Conferencia de Stuart Clark en Ciencia y Sociedad


Published on

  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Conferencia de Stuart Clark en Ciencia y Sociedad

  1. 1. Will we ever understand the universe? Stuart Clark Lecture given on 14th March 2013 Part of the Santander Foundation Science and Society Lecture Series Madrid, SpainOne hundred and thirty-four years ago today, on 14 March 1879, Albert Einstein was bornin Ulm, Kingdom of Württemberg, in the German Empire. He stands today as an icon ofscience, a man who gave us a way to understand the universe as never before. The discipline of cosmology starts with Einstein and his theory of General Relativitybecause, for the first time in history, he gave scientists a way of writing down an equationthat encapsulated the whole universe. All the individual objects in the universe could bereduced to a single mathematical term, and the behaviour of the universe calculated fromit. The future of the universe and its past could be calculated if we know the precisestate of it today. In centuries past, such knowledge would have been seen as that of thegods. It was a work of supreme self-confidence and mathematical competence. At an after dinner speech in Einsteinʼs honour, the English playwright GeorgeBernard Shaw encapsulated the progressive nature of science when he said, “Ptolemymade a universe, which lasted 1400 years; Newton also made a universe, which haslasted 300 years.” Then he quipped, “Einstein has also made a universe and I canʼt tellyou how long that will last.” As we shall see, it is possible that Einsteinʼs General Relativity will not even make itto its centenary in 2015. The question is: what will replace it? Will that be a final theory orwill it be another incremental step? In short, will we ever understand the universe?At the time when general relativity was born, Europe was a desperate place. The FirstWorld War was raging and Einstein was becoming an increasingly isolated figure. Hisoutspoken views against nationalism were an embarrassment to his colleagues and toGermany. Although born there, he had taken Swiss citizenship in 1900. Removed from the war effort, with very little support, he worked on his pet theory toextend the work by Isaac Newton and find a more complete description of gravity. Page 1 of 9
  2. 2. Newtonʼs work had been a landmark, a way of describing how objects would movein the presence of each other through the force of gravity. It explained why the planetsmove across the sky, why dropped objects fall to the floor yet the Moon in the sky doesnʼt. At the time, it was called a ‘System of the World’ – the seventeenth century phrasefor what we would now call the theory of everything. British astronomer Edmond Halley,who was a friend of Isaac Newton, even described the work as the perfection ofastronomy. By the end of the nineteenth century, this view had become so entrenched in somequarters of the scientific establishment that science was thought to be essentiallycomplete. In 1900, the great British scientist Lord Kelvin addressed the meeting of the BritishAssociation for the Advancement of Science. He famously said, ‘There is nothing new tobe discovered in physics now. All that remains is more and more precise measurement.’ What he forgot was that scientific advances are often made through bettermeasurement. Only when you develop better instruments, do movements and readingsthat do not fit the current theory become obvious. If they persist with repeated observation,then it is science’s job to explain them. By 1900, one such anomaly was clearly apparent: the movement of Mercury. It wasdrifting away from where Newton’s law of gravity said it should be. Initially, it was thoughtthat an undiscovered planet was pulling it off course but Newtonian gravity could notprovide an adequate solution to where the unseen planet was, and observational searchesfor it during total eclipses were not finding it either. Working in an office with a portrait of Newton on the wall, Einstein solved theproblem in 1915. At the moment of his triumph, he experienced heart palpitations.Obviously he survived but imagine for a moment the legend of Einstein if he had died withthe shock of success! Conceptually, Einstein’s universe is not too difficult to grasp but, to appreciate thebig difference, we have to look first at Newton’s universe. To him, space and time werefixed. They were a rigid framework within which to measure things. Einstein allowed thisframework to be distorted by the masses of the celestial objects it contained. Space andtime were warped in the presence of matter. These distortions create the effect of gravityand explain why things accelerate in the presence of a gravitational field. Despite finding the correct mathematical framework in 1915, it took four more yearsbefore it came to the attention of the rest of the world. This was because the mathematicsis difficult to work through. The theory needed to be verified by somebody independently. Page 2 of 9
