In tonight’s episode of The Big Bang Theory, the writers dared to go where most physicists will not, to the philosophical underpinnings of quantum mechanics. When Penny asks Sheldon to dance with her, he replies:
To tackle a description of the Many Worlds Theory of quantum mechanics (more often called an “Interpretation” rather than “Theory”), we first need to delve into quantum mechanics itself.
Before physicists realized that quantum mechanics was necessary, their view of the world held that the outcome of any event could be completely determined–as long as you had precise enough measurements beforehand. If you dropped a rose petal into a hurricane, all you needed was the positions and velocities of every molecule of air at just one moment and then you could calculate the final location of the rose petal with certainty. (OK, you would also need to know all about all the birds and pieces of vinyl siding flying around too.) As a practical matter, you could never really do this, but at least in principle it would have been possible.
This view changed forever with the discovery of quantum mechanics in the 1920’s and its experimental tests over the following decades. The outcome of many situations can never be predicted with certainty. Take for example carbon-11, a radioactive atom used in life-saving medical imagers called PET scanners. That carbon atom’s “half-life” is 2 minutes, meaning that half the carbon-11 atoms you possess at any one time will decay and disappear in 2 minutes. But what if you had only one atom of carbon-11? No physicist can ever say when it will decay with certainty. The best we can do is say that it has a 50% chance of still being around 2 minutes from now; a 25% chance 4 minutes from now; and 1 in a billion chance of being around an hour from now. To know the exact fate of any one atom is not a matter of not being able to see well enough inside the atom. There is no way of ever knowing.
This fundamentally probabilistic description of nature bothered some physicists. Even Albert Einstein objected famously: “I am convinced that He [God] does not play dice”. Convinced as he may have been, and as brilliant as he may have been, experiment trumps genius. Clever experiments have shown that there is no room for what Einstein was sure of: hidden deterministic variables that underlie the probabilistic laws of quantum mechanics.
Philosophical questions arise when you put the atom in a box for say 2 minutes and let no one check on it. Meanwhile the condition of the atom during that time can still affect other measurements, so descriptions of the atom during this time turn out to be both important and open to interpretation. The founders of quantum mechanics, working largely in Copenhagen, believed that the best way to view the situation was that the atom was simultaneously in a decayed and un-decayed state. They said that only after some observer comes along and looks in the box would the atom be forced into one state or another. In the Copenhagen Interpretation, the act of observation changes the universe. Such is the typical training that physicists such as myself received as undergraduates.
The Copenhagen Interpretation raises difficult, perhaps unanswerable questions: How large does something have to be to constitute an observer? If the atom bounces into another atom that detects its presence, is that other atom an observer? Is a large, complicated detector a valid observer? Must the observer possess consciousness? Must it be human consciousness or can it be a cat’s? Does another observer that does not know the outcome possess a different “reality”? The most likely answer to such question is “Shut up and calculate!”.
Thus things stood for decades. An alternative, the so-called “Many Worlds Interpretation”, emerged from the 1957 Ph.D. thesis of Hugh Everett at Princeton. Everett never labeled his interpretation “Many Worlds” but rather originally titled his paper “Wave Mechanics without Probability” ( “wave mechanics” meant “quantum mechanics”) He later changed the title to something more abstruse to placate his Ph.D. committee.
In Everett’s interpretation, the probabilities were only a consequence, not an elemental part of the theory. Not only is the state of the atom described by the equations of quantum mechanics, so are all the detectors and observers in the world. When an object and observer meet, the two affect each other according to the usual rules of quantum mechanics. No new process happens at the moment of observation. Of course when an experimentalist observes the atom he or she perceives it as decayed or not; but the experimenter is now part of the system including the atom, experiencing only an “inside” view. Meanwhile someone else, with an “outside” view, still entertains all outcomes. The interpretation does not rest on dice.
The persistence of both outcomes in Everett’s interpretation is often described as two different worlds that propagate forward and independently in time: one where the atom decayed and one where it did not. Had the life of a cat hinged on outcome of the decay, it is often said that our world branches into two worlds, one with a live cat and one with a dead cat. (Here we are adopting Schrodinger’s cat, described in the season-one finale, S01E16. These are hypothetical experiments only—no cats were harmed.) The critical question of why we experience the world as 100% live or dead cats, never as a mixture, was left by Everett as an exercise for the reader.
Soon after completing his Ph.D. thesis, Everett ventured to Copenhagen to explain his ideas to Niels Bohr. He failed miserably. Everett left academia, never to return. The world took little notice of his work.
But times have changed. At a recent meeting of quantum physicists, the Many Worlds Interpretation received more votes than the old Copenhagen interpretation as the picture closest to the participants’ own views. Still, physics is not a popularity contest. These were personal opinions, much like Einstein’s unsubstantiated claim about dice. For the debate to be meaningful there needs to exist some prediction for which the two differ. Hope exists in some quarters to find such tests, but I can find no specific experiment put forward. It remains that no matter how opposed the views of two physicists on the topic, they will all calculate exactly the same outcomes of experiments. Without an experimental prediction that differs, the fight is just a war of words not physics. A distinction without a difference.
Except for one. Science fiction writers and physicists alike have entertained one intrepid experiment. The means of distinction rely on an experimenter having no fear of approaching quantum suicide–by playing a game of quantum Russian Roulette. The observer shoots a gun at his or her own head with a 50% chance of having a bullet vs. a blank. After many trials the physicist will know if Many-Worlds is favored, since there is no experiencing a world where you are dead. Many Worlds predicts that the observer will eventually live in a world having survived the game 100 times or more. Unfortunately, we can never can never know what result our brave physicist friend found. There is no way for the survivor(s), if any, to tell us.
I’ve done some stupid things for physics…
but I won’t be volunteering for the quantum suicide experiment. It violates my university’s ethics protocols for experiments involving human subjects.
In the Many Worlds Interpretation, some measurements can encompass an infinite number of final states, or as Sheldon characterizes it: an infinite number Sheldons in an infinite number of universes. Luckily for us, the number of Sheldons is not just an ordinary infinity, but an even larger one called uncountably infinite number of Sheldons.