New York ? If you saw a penny on the sidewalk, would you pick it up?

You may think it’s not worth the effort, but a breed of investors who have been in the news do. Using super-fast computers, high-frequency traders in effect bend down to pick up pennies lying about in the stock market — then do it again, sometimes thousands of times a second.

More than a week after the Dow Jones industrial average fell nearly 1,000 points, its biggest intraday drop ever, regulators are still sifting through buy and sell orders to figure out what sparked it. One big focus are orders placed by high-frequency traders, or HFTs, and for good reason. These quick-buck firms barely existed a few years ago but now account for two-thirds of all U.S. stock trading.

In other words, all those TV pictures of the stately New York Stock Exchange building on the evening news are an illusion. The real action on Wall Street is far away in Kansas City, Mo., and in New Jersey, in towns like Carteret and Red Bank, where HFTs named Tradebot and Wolverine and Tradeworx ply their trade.

High-frequency trading firms, which number more than 100, use computers programmed with complex mathematical formulas to comb markets for securities priced too high or too low because traders haven’t had to time to react to the latest data. The computers then buy or sell in a split second, locking in a profit.

The opportunities seem hardly worth noting. They’re not just fleeting, but small, often a penny or less.

But those pennies can add up to a lot of money, enough to draw the attention of Goldman Sachs Group Inc., the giant Chicago hedge fund Citadel Investment and other big financial firms. In recent years they’ve paid hundreds of millions of dollars for stakes in high-frequency trading companies.

The money has stoked what was already fierce competition among the firms for a leg up.

To spot opportunities and act on them before others, HFTs are constantly hunting for faster computers. They also locate themselves close to the big exchanges’ data centers. That can cut their trade times by milliseconds.

One way these traders make money is by exploiting the fact that stock indexes sometimes don’t immediately reflect falling or rising prices of their component stocks, said Manoj Narang, chief executive at Tradeworx of Red Bank, N.J. If Microsoft shares rise 5 percent but an index fund that includes it such as the SPDR S&P 500 lags by a fraction of second to adjust, his computers will automatically buy shares of SPDR S&P 500 at the lower price and then sell them again when they are fully valued.

Or maybe Microsoft is trading in London at a penny less than it’s trading at the same moment in New York. A high-frequency trader will buy shares in London and wait for them to rise.

Since the discrepancy lasts a mere fraction of a second, speed is key.

Narang boasts it takes only 15 millionth of a second for his computers to place a buy or sell order after detecting an opportunity.

Or, as he puts it, “If you try to pick up the penny, we’ll probably beat you to it.”

So is that good or bad for the market?

If you listen to HFTs, all their fast trading benefits big and small investors alike. More trading means more bids and asks for shares, and that cuts the time needed to find someone willing to buy what you’re selling or vice versa. Costs also fall. With more bids and asks, the difference between the price you seek and the price offered (what traders call the “spread”) will likely narrow. You get to keep more of your money.