What is human hibernation?

No, its not possible may be due to evolution it may happen in future NASA is investigating about possible induced hibernation for astronauts for very long space journeys. There are two...

No one will probably ever know this answer.

Probably, yes. The Sun goes in an obvious 11-year cycle of activity that we call the "sunspot cycle"; the number of sunspots increases and decreases in a cycle that averages about 11 years long. But...

It would be useful in space flight.

You can't possibly do that. It is impossible. As in not possible. You can't. Do some research before making yourself look not smart.