Interstellar Travel V: Warp Drives, Wormholes, and Halo Drives

Welcome back to our series on Interstellar Travel, where we look at the many proposals made for sending missions to the stars since the dawn of the Space Age. In our first installment, we examined how Cold War-era developments in nuclear weapons and rockets paralleled advances in space exploration, resulting in proposals for nuclear rockets. We then examined how the creation of thermonuclear weapons led to applications in fusion propulsion.

Then we looked at how the discovery and creation of positrons and anti-protons fueled proposals for spacecraft driven by antimatter propulsion. In our fourth installment, we addressed the most practical options for sending missions to interstellar space, including solar sails, magnetic sails, and directed-energy propulsion (DEP). In our fifth installment, we'll examine the most exotic proposals ever made that could (in theory) enable interstellar spaceflight someday.

As we explored in a previous article, a ship relying on conventional propulsion methods would take thousands of years. And while theoretical methods exist that could shorten transits to a few decades, they would be very expensive to build and require tremendous amounts of propellant. Alas, there are always those long-shot proposals, the kind that straddle the line between science fiction and fact, that still give us hope.

Among them are the warp drive, wormhole travel, and using black holes to achieve velocities approaching the speed of light.

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to our Solar System. Credit: ESO/M. Kornmesser.

Nearest Star

As noted in our inaugural installment, there is no shortage of planets beyond the Solar System (exoplanets) that are well worth exploring. To date, scientists have confirmed the existence of 6,354 exoplanets in 4,756 star systems. In fact, that number has increased since this series began on July 31st! At that time, the census was 6,333 confirmed exoplanets in 4,747 systems. Doing the math, that means that 21 more exoplanets in 9 star systems have been confirmed in less than a month.

What's more, while "Earth-like" (rocky) planets are rare, with the total still at 222 confirmations, they are surprisingly abundant in our stellar neighborhood. In fact, within 50 light-years of Earth, there are 31 known terrestrial planets, 30 of which orbit low-mass, M-type red dwarf stars. The closest of these is Proxima b, a rocky planet orbiting Proxima Centauri, a red dwarf star 4.25 light-years from Earth.

Discovered in 2016, the planet has an estimated surface temperature of -39 °C (-38 °F), compared to 15 °C (59 °F) for Earth. While scientists are divided on whether this planet is habitable, there are scenarios in which Proxima b could support life. Regardless, Proxima b's proximity to Earth makes it the natural choice for an interstellar mission and likely the first stop in humanity's exploration beyond the Solar System (should that ever occur).

So what can the most exotic propulsion methods ever devised do to get us there?

Alcubierre Warp Drive

Fans of science fiction are certainly familiar with the concept of the Warp Drive. While the idea has existed for decades, it has largely been confined to the realm of science fiction and pure speculation. It was not until 1994 that an actual proposal was made for how a warp drive could work within the realm of known physics. This proposal is credited to Mexican theoretical physicist Miguel Alcubierre, who proposed what would become known as the "Alcubierre Drive" as part of his Ph.D. studies at Cardiff University, Wales.

Alcubierre described the concept in a 1994 paper titled "The warp drive: hyper-fast travel within general relativity," in which he argued that it was possible to move faster than the speed of light (FTL) without violating Einstein's Theory of Relativity. More specifically, he distinguished between Einstein's Special Theory of Relativity (SR) and his General Theory of Relativity (GR).

Annotated graphic of a spacecraft within a warp bubble, showing the 1) vertical dimension (expanding spacetime), 2) interior of the warp bubble, and 3) negative dimension (contracting spacetime). Credit: Kris Holland/Matt Jeffries

"When we study special relativity, we learn that nothing can travel faster than the speed of light," he wrote. "This fact is still true in general relativity, though in this case one must be somewhat more precise: in general relativity, nothing can travel locally faster than the speed of light." The concept involves stretching the fabric of space-time in a wave (a "warp bubble"), which would theoretically cause the space ahead of it to contract and the space behind it to expand. An object inside this wave could then ride it to super-relativistic speeds. As he explained it:

The basic idea can be more easily understood if we think for a moment in the inflationary phase of the early Universe, and consider the relative speed of separation of two comoving observers. It is easy to convince oneself that, if we define this relative speed as the rate of change of proper spatial distance over proper time, we will obtain a value that is much larger than the speed of light. This doesn’t mean that our observers will be traveling faster than light: they always move inside their local light-cones. The enormous speed of separation comes from the expansion of spacetime itself.

Since the ship is being carried along as it moves (but is not moving itself), the rules of spacetime and GR would cease to apply since this method does not rely on moving faster than light in the local sense. It is only FTL in the sense that a ship inside the bubble would reach another star before a beamed transmission traveled the same distance. The theoretical basis of this drive system, where Alcubierre proposed resolutions to Einstein's field equations for GR, is known as the Alcubierre Metric.

