By bringing together like-minded individuals who support a business, enjoy a specific entertainer or are passionate about a specific cause, online communities can reach a significant level of activity and connectivity, which then encourages its members to spend more time on the site, visit more regularly and interact more with others there.

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The Sommerfeld effect, a quantum-mechanical process that makes particles more likely to interact with one another in some circumstances and less likely in others, is mostly ignored in particle physics because it applies only in a limited range of conditions.

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These patients suffered less, were physically more capable, and were better able, for a longer period, to interact with others.

NEWYORKER: Letting Go