You could say that this is exactly it Isaac Newton’s Image of Gravity does — by relating the mass of an object to the gravitational force it exerts. And you’d be right. But the concept of spacetime curvature gives rise to a much richer range of phenomena than a elementary force. It allows for the kind of repulsive gravity that drives the expansion of our universe, creates time dilation around massive objects and gravitational waves in spacetime, and — at least in theory — makes warp drives possible.
Alcubierre approached his problem in the opposite direction than usual. He knew what kind of spacetime curvature he wanted. One in which an object could surf a region of warped spacetime. So he worked backwards to determine the kind of matter configuration needed to create it. It was not a natural solution to the equations, but rather something “made to order.” But it wasn’t exactly what he would have ordered. He found that he needed exotic mattersomething with negative energy density to curve space appropriately.
Exotic matter solutions are generally viewed with skepticism by physicists, and rightly so. While mathematically you can describe material with negative energies, almost everything we know seems to have positive energy. However, in quantum physics we have observed that there can be miniature, short-lived violations of the positivity of energy, and therefore “no negative energy” cannot be an absolute, fundamental law.
From Warp Drives to Waves
Given the Alcubierre hyperdrive space-time model, we can begin to answer our original question: What would the signal from this drive look like?
One of the pillars of newfangled gravitational wave observations and one of their greatest achievements is the ability to accurately predict wave forms based on physical scenarios using a tool called “numerical relativity.”
This tool is crucial for two reasons. First, because the data we get from the detectors is still very boisterous, which means that we often need to know more or less what the signal looks like in order to be able to extract it from the data stream. Second, even if the signal is so deafening that it stands out above the noise, we need a model to interpret it. This means that we need to have many different types of events modeled so that we can match the signal to its type; otherwise we might be inclined to dismiss it as noise or wrongly label it as a black hole merger.
One of the problems with a warp drive is that it doesn’t naturally produce gravitational waves unless it’s started or stopped. Our idea was to explore what happens when a warp drive stops, especially if something goes wrong. Suppose the warp drive’s confinement field collapses (a staple of science fiction); presumably there’s an explosive release of both exotic matter and gravitational waves. This is something we can, and have, simulated using numerical relativity.
We have discovered that the collapse of a warp drive bubble is indeed an incredibly violent event. The enormous amount of energy required to warp spacetime is released as both gravity waves and waves of positive and negative matter energy. Unfortunately, this is likely the end of the road for the ship’s crew, who would be torn apart by tidal forces.