werner heisenberg experiment

Quantum physics experiment shows Heisenberg was right about uncertainty, in a certain sense

Director, Centre for Quantum Dynamics, Griffith University

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Howard Wiseman does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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The word uncertainty is used a lot in quantum mechanics. One school of thought is that this means there’s something out there in the world that we are uncertain about. But most physicists believe nature itself is uncertain.

Intrinsic uncertainty was central to the way German physicist Werner Heisenberg , one of the originators of modern quantum mechanics, presented the theory.

He put forward the Uncertainty Principle that showed we can never know all the properties of a particle at the same time.

Read more: Explainer: Heisenberg’s Uncertainty Principle

For example, measuring the particle’s position would allow us to know its position. But this measurement would necessarily disturb its velocity, by an amount inversely proportional to the accuracy of the position measurement.

Was Heisenberg wrong?

Heisenberg used the Uncertainty Principle to explain how measurement would destroy that classic feature of quantum mechanics, the two-slit interference pattern (more on this below).

But back in the 1990s, some eminent quantum physicists claimed to have proved it is possible to determine which of the two slits a particle goes through, without significantly disturbing its velocity.

Does that mean Heisenberg’s explanation must be wrong? In work just published in Science Advances , my experimental colleagues and I have shown that it would be unwise to jump to that conclusion.

We show a velocity disturbance — of the size expected from the Uncertainty Principle — always exists, in a certain sense.

But before getting into the details we need to explain briefly about the two-slit experiment .

The two-slit experiment

In this type of experiment there is a barrier with two holes or slits. We also have a quantum particle with a position uncertainty large enough to cover both slits if it is fired at the barrier.

Since we can’t know which slit the particle goes through, it acts as if it goes through both slits. The signature of this is the so-called “interference pattern”: ripples in the distribution of where the particle is likely to be found at a screen in the far field beyond the slits, meaning a long way (often several metres) past the slits.

But what if we put a measuring device near the barrier to find out which slit the particle goes through? Will we still see the interference pattern?

We know the answer is no, and Heisenberg’s explanation was that if the position measurement is accurate enough to tell which slit the particle goes through, it will give a random disturbance to its velocity just large enough to affect where it ends up in the far field, and thus wash out the ripples of interference.

What the eminent quantum physicists realised is that finding out which slit the particle goes through doesn’t require a position measurement as such. Any measurement that gives different results depending on which slit the particle goes through will do.

And they came up with a device whose effect on the particle is not that of a random velocity kick as it goes through. Hence, they argued, it is not Heisenberg’s Uncertainty Principle that explains the loss of interference, but some other mechanism.

As Heisenberg predicted

We don’t have to get into what they claimed was the mechanism for destroying interference, because our experiment has shown there is an effect on the velocity of the particle, of just the size Heisenberg predicted.

We saw what others have missed because this velocity disturbance doesn’t happen as the particle goes through the measurement device. Rather it is delayed until the particle is well past the slits, on the way towards the far field.

How is this possible? Well, because quantum particles are not really just particles. They are also waves .

In fact, the theory behind our experiment was one in which both wave and particle nature are manifest — the wave guides the motion of the particle according to the interpretation introduced by theoretical physicist David Bohm , a generation after Heisenberg.

Let’s experiment

In our latest experiment, scientists in China followed a technique suggested by me in 2007 to reconstruct the hypothesised motion of the quantum particles, from many different possible starting points across both slits, and for both results of the measurement.

They compared the velocities over time when there was no measurement device present to those when there was, and so determined the change in the velocities as a result of the measurement.

Read more: We did a breakthrough 'speed test' in quantum tunnelling, and here's why that's exciting

The experiment showed that the effect of the measurement on the velocity of the particles continued long after the particles had cleared the measurement device itself, as far as 5 metres away from it.

By that point, in the far field, the cumulative change in velocity was just large enough, on average, to wash out the ripples in the interference pattern.

So, in the end, Heisenberg’s Uncertainty Principle emerges triumphant.

The take-home message? Don’t make far-reaching claims about what principle can or cannot explain a phenomenon until you have considered all theoretical formulations of the principle.

Yes, that’s a bit of an abstract message, but it’s advice that could apply in fields far from physics.

• Quantum mechanics
• Quantum physics
• Particle physics
• Uncertainty
• Scientific experiments

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Erwin schrödinger and werner heisenberg devise a quantum theory.

