Friday, 22 January 2016

Measurement problem: A big thorn at the heart of quantum mechanics

Quantum Mechanics blurs the distinction between the observer and the observed. Even after eighty decades since the inception of Quantum Mechanics the measurement problem is not resolved and still haunts the theory in every imaginable way. Physicists however have no problems in continuing to carry out their experiments and making predictions from quantum theory because they can safely ignore the measurement problem for all practical purposes.

Schroedinger devised his cat paradox to show the absurdity of the Copenhagen Interpretation which was the instrumentalist view of Neil Bohr. It was here that he coined the term entanglement. The quantum state of the radioactive atom |psi> = |decayed> + |not decayed> and the quantum state of the poisonous gun |psi> = |fired> + |not fired> and the quantum state of the cat |psi> = |dead cat> + |live cat> are all entangled and exist in a superposition of states and evolve according to the Schroedinger equation. Quantum particles behave like ghost particles where each particle passes through both the slits at once and tunnel through walls and change their properties according to the context of the choice of measurements we make on them. Matter exists in a state of potentialities and not in a state of actualities. 

If this is the true nature of matter then why does our world which is made up of matter appears to us in a state of actualities rather than in a state of potentialities. What prevents billiard balls from being smeared out as waves and existing at multiple locations all at once or what prevents them from tunnelling through walls? Physicists have sought to answer this question in many ways while some propose decoherence through environment and while others speculate the existence of parallel worlds and few others taking the wavefunction to be real postulate that the weavefunction collapses to produce a specific outcome which can be recorded in the form of detector clicks and events none of these answers solve the question in hand. Decoherence theory fails to account for Quantum interference observed in molecules as big as Buckminster fullerenes which are so hot so much that it is never isolated from the environment. Quantum entanglement has been observed in room temperatures. The parallel worlds carry too much baggage and no experimental evidence supports them and it is highly unlikely for the wavefunction to be real and objective collapse theories are not the way to move forward.

The contextuality of Quantum Mechanics can be explained by a pair of colourless cosco balls. Let a bowling machine emit a pair of colourless cosco balls which will travel through different tunnels where it passes through a color detector at the each end of a tunnel. Let there be a mathematical constraint in the theory such that if you see one pair of a ball having one color the other ball at the other end of the tunnel should also have the same color. What quantum mechanics is saying to us is that matter does not possess any objective properties prior to measurements or in other words it is the very act of measuring the ball by the color detector which assigns a value say the color red to the ball in an absolutely random way, we just don't know in what color the ball will pass out through the tunnel, it may be red, pink, yellow, blue or green and there is no way of knowing it which is the presence of inherent randomness in the quantum system. The weirdness does not stop here the measurement made by the color detector not only assigns a color to the ball at this end of the tunnel it also instantaneously assigns a color value to the ball at the other end of the tunnel which might be hundreds of kilometres apart violating locality. Numerous experiments have confirmed this behaviour and quantum entanglement is an empirical fact now that measurements not only changes the system which is being measured here it also changes the system which may be situated at the other end of the universe.

All the strangeness, randomness and weirdness of Quantum mechanics is not because of the Schroedinger equation which is perfectly deterministic instead it is due to the measurement made on the quantum system. Every quantum physicist knows that they have to understand what is meant by measurement if they have any chance of clearly understanding quantum mechanics. Given the importance of measurement and observation in the theory of quantum mechanics which was recognized by Fritz London and Edmond Bauer who advocated for the advent of Phenomenology we certainly need to reconsider our notion of perception.

Below I will speculate a lot on how this measurement problem may be connected to the theories of perception in Indian philosophy.

“According to Western medical science, light vibrations from outside strike the retina and an inverted image is formed there. These vibrations are carried through the optic tract and optic thalamus to the centre of vision in the occipital lobe of the brain in the hind part of the head. There a positive image is formed. Only then we see the object in front of us. The Vedanta theory of perception is that the mind comes out through the eye and assumes the shape of the object outside.” (Mind and Its Mysteries: p. 70)

“The mind assumes the shape of any object it intensely thinks upon.” “When you pass through a mango garden, a ray of the mind comes out through the eye and envelops the mango. It assumes the shape of the mango. The ray is termed a Vritti. The enveloping process is called Vritti-vyapti. The function of a Vritti is to remove the Avarana (veil) that envelops the object and the Upahita-chaitanya (consciousness defined by an adjunct). The veil that envelops the mango is removed by the Vritti or the mental ray. There is Chaitanya (consciousness) associated with the Vritti. This Chaitanya illuminates the object ‘mango.’ This result is termed Phala-vyapti. Just as a torch-light illuminates an object in a flash, this Vritti-chaitanya (consciousness conditioned by the mental mode) illumines the object. Only then does perception of the mango take place” (Mind and Its Mysteries: P. 194). “According to the Advaita theory of perception, it is the Chaitanya within us that makes perception possible. The Chetana (intelligence) within us unites with the Chetana (intelligence) in the object, and the result is perception. It does not follow from this that the mind and the senses are useless,…..for they serve the purpose of determining the special object of each sense” (Ibid, p. 205)


