Universes: In recent decades multiple universes, parallel worlds, the many worlds interpretation of quantum mechanics have become important subjects in physics and astronomy in the scientific literature, in popular books on science, in magazines and newspapers and in television. Yet in the scientific world there is ongoing controversy: do these subjects belong in physics or belong in philosophy or belong in mathematics? In this blog I give my view of the multiple universe enterprise, confessing that I am a hands-on physics experimenter. I was trained in chemical engineering and before going to Columbia University for my physics Ph.D. I worked as an engineer in the General Electric electron tube division. First some examples of multiple universe ideas and some references.
Examples: I start with a simple example. Recall that our Universe started with the Big Bang and the observations that it is 13.7 billion years old and has a visible radius of 5 x 1010 light years. Our presently accepted view is that space is approximately flat and infinite in extent. We think of our Universe as occupying all of space.
But suppose that our Universe does not occupy all of space. Then there could be a second Universe in another part of space. Of course the mathematical-physical description of space containing two universes would have to be different and more complicated than the general relativity description of our Universe. The ultimate question for an astronomical observer in one universe is whether evidence can be found for the existence of the second universe.
Now we can add variations. We can suppose that there are many universes. We can suppose that since space is infinite in extent, that universes are being created all the time, and there is always room for more. Going further there is no need to suppose that the physical constants in the different universes are the same, the charge of the electron or the velocity of light might be different. And further still, the physical laws might themselves be different, no Maxwell’s equation, no quarks in a neighboring universe. This raises the thought that two universes could coincide but not be able to detect each other.
Some References: In the large literature on multiple universes I have some specific recommendations. Max Tegmark [arXiv:0905.1283] presents a fascinating and intriguing classification of different levels of universes. Leonard Susskind in his book The Cosmic Landscape [Little, Brown and Company, New York, 2005] discusses multiple universes from the string theory viewpoint and also argues against various teleological and “intelligent design” theses that have permeated this subject. Andre Linde, a leader in multiverse theory, has a website [http://www.stanford.edu/~alinde/]. The term multiverse is often used to denote a world containing many universes.
On the other hand, for an example of withering criticisms of multiple universe ideas, see one of my favorite blogs Not Even Wrong written by Peter Woit. See for example his Nov. 28 posting [http://www.math.columbia.edu/~woit/wordpress].
Finally, an historical overview of multiple universes and much else is given in the excellent book Higher Speculations by Helge Kragh [Oxford University Press, New York, 2011]. The Amazon price for the book is $48 and it is well worth it. Higher Speculations has been reviewed in the July 8, 2011 posting of Not Even Wrong.
Much Interest and Sophisticated Work: In part of the professional physics and astronomy communities there is much interest in, and sophisticated theoretical work on, multiple universes; in spite of the usually acknowledged impossibility of experimenters and observers testing most of these theories. Indeed it is a long tradition in the science world to develop explanations and world views that go beyond the possibility of experimental or observational verification. A nice example of a theory that could not be verified is the Victorian model of vortex atoms as described in chapter 2 of Higher Speculations. In this speculative model, the vortexes were supposed to occur in a universal fluid, perhaps the ether. But neither the universal fluid or its vortices had been discovered.
Unlike many experimenter and theoretical colleagues I am not bothered by the nature of the professional work on multiple universes, and as a professional physicist I am not concerned whether it is called physics or metaphysics or philosophy or mathematics.
The Public: There are concerns by some scientists and educators about the impact on the interested public of multiple universe theory as presented in popular science books, websites and television. Does this public distinguish between the validity of theories requiring verification by experiment or observation and the validity of theories being based on their beauty or their mathematical perfection? And how does one know the boundary between the work of the professional physicist and the work of the crank and quack physicists? Of course, some of the public following physics and astronomy understand the two different types of validity and can detect crank and quack physics, but not everyone.
I do not believe that the research and teaching communities can do anything about this. We are a small part of an enormous media world permeated by astrology and homeopathic medicine and psychic phenomenon and creationism and claims for infinite amounts of energy available from the quantum mechanical zero-point energy of the vacuum. (See the Wikipedia article entitled Zero-point energy.)
The public interest in multiple universe ideas is that the ideas are often fascinating and spectacular. The development and exposition of new ideas can move quickly, particularly with the use of the Internet. Contrast this situation with the pace of extraction of experimental results from the Large Hadron Collider (LHC). A powerful, beautiful proton-proton collider giving the highest energy collisions ever obtained, powerful experimental groups with wonderful particle physics detectors, yet Nature holds its secrets closely.
Experimental and Observational Research
My training and my research over the years leads me to restrict scientific reality to ideas, models, laws, and theories that have been confirmed by experimental or observation research. The multiple universe world does not meet my criterion for scientific reality. But I like to think occasionally about multiple universes as a way of stimulating my search for new research technology ideas. Will a new technology be found for searching for evidence for multiple universes? Of course I don’t know what it would be, but I don’t see this technology goal as surely hopeless.
An example of a search for effects of a multiple universe model is the paper First Observational Tests of Eternal Inflation by S. M. Feeney et al [arXiv:1012.1995v3]. They find no evidence for the existence or non-existence of a particular type of multiple universe proposed by Linde. It is interesting to look at their search method, looking for anomalies such unexpected symmetries and boundaries in WMAP data. This search used existing technology for studying the cosmic microwave background.
I end this blog with a fable. It is 1910 and the mass, charge and low energy behavior of the electron has been elucidated. A kindly twenty first century physicist sends back a message though time – congratulations on the electron and by the way there is a particle related to the electron that has zero charge and a much smaller mass, we call it an electron neutrino. But the 1910 physicists cannot do anything about this kindly hint, the message is incomprehensible. The technology to look for the neutrino did not exist.
It was not until the early 1950s that Clive Cowan and Fredrick Reines detected reactor neutrinos. [Wikipedia Cowan–Reines neutrino experiment.]
The Current Situation in High-Energy Physics
After wandering in the speculative physics of multiple universes I end this blog recommending the summary talk of Michael Peskin at the 2011 Lepton-Photon conference [arxiv.org/abs/arXiv:1110.3805]. He gives his perspective on the current situation in high-energy physics.
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