The ancient Greeks had a great idea: The universe is simple. In their minds, all you needed to make it were four elements: earth, air, fire, and water. As theories go, it's a beautiful one. It has simplicity and elegance. It says that by combining the four basic elements in different ways, you could produce all the wonderful diversity of the universe. Earth and fire, for example, give you things that are dry. Air and water, things that are wet. But as theories go, it had a problem. It didn't predict anything that could be measured, and measurement is the basis of experimental science. Worse still, the theory was wrong. But the Greeks were great scientists of the mind and in the 5th century B.C., Leucippus of Miletus came up with one of the most enduring scientific ideas ever. Everything we see is made up of tiny, indivisible bits of stuff called atoms. This theory is simple and elegant, and it has the advantage over the earth, air, fire, and water theory of being right. Centuries of scientific thought and experimentation have established that the real elements, things like hydrogen, carbon, and iron, can be broken down into atoms. In Leucippus's theory, the atom is the smallest, indivisible bit of stuff that's still recognizable as hydrogen, carbon, or iron. The only thing wrong with Leucippus's idea is that atoms are, in fact, divisible. Furthermore, his atoms idea turns out to explain just a small part of what the universe is made of. What appears to be the ordinary stuff of the universe is, in fact, quite rare. Leucippus's atoms, and the things they're made of, actually make up only about 5% of what we know to be there. Physicists know the rest of the universe, 95% of it, as the dark universe, made of dark matter and dark energy. How do we know this? Well, we know because we look at things and we see them. That might seem rather simplistic, but it's actually quite profound. All the stuff that's made of atoms is visible. Light bounces off it, and we can see it. When we look out into space, we see stars and galaxies. Some of them, like the one we live in, are beautiful, spiral shapes, spinning gracefully through space. When scientists first measured the motion of groups of galaxies in the 1930's and weighed the amount of matter they contained, they were in for a surprise. They found that there's not enough visible stuff in those groups to hold them together. Later measurements of individual galaxies confirmed this puzzling result. There's simply not enough visible stuff in galaxies to provide enough gravity to hold them together. From what we can see, they ought to fly apart, but they don't. So there must be stuff there that we can't see. We call that stuff dark matter. The best evidence for dark matter today comes from measurements of something called the cosmic microwave background, the afterglow of the Big Bang, but that's another story. All of the evidence we have says that dark matter is there and it accounts for much of the stuff in those beautiful spiral galaxies that fill the heavens. So where does that leave us? We've long known that the heavens do not revolve around us and that we're residents of a fairly ordinary planet, orbiting a fairly ordinary star, in the spiral arm of a fairly ordinary galaxy. The discovery of dark matter took us one step further away from the center of things. It told us that the stuff we're made of is only a small fraction of what makes up the universe. But there was more to come. Early this century, scientists studying the outer reaches of the universe confirmed that not only is everything moving apart from everything else, as you would expect in a universe that began in hot, dense big bang, but that the universe's expansion also seems to be accelerating. What's that about? Either there is some kind of energy pushing this acceleration, just like you provide energy to accelerate a car, or gravity does not behave exactly as we think. Most scientists think it's the former, that there's some kind of energy driving the acceleration, and they called it <i>dark energy</i>. Today's best measurements allow us to work out just how much of the universe is dark. It looks as if dark energy makes up about 68% of the universe and dark matter about 27%, leaving just 5% for us and everything else we can actually see. So what's the dark stuff made of? We don't know, but there's one theory, called <i>supersymmetry</i>, that could explain some of it. Supersymmetry, or SUSY for short, predicts a whole range of new particles, some of which could make up the dark matter. If we found evidence for SUSY, we could go from understanding 5% of our universe, the things we can actually see, to around a third. Not bad for a day's work. Dark energy would probably be harder to understand, but there are some speculative theories out there that might point the way. Among them are theories that go back to that first great idea of the ancient Greeks, the idea that we began with several minutes ago, the idea that the universe must be simple. These theories predict that there is just a single element from which all the universe's wonderful diversity stems, a vibrating string. The idea is that all the particles we know today are just different harmonics on the string. Unfortunately, string theories today are, as yet, untestable. But, with so much of the universe waiting to be explored, the stakes are high. Does all of this make you feel small? It shouldn't. Instead, you should marvel in the fact that, as far as we know, you are a member of the only species in the universe able even to begin to grasp its wonders, and you're living at the right time to see our understanding explode.
