Boston ? It was a scene beyond insult. In the Pakistani courtroom, the grisly videotape of Danny Pearl’s murder had played three times.

This tape was made as a boast, not a confession. Its “highlights” now circulate as a recruitment tool, not a guilty plea. Join us and you, too, can murder a young American journalist.

But the lawyer for the four men accused of murder dismissed this video airily as “very weak evidence.” He claimed it was a fraud, a fake.

Doing so, he played to the international audience of conspiracy theorists. He spoke to the crowd that believes that the moon landing was staged, the Holocaust was a fraud, the terrorist attacks were carried out by U.S. government officials. To them, “Wag the Dog” was a documentary.

Reading this coverage, I found it easy to dismiss his outrageous claim, easier still to stand by the video as the truth.

But now I wonder how long we will be able to believe our own eyes. On the very day the lawyer slandered this evidence, there was an unsettling tale from the cutting edge of technology: Scientists at MIT have created the first realistic videos that can show people saying things they never actually said.

The researchers taught a machine to read a face, to learn what people look like when they are talking and then to apply those expressions to other lines or languages. Even in its first stage, this talking head is far better than the “talking babies” of Hollywood fame. It is impossible to tell real from unreal. The researchers can now literally put words in our mouths.

This is not without precedent. Other, older technologies that captured reality also distorted it. Abraham Lincoln once gave Mathew Brady credit for helping him win an election with an altered and improved portrait. Joseph Stalin had the photographs of his inner circle airbrushed and updated to fit every new purge.

Now with computers animating and generating and editing, we have already gone beyond airbrushing and cropping. We have gone beyond still images to mobile ones.

When the producers of movies like “Serendipity” and “Zoolander” wanted to get rid of the Twin Towers backdrop, they simply deleted the image. When the sponsors for a sitcom change, products can be digitally placed and replaced. A Sprite on the table in prime time can turn into a Pepsi in reruns.

We have brought actors back to life with technology. Fred Astaire was exhumed to dance with a vacuum cleaner. Humphrey Bogart was resurrected to sell Coke.

Today sports fans see virtual ads on the TV field that do not grace the actual Astroturf. And one New Year’s Eve, CBS changed the face of Times Square by superimposing its own logo over NBC’s.

We have grown used to all this. We have become cynical about ads. We laugh and give Academy Awards to the hi-tech tricks that fool us in the name of entertainment. And I suppose that if MIT’s new technology is just used to allow Josh Hartnett to speak Japanese and Julia Roberts to be dubbed into Italian, few of us will notice.

But if science can erase the line between the real and the realistic, its most successful product will be doubt. Its most earnest and edgy creation will be mistrust.

Consider this: The MIT team is using a tape of “Nightline’s” Ted Koppel to see if they can dub it in Spanish. If they can put Spanish into the newscaster’s mouth, why not lies?

If we can’t tell truth and fiction apart, what happens to our trust in, say, the Rodney King tapes, or surveillance tapes, or presidential speeches? Who will draw the line then between paranoia and skepticism? Truth and propaganda?

Is this technological blowback? After all, captured images have become our proof, our truth. We rely on photographs to replace memory. We trust images more than our own eyes. When we want to check the facts we say, “Let’s go to the videotape.”

But in the Information Age the same technology that informs us can also misinform us and disinform us. We have seen that on the Internet. Now will we see it, literally, on phony tapes? What can we trust?

Rumors and research circle the globe at the same speed. Fact and myth vie for attention in courtrooms and classrooms. We need, more than ever, to tell the difference. But today science, our fact-finder, is also erasing the difference.

If we can’t believe what we see, will we see only what we already believe?

Ellen Goodman is a columnist for Washington Post Writers Group.