When the multiverse and many-worlds collide

[div class=attrib]From the New Scientist:[end-div]

TWO of the strangest ideas in modern physics – that the cosmos constantly splits into parallel universes in which every conceivable outcome of every event happens, and the notion that our universe is part of a larger multiverse – have been unified into a single theory. This solves a bizarre but fundamental problem in cosmology and has set physics circles buzzing with excitement, as well as some bewilderment.

The problem is the observability of our universe. While most of us simply take it for granted that we should be able to observe our universe, it is a different story for cosmologists. When they apply quantum mechanics – which successfully describes the behaviour of very small objects like atoms – to the entire cosmos, the equations imply that it must exist in many different states simultaneously, a phenomenon called a superposition. Yet that is clearly not what we observe.

Cosmologists reconcile this seeming contradiction by assuming that the superposition eventually “collapses” to a single state. But they tend to ignore the problem of how or why such a collapse might occur, says cosmologist Raphael Bousso at the University of California, Berkeley. “We’ve no right to assume that it collapses. We’ve been lying to ourselves about this,” he says.

In an attempt to find a more satisfying way to explain the universe’s observability, Bousso, together with Leonard Susskind at Stanford University in California, turned to the work of physicists who have puzzled over the same problem but on a much smaller scale: why tiny objects such as electrons and photons exist in a superposition of states but larger objects like footballs and planets apparently do not.

This problem is captured in the famous thought experiment of Schrödinger’s cat. This unhappy feline is inside a sealed box containing a vial of poison that will break open when a radioactive atom decays. Being a quantum object, the atom exists in a superposition of states – so it has both decayed and not decayed at the same time. This implies that the vial must be in a superposition of states too – both broken and unbroken. And if that’s the case, then the cat must be both dead and alive as well.

[div class=attrib]More from theSource here.[end-div]