  3. 3. Luckily for Einstein, this was possible because the theory made a clear prediction aboutthe effect of gravity on a beam of light. It would be bent by a precise amount. By 1919, the world was in the aftermath of the First World War; countries andempires were collapsing. A new political landscape was being drawn. Change was in theair and a British astrophysicist was on a small African island, setting up his telescope. Hiswork would turn Einstein into an icon. Arthur Eddington was born in Cumbria, England, and educated at Cambridge. Anexceptional mathematician, he was certainly aware of the scientific importance ofEinsteinʼs work and argued that if Newtonʼs theory of gravity was to be overthrown, thenan Englishman should be involved in doing it. Eddington waited until the eclipse of 1919 and measured the deflection of starlightaround the Sun. The amount of this deflection, caused by the distortion in space and timearound the Sun, was clearly predicted by Einsteinʼs General Relativity and alsosubstantially differed from the value offered by Newtonʼs work. Eddington measured a deviation consistent with General Relativity, announced theresult in early November 1919 and Einstein became world famous. The New York Timesdeclared it to their readers with the amazing headline: Lights All Askew in the Heavens Men of Science More of Less Agog Over Results of Eclipse Observations Einstein Theory Triumphs Stars Not Were They Seemed or Were Calculated to be but Nobody Need Worry. A book for 12 Wise Men No More in all the World Could Comprehend It, Said Einstein When His Daring publishers Accepted It But Einstein himself, like Newton before him, knew that from the lofty pinnacle ofachievement, they saw not a final solution but a whole new landscape of possibilitiesstretched out before them. Newton expressed it best when he wrote, “I do not know what Imay appear to the world, but to myself I seem to have been only like a boy playing on theseashore, and diverting myself in now and then finding a smoother pebble or a prettiershell than ordinary, whilst the great ocean of truth lay all undiscovered before me.” Page 3 of 9
  4. 4. General Relativity extended human knowledge into a whole new realm of understanding.Now it was possible to compute the behaviour of the whole universe. The ability of spaceand time to be distorted meant that the universe could be in a state of overall expansion orcontraction. Belgian Georges Lemaître predicted this expansion in 1927, two years beforeAmerican Edwin Hubble ʻdiscoveredʼ it. It is in the interest of historical justice that we must work harder at giving Lemaîtrethe credit he deserves. He also used General Relativity to predict a beginning to theuniverse. We now call this the big bang, yet Lemaître called it ʻThe Day WithoutYesterdayʼ. In June 1966, word reached Lemaître on his deathbed that a spectacular discoveryhad been made. The universe was filled with microwaves – so many that theyoutnumbered the atoms by a billion to one. Extraordinarily, the existence of thesemicrowaves had been mathematically predicted by Ralph Alpher, an Americancosmologist. You could think of the microwaves as the remains of the fireball thataccompanied the universeʼs birth. So there had indeed been a day without yesterday. The discovery of the microwave background was a stunning vindication of scienceʼsgreat belief that mathematics is the language in which the universe is written. In 1623,Galileo Galilei wrote one of the most famous expressions of this belief. It occurs in hisbook The Assayer. He wrote, “The universe cannot be read until we have learned thelanguage and become familiar with the characters in which it is written. It is written inmathematical language, and the letters are triangles, circles and other geometrical figures,without which means it is humanly impossible to comprehend a single word. Withoutthese, one is wandering about in a dark labyrinth.” But why should numbers describe the universe so well? Does it mean that reality ismathematical or are we being fooled by fitting imprecise theories to approximateobservations? If so mathematics is just a tool rather than a fundamental property of the universe.Yet even if that is the case, maths can still be useful.The man who originally proved the value of mathematical astronomy was GermanLutheran, Johannes Kepler. Indeed to George Bernard Shaw’s list of ‘universes’ I wouldadd one more: Kepler’s mathematical description of planetary motion in the early Page 4 of 9
  5. 5. seventeenth century. Kepler was the first astronomer in history to show that heavenlymotion could be been distilled into general mathematical formulae. His three laws of planetary motion hold not just for all the planets in the solarsystem that he knew about, but the three that were subsequently discovered. They arealso true for all the asteroids and comets that circle that Sun, and the almost 1000 planetsthat have been found around stars other than the Sun during the last 17 years. In his laws, Kepler discovered a piece of universal truth so profound that it appliesacross the universe. For astronomers it was the herald of a new way of working and gavethem every reason to believe that the universe was rational and could be captured inmathematics. If something is possible mathematically then, goes the reasoning, it stands a goodchance of being true in reality. By the twentieth century, such mathematical predictionswere truly paying off. As well as Lemaître’s anticipation of the expanding universe, KarlSchwarzschild predicted the existence of black holes, for which there is now overwhelmingevidence. For the particle