You could say that this is exactly what Isaac Newton’s image of gravity Yes, it does, giving a relationship between the mass of an object and the gravitational force it exerts. And you would be right. But the concept of spacetime curvature gives rise to a much richer range of phenomena than a simple force. It allows for a kind of repulsive gravity that drives our universe to expand, creates time dilation around massive objects and gravitational waves in spacetime, and, at least in theory, makes warp drives possible.
Alcubierre approached his problem from the opposite direction than usual. He knew what kind of space-time curvature he wanted. It was one where an object could surf in a region of warped space-time. So he worked backwards to determine the kind of configuration of matter that would be needed to create this. It wasn’t a natural solution to the equations, but something “tailor-made.” However, it wasn’t exactly what he would have asked for. He found that he needed exotic mattersomething with a negative energy density, to warp space in the right way.
Physicists are often skeptical of exotic matter solutions, and rightly so. While matter can be described mathematically with negative energies, almost everything we know seems to have positive energy. But in quantum physics, we have observed that small, temporary violations of energetic positivity can occur, and therefore the “absence of negative energy” cannot be an absolute fundamental law.
From warp engines to waves
Given Alcubierre’s model of warp-driven spacetime, we can begin to answer our original question: What would a signal from it look like?
One of the cornerstones of modern gravitational wave observations, and one of their greatest achievements, is the ability to accurately predict waveforms from physical scenarios using a tool called “numerical relativity”.
This tool is important for two reasons. First, because the data we get from detectors is still very noisy, meaning we often have to know roughly what a signal looks like in order to extract it from the data stream. And second, even if a signal is so strong that it stands out above the noise, we still need a model to interpret it. That is, we need to have modeled many different types of events, so that we can match the signal to its type; otherwise, we might be tempted to dismiss it as noise or mislabel it as a black hole merger.
One problem with warp drive spacetime is that it doesn’t naturally produce gravitational waves unless it’s started or stopped. Our idea was to study what would happen when a warp drive was stopped, particularly in the case where something went wrong. Suppose the warp drive containment field collapsed (a staple storyline in science fiction); presumably there would be an explosive release of both the exotic matter and the gravitational waves. This is something we can, and did, simulate using numerical relativity.
What we discovered was that the collapse of the warp drive bubble is indeed an extremely violent event. The enormous amount of energy required to warp space-time is released in the form of gravitational waves and waves of positive and negative matter energy. Unfortunately, it will most likely be the end of the road for the ship’s crew, who would be torn apart by tidal forces.