As the myth about the “role of observer”, “the loss of objective reality” , that was initiated by the use of the word "measurement" in Copenhagen interpretation spread out not only among physicists but in all areas of human thinking because of its philosophical consequences(I will discuss them in “reflections”) , the majority of physicists accepted the Copenhagen interpretation as it is without bothering about the philosophical consequences. They thought that the world at microscopic scales doesn’t have to be fully imaginable with our terminology since our terminology (wave. particle) reflects our experience in macroscopic world. They thought “we have to live with this mystery without further making lengthy philosophical discussions on it”. They said “Copenhagen interpretation gives us a framework where we can make our calculations and compare the experimental results with the calculations. That is what one can all want from a theory. All these philosophical discussions about the nature of duality and about the role of the observer in measurement process don’t affect how we do our work so why bother about these things. Our framework is enough for our purpose of doing science. “

Although majority gave up to discuss these issues, some took the problems of Copenhagen interpretation serious and tried to find a cure for the iatrogenic problem 1 by proposing alternatives to Copenhagen interpretation.

The entanglement of distant particles presents a problem for hidden variable theories. The entanglement of photons in EPR paradox means that there cannot be two separate independent hidden variables for each photon since the outcomes of spin measurements are correlated. (see chapter 8)

One alternative to conceal wave and particle aspects is to assume that waves are like force fields guiding the pointlike particle. This approach was originally the idea of De Broglie and is known as Pariser school or pilotwave approach. The most successful of these theories is Bohm’s theory. It consists of two axioms ².

It is criticized mainly because of this nonlocality. I think however that nonlocality is a fact (see chapter 8) that can not be explained away by considerations like “although there are nonlocal correlations there is no possibility of signal transfer at superluminal speeds since the quantum events have a random nature” . Or by stating that wave function is merely a mathematical construction ^{3, 4}. Therefore it is unjust to eliminate Bohm's model merely because of the inherent nonlocality. We think however there is not enough direct evidence for the existence of pointlike entities since the bright dots on the scintillating screen can well be interpreted as transition of the wave-function from a spatially extended form to a localized form around a nucleus as Schrodinger suggested. In chapter 13 We will rediscuss the objections against Schrodinger's viewpoint mentioned in chapter 6 and try to show that they are unjustified prejudices or pseudoproblems and that they are not unsolvable.

Contrary to the assumption for fermions (material particles like proton electron etc) Bohm's model assumes bosons(photon graviton etc.) as fields without pointlike entity. It argues that despite the missing of pointlike entities the pointlike hits observed in photon detection process can be understood as a consequence of instability and an extreme sensitivity to initial conditions where depending on initial conditions only one of the atoms in the screen absorbs the photon ⁵. As I will discuss in chapter 14 and chapter 15 I think that a similar kind of instability occurs with electron beams when interactin with the atoms in the scintillating cristall so that we don’t need the pointlike entities for fermion fields either .

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One of the suggestions that at first look seem to eliminate subjectivism and indeterminism altogether is Everetts many worlds interpretation.^{6, 7} According to this view, in a measurement all the probabilities that are not realized in our universe are in truth realized in other parallel universes. There are two ways to imagine these parallel universes. Either a universe splits into many universes at the very moment of measurement i.e. these universes are created at this moment or all these universes already exist together with the common history until the very moment of measurement but they take different histories after the measurement. In The first view a measurement is some very special event class creating new universes, But like the Copenhagen interpretation many worlds interpretation cannot define the objective circumstances that lead to such a special event. The problem is similar to the problem in the objectivist version of Copenhagen interpretation where the role of observer is eliminated but no other consistent criteria (objective criteria) is proposed for defining the conditions when the unitary evolution of the wave function is interrupted . The second view with already existing parallel universes imply that for any possible slightest difference in the history and in the future , a separate universe already should exist at the very beginning. Even in this case the measurement remains as a special event class in the sense that it is the only event class that creates different results in different universes while non measurement events occur with exactly same course in parallel universes. Thus many world interpretation cannot give the objective criteria defining the conditions that leads to such a splitting of histories.

Figure 12 two succesive measurements of spin with Stern Gerlach measuring devices with a nonzero relative alignement angle between the magnetic fields of the two measuring devices

There are 2²⁰ possible combinations. 2²⁰ (roughly 69 billion) universes must be involved. Since the relative alignement of the two measurement apparatus is the same in each universe under consideration we should expect for each universe that the ratio of (number of spin up outcomes / number of spin down outcomes) should be close to 3 ( ¾ divided by ¼). especially for large number of measurements . Of course it doesn't mean that there are no odd universes with more downs and less ups. Ours could be one of these. For small number of measurements it is not so unlikely for a universe to be an odd universe. However it should become less and less probable for a universe to remain an odd universe as the number of measurements increase. Although the number of the ODD universes increase with increasing number of measurements since the number of ALL possible universes to be considered increase the ratio (number of ODD universes / number of ALL universes) should go to 0 if number of measurements increase. We expect that in the limit of very large numbers in every universe the ratio of (number of spin up outcomes / number of spin down outcomes) asymptotically approaches the value 3

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The consistent histories approach takes the results of measurement events as facts and tries to find an history for the evolution of the wave function that is consistent with the outcomes of these measurements ⁸.

