"In your life! Have you ever seen anything like that?" (Verne Lundquist, CBS commentary, 2005 Masters).
If the many-worlds interpretation of quantum theory is correct, the universe branched when Tiger Woods tried to hole his chip on the 16th green of the 2005 Masters. In our branch of the universe, the ball rolled up to the side of the hole, paused on the precipice, then toppled in. In other branches, the quantum fluctuations in the centre of mass of the ball were such that it remained poised on the edge of the cup, and Tiger failed to win the tournament.
In most branches of the universe, 2010 was not one of Tiger's better years.
The point, however, about Tiger's chip is that it demonstrates the ubiquity of instability in the physical world, and the manner in which that instability opens the door for quantum effects to be amplified onto macroscopic length scales. The macroscopic world cannot be sealed off from the quantum world.
Despite this fact, in a recent paper on the interpretation of quantum mechanics, Lev Vaidman invites the reader to consider the 'Tale of a single-world Universe':
"Let us assume that we are the only civilization and that we live under a very strong dictatorship which has laws against quantum measurements. It is forbidden to perform quantum experiments in which there is a nonzero probability for more than one outcome. Manufacture of Geiger counters is banned, quantum random number generators are forbidden, and a special police prevents world splitting devices of the kind that can be found in Tel-Aviv university. There are even laws that under the threat of death enforce disposal of neon light bulbs after six months of operation, to avoid operating an old bulb, which, when flicking, splits our world. In this tale Nature does not arrange quantum experiments accidentally: no macroscopically different superpositions of a macroscopic object ever develop."
The existence of instability throughout the natural world makes it impossible to satisfy Vaidman's hypothesis. Consider, for example, the widespread presence of chaotic systems. In such systems, no matter how closely one chooses a pair of possible initial states, the distance between them will diverge exponentially with time. Thus, initial states separated by a quantum perturbation will diverge exponentially with time, amplifying the quantum perturbation onto macroscopic scales. As philosopher of physics David Wallace points out, "Chaos...is everywhere, and where there is chaos there is branching, (the weather, for instance, is chaotic, so there will be different weather in different branches."
Quantum effects can also kill you. On average, every cell in your body is traversed by a track of ionising radiation every year. If quantum theory provides a maximal description of the physical world, then the interactions between such radiation tracks and the particles in your body are objectively probabilistic. If you're unlucky, one such radiation track might undergo elastic Compton scattering with an electron in the outer shell of an atom close to the DNA within the nucleus of a cell. The liberated electron might then proceed to wreak havoc in its vicinity, causing numerous further ionisations on its trail of destruction. These ionisations might create free radicals, which proceed to cause double-strand breaks in the nearby DNA. Such damage might be mis-repaired, creating a mutation in the DNA. The mutation might lie within that part of the DNA which controls the cell cycle mechanism, and the mutant cell might then reproduce uncontrollably, creating a cancerous growth within the host organism. Thus, a single, objectively probabilistic quantum event has the capability to kill you.