Suppose, for example, we get a message that says, “You must re-broadcast this message for 50 years at one million watts or we will destroy you.” Maybe the message also includes some indecipherable data, just to make it more mysterious. Is it a dumb alien chain letter? Do we ignore it and risk the end of the world? Or maybe the message includes instructions to build a machine, and says “We’re going to knock a massive asteroid into your planet if you don’t comply.” After a huge international effort build the “X”, it just flashes a big sign that says, “Sucker!”
If you remember the early days of the internet, you know that it sounded at least somewhat plausible the first time you got a message that said the government was going to start putting a tax on e-mail messages. So how could we know what’s intergalactic spam and what isn’t? There could be jokes, viruses, sincere messages, requests for information, space porn, and religious appeals. All-species Viagra! Order today. Just send a us a copy of the human genome!