It’s not quite as simple as that though. Nuclear powered (fission or fusion) craft could conceivably reach maybe 0.1c - 0.2c. Various schemes have been proposed (see Daedalus and Orion projects in particular), also Bussard nuclear ramjets. At that speed you can get somewhere interesting in a few decades. It’s not far fetched to think that even humans with some kind of hibernation technology could pull this off. Of course we don’t know how long aliens live also. Maybe their lifespans are like those of the the Sequoia, maybe they have perfected nano-medicine and live far longer than they used to and a journey of a few hundred years is not such a big deal. Maybe they send sophisticated AIs that are immortal for all practical purposes.
Now, that said, I don’t believe in the saucer people, the Mars face or the probe-me-harder-baby aliens at all. My guess is the aliens aren’t here not because they can’t get here but because they don’t exist nearby. My bet would be that the nearest ET civililization is extra-galactic and you get maybe one per galaxy extant at a time. Otherwise Fermi’s paradox kicks in and you wonder where all the alien probes are.
One more bit of Devil’s advocate on behalf of the conspiracy nutters - Assume that I’m wrong and there ARE lots of technically advanced ET civilizations in the galaxy and that they DO come here. Also assume that the typical one lasts, say, 100,000 years, then a random sampling of any old ETs that visit puts 99,900* out of 100,000 of them ahead of us in tech and the vast majority would be at least millenia beyond us. This is just the old Drake equation at work. So it may be that the govies have the saucers after all but the tech is so far beyond us that we don’t know how to start on it. Of course that blows away the idea that we got everything from Spam to Microsoft Word from the aliens too.
- I got this by assuming that we have been “technically advanced” for 100 years. Mostly, I just took the “radio age” to be my guide here.