physics

Mermin's device

Mermin's device is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mermin's device rather than just read about it. In short: In physics, Mermin's device or Mermin's machine is a thought experiment intended to illustrate the non-classical features of nature without making a direct reference to quantum mechanics. The challenge is to reproduce the results of the thought experiment in terms of classical physics.

Key takeaways

Mermin's device belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mermin's device to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mermin's device from memory before moving on to harder problems.

Reference excerpt

In physics, Mermin's device or Mermin's machine is a thought experiment intended to illustrate the non-classical features of nature without making a direct reference to quantum mechanics. The challenge is to reproduce the results of the thought experiment in terms of classical physics. The input of the experiment are particles, starting from a common origin, that reach detectors of a device that are independent from each other, the output are the lights of the device that turn on following a specific set of statistics depending on the configuration of the device. The results of the thought experiment are constructed in such a way to reproduce the result of a Bell test using quantum entangled particles, which demonstrate how quantum mechanics cannot be explained using a local hidden variable theory. In this way Mermin's device is a pedagogical tool to introduce the unconventional features of quantum mechanics to a larger public.

History The original version with two particles and three settings per detector, was first devised in a paper called "Bringing home the atomic world: Quantum mysteries for anybody" authored by the physicist N. David Mermin in 1981. Richard Feynman told Mermin that it was "One of the most beautiful papers in physics". Mermin later described this accolade as "the finest reward of my entire career in physics". Ed Purcell shared Mermin's article with Willard Van Orman Quine, who then asked Mermin to write a version intended for philosophers, which he then produced. Mermin also published a second version of the thought experiment in 1990 based on the Greenberger–Horne–Zeilinger experiment analysis by Robert K. Clifton, Michael Redhead and Jeremy Butterfield, with three particles and detectors with only two configurations. In 1993, Lucien Hardy devised a paradox and Mermin made it into a Mermin-device-type thought experiment with two detectors and two settings.

Original two particle device

Assumptions

In Mermin's original thought experiment, he considers a device consisting of three parts: two detectors A and B, and a source C. The source emits two particles whenever a button is pushed, one particle reaches detector A and the other reaches detector B. The three parts A, B and C are isolated from each other (no connecting pipes, no wires, no antennas) in such a way that the detectors are not signaled when the button of the source has been pushed nor when the other detector has received a particle. Each detector (A and B) has a switch with three configurations labeled (1,2 and 3) and a red and a green light bulb. Either the green or the red light will turn on (never both) when a particle enters the device after a given period of time. The light bulbs only emit light in the direction of the observer working on the device. Additional barriers or instrument can be put in place to check that there is no interference between the three parts (A,B,C), as the parts should remain as independent as possible. Only allowing for a single particle to go from C to A and a single particle from C to B, and nothing else between A and B (no vibrations, no electromagnetic radiation). The experiment runs in the following way. The button of the source C is pushed, particles take some time to travel to the detectors and the detectors flash a light with a color determined by the switch configuration. There are nine total possible configuration of the switches (three for A, three for B). The switches can be changed at any moment during the experiment, even if the particles are still traveling to reach the detectors, but not after the detectors flash a light. The distance between the detectors can be changed so that the detectors flash a light at the same time or at different times. If detector A is set to flash a light first, the configuration of the switch of detector B can be changed after A has already flashed (similarly if B set to flash first, the settings of A can be change before A flashes).

Expected results The expected results of the experiment are given in this table in percentages:

Every time the detectors are set to the same setting, the bulbs in each detector always flash same colors (either A and B flash red, or A and B flash green) and never opposite colors (A red B green, or A green B red). Every time the detectors are at different setting, the detectors flash the same color a quarter of the time and opposite colors 3/4 of the time. The challenge consists in finding a device that can reproduce these statistics.

Hidden variables and classical implementation In order to make sense of the data using classical mechanics, one can consider the existence of three variables per particle that are measured by the detectors and follow the percentages above. Particle that goes into detector A has variables ( a 1 , a 2 , a 3 ) {\displaystyle (a_{1},a_{2},a_{3})} and the particle that goes into detector B has variables ( b 1 , b 2 , b 3 ) {\displaystyle (b_{1},b_{2},b_{3})} . These variables determine which color will flash for a specific setting (1,2 or 3). For example, if the particle that goes in A has variables (R,G,G), then if the detector A is set to 1 it will flash red (labelled R), set to 2 or 3 it will flash green (labelled G). We have 8 possible states:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Mermin's device

Start with the simplest possible case. Write down what Mermin's device claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mermin's device before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mermin's device ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mermin's device

In research: Mermin's device appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mermin's device in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mermin's device is common in secondary-school and first-year university syllabi. It links to neighbouring topics Physical paradoxes, Quantum measurement, Thought experiments in quantum mechanics, so understanding it makes those chapters shorter.
In everyday life: Look for Mermin's device outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Mermin's device in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mermin's device means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mermin's device out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mermin's device in simple terms?

Why does Mermin's device matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mermin's device?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mermin's device.