Indian philosophy does recognize the existence of a physical world independent of observers which is made up of five elements: fire, earth, water, air and space called as Panchabhoothas and all the objects including our bodies which we can see are made from the mixtures of these five elements. The non-physical mind along with its sense organs which exists in the subtle body reaches out to the external objects made up of Bhoothas and alters the Bhoothas giving rise to the appearance of a classical world and these Panchabhoothas does not possess any objective properties with in themselves and all objective and cognitive properties of an object like speed, position, momentum, spin, angular momentum, velocity, mass, color, smell, dimension and time etc etc are assigned to this object by the mind and in turn alters the state of these Panchbhoothas. It is not that reality is what you make it or that you can change your reality just by observing it differently instead reality is what has been constructively given to us by the external non-physical mind. We don't construct reality inside our brains in fact brain is just a construct of the mind.

Immaneul Kant long argued this in his theory of transcendental Idealism where the categories of the mind constructs the phenomenal reality including the space-time continuum and quantum mechanics upholds this view more strongly than anything ever has.

"Kant taught that space and time are not part of external reality but are rather preexisting structures in our minds that allow us to relate objects and events."

- Steven Weinberg

So again why does billiard balls appears to us to always possess objective properties irrespective of whether we observe them or not while the same is not true for electrons and photons when both billard balls and electrons and photons are made from the same Panchabhoothas? Caslav Brukner and Kofler have explained the emergence of the classical world from the quantum world in their paper answering this very question.

Do classical laws arise from quantum laws?
A coarse-grained Schrödinger cat
“The physics community is mostly divided into two groups,” Johannes Kofler tells PhysOrg.com. “One group believes that quantum theory is underlying the classical world, and that classical physics comes from the quantum. The other group thinks that quantum physics has to be altered. It forbids that quantum mechanics works on a macro level in the classical world by postulating additional laws.”

Kofler belongs to the former group


Read more at: http://phys.org/news/2007-11-classical-laws-quantum.html#jCp
“The physics community is mostly divided into two groups,” Johannes Kofler tells PhysOrg.com. “One group believes that quantum theory is underlying the classical world, and that classical physics comes from the quantum. The other group thinks that quantum physics has to be altered. It forbids that quantum mechanics works on a macro level in the classical world by postulating additional laws.”

Kofler belongs to the former group


Read more at: http://phys.org/news/2007-11-classical-laws-quantum.html#jCp

“The physics community is mostly divided into two groups,” Johannes Kofler tells PhysOrg.com. “One group believes that quantum theory is underlying the classical world, and that classical physics comes from the quantum. The other group thinks that quantum physics has to be altered. It forbids that quantum mechanics works on a macro level in the classical world by postulating additional laws.”

Kofler belongs to the former group


Read more at: http://phys.org/news/2007-11-classical-laws-quantum.html#jCp
“The physics community is mostly divided into two groups,” Johannes Kofler tells PhysOrg.com. “One group believes that quantum theory is underlying the classical world, and that classical physics comes from the quantum. The other group thinks that quantum physics has to be altered. It forbids that quantum mechanics works on a macro level in the classical world by postulating additional laws.”

Kofler belongs to the former group


Read more at: http://phys.org/news/2007-11-classical-laws-quantum.html#jCp




 Above image taken from Caslav Brukner and Kofler paper.

So the classical world is classical because the mind by making coarse grained measurements on external objects has constructed a classical world to us. This view obviously introduces problems for quantum cosmology and also a classical detector can act as a measuring device and there is no need for conscious observers or human beings. So does this mean that even measuring devices have a mind of their own and this leads to some kind of panpsychism? I do not wish to speculate further on this path but the world might be like this who knows but the conclusion of this article is very much this:
It from bit. Otherwise put, every 'it'—every particle, every field of force, even the space-time continuum itself—derives its function, its meaning, its very existence entirely—even if in some contexts indirectly—from the apparatus-elicited answers to yes-or-no questions, binary choices, bits. 'It from bit' symbolizes the idea that every item of the physical world has at bottom—a very deep bottom, in most instances—an immaterial source and explanation; that which we call reality arises in the last analysis from the posing of yes–no questions and the registering of equipment-evoked responses; in short, that all things physical are information-theoretic in origin and that this is a participatory universe.

-- Wheeler, John A. (1990), W. Zurek, ed., "Information, physics, quantum: The search for links", Complexity, Entropy, and the Physics of Information (Redwood City, California: Addison-Wesley)