Extraterrestrial (non-Earth) intelligence is intelligence that originated somewhere other than Earth and that is at least as intelligent as human society was in July 2002. It could be artificial (computer) intelligence, a flesh-and-blood biological being, some combination of the two, or some other form not generally anticipated. Traditional SETI projects use radio or optical telescopes in hopes of finding some artificial signal, beacon, or message that originated several light-years from Earth. After 42 years of looking, this type of SETI project has yet to achieve a confirmed detection.
I am betting that extraterrestrial (non-Earth) intelligence, in one form or another, has already reached our solar system and will be confirmed first.
Here is my rationale. Most SETI scientists agree that any ETI we detect will likely be thousands or millions of years ahead of us (because our sun and our science are so young). Such an advanced society will likely have the CAPACITY to build and launch cheap smart autonomous probes to explore the galaxy. This statement is supported by our recent theoretical and engineering advances in robotics (Ray Kurzweil, Hans Moravec), molecular manufacturing (nanotechnology), and interstellar propulsion (beam propulsion and even more radical possibilities). Also, an advanced society will likely be MOTIVATED to send out exploratory probes, judging by our own society, which sends spacecraft to explore everything within reach.
If such a probe were sent a few centuries ago to explore Earth, it will likely be here by now. We might stumble on it somewhere on Earth by accident. Or we might find it through the ongoing monitoring carried on by scientists, security and intelligence agencies, and the military. Or one report in the flood of UFO and abduction reports might turn out to be a genuine alien. Or one of our space exploration programs might find some alien artifact somewhere in the solar system. Or ETI might reply to the Invitation to ETI issued by 100 scientists, artists, and others at http://www.ieti.org. (As the founder and Chief Scientist of this innovative SETI project, I am naturally optimistic about its chances of success.)
Evidence of Dyson spheres or other astroengineering projects from several light-years away, or evidence of the extraordinary energy use of a highly advanced civilization, is a third type of evidence. It is excluded from this bet. If this third type of evidence is found first, the bet continues until one of the other two types of evidence is confirmed.
The bet refers to evidence of the authenticity of extraterrestrial intelligence being "confirmed." The most likely group of scientists to be responsible for confirming or disconfirming the evidence is the community of SETI scientists. (For this bet, confirmation by the UFO community will be ignored.)
The confirmation process will depend on the evidence. If the evidence is clearly irrefutable, the confirmation process may be rapid and informal. If authenticity of the evidence is not self-evident, the process may have to be ponderous and formal, likely involving the SETI Permanent Study Group of the International Academy of Astronautics.
I'm betting against my respected colleague Allen Tough, not because I think he's wrong about interstellar probes, but because we don't yet know how to detect them. It's a matter of instrumentation, and though we've gotten very good at intercepting electromagnetic waves, our record for detecting even nearby natural space debris is not too stellar (pun completely intentional).
Although I do not at all question the likely existence of interstellar probes within our solar system, I consider interstellar electromagnetic leakage (or, if we are extremely fortunate, deliberate radio or optical beacons) much easier to detect with our present level of technology. Thus, selectivity factors in the instrumentation favor detection of interstellar EM artifacts first. (As our techniques improve, the odds of detection will ultimately shift. So, I guess by betting on interstellar EM signals, I'm siding with those who believe contact will occur in the short term.)
Allen has argued (most convincingly, I might add) that any super-smart alien robot that is sophisticated enough to come here will be quite capable of initiating contact, or even of responding to our joint invitation at http://ieti.org. Although I certainly hope this is the case, it does put us in the role of passive communications partners, waiting for ETI to take the first step. Being no wallflower, I personally advocate a more proactive approach, preferring to put our eventual entry into the Galactic Community squarely in the hands of humanity. Assuming the existence of extraterrestrial intelligence (without it, this bet can neither be won nor lost), traditional interstellar SETI makes us solely responsible for the success or failure of our efforts. Should ETI decide to step in and shortcut the process, I for one will be delighted at the contact, though dismayed that my friend Allen will end up winning this bet!
Prof. Tough has generously stated that if he wins his bet, the sums on deposit will be contributed to the nonprofit SETI League, an organization near and dear to my heart. In putting up money against him, I too designate that worthy grassroots organization as recipient of any funds, once the bet is settled. That way, this will be a no-lose bet!
One final note to any nearby alien lurkers who might happen to be browsing this web page: I encourage you to step forward and make yourself known. Even if I lose this bet, humanity wins.
The logical arbiter of this bet is the SETI Permanent Study Group of the International Academy of Astronautics. Any evidence of extraterrestrial intelligence publicly announced as definitive by that body will be deemed credible for purposes of settling this bet. The present Chairman of that august body has already agreed that they will be willing to arbitrate this bet, should a detection be announced. If the IAA SETI Permanent Study Group is no longer in existence at the time a detection is made, some other organization involved in credible SETI science should be approached to make the decision.
For the purpose of this bet the definition of the edge of our Solar system is one parsec. Since the nearest star is over one parsec away, anything closer than that is regarded as being in our solar system; anything farther than one parsec can be considered "interstellar".
For purposes of SETI science, intelligent signals are defined as those generated by technology, and clearly not of natural astrophysical origin. Probes and radio signals are manifestations of technology, thus "intelligent" within the SETI community's limited definition.