When talk turns to SETI, there's one question that's as common as catfish: "We're not broadcasting to the aliens; so what makes you think they'll be broadcasting to us?"
This may strike the innocent mind as a good point. But then again, hanging out in a bar all night without uttering a word hardly ensures that I won't hear conversation. When it comes to interstellar communication, our muteness has nothing to do with theirs, except insofar as our motives for not broadcasting may be widely shared.
Is that plausible? Could it be that our reasons for not firing up the transmitters and beaming a big hola to the universe have also occurred to other galactic inhabitants, ensuring a "great silence" and grim prospects for SETI?
Maybe, but I don't think so. I believe our decision to just listen, and (perhaps) transmit later, has more to do with considerations of the moment.
The merits and pitfalls of deliberate broadcasting (as opposed to the accidental, and probably ineffectual oozing of our commercial radio and television into space) have certainly been weighed. In a recently published study ("SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence"), there's a page of gracefully worded verbiage explaining why many SETI scientists think it's too early to throw the big knife switch and put Radio Terra on the air. Listed below are the principal arguments of this study: after all, lists are good (unless you're a ship). But after each argument, I've added a paragraph in which I consider whether aliens would come to the same conclusions we have. If not, if our reasoning is too provincial, then our decision not to send intentional signals is merely that: our decision. It will give little insight into the broadcast policies of other worlds.
Argument for not transmitting #1
It's important to note that our first contact with an extraterrestrial society probably won't be their first contact. Why? Because anyone we hear is going to be far beyond our technological level. After all, alien Neanderthals won't build microwave transmitters (or if they do, they hobble their usefulness by building them out of stone). We'll only receive a solid signal from technically accomplished societies at our level or beyond. So it's quite possible they've been engaged in planet-to-planet communication for centuries or millennia. Consequently, these societies could be using an established, galactic protocol for information interchange, an orderly way of doing things that it behooves us to imitate. Since this seems inevitable (think of the protocols for exchanging information on the internet), it would clearly be better for us to eschew transmitting. Listen first and learn the protocol. After all, we'll be new members in the galactic club.
At first flush, this sounds reasonable. If you post a web page without knowledge of HTML, it will display as fourth-rate gibberish to anyone stumbling across it. One must have standards, after all. But then again, the mere existence of a signal from Earth - appropriately formatted or otherwise - might intrigue retired aliens with time on their hands. Don't you think that if an extraterrestrial society found any transmission from us, they would spend the effort required to figure out how it's encoded, and possibly extract our message? Even new club members are allowed to talk, despite the fact that they haven't yet learned the club's secret handshake. Not being versed in the niceties of information exchange seems a rather artificial reason for our decision not to broadcast, and would be even less compelling for them. This is an argument for procrastination, not ceaseless silence. I can't imagine that all the alien societies in our galaxy are sitting on their prehensile appendages, awaiting technical manuals.
Argument for not transmitting #2
We can't afford to broadcast. After all, transmitting is comparable in cost to receiving, but unlike the latter, might not elicit a payoff for centuries. Given that SETI's budgets are anorexic, the best thing to do now is listen - a strategy that has at least a chance of an early payoff. In addition, there's no point in going on the air for a few minutes or a few hours. Any transmitting project that hopes to be heard must be persistent, which means the transmitter needs to stay warm for hundreds or thousands of years.
Well, it's true that there aren't easy resources for SETI researchers to mount major transmitting projects today. But of course, this says nothing about what the aliens might choose to do. Furthermore, it's unclear that a transmission project needs to beam away for eons. Schemes that provide some "synchronization" event - some clue as to when a society might be listening - can reduce the necessity for an unremitting 24/7 broadcast schedule. One example of such a synchronization event is when a planet passes in front of its sun as seen from another star system. But the point here is that a bit of cleverness might pay off big, allowing a short-term transmitting scheme to work as long as those you are trying to reach have a long-term interest in receiving. These sorts of considerations could make beaming signals far more attractive, at least for our more-advanced galactic brethren.
Argument for not transmitting #3
Transmitting is a "diplomatic act". Letting others know of our presence might be dangerous, and we shouldn't do it without a bit more cultural maturity. This point of view was expressed most famously in 1974 by Sir Martin Ryle (Britain's Astronomer Royal at the time) when he reacted unhappily to the well-known Arecibo demonstration broadcast of 1974. Ryle was not down with this short, three-minute message, beamed to a big ball of stars 21,000 light-years away. After all, he said, "any creatures out there [might be] malevolent or hungry."
We don't know a great deal about alien behavior or eating habits, so you could argue that Sir Martin's concern was mere paranoia. Still, there are reasonable people who would come down on the side of caution. Even if the chance that an interstellar ping would be answered with a phalanx of incoming rockets is miniscule, it's hard to guarantee that it's zero. Given the ruinous consequences, it's better to be silent and safe.
In truth, of course, we can't assume that extraterrestrial societies would be so cautious. And seriously advanced cultures could surely protect themselves by using "off-shore" transmitters or signaling schemes that don't betray their whereabouts. An argument that all aliens will be silent, premised on our analysis of their defense needs, seems weak at best.
The bottom line is this: while we can concoct various reasons why broadcasting might be inconvenient or possibly dangerous, none of the arguments proffered allows us to conclude that our reasons for silence are universally shared. We should therefore continue to keep our metal ears attuned to the skies. Someone, somewhere is surely curious enough, and brave enough, to speak up.