Stari su Grci imali sjajnu ideju: svemir je jednostavan. Mislili su da se sastoji od samo 4 elementa: zemlje, zraka, vatre i vode. Lijepa je to teorija. Jednostavna i elegantna. Prema njoj, različitim kombinacijama četiri osnovna elementa nastaje sva divna raznolikost svemira. Zemlja i vatra, na primjer, tvore suhe stvari. Zrak i voda – vlažne stvari. Ipak, postojao je problem s tom teorijom. Nije predviđala ništa što je moguće mjeriti, a mjerenja su osnova eksperimentalne znanosti. Još gore, teorija je bila pogrešna. No Grci su bili sjajni znanstveni mislioci te je u 5. stoljeću pr. Kr. Leukipu iz Mileta pala na um jedna od najtrajnijih znanstvenih ideja u povijesti. Sve što vidimo sastoji se od sićušnih, nedjeljivih djelića tvari koji se nazivaju atomima. I ova je teorija jednostavna i elegantna, ali, za razliku od teorije zemlje, zraka, vatre i vode, ona je i točna. Stoljeća znanstvene misli i eksperimentiranja utvrdila su da je prave elemente, kao što su vodik, ugljik i željezo, moguće rastaviti na atome. Prema Leukipovoj teoriji atom je najmanji, nedjeljivi djelić tvari koji i dalje ima osobine vodika, ugljika ili željeza. Jedina je pogreška u Leukipovoj ideji to što su atomi zapravo djeljivi. Osim toga, njegova ideja o atomima tek u maloj mjeri objašnjava od čega se sve svemir sastoji. Ono što nam se čini uobičajenim dijelom svemira ustvari je prilično rijetko. Leukipovi atomi, i njihovi dijelovi zapravo čine samo oko 5 % poznatog dijela svemira. Fizičari smatraju preostalih 95 % tamnim svemirom, sastavljenim od tamne tvari i tamne energije. Kako to znamo? Pa, znamo jer gledamo u stvari i vidimo ih. To možda zvuči površno, no zapravo je vrlo duboko. Sve stvari sastavljene od atoma vidljive su. Odbijaju svjetlost, pa ih možemo vidjeti. Kad gledamo u svemir, vidimo zvijezde i galaktike. Neke od njih, poput ove u kojoj živimo, prekrasnog su spiralnog oblika i graciozno se vrte svemirom. Kad su znanstvenici prvi put mjerili kretanje skupina galaktika u 1930-ima i izmjerili količinu tvari koju one sadržavaju, iznenadili su se. Otkrili su nedovoljno vidljive tvari u tim skupinama da bi ih držala na okupu. Kasnija su mjerenja pojedinačnih galaktika potvrdila taj zbunjujući rezultat. U galaktikama nema dovoljno vidljive tvari koja bi osigurala gravitaciju što ih drži na okupu. Iz toga slijedi da bi se trebale raspasti, no to se ne događa. Znači, postoji nešto što ne vidimo. To nešto nazivamo tamnom tvari. Najbolji dokaz postojanja tamne tvari pružaju mjerenja kozmičkog pozadinskog mikrovalnog zračenja, zaostalog odbljeska Velikog praska, no to je druga priča. Svi dokazi upućuju na to da tamna tvar postoji i tvori veći dio prekrasnih spiralnih galaktika što ispunjavaju nebo. Koji je, dakle, zaključak? Već dugo znamo da se nebesa ne okreću oko nas i da nastavamo prilično običan planet, koji kruži oko obične zvijezde u spiralnom rukavcu prilično obične galaktike. Otkriće tamne tvari odvelo nas je korak dalje od središta svega. Pokazalo nam je da je ono od čega smo sačinjeni samo djelić onoga od čega se sastoji svemir. No to nije bilo sve. Početkom stoljeća proučavanjem dalekih predjela svemira znanstvenici su potvrdili ne samo da se sve odmiče od svega ostalog, što se i očekuje u svemiru nastalom vrućim, gustim velikim praskom, već i da se širenje svemira naizgled ubrzava. O čemu je riječ? Ili postoji neka vrsta energije koja potiče to ubrzavanje, kao što se automobil ubrzava s pomoću energije, ili gravitacija ne djeluje onako kako mislimo. Većina znanstvenika misli da je u pitanju ovo prvo, tj. da je uzrok tom ubrzanju neka vrsta energije koju nazivaju tamnom energijom. Najbolja današnja mjerenja pokazuju koliki je dio svemira taman. Izgleda da tamna energija čini oko 68 % svemira, a tamna tvar oko 27 %, što ostavlja samo 5 % za nas i sve ostalo što doista vidimo. Od čega se sastoji tamna tvar? Ne znamo, ali postoji teorija tzv. supersimetrije, koja bi to mogla donekle objasniti. Supersimetrija (skraćeno SUSY) predviđa cijeli niz novih čestica od kojih neke možda tvore tamnu tvar. Dokažemo li SUSY, mogli bismo, umjesto 5 % našeg svemira, onoga što zapravo vidimo, razumjeti otprilike trećinu. Nije loše za tako kratko vrijeme. Tamnu energiju vjerojatno bi bilo teže shvatiti, no postoje spekulativne teorije koje nas mogu usmjeriti. Među njima su teorije koje se vraćaju na onu prvu sjajnu ideju starih Grka, ideju kojom smo prije nekoliko minuta započeli: da svemir mora biti jednostavan. Prema njima postoji samo jedan element od kojeg potječe sva prekrasna raznolikost svemira: vibrirajuća struna. Ideja je da su sve danas poznate čestice samo različite frekvencije na struni. Nažalost, teorije struna još je nemoguće testirati. No budući da je dobar dio svemira neistražen, ulozi su visoki. Osjećate li se zbog toga neznatnim? Ne biste trebali. Naprotiv, treba vas zadiviti činjenica da, koliko znamo, pripadate jedinoj vrsti u svemiru koja može barem donekle dokučiti njegova čudesa i živite u vremenu u kojem možete svjedočiti naglom porastu naših spoznaja.