The current series of Horizon seems to be filled with doom and gloom. We’ve looked for better ways to kill people, locked them up in solitary for 48 hours, and now it seems we’ve found something wrong with that most fundamental force in the universe: gravity (tonight’s programme is called: “What on Earth is Wrong With Gravity?”).

Although there are many theories about how gravity works, the reality of it never quite fits them, so a true understanding still eludes us. The holy grail of science – the Grand Unified Theory – depends on uncovering the fundamentals of gravity, and to explain the problem one of today’s foremost physicist goes on a road trip around America trying to work out what on Earth is going on.

Horizon: What on Earth is Wrong With Gravity?: BBC Two, Tuesday 29 January, 9pm