A century ago, the answer for all events of unknown cause was fairies, ghosts or spirits, things that can pop in and out of existence leaving very little evidence behind.

Today, aliens from another planet  have become the silastic for all the gaps in our education.  Why is this?  What evidence is there?  The SETI program which scans the universe for artificial radio signals has so far detected nothing.  Eyewitness accounts of alien encounters are unreliable and follow a pattern of earlier accounts influencing later ones.  Physical or photographic evidence has never been found that did not also have several more likely earth-based explanations.  Crop circles, for example, have been proven beyond interest to be the work of clever pranksters.  The original perpetrators confessed that it was a hoax, while others have copied them and have elevated the technique to an art form.

You can judge for yourself whether the alien hypothesis is a reasonable one, once you discover what that hypothesis involves.    Keep reading!

If aliens are not fairies, spirits or ghosts but physical organisms from a different planet, then we already know a great deal about them.  For fairies, you can make up any rules you like.  Their capabilities are limited only by your imagination.  For physical beings like us, the universe makes the rules and we have no choice but to follow them. 

We have discovered through observation exactly how those laws operate on our planet.  What's more, astronomers have also discovered that these laws operate on other planets precisely as they do here.  How do we know this?  We have directly observed that even the most distant stars follow the same basic laws of the universe.  They are composed of exactly the same kind of matter as our sun is.  They have the same electrons, protons and neutrons that we do, and their particles follow the same laws of attraction and repulsion. 

That means the same chemical elements that are available to us are also available to aliens.  The same goes for chemical compounds.  Energy, mass and momentum work in the same way.  On distant stars as well as ours, the speed of light is the one and only possible speed through four-dimensional space-time, and hence it is the maximum possible speed through space.  Countless experiments and observations have proven this to be the case both here and throughout the universe. 

What, therefore, would an alien being have to do in order to visit us here on the earth? First, they have to get off their own planet.  Physics being what it is, about 90% of the mass of the alien spacecraft will be the fuel needed just for getting out into space one time.  The vast majority of the rest will be the fuel required to accelerate up to a reasonable speed.  Getting up to a tenth of the speed of light would require the energy equivalent of about a million barrels of oil for each kilogram of mass on board.   Most likely, space travellers will need to be very small.

The closest known planet with the highest probability for supporting life is Gliese 581 d, discovered in April 2009.  It is 20 light-years away.  If our hypothetical alien were starting from this planet and reached a speed of one tenth the speed of light, the trip would take about 200 years.  After that, the alien who may well have spent his planet's entire energy reserve trying to get here will not only have no way of getting back, he-she-it will not even have a way of stopping or even slowing down for a chat.  The spacecraft will whiz past us and keep right on going through interstellar space.  Unless they brought along an extraordinary quantity of additional fuel, which multiplies the original amount of fuel needed enormously.

Of course if aliens aren't real, you are free to imagine them popping in for a visit and vanishing again without a trace.  But if they're made of matter, then that matter must obey the laws that the universe has set.

Does this prove that aliens are not visiting our planet?  No.  It only means that in order to believe that aliens are here, one also accepts that:
1.  They started the trip hundreds or even thousands of years ago, long before they could have been aware of our existence.
2.  They consumed phenominal amounts of energy both to start the journey and to end it.
3.  They have no way of ever returning, since energy-wise we're not really in a position to help them out.

What seems to you to be the most reasonable explanation for any unfamiliar phenomena you may see?   If there's something you can't explain, does it have to be from another planet?  Personally, I always assume that I still have a lot to learn about this planet.