• Question: What is the LIGO discovery?

    Asked by Eleanor to Alice, Bose, Christian, Emma, Steve on 4 Mar 2016. This question was also asked by Maisie.
    • Photo: Christian Killow

      Christian Killow answered on 4 Mar 2016:

      The recently announced discovery by the LIGO Scientific Collaboration was the first direct detection of gravitational waves. This was such a big deal because, although they were predicted by Einstein 100 years ago, it is only after decades of developing ultra-precise detectors that we have finally made a detection. More importantly, the era of gravitational wave astronomy has now been opened – we will be able to detect events like black holes crashing into each other, other massive objects being eaten by black holes, and lots of other events that previously we were blind to.

      We usually describe gravitational waves as ‘ripples in spacetime’. Not an easy concept to visualise! They are passing through you right now, and as they do so they stretch you in one direction and squeeze you in the other, and then squeeze and stretch, and then stretch and squeeze, and so on, as the wave passes.

      Why don’t you notice it? The effect is incredibly small. The LIGO interferometers suspend ‘test masses’ (big lumps of glass) at the end of 4 kilometre long ‘arms’. There are two arms and they form an ‘L’. By monitoring the movement of these test masses the stretching and squeezing motion was measured. The test mass positions had to be measured to around 1/10,000,000,000,000,000,000 of a metre!! Or 10 raised to the power of -19.

      To put this in perspective, we need to measure changes about 10,000 times smaller than the diameter of a proton – and they are already really very small!

    • Photo: Steve Marsden

      Steve Marsden answered on 4 Mar 2016:

      LIGO discovered the existence of gravitational waves, which were caused by two black holes orbiting around each other and eventually colliding.

      A very hand wavy way of thinking about gravitational waves is to compare them to the Doppler effect you can hear when an siren drives past you at high speed. As it drives towards you the frequency is shifted up, as it drives away the frequency is shifted down. As the two black holes orbit each other they alternately more towards you, and away from you, in quick succession at speeds close to the speed of light. This essentially results in the gravitational force to be Doppler shifted, increased as the black hole comes towards us, and decreased as the black hole moves away. This quick succession of the gravity increasing and decreasing repeatedly travels through space at the speed of light, and are referred to as gravitational waves.

      This is important as relativity states that gravity must travel at the speed of light, and cannot be instantaneous. So if say the sun were to vanish, we would continue orbiting where it was for several minutes before flying off in a tangent. If gravity were instantaneous, and not limited by the speed of light, this Doppler shift would not be present. So by seeing these gravitational waves, we know that gravity is limited to the universal speed limit, and that once again general relativity is correct.

      (Just want to stress that the Doppler analogy is far from perfect, but gives the general idea.)

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