If there were a hydrogen atom sitting out in the middle of nowhere - nothing with which to interact - would it exist forever, or would it eventually wind down?

 

Matter and energy cannot be destroyed, and perpetual motion supposedly does not exist, so what happens in this case?

 

I am just a layperson; please keep answers simple. Thanks!

Some atoms of course do decay; that's called radioactivity. Hydrogen atoms are supposed not to decay. Some exotic physics proposes them to decay. This decay has actually been looked for in huge experiments like the Kamiokande experiment. No decay of the Hydrogen atom has been found. Note that pretty much the same question has been asked in this forum not too long ago. You might be interested in reading the other thread about it.

Edited March 7, 201114 yr by timo

Things decay because there is a lower energy state available. For a proton, there is no evidence that there is lower energy state, and no lower state for the electron than the ground state.