In the 1920s, physicists were trying to apply Planck's concept of energy quanta to the atom and its constituents. By the end of the decade Erwin Schrödinger and Werner Heisenberg had invented the new quantum theory of physics. The Physical Institute of the University of Zürich published Schrödinger's lectures on Wave Mechanics  (the first from 27 January 1926) and in 1930 Heisenberg's book The physical principles of the quantum theory appeared.

The problem now was that quantum theory was not relativistic; the quantum description worked for particles moving slowly, but not for those at high or "relativistic" velocities, close to the speed of light.

March 8, 2012

One Thing Is Certain: Heisenberg's Uncertainty Principle Is Not Dead

Experimenters violate Heisenberg's original version of the famous maxim, but confirm a newer, clearer formulation

By Aya Furuta

What Einstein's E = mc 2 is to relativity theory, Heisenberg's uncertainty principle is to quantum mechanics—not just a profound insight, but also an iconic formula that even non-physicists recognize. The principle holds that we cannot know the present state of the world in full detail, let alone predict the future with absolute precision. It marks a clear break from the classical deterministic view of the universe.

Yet the uncertainty principle comes in two superficially similar formulations that even many practicing physicists tend to confuse. Werner Heisenberg's own version is that in observing the world, we inevitably disturb it. And that is wrong, as a research team at the Vienna University of Technology has now vividly demonstrated.

Led by Yuji Hasegawa, the team prepared a stream of neutrons and measured two spin components simultaneously for each, in direct violation of Heisenberg's version of the principle. Yet, the alternative variation continued to hold. The team reported its results in Nature Physics on January 15. ( Scientific American is part of Nature Publishing Group.)

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Heisenberg inferred his formulation in 1927 via his famous thought experiment in which he imagined measuring the position of an electron using a gamma-ray microscope. The formula he derived was ε ( q ) η ( p ) ≥ h /4π. This inequality says that when you measure the position of an electron with an error ε ( q ), you cannot help but alter the momentum of the electron by the amount of η ( p ). An experimenter cannot know both the position and the momentum precisely; he or she must make a tradeoff. "For that reason everything observed is a selection from a plenitude of possibilities and a limitation on what is possible in the future," Heisenberg wrote.

The same year, Earle Kennard, a less-known physicist, derived a different formulation, which was later generalized by Howard Robertson: σ ( q ) σ ( p ) ≥ h/ 4π. This inequality says that you cannot suppress quantum fluctuations of both position σ ( q ) and momentum σ ( p ) lower than a certain limit simultaneously. The fluctuation exists regardless whether it is measured or not, and the inequality does not say anything about what happens when a measurement is performed.

Kennard's formulation is therefore totally different from Heisenberg's. But many physicists, probably including Heisenberg himself, have been under the misapprehension that both formulations describe virtually the same phenomenon. The one that physicists use in everyday research and call Heisenberg's uncertainty principle is in fact Kennard's formulation. It is universally applicable and securely grounded in quantum theory. If it were violated experimentally, the whole of quantum mechanics would break down. Heisenberg's formulation, however, was proposed as conjecture, so quantum mechanics is not shaken by its violation.

In 2003 Masanao Ozawa of Nagoya University developed a new formulation of the error–disturbance uncertainty that Heisenberg aimed to express, but this time on much firmer footing. Derived mathematically from quantum measurement theory, the new formulation describes error and disturbance as well as fluctuations: ε ( q ) η ( p ) + σ ( q ) η ( p ) + σ ( p ) ε ( q ) ≥ h /4π . Hasegawa's team is the first to have demonstrated the violation of Heisenberg's inequality and the validity of Ozawa's inequality. It did so by directly measuring errors and disturbances in the observation of spin components. Even when either the source of error or disturbance is held to nearly zero, the other remains finite.

"I think it is significant, especially for experimental physics, that measurement errors and disturbances are clearly distinguished from quantum fluctuations in Ozawa's formulation," said Shogo Tanimura of Nagoya, who is independent from Ozawa's group. "Physicists thought that the only way to reduce errors is to suppress fluctuations. But Ozawa's inequality suggests that there is another way to reduce errors by allowing an object system to have larger fluctuations, although it may sound contradictory."

