By Marcus Wilson 12/02/2016 7


The big breaking physics news is the detection of gravitational waves. These waves are distortions in space-time, caused by a large mass doing something spectacular (two colliding black holes in this case)  that propagate across the universe and create tiny changes in space when they reach us.

The commentary here describes what goes on. Essentially, things change their length/width. When a gravitational wave passes through my office (say ceiling to floor) one can imagine the length of the office increasing slightly, coupled with a decrease in the width of the office, followed by the reverse – a decrease in the length and an increase in the width.  But its not just that the bricks that make up the room vibrate (e.g. as in a seismic wave) – its the whole of space that does it.

These waves were predicted by Einstein in 1916, just after the publication of his theory of General Relativity. Their discovery is further evidence for the theory. But it’s not just about Einstein. Gravitational waves provide another way of observing the universe – ‘seeing’ what’s going on. Up to now, we’ve been stuck with light-based observations (be it visible light, infra-red, microwave – they all are electromagnetic waves). There are neutrino observations too, but these aren’t exactly easy. But gravitational waves are something else – it’s like seeing AND hearing something, rather than just seeing it.

So how are they detected? The concept is rather simple, as explained in the commentary. Build a (large, meaning 4 km in the case of LIGO) interferometer with two arms. Pass light up and down each arm. The light from the two different paths will interfere – such interference could be constructive (if a peak from one arm comes at the same time as a peak from the other) or destructive (if a peak comes with a trough). If everything is stable, the interference is stable. But when a gravitational wave passes, the arms change their lengths. Not by much. The light takes longer to pass up and down one arm, and shorter to pass up and down the other. Now the timings of the peaks and the troughs change, and the interference signal changes. We detect a gravitational wave.

The difficulty to now has been detecting the tiny signals amongst larger ‘noise’ signals, but a recent upgrade to the LIGO detector has done its job. Well done LIGO team!

Featured image: LIGO Hanford Observatory, Caltech/MIT/LIGO Lab.


7 Responses to “Gravitational Waves”

  • My problem with this experiment is the whole thing does not follow scientific method very well.
    There is no way to prove the signals picked up by the laser interferometer were caused by the collision of black holes a billion years ago. This is a claim that cannot be proved but is taken as fact.
    This Laser Interferometer had no signals since 2002 and the first signals it does get and the whole world knows about it.
    The last time I know of an interferometer being used was the Michelson Morley experiment and that was to dis prove the Ether which it did not do.
    So my question to the scientific community is How do they know these were gravity waves, what evidence is there that would convince me 100% that these are gravity waves, where is the science ?

  • The full article gives more details of how one can rule out other sources. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102
    Essentially, 1. the ‘same’ signal was detected simultaneously by two different detectors (in different locations) and 2. the shape of the signal is in agreement with numerical predictions of what it should look like. The first point suggests that the signal was of non-local origin (e.g. wasn’t a local earthquake or large aircraft flying low overhead) and the second that it is a gravity wave,. One event does not prove 100% beyond any doubt whatsoever that gravitational waves exist, but is certainly the first step and the publication is certainly justified. The LIGO collaboration will now want to be detecting many more of these cases.

  • Thanks I fully expect this is the last we will hear about gravity waves .
    Good to hear from science that it is not 100 % proven at last some rationality.
    So it is not a Certainty and never will be let us remember this.

    • I don’t intend my statement “One event does not prove 100% beyond any doubt whatsoever that gravitational waves exist” to mean “all science is bunkum”, which is what you are suggesting. It does not mean that you can pick and choose the truth based on what you want the truth to be. In the fullness of time, we shall see whether LIGO detects more gravitational waves or not. At present, I’m happy to believe that they have detected one. If there turns out to be some other explanation, I am not going to be ashamed of the fact that I believed their statement or give up on science because it is all wrong. I will modify my beliefs based on the evidence.

    • So, the BICEPS2 team at the South Pole announced they had indirectly seen a gravitational wave in 2014, then retracted that after further analysis of the data. That does not mean that the recent LIGO detection, by a completely different and much more direct method, is also false. It does not mean that gravitational waves do not exist.

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