Executable code is intimately linked to the operating system on which it runs. As many readers will already know, it is impossible to execute a piece of Linux binary code on a Windows system, and it is every bit as difficult to launch an Apple executable file in AIX, or at least to do so natively, without using emulators. There have been attempts, of course: for example, Windows NT 4 was able to execute not just DOS but OS/2 programs. But only in character mode and with a 16-bit structure. So any Martian creating a computer virus for use on planet earth would face some very daunting obstacles.
But the problem doesn't end there. How do Earth computers work? The most advanced users talk about binary logic, and data buses with a given number of bits and specific activation times for a signal to be recognized by a given device. How could an extra-terrestrial find out about this? It takes humans several years to understand it - basically, the length of a degree course in computer studies - and that's assuming a knowledge of the language and writing system in which the information itself is communicated.
I also have one last doubt, which is surely the clincher. How do they know we're here? If they use the same methods as we do (visual observation at the beginning, and radar now), they would need to have detected us in some way. We can rule out visual observation, as even astronauts in orbit are unable see human life from a few kilometres up, let alone from many light years away.
So the only hope is that they have detected us using the electromagnetic waves we send out. And the first signal of any intensity was sent on December 12, 1901 when Marconi received an "S" in Morse Code, sent from one side of the Atlantic to the other. This was just over 100 years ago, so even with a smooth journey the signal would only have travelled 100 light years, rather less than the distance of 900 years it is assumed that it would take to reach the nearest planetary system to Earth, the PSR 1257+12 pulsar.
Anyway, let's assume that these aliens - Klingons, for example - have managed to intercept this signal and have learnt about the behaviour of Earth computers solely on the basis of Morse Code transmissions. Being Klingons, and because they are really bored, they focus all their efforts on introducing a virus into Earth's computers. I'm sure they would have nothing better to do when encountering a life system and civilization very different from their own!
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