Police investigators analyse blood stains at crime scenes to build a detailed picture of what has taken place. Drips, smears and spatters are studied to work backwards to reconstruct the locations and actions of people and weapons involved. The procedure of Blood Pattern Analysis (BPA) has played a prominent role in murder trials, including those of sports star O J Simpson (1994–1995, verdict of not guilty) and music producer Phil Spector (2007–2009, retrial verdict of guilty).

But it is a field of forensics that has been called into question. A damning 2009 report from the US National Academy of Sciences (NAS) concluded that BPA lacks scientific rigour and valid accreditation for its practitioners. Since that landmark NAS report, physicists have been helping to develop deeper physical models to underpin BPA. Find out more in this article by physicist and science writer Sidney Perkowitz, originally published in the October 2019 issue if Physics World. Members of the Institute of Physics can enjoy the full issue via the Physics World app.