We assume that the way humans see the world has some correspondence to objective truth. That’s not necessarily correct. Let's do a thought experiment where we have a quick chat with an alien species to see how mistaken we are about the universe.
So we’ve made contact with aliens, and some people are celebrating and others are melting down with fear, but the overall consensus is that we should ask them over for tea or something. It’s likely that tea time conversation will be a bit stilted. What do people talk about other than what they have in common? Although we may assume that we have the whole universe in common, the aliens, if we just tweak their senses a bit, may be living in a different universe from ours.
Let’s start with the sense we use most to interact with the world. We knew there were aliens out there because we could see the sun, the moon, the stars, and even the planets in the solar system. And we could see those things with our naked eyes, because we can pick up certain wavelengths of light. All we had to do, even hundreds of years ago, was turn out eyes upwards (sometimes with the aid of telescopes, true) to see the potential for other worlds. The same is not necessarily true for the aliens.
The tweaks to their vision might be minor. They might just pick up, visually, on different stars than we do, seeing things in the infrared range or the ultraviolet range. Since we now have plenty of telescopes scanning the skies and picking up on parts of the electromagnetic spectrum that we can’t see, it would just be a matter of superimposing our visual star maps on their visual star maps to get some common ground.
But what if they don’t see anything remotely within our visual spectrum? Let’s say that they evolved on a planet not powered by light from a nearby star, but from vents from the planet’s core. Light, and vision, might not be a big component of their senses. Perhaps, for whatever reason, they only see ultra-high energy light given off by quasars, which perch at the centers of galaxies. They wouldn't realize there was the possibility of life in the outer reaches of a galaxy before we contacted them. Or they might only be able to see quickly changing light, and can only navigate space by pulsars. There’s even a possibility that they can only, visually, pick up on very faint microwaves, and it never occurred to them that there was life in the universe, because they only saw the faint, homogeneous smear of the cosmic microwave background. Never mind how we’d contact a society like that, how would we even begin to explain who we are and where we came from?
If we sorted through the first contact and they got to Earth, there could be even more problems with light. If they only saw in x-rays, we very well might look like bags of bones with weird, invisible flesh around them. (Also, any prolonged exposure to alien “flashlights” would kill us.) But what, for example, if they only saw polarized light? We know that some animals can see polarized light, and it’s thought that they navigate by the natural polarization that light goes through in the sky throughout the day. But what if a species can see polarized light and nothing else? Because vertical and horizontal surfaces polarize the light that hits them, the aliens would see a multicolored sky, blinding walls of light from windows, the hoods of cars, lakes, and puddles, but wouldn't be able to see any humans. (Although they would, in sunlight, be able to catch the intermittent glare coming from our glasses. How creepy would that be for them?)
The number of problems that can come with sound is near-infinite. We might have to communicate with aliens, once they get here, with devices that make ultra low frequency sound, or with dog whistles. We might have to scream at them through a loudspeaker. Or we might only talk in whispers, to keep from blowing out their eardrums. Even if they’re able to hear within the frequency and volume that we do, and we are able to communicate with them using our voices, their hearing might encompass a very small range of frequency. We’d have to keep to a monotone drone to keep from sporadically – from their perspective – hitting the mute button. Their aural range might also constrain our choice of ambassadors. They might only be able to comfortably communicate with high-voiced women or low-voiced men.
But our real problem might come with smell and taste. Humans, famously, have terrible senses of smell and taste. One of the reasons we have to label what’s in food is we don’t have the ability to smell or taste it ourselves. This has gotten us in trouble when we gulp down food that is filled with contaminants and bacteria, or touch something crawling with viruses without knowing it.
The problem here will not just be the social faux pas of offering the aliens, from their perspective, obviously filthy food. The problem will come when they distinguish people by smell in ways we can’t begin to understand. People are crawling with microfauna. Some estimate that the number of foreign cells in our body outnumbers our own cells ten to one. There’s evidence that fruit flies choose their mates based, in part, on the gut bacteria that their suitors carry. What if aliens do the same? They might see humans as entirely different species (or at least subspecies) depending on the bacteria, microbes, and viruses they have on them at any given time. If an ambassador takes a break from the initial talks with the aliens, and has some yogurt to eat, when they walk back into the room the aliens might consider them an entirely different person. The same might happen if they got a cold, or changed their brand of fabric softener, or started to sweat. (Which would be another creepy moment for the aliens. How terrifying would it be if, while you're talking to a guy, he slowly, without acknowledging it, morphs into a totally different dude?)
The big problem would come, though, if aliens started taking the microbes more seriously than they took us. Humans don’t do well without the bacteria in our guts and on our skin. The bacteria, on the other hand, do perfectly well without us, as long as they have a nutrient rich environment. If you notice that the person you are talking to is actually a sketchy framework upon which a thriving community of thousands of smaller living beings lived, would you assume that the large structure was sentient, or the small individuals living in it? Once they get a whiff of us, aliens may consider us no more sentient than we would consider an apartment building. They might think of us as creations of the bacteria and viruses. If that's the case, from their point of view they could disassemble us, while keeping the bacteria alive. Taken from their perspective, they aren’t committing murder any more than we would be if we trooped everyone out of a house, made renovations, and walked them back inside again. One second we’re contacting aliens. The next, we’re ushering in a well-meaning apocalypse. It could happen. We’re almost indifferent to smell and taste, and so we’re indifferent to our own microbial life. How would we even begin to deal with a species that considered that kind of thing important?
I say we don’t try to contact them at all.