Time

Time... The concept of time. We all know it,  we all live with it.  Day to day, year to year, and all of our life times. The concept of time and history... generations, years & centuries, even millions of earths history all put into perspective by time. Time is constant and linear. Although when working times seems to go slow,  and while relaxing time seems to go by quicker. 
THEN... There's the concept of time travel. Theoretically being able to physically go to another point in time... To the past or future. Two theories... You could go back in time as an observer with no interactions or consequences. And two, you could go back in time and any actions or interactions could change history and the future... creating a paradox changing everything in the future. When changing anything, good or bad could result in massively changing the future... But then again, no one in the future would know that there were changes... It would merely be their new here and now as well a they're new history.  And who's to say that this had not already happened,  perhaps numerous times... No one would ever know except the person who time traveled.  Time travel... a concept the ranges from wishful thinking going back just hours or days,  to going back in time years ago to prevent a war  ( like killing Hitler before he gains power).

Or from a scientific of going back in time to see history first hand. 
What about great minds, scientists,  that mysteriously disapeared without a trace.  Did they go back in history,  get stuck there or die,  and never returned to the present. Unknown. 

Paradoxes... Just something to think about.  Science is...