“A physicist is just an atom’s way of looking at itself.”
~ Niels Bohr
The microscopic world described by quantum theory seems a strange, confusing place – but some physicists argue it’s just us who are uncertain…
SNATCH a toy from the tiniest of infants, and the reaction is likely to disappoint you. Most seem to conclude that the object has simply ceased to exist. This rapidly changes. Within the first year or so, playing peekaboo also becomes fun. As babies, we soon grasp that stuff persists unchanged even when we are not looking at it.
Granted, at that age we know nothing of quantum theory. In the standard telling, this most well-tested of physical theories – fount of the computers, lasers and cellphones that our adult souls delight in – informs us that reality’s basic building blocks take on a very different, nebulous form when no one is looking. Electrons, quarks or entire atoms can easily be in two different places at once, or have many properties simultaneously. We cannot predict with certainty which of the many possibilities we will see: that is all down to the random hand of probability.
That’s not the way our grown-up, classical world seems to work, and physicists have been scrabbling around for the best part of a century to explain the puzzling mismatch. To no avail. Faced with reality at its most fundamental, we end up babbling baby talk again.
David Mermin thinks he has something sensible to say. An atomic physicist at Cornell University in Ithaca, New York, he has spent most of his half-century-long career rejecting philosophical musings about the nature of quantum theory. Now he’s had an epiphany. The way to solve our quantum conundrums is to abandon the ingrained idea that we can ever achieve an objective view of reality. According to this provocative idea, the world is not uncertain – we are.
The idea that an objective, universally valid view of the world can be achieved by making properly controlled measurements is perhaps the most basic assumption of modern science. It works well enough in the macroscopic, classical world. Kick a football, and Newton’s laws of motion tell you where it will be later, regardless of who is watching it and how.
Kick a quantum particle such as an electron or a quark, though, and the certainty vanishes. At best, quantum theory allows you to calculate the probability of one outcome from many encoded in a multifaceted wave function that describes the particle’s state. Another observer making an identical measurement on an identical particle might measure something very different. You have no way of saying for sure what will happen.
So what state is a quantum object in when no one is looking? The most widely accepted answer is the Copenhagen interpretation, so named after the site of many early quantum musings. Schrödinger’s notorious cat illustrates its conclusion. Shut in a box with a vial of lethal gas that might, or might not, have been released by a random quantum event such as a radioactive decay, the unfortunate feline hangs in limbo, both alive and dead. Only when you open the box does the cat’s wave function “collapse” from its multiple possible states into a single real one.
This opens a physical and philosophical can of worms. Einstein pointedly asked whether the observations of a mouse would be sufficient to collapse a wave function. If not, what is so special about human consciousness? If our measurements truly do affect reality, that also opens the door to effects such as “spooky action at a distance” – Einstein’s dismissive phrase to describe how observing a wave function can seemingly collapse another one simultaneously on the other side of the universe.
Then there is the mystery of how atoms and particles can apparently adopt split personalities, but macroscopic objects such as cats clearly can’t, despite being made up of atoms and particles. Schrödinger’s intention in introducing his cat was to highlight this inexplicable division between the quantum and classical worlds. The split is not only there, but also “shifty”, in the words of quantum theorist John Bell: physicists contrive to put ever-larger objects into fuzzy quantum states, for instance – so we have no set way of defining where the boundary lies.
The Copenhagen interpretation simply ignores these quantum mysteries, famously leading Mermin to dub it the “shut up and calculate” approach in an article he wrote in 1989. He counted himself as an adherent. Although alternatives did exist – such as the many worlds interpretation, which suggests the universe divides into different paths every time anything is observed – none quite seemed to crack the central mystery.
Now Mermin thinks one does. It is not his idea: in fact, he spent more than a decade arguing against it with its originators, Carlton Caves of the University of New Mexico in Albuquerque, Christopher Fuchs of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and Rüdiger Schack of Royal Holloway, University of London.
Known as quantum Bayesianism, its ideas stem from reassessing the meaning of the wave function probabilities that seemingly govern the quantum world (see diagram). Conventionally, these are viewed as “frequentist” probabilities. In the same way that you might count up many instances of a coin falling heads or tails to conclude that the odds are 50/50, many measurements of a quantum system tell you the relative frequency of its multiple states cropping up.
Despite its limitations, not least when dealing with single, isolated events, frequentist probability is popular throughout science for the way it turns an observer into an entirely objective counting machine. But an alternative, older approach to probability was devised by English clergyman Thomas Bayes in the 18th century. This is the sort of probability that crops up in a statement such as “there’s a 40 per cent chance of rain today”. Its value is not objective or fixed, but a fluid assessment based on many changing factors, such as current air pressure and how similar weather systems developed in the past. Acquire a new piece of information – see a bank of threatening cloud when you open the curtains in the morning, for example – and you might well update your prognosis to a 90 or 100 per cent chance of rain. The actual likelihood of rain has not changed; but your state of knowledge about it has.
