Humankind has long looked up to the stars and wondered what was out there. When space travel emerged from the realm of science fiction attention turned to whether we could go out there and explore over the horizon just like the ships of old that traversed the sea to venture to the New World. We have made great strides, gone to the moon and sent out various probes and rovers to other worlds within our solar system, but will we ever get beyond that? We may eventually have to. There may come a time when we are forced to leave this world of ours to colonize other planets, but how will this ever be possible? With our current technology there seems to be no way to feasibly traverse the vast sea of space, but there have been plenty of people trying to get past this problem, and the key seems to be going faster than light.
The main hurdle to interstellar travel is the speed of light, which through a vacuum is exactly 299,792,458 meters (983,571,056 feet), or 186,282 miles, per second. According to Albert Einstein's theory of special relativity, on which much of modern physics is based, that’s it, that’s the fastest anything in the universe can go. This is not just a speed bump, but more of a brick wall, the immutable universal speed limit which can never be surpassed. Although the reason for why this should be so is incredibly complex, the basic explanation is that the faster you go, the more your spatial dimension in the forward direction shrinks and the slower your clock runs when viewed by an external observer. Space and time are not a fixed background that are the same everywhere, but rather can warp and bend, so essentially this means that when you pass the speed of light, you reach a point in which your spatial dimension in the forward direction shrinks down to nothing and your clock slows to a stop. Essentially, since you are no longer moving forward and time is standing still, you are now in a reference frame that does not exist, and therefore space and time do not actually exist beyond this point. It is much, much more complicated than this, but that’s the gist, and since why the speed of light is the universal speed limit is beyond the scope of this article let’s move on.
The important takeaway here is that’s what we have, that’s as fast as we can go. That’s as fast as anything can go, and it is a fundamental part of the way the universe works, but the problem is that it’s just not fast enough. Let’s look at the closest star to Earth, which is Proxima Centauri, which is about 4.25 light-years away, around 25 trillion miles (40 trillion km), so we can’t drive it. 4.5 light years might not seem that bad, but to put it into perspective if we were to take the fastest Earth spacecraft ever made, which would be the Parker Solar Probe, with a top speed of 450,000 miles (724,000 km) per hour, it would still take us about 6,633 years to reach that star. Even if we could somehow reach the speed of light, which we can't, it would still take us 4.5 years to get there, which is a pretty long trip. If we ever want to visit other star systems, we are going to have to find a faster way. Unfortunately, there is that pesky speed limit and we can’t ever get past the speed of light, but is there some way we could? That is a question both scientists and science fiction writers alike have pondered for a very long time, but there could be some ways, so let’s take a look.
One challenge to getting around that annoying speed of light thing is that the speed of light is unbreakable, so there’s no way we can just try a little harder to make a spaceship that goes a little faster. “But it’s just a theory, right?” You may ask. Not exactly. In science a theory is not just an unproven or untested idea (that would be a hypothesis), but rather it is a well-tested principle that has been shown to be right time and time again and shown to be correct, or at least based on such principles to the extent that we can logically surmise that it is very likely true. We don’t just stop building buildings because structural theory is “just a theory.” In science, a theory is very strongly supported by copious evidence, and can be considered to be a reasonable assumption for how something actually works until someone somehow proves it’s wrong. Einstein’s theory of relativity has been tested countless times, survived every single observational and experimental test thrown at it, and it is always right. No one has been able to refute it, so it can be considered to be pretty solid, and it states unequivocally that we cannot pass the speed of light. Considering this, we can’t really look at “faster than light” travel so much as find ways we can get around it in a way that does not break the theory of relativity, some sort of loophole.
One of the ways that has been proposed for unfettering ourselves from the speed of light speed limit is using what are called wormholes. Wormholes are theoretical tunnels that cut through space and time to connect two points in space that could be many light years apart, essentially shortcuts through the universe. A good way to imagine it is to think of two towns on opposite sides of a mountain. To get from one to another, typically you’d have to walk all the way around the mountain to get to the other side. However, if you were to dig a tunnel through the mountain, you could get there much more quickly. That tunnel is the wormhole. Wormholes are even theorized to cut through time as well, meaning you could travel through one to another point in time, like a time machine. So the idea is that if you could create one of these wormholes, you could travel through it and leap many light years in minutes or hours, maybe even practically instantaneously. It is a concept that has long been used in science fiction, but is being increasingly seen as an actual possibility for interstellar travel by scientists as well, and since Einstein’s theory allows for wormholes it would not be breaking any of the rules.
