The probability that there is intelligent life somewhere other than earth increases daily as we discover more and more solar systems that seem capable of sustaining life.
What is likely to happen if we make contact with ET? Given the size of astronomical distances and assuming the speed of light as the maximum possible velocity, the most likely outcome is not real contact but merely an exchange of messages, perhaps at very long intervals. Little chance of harm there.
But there is still non-zero probability of real contact. Since we have no way of predicting with any certainty the outcome of such contact, it might seem that we have no reason to assume a bad rather than a good result. Why ?
well if ET travels half way across the galaxy to Earth, they wont just be coming to say “hi” and for sure, they will be after something worth the journey.
Contact with ET would have to result from their coming here, which would in all likelihood mean that they far surpassed us technologically. They would be able to enslave us, hunt us as prey, torture us as objects of scientific experiments, or even exterminate us and leave no trace of our civilization. They would, in other words, be able to treat us as we treat animals — or as our technologically more advanced societies have often treated less advanced ones badly.
Since there’s at least a probability that they will bring us catastrophic evil, enslavement or even death, why should we risk such an outcome?
ET could instead bring us enormous benefits: they might even lead us to a paradise of peace, wisdom and joy. But there is no reason to think that such a paradise is more probable than a hell of slavery or extermination. And enormous gains are not worth the equal risk of horrendous loss.
We cannot know what might happen to us from contact with ET. But we do know that there may well be unthinkably horrible outcomes.
ET go home!