How measuring a quantum particle turns it into an everyday object

Despite the fuzziness of the quantum world, measurements of quantum particles yield accurate results in our everyday world. How does measurement achieve this transformation? Credit: Institute of Physics

The quantum world and our everyday world are very different places. In a publication that appeared as the “Editor’s Suggestion” in Physical Assessment A this week, UvA physicists Jasper van Wezel and Lotte Mertens and their colleagues are investigating how measuring a quantum particle turns it into an everyday object.

Quantum mechanics is the theory that describes the smallest objects in the world around us, ranging from the constituents of single atoms to tiny dust particles. This microscopic realm behaves remarkably differently from our everyday experience – despite the fact that all objects in our human-scale world are themselves made of quantum particles. This leads to intriguing physics questions: why are the quantum world and the macroscopic world so different, where is the dividing line and what exactly happens there?

measurement problem

One particular area where the distinction between quantum and classical becomes essential is when we use an everyday object to measure a quantum system. The division between the quantum and the everyday world then comes down to the question of how ‘large’ the measuring device must be in order to display quantum properties in our everyday world. Figuring out the details of measurements, such as how many quantum particles are needed to make a measurement device, is called the quantum measurement problem.

As experiments investigating the world of quantum mechanics become more sophisticated and involve ever larger quantum objects, the invisible line where pure quantum behavior transitions into classical measurement results is rapidly approaching. In an article, UvA physicists Jasper van Wezel and Lotte Mertens and their colleagues take stock of current models that attempt to solve the measurement problem, especially those that do so by proposing minor adjustments to the only equation that all quantum behavior rules : Schrödinger’s equation.

The rule of born

The researchers show that such adjustments can in principle lead to consistent proposals for solving the measurement problem. However, it proves difficult to make models that satisfy Born’s rule, which tells us how to use Schrödinger’s equation to predict measurement results. The researchers show that only models with sufficient mathematical complexity (in technical terms: models that are non-linear and non-unitary) can give rise to Born’s rule and thus have a chance to solve the measurement problem and teach us about the elusive crossover between quantum physics and the everyday world.

The best of both worlds: combining classical and quantum systems to meet the demands of supercomputers

More information:
Lotte Mertens et al, Inconsistency of linear dynamics and Born’s rule, Physical Assessment A (2021). DOI: 10.1103 / PhysRevA.104.052224

Provided by University of Amsterdam

Quote: How the act of measuring a quantum particle transforms it into an everyday object (2021, November 30) retrieved December 1, 2021 from

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