The speed at which light travels through a vacuum, about 186,000 miles per second, is enshrined in physics lore as a universal speed limit. Nothing can travel faster than that speed. Einstein's theory of relativity would crumble, theoretical physics would fall into disarray, if anything could.

Two new experiments have demonstrated how wrong that comfortable wisdom is. Einstein's theory survives, physicists say, but the results of the experiments are so mind-bending and weird that the easily unnerved are advised--in all seriousness--not to read beyond this point.

In the most striking of the new experiments a pulse of light that enters a transparent chamber filled with specially prepared cesium gas is pushed to speeds of 300 times the normal speed of light. That is so fast that, under these peculiar circumstances, the main part of the pulse exits the far side of the chamber even before it enters at the near side.

It is as if someone looking through a window from home were to see a man slip and fall on a patch of ice while crossing the street well before witnesses on the sidewalk saw the mishap occur--a preview of the future. But Einstein's theory, and at least a shred of common sense, seem to survive because the effect could never be used to signal back in time to change the past--avert the accident, in the example.

The invisible man has come one step closer: scientists believe they have found a way to make flesh transparent for a few minutes at a time.

By manipulating the way light passes through tissue, a team at the University of Texas at Austin has moved into what was once the realm of science fiction. The engineers say they can create a temporary "window" in tissue, allowing doctors to see up to five times deeper than at present over an area of up to one or two square inches.

Although the technique has not yet been tested on human skin, the engineers believe it could have applications in diagnosis; helping to reveal the extent of skin cancer, for example, and in treatment, by allowing a laser beam to be targeted on underlying tissue.

By injecting various substances, the team made small areas of rat or hamster skin nearly transparent for 20 minutes or more. Prof Ashley Welch, the lead investigator, said: "We could see a blood vessel which had not been visible." Light does not usually penetrate skin because it is scattered, like a torch beam in fog.

Just as each water droplet in the fog scatters light, so small components of tissue also scatter light. To overcome this, his team used glycerol, a hygroscopic alcohol which pulls water out of tissue. The team has yet to look into the toxicity of the technique, which Prof Welch admitted was "an important question".

Kids at Philadelphia's George Washington High School don't cut many classes these days.

Bar-coded identification cards that monitor who comes into the building each morning and track the classes a student attends are making it pretty difficult to skip out. Students who do it anyway are greeted the next morning at school with the shriek of a siren that alerts the school principal and lets him know who was naughty.

The software that works with the cards is called the Comprehensive Attendance Administration and Security System. CAASS works by scanning each student's ID card upon entry to the school, then sends the student's name and picture to the CAASS screen, enabling the staff member to confirm the student's identity.

George Washington principal Sam Karlin said the program has done wonders for the high school. He used to get a 14-page stack of papers showing up to 3,000 reported cuts, he said. Now he gets a few pages worth of skipped classes by about 80 students. And the kids who insist on skipping class now get to face a stern-faced principal each morning.

While technology does seem to be helping schools keep kids in class, school officials like Sam Karlin say they have mixed feelings about programs like CAASS.

Karlin said he and a colleague watched the students of George Washington on the first day the school implemented the system.

"We were looking down the hall, seeing all the kids line up, and (my colleague) said, 'Look how wonderful this is that we have the technology to do this in a modern high school.'

"And then he said, 'And look how sad it is we have to.'"

The standard advice may be "take two aspirin and call me in the morning." But it soon may become "take these tablets and they will call me in the morning"—and in the afternoon and evening.

That is what some researchers are predicting for the fast-growing field of embedded computing—a futuristic place of microscopic medical devices packaged in grains of medicine as small as grains of sand.

Within the decade, some researchers say, patients will swallow these tiny grains, and down will go microscopic sensors, microscopic processors and microscopic radio transceivers, all part of a delivery and communications network. The devices will, for instance, track a medical condition and release drugs to treat it while constantly sending wireless data transmissions on the patient's physiological responses to a portable server in the home. The data will then be sent via the Internet to the doctor.

This medicine will be more than smart—it will be downright voluble. "Such applications are nearer to happening than even educated observers might think," said Dr. Gaetano Borriello, a professor in the department of computer science and engineering at the University of Washington in Seattle.

The warning signs of technological doom are there, unmistakably. I wrote recently about the grim predictions of the computer wizard Bill Joy of Sun Microsystems that the wondrous tools we're creating will eventually destroy us.

This is ''the century of danger," as Mr. Joy says. But it is also the century of opportunity—offering greater promise to cure disease, extend life and enrich human experience than any in history. That was the other theme of the Highlands Forum conference that I attended this month, where Mr. Joy delivered his message of doom to an audience of America's brightest technologists.

That is the essence of the dilemma that Mr. Joy poses. The hideous dangers he describes come wrapped inside the most exciting and beautiful container imaginable.

Two of the Highlands presentations were especially dazzling: The first was an explanation by Berkeley's Kris Pister of his research to create ''smart dust," which are computers so tiny they could literally go anywhere. And when he says tiny, he means something that's about one-tenth the diameter of human hair.

These smart sensors—and their active counterparts, called ''actuators"—are known as Micro-Electro-Mechanical Systems, and they will soon perform amazing tasks: They will travel in our blood stream to detect and eradicate disease; they will be painted on our cars, allowing them to change color and shape; they will sense the presence of biological or chemical weapons and warn us to take countermeasures; they will monitor every item in our refrigerator and tell us when something is stale.

Since the Pentagon sponsors the Highlands conference, some of the gizmos that Mr. Pister described are inevitably spy gear: a tiny spy plane, less than a foot long, that can disseminate smart dust over a battlefield; synthetic insects that can hover like mosquitoes or tunnel like inchworms or clamber across obstacles like cockroaches. Nothing—no person, place, signal or conversation—will be out of reach of these microspies.

The second astonishing presentation was Philip Kuekes's discussion of the work he is doing at Hewlett-Packard to create molecular-scale electronics, or ''moletronics." The computers are so tiny and finely woven that they begin to approach the architecture of human brain cells. Mr. Kuekes's team has built wires that are just 10 atoms wide.

These nano-computers will be self-assembling, using thermodynamics, rather than the lithography that is used to create computer chips.

Mr. Kuekes explained that, once built, the tiny grids will be attached to doctor computers, which will look for defects, and then to tutor computers, which will download the instructions to process digital information.

Mr. Kuekes imagines that these molecular computers will be as pervasive and powerful, and as cheap, as life itself. They will transform human existence in ways we can only begin now to imagine.

And that is the seduction: Technology is creating tools that will allow us to conquer disease, live twice as long, master our world at last. The package is dazzling, but what's inside?

Computers are becoming such an integral part of our lives that soon we won’t even notice them.

At the Invisible Computer conference at the Fashion Institute of Technology, speakers were talking about pushing the envelope further than the concept of just moving the computer from the office into the living room. They were touting bottles you open to get the weather report and watches that record every physical move you make.

"I never wanted my mom to boot up a PC, or learn how to use Internet Explorer. It’s irrelevant to her life," said Hiroshi Ishii, director of the Tangible Media Group at the MIT Media Lab in Cambridge, Massachusetts. So he created something as familiar to his mom as the bottle of soy sauce she used every day. His glass bottles announce the weather report when they’re opened.

Ishii quoted the late computer scientist Mark Weiser to sum up the concept of invisible computing: "The most productive technologies are those that disappear. They wear themselves into the fabric of everyday life until they are indistinguishable from it."