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Is anything truly random?

A dice roll is only unpredictable because you cannot see the details. Quantum measurement may be different, and there is now an experiment that certifies unpredictable bits without a model of the device that made them.

Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Inspect the question
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The question

A coin toss or a dice roll is a deterministic process you cannot track: the spin, the table, the tremor in a hand.

The question is whether there is anything left when you take the ignorance away.

What can be shown

Known. Strongly supported by evidence and broadly established.

Strongly supported by evidence and broadly established.

Randomness can be certified rather than assumed

The usual problem with a random number generator is that you have to trust it. A device could be producing a pattern nobody has noticed, or one somebody built in.

Bell tests provide a way out. If the two measurement stations are far enough apart that nothing travelling at light speed could pass between them in time, and the settings are chosen independently of the devices, correlations that violate the inequality cannot come from a device whose outputs were fixed in advance, so the violation itself certifies that they were not, and it certifies it without any assumption about what is inside the box.

A 2018 experiment did this with entangled photons, extracting 1,024 bits uniform to within a part in a trillion.

Two kinds of unpredictable

Observed
A coin toss cannot be predicted in practice, beyond a slight lean toward the side it started on.
Inferred
Ignorance of initial conditions. The physics is deterministic and a sufficiently detailed measurement would predict it.
Observed
Which detector a photon triggers cannot be predicted.
Inferred
Either the same thing at a level nobody can see, or genuine indeterminacy. Quantum mechanics as standardly used gives only a probability for each detector.
Observed
Outputs that violate a Bell inequality.
Inferred
Not fixed in advance by anything local. Unpredictability that does not depend on trusting the device.
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Unpredictable to us, or unpredictable to anything. The experiments reach the first and press on the second.

What is left assumed

Uncertain. Meaningful scientific, historical or philosophical disagreement remains.

Meaningful scientific, historical or philosophical disagreement remains.

Whether the universe is deterministic underneath

Interpretations differ. Some take the randomness as fundamental. Some describe a deterministic evolution in which all outcomes occur and the apparent randomness is about which branch an observer finds themselves in. One keeps determinism by adding hidden variables and giving up locality.

All of them agree with every experiment performed so far, so this is not a question the experiments above settle. Collapse theories, one way of taking the randomness as fundamental, predict different statistics in experiments not yet performed.

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What certification actually assumes

It assumes that the experimenters’ choices of what to measure are independent of the system being measured. A universe in which those choices were correlated with the particles could fake the result.

That assumption cannot be tested from inside. It is the residue that every version of this argument leaves behind.

The rest of what is trusted is more ordinary and is stated in the paper: the timing electronics that verify the separation, the finality of a result once recorded, the classical computers that process the data. In the run that produced the 1,024 bits the settings came from a standard pseudorandom generator, which the authors call predictable and unsuitable against an adversary. They argue that their devices had no physical means of predicting it.

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The evidence

Primary research, scientific institutions and archives. Links open at the publisher.

  1. Experimentally generated randomness certified by the impossibility of superluminal signalsBierhorst and others, Nature · April 2018
  2. A snapshot of foundational attitudes toward quantum mechanicsSchlosshauer, Kofler and Zeilinger, Studies in History and Philosophy of Modern Physics · 2013
  3. Fair coins tend to land on the same side they started: evidence from 350,757 flipsBartoš and others, Journal of the American Statistical Association · 2025
  4. The three-dimensional dynamics of the die throwKapitaniak, Strzalko, Grabski and Kapitaniak, Chaos · 2012
  5. Philosophical Issues in Quantum TheoryMyrvold, Stanford Encyclopedia of Philosophy · revised March 2022
  6. Bell’s TheoremMyrvold, Genovese and Shimony, Stanford Encyclopedia of Philosophy · revised January 2024
On a route

What a measurement is

The experiments first, and the interpretations last, once it is clear what any of them has to account for.

03 of 05This question is stop 3 of 5 on What a measurement is, a route through the Ocean of Reality.
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