The central argument of quantum Bayesianism, or QBism, is that, by applying this more subjective type of probability to the quantum world, whole new vistas open up. Measure the spin of an invisible electron, say, and you acquire new knowledge, and update your assessment of the probabilities accordingly, from uncertain to certain. Nothing needs to have changed at the quantum level. Quantum states, wave functions and all the other probabilistic apparatus of quantum mechanics do not represent objective truths about stuff in the real world. Instead, they are subjective tools that we use to organise our uncertainty about a measurement before we perform it. In other words: quantum weirdness is all in the mind. “It really is that simple,” says Mermin.
Mind you, it took six weeks of intense discussions with Fuchs and Schack in South Africa last year to finally convince Mermin that he had been a QBist all along. Last November, they published their conclusions together (arxiv.org/abs/1311.5253).
For Mermin, the beauty of the idea is that the paradoxes that plague quantum mechanics simply vanish. Measurements do not “cause” things to happen in the real world, whatever that is; they cause things to happen in our heads. Spooky action at a distance is an illusion too. The appearance of a spontaneous change is just the result of two parties independently performing measurements that update their state of knowledge.
As for that shifty split, the “classical” world is where acts of measurements are continuous, because we see things with our own eyes. The microscopic “quantum” world, meanwhile, is where we need an explicit act of measurement with an appropriate piece of equipment to gain information. To predict outcomes in this instance, we require a theory that can take account of all the things that might be going on when we are not looking. For a QBist, the quantum-classical boundary is the split between what is going on in the real world and your subjective experience of it.
Quantum theorist William Wootters of Williams College in Williamstown, Massachusetts, thinks this is the most exciting interpretation of quantum theory to have emerged in years, and points to historical precedents. “It addresses Schrödinger’s concern that our own subjective experience has been explicitly excluded from physical science, and both requires and provides a place for the experiencing subject,” he says.
Others are less keen. Carlo Rovelli of Aix-Marseille University in France proposed a similar, less extreme, observer-dependent idea called relational quantum mechanics in 1996. He worries that QBism relies too much on a philosophy espoused by German philosopher Immanuel Kant in the 18th century – that there is no direct experience of things, only that which we construct in our minds from sensory inputs. “I would prefer an interpretation of quantum theory that would make sense even if there were no humans to observe anything,” he says.
Antony Valentini of Clemson University in South Carolina also thinks it moves things in the wrong direction. He paints a picture of someone setting up equipment to measure the energy of a particle, and then going off for a cup of tea. During the tea break, did the pointer on the equipment’s dial have no definite orientation? A QBist would say maybe not, you can’t tell – even though experience tells us a macroscopic object such as a pointer does always have a definite orientation. That view can’t be taken seriously, says Valentini. “A physical theory should try to describe the physical world, not just some body of talk.”
Schack counters that there is only one world out there, and we must find a way of unifying our classical and quantum interpretations of it – even if it means accepting we have no objective connection to reality in either sphere. “QBism abandons the idea that nature can be described adequately from the perspective of a detached observer,” he says.
For him the strongest sign that QBism is on the right track is a thought experiment called Wigner’s friend. Imagine you are standing outside a closed room where a friend is about to open the box containing Schrödinger’s cat. Your friend witnesses a clear outcome: the cat is either alive or dead. But you must assign a set of probabilities based on a superposition of all the possible states of the cat and the reports your friend might make of it. Who’s right? Both, say QBists: there is no paradox if a measurement outcome is always personal to the person experiencing it.
With all the zeal of a convert, Mermin has recently sought to convince detractors by applying QBist reasoning to the problems of an entity that has nothing to do with quantum theory, and nothing to do with probability: space-time (see “Lost in space”).
But Caslav Brukner of the University of Vienna in Austria wonders how far such approaches can take us. “I do not see in QBism the power to explain why quantum theory has the very mathematical and conceptual structure it does,” he says. Other theories about the world at its most fundamental could have similar Bayesian underpinnings – so why specifically does quantum theory come up with the right answers? Like many who have inspected the undercarriage of quantum mechanics, Brukner would prefer to reconstruct it from a core set of principles or axioms.
You might wonder whether all this matters, given that quantum theory does such a stupendous job of describing the world and supplying us with technological innovation. That is true up to a point, says Rovelli – but our lack of intuitive understanding hampers our search for some greater theory that can embrace all of physics from the smallest to the largest scales. “If we want to better understand the world, for instance, for quantum gravity or for cosmology, it does matter,” he says.
Faced with the prospect of abandoning scientific objectivity, the temptation to shut up and calculate might be as strong as ever. But perhaps quantum Bayesianism provides a way to have our cake and eat it. Shifting quantum theory’s weirdness into our own minds doesn’t diminish our power to calculate with it – but might just make us shut up about how shocking it all is.
Matthew Chalmers is a freelance writer based in Bristol, UK. This article is from the New Scientist Magasine, May 10-16, 2014
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