鈥淚f you think you understand quantum mechanics, you haven鈥檛 understood quantum mechanics.鈥� That jibe, supposedly made by physicist Richard Feynman half a century ago, still rings true today. Quantum theory has given us lasers, cellphones and any number of dodgy product promotion strategies, but its portrayal of reality continues to bamboozle even the finest minds.
Our everyday experience tells us that we live in a world of certainty, where things have a defined place and causes lead to effects. So how are we to understand a basic theory of reality that says everything is at root fuzzy and uncertain 鈥� where Schr枚dinger鈥檚 cat can be both dead and alive, where objects can be in two places at once, or be in one place and then suddenly pop up in another, or influence each other instantaneously from afar?
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We don鈥檛 have an answer. All we have are guesses 鈥� 鈥渋nterpretations鈥� that attempt to bridge the gulf between what quantum theory predicts and what common sense tells us.
So what are our best guesses? In 2011, 33 physicists and philosophers at a conference in Austria on 鈥淨uantum physics and the nature of reality鈥� were asked to , listed below. The percentages of the delegates backing the various options do not add up to 100 鈥� in keeping with the spirit of quantum theory, the poll allowed multiple answers.
The de Broglie-Bohm interpretation 鈥� no votes, 0 per cent
No one likes it 鈥� so why should we care? Because the idea has a long and rich history, being based on classic work by quantum pioneers Louis de Broglie and David Bohm, and isn鈥檛 entirely done yet. The bewildering nature of quantum theory implies that there must be additional stuff we鈥檙e not seeing 鈥� in this case it鈥檚 鈥減ilot waves鈥� that guide the evolution of quantum states on some hitherto unexplored layer of reality.
Einstein was an early fan, although he later cooled, and some experiments had seemed to rule out such hidden features. Since the 2011 poll, though, the de Broglie-Bohm interpretation has staged a comeback, buoyed by advances in quantum information theory 鈥� and that nagging feeling expressed by Feynman that, whatever the answer to quantum weirdness might be, we鈥檝e none of us yet quite understood the question.
Discover the de Broglie-Bohm interpretation Reality check: The hidden connections behind quantum weirdness
Quantum theory says that stuff doesn鈥檛 exist when we鈥檙e not looking at it. But weirder-than-weird experiments are resurrecting an long-derided alternative
The Copenhagen interpretation 鈥� 14 votes, 42 per cent
Critics of this view claim it is no explanation at all. But the Copenhagen interpretation, devised by quantum pioneer Niels Bohr and others in the Danish capital in the 1920s, remains by far the dominant way to explain away quantum weirdness. Often described as the 鈥渟hut up and calculate鈥� option, it basically says that since we are conditioned to think in terms of the classical world around us, the quantum world is in essence unknowable. Quantum theory is an extremely effective tool for making predictions, but no more than that. When we observe the quantum world, we force it to conform to our preconceptions 鈥� 鈥渃ollapsing鈥� it into a classical shadow of itself. So don鈥檛 ask who killed Schr枚dinger鈥檚 cat 鈥� you did.
The information interpretation 鈥� 8 votes, 24 per cent
Information-theory interpretations stem from a growing realisation among physicists that the most basic currency of reality might be not stuff, but stuff we know about stuff 鈥揵its of information. When we observe a quantum object, we extract information from it, and it is this that causes it to lose its quantum mojo.
Many worlds 鈥� 6 votes, 18 per cent
For a disciple of many worlds, the quantum realm is intrinsically fuzzy. Observing it does not create a single defined reality, but splits reality into as many parallel worlds as there were options for what might have been observed.
The theory was the brainchild of Princeton graduate student Hugh Everett III in the 1950s, and initially few were convinced. Everett described a journey to Copenhagen in 1959 to explain his idea to Niels Bohr as 鈥渉ell鈥� doomed from the beginning鈥�.
In recent decades, however, many worlds has enjoyed something of a revival as ideas of a 鈥�multiverse鈥� of parallel universes have permeated cosmology. That was not before Everett, who believed his theory guaranteed him immortality in some world or other, had eaten, drunk and chain-smoked himself to an early death in 1982.
Objective collapse 鈥� 3 votes, 9 per cent
In this picture, there鈥檚 no need for an observer to destroy an object鈥檚 quantum nature 鈥� it happens spontaneously all the time, like radioactive particles randomly decaying. The more particles there are, the more speedily this happens. We are a clodhopping, already decayed, non-quantum bundle that entangle ourselves with any quantum object we observe, infecting it with classical physics. Proponents of objective collapse are now claiming it could help explain grand mysteries of cosmology such as the nature of dark energy, why time flows, and why there鈥檚 any stuff in the universe at all.
Quantum Bayesianism 鈥� 2 votes, 6 per cent
Taking its cue from Bayesian probability, in which a 50 per cent probability of rain in the weather forecast is immediately updated to a 100 per cent probability when you open the curtain and see it鈥檚 actually raining, quantum Bayesianism asserts that quantum uncertainty is all in our minds. Our confusion about how reality works at the finest scales is merely a product of our imperfect information about it. In other words, it鈥檚 not the world that鈥檚 uncertain 鈥� it鈥檚 you.
Relational quantum mechanics 鈥� 2 votes, 6 per cent
In Einstein鈥檚 relativity there is no absolute answer to whether two events are simultaneous 鈥� it depends on your point of view. Similarly, this interpretation asserts that no single observer can ever be in possession of all the facts about a quantum state 鈥� we are part of any measurement we make, so lack any full view of it. The brainchild of Italian physicist Carlo Rovelli, it鈥檚 another variant on the idea that quantum weirdness is all an illusion born of imperfect information.
Read more: Has quantum theory鈥檚 greatest mystery been solved?
