Skip to main content
Home
plus.maths.org

Secondary menu

  • My list
  • About Plus
  • Sponsors
  • Subscribe
  • Contact Us
  • Log in
  • Main navigation

  • Home
  • Articles
  • Collections
  • Podcasts
  • Maths in a minute
  • Puzzles
  • Videos
  • Topics and tags
  • For

    • cat icon
      Curiosity
    • newspaper icon
      Media
    • graduation icon
      Education
    • briefcase icon
      Policy

      Popular topics and tags

      Shapes

      • Geometry
      • Vectors and matrices
      • Topology
      • Networks and graph theory
      • Fractals

      Numbers

      • Number theory
      • Arithmetic
      • Prime numbers
      • Fermat's last theorem
      • Cryptography

      Computing and information

      • Quantum computing
      • Complexity
      • Information theory
      • Artificial intelligence and machine learning
      • Algorithm

      Data and probability

      • Statistics
      • Probability and uncertainty
      • Randomness

      Abstract structures

      • Symmetry
      • Algebra and group theory
      • Vectors and matrices

      Physics

      • Fluid dynamics
      • Quantum physics
      • General relativity, gravity and black holes
      • Entropy and thermodynamics
      • String theory and quantum gravity

      Arts, humanities and sport

      • History and philosophy of mathematics
      • Art and Music
      • Language
      • Sport

      Logic, proof and strategy

      • Logic
      • Proof
      • Game theory

      Calculus and analysis

      • Differential equations
      • Calculus

      Towards applications

      • Mathematical modelling
      • Dynamical systems and Chaos

      Applications

      • Medicine and health
      • Epidemiology
      • Biology
      • Economics and finance
      • Engineering and architecture
      • Weather forecasting
      • Climate change

      Understanding of mathematics

      • Public understanding of mathematics
      • Education

      Get your maths quickly

      • Maths in a minute

      Main menu

    • Home
    • Articles
    • Collections
    • Podcasts
    • Maths in a minute
    • Puzzles
    • Videos
    • Topics and tags
    • Audiences

      • cat icon
        Curiosity
      • newspaper icon
        Media
      • graduation icon
        Education
      • briefcase icon
        Policy

      Secondary menu

    • My list
    • About Plus
    • Sponsors
    • Subscribe
    • Contact Us
    • Log in
    • Birdwatching icon

      Watch and learn

      Marianne Freiberger
      20 November, 2016
      1 comments

      Scientists are a bit like bird watchers. They set up their experiment and then watch quietly from their hide to see what nature reveals to them. Thousands of years' worth of science are built on this approach.

      David Wallace and Adrian Kent talk about the role of the observer in quantum mechanics.

      One of the theories that has emerged from such careful observation is quantum mechanics. It's been hugely successful and it dominates modern physics. But perhaps ironically, the theory seems to suggest that passive observation is impossible: the very act of looking at something can change what's being looked at. This apparent fact is the theory's greatest mystery.

      Quantum mechanics was developed in the 1920s to describe the smallest building blocks of matter; particles such as electrons or protons. The theory worked so well, it had to be taken seriously, but it also seemed to suggest that at very small scales, reality becomes fuzzy. "Quantum mechanics, if you take it literally, seems to say that particles can be in two places at once, or go at two speeds at once," explains David Wallace, a philosopher of physics at the University of Oxford. "The mathematics of the theory also seems to say that when you make a measurement of a quantum system, then the macroscopic world also starts doing two things at once." That is, if you measure a particle that's in such a superposition state, such as being in two places at once, then, so the theory suggests, you yourself go into a superposition state of seeing it here and there. Developing this idea further, you can come up with thought experiments like that of Schrödinger's famous cat: because its life depends on a radioactive particle that can be simultaneously decayed and non-decayed, the cat is simultaneously dead and alive.

