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What If String Theory Is If It Is, Then Dark Matter, Dark Energy, WRONG? and Cosmic Inflation Are in Big Trouble

What If String Theory Is If It Is, Then Dark Matter, Dark Energy, WRONG? and Cosmic Inflation Are in Big Trouble

What if is If it is, then , dark energy, WRONG? and cosmic inflation are in big trouble. ⁄⁄⁄ BY STEN ODENWALD

cientists pin their hopes of unifying nature’s fundamen- tal forces to a single concept crafted over the course of 3 decades by thousands of mathematicians and physicists. It’s calledS . Its basic ingredients are one-dimensional strings of pure energy that exist in a 10-dimensional universe, four of which are our familiar spatial plus . Strings’ vibration and movement create the fabric of itself, along with all of nature’s fundamental forces and elementary particles. Now, imagine a future in which physicists are delivered the bad news: irrefutable data proving superstring the- ory doesn’t work in our physical world. We’re IF CURRENT about are correct, every has a nowhere near such a crisis today, but it’s sober- massive superpartner not yet detected. Scien- ing to realize what we stand to lose if tists hope to detect some of these particles using the , which smashes ’ best bet proves to be a atomic nuclei to liberate the fundamental complete dead end. constituents of matter. DON DIXON FOR

© 2015 Kalmbach Publishing Co. This material may not be reproduced in any 78 ’s Greatestform without Discoveries permission from ⁄⁄⁄ the 2009 publisher. www.Astronomy.com www.Astronomy.com 79 Cosmic constituents National Accelerator Laboratory in Bata- In some respects, a world without super- STRING SPEAK via, Illinois. “That’s why we’re so excited isn’t so bad. The standard Atoms about looking for them.” model and ordinary hold GENERAL RELATIVITY 5 percent Albert Einstein’s formulation of grav- If scientists validate superstring theory, that all astronomers need to describe accu- ity, which shows that space and time rately most of the phenomena they study, we’ll see the birth of the first unified communicate the gravitational force Dark energy description of the physical world. Along the from evolution and supernova deto- 72 percent through their curvature. way, we’ll have to make peace with the idea nations to the extreme physics of neutron of a in which trillions of other and black holes. GLUON universes may exist, each with its own nat- As for , the late physicist Rich- The smallest bundle of the strong ural laws, forces, and particles. ard Feynman once said, “Perhaps it is dif- nuclear force, which holds atomic Dark matter But what would happen if all these ficult for physicists to unify with the nuclei together. 23 percent ideas are wrong? String theory would go other forces because nature never intended down in history as the most spectacular for them to be unified in the first place.” wrong turn science has ever taken. It will Laws governing the universe on sub- atomic scales in which uncertainty and have sent thousands of physicists on a Cosmic trouble SCIENTISTS AT CERN will use the Large Hadron Collider’s ATLAS detector to smash COSMOLOGISTS’ best tally of cosmic the dual nature of waves and particles spectacular wild-goose chase, while simul- Astronomers have also discovered there is together high-velocity proton beams. The resulting subatomic debris should reveal funda- content comes from NASA’s Wilkinson become apparent. taneously revolutionizing obscure subjects far more to the universe than what we can mental particle processes. The ATLAS experiment is one of the largest collaborative efforts Microwave Anisotropy Probe (WMAP). ever attempted in the physical sciences. The project involves 2,500 physicists from 169 Normal matter accounts for only about in pure . see. The has absolutely no STANDARD MODEL institutions in 37 countries. CERN/CLAUDIA MARCELLONI 5 percent of the total. ASTRONOMY: ROEN KELLY place in it for dark matter and dark energy, A theory of interactions of nature’s Forget supersymmetry? ingredients that constitute 95 percent of the most fundamental known particles If don’t exist, universe’s gravitating “stuff.” (e.g., electrons and ) developed Extra dimensions ing into a gluon. Luckily, other ways to properties as they move through our three- then supersymmetry doesn’t And it gets worse. in the late 1960s and early 1970s. Its Superstring theory is based on three key search for supersymmetry exist. dimensional world. There are trillions of either. Without both of Without supersym- inability to explain gravity suggests a ideas that remain experimentally unproven If nature follows supersymmetry, then possible six-dimensional , each these cornerstones of LITERALLY metry, some physicists more fundamental theory exists. after 30 years of research: the principle of every normal particle has a superpartner: calling into existence a unique universe string theory, physi- have proven that the STRING THEORY supersymmetry, additional spatial dimen- Electrons partner with “selectrons,” pho- with specific physical laws, particles, and cists would be left trillions of possible energy of empty A unified theory of the universe postu- sions, and gravity as a force defined by the tons with “photinos,” quarks with “squarks,” forces. Which one is