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Time machines get a step closer


(January 1997)


Hendrik Casimir predicted the weak force between two plates in a vacuum which we now call the Casimir effect in 1948. Steven Lamoreaux at the Los Alamos National laboratory in New Mexico has just succeeded in measuring the force of the effect, using a torsion pendulum. The result: a force within 5% of the predicted level was measured, a very good result indeed.


The importance of this is that any time machine that we can now predict will need to use two sets of plates experiencing the Casimir effect. But until we have a working time machine available to see what happens, the best we can say is that it is early days yet.


The Casimir effect is only measurable when two parallel plate are set up, just a fraction of a millimeter apart in a vacuum, and the result is that a weak force then operates to push them together. Empty space is not really empty, according to quantum theory. Instead, virtual photons are continually popping into existence and then disappearing again.


In the narrow gap between the plates, the only photons which can exist are those with wavelengths which are a equal to the gap distance divided by an integer. All other photons are excluded from the gap, and this means there are more photons pressing on the outside of the gap than on the inside, producing the force we call the Casimir effect. According to Lamoreaux, the force he measured, with a separation of just 0.75 micrometre, was about one billionth of a newton.


This is the third major breakthrough in physics that has been achieved with a torsion pendulum, after a wait of almost two centuries. Charles Coulomb used a torsion pendulum to measure the forces between electrical charges in 1785, and soon after, Henry Cavendish had used a similar device to measure the force of gravitation in 1798.