At one extreme in alien-invasion movies, there's the slimy, razor-toothed, grotesque creature full of powerhouse violence -- along with over-the-top special effects -- and a too-eager desire to feast on human filet. These aliens, never cordial in social settings, are the worst-nightmare kind of beast that one finds in the "Alien" series or "The Thing," among many others.
At the other extreme exist kinder, gentler alien visitors -- "E.T.: The Extra-terrestrial," "Paul" and "Close Encounters of the Third Kind," along with those using human form to hide their extraterrestrial identities (and save on special effects) as happened in "Starman" and "Cocoon."
And as crazy as it sounds, those extremes reflect opposing positions of an actual scientific debate about extraterrestrial invaders, or at least a well-publicized intellectual disagreement that occurred this spring between two renowned scientists, Jill Tarter and Stephen Hawking.
Ms. Tarter, retiring director of the SETI (Search for Extraterrestrial Intelligence) Institute after 35 years in the field, and the inspiration for the Jodie Foster character in the 1997 film "Contact," will participate in a debate about alien-invasion scenarios during the institute's SETIcon event June 22-24 in Santa Clara, Calif.
From the "E.T." and "Paul" school of alien visitation, she has taken public issue with Mr. Hawking, the famed British scientist considered one of the world's most intelligent people, after he stated on his Discovery Channel television series, "Into the Universe With Stephen Hawking," that an alien invasion of Earth would cause the kind of devastation to inhabitants that a group of earthlings experienced centuries ago when "aliens" invaded their land beginning in 1492.
"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the native Americans," Mr. Hawking said. Native Americans experienced disease, death and displacement as European then American invaders fought them, seized their land and forced them onto reservations sometimes after death marches. It's like "Avatar" with a more depressing ending for the native population.
But Ms. Tarter says movie-version invaders better depict human psychology, fears and imagination than science or serious analysis of what an alien visitation would portend for earthlings.
"While Sir Stephen Hawking warned that alien life might try to conquer or colonize Earth, I respectfully disagree," she's quoted as saying, but confirmed in a recent interview. "If aliens were able to visit Earth that would mean they would have technological capabilities sophisticated enough not to need slaves, food or other planets. If aliens were to come here it would be simply to explore."
Mr. Hawking was traveling and unavailable to discuss the issue, his spokeswoman Judith Croasdell said.
Extraterrestrial life with enough intelligence to travel light-years through the Milky Way already would be kinder and gentler with no need to pirate resources or use Earth as an outpost. With technology and science she's helped develop at SETI, and its continuing efforts to scan space for signals from intelligent life, humans would be aware of any approaching aliens, with clues about their technology and IQ, long before their arrival, Ms. Tarter said.
The SETI Institute, in operation for 28 years and now with 150 scientists, has yet to receive one signal from intelligent beings elsewhere, she said. Nor is there solid evidence that basic life forms -- bacteria or microbes, for example -- exist out in the solar system or beyond. SETI's mission includes finding life of any form elsewhere in the universe.
So the alien-invasion debate must be based solely on science-based speculation.
"Stephen [Hawking] has a brilliant mind, but he doesn't have expertise in extraterrestrial intelligence," Ms. Tarter said. "Aliens coming here hellbent on trashing the neighborhood and claiming all our resources -- I'm skeptical of that.
"Certainly, we don't see evidence of intelligent life," she said. "The case is still open."
With retirement plans to raise funds for SETI, Ms. Tarter said she's disappointed she never detected a signal from extraterrestrial intelligence, which would have been "the game changer."
"But nevertheless, I have been able to advance the state of the art in the field, and today our searchers have the capability of 14 orders of magnitude" of scanning ability than initially existed to gather signals, interpret them and determine whether intelligent life sent them.
Our own radio, communications and television signals have been traveling through space at the speed of light for more than 90 years, ever since the invention of radio. That means our signals could have reached the solar systems of more than 100 stars within 45 light-years of Earth with time for any intelligent being to send a reply. Earth could have received a reply as early as 1930 from the closest star's solar system.
No great fan of alien-invasion movies, Ms. Tarter said only a handful of sci-fi movies are scientifically plausible and serve as "a fantastic gateway into all kinds of scientific careers."
The plot of "Contact," inspired by scientist Carl Sagan's novel, involves a scientist like Ms. Tarter who eventually does make contact with extraterrestrials and travels briefly to their world. Ms. Tarter said that movie inspired females to seek careers in science. Others include the 1968 Stanley Kubrick film, "2001: A Space Odyssey," and the 1998 documentary-style movie "Apollo 13," which detailed how quick-witted science saved the crew of the failed American moon mission.
"I really like the intersection of the imagination and real science," Ms. Tarter said. "But I wish there were good science in science fiction. Some of it is funny and makes you laugh, but I'm disappointed when scientists are portrayed as being out to save the world or destroy it."
While alien-invasion movies typically portray fantastic types of alien life, "thus far they haven't been as creative and imaginative as nature might have produced somewhere else," she said. Fossil records on Earth reveal general bilateral and symmetrical forms of life, which also might be "generalizable" for the entire universe.
As for the debate about how aliens would behave should they ever arrive on Earth, she and other scientists can offer only knowledgeable guesses.
"I don't have the expertise nor does Stephen to show that he's right or that I'm right," she said. "But I don't think it should stop us from answering old questions or from looking to see what's out there."
Post-Gazette movie editor Barbara Vancheri contributed to this article. First Published June 13, 2012 4:00 AM