2013 01 Dont Infinite Scientists

You don't exist in an infinite number ofplaces, say scientists25 January 2013, by Lisa Zyga

Infinite repetition, the idea that planets and living beingsmust be repeated an infinite number of times, cannot belogically deduced from current physics and cosmologytheories. Credit: NASA/Apollo 17

(Phys.org)If you've read about how moderncosmology may imply that, in an infinite universe,the existence of planets and the life forms that liveon them must be repeated an infinite number oftimes, you may have been just a little bit skeptical.So are a couple scientists from Spain, who haveposted a paper at arXiv.org criticizing the conceptof the infinite repetition of histories in space, anidea closely related to the concepts of "alternatehistories," "parallel universes," and the "manyworlds interpretation," among others.

Francisco Jos Soler Gil at the University of Sevillaand Manuel Alfonseca at the AutonomousUniversity of Madrid have looked at two differentproposals one based on classical cosmology andthe other on quantum mechanics that contendthat we live in an infinite universe in which historyis repeated an infinite number of times in space.They have picked apart both proposals and arguethat both are highly speculative, despite oftenbeing presented as plausible ideas. Moreover, theyargue that we really don't know whether we live inan infinite universe, as a finite one seems equallylikely.

The basic idea of the infinite repetition of historiesin space is that, if you take yourself right now andchange one thing (say make your red shirt a blueone), then there's another you somewhere who isexactly the same except for that one difference.Change your shirt to purple, and that's a third you.

Change the drink in your hand from soda to tea, andthere's another one. Plus, there are copies of all ofthese universes an infinite number of copies. Intheir paper, Soler Gil and Alfonseca quote thepopular science book "The Music of the Big Bang,The Cosmic Microwave Background, and the NewCosmology" by Amedeo Balbi: "In an infiniteuniverse, every possible event does happen. Notjust that: it happens an infinite number of times."

This infinite repetition idea can be found in earlyphilosophy, ancient mythology, and today's sci-filiterature. But can it be derived from physicaltheories about the universe, and does it have aplace in science?

In the first proposal that Soler Gil and Alfonsecaanalyze, Ellis and Brundrit argue that infiniterepetition logically arises from classic relativisticphysics. A more detailed summary can be found inthe arXiv paper, but the general argument is asfollows. If the universe, the number of planets andgalaxies, and the number of possible histories (theone we're familiar with is our 13.7-billion-yearhistory) are all infinite; and if the probability of DNA-based life is greater than zero; and if the number oftypes of DNA-based living things is finite (becausethe size of the DNA molecules cannot be arbitrarilylarge); then an infinite universe must contain aninfinite number of copies of the finite number ofDNA-based living things, and some of these livingthings will follow very similar and even identicalhistory lines. In other words, infinite histories plusfinite types of living things means that those livingthings' histories are infinitely repeated.

Soler Gil and Alfonseca take issue with several ofthese assumptions. One of their maincounterarguments at first seems odd: they say thatwe can't be sure that the probability of DNA-basedlife is greater than zero. Neither our existence nor

1 / 3

our discovery of a finite number of cases of life onother planets can, at least in the logical sense, beused to deduce that the probability is greater thanzero. As a result, the infinity of histories is largerthan the infinity of living individuals, so each planetcompatible with life could have its own uniquehistory.

"If there is an infinite number of possible histories,the fact that there is a given history (or a finitenumber) leading to life does not make that historyprobable: its probability would be 1 divided byinfinity, which is zero," Alfonseca explained to Phys.org. "To have a greater-than-zero probability,you need an infinite number of approaches. But inany case, with this scenario, the number ofhistories would always be larger than the number ofbeings, so the same beings infinitely repeatedwould still have different histories."

The second proposal, by Garriga and Vilenkin,does involve a finite number of histories, but isrooted on the idea in quantum theory that discreteregions of space have finite amounts of energy. Inthe decoherent histories (DH) interpretation ofquantum mechanics, the infinite universe can bedivided into an infinite number of regions thatcannot influence one another (i.e., they're causallydisconnected) because they are separated byevent horizons. Then Garriga and Vilenkin deducethat the number of possible histories in each regionis finite because the energy in each region is finiteand, according to quantum mechanics, energy isquantified. To put it briefly, an infinite number ofregions plus a finite number of possible histories ineach region means that every history must berepeated an infinite number of times.

Soler Gil and Alfonseca criticize almost all of theassumptions in this proposal, starting with theapplication of quantum theory to cosmology, whichis currently mere conjecture without evidence.Other problems arise when considering thegravitational effects of black holes and theexpansion of the universe, which can potentiallyincrease the number of possible historiesindefinitely, preventing repetitions.

But the scientists' biggest criticism of the idea ofinfinite repetition in both proposals is theassumption that the universe is infinite. Whetherthe universe is infinite or finite is a big open-endedquestion in cosmology that scientists may neveranswer. Soler Gil and Alfonseca note that, lookingback at the history of physics, situations emergedwhere infinities seemed impossible to avoid, yetimproved theories eliminated the infinities.Currently the two basic theories in physics, generalrelativity and quantum theory, both predict infinities.In relativity, it's gravity singularities in black holesand the big bang. In quantum theory, it's vacuumenergy and certain parts of quantum field theory.Perhaps both theories are simple approximations ofa third more general theory without infinities. SolerGil and Alfonseca also note that, Paul Dirac oncestated that the most important challenge in physicswas "to get rid of infinity."

While Soler Gil and Alfonseca can't disprove theproposals of infinite repetition, they emphasize thatthe point of their critique is to show that the idearemains in the realm of philosophy, mythology, andsci-fi tales, not modern cosmology. They call thespeculation "ironic science," a term used byscience journalist John Horgan to describe optionsthat do not converge on truth but are at best"interesting." Despite the accounts of many popularscience books, the idea that our lives are beingrepeated an infinite number of times somewhereout in the universe is in no way certain and far fromeither probable or plausible.

More information: Francisco Jos Soler Gil andManuel Alfonseca. "About the Infinite Repetition ofHistories in Space." arXiv:1301.5295 [physics.hist-ph]

2013 Phys.org

2 / 3

APA citation: You don't exist in an infinite number of places, say scientists (2013, January 25) retrieved 19March 2015 from http://phys.org/news/2013-01-dont-infinite-scientists.html

This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no partmay be reproduced without the written permission. The content is provided for information purposes only.

Powered by TCPDF (www.tcpdf.org)

3 / 3