A central aim of physics education research is to understand the processes of learning and use that understanding to inform instruction. To this end, researchers often conduct studies to measure the effect of classroom interventions on student outcomes. Many of these intervention studies have provided an empirical foundation of reformed teaching techniques, such as active engagement. However, many times there is not sufficient evidence to conclude that the intervention had the intended effect, and these null results often end up in the proverbial file drawer. In this paper, we argue that null results can make significant contributions to physics education research, even if the results are not statistically significant. First, we review social science and biomedical research that documents widespread publication bias against null results, exploring why it occurs and how it can hurt the field. We then present three cases from physics education research to highlight how studies that yield null results can contribute to our understanding of teaching and learning. Finally, we distill from these studies some general principles for learning from null results, proposing that we should evaluate them not on whether they reject the null hypothesis but according to their potential for generating new understanding.