      This seems like utter rubbish. We never find ourselves, or cats, in superposition. How can it be that such a crazy theory says anything useful about the world? The answer lies in probabilities. We all know that when we measure the location of a particle, we will always find it in exactly one place. The theory may appear to dispute that it really is in one place, but it also provides us with a set of numbers that form a probability distribution: for each possible location we could find the particle at, the theory gives the probability of finding it there. This really works. If the theory says the probability of finding the particle at location x is 1/3rd and the probability of finding it at location y is 2/3rds, then if you do a large number of identical particle-spotting experiments, you will find the particle at x 1/3rd of the time, and at y the other 2/3rds of the time. Things like MRI scanners, lasers and the internet heavily rely on the reliable predictions of quantum mechanics. But the theory also seems to suggest a deep distinction between what we observe, or measure, and what is really going on.

      Schrödinger's

      Erwin Schrödinger devised a famous thought experiment in which a cat in a box is in superposition of two states: dead and alive. Image: Dc987.

      "This brings the observer into the game," says Adrian Kent, a theoretical physicist at the University of Cambridge. "You can't understand the theory without talking about measurements. So what is this thing, this process of measurement? Maybe it's to do with the [measurement] apparatuses, things with dials on, but that's a pretty funny thing to have right at the heart of what's supposed to be a fundamental physical theory. Maybe it's to do with us somehow. Physics is telling us about the probability of things we observe. That's also a pretty weird thing to say, because as far as we understand we are not really any more special than experimental apparatuses, or planets, or electrons. But somehow you have to break this chain — you have to say something about what's happening in physics that gives you a [definite] measurement outcome."

      All this leaves us with essentially three options. Accept that the observer plays a fundamental role in physics, which poses the awkward task of defining what counts as an observer (a dog, a snail, an apparatus?) and explaining the mechanism by which an observer changes reality. The second option is to change something about the theory of quantum mechanics to bring it in line with our experience of the world. It's not a popular option with many physicists because the theory works so well. The third option is to take the mathematics literally: to admit that several things really can be happening at once without us noticing. This leads us into the strange realm of parallel worlds. We'll explore all three options in the next articles.


      About this article

      Adrian Kent is Professor of Quantum Physics in the Centre for Quantum Information and Foundations, DAMTP, University of Cambridge. He is currently working on a FQXi funded project developing a formulation of relativistic quantum theory in which simple additional mathematical postulates give us a description of a single real world consistent with cosmological observations and quantum experiments. He co-edited Many Worlds? Everett, Quantum Theory and Reality; one of his contributions to the volume was the question mark in the title.

      David Wallace is a philosopher of physics at the University of Southern California, having previously received PhDs in physics and in philosophy at the University of Oxford. His 2012 book on the many-worlds interpretation of quantum mechanics, The Emergent Multiverse, was joint winner of the 2013 Lakatos Prize for philosophy of science.

      Marianne Freiberger is Editor of Plus. She interviewed Kent and Wallace at the 5th FQXi International Conference in Canada in August 2016.

      FQXi logo

      This article is part of our Who's watching? The physics of observers project, run in collaboration with FQXi. Click here to see more articles and videos about questions to do with observers in physics.

      • Log in or register to post comments

      Comments

      Lucian M Ionescu

      2 January 2023

      Permalink

      Observation or measurement, is a reduction process (Birkhoff-von Newman article): from quantum logic (information), involving superposition to describe complex states of complex systems to simple models, and classical logic, like in the Court of Law: "Yes XOR No"!
      The perturbation (intrusion) via measurement is a different aspect: destruction of the quantum system, like an autopsy on a live subject! It is modeled by the collapse of the wave function, which corresponds to a real collapsing process, of the fermionic circuit transferring quantum information (bosons) in an interaction, like in a 2-slit experiment or entanglement ...

      Happy New Year!

      • Log in or register to post comments

      Read more about...

      FQXi2016
      The physics of observers
      quantum mechanics
      quantum superposition
      video
      University of Cambridge logo

      Plus Magazine is part of the family of activities in the Millennium Mathematics Project.
      Copyright © 1997 - 2025. University of Cambridge. All rights reserved.

      Terms