physicists, their mathematical theories of the way atoms and theirconstituent particles interact were predicting previously unknown particles of nature. Morethan that, experiments designed to capture these fleeting things were achieving results.Antimatter, predicted in 1928, was found in 1932; neutrinos, proposed in 1930, werediscovered in 1956. By the 1970s, the particles physicists were starting to believe that they could find anequation for everything. It would explain why there were four fundamental forces in theuniverse today and how they are linked together. However, such a theory requires further,as yet undiscovered particles. They are predicted to be different from atoms, hardlyinteracting with normal matter at all except through their gravity. If so, the astronomersshould be seeing movement in the universe that they can’t explain. As it turned out the astronomers had been struggling with some strange motions... The trouble all started because as measurements became more and more precise,so the reach of science became greater. Astronomers could peer out into the depths ofspace and measure great motions that had been rendered small only by their distance.That’s when they realised that things were not as they seemed. Prickly Swiss astronomer Fritz Zwicky emigrated to America in 1925 to work at theCalifornia Institute of Technology. By 1933, he had convinced himself that there was moreto the universe than meets the eye. He was studying a collection of a thousand galaxiesthat were all bound to each other through the gravity they generated. Each galaxy was Page 5 of 9
  6. 6. home to a hundred billion or more stars yet Zwicky’s analysis showed that this was notenough. If the stars alone made up the majority of matter in the galaxies, then the clustercould not be generating sufficient gravity to keep it together. Yet there it was. And it wasnot alone. Ever more powerful telescopes were showing clusters of galaxies spreadthroughout the universe. Something was keeping them together. Zwicky concluded that there must be extra reservoirs of matter providing anadditional gravitational force. Because these reservoirs were not easily visible, they couldnot be emitting any light. He published his original paper on the subject in German in apaper to the Swiss Physical Society and referred to the unseen stuff as dunkel Materie,dark matter. Initially, dark matter was thought to be clouds of ordinary atoms that had so farescaped detection because they had not collapsed into stars. Decades of searching,however, turned up very little. Radio telescopes and infrared telescopes increasingly foundstocks of once invisible atoms but not in nearly the quantities that were needed. By the1970s, things were looking grim for the astronomers. So the particle physicists’ suspicionsof undiscovered particles of nature, linked to a theory of everything, were something of amagic bullet. Not only do most astronomers think that dark matter holds clusters of galaxiestogether, they also think it provides the gravitational glue to stop individual galaxies flyingapart. The only problem is that, so far, no one can find a single piece of direct evidencethat it actually exists. Awkward. Things got worse. The astronomers weren’t done with their revelations. Theuniverse was hiding not just a big secret from us but the biggest secret. By the late 1990s,astronomers were ready to make the announcement, and they could have pinched theopening line of Star Wars to do it. A long time ago, in a galaxy far, far away... A star exploded. The light from that cataclysm reached Earth billions of years laterand astronomers analysed the light to compare it to their mathematical expectations basedon General Relativity. Astronomers expected to find that the expansion of space, sparked by the explosivecreation of the universe in the big bang, would be slowing down as gravity tried to pulleverything back together. That’s why they had gone looking for distant exploding stars tomeasure the effect from then to now. Instead, they saw that the universe was accelerating.Something was resisting gravity. But what? It could be an energy or a force. No one knows. Astronomers have called it darkenergy to signify its mystery. Their mathematical estimates are growing ever more Page 6 of 9
  7. 7. sophisticated and show that dark energy constitutes almost three-quarters of the universe.The putative dark matter makes up almost another quarter, and the normal atoms are thepuny four percent left over. In this view, the atoms are nothing but celestial froth. Everything we see around usis the most insignificant part of the universe. Then the particle physicists who were working on the theory of everything suffereda setback. Instead of a definitive mathematical description of the universe, physicistsfound a number of possible ones, each one seemingly as valid as the others. These werenamed string theories because they shared the common foundation of transforming theparticles of nature from point-like entities into wiggling knots of subatomic energy. But if mathematics was capable of providing more than one description of theuniverse, how could you decide between them? What did this mean for mathematics andits relationship to reality? How do we decide between such equals? Perhaps you don’thave to, suggested American