Saturday, March 16, 2013

Quantum theory...totally understood?

My response--no.

"Will we ever… understand quantum theory? "

Philip Ball

January 25th, 2013




That’s because quantum theory poses all sorts of strange questions that stretch the limits of our imagination – forcing us, for example, to conceive of objects like electrons that can, in different circumstances, be either waves or particles.

One of the most controversial issues concerns the role of measurements. We’re used to thinking that the world exists in a definite state, and that we can discover what that state is by making measurements and observations. But quantum theory (“quantum mechanics” is often regarded as a synonym, although strictly that refers to the mathematical methods developed to study quantum objects) suggests that, at least for tiny objects such as atoms and electrons, there may be no unique state before an observation is made: the object exists simultaneously in several states, called a superposition. Before measurement, all we can say is that there is a certain probability that the object is in state A, or B, or so on. Only during the measurement is a “choice” made about which of these possible states the object will possess: in quantum-speak, the superposition is “collapsed by measurement”. It’s not that, before measuring, we don’t know which of these options is true – the fact is that the choice has not yet been made.

This is probably the most unsettling of all the conundrums posed by quantum theory. It disturbed Albert Einstein so much that he refused to accept it all his life. Einstein was one of the first scientists to embrace the quantum world: in 1905 he proposed that light is not a continuous wave but comes in “packets”, or quanta, of energy, called photons, which are in effect “particles of light”. Yet as his contemporaries, such as Niels Bohr, Werner Heisenberg and Erwin Schrodinger, devised a mathematical description of the quantum world in which certainties were replaced by probabilities, Einstein protested that the world could not really be so fuzzy. As he famously put it, “God does not play dice.” (Bohr’s response is less famous, but deserves to be better known: “Einstein, stop telling God what to do.”)

Wonderful, wonderful Copenhagen

Schrodinger figured out an equation that, he said, expressed all we can know about a quantum system. This knowledge is encapsulated in a so-called wavefunction, a mathematical expression from which we can deduce, for example, the chances of a quantum particle being here or there, or being in this or that state. Measurement “collapses” the wavefunction so as to give a definite result. But Heisenberg showed that we can’t answer every question about a quantum system exactly. This is Heisenberg’s uncertainty principle: the more precisely you determine an electron’s momentum (as measured by mass multiplied by velocity), the less you can know about its position in space, and vice versa. In other words, there are some pairs of properties for which an increasingly accurate measurement of one of them renders the other ever fuzzier.


Bohr, Heisenberg and their collaborators put together an interpretation of quantum mechanics in the 1920s that is now named after their workplace: the Copenhagen interpretation. This argued that all we can know about quantum systems is what we can measure, and this is all the theory prescribes – that it is meaningless to look for any “deeper” level of reality. Einstein rejected that, but nearly two-thirds of those polled in Austria were prepared to say that Einstein was definitely wrong. However, only 21% felt that Bohr was right, with 30% saying we’ll have to wait and see.



So will we ever get to the bottom of these questions? Some researchers feel that at least some of them are not really scientific questions that can be decided by experiment, but philosophical ones that may come down to personal preference. One of the most telling questions in the Austrian poll was whether there will still be conferences about the meaning of quantum theory in 50 years time. Forty-eight percent said “probably yes”, only 15% said “probably no”. Twelve percent said “I’ll organize one no matter what”, but that’s academics for you.

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