QUESTION: Why hasn't anybody designed a computer that's conscious and has emotions, like HAL in the movie "2001: A Space Odyssey?"

ANSWER: We know exactly what would happen if we let computers think like humans: They'd turn on us. In every science-fiction movie, machines are double-crossers. Give them an ounce of emotion, and suddenly they turn into Ted Bundy.There's also a more fundamental problem: We don't know what consciousness is, precisely. We can describe it - it's what allows us to notice that we are thinking about consciousness, and to notice that we are thinking about ourselves thinking about consciousness - but it's hard to understand how it emerges out of the hardware, that big gloppy gray thing in the skull.

"When will computers be like human beings? I don't know," says Ramesh Jain, director of the Artificial Intelligence Lab at the University of Michigan. "The problem is, we know very little about human beings, how we see things, how we hear things, how we react to different things. The day we know human beings better, we'll be able to design computers like human beings."

The 17th-century philosopher Rene Descartes articulated the idea that the body is a kind of machine but the mind is a separate, immaterial essence. How did the mind interact with the body? Through the pineal gland, he said. The pineal gland! It's amazing how people used to be able to get famous and even respected by saying incredibly stupid things.

Nowadays the idea of "mind" no longer meets the standards of scientific inquiry. The prevalent view is that the brain works like a computer. Even things like love are therefore merely the result of electrochemical interactions among neurons. Marriage is what happens when two machines interface. You get the picture.

The corollary is that we should be able to design computers that have humanlike intelligence. One major problem is that the human brain has about 100 billion neurons, each firing at various moments, a simple binary code multiplied so many times as to be too complex to simulate. Even if this could be modeled in a computer program, how would the computer ever learn to be self-referential, to stand apart from its computations and discern itself as an entity? One famous dictum is that the huge electronic billboard at Times Square can flick its lights on and off in innumerable patterns but will never really "know" what its message is.

Perhaps computer intelligence would simply emerge by itself from a complex program. After all, that's how we think human intelligence appeared - it evolved from an earlier, brutish state. We needed about a couple of million years. Computers may need only a few decades.

And then they'll kill us.

QUESTION: Why, in movies, do car wheels sometimes suddenly seem to be turning backward even though the car is still moving forward?

ANSWER: Surely you've seen this phenomenon. It's particularly true of wagons with big wheels and prominent spokes. One moment they are rolling forward, and the next moment they shift into reverse. Why?

A movie is a series of still photographs. The camera shoots 24 frames every second. At certain speeds, the spokes will turn just enough to present to the camera an identical picture every 124th of a second. The wheel will therefore appear to be stationary on the film.

But if the wheel is turning slightly slower than that, the spokes will present to the camera the impression that they are slowly ticking counterclockwise and thus create the illusion of going backward.

The Mailbag:

Someone sent a question asking why there's that FBI warning at the start of every videotaped movie, saying you can't copy it. Does the FBI really think it could stop us?

We called the Motion Picture Association of America and the FBI - that's not one big title but rather two separate organizations - and found out a few things, namely that it's almost impossible for the average consumer with two VCRs to make a copy of a videotaped movie. They've all got some special encoding, done by a company called Macrovision, that prevents duplication. You need special equipment to do it. "We are not, as an industry, terribly concerned with home taping," says Mark Harrad, spokesman for the MPAA. What they are concerned about are big-time video pirates mass producing copies for resale. The FBI conducted 89 raids last year. So we're told.

Got a postcard: Andrew E. of Phoenix asks, "Isn't the Earth being destabilized in some way by all this pumping up of oil from the ground all over the world?"

Dear Andy: Geologically, no. Politically, yes. You're not going to fall through the crust of the Earth because of pumping. The oil is too deep, and the Earth too big by comparison. The real problem is that someday you might get drafted.