ours? with what they space would be so six-dimensional geom- lating that the fundamental ingredi- exchange of quantum particles. and so on. These superpartners haven’t Some researchers feel all geometric pos- refer to as the enormous that the ents of nature are not particles but Supersymmetry is a mathematical prin- been seen yet, but scientists built a new sibilities exist in the multiverse — the col- standard model. etries exist, each calling universe would one-dimensional filaments called ciple that allows force-carrying particles, accelerator, the Large Hadron Collider lection of all possible universes with all The standard instantly collapse. strings. String theory unites quantum such as photons and gluons, to transform (LHC), at the European Organization for possible physical laws logically allowed by model combines into existence a unique Only by under- mechanics and general relativity, into one another. It also allows the unifica- Nuclear Research (CERN) in Switzerland to superstring theory. Ours, then, is a Goldi­ electroweak theory universe. Which one standing physics which are otherwise incompatible; tion of gravity with other forces because its find them. Researchers hope experiments locks Universe, where exactly the right six- (EW) — itself a com- beyond the standard often short for superstring theory. particle, which some call the , can with the LHC will turn up at least a few of dimensional allows the existence bination of electromag- is ours? model can we hope to transform into one of the other force- the lightest superparticles. Such a success of life as we know it. There may be 10100 netism and the weak understand how the vac- carriers. Although this idea is mathemati- would be a prelude to confirming the most universes that fail this challenge. There may nuclear force — with a sepa- uum and the universe’s dark Dimensions cally elegant, and puts gravity on an equal bizarre physics idea of the last 80 years — be others in which new, unthinkable pos- rate theory of the strong nuclear side work. And only string the- footing with the other forces, no one has extra dimensions to space. sibilities for life could flourish. force called ory appears able to serve as a reliable math- to spare ever detected, for example, a photon turn- If extra dimensions really exist, they’ll (QCD). Each part behaves like a train on a ematical guide to that larger universe. Space-time is just the start change the behavior of gravity at small dis- separate track. But just as electricity and Another major disaster to astronomy Identity change Conventional superstring theory requires tances. In 2003, John Price of the University magnetism were unified into electromagne- would be in our understanding of the larg- extra dimensions to space beyond the usual of Colorado at Boulder and his colleagues tism, the standard model’s EW and QCD est structures in the universe and the uni- three (up-down, left-right, near-far). These announced an ultra-precise measurement are so similar that they, too, must be unified. verse’s large-scale smoothness revealed by additional dimensions give physicists extra of gravity at scales of one-tenth of a mil- Grand unification theories (GUTs) tried studies of the cosmic microwave back- “handles” from which they can fashion limeter. The team saw no departure from to do just this. Physicists worked with doz- ground (CMB). not just an improved theory of gravity gravity’s inverse-square law, in which the ens of prospects between 1975 and 1980; Inflation, a super-fast but the properties of the known par- force of attraction decreases by the square most failed. Many predicted the proton, the expansion of the uni- ticles themselves. Vastly smaller of the distance between bodies. So, the hunt foundation of atomic nuclei, would rapidly verse when it was than a proton, the extra dimen- resumes below this limit to test other extra- decay; some introduced faster-than-light less than a trillionth sions appear as six additional dimensional possibilities. as new physical particles. of a trillionth of a Width Distance Six additional spatial dimensions coordinates attached to “We have a number of hints from None turned out to work very well once second old, explains each point in space. experiments and theoretical ideas that physicists puzzled out their intricate math- observations of the Height EVERY POINT They actually form make us think they’re probably out there,” ematics and compared them to the real CMB made by satel- in our universe’s Photon their own closed, six- says Joseph Lykken, a physicist at Fermi world. Physicist Mark Srednicki at the Uni- lites such as NASA’s three-dimensional dimensional that versity of California, Santa Barbara, once Wilkinson Micro- Time space may contain Astronomer Sten Odenwald works at NASA’s STRING THEORY says that, now and must be everywhere. complained about one candidate, “It is wave Anisotropy six additional then, a photon can turn into a gluon. Gluon Otherwise, particles Goddard Space Flight Center in Maryland and is impossible to get this pile of junk to come Probe (WMAP). But dimensions. ASTRONOMY: ROEN KELLY would take on different investigating the cosmic infrared background. out anything like the standard model.” it can’t account for Three spatial dimensions ASTRONOMY: ROEN KELLY

