Category:	Science and Technology
Domain:	Physics
Keywords:	Theoretical physics - quantum, entanglement, uncertainty, quantum computing, encryption, relativity
Outlook:	Progress in quantum theory may make it more understandable and useful : a better understanding of foundational issues in physics may support developments in nanotechnology, quantum computing, superconducting materials and many other fields.
Summary Analysis:	Einstein said of quantum mechanics that it seemed like a 'system of delusions of an exceedingly intelligent paranoiac.' Following its inception in the 1920s, quantum theory has led to the laser, MRI and other forms of medical imaging, the transistor, and the birth of nanoscience. However, more than 100 years after its birth, quantum theory -- considered along with relativity to be the foundation of modern physics -- still doesn't fit with other theories of physics. First of all, whereas in classical physics an object is either a particle or a wave, in quantum physics these building blocks of the universe can behave as either. The mathematics of quantum theory says that a quantum object, such as a subatomic particle, can be in two states at the same time, called a superposition of states. According to the Copenhagen Interpretation of quantum mechanics, observing the particle will ‘collapse’ the wavefunction that describes its state and force the particle into a determinate state. In 1935, Erwin Schrödinger pointed out a bizarre corollary of the Copenhagen Interpretation’s emphasis on the role of observation in quantum theory: if a cat in a box were somehow dependent upon a quantum event (for instance, because it will be poisoned if a radioactive atom decays, but will survive otherwise) then the cat too would be in a superposition of states – both alive and dead – until it was observed. Another classic counterintuitive result from quantum mechanics is Heisenberg’s indeterminacy principle, which states that it is impossible to have total knowledge of a quantum object. For pairs of observable quantities such as speed and location – the more precisely you know one, the less you can say about the other. For instance, if you precisely measured the speed of an electron you would not be able to determine where it was at all. But the counterintuitive result that has been receiving the most attention from physicists over the past decade is ‘entanglement’ – which Einstein described as ‘spooky action at a distance’. Pairs of quantum objects can have their properties entangled, which is to say that measurement of a property of one object forces the corresponding property of the other object in to a determined state. Even if the quantum objects are separated by light years, measuring one fixes the results of measurement on the other. For instance, if you measure the spin on one of a pair of entangled photons and notice its spin is ‘up’ then a measurement of the spin on the corresponding photon will be ‘down’. If the spins were fixed from the moment of entanglement then this would not be remarkable. The ‘spooky’ aspect of entanglement is that a photon’s spin is indeterminate until its measured – the spin on the first photon could just as easily have been ‘down’ when it was measured, which would have instantly forced the other photon’s spin to be ‘up’. A generation of physicists adopted a pragmatic stance towards quantum theory. They didn’t understand it so they didn’t think about it. The Nobel Prize-winning physicist Richard Feynman said in a famous lecture on quantum theory, "It is my task to convince you not to turn away because you don't understand it. You see, my physics students don't understand it either. That is because I don't understand it. Nobody does." The physicist David Mermin said of the Copenhagen Interpretation of quantum mechanics (which for decades represented the consensus) “What (it) says to me is ‘shut up and calculate’”. This strategy was very successful for a long time. Huge advances in physics were made despite (or perhaps because of) physicists’ reluctance to grapple with the foundations of their subject. Trying to understand quantum mechanics, rather than just ‘use’ it, was actively discouraged for a while. This is the source of the myth about Einstein never coming to grips with the subject. Ask most physicists about Einstein and they’ll tell you that, brilliant as he may have been when he was younger, he was too committed to a classical world view to embrace quantum mechanics. Some go further and suggest that Einstein never really understood quantum mechanics. This is the fairy story we are told as students and it is repeated so often that it tends to be taken on face value but his reputation as a quantum stick-in-the-mud is not deserved. Einstein was far more willing than Bohr and Heisenberg to give up classical concepts. Contrary to the myth, Einstein didn’t challenge quantum mechanics. His great crime was to challenge the emerging consensus on the matter: the Copenhagen Interpretation. Foundational issues in physics have experienced a revival in interest recently – which vindicates Einstein’s scepticism. This interest has been inspired partly by new avenues of experimental research into entanglement and teleportation. Roger Penrose, an English mathematical physicist who has worked with Steven Hawking, has suggested a role for gravity in bringing about the ‘collapse of the wavefunction’ (the shift from a superposition of states to a single determinate state) which has opened up new experimental opportunities also. Quantum computing and cryptography is an exciting prospect (though at a very early stage of development) and has also been inspiring new approaches to quantum mechanics. Also, the problem of reconciling quantum mechanics with general relativity and the development of overarching ‘theories of everything’ such as ‘sting-theory’ and ‘brane-theory’ is driving new approaches to the foundations of physics. Partly as a result of these developments, it is more acceptable now to diverge from the old consensus position of the Copenhagen Interpretation and consider alternatives such as the ‘many worlds’ interpretation, in which wavefunctions do not ‘collapse’ – rather the universe splits when an observation is made of a quantum object in a superposition of states. (This approach, it has been noted, is cheap on assumptions but expensive on universes.) Increasingly sophisticated approaches to the interpretation of quantum mechanics are emerging. Perhaps within the next few decades, we will actually understand one of them.

