You could say that this is exactly what Isaac Newton’s picture of gravity does—providing a relationship between the mass of an object and the gravitational force it exerts. And you would be right. But the concept of space-time curvature provides a richer set of phenomena than a simple force. This enables a kind of repulsive gravity that drives our universe to expand, creates time dilation around massive objects and gravitational waves in space-time, and—in theory at least—makes warp drives possible .
Alcubierre discusses his problem from the opposite direction to the common. He knows what kind of space-time curvature he wants. It is one in which an object can surf through a region of warped space-time. So, he worked backwards to determine the type of configuration item you would need to do this. It is not a natural solution of equations, but rather something “made to order.” It wasn’t exactly what he would order. He found that he needed something strange, something with a negative energy density, to warp space in the right way.
Strange matter solutions are generally viewed with skepticism by physicists, and rightly so. Although mathematically, one can describe matter as having negative energy, almost everything we know appears to have positive energy. But in quantum physics, we observe that small, temporary violations of the positivity of energy can occur, and thus, “no negative energy” cannot be an absolute, fundamental law.
From Warp Drives to Waves
Given Alcubierre’s space-time warp drive model, we can begin to answer our original question: What would a signal from this look like?
One of the cornerstones of modern gravitational wave observations, and one of its greatest achievements, is the ability to accurately predict waveforms from physical situations using a tool called “numerical relativity.”
This tool is important for two reasons. First, because the data we get from the detectors is still very noisy, which means we often need to know what a signal looks like to extract it from the data stream. And second, even if a signal is so strong that it stands above the noise, we need a model to interpret it. That is, we need to model 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 space-time is that it doesn’t naturally give off gravitational waves unless it starts or stops. Our idea was to study what happens when a warp drive stops, especially in the case of something going wrong. Suppose the warp drive containment field collapses (a staple storyline in sci-fi); there would likely be an explosive release of both strange matter and gravitational waves. This is something we can, and have, simulated using numerical relativity.
What we found was that the collapse of the warp drive bubble was indeed a very violent event. The enormous amount of energy required to warp space-time is released as both gravitational waves and waves of positive and negative matter energy. Unfortunately, this is likely to be the end of the line for the ship’s crew, who will be destroyed by the tidal force.
