FINDING evidence to back up a theory of everything was always going to be difficult. Now two particle physicists say they may have tracked some down 鈥� lurking inside black holes.
The pair鈥檚 calculations have revealed that black holes might be harbouring enigmatic hypothetical entities called magnetic monopoles. If they are, not only would physicists have stumbled upon a key ingredient for a theory of everything, but it may explain why some black holes rotate.
Physicist first proposed magnetic monopoles in 1931. Magnetic poles usually come in north and south pairs. By contrast, monopoles carry just a single magnetic 鈥渃harge鈥�. Dirac proposed that monopoles are necessary to explain why electrons carry just a single electric charge, and why all other charged particles carry multiples of this charge.
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Today, they feature in almost all 鈥済rand unified theories鈥� (GUTs) 鈥� theories that attempt to link the and forces, a necessary first step towards an even grander 鈥渢heory of everything鈥�. Most GUTs predict that monopoles litter the universe. 鈥淢onopoles are remarkably resilient, coming back again and again in theories of electromagnetism, quantum theories and GUTs,鈥� says Claudio Bunster, a physicist at the Centre for Scientific Studies (CECS) in Valdivia, Chile. 鈥淎nd yet we can鈥檛 find a single one.鈥�
聯Monopoles are remarkably resilient, coming back again and again in theories, and yet we can鈥檛 find a single one聰
That dilemma prompted Bunster and colleague Marc Henneaux of the Free University of Brussels (ULB) in Belgium to begin a new hunt. 鈥淢y immediate instinct was that they are hiding in black holes,鈥� says Bunster. 鈥淓ven a small child could have that naive idea because that鈥檚 exactly what black holes are famous for doing 鈥� hiding things.鈥�
Their calculations indeed show that could be hiding magnetic monopoles. What鈥檚 more, that could also explain how black holes begin to rotate.
Dirac had shown that monopoles should impart quantum spin to other objects. What Bunster and Henneaux鈥檚 calculations reveal is that, for a black hole, this effect is dramatic. As a monopole crosses a black hole鈥檚 event horizon 鈥� the boundary beyond which nothing can escape the black hole鈥檚 gravitational pull 鈥� this quantum spin is converted into classical mechanical motion, and the monopole sets the black hole spinning ().
鈥淚t seemed bizarre at first, but then we realised it鈥檚 quite natural,鈥� says Bunster. 鈥淲e set out to explain where the missing monopoles had gone, but we ended up solving two mysteries for the price of one. That鈥檚 the sign of a good theory.鈥�
聯We set out to explain where the missing monopoles had gone, but we ended up solving two mysteries for the price of one聰
鈥淚t鈥檚 a very beautiful idea,鈥� says Nobel laureate David Gross, director of the Kavli Institute for Theoretical Physics in Santa Barbara, California. 鈥淲e know very little about the mechanisms of black hole formation and it鈥檚 certainly conceivable that you could pump them up and make them rotate by throwing in monopoles.鈥�
One thing might scupper the idea, however: it could violate the second law of thermodynamics, says physicist Paul Davies at Arizona State University in Tempe. Earlier studies have suggested that dropping a monopole into a black hole would make the black hole shrink. That would reduce the amount of disorder that could be created within the black hole, reducing its entropy and so violating the law.
However, Davies has shown that monopoles will not violate the second law if they are large enough 鈥� by which he means a sphere around 10-15 metres in diameter, roughly the size of an atomic nucleus.
That may not sound much, but Dirac originally envisaged magnetic monopoles as being a single point without volume. Davies believes that if magnetic monopoles have size, and therefore mass, then adding them to a black hole would increase its entropy, even if it is also shrinking (). 鈥淚t turns out that there鈥檚 a very subtle balance between these effects, which help to save the monopole,鈥� he says.
Stephen Hawking, of the University of Cambridge, likes the way that Bunster and Henneaux have exploited the monopole鈥檚 ability to make things spin. 鈥淸The paper describes] an interesting property that a monopole and an electric charge have angular momentum even when they are at rest,鈥� he told New 杏吧原创.
However, he points out that there are alternative explanations for why we have not yet found any monopoles. For instance, cosmologists believe that our visible universe might have grown from a tiny region of space, which went through a rapid period of expansion known as 鈥渋nflation鈥�, soon after the big bang. If that initial region of space was small enough, it would contain just a single monopole. According to inflation theory, that monopole is floating somewhere within our visible universe, but the chance of us bumping into it is small. 鈥淚 think inflation is a more likely explanation of the observed lack of monopoles,鈥� says Hawking.
