Wormholes

FIELDS OF STUDY: Theoretical Astronomy; Astrophysics; Cosmology

ABSTRACT: A wormhole is a hypothetical passage or tunnel through space-time that could theoretically allow travel from one place in time and space to another. The theory of general relativity allows the warping of space-time in a way that could allow for the formation of wormholes. However, no direct evidence of their existence has been discovered. The study of wormholes remains important because investigating the properties of space-time leads to greater understanding of its nature.

Tunneling through Space-Time

In 1915, German-born physicist Albert Einstein (1879–1955) proposed his theory of general relativity. According to this theory, the presence of matter in the universe causes time and space to curve around it. This curvature gives rise to the phenomenon of gravity, which in turn affects how matter moves through space. Rather than being independent and progressing in a straight line, time and space are affected by each other and by any matter within them.

One consequence of Einstein’s theory is the idea that gravity can bend light. According to classical physics, this would be impossible, because gravity is the result of one body with mass acting on another body with mass. However, if gravity is instead the result of space-time being curved, any particle in the universe would still have to follow the curve. This is true even of massless particles, such as photons (light particles).

One example of gravity exerting overwhelming force on everything around it is a black hole. The singularity at the center of a black hole is created when the remnant of a dying star is overwhelmed by its own gravity and collapses. As all of the star’s matter falls toward its center, it is condensed past the point where even the neutrons in its atomic nuclei are crushed. The end result is a dimensionless point in space with a mass many times that of the sun—in essence, a point of infinite density. A singularity can swallow vast amounts of gas and debris. It is surrounded by an event horizon, a boundary in space-time that defines the radius of a black hole. Within the event horizon, the gravitational pull of the singularity is so strong that not even light can escape.

In the 1930s, Einstein and his assistant Nathan Rosen (1909–95) reviewed a paper that had been published by Vienna-based physicist Ludwig Flamm (1885–1964) in 1916. Flamm’s paper summarized German physicist Karl Schwarzschild’s (1873–1916) solutions to Einstein’s general relativity theory. These solutions described some characteristics of black holes, which at the time were not yet proved to exist. In his summary, Flamm noted that Einstein’s theory allowed for the possibility of a second solution mathematically connected to the first. This second solution described what was later called a "white hole." A white hole is a hypothetical region of space with the reverse properties of a black hole, unable to be entered but capable of emitting light and matter. Einstein and Rosen proposed that a black hole could combine with a white hole to form a bridge between two areas of space-time. Matter would be drawn into the black hole on one side of the bridge and then expelled through the white hole on the other side. This theoretical model came to be called an Einstein-Rosen bridge.

In the 1950s, Princeton University–based theoretical physicist John Archibald Wheeler (1911–2008) began to study Einstein-Rosen bridges. It was Wheeler who first coined the term "wormhole." He noted that the path wormholes describe is much like the hole a worm makes through an apple, if the surface of the apple represents conventional space-time. Although Wheeler determined that the Einstein-Rosen model would be too unstable for any matter to travel through it, the idea of a possible shortcut through space-time remained.

The Challenges of Wormholes

While general relativity allows for the existence of wormholes, research strongly indicates that even if they exist, they would not be viable for human travel. An Einstein-Rosen bridge between two singularities would be too unstable, collapsing almost immediately after it formed. Anything attempting to travel through the bridge would be destroyed.

In the 1980s, American astronomer and popular science writer Carl Sagan (1934–96) worked with theoretical physicist Kip Thorne (b. 1940), then a professor at the California Institute of Technology, to devise a plausible method of faster-than-light travel for Sagan’s 1985 novel Contact. Thorne and his graduate students proposed that a wormhole could be stabilized using exotic matter. This hypothetical matter would have negative energy, meaning that its energy per unit volume would be less than zero. Such matter would be repelled by gravity rather than attracted by it.

No exotic matter with negative energy has been found to exist naturally in the universe, but very small amounts can be created in laboratory conditions. As a result, Thorne and other scientists agreed that such stable wormholes could not form naturally. Even if a wormhole could be stabilized in this manner, the introduction of nonexotic matter—for example, a person or a spaceship—could still cause it to collapse.

