![\"\"](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2026\/04\/29105132\/SEI_294978299.jpg\")
<\/div>A so-called pilot wave is key to David Bohm\u2019s interpretation of quantum mechanics<\/p>\n
Courtesy of Daniel Harris and John Bush, MIT<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
Sometime in the 1940s, US physicist David Bohm decided that the only way to understand quantum mechanics was to write a book about it. He wanted it to offer the reader something they could understand in the \u201ccustomary imaginative sense\u201d, including an interpretation of the equations of quantum mechanics that didn\u2019t require tangling with advanced mathematics. This quest stayed with him for over a decade, through years of personal and professional tumult, until he ended it by developing his own interpretation of quantum theory.<\/p>\n
<\/p>\n A pair of papers announced this \u201cBohmian mechanics\u201d in 1952, but, save a few ardent supporters, it never really took off. Bohm had become a politically controversial figure, and his work was weighed down by some of that baggage, on top of its already heterodox approach to quantum physics. In 2025, however, an experiment with particles of light brought it back into focus \u2013 and reignited the idea that Bohm may have been onto something after all.<\/p>\n Bohm\u2019s political trouble lay in his affiliation with several communist organisations during his doctoral studies and his unwillingness to testify and provide evidence against his colleagues in front of the House Un-American Activities Committee at the height of the post-second world war Red Scare. What made him somewhat heretical among physicists, on the other hand, was that he thought that reality is, well, real.<\/p>\n One of the big open questions in quantum mechanics is what exactly happens when an experimenter interacts with a quantum object. The mathematics on which the forefathers of quantum mechanics \u2013 such as Niels Bohr and Werner Heisenberg \u2013 built up the theory seem to accommodate an odd situation: while it is unobserved, an object is an existentially fuzzy mix of all of its possible states, but when it is observed, that cloud becomes one concrete state. Light and electrons give us the most extreme example of this. They exhibit \u201cwave-particle duality\u201d, where light that typically seems to be a wave can be corralled into materialising as a particle in some experiments, while other experiments can nudge objects that ought to be particles, such as electrons, to assume a wave-like nature instead.<\/p>\n How exactly this happens, and whether there is a physical mechanism that literally changes the object, or if the effect stems from how we process information, remains frustratingly unclear. But back up a little and we end up in an even murkier situation: what does quantum mechanics have to say about the reality of the world before an observation or a measurement can be made?<\/p>\n Heisenberg held that \u201cthe idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them\u2026 is impossible\u201d. This view became quantum orthodoxy; now called the Copenhagen interpretation, it is widely held among physicists. But that wasn\u2019t enough for Bohm. In the 1990s, he told New Scientist<\/em> he found Bohr\u2019s idea that quantum mechanics could offer only formulae that make great predictions, but doesn\u2019t engage with a description of reality deeply unsatisfying. \u201cI said, that\u2019s not enough. I don\u2019t think I would be very interested in science if that were all there was,\u201d he said.<\/p>\n Bohm\u2019s major intervention into quantum mechanics concerned a mathematical object called the wave function, which physicists use to represent an object\u2019s quantum state. When a mathematical operation that represents a measurement is used on a wave function, it changes. This is infamously known as wave function collapse, and it was the first thing that alerted physicists to the idea that quantum objects may congeal from a cloud of probability into something more tangible. As Bohm saw it, the trouble was that the wave function itself wasn\u2019t physical, but rather an abstract object that lived in a purely mathematical space, at least according to the likes of Bohr and Heisenberg. Bohm\u2019s wave function, on the other hand, was never fuzzy, never collapsed and stood a chance of being a lot more real.<\/p>\n For instance, there is no wave-particle duality in Bohmian mechanics because particles are always particles and don\u2019t have to be observed in some special way to show up to us in their particle form. But they still behave like waves in some experiments, so what did Bohm make of that? He proposed that it is because their behaviour is guided by a \u201cpilot wave\u201d, which is caused by fields similar to electromagnetic fields but associated with some set of as-yet undiscovered, inherently quantum forces.