Possible Alien Invasion:

This week's collection of scary stories on the web features Stephen Hawking's warning that there may be unfriendly aliens lurking out there in the cosmos.

Let's look at the possibilities.

The nearest star outside the Solar system is variously estimated to be about four light years away. That means that if it were possible for solid objects (ETs) determined enough to come here at light speed, they would have to be in transit for more than four years. 

Since our science has established that light speed is the speed limit of the universe, interstellar travel for biological beings is unlikely if not impossible. 

So, aliens arriving here would have to be robots like our Mars probes or androids specifically designed to survive in the vacuum and radiation levels of interstellar space.

First evidence of intelligent life out there in the void would be gathered by the Search for Extra Terrestrial Intelligence (SETI) group, who are scanning the cosmos for coherent electromagnetic signals and, so far, not finding any.

Those signals can travel at the speed of light and until we detect them, it is correct for us to go on believing that as an intelligent species, we are pretty much alone, at least in our sector of the galaxy, and out of reach of marauders from space.

Sorry, Stephen, but unless and until we detect alien electromagnetic probes trying to monitor our environment, it ought to be okay for us to continue sleeping peacefully at night.