Definitively, means, to you, prove to me so i have no doubt.
And that, another person can never do.
Can't even definitively disprove the theory of relativity. And the light speed has barrier been broken.
You can get data from people who have actually experienced these things and have come back, or from people who remember these times in between their lives.
However, most people believe the pastor in Sunday school when they were taught from a book that was corrupted. The Bible basically says, Work hard for your master (lord) in this life, and then you get to go to heaven, else hell awaits you. (that is what it was changed to say, by the masters who want you to keep working for them)
You can learn to go explore these times in your history. The information is available to each person. A lot of people give up along this journey. So, i can't even say, if you use this path, you will get there.