Thursday this week saw a wonderful gift from a large international team of scientists and engineers to the rest of humankind. They reported the first detection of gravitational waves following their prediction by Albert Einstein a century before (in Einstein, A., Annalen der Physik 49, 769-822 (1916)] .
The discovery is the first-fruits of overwhelming human imagination, to conceive of such a thing as a wave that travels in the warp and weft of space and time itself, of the extraordinary talent, skill and care taken in building the exquisitely sensitive LIGO detectors, and of the deeply impressive patience, over decades, of researchers willing to devote fruitless decades to the long search in the hope that it will one day open our gravitational ‘ears’ to the sounds of the universe.
The astounding work deserves to be shared and enjoyed widely, and the scientists and science journalists have done an excellent job in trying to do so. Even so several people have asked me, quite baffled, to explain what has happened. I have found that the analogy of ripples on a lake works well:
Imagine that you are standing on the shore of a calm lake. You have a small toy boat with you that you set afloat at your feet. Then you find the largest stone within reach and heft it away into the lake. On landing and sinking it creates a large splash a few metres away from you. Then ripples start to spread out in circles from the entry point, widening and weakening as they travel. After a few moments, the toy boat at your feet is disturbed by the ripples as they arrive at the shore, and very gently starts bobbing up and down. The splash is the equivalent of the merging black holes, the surface of the lake behaves like the space between the Earth and the distant galaxy where the black holes dwell, the ripples model the gravitational waves, and the toy boat corresponds to the sensitive laser-beam detectors, which also were caused to ‘bob’ by tiny amounts as the waves passed by.
If you understand that, then you have understood everything about the LIGO discovery, apart from the details of the particular space-time geometries of the gravitational waves, and some rather different numbers. The splash was not metres away but 1.3 billion light years, in a galaxy far, far away. We know that very simply from being able, thanks to Einstein, to work out how much energy was released at the source, and therefore how far away it must have been to produce the signal strength observed here on Earth. The wave speed was not a few metres a second, but the speed of light, fast enough to circle the earth 7 times a second. The detector bob was not a few millimetres, but one thousandth of the diameter of a single proton. The sheer sensitivity achieved by the experimentalists to achieve that measurement is perhaps the most impressive aspect of the whole work.
The opportunity to enjoy, celebrate and contemplate this beauty is an opportunity I looked forward to in Faith and Wisdom in Science (which, coincidentally, appeared in paperback this week). The story embodies many of the aspects of the cultural narrative of science that I advance there: the long human story of looking into nature and seeing with our minds into its hidden structure, the experience of pain and longing in the often painful process of gaining wisdom about nature, the extraordinary human ability to do this – to reconceive of cosmic structure almost as if we were creating a universe, the experience of the joy of a sort of reconciliation when we do it, the faith in the rationality of the universe and our continuing ability to understand it. I cannot think of a better example to illustrate how science ought to be thought of as a humanity, rather than in some form of opposition to the disciplines of language, art and poetry. I found myself writing this brief facebook post on the day itself:
This beautiful discovery is utter human joy. The last time we saw such a wonder was in 1888. Heinrich Hertz detected waves of the electromagnetic field predicted by James Clerk Maxwell’s field equations of electromagnetism in 1865. Einstein wrote the field equations of gravity in 1915. They have wavelike solutions. It took 100 years to see them. The last 40 or so required men and women to dedicate fruitless decades to this beautiful idea. Today their gift is inestimable. It is a sort of poetry – wild imaginative force that entertains the dance of death between black holes of immense proportion, constrained by the tight form of space time curvature and the displacement of a thousandth part of a proton. Yet we noticed just that. The contemplation of all this is a gift. Thank you my patient, loving, enduring, believing fellow scientists.
There are still some glimmers of hope that there are still human activities that have not entirely been monetarised in our Western society. There is hope that there may be more if we can learn to draw from the ancient resources of Wisdom thinking for today. A celebration of ‘natural philosophy’ – of ‘the love of wisdom to do with natural things’ that is the real name for ‘science’ – as a deeply human activity, and a necessary part of our individual humanity, has been enriched this week.
Finally, here are the actual signals detected at the two sites, showing the rapidly increasing and quickening oscillations as the two giant black holes circle each other and merge into silence. Superimposed on the data are the calculations from Einstein’s equations for general relativity, supposing just this scenario. The ‘chirp’ song could not be clearer. Not only have we seen gravitational waves for the first time, but also made the first direct detection of a black hole (in fact of course a pair of them).
The original paper in Physical Review Letters is open-access, very readable, and here. What a gift!
The Lord brought me forth as the first of his works,[c][d]
before his deeds of old;
23 I was formed long ages ago,
at the very beginning, when the world came to be.
24 When there were no watery depths, I was given birth,
when there were no springs overflowing with water;
25 before the mountains were settled in place,
before the hills, I was given birth,
26 before he made the world or its fields
or any of the dust of the earth.
27 I was there when he set the heavens in place,
when he marked out the horizon on the face of the deep,
28 when he established the clouds above
and fixed securely the fountains of the deep,
29 when he gave the sea its boundary
so the waters would not overstep his command,
and when he marked out the foundations of the earth.
30 Then I was constantly[e] at his side.
I was filled with delight day after day,
rejoicing always in his presence,
31 rejoicing in his whole world
and delighting in mankind.
Hymn of Wisdom from Proverbs chapter 8