If you have an interest in physics, chances are that in the past few weeks you heard talk of something called LIGO, or perhaps the term “gravitational waves”. Physicists the world over have been talking after a rumour began on Twitter. Posted by physicist and science communicator Lawrence Krauss, the rumour is that an experiment called LIGO has discovered a signal believed to be caused by gravitational waves. This comes no more than a week after the experiment restarted.
LIGO stands for Laser Interferometer Gravitational-Wave Observatory. It has been running since 2002, and in its first run, which lasted until 2010, it failed to discover any trace of gravitational waves. In the interim it received a huge upgrade, tripling the sensitivity of its detectors. The renewed experiment, known as Advanced LIGO, began on the 18th of September, and if the rumours are true, this is massive news for the physics community.
But what are gravitational waves anyway, and how is LIGO supposed to detect them? Gravitational waves were first predicted to exist by none other than physicist and big hair enthusiast Albert Einstein. They are the most elusive, and one of the most interesting phenomena predicted by his general theory of relativity. In theory, gravitational waves are produced by cataclysmic events, for example by the merger of two black holes. If such an event happened close enough to the Milky Way, the resultant waves should be seen by LIGO. What scientists really want to detect is a “chirp”; the signature of two black holes spiralling toward each other. These would emit waves that increase in pitch and volume over time, eventually reaching the threshold for detection by LIGO.
In order to detect the waves, LIGO has two mirrors at either end of a four kilometre long vacuum filled tunnel. A laser is bounced between these mirrors. In theory, passing gravitational waves would change the length of the tunnel by miniscule amounts which could then be detected. The sensitivity of the detectors is so great that previously it could be knocked offline by trucks passing kilometres away. Since the upgrade, it has been online for much longer periods of time.
Is there anything to the rumours? There are a number of possible explanations. The first is simple: it is nothing but a rumour, started by some troublemaker and unwittingly (or even wittingly) spread by Dr. Krauss. The second is that waves really have been detected. If this is the case the announcement may take some time. The data must be painstakingly analysed to make sure that it is a true detection and not an error. It should also be noted that while the experiment officially started in September, the detectors had been active since June. Any detections in this period may have been caused by the team calibrating the equipment. The third possible explanation is even more curious. A team of three calibration experts on the experiment have the power to “insert” a wave into the experiment by manually moving the mirrors. The purpose of this is to allow the team to practise the analysis techniques that will be needed for a real event. Two such rehearsals took place in the initial run of the experiment, and it was only when the team was ready to announce the results that they were notified that it was a test run.
The experiment is due to be upgraded on a continual basis until 2021, and it is hoped that the next round of upgrades, which should be completed by the end of the year, will allow scientists to detect events around ten times per year. If this is the case, and if detections become regular and routine, then the team will learn to recognise them almost in real time. If this is the case, astronomers could be notified and could try to scan the sky for possible causes of the waves.
Einstein published his theory of general relativity 101 years ago. Within the past century, many advances have been made in science and technology and maybe we will soon witness the first signs of its last unobserved predictions.