With ever more precise measurements, physicists are probing accepted truths at the limits of physics.
The beams of protons that circulate around the 27km-circumference ring of the Large Hadron Collider (LHC), the world’s biggest particle accelerator, carry as much kinetic energy as an American aircraft-carrier sailing at just under six knots. Andrew Geraci’s equipment, on the other hand, comprises a glass bead 300 billionths of a meter across, held in a lattice of laser light inside an airless chamber. The power it consumes would run a few old-fashioned light bulbs. Like researchers at the LHC, Dr. Geraci and his team at the University of Nevada, in Reno, hope to find things unexplained by established theories such as the Standard Model of particle physics and Newton’s law of gravity. Whereas the LHC cost around SFr4.6bn ($5bn) to build, however, Dr Geraci’s set-up cost a mere $300,000 and fits on a table about a meter wide and three long.
A century ago these were the normal dimensions for experiments in fundamental physics. The electron, the proton and the neutron were all found using kit this size. (J.J. Thompson and his electron-discovery device are pictured above.) But digging deeper into theories of reality requires more energy, and thus bigger machines — of which the LHC is the latest. Since finding the Higgs boson in 2012, though, this behemoth has drawn a blank. Dr. Geraci and those like him aspire, by contrast, to find evidence for those theories’ veracity by making precise measurements of the tiny forces that the particles they predict are expected to exert on other objects.
In Dr. Geraci’s experiment the suspended bead scatters laser light onto a detector. If a force displaces the bead, the pattern of light changes, permitting the bead’s new position to be calculated. In work published last year in Physical Review A, his team showed that the apparatus can detect forces of a few billionths of a trillionth of a newton. (A newton is about the force exerted by Earth’s gravity on an apple.) Their next step will be to move a weight past the bead at a distance of five microns (five thousandths of a millimeter), to measure the gravitational attraction between them. That experiment is now under way.
Dr. Geraci is looking for deviations from Newton’s inverse-square law of gravity (that the gravitational force between two objects is inversely proportional to the square of the distance between them). Any departure from this law would provide support for theories which hope to solve what is known as the hierarchy problem of physics. This is the question of why gravity is so much weaker than the other three fundamental interactions between particles, namely electromagnetism and the weak and strong nuclear forces. The disparity between gravity and these forces explains, for example, why a small magnet can pick up a paper clip against the gravitational force of an entire planet…
If either Dr. Geraci or Dr. Adelberger do overthrow the inverse-square law, they will open the way to a test of string theory — an attempt to explain physics at the most fundamental level. A recent version of string theory posits the universe to have 11 dimensions, seven of which are beyond human ken. Bringing even one or two of these within the realm of experiment, as ADD would if proved correct, would be a huge advance in understanding.
A second area in which tabletop experiments may beat the big guns is the search for dark matter. This mysterious stuff, not composed of the familiar protons, neutrons and electrons that make up atoms, is thought to pervade space and to constitute about 85% of the matter in the universe. Its gravitational effects can be seen on the ways that galaxies move. But, in a topsy-turvy parody of the hierarchy problem, it shows little or no sign of interacting with atomic matter through any of the other three known forces. Many physicists, however, suspect that it may do so through forces as yet unknown. Some theories of dark matter predict the existence of force-carrying particles called axions and dark photons — and that these things interact, albeit weakly, with familiar matter. One searcher after such interactions is Hendrick Bethlem of the Free University of Amsterdam, in the Netherlands. He hopes to see signs of them in the spectra of individual molecules.
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Searching for particles on a benchtopMaking precise measurements of tiny forces