Interstellar travel continues to thrill the minds of the public. Warp drives have long been featured prominently in pop culture, both in literature and cinema. But will humanity be able to create a technology that can manipulate space-time, providing travel faster than light?
We see this all the time in science fiction: warp-powered ships allow characters in a story, novel, movie, or television series to explore new planets and even galaxies. These vehicles could even fly faster than the speed of light, even if General Relativity teaches us that no one can travel faster than light. Is not it? After all, light has no mass, which means it can travel at a speed of 299,792,458 meters per second.
This is all true. Nothing can exceed the universal speed limit. However, we may still have the ability to build a warp drive without breaking any physical laws.
In 1994, theoretical physicist Miguel Alcubierre from Mexico wrote an article in which he presented the mathematical calculations and scientific basis for creating a real warp drive that would not contradict General Relativity. He became interested in this method of interstellar travel after seeing it in action - covering giant distances in science fiction works.
The warp drive expands and contracts the fabric of spacetime around the ship and its bubble. The apparatus, in principle, does not accelerate or move. The fabric moves around it and thus pushes it forward. As an example, imagine that you are standing on a conveyor belt - you are moving, but you yourself are not walking. The fabric of the tape moves you forward. The contraction of space-time in front of the spacecraft will pull it, and the expansion behind it will continue this forward movement. Einstein showed that spacetime can be bent by mass or energy, hence it can be manipulated in other ways as well. The reason this ship could travel faster than light is because General Relativity says that nothing in space can overcome the speed limit, but there is no speed limit for the expansion or contraction of space itself. We do not move anything in space - we move space itself.
Alcubierre's work was encouraging and impressive, but there were also many holes in it. In the original work, he theorized that to provide such a ship with sufficient power, it would take more negative energy than there is in the universe at all, namely, thanks to it, space expands. The problem is that negative energy is elusive, even many physicists doubt its existence, not to mention that we will be able to produce huge amounts of it.
Our observations of what could be negative energy are, to put it mildly, insufficient. Presumably, what we think of as empty space is by no means empty - it has an energy density that is also called zero. According to quantum mechanics, empty space is filled with particles of energy appearing and disappearing. If we manage to stop their appearance, we will receive negative energy.
Scientists tried to create it in the laboratory by squeezing two metal plates (which were so flat that they were perfectly smooth at almost an atomic level) to a distance much less than the thickness of a human hair. The remaining space was so small that particles could not exist in it, due to which the force around the plates increased and manifested the properties of negative energy. Of course, these observations are not enough - this is just a small experiment, the results of which are far from being able to draw final conclusions.
If in the future we still manage to figure out how to get more negative energy, it may not be needed as much as Alcubierre assumed. Recent refinements of its work, carried out by NASA scientists, have significantly reduced the amount of energy that would be required for a warp drive by vibrating parts of the device at high frequencies. This would make it easier to move through spacetime and reduce the amount of energy needed. Current theories about how much negative energy a warp drive might need to run range from 65 ecjoules to multiple negative and positive solar masses. 65 exajoules is roughly what the US uses in a year. This is still a lot, but quite real. If we can use dark energy, then we will need no more than the mass of Jupiter. The only problem is that we don't really understand what dark energy is and how it works. And it could end up being the exotic material needed to power a warp drive.
For comparison: for interstellar travel on traditional rockets not only will it take hundreds of thousands of years, but also the fuel reservoir is larger than the Universe. And this is not to mention the fact that it is still necessary to find material that can withstand such a long journey.
In some models - for example, in the concept of Harold White - a spacecraft powered by a warp drive can travel 10 times faster than light. At this speed, we could reach the nearest exoplanet - Alpha Centauri B b - in just six months, despite being more than four light years from Earth. The fastest modern devices can reach speeds of just over 32 thousand kilometers per hour: the journey to Alpha Centauri B b at this speed will take 142 thousand years. Thirty-two thousand kilometers per hour is about 0.003% of the speed of light.
Traveling at such a speed would allow humanity to cross the cosmological horizon and explore not only its Universe, but also the Multiverse. In theory, there is a limit to the speed of a warp drive, but even those theoretical limits would enable us to travel to new galaxies in a fraction of a second. As an advantage, the ship would be able to accelerate and decelerate, and the passengers would not experience time dilation. Simply put, it would have been possible to avoid a situation where you arrived at your destination and found yourself so far ahead in time that everyone on Earth that you knew is long dead.
In addition to energy sources, particles accelerated during travel are also considered a problem, which inadvertently can be launched when braking and destroy entire worlds. Moreover, there is a possibility that it will be impossible to slow down as soon as you start moving, and the crew may die for a number of reasons. Yet mathematics and experimental data show that warp drives may have a chance.
If we really succeed in creating this technology, not a century will pass until we learn to apply it. Like wormholes, the possibilities that warp drives can provide are incredible, but they won't be easy to achieve.