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Looking for Life in the Multiverse Universes with Different Physical Laws Might Still Be Habitable by Alejandro Jenkins and Gilad Perez

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Looking for Life in the Multiverse Universes with Different Physical Laws Might Still Be Habitable by Alejandro Jenkins and Gilad Perez COSMOLOGY Looking for Life in the MULTIVERSE Universes with different physical laws might still be habitable BY ALEJANDRO JENKINS AND GILAD PEREZ he typical Hollywood action hero skirts very little of the carbon and other elements that KEY CONCEPTS death for a living. Time and again, seem necessary to form planets, let alone life. If scores of bad guys shoot at him from the proton were just 0.2 percent heavier than it ■ Multiple other univers- T multiple directions but miss by a hair. Cars ex- is, all primordial hydrogen would have decayed es—each with its own plode just a fraction of a second too late for the almost immediately into neutrons, and no atoms laws of physics—may fireball to catch him before he finds cover. And would have formed. The list goes on. have emerged from the same primordial vacuum friends come to the rescue just before a villain’s The laws of physics—and in particular the that gave rise to ours. knife slits his throat. If any one of those things constants of nature that enter into those laws, happened just a little differently, the hero would such as the strengths of the fundamental forc- ■ Assuming they exist, many be hasta la vista, baby. Yet even if we have not es—might therefore seem finely tuned to make of those universes may seen the movie before, something tells us that he our existence possible. Short of invoking a su- contain intricate struc- will make it to the end in one piece. pernatural explanation, which would be by tures and perhaps even In some respects, the story of our universe re- definition outside the scope of science, a num- some forms of life. sembles a Hollywood action movie. Several ber of physicists and cosmologists began in the ■ These findings suggest physicists have argued that a slight change to 1970s to try solving the puzzle by hypothesiz- that our universe may not one of the laws of physics would cause some di- ing that our universe is just one of many exist- be as “finely tuned” for saster that would disrupt the normal evolution ing universes, each with its own laws. Accord- the emergence of life as of the universe and make our existence impos- ing to this “anthropic” reasoning, we might previously thought. sible. For example, if the strong nuclear force just occupy the rare universe where the right —The Editors that binds together atomic nuclei had been slight- conditions happen to have come together to ly stronger or weaker, stars would have forged make life possible. FILMS SLIM 42 SCIENTIFIC AMERICAN © 2009 SCIENTIFIC AMERICAN, INC. January 2010 © 2009 SCIENTIFIC AMERICAN, INC. © 2009 SCIENTIFIC AMERICAN, INC. Amazingly, the prevailing theory in modern which our universe neither recollapsed into noth- WHAT IS THE cosmology, which emerged in the 1980s, sug- ingness a fraction of a second after the big bang, MULTIVERSE? gests that such “parallel universes” may really nor was ripped part by an exponentially acceler- Alternative universes have exist—in fact, that a multitude of universes would ating expansion. Nevertheless, the examples of al- now become a legitimate field incessantly pop out of a primordial vacuum the ternative, potentially habitable universes raise in- of study, in part because they way ours did in the big bang. Our universe would teresting questions and motivate further research may actually exist. According be but one of many pocket universes within a into how unique our own universe might be. to the prevailing cosmological theory, our universe was wider expanse called the multiverse. In the over- spawned from a microscopic whelming majority of those universes, the laws of The Weakless Way of Life region of a primordial vacuum physics might not allow the formation of matter The conventional way scientists find out if one by a burst of exponential as we know it or of galaxies, stars, planets and particular constant of nature is finely tuned or expansion called inflation. But life. But given the sheer number of possibilities, not is to turn that “constant” into an adjustable the vacuum may continually spawn other universes as well. nature would have had a good chance to get the parameter and tweak it while leaving all other Each universe might have its “right” set of laws at least once. constants unaltered. Based on their newly mod- own physical laws; some would Our recent studies, however, suggest that ified laws of physics, the scientists then “play the be hospitable to life, some not. some of these other universes—assuming they movie” of the universe—they do calculations, exist—may not be so inhospitable after all. Re- what-if