Archived

Math & Science- Mar 22, 2016

Gravitational Waves: Yet Another Grand Victory For Theoretical Physics

Physicists had accurate enough equipment. Data analysis methods made sure they were able to dig the signal from background noise. The team at LIGO was ready to make history one way or another – by finding them or by realising they do not exist.

Gravitational Waves, Photo: Caltech/MIT/LIGO Laboratory

Gravitational Waves, Photo: Caltech/MIT/LIGO Laboratory

Even though general relativity is over a 100 years old, it is the most accurate theory explaining gravitation. The theory predicts many phenomena: black holes, changes in time observation based on speed, gravitational waves zooming through the universe at the speed of light, and many more. When scientist expected them to show up in observations they were all found to be correct: we can observe black holes, like the one in the centre of our galaxy, and minute corrections to the clocks on satellites makes our GPS system work. And then there’s gravitational waves.
These waves are ripples in space-time itself. When an object is accelerated, it produces gravitational waves. Therefore, according to general relativity even by something as simple as moving your hand you make tiny disturbances in the space-time – but they are so tiny that they are, in fact, immeasurable. It requires huge objects to make gravitational wave that we could observe.
But up until now, we could not see them. This all changed with the creation of LIGO: The Laser Interferometer Gravitational-Wave Observatory. Designed by some of today’s world leading experimental physicists, it became accurate enough to measure even the smallest of disturbances in two rays of light. Like ones produced by two black holes colliding.
But the story of the discovery of gravitational waves begins long before today.

A waves sensation was just dust

The efforts to prove the existence of gravitational waves began much earlier than LIGO was built. In 1974 Hulse and Taylor found the first binary pulsar star. They analyzed this binary star and found that it lost energy as it rotated. This energy loss could be explained exactly by the energy carried away by the gravitational waves. This was considered indirect evidence and not as proof.
Many years later, BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) came close to find direct evidence of these waves. It analyzed polarisation of the cosmic microwave background radiation which is left over from the big bang. According to the theory of inflation, the early universe expanded rapidly. This expansion, in much the same way you move your hand, should have produced gravitational waves. In an early universe filled with hot plasma these waves would have changed the polarisation of light (pdf). These changes in polarisation would then show up as certain patterns in background radiation.
BICEP2 was built to find these patterns.
BICEP2, and lo and behold: the team of research found the right polarisation and announced on March 17th, 2014, the discovery of the primordial gravitational waves!
It was a huge discovery, providing strong evidence for the inflation and demonstrating the existence of the gravitational waves it caused.
But as with any good science, doubts remained, leading scientists to always try and show that previous discoveries are indeed true – or false. When half a year later data from the Plank satellite was published, scientists realised for their great disappointment that the polarisation was not due to the primordial gravitational waves. It was caused by interstellar dust, and once again, they had to go back to work to find a direct proof of inflation – and gravitational waves.

Provided to you by: two large black holes

0.7-1.9 billion years ago in a galaxy far far away two black holes collided, reaching a final speed of over half the speed of light. This collision produced huge disturbance in space-time – approximately three times the mass of the sun was lost in form of the energy of gravitational waves. The firs-ever detected gravitational wave started spreading through space, eventually hitting LIGO and leading to the discovery long awaited.

Laser interferometer, Photo: Caltech/MIT/LIGO Laboratory
Laser interferometer, Photo: Caltech/MIT/LIGO Laboratory

LIGO is an instrument that uses laser interferometry to detect gravitational waves. It has two pipes that are perpendicular, inside of which are laser beams bouncing off mirrors at the end of these pipes. According to their structure, a gravitational wave would stretch one arm and shorten the other. Due to the length difference, the laser beams would go off the same phase. This would make an interference pattern, in the same way to waves in a pond create a mixed pattern of waves, and thus a detectable signal – even though the stretching is smaller than the diameter of a proton.
There are two observatories of this kind in America, placed on opposite ends of the country, and both of them recorded same signal that matched the predicted gravitational wave nicely. Yet still the team at LIGO couldn’t be sure that the signal was real: in the testing of the machine, some people were instructed to inject signals that look like gravitational waves, in order to see how these would present. This forces the group to analyze the data well and to make no mistakes. After the false alarm of BICEP2 the team wanted to be extra confident that they had caught the real thing. The discovery was already peer reviewed before the announcement and the celebration of the discovery could begin.

The meaning behind the hype

The discovery means that every accelerated body generates waves in space and time. It’s really awesome to think that by drinking your morning coffee you produce ripples in the space-time that move at the speed of light. But aside from the coffee time thoughts are many other reasons for being excited about gravitational waves:
There aren’t any technological applications in sight, but there are ways to use this for science. For the first time in the history of astronomy, there is another way to study cosmos other than light. It is like a whole new sense for humankind. This is a new channel to study massive collisions far away in the universe, and, for example, collisions of black holes were completely out of reach because they produce no light. In future, there will be more powerful instruments to detect these waves, for example eLISA, which consists of three probes orbiting sun with one million kilometres distance from each other. Its planned launch is in 2032.

LIGO Locations, Photo: Caltech/MIT/LIGO Laboratory
LIGO Locations, Photo: Caltech/MIT/LIGO Laboratory

Technological applications are hard to find. Electromagnetic waves like light are easy to produce and detect. That is why we use them to carry information. Gravitational waves on a detectable scale are hard to make, and we need huge hi-tech observatories to spot them. Also their effects on matter are tiny so they can be neglected in designing the technology of near future. In the distant future we might have to take into account these small effects of gravitational waves, because we will learn to manipulate matter more and more delicately. Additionally, they might be important for sending information over vast distances, because they go through matter almost completely unchanged. First we would have to get over of the huge difficulties of making in observable amounts and learn to detect smaller waves.
This discovery is a huge thing for theoretical physics. Like the Higgs boson but also in sense of approval for the theoretical physics of our time. Einstein made his theory of gravity using almost only logic and math. The precession of the perihelion of Mercury helped Einstein but that is not much for jumping to a theory with a flexible space-time. The current state of theoretical physics is similar. The theory is way too far to be tested with experiment and the advancement of theories is based entirely on logic and math. This discovery doesn’t only prove a prediction of the General Relativity to be correct but also it is a proof that theory can progress with math and logic. The discovery of gravitational waves proves that nature can be predicted with tools of logic and mathematics. It gives hope that the direction in which theoretical physics is going is valid.

Joonas Hirvonen Joonas Hirvonen is an eager freshman of theoretical physics trying to peek deeper into the physical world.