In Aeon, quantum physicist Adrian Kent outlines the problem quantum physics presents to a scientific picture of reality:
Bell was one of the last century’s deepest thinkers about science. As he put it, quantum theory ‘carries in itself the seeds of its own destruction’: it undermines the account of reality that it needs in order to make any sense as a physical theory. On this view, which was once as close to heresy as a scientific argument can be but is now widely held among scientists who work on the foundations of physics, the reality problem is just not solvable within quantum theory as it stands. And so, along with the variables that describe potentialities and possibilities, we need to supplement our quantum equations with quantities that correspond directly to real events or things – real ‘stuff’ in the world.
Kent suggests that quantum collapse theories as put forward by Giancarlo Ghirardi, Alberto Rimini, Tullio Weber and Philip Pearle I th 1980s offer the best way forward:
Their approach became known as the ‘spontaneous collapse’ model and their brilliant insight was that we can find mathematical laws that describe how the innumerable possible outcomes encoded in a quantum description of an experiment get reduced to the one actual result that we see. As we have already noted, the tension between these two descriptions is at the heart of the quantum reality problem.
When using standard quantum theory, physicists often say that the wave function – a mathematical object that encodes all the potential possibilities – ‘collapses’ to the measured outcome at the end of an experiment. This ‘collapse’, though, is no more than a figure of speech, which only highlights the awkward fact that we do not understand what is really happening. By contrast, in Ghirardi-Rimini-Weber-Pearle models, collapse becomes a well-defined mathematical and physical process, taking place at definite points in space, following precise equations and going on all the time in the world around us, whether or not we are making measurements. According to these new equations, the more particles there are in a physical system, the faster the collapse rate. Left isolated, a single electron will collapse so rarely that we essentially never see any effect. On the other hand, anything large enough to be visible – even a dust grain – has enough particles in it that it collapses very quickly compared to human perception times. (In Schrödinger’s famous thought experiment, the cat’s quantum state would resolve in next to no time, leaving us with either a live cat or a dead one, not some strange quantum combination of both.)
In passing, Kent touches on the “many worlds” theory (which I wrote about here) developed by Hugh Everett III, and what has since become of it:
On this view, every time any of us does a quantum experiment with several possible outcomes, all those outcomes are enacted in different branches of reality, each of which contains a copy of our self whose memories are identical up to the start of experiment, but each of whom sees different results. None of these future selves has any special claim to be the real one. They are all equally real – genuine but distinct successors of the person who started the experiment. The same picture holds true more generally in cosmology: alongside the reality we currently habit, there are many others in which the history of the universe and our planet was ever so slightly different, many more in which humanity exists on Earth but the course of human history was significantly different from ours, and many more still in which nothing resembling Earth or its inhabitants can be found.
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The problem is that, from Everett and his early followers onwards, they have never managed to agree on a clear story about how exactly this picture of branching worlds is supposed to emerge from the fundamental equations of quantum theory, and how this single world that we see, with experimental outcomes that are apparently random but which follow definite statistical laws, might then be explained. One of the blackly funny revelations in Peter Byrne’s biography The Many Worlds of Hugh Everett III (2010) was the discovery of Everett’s personal copy of the classic text The Many-Worlds Interpretation of Quantum Mechanics, put together in 1973 by the distinguished American physicist Bryce DeWitt and a few of Everett’s other early supporters. To DeWitt’s mild criticism that ‘Everett’s original derivation [of probabilities]… is rather too brief to be entirely satisfying’, Everett scribbled in the margins ‘Only to you!’ and ‘Goddamit [sic] you don’t see it’. On another paper addressing the same issue, his comment was the single word ‘bullshit’. Although generally in more civil terms, Everettians have continued to argue over this and related points ever since.
Indeed, the big unresolved, and seemingly unsolvable, problem here is how statistical laws can possibly emerge at all when the Everettian meta-picture of branching worlds has no randomness in it. More.
Presumably it has no randomness because everything actually happens (probability: one).
Is there cosmic justice in the fact that the many worlds proponents never managed to agree on a reality single enough to meet the standards of most contemporary physicists?
Kent closes on the well-advised note that “Quantum theory might not be fundamentally correct, but it would not have worked so well for so long if its strange and beautiful mathematics did not form an important part of the deep structure of nature.” Yes, and now what are the other parts?
See also: As if the multiverse wasn’t bizarre enough …meet Many Worlds
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