Stephen Hawking (of all people) and Hollywood have mistakenly identified a potential desire for our resources as the main motivation for this threat scenario. But in reality there is nothing on Earth that couldn't be much easier mined in any arbitrary star system, in any arbitrary asteroid belt.
There are, however, still factors imaginable that might lead to a hostile (=genocidal) reaction from an advanced race - for example a reaction motivated by religion or paranoia. If we are within the reach of such a civilization they're already looking for us anyway though, it's unlikely additional harm will come from sending them a signal if they're already in the process of scouring the galaxy for places that are in need of a thorough ethnic cleansing.
The probability of such a scenario can currently not be estimated since we don't even have a single data point on other civilizations in our neighborhood. We don't even know if FTL is possible in practice, a technology without which an assault on Earth may very well be impractical for the foreseeable future.
One thing that might concern us, related to a potential explanation for the Fermi Paradox, is the possibility of attracting the attention of an embodied "great filter", for example in the shape of automated replicating drones that sweep the galaxy to eradicate all instances of technology they can find. But if such a filter exists, our eventual discovery will be inevitable anyway so sending an additional signal out into space would probably not make a difference.
It's difficult to construct a credible scenario where sending a signal will be instrumental in our destruction, mainly based on the assumption that interstellar civilizations or entities with the motivation to destroy us would likely not be content with sitting around on the off chance that another planet might contact them.
So who's our most likely recipient then? It's a civilization on a tech level comparable to our own. So for the sake of argument let's assume the roles are reversed:
One day we get an interstellar call. There's another inhabited star system out there, maybe not too far, let's say 200 light years away. Let's assume the worst and, upon receiving a friendly "hello" type message, the world goes completely crazy and decides the remote greeters need to DIE! At this point, there is really not much we can do to actualize that desire. We could build a huge generation ship and send it on its way but it's going to be traveling for thousands of years. When (if!) it arrives, the other civilization will either be gone or so far advanced that it's beyond the point where our puny aging low-tech generation ship can do any damage.
These are the assumptions we're operating under. Yes, they're assumptions, but for the most part they're based on the best we can do for reasoning and scientific knowledge today.
It seems like the purpose of sending a signal is ultimately to get aliens to visit us. But the diseases they bring with them do have a high probability of wiping us out (or us wiping them out).
I suppose if both sides are very careful, or if we restrict ourselves to communication only, then it could be all right. But why go looking for more problems when we haven't even solved our own? And we probably never will.
One reason to be searching for a new civilization is to steal their technology. The first nation that's willing to do this stands to dominate every other nation, so there's a big incentive for us to steal their tech as long as the concept of nations still exist.
And of course, if the aliens somehow didn't already have nuclear technology, then transmitting our Wikipedia to them might prove disastrous for them.
> It seems like the purpose of sending a signal is ultimately to get aliens to visit us.
That's almost certainly beyond the capabilities of either civilization, but sure in the long term there's the benefit of already having some like-minded friends when we do get that capability.
> But the diseases they bring with them do have a high probability of wiping us out (or us wiping them out)
Again, considering the type of civilization we're trying to contact that's so far in the future it's not of immediate concern. What dangers both world pose to each other biologically would have to be studied carefully as data becomes available. There are certain things we can almost certainly exclude as disease carriers, such as viruses or other vectors that are tightly coupled to the host's biochemistry. Parasites and vermin might become a problem on both ends if living conditions are compatible enough (which is another big question mark).
> But why go looking for more problems when we haven't even solved our own?
We're not looking for problems, we're trying to contact people to at least gain some scientific knowledge but if possible also to get some serious motivation going towards technological advancement. Cooperation between two comparable but entirely separate civilizations has the potential to be very fruitful for both.
> One reason to be searching for a new civilization is to steal their technology.
Well, it's not like we'll pop in and steal their iPhones ;) If they choose to transmit scientific and engineering data, that's not stealing either (copying is not stealing). I would imagine such an exchange to be very cautious in the beginning. It might not even be feasible to transmit time-critical knowledge, however. Still, it's always of interest to compare notes, and I imagine scientists on both sides would welcome an extended conversation if posssible.
> so there's a big incentive for us to steal their tech as long as the concept of nations still exist
If they transmit the blueprint for some really advanced stuff, sure, there'd be the usual power shenanigans here on Earth. But at the moment, that's not a concern as the first messages back and forth would most likely be very limited in scope. Even establishing a common communications format might take several hundred years.
> if the aliens somehow didn't already have nuclear technology, then transmitting our Wikipedia to them might prove disastrous for them
It's unlikely they'd be able to communicate but at the same time not have mastered basic physics. There will be things that each side is more advanced at - for example it's conceivable that Earth has better information technology but lags behind its potential in energy production.
The biggest boost an open phone line can provide would probably be psychological, long before a meaningful science collaboration could be orchestrated. Just knowing there are other people out there is of tremendous value.
>for example a reaction motivated by religion or paranoia.
It's also possible they might simply want to quarantine Earth as wars, religion and poverty are still present, and they wouldn't want them to spread to the rest of the galaxy.
There are, however, still factors imaginable that might lead to a hostile (=genocidal) reaction from an advanced race - for example a reaction motivated by religion or paranoia. If we are within the reach of such a civilization they're already looking for us anyway though, it's unlikely additional harm will come from sending them a signal if they're already in the process of scouring the galaxy for places that are in need of a thorough ethnic cleansing.
The probability of such a scenario can currently not be estimated since we don't even have a single data point on other civilizations in our neighborhood. We don't even know if FTL is possible in practice, a technology without which an assault on Earth may very well be impractical for the foreseeable future.
One thing that might concern us, related to a potential explanation for the Fermi Paradox, is the possibility of attracting the attention of an embodied "great filter", for example in the shape of automated replicating drones that sweep the galaxy to eradicate all instances of technology they can find. But if such a filter exists, our eventual discovery will be inevitable anyway so sending an additional signal out into space would probably not make a difference.
It's difficult to construct a credible scenario where sending a signal will be instrumental in our destruction, mainly based on the assumption that interstellar civilizations or entities with the motivation to destroy us would likely not be content with sitting around on the off chance that another planet might contact them.
So who's our most likely recipient then? It's a civilization on a tech level comparable to our own. So for the sake of argument let's assume the roles are reversed:
One day we get an interstellar call. There's another inhabited star system out there, maybe not too far, let's say 200 light years away. Let's assume the worst and, upon receiving a friendly "hello" type message, the world goes completely crazy and decides the remote greeters need to DIE! At this point, there is really not much we can do to actualize that desire. We could build a huge generation ship and send it on its way but it's going to be traveling for thousands of years. When (if!) it arrives, the other civilization will either be gone or so far advanced that it's beyond the point where our puny aging low-tech generation ship can do any damage.
These are the assumptions we're operating under. Yes, they're assumptions, but for the most part they're based on the best we can do for reasoning and scientific knowledge today.