1. The Everett approach ↑
The measurement problem, which shows that it is impossible for quantum and classical objects to interact, is resolved by accepting the idea that macroscopic objects are described by the same laws as quantum objects. If we accept this, then the preferred basis problem, which asks why the universe is split into the 'separate worlds' we experience if it is really part of a multiverse, may be resolved with decoherence theory.
The problem of action at a distance does not occur within Everett's approach because all events are actualised, and so information regarding a specific result does not need to be sent.
The Everett approach is also preferable to other approaches because it applies to the universe as a whole, and so it can be used on a cosmological scale.
The Everett approach claims to resolve all of these problems without modifying the basic equations of quantum theory, but, by taking the theory of quantum mechanics literally, it does challenge some common sense assumptions.
2. Everett and Ockham's razor ↑
Some are reluctant to accept the Everett approach because it relies on the existence of an infinite amount of other, unobservable worlds to account for our experiences in this one. It's sometimes claimed that this makes it unnecessarily extravagant, violating Ockham's razor, the idea that the simplest approach is preferable.
American physicist Bryce DeWitt admitted having these reservations when he first read about Everett's theory. He stated:
"I still recall vividly the shock I experienced on first encountering this multiworld concept. The idea of 10100+ slightly imperfect copies of oneself all constantly spitting into further copies, which ultimately become unrecognisable, is not easy to reconcile with common sense".
However, proponents of the Everett approach reject the idea that it contradicts Ockham's razor and argue that, in fact, Ockham's razor favours the Everett approach.
Ockham's razor states that entities must not be multiplied beyond necessity, but this does not refer to the number of unobservable objects that a theory invokes. It refers to the number of mutually independent assumptions that a theory makes, and their individual complexity.
The Everett approach is simpler in Ockhamist terms because it solves all of the problems faced by the collapse and Bohm approaches without adding extra structure to the theory of quantum mechanics. It is also mathematically simpler to describe a superpositional universe than to define all of the eccentricities of any particular one.
Everett responded to DeWitt's worries in a letter written in 1957. Everett stated that:
"A crucial point in deciding on a theory is that one does not accept or reject the theory on the basis of whether the basic world picture it presents is compatible with everyday experience. Rather, one accepts or rejects on the basis of whether or not the experience which is predicted by the theory is in accord with actual experience.
Let me clarify this point. One of the basic criticisms levelled against the Copernican theory was that 'the mobility of the earth as a real physical fact is incompatible with the common sense interpretation of nature.' In other words, as any fool can plainly see the earth doesn't really move because we don't experience any motion. However, a theory which involves the motion of the earth is not difficult to swallow if it is a complete enough theory that one can also deduce that no motion will be felt by the earth's inhabitants (as was possible with Newtonian physics). Thus, in order to decide whether or not a theory contradicts our experience, it is necessary to see what the theory itself predicts our experience will be.
Now in your letter you say, 'the trajectory of the memory configuration of a real physical observer, on the other hand, does not branch. I can testify to this from personal introspection, as can you. I simply do not branch.' I can't resist asking: Do you feel the motion of the earth?".
Everett argued that it may be easier for us to believe that the Earth is stationary than to learn Newtonian physics, but in doing so we are making many assumptions that cannot be verified. Newtonian physics is preferable to the idea that the Earth is stationary because it can be derived from very few basic assumptions, and explains a wide variety of phenomena.
If we reject theories on the basis that they postulate unobservable entities, then we are taking an instrumentalist approach to science. This may involve rejecting the existence of anything that we cannot verify with our own eyes, including objects such as bacteria and the moons of Jupiter.
Instrumentalism has not been acceptable in astronomy since the 1600s, and also seems implausible when applied to fields such as chemistry and biology, which manipulate microscopic objects in order to produce macroscopic results.
Everett stated that objections to his approach on the grounds that it contradicts common sense seem:
"to be founded on the notion that the only purpose of a theory is to serve as a summary of known data, and overlooks the second major purpose, the discovery of totally new phenomena. The major purpose of this viewpoint appears to be the desire to construct perfectly 'safe' theories which will never be open to contradiction"[4a].
Everett concluded that:
"strict adherence to such a philosophy would probably stifle the progress of physics"[4b].