80 Cosmology’s Greatest Discoveries ⁄⁄⁄ 2009 www.Astronomy.com 81 Hidden symmetry Particles

Electron Photon

Photino Selectron

Superpartners

SUPERSYMMETRY envisions a “shadow world” that links known particles to massive superpartners not yet detected. ASTRONOMY: ROEN KELLY the small-scale structure that led to galaxy In the 1980s, when at the tal aspects of the physical world would formation. Only by reaching back to the era Massachusetts Institute of Technology and devolve into semantic quibbles. Other before inflation, often called the quantum- , now at , impacts prevent us from simply turning gravity era or Planck Era, can cosmologists first developed the inflation idea, it had back the theoretical clock to the 1970s. explain why today’s universe looks the way nothing to do with string theory, extra “We would be ignoring the vast amount it does at galactic scales. dimensions, or supersymmetry. Refinements of mathematical knowledge that has been The standard model and ordinary gen- to inflation have come only by borrowing gained over the subsequent years, mostly eral relativity aren’t enough. Only some ideas from string theory. from string theory,” Susskind larger, more encompassing theory can lead explains. “That is just not the way. “It is hard to find a serious paper End game going to happen. The about particle phenomenology that doesn’t Without superstring changes in our theo- in some way use the tools of superstring theory, we’d lose the TO LEARN THAT retical understand- theory,” says Stanford University physicist intriguing philo- 95 percent of the ing of quantum . “Furthermore, we sophical prospects theory, grav- all agree that the origin of elementary par- for the multiverse, cosmos is unknowable ity, and black holes ticles is most likely at the Planck scale and with its infinite are completely cannot be understood without a good the- and eternal cre- would be a bitter pill irreversible. The ory of .” ativity in spawning for astronomers mathematics of new universes. We’d superstring theory have no mathematics to swallow. has even worked its Mediating for spanning the gap way into nuclear phys- between everyday physics ics and heavy ion colli- gravity and the high energies where sions, as well as into con‑ quantum gravity operates. The densed matter physics.” road to creating a quantum description of Historically, there are many examples of gravity will be a murky one. scientists forced to abandon ideas. Steady- More immediately, dark matter and dark state cosmology lasted nearly 30 years THE GRAVITY energy would remain imponderable enig- before science tossed it aside. While string FORCE we feel mas, shorn of any clues about where they theory is sparse on experimental validation, arises from hypo- come from. Astronomers can live without the situation is not so different from gen- thetical , knowing the quantum properties of gravity. eral relativity in its early days, when diffi- according to super- But to learn that 95 percent of the cosmos cult mathematics made calculating a string theory. is unknowable would be a bitter pill for prediction extremely challenging. ASTRONOMY: ROEN KELLY Graviton astronomers to swallow. Princeton University physicist Ed Wit- It would be even worse for physicists. ten once described string theory as 21st- Without a logical framework in which century physics glimpsed long before its to pose and answer questions, our time. Let’s hope it doesn’t represent a bit of inquiries into the fundamen- 25th-century mathematics, too.

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