How big is String? | Scale of the Universe

In string theory, a string is a proposed basic building block of nature: a tiny thread that has length but no width. Strings are hypothetical, and no experiment has detected one, 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. String theory itself does not say exactly how long a string is.

Size

String theory proposes that the smallest pieces of matter are not points but tiny loops of string. How big a string is depends on a quantity called the string length. The theory leaves its value open. It depends on other unknowns, such as how strongly strings interact and the size of the extra dimensions the theory requires.

Physicists usually assume a string length not far above the Planck length, 1.6 × 10⁻³⁵ meters, the scale at which the quantum effects of gravity are expected to matter. In the versions of the theory that physicists understand best, a string is longer than the Planck length. The figure shown above is a little over half the Planck length.

In string theory, the string length also acts roughly as a smallest distance. A string cannot probe anything shorter than itself, because at that scale it is fuzzy.

Size comparisons

About 100 billion billion (10²⁰) strings one Planck length long, laid end to end, would stretch across a proton, which is about 1.7 femtometers wide. A femtometer is 10⁻¹⁵ meters, and the Planck length is the next larger entry in the explorer.
If a string one Planck length long 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.
Experiments at the HERA particle collider found that quarks are less than 8.6 × 10⁻¹⁹ meters across. That limit is still about 50 million billion (5 × 10¹⁶) Planck lengths.

What strings would explain

In string theory, every kind of particle is a string vibrating in its own way, each with its own frequency. The theory began as an attempt to describe how particles such as protons interact. It grew into an attempt to describe all particles and forces in one theory.

Gravity is not optional in string theory: the theory requires it. Many physicists study string theory as a possible theory of quantum gravity, one that would join gravity with quantum mechanics. The theory also requires six or seven extra dimensions of space beyond the three that have been observed.

Why strings have not been tested

String theory is speculative. There is no experimental evidence that it describes the real world, and it does not yet make a single, definite prediction that experiments can check. In the picture developed around 1980, the extra dimensions are curled up at about the Planck scale, too small to test in the near future.

Some versions put strings within reach of the Large Hadron Collider, the world's largest and most powerful particle accelerator. The collider has searched for heavy particles that would be vibrating strings. As of the Particle Data Group's 2025 review, it had ruled out such particles lighter than about 8,400 times the mass of a proton.