In 2014, Thorne’s theories on wormholes, black holes, and the physics of general relativity were used as the basis for the Christopher Nolan film Interstellar. Thorne served as science advisor to the film and published a book, The Science of Interstellar, about the theoretical science depicted in the film.

Other Potential Portals

While wormholes may be unlikely, scientists working with the US National Aeronautics and Space Administration (NASA) have discovered another type of portal. Data from NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites and the European Space Agency’s (ESA’s) Cluster II probes have revealed points in Earth’s magnetosphere where the planet’s magnetic field connects directly to the magnetic field of the sun, a phenomenon known as "magnetic reconnection." This connection allows high-energy particles to pass from the sun into Earth’s upper atmosphere. Such transfers are called "flux transfer events." The interaction of these particles with Earth’s magnetic field causes geomagnetic storms and auroras, among other phenomena.

These magnetic portals can be located by identifying points where the magnetic fields of Earth and the sun cross each other. NASA scientists call these points "X-points." Once opened, the portals form a direct, uninterrupted path from Earth to the sun, more than ninety-three million miles away. In March 2015, NASA launched the Magnetospheric Multiscale Mission (MMS) to further study these portals, how they work, and their potential impact. Their existence also raises the possibility that other as-yet-unidentified portals might be waiting to be found.

In an article published in the journal Scientific Reports in 2015, researchers at the Autonomous University of Barcelona reported having designed a device that allowed them transfer a magnetic field from one point in space to another through a "magnetic wormhole," leaving no magnetic trace of its path. The design was based on a theoretical device, first proposed by mathematics professor Allan Greenleaf and colleagues in 2007, that would render electromagnetic waves undetectable. While the design can only transport magnetic fields, not actual matter, it does have potential broader applications. For example, the researchers suggested it could be used to perform magnetic resonance imaging (MRI) from a greater distance than is now possible, so that patients would not have to lie inside the small, enclosed space of the MRI machine.

Tiny Wormholes and Time Travel

In 1955, Wheeler proposed that if the universe could be viewed on the smallest possible scale (the Planck scale, about 10⁻³⁵ meters in length), virtual particles, black holes, and wormholes would appear to be constantly popping in and out of existence. This idea is called the "quantum foam" hypothesis because space-time at this scale would resemble a mass of churning foam.

In a 2010 article for the Daily Mail, renowned British theoretical physicist Stephen Hawking (1942–2018) wrote that the laws of physics allow for the possibility of trapping one of these short-lived, infinitesimal wormholes and expanding it enough to travel through. He also claimed that time travel through such wormholes may be theoretically possible, although that possibility raises many other issues. Traveling back in time would violate the universal law that says cause must happen before effect. It would leave the universe vulnerable to paradox—an event that could change the past in such a way as to prevent itself from happening. Further, if a wormhole were opened to a point in the past, the natural radiation passing through it would create a feedback loop that would ultimately destroy it. For these reasons, Hawking believed that a wormhole to the past could never exist.

However, it is known that time passes at different speeds in different parts of the universe. For the Global Positioning System (GPS) satellites that orbit Earth, time passes slightly faster than it does on Earth’s surface, while in the vicinity of a black hole, time slows nearly to a standstill. The faster an object is moving, the slower the rate at which it experiences time. In this sense, time travel to the future is already possible, although the technology does not yet exist to travel more than a short "distance." To go farther forward in time would require a form of transportation that could travel at near light speed.

Wormholes may seem to be too theoretical and improbable to be of any serious interest, but the universe is full of the unknown and undiscovered. While scientists concede that any real wormholes may bear little to no relation to those envisioned in science fiction, there is still a good chance that they exist, and researchers continue to investigate the possibilities.

Principal Terms

• black hole: a region of space that exerts such strong gravitational force that nothing can escape, not even light.
• exotic matter: any type of matter, whether real or hypothetical, whose physical properties violate the known laws of physics; for example, matter with a mass or energy density of less than zero.
• general relativity: Albert Einstein’s theory that gravity is the result of matter and energy causing space-time to curve, which in turn affects the movement and distribution of matter and energy in space.
• singularity: a theoretical point or region of infinite density where the curvature of space-time, and thus the gravitational force, likewise becomes infinite.
• time travel: the theoretical or imagined ability to travel between two points in time.

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