A face-to-face human conversation-the sort for which dinner tables and traditional seminar and meeting rooms are designedis a spatially coherent, corporeal, and strictly synchronous event. The participants are all present in the same place, everybody hears the words as they are spoken, and replies usually come immediately. The telephone and talk radio have allowed conversants to be dispersed spatially but have not altered this condition of synchrony. (Until the introduction of the answering machine, you had to be by the phone, at the right time, to take a call.)

But there is an alternative. Where necessary, as when Pheidippides was dispatched to run from Athens to Sparta and back, the ancient Greeks used messengers for asynchronous communication. The letter and the postal system, the fax machine, the humble home answering machine, and the fancy corporate voice mail system are all more up-to-date devices for asynchronous communication and so-more significantly in this context-are the network's e-mail and bulletin board systems. ¹⁹ In the asynchronous mode, words are not heard as they are spoken, but are repeated at some later point. Replies do not come immediately. The unity of the face-to-face conversation is fractured both spatially and temporally.

We usually find the laggardliness of the Postal Service's snail-mail, its enforcement of slug's-pace asynchrony, to be a nuisance. As much more efficient asynchronous communications systems have become commonplace, though, we have seen that strict synchrony is not always desirable; controlled asynchrony may have its advantages. We all know how inconvenient an unexpected demand for communication-a knock on the office door when one is deep in thought or a telephone call at the wrong time-can be. Business people and academics have gratefully discovered that it is usually much easier to communicate between Boston and Tokyo by fax than it is to find convenient times at both ends for telephone conversations. Answering machines and voice mail systems eliminate the frustration of telephone tag. You can attend to your e-mail whenever it is convenient to do so, not when you are unexpectedly and arbitrarily interrupted by a telephone ring. We are discovering that strictly synchronous communication is really just a limit case of asynchronous communication.

The tilt toward electronic asynchrony will have increasingly dramatic effects upon urban life and urban form. In the familiar, spatial, synchronous style of city, there is a time and a place for everything. ²⁰ Gathering spots like restaurants and cafés are open, and people come together in them, for well-defined periods. Workers carry out their tasks during standard business hours, and there are predictable rush hours as they travel to and from their workplaces. Buses and trains have schedules, appointments and meetings are arranged for specific moments, theatrical performances, television programs, and university classes are slotted for particular times. Just as each city has its characteristic spatial organization, so it has its own daily, weekly, and seasonal rhythms-very different for New York, Rome, Delhi, and Tokyo. As there is prime real estate, so there is prime time. But now extrapolate to an entirely asynchronous city. Temporal rhythm turns to white noise. The distinction between live events and arbitrarily time-shifted replays becomes difficult or impossible to draw (as it often is now on the television news); anything can happen at any moment. ²¹

When, for example, does an online forum take place, and where do you show up for it? You cannot say. The discussion unfolds over an indefinite period, among dispersed participants who log in and out at arbitrary moments, through uncoordinated posting and receipt of e-mail messages.

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