I’ve been watching a show called Invasion on Apple TV. The show is kind of a mish-mash of other alien invasion plots. The aliens arrive in giants ships a lot like the ones in Independence Day. The twist is that the ships don’t have impenetrable shields like the ones in Independence Day but they do have cloaking devices like the ones the Romulans had in Star Trek. That makes it very difficult to fight back against them. The aliens themselves aren’t humanoid but look like balls of tar that can extend spikes to either walk on or stab people with. And in season 2 they mutate into something very reminiscent of the aliens from 2011s Attack the Block, sort of like big glowing dogs.

There are so many alien invasions stories out there it’s hard to keep track of them. Just a decade ago Falling Skies ran for a few seasons and if you back to the 90s the X-Files had a long-running plot about aliens being covered up by the government. And a bit before that we had the gerbil eating aliens of V (that show came back about 15 years ago). And Long before that there was another show about invading aliens from the UK called UFO by the same folks who would later produce Space 1999. And let’s not forget The Invaders from the 1960s which was also about a secret alien invasion.

You can keep tracing this lineage back to the dawn of the television era. In 1951, Robert Heinlein wrote a novel called The Puppet Masters about parasitic aliens who arrive in flying saucers and take over people’s bodies. That idea has been ripped of more times than I can count including by The Outer Limits, Star Trek II: The Wrath of Khan and Star Trek the Next Generation.

There were lots of alien invasion films in the 1950s including The Thing from Another World, Invasion of the Body Snatchers (which seems like it might have been inspired by Heinlein), Invaders from Mars, The Day the Earth Stood Still, The Blob, The Crawling Eye, I Married a Monster from Outer Space, Earth vs the Flying Saucers and of course War of the Worlds. The novel War of the Worlds was written by H.G. Wells around 1896 and was famously performed as a radio drama by Orson Welles in 1938.

This is really just scratching the surface when it comes to alien invasion films. There are at dozens more from the 1960s through the last few years. Some of the aliens were friendly ET and some were not Predator. The point of all of this is that we’ve been entertaining (and scaring) ourselves with alien invasion stories for well over 100 years now. It’s one of the staples of science fiction along with time travel stories (also introduced by H.G. Wells).

And yet, over that same 130 years or so we haven’t seen any aliens, at least I’m not convinced we have. Lots of reports of UFOs over the decades and lots of movies and TV shows but no actual aliens landing on the Mall in Washington, DC or bringing a massive glowing craft to the Devil’s Tower in Wyoming as happened in Close Encounters of the Third Kind. One of the tropes in a lot of these movies is that the government is covering it up because the people in charge are convinced human beings aren’t prepared for the truth. But we’ve had this idea as part of our culture for more than a century. We’re probably as ready as we can ever be for the real thing. And still the aliens haven’t showed up.

Today the Washington Post has a story about the ongoing argument among scientists. The universe is a very big place but so far there’s really no solid evidence of anyone else inhabiting it, despite decades of looking.

For SETI researchers, the hypothetical existence of aliens is foundational. Nestled in the remote mountain town of Green Bank, the National Radio Astronomy Observatory has a role in one of the most ambitious SETI projects, called Breakthrough Listen. The project buys time on the towering Robert C. Byrd Green Bank Telescope, which has a steerable dish 300 feet in diameter. If there are aliens transmitting radio signals anywhere near us in the galaxy, that big dish is all ear…

Only a small fraction of our galaxy has been studied. Absence of evidence, as everyone knows, is not evidence of absence. Aliens may not consider radio waves to be a useful or dignified way to communicate. They could be pathologically shy. Or, at least with the kind of technology we have today, they could be just a little bit out of range.

For whatever reason, SETI has not found anyone out there, and at some point the silence could get deafening.

The argument for SETI and really the argument for aliens comes down to the universe being so big that the chances we’re alone seems terrifically unlikely.

The Big Numbers argument notes that our galaxy, the Milky Way, has something like 400 billion stars, and it’s just one of untold billions of galaxies in a universe that might be infinite. Moreover, in the past 30 years, astronomers have discovered that planets of all shapes and sizes are common in the universe.

With so much turf out there, even the most frowny-faced skeptic must admit it’s hard to run the numbers in a 13.8 billion-year-old universe like ours and wind up with just one self-aware, technological, telescope-constructing species.

Here’s the problem with that argument. There are millions of galaxies out there each with hundreds of millions of stars but based on our current understanding of physics we’ll never meet any of them. Anything outside our own galaxy might as well be infinitely far away. If there are aliens living in Andromeda, we’ll never know.

Even the size of our own galaxy is pretty daunting for creatures that only live about 80 years. If the speed of light is the universal speed limit for human beings then the volume of space we can visit and explore isn’t terribly large, especially if you plan on returning home. Even having a conversation with someone is basically impossible beyond the nearest stars. Imagine sending a message and waiting a decade for the answer. Maybe we’re not actually alone in the universe but we probably are functionally alone.

The bible for alien-life pessimists is the 2000 book “Rare Earth: Why Complex Life Is Uncommon in the Universe,” by Peter D. Ward and Donald E. Brownlee. The controversial book argued that Earth is unusually blessed with conditions that make complex life possible, such as having the giant planet Jupiter in just the right spot to run interference against dangerous comets.

But the book does not argue there are no aliens out there. Here’s where the debate gets subtle. Ward told The Washington Post that he assumes aliens do exist somewhere in the vast universe, but we’ll never know because they’re just too far away to make contact. We’re not literally alone, in his scenario, but we’re functionally alone.

“The chances that there’s one close enough to ever interact with is vanishingly small,” Ward said.

As the article notes, it took about 3 billion years for life on earth to go from single cells to multi-celled animals and then hundreds of millions more years to get to intelligence capable of complex language and technology. Life, in terms of single-celled organisms, may not be rare in the universe. There may be green slime growing on rocks in a few places in our own galaxy right now, but the chances of finding someone else to talk to, much less someone who has the technology to travel here and take over the planet seems pretty unlikely.

The aliens, if they’re out there, are facing the same problems getting around this massive universe that we are. Even if they want to invade the earth and make it their own, having the technology and patience to pull that off is not a trivial task.