The most intriguing mystery of the universe? WHITE holes

White Holes: Inside The Horizon Allen Lane, €19

Mind-boggling concepts abound in modern physics. Some may seem absurd. Yet, as the Italian physicist Carlo Rovelli points out in this small book about big ideas, many of our basic beliefs about the universe defy common sense. The notions that the Earth is spherical, and that it moves, look at first glance to be ludicrous. To the casual observer, it appears to be flat and still. However, we first figured out that the Earth is round two millennia ago and we have known that it revolves around the Sun since the 16th Century.

More recently, much weirder phenomena have entered the scientific arena. Black holes, for example.

Only 20 years ago many people, including some physicists, doubted the reality of black holes. Rovelli tells of how, when he moved from the USA to a French university in January 2000, his new department chair approached him. ‘You don’t really believe that black holes actually exist, do you?’ he asked. Today hundreds of black holes are known. In all likelihood there are billions upon billions of them in the universe. Photos of some are available. One is reproduced in this book.

Black holes are created when stars collapse in on themselves. This creates a region of spacetime inside which the force of gravity is so strong that it traps everything. Not even light can escape it. Hence the name. There are many different sizes of black hole. As Rovelli notes, they can vary ‘from a few kilometres in diameter to some that are truly colossal, as big as the entire solar system, or bigger still’. There may even be black holes as small as a pingpong ball, although no astronomer has yet observed one of these.

Now that the existence of black holes is an accepted reality, more adventurous thinkers in the world of cosmology have begun to speculate about even stranger phenomena. Perhaps there are such things as ‘white holes’. Carlo Rovelli is one of those scientists prepared to ponder the possibility of the existence of what he calls the ‘elusive younger siblings of black holes’.

A white hole would be a black hole in which time was reversed. Scientists have long known that time is far more mysterious than everyday experience suggests. Perhaps there are circumstances in which its arrow does not just head in one direction. ‘A white hole,’ in Rovelli’s words, ‘is how a black hole would appear if we could film it and run the film in reverse’. Just as it’s possible to enter a black hole but not to leave it, so you can exit a white hole but not enter it. And a white hole would be born in ‘the deepest inner regions’ of a black hole ‘where time and space melt’.

Rovelli is willing to admit that this hypothesis might be wrong. ‘I do not know if it is correct,’ he writes. ‘I do not even know if white holes actually exist.’ No one has ever yet seen one. Their proposed births in the depths of black holes, from which no light emerges, can never be observed. However, as he remarks, ‘If we cannot travel there physically… we can do so mentally’.

This new book by Carlo Rovelli, already renowned for bestsellers such as Seven

Brief Lessons On Physics and Reality Is Not What It Seems, provides a remarkable journey into one of the oddest, most intriguing mysteries of the universe.