Ozawa's formulation confirms an emerging trend in probing the foundations of physics: to hew closely to what experimenters directly see in the lab—a so-called operational approach. "The error–disturbance uncertainty relation is much more important than that of fluctuations," says Akio Hosoya, a theoretical physicist at Tokyo Institute of Technology, "because in physics the final say comes from experimental verification." Heisenberg would be pleased that the limitation we can know about the world, which he aimed to expressed, was this time clearly revealed with the new rigorous, experimentally verified formulation. The new uncertainty relation between measurement error and disturbance is no more just conjecture, but physical law.

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Credit line: Photograph by John H. Martin, courtesy AIP Emilio Segrè Visual Archives Description: Wermer Heisenberg (second from left) and Carlo Rubbia (far right) talking to students at Harvard University, Cambridge, Massachusets. Person(s): Heisenberg, Werner, 1901-1976 | Rubbia, Carlo, 1934

The Quantum Mechanic (1925-1927)

The leading theory of the atom when Heisenberg entered the University of Munich in 1920 was the quantum theory of Bohr, Sommerfeld, and their co-workers. Although the theory had been highly successful in certain situations, during the early 1920s three areas of research indicated that this theory was inadequate and would need to be replaced. These areas included the study of light emitted and absorbed by atoms (spectroscopy); the predicted properties of atoms and molecules; and the nature of light itself--did it act like waves or like a stream of particles?

During his work in Munich, Göttingen, and Copenhagen Heisenberg engaged intensively in the theoretical study of all three of these areas of research. By 1924 physicists in Göttingen and Copenhagen were agreed that the old quantum theory had to be replaced by some new "quantum mechanics."

"All of my meagre efforts go toward killing off and suitably replacing the concept of the orbital path which one cannot observe." —Heisenberg, letter to Pauli, 1925

Heisenberg set himself the task of finding the new quantum mechanics upon returning to Göttingen from Copenhagen in April 1925. Inspired by Bohr and his assistant, H.A. Kramers, in Copenhagen, Pauli in Hamburg, and Born in Göttingen, Heisenberg's intensive struggle over the following months to achieve his goal has been well documented by historians. Since the electron orbits in atoms could not be observed, Heisenberg tried to develop a quantum mechanics without them. He relied instead on what can be observed, namely the light emitted and absorbed by the atoms. By July 1925 Heisenberg had an answer, but the mathematics was so unfamiliar that he was not sure if it made any sense. Heisenberg handed a paper on the derivation to his mentor, Max Born, before leaving on a month-long lecture trip to Holland and England and a camping trip to Scandinavia with his youth-movement group. After puzzling over the derivation, Born finally recognized that the unfamiliar mathematics was related to the mathematics of arrays of numbers known as "matrices." Born sent Heisenberg's paper off for publication. It was the breakthrough to quantum mechanics .

"The present paper seeks to establish a basis for theoretical quantum mechanics founded exclusively upon relationships between quantities which in principle are observable." —Heisenberg, summary abstract of his first paper on quantum mechanics

Together with his other assistant, Pascual Jordan, Born worked toward the further development of a quantum mechanics based upon the abstract mathematics of matrices. After Heisenberg returned from his youth-movement travels, the Göttingen work resulted in a famous "three-man paper" setting forth the details of a new matrix-based quantum mechanics, the "matrix mechanics." With the introduction of additional concepts (electron "spin" and Pauli's "exclusion principle"), Heisenberg, Born, Jordan, Pauli, and others showed that the new quantum mechanics could account for many of the properties of atoms and atomic events.

Einstein, however, objected to Heisenberg's approach in which the new theory was based only on observable quantities. Heisenberg recalled a conversation with Einstein on this issue following a lecture Heisenberg delivered in Berlin in 1926.

Independently, and somewhat later, Austrian physicist Erwin Schrödinger proposed another quantum mechanics, an alternative "wave mechanics" in 1926. The wave mechanics appealed to many physicists because it seemed to do everything that matrix mechanics could do but much more easily and seemingly without giving up the visualization of orbits within the atom. This unleashed an intense debate between the followers of the alternative versions of quantum mechanics that formed the background to the later uncertainty relations and the Copenhagen Interpretation.

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How did Werner Heisenberg contribute to atomic theory?

Werner Heisenberg contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle , which states that a particle’s position and momentum cannot both be known exactly. The combined uncertainty in both measurements must be equal to or greater than h /(4π), where  h  is  Planck’s constant .

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