<\/p>\n Physicist David Bohm held the controversial view that reality is real<\/p>\n Keystone\/Getty Images<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n In Bohmian mechanics, the pilot wave is also to blame for everything odd that happens with quantum objects. If you can\u2019t know something about a quantum object, for instance, it isn\u2019t because that thing is fundamentally unknowable or because the world is probabilistic instead of deterministic, but because you disturbed the pilot wave. Details of the pilot wave are so-called hidden variables that we cannot access directly, but the pilot wave is physically there, a scaffolding for reality that exists even when absolutely no one is looking.<\/p>\n The price Bohm had to pay for going all in on pilot waves is that his idea by default allows for non-local effects \u2013 particles can stay inextricably connected across remarkable distances through \u201cquantum entanglement\u201d. Infamously, the idea of such non-locality was upsetting enough to Albert Einstein that he dismissed it entirely. There are many other so-called hidden variable theories that account for entanglement and other non-local effects, essentially trying to explain it away by invoking some unseen property or force. But Bohm was unbothered by the idea that incredibly distant parts of the cosmos could be sharing quantum states and affecting each other at all times.<\/p>\n His idea still relied on Erwin Schr\u00f6dinger\u2019s equation as the starting point of every quantum calculation, which is standard; he eliminated wave function collapse, which was bothersome to many; and some of his later philosophical ideas were even rather friendly to the notion that the whole cosmos may share some sort of implicit one-ness and connection.<\/p>\n Louis de Broglie, a pivotal figure in uncovering particles\u2019 wave-like behaviour in the early days of quantum theory, formulated an idea similar to pilot waves in the 1920s, but was persuaded to abandon it by his peers. After Bohm independently came up with and further fleshed out this very similar framework, he was also met with resistance.<\/p>\n In 1989, philosopher Ren\u00e9e Weber, speaking to John Stewart Bell, a visionary physicist whose work on quantum entanglement also challenged our view of reality, asked why Bohmian mechanics had seemingly been so easy to ignore. Bell, who was a champion of Bohm\u2019s work, demurred: \u201cLet\u2019s not go into that. That\u2019s another question. That\u2019s the psychology and history of physicists.\u201d Bohm\u2019s work always carried with it the shadow of his unfair political persecution. Yet there are also two major scientific problems with Bohmian mechanics.<\/p>\n The first gets to the core of how physics ought to work \u2013 can Bohmian mechanics suggest an experiment whose results would differentiate it from more orthodox interpretations of quantum mechanics? The word \u201cinterpretation\u201d itself suggests that the answer should be no, because it is only distinct theories rather than interpretations of the same theory that can typically be told apart through experiments. But in July 2025, a study in the journal Nature<\/em> suggested otherwise. Its title read \u201cEnergy-speed relationship of quantum particles challenges Bohmian mechanics\u201d, offering a counterpoint to Bohm\u2019s work more concrete than matters of philosophy or politics.<\/p>\n It didn\u2019t start out that way, says Jan Klaers at the University of Twente in the Netherlands, who worked on the experiment. He and his colleagues set out to study the quantum phenomenon of tunnelling, where a particle manages to enter a space that ought to be forbidden to it, when they realised that they could actually infer something about Bohm\u2019s work, too. They think they have identified a detail of his idea that could be experimentally tested after all.<\/p>\n Their experiment was similar to a ball rolling down a hill and colliding with a wall. Outside of the quantum world, this is a straightforward situation, but in the quantum realm, the ball can \u201ctunnel\u201d into the wall. This process is distinct from the ball having enough energy to physically break a hole in the wall. With quantum tunnelling, there is simply some probability of the ball showing up inside the wall even if it isn\u2019t very energetic at all. (In the mathematical world, the wave function of the ball \u201cspills\u201d into the wall.)<\/p>\n Klaers and his team made their own \u201cball\u201d by shining a laser into a thin layer of a liquid filled with fluorescent dye molecules and sandwiched between two mirrors. The mirrors, the liquid and the molecules had to be there to make the massless photons behave as if they had mass, like a ball does, and the bottom mirror was also adorned with special nano-sized patterns that made the photons move along two pre-set parallel paths or \u201cwaveguides\u201d. One was designed so that a photon would experience it as a downward ramp that ended in a bump \u2013 the wall at the bottom of a hill.<\/p>\n The researchers created photon after photon at the top of the hill and tracked what happened to them. Some jumped from one waveguide into another, and some tunnelled into the wall. Klaers says counting how many were hopping between the waveguides could serve as an internal clock for the system, with each hop like a tick \u2013 which let them determine the speed of the photons that were tunnelling through the barrier. The speeds they measured were rather high \u2013\u00a0 thousands of kilometres per second \u2013 but when Klaers and his colleagues used Bohm\u2019s approach to calculate what they ought to be, the answer was nearly zero. The discrepancy was large enough to put Bohmian mechanics into peril.