The universe is pretty big. We have to invoke our inner Carl Sagan and say “billions and billions” when talking about the universe. (He never used that phrase but finally titled one of his books “Billions and Billions.”)
We don’t really have an intuitive understanding of very large numbers. Billions — or even millions — don’t mean much to us. The largest numbers we intuitively grasp are a few thousands. The higher order numbers are beyond our comprehension. With that caveat, let’s talk of the universe.
It is estimated that our galaxy, the Milky Way, has around 200 to 400 billion stars; that there are around two trillion galaxies in the observable universe; that the average star has a bunch of planets on which life could have evolved; that the universe is around 13.8 billion years old; etc., etc.
We know for sure that intelligent life arose on one planet — the earth. Hundreds of millions of species have evolved on earth in the last four billion years or so, and around 99.9% of those species have gone extinct. Our species homo sapiens finally developed a technological civilization recently in the last couple of hundred years. Therefore we know for sure that technological civilization is possible through Darwinian evolution.
Given that there are trillions of planets that have existed for billions of years in the universe it is possible, perhaps inevitable, that technological civilizations have developed in many other planets in the universe. How many?
We can estimate that number using the Drake equation. (I am happy to report that I once met Frank Drake at a SETI event in the Bay Area.)
N = R* × fp × ne × fl × fi × fc × L
Plug in estimates of the various parameters into that equation, and you will have an estimate of the total number of technologically advanced civilizations in the universe. It could end up being billions and billions.
If there are billions and billions of them scattered around in the universe, then some of them must have developed the technology for interstellar and intergalactic travel. Enrico Fermi famously asked, “Where are they?”
Many answers have been proposed to the Fermi question. One answer is that there aren’t any technologically advanced civilization; or that there were some but they went extinct; or that they don’t bother to travel or don’t bother with earth, etc.
But some people appear to believe that extraterrestrial technologically advanced civilizations have visited, and are currently visiting, the earth. They believe that UFOs — unidentified flying objects — are alien crafts. They are now known as UAPs — unidentified anomalous phenomena.
I don’t think there are aliens on earth — now or even in the past. Here’s why.
The universe is huge, as previously noted. True there are trillions of planets but the distances are huge. The observable universe is 92 billion light years across and expanding all the time.
On a broad scale, galaxies are distributed in a cosmic web of filaments, walls, and voids. The typical distance between large galaxies (like the Milky Way) in relatively dense regions, such as galaxy clusters like the Virgo Supercluster, can range from a few hundred thousand light-years to a few million light-years.
For example, the Milky Way and its nearest major neighbor, the Andromeda Galaxy, are about 2.5 million light-years apart. Cosmologists estimate that, on average, galaxies are separated by roughly 5 to 10 million light-years, though this is a rough approximation because galaxy distribution is not uniform.
Within a galaxy, the stars are really far apart. Alpha Centauri, the closest star to the solar system, is around four light years away. Galaxies are mostly empty space. The Milky Way and Andromeda Galaxy are on a collision course, and when they do collide in a few billion years, none of the hundreds of billions of stars in the two galaxies will collide. The two galaxies will pass through each other, though they will be distorted through their gravitational interaction.
The main fact we need to remember here is that the distances are huge. Regardless of how technologically advanced a civilization becomes, the speed of light is the barrier that prevents intergalactic travel. The universe may be teeming with life but they are all essentially isolated, and for all practical purposes from their perspective they are the only ones in the entire universe.
We humans are alone.
Reports of UFOs and UAPs are total bunk. Maybe some advocates of aliens on earth are sincere in their beliefs but I suspect that most are grifters seeking to profit from the stupid, the gullible and the innumerate. (A grifter is a person who engages in dishonest schemes or scams to trick people out of money or other valuables, often by gaining their trust first.)
The modern UFO craze in the US can be traced to a reported crash of an alien craft in Roswell, New Mexico. In July 1947, something crashed on a ranch about 30 miles north of Roswell. The military initially called it a “flying disc,” sparking wild speculation, before backtracking to say it was just a weather balloon.
People have been reporting all sorts of anomalous events for decades. The curious thing is that there’s not one shred of credible evidence. No one has a compelling video of an alien craft even though billions of people have excellent cameras on their smartphones that they carry with them all the time. All we have are truckloads of conspiracy theories based on fuzzy pictures and videos.
The craziest of the conspiracy theories is one which says that the US government has in its possession alien crafts and even the biological remains of aliens (“little green men”). The government, it says, keeps all this secret because it “reverse engineers” alien technology for military purposes.
That reverse engineering story is total bovine feces.
One can reverse engineer a technology provided it is almost but not quite close to one’s technological capability. The Chinese (for instance) could steal American missile technology and reverse engineer it to build their own missiles. But a tribe in Papua New Guinea could not make head nor tail of American technology. And that tribe is perhaps only a few hundred years behind the Americans in technology.
In fact, Americans of 1925 would find our 2025 technology totally incomprehensible. Any sufficiently advanced technology, as Arthur C. Clarke pointed out, is indistinguishable from magic. To us, if we ever come across alien technology, we would not even recognize it, never mind comprehend it.
I have to admit that I like to waste a lot of time reading reports of UFO sightings. It is hugely entertaining.
Time for a bit of music.
Atanu Dey : Life is a Random Draw 
That nothing can exceed speed of light (even when relatively going in opposite direction) is possibly the most non intuitive stuff I ever learnt. I know it now from others’ received wisdom. But I still cannot wrap my ‘common’ sense around it.
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