observatories is in Livingston, Indiana, and the other is in Hanford, Washington State; making them 1,865miles (3,002km) apart. A laser travels down the length of each arm, and is reflected back on itself, making a very sensitive measurement of its length. If a gravity wave hits the instrument, the effect would be to change the length by a very small amount. Any event powerful enough to do that would be felt at both sites, as gravity waves would sweep through the entire planet.
And this seems to be exactly what happened. In a paper published in the journal Physical Review Letters (with over 1,000 authors), the team explained how both observatories detected a length-change equivalent to a thousandth of the diameter of a proton at both observatories at almost the same time; by an amazing freak of luck, the instruments were in ‘engineering mode’ to work out any system problems before the experiment proper was due to begin.
The event that LIGO detected is mind-boggling in its scope. The teams have interpreted the signal as coming from two black holes, one weighing 29 times as much as our sun and the other 36 times, orbiting each other. The energy given out by the gravity waves forced them to spin around each other faster and faster, and move closer together, until they were moving at half the speed of light, with each orbit taking milliseconds. Eventually they collided and merged into a single body with a mass 62 times that of the sun; the missing three solar masses were converted into the energy of the gravity ripple that reached the Earth. The new black hole then emitted more gravity waves as its shape smoothed out to a sphere.
This event happened 1.3 billion light-years away, which, as gravity waves moves at lightspeed, means that the waves had been travelling for 1.3 billion years. When the collision happened, the continents of the Earth weren’t yet in their current positions: North America, Antarctica, West Africa, Australia, the Amazonian part of South America and several other large landmasses had all recently collided to form a supercontinent known as Rodinia.
The discovery means that humans now have a new way of looking at the universe. For most of our history, we have only been able to use light; visible at first, then in the 20th century, using more and more of the electromagnetic spectrum from radio to X-rays, with each telling us about different objects at longer distances from Earth and further back in time. More recently, we’ve been able to use neutrinos, sub-atomic particles emitted by highly energetic events such as supernovas. But observing gravity waves will allow us a new way to probe the so-called ‘dark universe’ that emits no light or other particles we can detect. This first observation already tells us something we didn’t know before: black holes of this size were not thought to exist, and this is the first direct observation and measurement of a binary black hole merger. Gravity waves could also tell us about the early universe and how it behaved after the Big Bang, and maybe about the so-called ‘dark matter’ and ’dark energy’ that are believed to make up most of the mass of the universe.
One point of interest for the engineering community will be what effect the discovery has on ESA’s proposed eLISA mission, whose forerunner, LISA Pathfinder, was launched to its station at the L1 libration point of the sun-moon-Earth system earlier this year. eLISA is intended to detect gravitational waves using three spacecraft flying in formation in a triangle with sides a million kilometres long; each craft contains two isolated masses of a gold alloy floating free inside a container. The mission location is such that the gravity of the sun, moon and Earth cancel each other out so the only thing that can disturb the test masses’ position is a gravity wave. LISA Pathfinder is a single one of these three proposed craft, and is designed to test whether the measurement technology works. The funding for the full eLISA mission is yet to be confirmed; the LIGO discovery could be seen as giving it a target to investigate, now that we know there is something to investigate.