As Alcubierre argued, quantum field theory allows for regions of spacetime with negative energy densities to exist. This is known as the Casimir Effect, the attractive force between two surfaces in a vacuum. If a "ring" of negative mass could be created around a spacecraft, spacetime could theoretically contract in front of the ship and expand behind it. As he wrote:

By a purely local expansion of spacetime behind the spaceship and an opposite contraction in front of it, motion faster than the speed of light as seen by observers outside the disturbed region is possible. The resulting distortion is reminiscent of the "warp drive" of science fiction. However, just as it happens with wormholes, exotic matter will be needed in order to generate a distortion of spacetime.

However, the concept has its share of theoretical challenges. According to Alcubierre's original paper, the amount of negative mass (or "exotic matter") required to achieve a warp field was beyond anything humanity could achieve. There are also many uncertainties, including violations of causality, the effect on spacetime at the destination, and whether such a system would be safe for those riding the wave. In addition, all proposed warp-drive theories violate various energy conditions.

In the past fifteen years, the Alcubierre Drive has received renewed attention, thanks in large part to NASA scientist Harold "Sonny" White. Dr. White's involvement began in 2011 while preparing to deliver a speech at the first 100 Year Starship symposium, a joint project hosted by NASA and the Defense Advanced Research Projects Agency (DARPA). White chose to discuss Alcubierre's field equations, during which time he made some calculations and found that the energy requirements may actually be within the realm of possibility.

In 2012, White announced that he and his colleagues at the Advanced Propulsion Physics Research Laboratory (NASA Eagleworks) had begun researching the concept. In a paper titled "Warp Field Mechanics 101," White described how his team had built an interferometer for detecting spatial distortions produced by the expanding and contracting spacetime of the Alcubierre Metric.

In 2013, the Jet Propulsion Laboratory published results of a warp field test conducted under vacuum conditions, which were reported as "inconclusive." By 2019, NASA effectively ended its warp drive and advanced propulsion research after White left the agency. A year later, White began working with engineers and scientists at the Limitless Space Institute (LSI), a non-profit organization dedicated to education, outreach, and advanced propulsion methods, with the ultimate purpose of creating the first warp drive!

White's research builds on the work of his colleague, astrophysicist Richard Obousy, who co-founded Project Icarus with starship engineer Kevin Long in 2009. Also in 2009, Obousy and co-author Aram Saharian released a study titled "Casimir energy and the possibility of higher dimensional manipulation," which considered how next-generation particle accelerators could produce fields that alter the density of dark energy locally, changing the expansion of spacetime. Their calculations also indicated that this could be done with a negative mass about the size of Jupiter (1.898 x 10²⁴ kg; 4.18 x 10²⁴ lbs).

While their calculations indicated that it was mathematically possible to generate a warp bubble, the energy requirement is beyond anything humanity can currently conceive. But as White explained to Universe Today in a previous interview, he and his fellow researchers showed how reconsidering the "shell-thickness parameter" of the warp bubble could further reduce the energy requirement. In short, a thicker warp shell would reduce the strain on spacetime, thus allowing a spacecraft to achieve speeds of up to 10 times the speed of light (10 c) using only two metric tons (2.2 U.S. tons) of exotic matter.

Alas, while the concept has become much more feasible in recent years, it is still a theoretical proposal. At present, scientists have not confirmed the existence of negative mass, and a proof of concept is still a long way off.

Wormhole Travel

Here's another staple of science fiction, one which fans are sure to recognize. To break it down, wormholes are theoretical tunnels that connect two points in spacetime. The theory emerged in the early 20th century in response to Einstein's General Theory of Relativity, which established that the speed of light could not be reached or exceeded. The first to postulate their existence was German physicist and astronomer Karl Schwarzschild, whose solutions to Einstein's field equation (the Schwarzschild metric) provided the first theoretical basis for black holes.

A consequence of the Schwarzschild metric is what he termed "eternal black holes," which were essentially connections between different points in spacetime. Albert Einstein and Nathan Rosen similarly postulated the existence of wormholes, known as Einstein-Rosen Bridges, in a 1935 paper titled “The Particle Problem in the General Theory of Relativity.” According to their paper, this bridge would connect a regular black hole (from which nothing can escape) and a "white hole" (from which nothing can enter).

Unfortunately, the "bridge" is located behind the black hole's event horizon, so any spacecraft entering the black hole would be crushed into the singularity. What's more, the Schwarzschild Metric showed wormholes to be unstable, as they would collapse too quickly for anything to cross from one end to the other. For a wormhole to be stable (a traversable wormhole), it requires negative mass. While negative mass is yet to be discovered, negative energy exists in many quantum scenarios (again, the Casimir Effect),

In a series of papers, Prof. Juan Maldacena from the Institute for Advanced Study and Alexey Milekhin, a graduate student at Princeton University, explored the possibility of humans traversing wormholes. In the first, "Traversable wormholes in four dimensions," they proposed how charged massless fermions could give rise to a Casimir-like energy, supporting a stable wormhole. In their second paper, "Humanly traversable wormholes,," they argued that these wormholes were consistent with the Standard Model of Particle Physics.

