A null hypothesis is a hypothesis that says there is no statistical significance between the two variables. It is usually the hypothesis a researcher or experimenter will try to disprove or discredit. An alternative hypothesis is one that states there is a statistically significant relationship between two variables.
Simply so, what is the role of the alternative hypothesis?
The alternative hypothesis is the hypothesis used in hypothesis testing that is contrary to the null hypothesis. It is usually taken to be that the observations are the result of a real effect (with some amount of chance variation superposed).
What is an untestable hypothesis?
Most often, an "untestable hypothesis" is one that is impractical, or unethical, to explore with a controlled experiment. Most theories about the workings of our economy, for example, cannot put to the test using controlled experiments, because it is not possible to fully control large economic systems.