<\/p>\n Not everyone agrees. Hui Wang at the University of Science and Technology of China says the velocity of tunnelling photons that could be calculated from the team\u2019s setup cannot be directly compared to predictions of Bohm\u2019s theory. Other quantities the researchers calculated from their measurements are, in his view, correct, and present no challenge to Bohmian mechanics, but their definition of speed is too close to its classical, rather than quantum, definition.<\/p>\n \u201cI think Bohmian mechanics is still a promising interpretation of quantum mechanics,\u201d says Wang. \u201cCopenhagen interpretation is just a very powerful tool to correctly predict the experimental measurement results, but it provides very limited \u2018physical reality\u2019 of nature itself. Yet the core of science is to discover more of that reality in order to better understand nature. Bohmian mechanics is on this path.\u201d<\/p>\n In fact, Wang says his own team is preparing a paper that will also offer a resolution to the second big problem with Bohmian mechanics, namely that it has historically not been applicable for objects that move close to the speed of light and therefore have to obey Einstein\u2019s theory of special relativity \u2013 a pillar of physics that has withstood decades of testing. While their work has yet to be reviewed by other physicists, if successful, it could give Bohmian mechanics a real boost in the race to become the next best theory of physical reality. In that decades-old interview, Bell told Weber that he, essentially, felt similarly. Were it not for discrepancies with the mathematics of relativity, he would have simply adapted Bohm\u2019s view as correct instead of seeing \u201ca big, deep mystery in quantum mechanics\u201d, he said.<\/p>\n While Klaers disagrees with Wang on the interpretation of his team\u2019s data and how they used it to calculate photons\u2019 speeds, he doesn\u2019t think it\u2019s fully lights out for all ideas like Bohm\u2019s. There are two equations that are crucial for Bohmian mechanics, and they are the unavoidable: Schr\u00f6dinger\u2019s equation and the \u201cguiding equation\u201d, which is more specific to Bohm\u2019s work. This second equation determines the velocity of any one particle from the configuration made by all particles. The experiment with tunnelling photons specifically points to issues with this guiding equation, says Klaers.<\/p>\n \u201cThere are many guiding equations possible, and you can come up with additional models that actually produce the same particle density and are in agreement with our speed measurements. So, it\u2019s not really a distinction between Bohm mechanics and standard quantum mechanics. It\u2019s really a question of, OK, this guiding equation that the standard Bohmian mechanics choose, is that actually the physically correct one?\u201d he says. In a follow-up study, he and his colleagues have shown that tweaks to the guiding equation can push Bohm\u2019s mathematics to match their experiment.<\/p>\n Is this actually good news for realists and Bohmians in the ranks of modern-day physicists? There is no straightforward answer. Finding a way to match a Bohmian theory to experimental evidence should add more credence to it, but a theory that must be repeatedly tweaked to catch up with experiments is at risk of not being sufficiently well-defined or fully complete. It is more likely that it is a stepping stone on the way to a theory that would have fewer ambiguities and more predictive power. In this way, Bohmian mechanics has been handed a new challenge \u2013 but at least it is staying in the ring.<\/p>\n Topics:<\/p>\n<\/section><\/div>\n\n","protected":false},"excerpt":{"rendered":" A so-called pilot wave is key to David Bohm\u2019s interpretation of quantum mechanics Courtesy of Daniel Harris and John Bush, MIT Sometime in the 1940s, US...<\/p>\n","protected":false},"author":1,"featured_media":553,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"fifu_image_url":"https:\/\/images.newscientist.com\/wp-content\/uploads\/2026\/04\/29105132\/SEI_294978299.jpg","fifu_image_alt":"","footnotes":""},"categories":[1],"tags":[621],"class_list":["post-552","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-rj","tag-quantum-mechanics"],"_links":{"self":[{"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=\/wp\/v2\/posts\/552","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=552"}],"version-history":[{"count":0,"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=\/wp\/v2\/posts\/552\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=\/wp\/v2\/media\/553"}],"wp:attachment":[{"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=552"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=552"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rjbarrett.redirectme.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=552"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2026\/04\/29105143\/SEI_294978646.jpg\")
<\/div>Putting Bohm to the test<\/h2>\n