Quantum foam is a proposed structure of space and time at the very smallest scales, where they would churn and fluctuate instead of being smooth. It has never been observed directly, so the 9.3 × 10⁻³⁶ meters shown above is not a measurement. That number is 9.3 divided by a 1 followed by 36 zeros. Physicists expect the foam, if it exists, to appear at about the Planck length, 1.6 × 10⁻³⁵ meters, about 1.7 times the figure shown.

Size

Space-time is the three dimensions of space together with time. At every scale that can be measured, it looks smooth. But parts of quantum mechanics, the theory that explains atoms and particles, predict that it would not stay smooth at far smaller scales. Tiny regions would keep changing, and space and time would stop having definite values.

Foam like this has no edge to measure, so what the theories give instead is a scale: the distance at which the foam would show. NASA puts it at about the width of a hydrogen atom's nucleus divided by 100 billion billion (10²⁰). That works out to about the Planck length, the scale at which the quantum effects of gravity are expected to matter. The exact scale depends on the theory, and physicists do not yet have a tested theory that joins gravity with quantum mechanics.

Size comparisons

The foam's expected scale is about the same as the Planck length, the next larger entry in the explorer.
A proton, about 1.7 femtometers across, is about 100 billion billion (10²⁰) times as wide as the foam's expected scale. A femtometer is 10⁻¹⁵ meters.
If the foam's scale were enlarged to the width of a human hair, about 0.1 millimeters, a proton would be about a light-year across. A light-year is the distance light travels in one year.
The quasars that Chandra observed for the 2015 test described below are about 12 billion light-years away. That distance is about 7 × 10⁶⁰ times the foam's expected scale, which is why tiny effects might add up along the way.

Where the idea came from

The physicist John Wheeler of Princeton University came up with quantum foam and its name. Quantum mechanics already predicted that particles could briefly pop into and out of existence. Wheeler pictured more than that: space-time itself whipped into a froth of distorted shapes. Wheeler also made the term "black hole" famous.

Testing for foam

The foam is far too small to see directly, so astronomers look for its effects on light. If space-time is foamy, the distance light travels would jitter by tiny amounts. In some models, these jitters add up over billions of light-years until distant objects blur out of sight.

In 2015, a team led by Eric Perlman of the Florida Institute of Technology tested this with quasars. A quasar is a very bright object powered by matter falling toward a giant black hole. NASA's Chandra X-ray Observatory, NASA's Fermi Gamma-ray Space Telescope and the VERITAS telescopes still detected distant quasars clearly. The team concluded that this ruled out two models of the foam, including one called holographic foam.

The question is not settled. In a 2026 conference paper, Eric Steinbring and Jack Ng, one of the 2015 authors, argued that holographic foam fits the spread in measured positions of an extremely bright gamma-ray burst after all. They present this as one way to read the data. Whether the foam's effects can build up over long distances has been debated for more than 20 years.

Sources

NASA Telescopes Set Limits on Space-time Quantum "Foam". NASA Chandra X-ray Center, 2015.
New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations. Perlman et al., The Astrophysical Journal, 2015.
The Great and the Small: Is Quantum Foam Losing its Fizz?. Eric Perlman, Chandra X-ray Center, 2015.
BR 0331-1622: NASA Telescopes Set Limits on Space-time Quantum Foam. NASA Chandra X-ray Center, 2015.
Holographic Quantum Foam: Theoretical Underpinnings and Observational Evidence. Steinbring and Ng, arXiv preprint for Proceedings of Science, 2026.
Leading physicist John Wheeler dies at age 96. Princeton University, 2008.
Planck length. NIST CODATA, 2022.
Proton rms charge radius. NIST CODATA, 2022.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Measuring the Universe. International Astronomical Union.

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