	Implications:	Deeper understanding of the nature of reality Increased knowledge about the history and future of the universe Innovations in nanotechnology Next-generation paradigms for computing Novel methods of encryption Dramatically faster telecommunications New models of consciousness
	Early Indicators:	Demonstration by Caltech physicists in 1998 of quantum teleportation Establishment by BBN Technologies in 1994 of the first computer network secured by quantum cryptography Receipt by UC Berkeley in 2003 of $4.5 million from the US National Science Foundation to analyze the practicality of quantum computers Construction by University of Bonn researchers in 2004 of the fundamental memory component of a computer out of a string of atoms trapped in a laser beam Receipt of venture funding by D-Wave, a Vancouver-based company that aims to develop quantum computing platforms Successful attempt in 2001 by researchers at the Harvard Smithsonian Center for Astrophysics, Harvard University, and the Rowland Institute to stop light in its tracks Successful attempt in 2004 by a team led by physicists from UC Berkeley to slow the speed of light as it traveled through a semiconductor
	What to Watch:	Breakthroughs in theoretical physics lead to new experiments. Increased funding from the private sector goes to quantum computing, quantum encryption, and nanotechnology.
	Parallels/Precedents:	Einstein's formulation of his theories of relativity Search for a Grand Unified Theory, a 'theory of everything'
	Enablers/drivers:	The end of Moore's Law Demand for increased data security Interdisciplinary collaboration across myriad scientific and engineering disciplines
	Leaders:	Institutions: Max Planck Institute for the Physics of Complex Systems Pennsylvania State University at University Park Tata Institute of Fundamental Research in Mumbai NEC Research Institute in Princeton University of North Carolina in Chapel Hill MIT University of Washington Brookhaven National Laboratory National Institute for Research in Nuclear Physics in Padua, Italy Imperial College, London [link] University College, London [link] University of Edinburgh [link] University of Oxford [link] University of Manchester [link] University of Glasgow [link] University of Nottingham [link] Organisations Particle Physics and Astronomy Research Council (UK) [link] Institute of Physics (UK) [link]
	Figures:
	Sources:	Ferris, Timothy. The Whole Shebang. New York: Simon & Schuster, 1997. Folger, Tim. "If an Electron Can Be in Two Places at Once, Why Can't You?" Discover, June 2005. Seife, Charles. "Do Deeper Principles Underlie Quantum Uncertainty and Nonlocality?" Science, 1 July 2005. "Special Reports: Quantum World." New Scientist [link] Cushing, James T. 1994, Quantum Mechanics: Historical Contingency and the Copenhagen Interpretation, (Chicago and London: University of Chicago Press) ISBN: 0 226 13204 8 Zee, A and Bern, Z, 2004, "Quantum Field Theory in a Nutshell," Physics Today 57, 4 page 88

	At A Glance:	When:	11–20 years
		Where:	Global
		How Fast:	Years
		Likelihood:	Medium-High
		Impact:	Medium-High
		Controversy:	Medium

Related Outlooks: