The Wormhole

From Einstein’s wall to Thorne’s door. How a shortcut through spacetime went from mathematical curiosity to a real question in physics — and why it still lives in the equations, waiting.

In 1935, Einstein and his colleague Nathan Rosen were trying to do something admirable: get rid of singularities. General relativity kept producing these points where the math broke down, where density went to infinity and the equations gave nonsense. They didn’t like it. So they looked for a way to describe a particle — an electron, say — not as a point, but as a bridge between two sheets of spacetime. Two separate regions of the universe, joined at a throat.

They published the idea in Physical Review in 1935. The structure they found was real — a genuine solution to Einstein’s equations. Later it would be called an Einstein–Rosen bridge. But there was a problem, and it was decisive: the bridge was not a door. It snapped shut instantly. Nothing — no signal, no particle, not even a photon moving at the speed of light — could pass through before the throat pinched off. It was a wall wearing the shape of a passage.

Wheeler names it

For two decades the bridge sat in the literature as a mathematical object, more curiosity than physics. Then, in 1957, John Archibald Wheeler came along. Wheeler was one of the great renamers of physics — he gave us “black hole,” “quantum foam,” a dozen others — and he gave us this one too. He called it a wormhole. He also described quantum foam: the idea that at the very smallest scales of spacetime, at distances so tiny they make an atom look like a galaxy, the geometry is not smooth but seething — wormholes blinking into and out of existence at the Planck scale, a constant fizz beneath the fabric of everything.

The name stuck. The concept was vivid and precise. But the original verdict held: wormholes, as Einstein and Rosen found them, were not traversable. The question of whether any wormhole could be traversable lay dormant for another three decades.

The novelist’s question

In 1985, Carl Sagan was writing his novel Contact. He needed a way for his protagonist to travel across interstellar distances in the span of a human life. He tried a black hole — it didn’t work physically. So he called his friend Kip Thorne at Caltech and asked, roughly: is there anything the equations actually allow?

Thorne looked at the problem seriously. What would a wormhole need to be traversable — wide enough to fly a ship through, stable enough not to collapse in transit, gentle enough that tidal forces didn’t kill the traveler? He found that the equations of general relativity did not forbid it. They demanded a price: exotic matter, a material with negative energy density, threaded through the throat to hold it open. Whether such matter exists in usable form is an open question. The Casimir effect shows that negative energy densities can appear between two conducting plates — that much is measured, real. But enough exotic matter, in the right configuration, to hold a wormhole open? No one has found it. No one has ruled it out.

The consultation spawned real physics. Thorne and his students turned the problem into a paper.

Morris and Thorne, 1988

In 1988, Michael Morris and Kip Thorne published “Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity” in the American Journal of Physics. It is one of the more unusual papers in the canon. The explicit audience is physics teachers; the real audience is anyone who wants to know what a traversable wormhole would actually require. The paper works out in careful detail how a wormhole could be wide, stable, and gentle enough that a traveler survives the crossing. It is the founding document of traversable wormhole physics. Not speculation in the loose sense — speculation in the scientific sense: a coherent, falsifiable set of conditions derived from equations we trust.

The honest status of all of this: no wormhole has ever been observed. This is physics that the equations permit, not physics that nature has demonstrated. The gap between “allowed by general relativity” and “found in the universe” is not a small one. It is the gap between a proof of concept and a fact.

From the page to the screen

When the Interstellar team built the wormhole near Saturn, they built a Morris–Thorne-class traversable wormhole. Thorne worked with the visual effects team on the renderer. The result looked nothing like the flat vortex that science fiction had been drawing for decades. A correctly-rendered wormhole is spherical — a crystal ball showing you the other side, the starfield on the far end curved across its surface like a fisheye lens. Light bending around the throat from both directions at once. It was the first scientifically-rendered wormhole in cinema, and it produced genuine published research about what imaging a wormhole would look like.

The path from Einstein’s wall to that image on the screen is ninety years of stubborn physics: one closed door, one name given in 1957, one novelist’s question in 1985, one paper in 1988 that worked out the terms. The door is not yet open. But at least now we know what it would take.

What is especially strange is that a traversable wormhole — a connection between distant regions of our universe — is not forbidden by the known laws of physics. It just requires something we haven’t found yet. — derived from Morris & Thorne, Am. J. Phys. 56, 395 (1988)