Here is a concrete example: write a program that systematically tests all numbers for counterexamples to the Goldback conjecture. Every time a number fails to be a counterexample, output a zero, otherwise output a 1. Consider the output of that program to be a real number expressed as a binary decimal. If that number is not zero you will know in a finite amount of time, but if it is zero you will not know that until someone proves the Goldbach conjecture.