CAMBRIDGE

THE iPHONE 3G changes nothing, and yet it may change everything. Lots of people are going to know exactly where lots of other people are, right now.

The new iPhone has a Global Positioning System (GPS), an option that cars and some cell phones have had for years. By listening to precisely timed radio signals from low-orbit satellites, a GPS can pin down its own location to within a few feet.

The Apple iPhone is such a well-designed, well-engineered product that it may push this capability — “geolocation” — over the tipping point. In a year or two, we may find it so natural and useful that only fuddie-duddies would go without it.

The change will come if we no longer think of location-tracking the way we used to, as something that intrusive authorities might try to do to us. It is rapidly becoming just something we do to ourselves and to each other, collectively and cooperatively, like sharing our vacation photos on Flickr or joining Facebook.

What happens to the location information, once a GPS has located itself? Your car probably keeps the data to itself, simply directing you to your destination. But not necessarily: Some car-rental companies added radio transmitters to their cars, and then watched where the cars were going in order to impose surcharges for driving too fast or going out of state.

Even without GPS, phones can be located approximately by the strength of the signals reaching nearby cell-phone towers. The cell-phone company can estimate a phone’s location, as long as its power is on, and can use the information to direct emergency responders — and also to assist criminal investigations.

The future, however, is in the vast commercial value of location data.

British shopping malls use geolocation data from cell phones to take a census of how many people are in which stores. You’d have to power off your phone to be omitted from the census.

Malls say that they’re interested in this aggregate data because it enables them to serve their customers better. If the stores are full at the 10 p.m. closing time, for example, perhaps the mall should stay open later. The system can tell one phone from another, so it could tell if a shopper at the Gap later wound up at Banana Republic — information that would be useful for laying out the locations of stores. With a little electronic sleuthing, the same data could be used to identify individuals by correlating the location data with credit card and cash register data.

But if we willingly supply our location data, perhaps in exchange for a service or entertainment, sleuthing is unnecessary. With so many people now carrying geolocation devices with that crucial radio transmitter, privacy norms are shifting rapidly. (Of course, a cell phone has a radio transmitter, though you may not think of it that way!) A Facebook application makes it easy to locate friends who happen to be nearby — and hometown neighbors, too, even if they haven’t yet agreed to join your circle of friends. Loopt invites you to “turn your mobile phone into a social compass with detailed maps that show where your friends are and what they are doing” — or what they will be doing in an hour, and where.

Citysense analyzes billions of bits of location data from cell phones in search of patterns of how people move around cities. By correlating the location data with calendar and weather information, the service can figure out what clubs twenty-somethings are frequenting on rainy Saturdays. Sense Networks, which offers this service, counts on our identifying ourselves in exchange for the information it will feed back to us about what people like us are doing. Such data would be a goldmine for advertisers targeting their ads at cell phones — they would love to know not only who you are, but where you are. And it would be a boon for shoppers, too — imagine being able to ask, when Nordstrom’s doesn’t have your favorite stockings in your size, if any nearby store has them in stock.

Cultural standards change very rapidly when intrusive technologies have a bright side. A few years ago, tracking the movements of law-abiding citizens would have sparked outrage. But with OnStar helping lost motorists and Facebook helping friends meet at Starbucks, tracking by the government seems less threatening too.

If the police were the only ones who could do it, it would be awful — but since we all can do it, then perhaps it doesn’t seem so bad.

Hal Abelson is professor of computer science and engineering at the Massachusetts Institute of Technology. Ken Ledeen is CEO of Nevo Technologies. Harry Lewis is a professor of computer science at Harvard and a fellow of the Berkman Center for Internet and Society. Together they are the authors of Blown to Bits: Your Life, Liberty, and Happiness After the Digital Explosion (bitsbook.com).