Science Seen Time One author Colin Gillespie helps you understand the physics of your world.
Multiverses versus Universe
In the physics stratosphere a great debate is going on. Physicists say it is about the future of physics and the nature of everything.This may be true, but the issues are also personal.
The story starts (or so it seems) with multiverses. Not long poems (though those still get more Google hits), I mean multiple-universe schemes. The idea has ancient roots in mythology. Modern multiverses may be found in comic books and fiction. The reason why the story has turned serious is that many physicists for whose smarts I have high regard now embrace one or another kind of multiverse. Of this, Italian physicist Carlo Rovelli says:
Today, many scientists do not hesitate to take seriously speculations such as … multiple universes, for which there isn’t a wit of empirical evidence.
Multiverses now appear in leading scientific journals. For example, in a recent article on quantum black holes in Science, unconstrained by referee or editor Argentinian astrophysicist Juan Maldacena can in all seriousness say:
Of course, the 10-dimensional space under consideration here is not the same as the four-dimensional region of the multiverse where we live.
Though they come in many versions, most multiverses seem to me to fall broadly in three groups (Max Tegmark finds four). Some suppose distinct or distant aspects of this universe. Others imagine universes that are, as South African cosmologist George Ellis puts it, ‘completely disconnected from our spacetime’. Then there is an in-between species, of which American physicist Sean Carroll says:
In inflationary cosmology, however, these different regions can be relatively self-contained — “pocket universes,” as Alan Guth calls them. When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be very different.
So choose from: far-flung regions of this universe; other universes that are ‘disconnected from our spacetime’, wherever that is; and not-quite-disconnected pocket universes. It’s an inconsequential choice because you’ll never get to check what anybody says about them; all three kinds are utterly beyond any possibility of observation.
Physics that is beyond observation? Some say that this is a contradiction.
Next: How do physicists think themselves into such strange confabulations?
Carlo Rovelli (2009), “Unfinished Revolution”, in Approaches to Quantum Gravity, Daniel Oriti (ed.), New York: Cambridge University Press, p. 4; http://arxiv.org/pdf/gr-qc/0604045.pdf
Sean Carroll (2011), “Are Many Worlds and the Multiverse the Same Idea?”, Discover Magazine, Waukesha WI: Kalmbach Publishing, blog post May 26, http://blogs.discovermagazine.com/cosmicvariance/2011/05/26/are-many-worlds-and-the-multiverse-the-same-idea/
Juan Maldacena (2014), “Testing gauge/gravity duality on a quantum black hole”, Science, Washington: AAAS, vol. 344, p. 806; http://www.sciencemag.org/content/344/6186/806.full.pdf
Image: Justice Society of America #20 by Geoff Johns, Alex Ross, Jerry Ordway, and Dale Eaglesham
Max Tegmark (undated), “Welcome to My Crazy Universe”, http://space.mit.edu/home/tegmark/crazy.html