physicist Hugh Everett. Another way to read the stringtheories is that there is a multiverse, of which our universe is just one small part. Truereality is a multitude of universes in which all possibilities are played out somewhere. Wejust happen to be trapped in one small portion of it. If this is the case, science will never be able to explain why our universe exists.Instead, it just does because it can – and so do countless others. All in all, we seem further from an understanding of the universe now than at anytime since Newton. To turn this around we need to critically look at the assumptions thatunderpin the thrust of our research, and we need to look for any puzzling observations thatmay provide a clue as to where to go next. One assumption is that gravity can be linked to the other forces of nature using thequantum theory, which was developed largely in Germany between the world wars. Here,everything was reducible to particles. Even forces were carried by particles. But the searchfor the quantum theory of gravity, which led to string theory, has all but stalled. By seeming to be able to describe everything (including universes beyond our own),string theory loses its power to tell us much about our own. There is currently no realprediction from string theory that we can test. So how do we continue? To make progress, we need new leads, and that means new experiments. They could show us where we are going wrong with string theory or take us in anentirely different direction. Page 7 of 9
  8. 8. One experiment is currently being studied by a working group of scientists andengineers. To my mind, it is the most important gravitational experiment since Eddington’s1919 eclipse expedition. The European Space Agency is building a mission called LISA-Pathfinder. It wasdesigned solely to test the technology needed for a larger mission called LISA (LaserInterferometer Space Antenna) but it is now being realised that the mission is capable ofso much more. An alternative to dark matter and dark energy is to modify the behaviour of gravity.While many researchers think that this is a long shot, progress is so slow in finding darkmatter or understanding the nature of dark energy that more and more people are willingto entertain what was once thought to be a wacky idea. Modifying gravity is not easy. Classic Newtonian and Einsteinian gravity works sowell in the solar system that we must be careful not to destroy this with any tinkering thatwe do. So, for example, you cannot make gravity pull a little harder. You have to be subtlerthan that. In the 1980s, Mordehai Milgrom, then at Princeton University, tweaked Newtonslaws so that an object in a very weak gravitational field experiences a slightly stronger pullthan Newton would have predicted. He showed that this revised version of gravity, nowcalled Modified Newtonian Dynamics (MOND), can neatly describe the observed rotationof stars in giant spiral galaxies without the need for dark matter. But how can we test this? We will never be able to send a probe tens of thousandsof light years to the edge of our galaxy. LISA-Pathfinder may be able to do it just a fewmillion kilometres away from Earth. The gravitational field of the Sun is overwhelming in the solar system, but there areplaces where the gravity of the planets cancels it out. These are called ‘saddle points’. Theone between the Earth and the Sun occurs 260,000 kilometres away. If LISA-Pathfindercan be sent through this saddle point, its instruments will measure the acceleration due togravity so precisely that we will see if MOND – or some other unexpected gravitationalbehaviour – is at play. The working group are still defining the requirements of this mission and will reportback to ESA this year. Although officially slated for a launch in 2015, the launch may slipto 2017. If so, it means that the saddle-point experiment could take place in 2019, acentury after Eddington’s eclipse confirmed General Relativity. Page 8 of 9
  9. 9. Next week, we will learn about our latest clue to the universe. The European SpaceAgency reports the results from the Planck spacecraft. This has been mapping the cosmicmicrowave background radiation that Lemaître learned about the week before his death. Spacecraft have studied this radiation before. It is effectively the blueprint for theuniverse. The importance of next week is that it is almost certainly impossible to takebetter pictures of the microwave background. Although we can build better microwavedetectors, the image itself is blurred on its way through space. What we see next week is the best image we will ever see of the universeʼsblueprint. There will be other ways to investigate it in the future but it is sobering to thinkthat in just over 400 years since the first astronomical use of a telescope, we have gonefrom Galileoʼs spyglass to the most precise map of our origins it is possible to take with aʻsimilarʼ telescope. The question is: will we be able to decode its message and then testour hypotheses? If so, linked with the other experiments that I have talked about, we stand a chanceof taking the next revolutionary leap. But will that be an incremental step or the finaltheory? We donʼt know. We canʼt know. Will we ever understand the universe? Maybe – but I suspect not for a long time. Page 9 of 9