# Show that the question “Is there life beyond earth?” is decidable

I was given a question to prove that there exists a turing machine that solves the question

Is there life beyond earth?

and is decidable. I actually don't understand how to show a turing machine decides this.

Thanks.

• I'll argue that it is not decidable. Step 1: destroy all other life in the universe. Step 2: run the Turing machine. Step 3: launch an astronaut into deep space. Step 4: run the Turing machine again, from the same initial state. Since the outputs given at steps 2 and 4 were the same, and the correct answers were different, the Turing machine does not correctly solve the problem. (Trick questions deserve trick answers.) – David Jun 2 '15 at 19:58
• The correct answer is a poor joke about a sloppy definition. A suitable definition of "decidable" would be that there is an algorithm that decides whether the given statement is true and not that there is an algorithm giving the correct result which is always the case because one algorithm produces "no" and the other "yes". Such a definition is completely useless because in this sense , almost EVERY yes-no-question is decidable. – Peter Sep 17 '18 at 8:09

• @Dan that is what I thought first, when I thought about the question. But this is not Yuval's explanation, which is a consequence of computing a function without input. Oh and obj.alive would be hard to implement as well. – mvw Jun 2 '15 at 14:38