If a batter's follow through hits the catcher while a runner is standing at first base but making no attempt to advance, what is the ruling?

In this scenario, the correct ruling is that the batter is called for interference. This situation falls under the category of "batter's interference," which occurs when the follow-through of a batter's swing makes contact with the catcher or interferes with the catcher's ability to play the ball, regardless of the runner's actions on the basepaths.

When a batter swings and their follow-through inadvertently strikes the catcher while he is making a play—such as attempting to catch a pitch or make a throw—it constitutes interference. This rule is in place to ensure that the catcher can perform his duties without being hindered by the actions of the batter. The fact that the runner at first base is not attempting to advance, while relevant to overall base running, does not change the batter's responsibility to avoid interfering with the catcher.

In this context, it's important to recognize that the intent or actions of the runner do not mitigate the infraction caused by the batter's swing. Thus, the umpire's call reflects the necessity of maintaining a fair and unobstructed play, ensuring that the catcher can operate effectively. The ruling, therefore, reinforces the integrity of the game by penalizing the batter for interference on the play.