At present, interstellar travel seems impossible for us, but if we could develop a spacecraft that bypasses the speed of light limit, that would change everything.
Since ancient times, humans have been filled with curiosity and a thirst for the cosmos, from ancient astronomical observations to modern aerospace technology, we continually explore this vast and mysterious space.
However, the scale of the universe is so immense that we measure distances in light-years, with one light-year equating to about 9.46 trillion kilometers. Even the closest star to us, Proxima Centauri, is 4.3 light-years away, meaning that even with our fastest spacecraft, it would take tens of thousands of years to reach it. Such time and distance make us feel helpless and frustrated about interstellar travel. So, is there a way to reach distant galaxies? Is there technology that allows us to break the speed of light barrier?
Is there technology that allows us to break the speed of light barrier? (Illustration: Zhihu).
The answer is yes, and that is the Alcubierre drive. The Alcubierre drive is a hypothetical device that allows a spacecraft to move faster than the speed of light without violating the laws of physics. Its principle of operation involves using energy to change the shape of the surrounding space, thereby reducing the distance between the spacecraft and its destination. The Alcubierre drive has not yet been realized in practice, but it has become one of the most popular and intriguing elements of science fiction. So what exactly is the Alcubierre drive? Can it actually achieve interstellar travel?
The Alcubierre drive is a hypothetical device that allows spacecraft to move faster than the speed of light. (Illustration: Zhihu).
The prototype of the Alcubierre drive appeared in 1957, when German physicist Kerhard Heim proposed a physical framework known as the Heim Theory. This theory attempts to resolve the conflict between quantum mechanics and relativity within a six-dimensional spacetime framework.
Although this theory is not widely recognized in the scientific community, it has sparked a strong reaction among science fiction enthusiasts. This theory provides the theoretical foundation for the appearance of warp drives in science fiction films.
The most famous application of the warp drive in science fiction is in the “Star Trek” series, where the USS Enterprise uses warp drive. The Enterprise utilizes energy generated from antimatter fuel to alter the shape of the surrounding space and achieve faster-than-light travel.
According to the film’s description, the Enterprise can move at a speed 9,000 times faster than light, meaning that in a place where light would take 9,000 years to arrive, the Enterprise would only take one year with its warp drive. The “Star Trek” series significantly boosted the popularity of the warp drive in the science fiction world, simultaneously igniting interest and anticipation for warp technology.
The spacecraft itself is surrounded by a non-distorted warp bubble. (Illustration: Zhihu).
In 1994, Mexican physicist Miguel Alcubierre proposed a mathematical model of spacetime known as the Alcubierre metric based on general relativity. This model describes a scenario where a spacecraft can compress the space in front and expand the space behind by distorting the surrounding space during navigation.
The spacecraft is surrounded by a non-distorted warp bubble, as it always resides within the curved bubble formed by the distorted spacetime, remaining in a static state relative to the surrounding space. This means that the spacecraft is not actually moving; it is navigating through the distortion of space.
As a result, effects such as time dilation and mass increase in relativity would not occur during navigation. This mode of travel resembles a person walking on an escalator; although the person moves slowly on the escalator, the escalator moves very quickly. Therefore, the spacecraft is not limited by the speed of light and can accelerate indefinitely.
In this way, it seems possible to achieve movement exceeding the speed of light. In reality, the spacecraft does not fly faster than the speed of light; the warp drive continuously compresses and expands space, causing the surrounding spacetime to be strongly distorted, creating a high-speed channel that shortens the distance traveled by the spacecraft during interstellar journeys.
If we had a warp drive, it would only take us 155 days to reach Proxima Centauri. (Illustration: Zhihu).
Suppose we have a warp drive applied to a spacecraft, allowing it to move ten times faster than the speed of light, then we would only take 155 days to reach Proxima Centauri. If our speed is even greater, such as one hundred times the speed of light, then it would only take 90 days to reach the Gliese 581 galaxy, which is 25 light-years away from Earth. If our speed increases further, such as a thousand times the speed of light, then we would only take 4 years to reach the Eagle Nebula, which is 4,000 light-years from Earth. Currently, the warp drive still faces many technical and theoretical issues, the biggest of which is negative energy density.
Negative energy density refers to a state of energy lower than that of a vacuum. Negative energy density is the core element of the warp drive, as it can create an attractive effect in space, thereby realizing the distortion of space. However, negative energy density is extremely rare and inherently unstable, and there is still no reliable and effective method to create and maintain negative energy density.
So far, scientists have only observed the presence of negative energy density in a phenomenon known as the Casimir effect. The Casimir effect is an attractive force that occurs between two very closely spaced parallel metal plates, and this attraction is due to the vacuum energy between the plates being lower than the vacuum energy outside the plates.
This means that there is negative energy density between the metal plates. However, the negative energy density created by the Casimir effect is very weak and not sufficient to create a warp drive. According to Alcubierre’s estimates, for a spacecraft to move ten times faster than light, the necessary negative energy density would be equivalent to converting the mass of Jupiter into energy.
