Traveling faster than light: if a warp drive is possible, why hasn't it been developed yet?

In 1994, theoretical physicist Miguel Alcubierre sought to determine whether a warp drive could enable faster-than-light travel in space. Remarkably, the scientist discovered a way to make it feasible, although it remains unclear if such a drive could ever function in reality, as noted by astrophysicist Paul Sutter in an article for Space .

How Does a Warp Drive Work?

While current laws of physics state that moving faster than light is impossible, this limitation applies only to local measurements. It is possible to manipulate spacetime in a way that allows superluminal movement to become achievable. For instance, the expansion of the universe pushes galaxies apart at speeds exceeding that of light, but since each galaxy exists in its own local region of space, this is permissible, Sutter explains.

Alcubierre's idea was to employ a similar trick. His solution for a warp drive within the framework of general relativity utilizes a region of perfectly flat space. According to the theory, a warp drive installed on a spacecraft creates a warp bubble. In front of this bubble is a region of compressed space, while behind it is a region of expanded space. The compression of space allows the bubble and the spacecraft within it to move faster than the speed of light.

According to Sutter, surprisingly, the occupants of the bubble would not feel anything unusual. In fact, from their perspective, they would not be moving at all. Instead, their destination would simply come closer to them. However, there is one issue: to create a spacetime with such precise geometry, we would need to use negative mass, which apparently does not exist in the universe and would violate everything we know about motion, inertia, and energy.

Negative Energy

Although there is no negative mass known to exist in the universe, there is negative energy, Sutter notes. If you take two metal plates and hold them very close to each other, the quantum fields between them will be restricted. They can only have certain allowed wavelengths. This restriction, known as the Casimir effect, leads to an attractive force between the plates and a region of negative energy.

However, such a minuscule amount of negative energy is insufficient to power a warp drive, and it may not function. Could a warp drive exist? This is more a question of quantum gravity, but physicists have yet to develop a theory that unifies relativity (which best explains gravity) and quantum mechanics.

At the same time, scientists have been trying for the last few decades to understand what might happen to quantum fields in extremely strange gravitational environments. This has led to some intriguing, albeit sometimes contradictory, ideas about the nature of warp drives.

Challenges with the Warp Drive

As Sutter writes, some calculations suggest that the quantum fields at the edge of the warp bubble, which essentially encompass the boundary between its inner parts and outer space, explode to infinity as soon as you activate the warp drive. Other calculations indicate that this only applies in limited cases, and if you gradually increase the power of the warp drive, everything should be fine.

Some other calculations sidestep all this and show how much negative energy is actually needed to create a warp drive. It turns out that to generate a single macroscopic warp bubble with a diameter of about 100 meters, you would require ten times more negative energy than the positive energy available in the universe. And that poses a significant problem.

However, other calculations suggest that this enormous amount applies only to the traditional warp bubble as defined by Alcubierre. It might be possible to modify the shape of the bubble so that it has a tiny "neck" at the front that does the work of compressing space, followed by a bulging region to contain the warp bubble. This minimizes any quantum oddities, so the amount of negative energy needed to power the warp drive would be equivalent to the energy emitted by a star.

According to Sutter, further calculations indicate that even if you obtain some negative energy or negative mass, once you start moving, you will encounter problems. Negative mass would immediately begin to leak from the edge of the bubble at a speed exceeding that of light. Ultimately, the exotic matter forming the warp bubble cannot keep pace with the bubble itself, causing it to tear itself apart.

While it may seem impossible to create a warp drive, it cannot be definitively stated that it is. Contemplating the feasibility of even the theories surrounding warp drives aids in exploring some remarkable connections between general relativity and quantum mechanics, Sutter summarizes.