Knocking on Heaven's Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World

  • Lisa Randall
Bodley Head: 2011. 464 pp. £20 9780061723728 | ISBN: 978-0-0617-2372-8

Mentioning particle physics may silence many dinner parties, but that has not deterred its funders. By the end of 2010, more than €7 billion (US$10 billion) had been ploughed into the current world-leading machine in experimental particle physics — the Large Hadron Collider (LHC) at CERN, Europe's high-energy physics lab near Geneva, Switzerland. So it behooves the researchers involved to communicate the relevance of the LHC's science goals to the public.

Lisa Randall's Knocking on Heaven's Door is the latest attempt to do so. Her eloquent book details the trials and tribulations of the LHC, from conception to implementation, and takes us on a grand tour of the underlying science. Randall, a professor of physics at Harvard University in Cambridge, Massachusetts, and a leading contributor to particle-physics theory, borrows her title from Bob Dylan's soundtrack to the 1973 Sam Peckinpah film, Pat Garrett and Billy the Kid. The film is a lament on the death of a gunslinger — and the book's title may be a reference to the prediction that turning on the LHC would result in the destruction of Earth. Fortunately, as Randall describes, this did not happen.

Building the Large Hadron Collider proved tricky, not least because of fears it would create tiny black holes. Credit: C. MARCELLONI/CERN

That prediction provides a measure of the LHC's reach, and its hold on the public's imagination. Physicists' ultimate dream is to unify the fundamental interactions of physics. This involves combining gravity with quantum theory and, in particular, with the forces that hold particles and atoms together in higher dimensions of space and time.

String theorists, whose ranks purportedly include the greatest brains in physics, have predicted that infinitesimal, very short-lived black holes are the unifying factor, the missing glue. Some have predicted that these microscopic objects could be recreated in sufficiently high-energy particle collisions with a powerful particle accelerator such as the LHC. However, most particle physicists doubt that they will actually see such events — Stephen Hawking predicted that, owing to quantum physical effects, microscopic black holes should decay in a fraction of a nanosecond.

But even Hawking might be fallible. Richard Feynman famously said that “nobody understands quantum mechanics”. If Hawking was wrong, an escaping black hole might suck up its surroundings: the LHC itself and Geneva (to which humanity could no doubt adapt) and even Earth. Pursuing this logic, a teacher in Hawaii combined forces with a Spanish writer in 2008 to file a lawsuit against CERN, the US Department of Energy and the US National Science Foundation that threatened to block the start-up of the LHC.

As Randall describes, scientists responded with fervour. It turns out that nature provides an answer to such concerns. Cosmic rays pervade space and bombard Earth continuously. Their energies extend to billions of times that achievable by the LHC. Had microscopic black holes been created in high-energy collisions of cosmic-ray particles, Earth and the stars would have been swallowed up long ago. Physicists could relax: the LHC risk-assessment exercise was favourably resolved.

On 20 November 2009, the LHC first powered up for experiments. By the end of 2012 it will reach a high enough energy to test the standard model of particle physics and to detect the Higgs boson, the elusive, mass-giving 'God particle' — if it exists. Knocking on Heaven's Door describes how that discovery would confirm one of the great predictions of physics. In parallel, the LHC will search for physics beyond the standard model. One of the most anticipated signatures will be that of supersymmetry, a new field that provides a candidate particle for dark matter.

Given her background, Randall naturally complements her discussion of the LHC by describing ongoing searches for dark matter that are mostly led by particle physicists. For them, the driving question is: what is it? But Randall largely ignores astronomers' contribution to the problem — namely, giving the empirical motivation for dark matter (it is the dominant form of matter in the Universe) and mapping its location.

The LHC could resolve the greatest mysteries of the Universe: one microscopically small, and the other macroscopically large. But suppose physicists fail to detect any sign of the Higgs boson or supersymmetry? Will we have wasted those billions? Failure would shift the goal posts. Exploration of the next particle-physics frontier will require more powerful, more expensive and less attainable machines. But we would also be unsure as to how high we would need to go in terms of energy or luminosity to achieve a breakthrough in new physics. Visionary ideas would be needed.

Let us hope that the LHC does find something. And that, regardless of the outcome, the inspired efforts of its builders will combine with theorists' dreams to develop new and affordable probes of the ultimate horizons of the Universe.