The Computer for the 21st Century

In 1991, Mark Weiser, head of the Computer Science Laboratory at Xerox PARC, published The Computer for the 21st Century in Scientific American. It opens with one of the most quoted sentences in computing:

The most profound technologies are those that disappear. They weave themselves into the fabric of everyday life until they are indistinguishable from it.

Weiser called his vision ubiquitous computing: instead of one personal computer that demands our attention, hundreds of small computers spread through every room, so well integrated that we stop noticing them.

A quarter of the 21st century later, do we live in Weiser’s world?

Technology That Disappears

Weiser’s model is writing. Text is everywhere, from books to street signs, and we take it in at a glance without consciously reading. Computers were far from this, and the problem was the idea of a personal computer itself: even the most powerful notebook “still focuses attention on a single box.”

To get there, his team at PARC built computers in three sizes, modeled on things already found in a room:

• Tabs are inch-scale, like active Post-it notes. The Active Badge told the building where you were, so doors opened for you and calls followed you.
• Pads are foot-scale, like paper: “scrap computers” you grab anywhere and leave behind, with no owner.
• Boards are yard-scale, like a whiteboard, controlled with electronic “chalk.”

The goal was “hundreds of computers per room,” and they “must know where they are” to adapt to their surroundings.

A Day in Sal’s Life

Weiser ends the paper with a story about a woman named Sal. Read today, it sounds like a list of product launches:

In Sal’s day (1991)	Today
Her alarm clock notices her restless rolling	Sleep-tracking alarms and smart coffee machines
She circles a newspaper quote and it’s sent to work	Google Lens or Live Text
A lost manual beeps from behind boxes in the garage	AirTags and the Find My network
Her office logs her in when she walks through the door	Unlocking a Mac with an Apple Watch

What Already Happened

Weiser said ubiquitous computing needs cheap, low-power computers, a network that ties them together, and software to run across them. Most of this has arrived.

Hardware: Today’s phones beat every spec Weiser predicted for the late 1990s. In 1991, about 50 million personal computers had been sold in total. In 2025 alone, 1.26 billion smartphones were shipped, with an estimated 21.1 billion IoT devices in use.

Location: Computers once had “no idea of their location.” Every phone now knows where it is through GPS, Wi-Fi and Bluetooth.

Networks and software: Phones switch between radios on their own, and apps move between screens: you can cast a video to the TV or continue an email on your laptop.

So, Are We Living in It?

My answer: computers are ubiquitous, but they haven’t disappeared. And for Weiser, disappearing was the whole point.

Our most important computer is exactly what he argued against: a single box that focuses our attention. He compared carrying a super-laptop to “owning just one very important book,” and the smartphone has become that book. Adults online spend 6 hours and 38 minutes on the internet every day, and our devices compete for attention with notifications and feeds.

Pads went the other way, too. Weiser wrote that “the pad that must be carried from place to place is a failure,” yet our phones and tablets belong to one owner and go everywhere with us. We made personal computers smaller instead of making computing ubiquitous.

Privacy

Weiser saw the risks:

hundreds of computers in every room, all capable of sensing people near them and linked by high-speed networks, have the potential to make totalitarianism up to now seem like sheerest anarchy.

In Sal’s world, “privacy conventions” prevent showing video of the neighbors. In ours, Amazon confirmed in 2022 that Ring had given doorbell footage to police without owners’ consent, and AirTags were misused for stalking until Apple and Google added unknown tracker alerts.

Weiser hoped cryptography, “designed into systems from the outset,” could protect privacy. But when data collection is the business model, it rarely is.

What Still Needs to Happen

Calm technology: In 1995, Weiser and John Seely Brown described technology that stays in the periphery until we need it. That needs designers who treat attention as a cost, not a metric.

Devices that work together: For Weiser, the power comes “from the interaction of all of them.” Today, devices mostly cooperate within one company’s ecosystem. Matter is a first step toward open standards.

Power: Most phones still need daily charging, and hundreds of computers per room only work if nobody has to charge them. E-paper and energy harvesting like Power-over-Skin could help.

Background AI: Weiser insisted “no revolution in artificial intelligence is needed.” We got one anyway, and it could give devices the context they’ve been missing, like the app I imagined in killer apps for mixed reality. But AI could also become another screen demanding attention, and camera glasses raise Weiser’s privacy concerns again.

Conclusion

So, are we living in a world of ubiquitous computing? Partly. The hardware, networks and location awareness are here, and most of Sal’s day has come true. What’s missing is what Weiser cared about most: the disappearance.

He hoped using computers would one day be “as refreshing as taking a walk in the woods.” Getting there depends less on new hardware than on calmer design, open standards and privacy built in from the start.

References

1. Mark Weiser. 1991. The Computer for the 21st Century. Scientific American 265(3), pp. 94-104. doi:10.1038/scientificamerican0991-94, PDF
2. Mark Weiser and John Seely Brown. 1995. Designing Calm Technology. Xerox PARC. calmtech.com

Image Credits

• PARC Tab by Roy Want, CC BY-SA 4.0