Entanglement

UNANSWERED QUESTIONS

Quantum information technology has progressed in leaps and bounds over the last few years. We are starting to witness a frantic effort by companies to make quantum computers commercially available. IBM, Google, and Microsoft—all have very strong research programs involving quantum computers. Recently scientists have taken a major step toward realizing the ultimate goal of a quantum internet where exchanging ultra-secure encrypted messages would be a reality. Today, quantum information technology is not in a “if” stage any more, the question is “when?”.

All of this started with the phenomenon of entanglement and Einstein’s efforts to come to terms with the notion of reality. The modern intellectual description of reality is quantum mechanical in nature and the world is indeed stranger than anything our rational mind can grasp. It therefore seems fitting to conclude this article by reflecting upon the philosophical question: what is reality?

Earlier in this article, we have talked about local realism, a common sense premise introduced by Einstein, Podolsky, and Rosen (EPR) and later considered by Bell and others in the context of the now famous EPR argument leading to Bell’s famous theorem. In 1984, Arthur Fine proved that observing Bell inequalities are both necessary and sufficient conditions for local realism. These inequalities were repeatedly violated in experiments with entangled particles conducted during the past decades, ruling out local realism in quantum systems (for example, Alain Aspect’s measurements were performed with single photons). Quantum mechanics does not allow transmission of information faster than the speed of light—a violation would be in contradiction with causality, whose implication is allowing communications into the past. In Anton Zeilinger’s words “…on a fundamental level, quantum randomness keeps entanglement from violating Einstein’s own theory of relativity. The reason is that although the observations on two entangled particles are perfectly correlated, this cannot be used for sending messages faster than the speed of light, because the observers have no control over what the measurement outcome will be, a result of its randomness.”

randomness
Source: Centre for Quantum Technologies.

However, the validity of local realism at a more macroscopic level is still an open question. Quantum effects are rarely observed in our ordinary macroscopic world. Schrödinger’s cat perhaps best illustrates the odd results of applying quantum mechanics to macroscopic objects, where a cat can be put into a superposition state of being both dead and alive. Clearly, quantum predictions are in stark contradiction to our everyday experience and a number of questions regarding the boundary between the quantum and the classical world still remain unanswered.

The notion of reality at the macroscopic level implicitly makes two assumptions:

  • Macroscopic realism: A macroscopic system with two or more macroscopically distinct states available to it will at all times be in one or the other of these states.
  • Noninvasive measurability at the macroscopic level: It is possible, in principle, to determine the state of the system with arbitrarily small perturbation on its subsequent dynamics.

In a classical world, objects have pre-existing properties, physical influences are local and cannot travel faster than the speed of light (local realism), and, in principle, it is possible to measure the properties of a macroscopic system without altering its state (macroscopic realism).

In 1985, Anthony Leggett and Anupam Garg defined a set of inequalities similar to Bell’s. But in contrast to the Bell inequalities which probe entanglement between systems separated in space, the Leggett–Garg inequalities test the correlations of a single system measured at different intervals of time. Originally designed as a test for quantum coherence in macroscopic systems, a violation of a Leggett–Garg test imply either the absence of realism or the impossibility of measuring a system without disturbing it. Not surprisingly, Leggett-Garg inequalities were seen to be violated in a variety of microscopic systems such as superconducting qubits, nuclear spins, and photons. But whether similar violations occur in macroscopic systems is unknown at this time and is one of the most exciting open questions in the foundations of physics. And also open is the question whether the laws of quantum mechanics are universally valid and hold for macroscopic objects as well.

Clearly, quantum mechanics has come a long way since the famous Bohr-Einstein debates concerning reality. Einstein was never persuaded by Bohr’s arguments and he was instinctively convinced of some fundamental conceptual weaknesses in quantum theory which disturbed him throughout his life. Often in the history of science we see instances where scientists turn to philosophy for answers to fundamental questions and Einstein was no different—his keen interest in philosophy is well known.

Einstein’s discussions with Rabindranath Tagore, the famous Bengali poet and Nobel Laureate, in 1930 concerning the nature of reality and the relationship of determinism to free will is a noteworthy event, not merely a “tiny footnote in the history of quantum theory.” During the early 1900s, Tagore was well known in the West for his poetic virtuosity, his philosophical views, and his “aura of oriental wisdom”. In the decades of 1920s and 30s he met with Einstein several times in Europe when Einstein was grappling with his own views about realism. Written accounts about their discussions on the nature of reality and the “relationship of determinism to free will” were published by the New York Times and by Tagore himself in 1931 in his book The Religion of Man, with an appendix including his conversation with Einstein.

Tagore’s position on reality is based on consciousness. Here is an extract from their conversation (as vetted by Einstein himself) that appeared in the New York Times (an excellent account of Einstein and Tagore’s dialog can be found in Home & Robinson, Einstein and Tagore: Man, Nature and Mysticism, 1995):

  • Einstein: There are two different conceptions about the nature of the universe – the world as a unity dependent on humanity, and the world as reality independent of the human factor…
  • Tagore: This world is a human world – the scientific view of it is also that of the scientific man. Therefore, the world apart from us does not exist; it is a relative world, depending for its reality upon our consciousness17.

Does consciousness somehow play a crucial role in the fundamental laws of physics? Einstein did not think so18. but the dialog between the two great thinkers managed to kindle interest among physicists and philosophers alike. Tagore’s view is metaphysical and perhaps untestable at this time, although interestingly, a handful of renowned scientists have expressed their support in its favor. Notable is lIya Prigogine, the 1973 Nobel Laureate in chemistry, who went on record to say: “Curiously enough, the present evolution of science is running in the direction stated by the great Indian poet.” (Prigogine and Stengers, 1984, p. 293).

According to Stuart Hameroff and Roger Penrose, consciousness resides in the microtubules of the brain cells which are the primary sites of quantum processing. The Penrose-Hameroff model (orchestrated objective reduction: “OrchOR”) suggests that quantum superposition and a form of quantum computation occur in microtubules—quantum computation with objective reduction may be involved in consciousness.

Needless to say these ideas are controversial and perhaps too extravagant for mainstream physics. But a flurry of activities has already started because when influential people like Sir Roger Penrose speak, others take notice. Often great ideas are controversial, or have been at one time.

It will be interesting to see as our knowledge of the functioning of the brain gets profounder and as we obtain deeper insights into quantum mechanics and molecular biology, whether consciousness will be understood in terms of the existing laws of physics and whether quantum mechanics has a part to play in the origins of consciousness.


17. Tagore’s philosophy about consciousness is brilliantly summarized in his immortal poem:

tagore-poem

18. Most of the quantum theoreticians agree. Sean Carroll in From Eternity to Here: The Quest for the Ultimate Theory of Time remarked “Surely we don’t want to suggest that the phenomenon of consciousness is somehow playing a crucial role in the fundamental laws of physics? (No, we don’t.)”

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