If you’ve read about how modern cosmology may imply that, in an infinite universe, the existence of planets and the life forms that live on them must be repeated an infinite number of times, you may have been just a little bit skeptical. So are a couple scientists from Spain, who have posted a paper at arXiv.org criticizing the concept of the infinite repetition of histories in space, an idea closely related to the concepts of “alternate histories,” “parallel universes,” and the “many worlds interpretation,” among others.
Francisco José Soler Gil at the University of Sevilla and Manuel Alfonseca at the Autonomous University of Madrid have looked at two different proposals – one based on classical cosmology and the other on quantum mechanics – that contend that we live in an infinite universe in which history is repeated an infinite number of times in space. They have picked apart both proposals and argue that both are highly speculative, despite often being presented as plausible ideas. Moreover, they argue that we really don’t know whether we live in an infinite universe, as a finite one seems equally likely.
The basic idea of the infinite repetition of histories in space is that, if you take yourself right now and change one thing (say make your red shirt a blue one), then there’s another you somewhere who is exactly 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, and there’s another one. Plus, there are copies of all of these universes – an infinite number of copies. In their paper, Soler Gil and Alfonseca quote the popular science book “The Music of the Big Bang, The Cosmic Microwave Background, and the New Cosmology” by Amedeo Balbi: “In an infinite universe, every possible event does happen. Not just that: it happens an infinite number of times.”
This infinite repetition idea can be found in early philosophy, ancient mythology, and today’s sci-fi literature. But can it be derived from physical theories about the universe, and does it have a place in science?
Soler Gil and Alfonseca criticize almost all of the assumptions in this proposal, starting with the application of quantum theory to cosmology, which is currently mere conjecture without evidence. Other problems arise when considering the gravitational effects of black holes and the expansion of the universe, which can potentially increase the number of possible histories indefinitely, preventing repetitions.
But the scientists’ biggest criticism of the idea of infinite repetition in both proposals is the assumption that the universe is infinite. Whether the universe is infinite or finite is a big open-ended question in cosmology that scientists may never answer. Soler Gil and Alfonseca note that, looking back at the history of physics, situations emerged where infinities seemed impossible to avoid, yet improved theories eliminated the infinities. Currently the two basic theories in physics, general relativity and quantum theory, both predict infinities. In relativity, it’s gravity singularities in black holes and the big bang. In quantum theory, it’s vacuum energy and certain parts of quantum field theory. Perhaps both theories are simple approximations of a third more general theory without infinities. Soler Gil and Alfonseca also note that, Paul Dirac once stated that the most important challenge in physics was “to get rid of infinity.”