Where were you when gravitational waves were detected?

About a billion years ago, a billion light-years away, two black holes collided. Out of their total mass of some 60 times that of our Sun, about three solar masses were converted into energy. The amount of energy thus released can be calculated with Albert Einstein’s famous 1905 equation, E = mc*2 {energy = mass x (speed of light squared)}.1

The resulting disturbance in space-time was detected as gravitational waves on 14 September 2015, whose existence was predicted by Einstein 100 years previously, and announced (after rigorous checking) on 11 February 2016. This is yet another confirmation of Einstein’s General Theory of Relativity.

Following on from his 1905 Special Theory of Relativity, Einstein extended his theory to include gravity, publishing his findings in his General Theory in 1915. The Special Theory stated that: the speed of light in vacuum was a constant and could not be exceeded; space and time were aspects of each other and should be called space-time; for fast-moving objects, time passes more slowly, lengths are decreased, and mass is increased; and light from approaching or receding objects is blue-shifted or red-shifted respectively. All of these are contrary to “commonsense” and yet have all been verified.

In his General Theory, Einstein says that: mass distorts (curves) space-time; light heading towards or away from massive objects is blue-shifted or red-shifted respectively; time passes more slowly close to massive bodies; and moving masses produce ripples or waves in space-time. It also led to the prediction of black holes.

The first success of the theory was that it explained the anomalous orbit of Mercury, an ellipse whose major axis moves round gradually by a greater amount than predicted by Newton’s theory. This results from the curvature of space-time by the mass of the Sun, altering the geometry of the orbit from that in a perfectly flat space-time. This ‘precession’ of Mercury’s orbit had been a major defect in Newtonian physics. The theory also predicted the bending of light from distant stars passing close by the Sun: it was a major triumph for the theory and made Einstein famous when this was observed during the total solar eclipse in 1919.

The gravitational red- or blue-shift was demonstrated about 50 years ago; gravitational time dilation was shown over 40 years ago and is corrected for continually in GPS systems and other orbiting satellites to keep them synchronised with Earth-based clocks. Black holes have been adequately demonstrated more than once.

In 1916, Einstein realised that if a mass moves, the distortion in space-time should also move, spreading out like ripples on a pond: these ripples in space-time are gravitational waves. These should also be detectable because they would cause changes in length of objects in their path, a rhythmic squashing and stretching at right angles to the direction of the waves. The first gravitational wave detecting system was built nearly 50 years ago but was unsuccessful. This is because the effect of the waves is so small that the apparatus was nowhere near sensitive enough. Indirect evidence of gravitational waves was obtained some 40 years ago by observing binary pulsars. These were found to be spiralling in towards each other as predicted if they were radiating away energy in the form of gravitational waves.2

The recent detection of gravitational waves was by LIGO (Laser Interferometry Gravitational-wave Observatory) detectors in Hanford and Livingston, US. These consist of two 4-km-long vacuum tubes at right angles. Laser beams are split, sent down each branch, reflected back and forth 400 times, recombined, and detected. The beams interfere3 with each other and, if gravitational waves arrive and change the length of one arm more than the other, the interference pattern will change.

Unfortunately, the change in length is predicted to be less than a millionth of the width of an atom, even if the mass involved is large. The waves detected came from the movement of a very large amount of mass, the collision of two black holes of total mass about 60 times that of our Sun. In this, about three solar masses were converted to gravitational wave energy in about a fifth of a second.1 In the signals detected, this is seen as rapidly increasing oscillations which then cease as the black holes form one large one. This is the same “shape” as the waves of a ‘chirp’ sound.4 It was also proved that gravitational waves travel at the speed of light and that the graviton (the particle associated with gravitational waves) must be massless, like photons of light.

Now this has been achieved, more and better gravitational wave detectors will be designed. Some will be space-based, with much longer distances so that they will be more sensitive. Other LIGO detectors in different countries will enable us to pinpoint more accurately where the waves are coming from. It may be possible to use pulsars (very regularly pulsing stars) as gravitational wave detectors by observing slight delays in their signals caused by the passage of waves. Since gravitational waves can pass through everything, while light can’t, we will be able to “see” hitherto invisible regions of the universe (like the centre of our galaxy). Different types of gravitational waves are produced by different events, such as stars being swallowed by black holes, neutron stars spiralling into each other, or even relic waves from the ‘big bang,’ giving us a source of information about that in addition to the cosmic microwave background.

Government bean-counters (and not just them) constantly question the value of basic, curiosity-driven, ‘blue skies’ research. Why can’t the money be spent on more practical things or given back to taxpayers? This ignores the importance and interest of knowledge about us and our universe. Why should we only know what is of value to our employers? It also ignores the spin-offs of basic research, some of which have transformed our lives. These include transistors, lasers, LEDs, nuclear power, computers, microwave ovens, accurate GPS, X-ray machines, the structure of DNA, MRI scanners, proton beam cancer therapy, genetic engineering, DNA fingerprinting…and the internet! ………………………………………………………………………………………………………………….. 1 This sounds like a lot and it is (6 x 10*30 kg x (3 x 10*8)*2). But remember The Hitch-hiker’s Guide to the Galaxy: “Space is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is.” My back-of-an-envelope calculations show that, if this energy is spread out across a sphere of radius 1.3 billion light-years, the energy reaching us is about 1 milliwatt per metre squared. I don’t know how reliable this calculation is as the first time I did it I got a number 10 billion times smaller!

2 The Earth is spiralling in towards the Sun through gravitational wave radiation but will take 10 trillion times the current age of the universe to hit the Sun. Don’t panic!

3 The light waves meet and recombine. The apparatus is designed so that they arrive out of step and destructively interfere, leaving darkness! If a gravitational wave arrives, altering the length of the arms differently, the light beams will start to interfere constructively and some light will appear.

4 https://caltech.app.box.com/s/ta7y0m97lqemz99lj1oztvf3mr8758je/1/3517143543/29359315721/1

Excellent explanations of gravitational waves and their discovery:




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