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Envision that tremulous moment when little Bobby reached to place the nine-hundred thirty-second domino (with only sixty-eight remaining) just as the telephone rang. He twitched, and the tap from his finger sent the entire chain spilling dramatically in spirals and branches, mere moments before he could call his mother in to watch. Without knowing it (and certainly without being happy about it), Bobby just learned about metastable states.

A metastable state occurs when the energy of a given system is at a localized minimum. It is stable, so long as nothing disturbs it by adding enough energy for it to change (or transition) to another, usually lower, energy state. Consider the case of the domino. When a domino is standing on its end, it has a fair amount of gravitational potential energy (based on the average distance from the ground - half the height of the domino). It would have much less energy if it was laying flat on its side, but it can't simply fall over. (We're assuming that you have a level surface and a domino with a flat end, of course.) For the domino to fall over, it first has to tip, and tipping requires that the end of the domino actually rises a little bit further from the table than it was, increasing its potential energy. (If you doubt this, grab a ruler. See how tall a standing domino is, then tip it just enough to make the top edge of the back side lean over the bottom edge of the front side and see how far from the ground that topmost edge is.) That little bit of tipping requires an input of energy from outside. It doesn't take much - a breath of air, the tap of a finger - but it is still greater than zero. The fact that, if left entirely alone, the domino would remain standing indefinitely describes a metastable state. It is only stable so long as it doesn't have the necessary energy to fall over into a more stable (lower energy) state.

When scientists describe energy levels and metastable states, they usually draw graphs with different states along the x-axis and the energy of each state along the y-axis. Working with the domino, the plot would look something like the top of the emblem employed by the Arby's restaurant chain. At the center would be the metastable state, which has a fairly high energy, but not nearly as high as the energy to either side, which represents the slightly tipped - and tallest - position of the domino. Off to the sides, the energy decreases rapidly, representing positions where the domino falls closer to the

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