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New York

THE ground is shifting under our feet. Fundamental properties of the Universe
are changing, and physicists can鈥檛 explain how or why. Now researchers say an as
yet undiscovered fifth force could be behind these mysterious changes.

Physicists combine supposedly unchanging physical properties, such as the
speed of light and an electron鈥檚 charge, into a number called the 鈥渇ine
structure constant鈥� that describes how our Universe hangs together. But in 1999,
astronomers analysing 10-billion-year-old light from distant quasars got a shock
when they found that it was different from what we鈥檇 expect to see today.

They concluded that the fine structure constant, or alpha, must have been
different 10 billion years ago. If so, the host of fundamental values tied to
alpha could be changing too: light may be slowing down, the electron鈥檚 charge
growing, and atomic nuclei losing mass.

If the Universe is four dimensional, then a fifth force is the only thing
capable of triggering these changes, say Gia Dvali and Matias Zaldarriaga of New
York University. The four fundamental forces known so far are gravity,
electromagnetism and the strong and weak nuclear forces. But the new force would
have a repelling effect, showing itself via tiny particles called 鈥渁lpha ions鈥�
slowly emitted from protons and neutrons. As the repelling alpha ions were lost,
atomic nuclei would lose mass, but atoms would become more strongly bound
together.

The force would be incredibly weak, 100,000 times fainter than gravity. But
it would work over long ranges, meaning it could serve as the mysterious
repulsive energy called quintessence, Latin for 鈥渇ifth element鈥�, that some
scientists say explains why the Universe is flying apart faster and faster.

鈥淭he possibility that you could find a [force] that is both making the fine
structure constant vary and causing the Universe to accelerate is
extraordinarily exciting. It鈥檇 be a fantastic discovery,鈥� says theoretical
physicist Sean Carroll of the Enrico Fermi Institute at the University of
Chicago.

There are several theories to explain the variation in alpha, but they all
require some kind of fifth force, says John Barrow of the University of
Cambridge. 鈥淭hey would each have different consequences for observations very
close to the surface of neutron stars.鈥� But Dvali and Zaldarriaga say there鈥檚 an
easier way to detect traces of alpha ions. They would be shed at different
energies depending on a particle鈥檚 mass, and that would make protons and
neutrons fall at very slightly different speeds.

A satellite programme called STEP is already being planned to test the
theory. Free-floating masses will be isolated in an ultra-high vacuum. If they
fall at different rates, it would support the existence of alpha ions.
Otherwise, we may have to look to higher dimensions for an answer. Dvali and
Zaldarriaga say that without a fifth force, that鈥檚 the only thing that could
explain why alpha is changing.