A photon wave packet a enters a beam splitter B1 (an half silvered mirror that reflects half of the incident light and lets the other half pass through) The ordinary Quantum mechanics tells us that the wave function of even a single photon is split into two parts |c > and |d > . The photon is detected either by detector E or by detector F so that the photon undergoes a transition either to the state |e > or the state |f > . The detection by detector F implies that the wave function part |d> must suddenly disappear during the detection and join the part |c > in the detector F to create |f >. Thus superluminal currents must be associated with the detection process as discussed in chapter 6 if one interprets the wave function as a real physical field.

|a > ---> 1/Ö 2 (|c > + |d >) -----> |f > (detection by F)

|a > ---> 1/Ö 2 (|c > - |d >) -----> |e > (detection by E)

on the other hand the history |a > -----> |c > -----> 1/Ö 2 (|e > + |f >) is also consistent. But since the photon can be detected by only either E or by F, but not by both detectors simultaneously , this history is not physically realized.

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This approach focuses on the collapse process during the measurement and tries to describe the collapse process mathematically. It uses the so called the density matrix representation. The offdiagonal elements in the density matrix represent the quantum correlations without classical analogy. During the collapse process the offdiagonal elements vanish. Zurek tried to show that if one takes the effect of the environment on the combined system quantum system + measuring device one can understand how the offdiagonal elements in density matrix vanish because of decoherence so that we are left with a classical probability distribution given by the diagonal elements. ^{9,10,11 .}The superposition of dead and alive states of the Schordingers cat is unstable . It either becomes a dead cat or an alive cat very rapidly. (in about 10^-19 seconds) The vanishing of the offdiagonal elements is however only one aspect of the collapse process. At the end of the measurement only one of the diagonal elements is realized. Which one of the diagonal elements is realized can be predicted only probabilistically where the probability is given by the value of the diagonal element. Thus although decoherence describes while a superposition of dead and alive states of the Schrodinger’s cat is unstable it doesn’t provides us the answer if the final state will be the dead state or alive state. The decoherence approach assumes the existence of a macroscopical measuring device. The indeterministic transitions occur however in purely microscopical events like spontaneous emission and radioactive decay. Thus although decoherence is useful to understand how some properties of the macroscopic world emerge ¹¹ it cannot explain the mechanism of the apparently indeterministic crucial step that, so I think, is the same independent if there is a macroscopic device present or if we have a purely microscopical process like spontaneous emission.

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It may appear to the knowledgeable reader surprising that I will discuss QED and TPDT under the heading alternative interpretations. Everyone believes that they are extensions of ordinary quantum mechanics without a new interpretation. Yet there is an important terminology shift that has occurred and that is not widely recognized as such .

The calculations showed that a radiation of frequency w could effect a system of charged particle(s) in two ways: ¹²

Figure 15 - a system with the energy eigenstates |1> |2> and |3> equidistantly located on the energy scale where the difference between two neighboring states is hw . Initially the system is in the intermediate state |2 > and a photon of the energy hw is present.

Lets assume that the system is in the state |2> initially and radiation of the frequency w is present . According to the Schrodinger equation alone, both processes described above occur simultaneously because of the linearity of the equation and the contribution of both the upmost and lowest states should increase continuously simultaneously . So that the contribution of |2 > decreases and the system proceeds towards a superposition of |1> and |3> in the form a|1> + b|3>. But in reality the system doesn’t do that but it either makes a transition to |1> by emitting a photon or a transition to |3> by absorbing a photon. So is the prediction of Schrodinger equation wrong?

Later the field quantization techniques were developed (see chapter 10). The concept of photon, that was born as a vague concept in order to explain the black body radiation and photo electric effect, established itself and got a sound theoretical basis. The interaction of radiation with matter was described as interaction between quantized fields. The resulting theory is called Quantum electrodynamics(QED) . All interactions with electromagnetic field is considered as absorption and emission of photons. Even the interaction with static field is considered as exchange of so called virtual photons ¹³ The field quantization has legitimated the terminology of the TDPT namely that “either this or that transition can occur but not both simultaneously” by asserting that a photon can either be emitted or absorbed but not both can happen simultaneously or by saying that half of the photon can not be emitted or absorbed.

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Notes and references