More than 40 years ago, a leading relativity theorist made a surprising prediction. Whereas empty space should feel immeasurably cold to any observer gliding along at a constant speed, one who is accelerating, say because he's riding a rocket, would find empty space hot. This so-called Unruh effect seemed practically impossible to measure, but now four theorists claim they have devised a doable experiment that could confirm the underlying physics. Skeptics say it will do no such thing—but for contradictory reasons.
"The hope is that this will convince skeptics that the whole thing is coherent," says Stephen Fulling, a theoretical physicist and mathematician at Texas A&M University in College Station who was not involved in the work. But Vladimir Belinski, a theorist at International Network of Centers for Relativistic Astrophysics in Pescara, Italy, says, "The Unruh effect is nonsense, it's based on a mathematical mistake."
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