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(Reindirizzamento da Interpretazione a molti mondi della meccanica quantistica)
L' interpretazione a molti mondi (abbreviata spesso in MWI , dall'inglese Many Worlds Interpretation ) è una delle strade nate per dare una spiegazione al significato ultimo della meccanica quantistica [1] . L'interpretazione in questione ha visto la luce nel 1957 by the physicist Hugh Everett III [2] and has since met with moments of strong resonance, as well as moments of utter oblivion. It must be said that until not long ago (and to some extent still today) the idea was branded by the stigma of more "too absurd to be true." Just think, for example, that in most basic texts in quantum mechanics this topic is not even touched it for more interpretation by presenting the "classical" - that usually interpretation of Copenhagen - as if it were the one and only chance.
The collapse of the quantum state [edit]
In the '20s to those who may well be defined on the pioneers of quantum theory, was presented a dilemma far from trivial: if indeed any physical system is completely determined by a vector in a Hilbert space (and this is the basic postulate of quantum mechanics), then linear combinations of vectors are "good" states for a system; this is just the principle of superposition, which is far from questioned, given the huge amount of experimental evidence which has had over the decades.
However, if this principle is valid then why were only observed in nature is never defined and strange combinations of states? It was the same Schrödinger who first raised the question: if microscopic objects like electrons can be combined because of different states should not be so also for the gross? After all, just think of any event "purely quantum", such as the decay of a metastable state, which affects a "classic" or not as the death of a cat. The famous thought experiment of Schrödinger's cat makes us think of the problem in all its obviousness.
The "recipe" to get out of this impasse is the Copenhagen interpretation: the measure, the act of the observer "breaks" the evolution of quantum dynamics (driven from the Schrödinger equation) and causes the collapse of the quantum state : the observer will see a defined state for the system (the cat alive or dead) and not a combination of states where the measure has shown the system in a specific state. What is the state where the system collapses is known only probabilistically, as first suggested by Max Born. After adding this postulate, it eliminates the problem of why nature seems to classical. "
So far nothing new dato che quella che si è brevemente descritta è l'interpretazione "ortodossa".
L'interpretazione di Everett [modifica]
L'idea di Everett parte da una premessa davvero semplice: in effetti si tratta semplicemente di rimuovere il postulato del collasso quantistico. Quello che potremmo chiamare il postulato di Everett (anche se in realtà è più un non-postulato) si può enunciare banalmente: tutti i sistemi isolati evolvono secondo l'equazione di Schrödinger.
Questo postulato riproduce esattamente le stesse previsioni, per un'operazione di misura, dell'interpretazione di Copenaghen. Vediamone un esempio.
Supponiamo di dover eseguire una misura di spin per un sistema fisico con spin 1 / 2 e denotiamo con e le proiezioni dello spin sull'asse z. Ipotizziamo di essere contenti se troviamo spin up e tristi se invece misuriamo spin down: possiamo quindi denotare con
gli stati in cui abbiamo misurato spin up, spin down e prima della misura, rispettivamente. Lo stato iniziale del sistema sarà in generale una combinazione del tipo
(dove α,β sono in generale numeri complessi), mentre l'effetto dell'osservazione sul sistema deve essere implementato da un operatore unitario di evoluzione, precisamente
con τ tempo caratteristico di risposta del sistema e H è l'Hamiltoniana the whole system-observer. From the above it must be inferred
and therefore, according to the idea of \u200b\u200bEverett:
that the resulting state is a combination of us happy to have found spin up and sad for us to have found spin down.
This means that after the measurement there are two observers: one that received the spin up and the other received the spin down. That is, the universal wave function would count a huge number of branches in different "perceived reality" that have been called just many worlds. This is a result of the interpretation of Everett that has caused the strong skepticism of the scientific community towards the MWI.
However it should be recognized that, once digested the surprise initially test the consequences of the MWI, the theory is without doubt an elegance and surprising simplicity. It should be noted that the interpretation of Everett plays exactly the same predictions of the Orthodox Church. Probabilism limitation inherent in the Born and the school in Copenhagen (the "God play dice" Einstein's) is replaced by a behavior that apparently is probabilistic, but intrinsically it is perfectly deterministic: each observer after a measure is unaware of his alter ego and what they perceived, from his point of view, nature is random. In terms of outward instead - that is, from a viewpoint that is independent of the observer himself - before the measure was perfectly able to say what will happen, simply by applying the Schrödinger evolution.
not clear [edit]
Clearly the matter is not exhausted here, in fact it is natural to ask why Nature should always be observed in macrostates which are eigenstates of the operator position or momentum eigenstates and not other operators. This is a serious problem of quantum theory, which in reality is not peculiar to one MWI, but it is broader. Only recently has found that there is a mechanism known as quantum decoherence, which seems to give a clear and elegant answer to the question.
But this is not the only "hiccup". For example, it is said that the MWI is a deterministic theory instead of quantum mechanics "orthodox." This is technically correct, but if you look closer you realize that, ultimately, nothing changes: the MWI is deterministic only in terms of the universal wave function, ie a hypothetical observer could follow the evolution of all worlds, for a real observer, however, the theory has the same uncertainty that we have used quantum mechanics. However, this observation can be answered that the two uncertainties non sono proprio uguali: quella dell'interpretazione di Copenaghen è ontologica essendo parte stessa della natura, quella dell'interpretazione a molti mondi è invece solo gnoseologica, poiché è indeterminato solo ciò che noi sappiamo.
Un altro problema piuttosto evidente è che l'interpretazione non risponde alla domanda importante sul meccanismo fisico secondo il quale i mondi si diramerebbero, e neppure spiega come questo possa essere in accordo con principi altamente condivisi come la conservazione dell'energia ecc...
Ci sono inoltre numerosi altri "problemi tecnici" e anche di natura più "filosofica" che rendono questa interpretazione (come tutte le altre) non universalmente accettata dalla scientific community. See the bibliography for the opportunity to delve.
^ See also the entry: Interpretation of quantum mechanics
^ See this site for a detailed biography
Interpretation of quantum mechanics, quantum decoherence
Multiverse
Collapse of the function d 'wave
time evolution operator of quantum mechanics
Wave function
Suicide quantum
http://it.wikipedia.org/wiki/Interpretazione_a_molti_mondi_della_meccanica_quantistica
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