Saturday, 19 September 2015

Waves in Space and Time: LIGO reopens UPDATED

Back in September I made the post below about LIGO reopening. Well, almost immediately after reopening, they made a discovery! The news made headlines and rightly so, they simultaneously confirmed that gravitational waves exist, added further credence to General Relativity, and made the first observation of a binary black hole system and merger.

It's worth pausing to consider what they have achieved here. Something that was first predicted in 1916 has been discovered 100 years later. The signal detected caused the length of LIGO's 4km long arms to change by a fraction of a protons width, the waves themselves originated from the merger of 2 black holes, each of about 30 times the mass of the Sun, which occurred 1.3 billion years ago. The event liberated something like 3 times the mass of the Sun in pure energy as gravitational waves, and the black holes reached velocities of ~ 60% of the speed of light immediately before their event horizons merged.

The next step is to carry on making observations - there are several sources that could be detected in theory, from compact object collisions, to the cosmological background thought to originate from quantum fluctuations during inflation!

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LIGO (Laser-Interferometer-Gravitational-Wave-Observatory) in Louisiana has restarted after a 5 year hiatus. It is now around 10 times more sensitive. But what does LIGO do?

LIGO is an experiment to detect gravitational waves (GWs), ripples in space-time predicted by General Relativity. They have never been directly detected, although their existence has been strongly inferred from the mechanics of systems such as binary neutron stars. Essentially, GWs are though to be emitted from accelerating masses in much the same way electromagnetic waves are radiated from accelerating charges. The problem is, GWs are weak and the most powerful sources (e.g. black hole mergers) are so far away, the signal here on Earth is tiny.

LIGO aims to measure the stretching and compression of space caused by a GW passing through the experiment. It consists of two 4 km long evacuated tunnels, orientated at right angles and meeting at one end. From the corner they meet, laser beams from the same source are fired along the tunnels at right angles. The two beams are bounced off mirrors at the far ends. They come back along the tunnels and recombine very near to where they started. If the experimental set up is perfect (ie perfect vacuum, no vibrations, exact distances), each beam will travel exactly the same distance and thus when they recombine, they will be perfectly in phase still. Another way of thinking of it is to say that the time of flight of the photons is identical for each beam. The diagram below shows the set up, which is very similar to a Michelson-Morley interferometer.

If one beam is shortened/ lengthened (an interference pattern produced), we can determine that a GW has passed along one of the beams. The ideal situation is a GW parallel to one tunnel and perpendicular to the other.

LIGO should now be capable of detecting GWs of frequency ~100Hz, which is similar to the lower end of human hearing (hence 'listening to the comos'). For different frequencies, and more sensitivity, the next experiment will be LISA (Laser-Interferometer-Space-Antenna), now eLISA (e for evolved). This will involve 2 spacecraft separated by millions of km. The same principle will be used, but now the 'arm length' is millions of km, not 4 km. Hence greater sensitivity can be achieved. The first prototype of this experiment, LISA pathfinder, is in construction and will be launched later this year. 

It's an exciting time: we are on the eve of testing one of the final predictions of General Relativity, and opening up a whole new type of astronomy.


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