(This article was originally posted on blogger.com on Oct 13, 2005)
At least this is the impression you could get by reading a recent article by the science columnist Margaret Wertheim, working at the L.A. Weekly. Margaret is also writing a book about ‘the role of imagination in theoretical physics’. Commenting on the present status of theoretical physics, she says, that:
THERE HAS BEEN much talk of late about the scientific method, which usually takes place in the context of distinguishing science from other “less rational” practices, such as religion and magic. But in recent years science itself has been showing increasingly magical tendencies. In the field of theoretical physics, it is now common practice to talk about other dimensions of reality, entire landscapes of universes for which there is no empirical evidence whatever.
I’m not sure what she means by ‘other dimensions of reality’, but if she’s thinking about the idea, that our universe could have more than four dimensions (three spatial and one time), then it’s an idea, which is almost 90 years old. (Thanks to Klein, who in the 1920s tried to unify gravity and electromagnetism in a five-dimensional extension of our universe). She also comments on Prof. Lisa Randall’s new book, Warped Passages, as:
[…] According to general relativity, the universe we live in has four dimensions: three of space and one of time. Randall’s work extends this framework and posits the existence of a fifth dimension. The fifth dimension is the bulk, and within its immeasurably expanded space, there is no reason to assume that ours is the only cosmos.
How I understand it, special relativity – and so much more for general relativity – did not teach us, that our universe has four dimensions. It’s more, that ‘space’ and ‘time’ are not separate quantities, and we should instead talk about ‘space-time’ (since any motion is space involves a motion in time and vice versa). A less important detail is, that the fifth dimension is not the bulk, but rather the bulk is the five dimensional space-time, which includes our (presently) observed universe and an extra fifth dimension.
According to this picture we live on a braneworld, which you can think of as an infinite plane to which the fifth dimension in transverse. It is possible, that there could be other universes ‘next to’ ours, like a stack of loafs of bread, but this possibility can to some extend actually be tested experimentally with the Large Hadron Collider (LHC), being built at CERN, which starts operating in 2007. Clever tests can also limit the number of extra transverse dimensions, and the ‘distance’ between our braneworld and other braneworlds in the fifth dimension.
As Margaret correctly says, the “evidence for this new dimension is nonexistent”. But I would only call Prof. Randall’s ideas as much religion as physics, if it in principle was impossible to test them (and most likely, this test could be carried out in the next decade or so, when LHC has collected enough data). Incidentally, one ‘theory’ of biology, which belongs to this class, is the idea of Intelligent Design. Margaret also wrongly claims, that:
IN MANY WAYS, string theorists’ extension of the universe is just one more step in a historical chain of cosmic expansions. […]. The difference here is that the prior extensions were prompted by observations of distant phenomenon. The extra dimensions […] have never been observed and, in principle, they may not be observable, at least not directly. […]. String theory is so fecund in its descriptive power that one physicist has estimated there may be as many as 10 to the power of 100 different versions of its equations!
Once upon a time, the sine qua non of scientific practice was supposed to be empirical verification. […]. In truth, the picture has always been more complex. Science is also an engine of the imagination, leading our minds beyond the mundane realm of what is to the enchanted regions of what might be. Nowhere is the speculative dimension of science more prominent than theoretical physics, which has given us such magical possibilities as time machines made from spinning black holes, wormholes that become portals to the far ends of the universe and the “parallel worlds” of quantum mechanics, which, in theory, make every possible version of history a realized physical fact. […]. Unchained by the fetters of verification, string theorists are free to dream, articulating through their equations vast imagined domains in which almost anything that is mathematically possible is deemed to be happening “somewhere.”
There are many places here, where Margaret misinterprets the history of physics. Much of the early research in physics were not guided by experiment (but of course she is correct in saying, that the picture is more complex). Einstein’s theories of relativity were not directly prompted by observations of distant phenomena. The special theory of relativity, for example, originated from Einstein’s ingenious ideas about the nature of light and matter – only years later could Einstein see his general theory of relativity being supported by experiments, like the non-Newtonian periapsis precession, which was confirmed for Mercury and later observed in several binary pulsars. Also, the deflection of light by the Sun, observed in 1919, helped to cement the status of general relativity. The idea of black holes started as pure speculation in 1783, while they where found as solutions of general relativity in 1915 (by Karl Schwarzschild). Another point is, that I disagree with the statement, that “almost anything that is mathematically possible is deemed to be happening “somewhere”. One things is that even though there are about 10^500 vacua, only one of them should describe our universe – I don’t think anybody believes, that there are 10^500 – 1 other ‘universes’. So, there are basically an infinite number of solutions to string theory, but virtually all of them are inconsistent. The job is to find out, what solutions are relevant for us, and not as much for mathematics.