Researchers behind the study speculate that other life-forms may have already sent us messages, perhaps even as organic material embedded in asteroids that have struck Earth.

"Any contact that we might establish with extraterrestrial life-forms is more likely to occur from a physical artifact than from electromagnetic communication," said Christopher Rose, a professor of electrical and computer engineering at Rutgers University in New Brunswick, New Jersey. Rose is a co-author of the study.

The theory is not new, but it is the first time it has been quantified. The research is described in this week's issue of the science journal Nature.

Diluted Signals

Humans have already sent inscribed information into outer space in the hope of reaching alien life.

The Voyager 1 spacecraft has traveled more than 13.5 billion kilometers (8.4 billion miles) since its launch in 1977. It is carrying a 12-inch (30-centimeter) disk containing a message from Earth: sounds and images selected by the late astronomer Carl Sagan and spoken greetings in 55 languages.

But the main focus among alien-hunters has been on electromagnetic radiation—radio and optical waves.

The private SETI Institute in Mountain View, California—its name is an acronym for Search for Extraterrestrial Intelligence—is the most prominent organization involved in the search for intelligent life beyond Earth.

The institute listens for radio signals in interstellar space. So far, it has not detected any signs of alien life.

That, however, does not prove there is no alien life. Instead, it could be because radio signals get diluted as they travel across great distances in space, says Rose, whose research grew out of his work at the Wireless Information Network Laboratory (WINLAB) at Rutgers University.

"Our original question was, How do you get the most bits per second over a wireless channel?" Rose said.

He learned that the "energy budget" required for sending a radio signal increases with distance. Beams of radiation are cone-shaped and grow in size as they travel outward, meaning the great majority of their energy is wasted.

Potential recipients may not even be capable of receiving the message. "There's no guarantee that there's anything out there that is evolved that can listen to the message, hear it, and decode it," Rose said.

A far more energy efficient way of communicating over great interstellar distances is to send a physical object with encoded information, Rose said. Such a package is not diluted as it travels across space. It also stays where it lands.

"Our calculations suggest that it's not only more energy efficient to send a package than radio waves," he said, "it's outrageously more efficient."

Humans already have the technology to encode dense information—genetic material, for example—in incredibly small packages. What we lack is the navigational capability to make sure the message gets to its destination.

"But if we can envision doing that in a hundred years, odds are that, if there's anything out there like us, they can probably do it too," Rose said.

Sea Post vs. Airmail

It is far from clear, however, that the key criterion for choosing a message's medium is energy expanded per data bit.

"There's little doubt that physically transporting information can be a very efficient way to get it from one place to another, if all you care about is the energy costs," said Seth Shostak, a SETI scientist. "I could ship the Library of Congress to Australia in a tanker, and the cost per bit would be low."

Speed is a factor equally worth considering, Shostak says.

"Do I want to package up my message, put it in a rocket, and wait 10,000 years for it to arrive?" he said. "Or send it on a light beam that's transmitting for a month, but that only takes a decade to arrive at that far-off world? It's like the difference between sea post and airmail."

Rose acknowledges that the choice of medium depends on the message being sent. "If you just want to say, I'm here, that's not that many bits of information. So it's probably better to communicate via radio waves than sending a physical message," he said.

Alien Packages

Rose says that humans may have been too concerned about trying to send and receive interstellar communication in their own lifetime. It may take tens of thousands of years for a message to reach an alien civilization and just as long for it to get back, Rose says. Trying to set up two-way communication may be a mistake.

"You don't know when an extraterrestrial civilization will spring up, and you don't know if it's going to last," he said. "We only started to see beyond our planet in the past few hundred years. The probability of contacting someone in our galaxy and setting up two-way communication is reasonably low. You have to hit them at the exact right time."

If the sender isn't concerned about reaching the recipient and getting an answer in his or her own lifetime, inscribing and sending material is the way to go, Rose said.

"The nice thing about matter is that you send it once and it more or less stays there," he said.

The researchers speculate that there may be extraterrestrial packages already here in our planetary backyard, perhaps in the vicinity of Jupiter, the sun, or the moon—or maybe even here on Earth in the form of organic material embedded in an asteroid.

"There might be many different messages from many different places sitting all around us," Rose said.

In an accompanying Nature article, Woodruff Sullivan, an astronomy professor at the University of Washington in Seattle, wrote that SETI should continue its radio wave research. But the institute should also be open to the idea of one day finding an information-drenched artifact, sent by an alien civilization interested only in one-way communication.

"It is a scenario reminiscent of 2001: A Space Odyssey, wherein a monolith discovered on the moon is … left by an extraterrestrial intelligence," Sullivan writes. "If astro-archaeologists were to find such, it would hardly be the first time that science fiction had become science fact."

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