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Why does a stock's price fluctuate so often, even when fresh news isn't available?
Investors are "forever" comparing the prices of stocks to other stocks. As others have pointed out, this is done faster and more frequently nowadays with high-speed computer programs. There may be no "fresh" news on stock A, but if there is fresh news on stock B (as there usually is), the news on B affects the COMPARISON with stock A. That could be what causes trading in stock A that has "no news."