Data analyst Yung Lin Ma offers fifteen reasons, including some new to us. He begins by observing,

There are about 1 billion stars that can produce an environment similar to the Earth. The environment of the earth does not necessarily have life, and this ratio is lower than 1 in 10,000. The reasoning is that at least in our galaxy, there should be 100,000 civilizations. Then why haven’t we seen even any single one civilization?

Yung Lin Ma, “15 Reasons Why We Can’t See Aliens” at Medium (July 14, 2020)

So, it’s an active question.

Of his fifteen reasons, here are three:

3. Extraterrestrial life does exist, and has visited the earth. It was just a long time ago. Later, the aliens have gone. If you have seen the movie — Interstellar should know that the flow of time is different for different people. It may be that humans think that ancient aliens have been away for a long time, but aliens have just left.

Yung Lin Ma, “15 Reasons Why We Can’t See Aliens” at Medium (July 14, 2020)

Interstellar (2014):

Flow of time is a concept worth thinking about. If that’s the answer though, we should eventually find artifacts, just as we do with humans who lived a long time ago.

4. In fact, there are already many aliens around us, but we are not aware of it. Perhaps in the visions of alien civilizations, human beings are very low life. We are unable to understand the existence of extraterrestrial life. Just like humans look at ants. Ants do unable to understand the existence of human beings. Ants may have noticed human beings, but they certainly cannot understand.

Yung Lin Ma, “15 Reasons Why We Can’t See Aliens” at Medium (July 14, 2020)

Not so sure about this one. We don’t need to understand what the extraterrestrials are doing so much as we need to confirm their existence. That is well within our grasp, just as confirming humans’ existence is within an ant’s grasp. All the rest depends on what follows.

and

7. We communicate with alien civilizations differently. We use electromagnetic waves and light to communicate. Alien civilizations may communicate with something we do not understand. It is impossible to find each other if communication fails.

Yung Lin Ma, “15 Reasons Why We Can’t See Aliens” at Medium (July 14, 2020)

That’s certainly possible. Recently, it was suggested that extraterrestrials could be using starlight (photonics, essentially). We would have no idea that the apparent winking of stars is a form of communication, certainly not before we started using photonics ourselves. Likewise, the extraterrestrials may never have considered using sound waves, which is how practically all humans communicate in everyday life.

Read and enjoy Ma’s other twelve hypotheses!

You may also enjoy these accounts of why we do not see aliens. The all offer thoughtful science fiction potential while we wait…

1.What if extraterrestrials can’t afford to take chances with us?
That’s the Dark Forest Hypothesis, riffing off the title of one of famed Chinese sci-fi author Liu Cixin’s novels. The Dark Forest Hypothesis assumes that we can use sociology to figure out what extraterrestrial intelligences might be like or might want. But can we?

2.Are the Aliens We Never Find Obeying Star Trek’s Prime Directive? The Directive is, don’t interfere in the evolution of alien societies, even if you have good intentions. Hence the Zoo hypothesis. Assuming the aliens exist, perhaps it’s just as well, on the whole, if they do want to leave us alone. They could want to “fix” us instead…

3.How can we be sure we are not just an ET’s simulation? A number of books and films are based on the Planetarium hypothesis. Should we believe it? We make a faith-based decision that logic and evidence together are reasonable guides to what is true. Logical possibility alone does not make an idea true.

4.Did the smart machines destroy the aliens who invented them? That’s the Berserker hypothesis. A smart deadly weapon could well decide to do without its inventor and, lacking moral guidance, destroy everything in sight. Extinction of a highly advanced civilization by its own lethal technology may be more likely than extinction by natural disaster. They could control nature.

5.Researchers: The aliens exist but they are sleeping… And we wake them at our peril. The Aestivation hypothesis is that immensely powerful aliens are waiting in a digitized form for the universe to cool down from the heat their computers emit.

6.Maybe there are just very few aliens out there… The Rare Earth hypothesis offers science-based reasons that life in the universe is rare. Even if life is rare in the universe, Earth may be uniquely suited to space exploration, as the Privileged Planet hypothesis suggests.

7.Does science fiction hint that we are actually doomed? That’s the implication of an influential theory, the Great Filter hypothesis, as to why we never see extraterrestrials. Depending how we read the Kardashev scale, civilizations disappear somewhere between where we are now and the advanced state needed for intergalactic travel.

8.Space aliens could in fact be watching us. Using the methods we use to spot exoplanets. But if they are technologically advanced, wouldn’t they be here by now? The Hart-Tipler conjecture (they don’t exist) is, of course, very unpopular in sci-fi. But let’s confront it, if only to move on to more promising speculations.

9.Is the brief window for finding ET closing? According to some scenarios (the Brief Window hypothesis), we could be past our best-before date for contacting aliens. Of course, here we are assuming a law of nature as to how long civilizations last. Can someone state that law? How is it derived?

10.What if we don’t see aliens because they have not evolved yet? On this view, not only did we emerge during a favorable time in the universe’s history but we could end up suppressing them. The Firstborn hypothesis (we achieved intelligence before extraterrestrials) lines up with the view that humans are unique but sees that status as temporary.

11. The aliens exist—but evolved into virtual reality at a nanoscale. That’s the Transcension Hypothesis, the latest in our series on science fiction hypotheses as to why we don’t see extraterrestrials. On this view, after a Singularity the ETs become virtual intelligences, exploring inner space at an undetectably small scale.

12.Is intelligent life in the universe living in interior oceans of planets and moons? The Ocean Planets Hypothesis is that intelligent beings may flourish in the interior oceans of the moons of gas giant planets — or within exoplanets — but they are trapped there. If intelligent life forms are trapped in the interior oceans of rocky moons and planets, Earth is a special planet—much better suited to space exploration.

13.Is real-world space travel just too daunting for ET? That’s the Percolation Hypothesis as to why we don’t make contact with aliens. They can’t overcome the laws of physics, any more than we can. If there is a purpose behind the universe, maybe the aliens and we weren’t intended to meet. That’s worth considering, given the physics barriers.

14.The Aurora Hypothesis: ET could risk only rare contact with us. Given the difficulties and risks of space travel, extraterrestrials with advanced technology may have visited Earth only one in a million years, researchers say. After centuries of modern science, we are just now looking for fossil bacteria on Mars, not without risk. ET may be in the same position.