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IT's become the basic tool of practicing physicists. It's one of the most accurate sciences ever developed, routinely predicting results out to nine decimal places. Its calculations underlie a substantial chunk of the Western world's economy - making possible everything from transistors to televisions, calculators to computers, fiber optics to fission reactors. It's called quantum mechanics, and it's more than 60 years old. Yet despite a spate of new books on the subject, the public still seems largely unaware of it - with the exception of the phrase ``a quantum leap.''

That's not surprising. It's difficult to grasp. To explain it, physicists resort to mathematics - or talk about such things as particles with no size, a 10-dimensional space, an observer-created reality, a multitude of parallel universes, and a world where probability replaces certainty. Many of them, in fact, admit that they don't really understand it.

Yet on several points most physicists agree:

It has only just begun to change our technologies - and therefore our life styles.

Its implications are revolutionary - probably more so than anything put forward by Copernicus, Galileo, Newton, or Einstein.

As its surprising conclusions about the nature of matter become better known, it will almost certainly have a profound influence on our thinking.

Why? Because, down through history, every major upheaval in physics has had immense impact on mankind's world view. When Copernicus banished man from the center of the universe, and when Newton pictured a deterministic universe ticking away like a great clock, the shock waves rumbled through philosophy, theology, and the arts - and reverberated for centuries.

What will be the legacy of quantum mechanics? Will it change the way we come to terms with matter and the universe? Will it fundamentally alter our sense of humanity's role and capabilities? Will it reinforce or undercut our awareness of energy, time, and space? Will it blur or sharpen the distinctions between consciousness and body, the observer and the object, the individual and the cosmos?

For answers, we need to descend into what physicists call ``the quantum world'' - the world of the subatomic particle.