A Weighty Matter for Physics

In January, five teams of physicists made headlines when they announced, “It is quite clear now that the universe will expand forever.” Last week, another group of physicists announced that the universe is likely to eventually collapse on itself. Because of their discovery that a tiny particle called the neutrino has physical mass, the second group said, the universe appears to have enough gravity to eventually stop expanding.

The about-face may lead many people to declare the whole of physics inscrutable. But physics is a young and searching field; it is less akin to a Zen master bearing impenetrable secrets than to a child just beginning to fathom the world.

Physicists may seem to have the answers, but consider this: The neutrino has just now been identified as the most common form of matter in the universe, and physicists are only beginning to understand its basic nature.

The dispute over whether the universe will expand or implode isn’t as confusing as it first seems. The first group of physicists based its calculations only on kinds of matter it could see and understand, thus excluding the ghostlike neutrinos. By contrast, the second group realized that the behavior of the universe could not be explained only by particles that physicists could see. Thus its members hypothesized that the elusive neutrinos could explain this behavior and then spent the last 25 years searching for evidence of mass in the particles.

Their perseverance confirms the wisdom in an old physics proverb: One does not see what one is not looking for.