However, they found this scenario would apply only to microscopic black holes, since the entire process is in the quantum realm. To be traversable by humans, wormholes would need to be significantly larger, requiring physics that go beyond the Standard Model. To that end, they adopted the Randall-Sundrum II model (aka. 5-dimensional warped geometry theory), from which they derived intermediately-sized charged black holes. But these would generate powerful tidal forces, so a spacecraft would need a significant boost factor to bypass them and enter the wormhole.

Meanwhile, research on Einstein-Rosen Bridges by Daniel Jafferis of Harvard University showed that while possible, stable wormholes would take longer to traverse than normal space. From the traveler's perspective, virtually no time would have passed but the travel time would seem much longer to an outsider. This is consistent with General Relativity and the time dilation caused by relativistic travel.

Once again, though, the issue comes down to negative mass, which is required in all cases to keep wormholes stable. Since scientists have no idea how to generate negative mass, or find it as a natural by-product of cosmic processes, wormhole travel remains the stuff of theory and science fiction.

Halo Drive

To recap, traveling to even the nearest stars beyond our Solar System is always subject to the tyranny of General Relativity. To reach a significant velocity and get there in a human's lifetime, an incredible amount of energy is required. This means a tremendous amount of propellant, tiny spacecraft, massive laser arrays, exotic physics, or a combination of these. But what if it was possible to accelerate a spacecraft to relativistic speeds without the need for propellant?

This is the logic of the Halo Drive, a concept proposed by Prof. David Kipping of Columbia University and the leader of its Cool Worlds lab. Prof. Kipping described this concept in a 2019 paper, where he addressed these challenges and devised a rather exotic solution. To this, Kipping suggests a modified version of what is known as the "Dyson Slingshot," an idea proposed by venerated theoretical physicist Freeman Dyson (inventor of the Dyson Sphere).

In the 1963 anthology Interstellar Communications (Chapter 12: "Gravitational Machines"), Dyson described how spacecraft could slingshot around compact binary stars to gain a significant boost in velocity. As Dyson described it, a ship would be sent to a compact binary system where it would perform a gravity-assist maneuver with the two stars. The spacecraft would pick up speed from the stars' intense gravity, adding the equivalent of twice their rotational velocity to its own, before being flung out of the system.

Expanding on this, Kipping proposed that the same gravity-assist maneuver could be accomplished using a rapidly rotating black hole. This idea calls to mind the Penrose Process, proposed by English mathematical physicist and Nobel-prize winner Roger Penrose. In 1971, Penrose proposed that energy could be extracted from a black hole's ergosphere, the region just outside the event horizon. In this region, infalling matter is accelerated to velocities approaching the speed of light, emitting tremendous energy in the process.

By harnessing the multi-wavelength radiation emitted by the infalling matter, an intelligent species would have a virtually inexhaustible supply of energy. In the case of the Halo Drive, however, a spacecraft would insert itself into the ergosphere, allowing the black hole's rapid rotation to accelerate it to near-to-light (NTL) velocities. And just like Dyson's gravitational machines, the spacecraft would break off from the ergosphere and travel towards its destination at incredible speeds.

This power of gravitational slingshots is supported by the relatively recent discovery of hyper-velocity stars. According to research from the Harvard-Smithsonian Center for Astrophysics (CfA), these stars are a result of galactic mergers and interaction with massive black holes, which causes them to be kicked out of their galaxies at speeds of ~30,000 to 100,000 km/s (18,600 to 62,000 mps) - one-tenth to one-third the speed of light.

But of course, the Halo Drive presents numerous challenges and some notable disadvantages. In addition to requiring a spacecraft capable of withstanding intense radiation, it would also need to be strong enough structurally to handle the stresses of the intense acceleration it imparts. The process also demands tremendous precision; otherwise, the ship and crew could be pulled apart in the black hole's maw. On top of that, there's the matter of reaching the black hole, which would require significant propellant and long transit times.

Ultimately, the most exotic concepts for achieving interstellar travel suffer from the same problems as many of the less exotic ones. In short, they require tremendous amounts of energy, significant advances in science, and/or the discovery of exotic physics and matter. At the same time, these concepts remain fixed in the popular imagination and continue to be the subject of research because they instill hope. With the right scientific breakthrough and commitment, an FTL or NTL concept could be realized that would deliver us to distant worlds.

The kind of hope that says "someday this will be possible" is always a welcome relief whenever we're faced with the harsh realities of spaceflight and the sheer grandeur of the Universe.

Sources:

The warp drive: hyper-fast travel within general relativity - Alcubierre, M. Classical and Quantum Gravity, vol. 11, no. 5 (1994)
Warp Field Mechanics 101 - Dr. Harold "Sonny" White (2011)
Casimir energy and the possibility of higher dimensional manipulation - R.K. Obousy & A.A. Saharian (2009)
The Particle Problem in the General Theory of Relativity - Einstein, A. & Rosen, N. Physical Review Journals, vol. 48, no. 73 (1935)
Traversable wormholes in four dimensions - Maldacena, J. et al. Classical and Quantum Gravity, vol. 40, no. 15 (2023)
Humanly traversable wormholes - Maldacena, J. & Milekhin, A. * Physical Review D*, vol. 103 (2021)
The Halo Drive: Fuel-Free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons - Kipping, D. (2019)