From the Super Mario Wiki
The Physics Equation is an item in the NES release of Mario's Time Machine. It is a scrap of paper that holds Albert Einstein's famous formula for mass–energy equivalence, E=mc2 (energy equals mass times the speed of light squared). It is stolen by Bowser after he travels back in time to 1905, and it then placed inside Bowser's Museum next to other time-displaced items and some Koopa Troopas. Later, Mario fights off the Koopa Troopas, steals back the Physics Equation, and travels back in time to 1905 to return the paper. Mario then arrives at a laboratory and leaves it in front of a nondescript chalkboard. Afterwards, a news article appears that congratulates Mario for annoying future physics students.