Decoherence

The search for understanding the true nature of the physical world has created the strangest of psychological conundrums.

In the late 1800’s there was such a feeling of certainty among scientists that simply by knowing some initial conditions and the laws of causality one could more or less predict everything. It was patently obvious that from the smallest of the small to the vastness of the cosmos that we only had to observe some initial conditions to understand and explain why things are as they are, everywhere and for all time. Reality, with a bit of work, was objectively knowable.

Physics was rocked in the beginning of the 20th century by Einstein with his ideas that reality could appear differently to various observers depending on the relative conditions of the observers and observed. Einstein’s work suggested that what our senses told us wasn’t necessarily True in the sense that it was the same for everyone everywhere. Truth it seemed might just be a local phenomenon which could be false for some other observer. Not easy to comprehend if you thought that there was an objective, knowable reality, Einstein’s proposals shocked the scientific and intellectual worlds and made intelligent people very uncomfortable.

But even the pioneeering genius and great thinker that he was, Einstein couldn’t accept the new field of quantum mechanics which was rapidly evolving at the same time into a powerful way to understand the world. Quantum mechanics allowed scientists to successfully predict things that no other scientific theory could successfully predict and gave insight where there was none previously. When quantum mechanics explanations were pitted against Einstein’s ideas, quantum mechanics won. So to speak.

What made everyone uncomfortable with quantum mechanics was that it so heavily relied on a field of mathematics which gave multiple right answers to problems. The real world doesn’t seem to have multiple right answers. Call a friend and they either answer their phone or they don't but they don't both answer and not answer. On any given day you are either late for work or you're on time but you’re not both. Experience tells us that only one reality will arise from any set of possible outcomes.

For future events for any observer reality is a set of possible outcomes with some being more probable to happen than others in the mind of the observer. But uncertainty is the key feature. This uncertainty can become a significant reality in itself for certain outcomes (does she love me, do I have aids, will the carrot harvest be good, etc).

Yet when the "future" arrives all uncertainty dissipates, that which apparently has happened is real and the other possiblities which didn't "happen" become faeries in the mist, a fiction that was never going to be real had we only known the future.

But a far stranger explanation, sometimes known as the multiple universe or many worlds theory is far more powerful in a predictive sense. The Many Worlds theory says that all the possibilities which didn't "happen" actually did happen but in a different universe. In one universe your friend answered the phone and in an otherwise identical universe your friend didn't answer the phone. And not just for big important things like chatting with your mates but for every possible event. Seem implausable? We just don't live our lives this way.

Yet it’s what Nobel prize winners in physics like Stephen Hawking and Murry Gell-Man believe. In fact, it’s accepted theory today. Absolutely every potential outcome for every potential event all exist in a cascading plethora of universes each splitting off at the moment of observation by an observer into many worlds each containing one of the many possible outcomes.

Think of the set of potential outcomes of some future event as a kind of ocean wave, a wall of water moving across the ocean's surface. An observer is standing in the shallow water. The entire wave of possible outcomes is real but only that part of the wave which passes over the mentioned observer standing in its path, only that part of the wave of possible outcomes becomes “entangled” with the observer. For this particular observer, this part of the wave with which he becomes entangled, is what this observer considers "real". The rest of the wave of possiblities which aren't striking this observer seem to this observer to have "not happened" when "in reality" the wave is also entangling with other observers standing along the shore. A plethora of observers, a plethora of other universes as the wave of possiblity becomes entangled with any number of particular observers.

Each observer who becomes entangled with a different part of the wave of possible outcomes is a version of you but in a different universe. Each version of you becomes entangled with a different part of the wave of possible outcomes. All outcomes are real but are happenning to a different version of you in a different universe. "You" are only one of those versions!

Decoherence is the scientific term to describe the entanglement of one part of the wave of all possible outcomes with one particular observer creating for that observer what seems to be the one true reality.

Eric Perlberg
London July 2010