scenarios or computer simulations—to Universe Universe not markably, we have found examples of alternative see what disaster occurs first. But there is no congenial congenial to life to life values of the fundamental constants, and thus of reason why one should tweak only one param- alternative sets of physical laws, that might still eter at a time. That situation resembles trying to lead to very interesting worlds and perhaps to drive a car by varying only your latitude or only life. The basic idea is to change one aspect of the your longitude, but not both: unless you are laws of nature and then make compensatory traveling on a grid, you are destined to run off changes to other aspects. the road. Instead one can tweak multiple param- Our work did not address the most serious fine- eters at once. tuning problem in theoretical physics: the small- To search for alternative sets of laws that still ness of the “cosmological constant,” thanks to give rise to complex structures capable of sustain- [THE BASIC IDEA] HOW TO FIND HOSPITABLE UNIVERSES Many features in the laws of ●1 TWO ●2 TWEAK ONE nature appear to be finely CONSTANTS CONSTANT tuned: a small change to any Physicists can Changing the con- one of the constants that plot the observed stant A (while keep- appear in physics equations values of two ing everything else different constants the same) is repre- typically leads to a “disaster.” A and B as two sented by moving on Small range For example, atoms cannot B coordinates of a congenial to life a horizontal line. form, or matter gets dispersed point on a plane. Going beyond a tiny Each point on the change usually results in space so thinly that it cannot alue of plane represents in a disaster, and the condense into galaxies, stars a different pair Disaster� universe would be or planets. Changing two Actual v of values. (universe not unsuitable for life. constants at a time, however, congenial to life) can sometimes lead to sets Actual value of A of possible values that are 3 4 compatible with the formation ● TWEAK ● CHANGE BOTH ANotHER More values CONSTANTS congenial to life of complex structures and CONSTANT The ability to change perhaps even some forms Changing the both A and B at ) of intelligent life. Changing constant B and once—for example, � charts keeping every- Disaster moving along a three or more constants and thing else the diagonal—can lead widens the range of New range same is represent- of values to new sets of values ed by moving on that are con­­genial to possibilities even more. congenial to life multiverse Small range a vertical line. life. Farther away congenial to life Beyond a small from the known KKANDA ( Disaster� tweak, this also values could also lie I often results in other “islands” of a disaster. congenial values. LUCY READING- 44 SCIENTIFIC AMERICAN © 2009 SCIENTIFIC AMERICAN, INC. January 2010 ing life, one of us (Perez) and his collaborators the universe would unfold. It is quite possible that did not make just small tweaks to the known laws a wide range of other “weakless” universes exist OTHER NOTIONS of physics: they completely eliminated one of the that are habitable but look nothing like our own. OF “PARALLEL four known fundamental forces of nature. In the weakless universe, the usual fusing of UNIVERSES” By their very name, the fundamental forces protons to form helium would be impossible, Physicists and cosmologists— sound like indispensable features of any self-re- because it requires that two of the protons con- and often science-fiction specting universe. Without the strong nuclear vert into neutrons. But other pathways could ex- writers—talk of parallel uni- force to bind quarks into protons and neutrons ist for the creation of the elements. For example, verses in several different contexts: at least three notions and those into atomic nuclei, matter as we know our universe contains overwhelmingly more exist distinct from the multi- it would not exist. Without the electromagnetic matter than antimatter, but a small adjustment verse described in this article. force, there would be no light; there would also to the parameter that controls this asymmetry be no atoms and no chemical bonds. Without is enough to ensure that the big bang nucleosyn- HUBBLE BUBBLE gravity, there would be no force to coalesce mat- thesis would leave behind a substantial amount Our universe is probably much larg- er than the part we can observe, ter into galaxies, stars and planets. of deuterium nuclei. Deuterium, also known as our “Hubble bubble.” If it is infinite The fourth force, the weak nuclear force, has hydrogen 2, is the isotope of hydrogen whose in size, then infinitely many sepa- rate Hubble bubbles (centered on a subtler presence in our everyday life but still nucleus contains a neutron in addition to the observers in re- has played a major role in the history of our uni- usual proton.
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