The problem with wormholes is that there has never been found any evidence that they actually exist, although Einsteins theory accounts for them and they are presumed to be out there somewhere. Another problem is whether we would be physically able to actually enter and travel through one, and a traversable wormhole is a bit tricky. The trick to building a wormhole is to find a configuration of matter and energy that allows you to form a tunnel between two points in space. However, a wormhole build to the specifications of Einstein’s theory of general relativity would be hidden behind an event horizon, which is basically a one-way barrier in space, meaning that it would not be traversable. Wormholes are also incredibly unstable, and would collapse as soon as anything entered one due to the immense gravity it would generate. Anything that were to try and enter the tunnel, even a single photon or light particle, would be catastrophically crushed as the wormhole violently collapsed, so the trick here is to keep the wormhole from doing that. Some researchers believe there is a way, and it entails tweaking gravity a bit.
One way around this obstacle and to stabilize a wormhole to make it traversable is to fashion it from a form of matter that has negative energy or negative mass, also known as “exotic matter.” The idea behind negative mass is relatively simple. Negative mass is exactly what it sounds like and negative energy is a state in which the energy in a particular location is negative relative to its surroundings. This would theoretically counteract or cancel out the crushing gravity within the wormhole and allow passage. The problem is that negative mass has never actually been observed and negative energy has only been demonstrated at extremely small scale in the quantum realm. Even if there is negative mass, which scientists believe must exist somewhere, it would be incredibly rare and exist in such miniscule quantities that it seems unlikely that we would ever be able to gather enough to make our wormhole. However, Einstein’s theory has been unable to completely explain the intricacies of gravity and it delves little into quantum physics, so there could still be a way. But without this negative energy or matter, a traversable wormhole would be impossible. João Rosa, a physicist at University of Tartu, in Estonia, says of the possibility of travelling through wormholes:
The possibility to visit other stars or even other galaxies, possibly finding alien civilizations, and the possibility to revisit the past or not having to wait for the future have been part of the human imagination and fantasy for a long time, and wormholes provide a relatively simple and unified solution for both of these problems. The presence of this negative matter is essential as it prevents the wormhole throat from collapsing upon a traveler. Any traveler trying to cross a wormhole that does not satisfy this will be crushed inside as the tunnel collapses.
Rosa has been trying to virtually build a stable, traversable wormhole, utilizing a modified form of weird gravity called generalized hybrid metric-Palatini gravity. This tweaked form of gravity follows Eintein’s theory, but would allow for more leeway and flexibility in the connections between matter and energy, as well as space and time. This had been theorized by other scientists before, but Rosa found a theoretical way to do it without requiring negative mass or energy. To do this, he believes that it is possible if one is to layer the entrance to the wormhole with a series of double thin shells of regular matter. Rosa explains:
What happens is that these gravitational effects needed to guarantee the traversability of the wormhole happen naturally if one modifies gravity, and exotic matter [matter with negative mass] is no longer needed to serve this purpose. This is just a very small step towards the final goal: One must now use experimental data and observations (e.g. gravitational waves and trajectories of stars near the center of the Milky Way) to test and hopefully confirm the validity of these theories.
For now, the use of wormholes for actual travel across the sea of stars remains in the realm of science fiction, but who knows where the future will take us? Another theory for beating the speed of light is to use what are commonly referred to as "warp drives," also a trope of countless science fiction stories. The basic concept is that we can skirt around the universe’s ultimate speed limit by bending the fabric of reality, actually warping space and time so that it flows around us and brings two points closer together. The concept was first seriously pursued in 1994 by physicist Miguel Alcubierre, who believed there was a hypothetical way to create a “warp bubble,” that is, a bubble that bends spacetime in front of itself and expands it to the rear in a way that allows a “flat” area inside the bubble to travel faster than light. To visualize what “flat” means in this case, imagine the universe as a mat that curves in the presence of matter and energy. Gravity is basically the tendency of objects have to roll into the dents or curves of this mat, which would be created by things like stars and planets, so “flat” would be like an un-curved mat with nothing on it.
Essentially, a warp drive would contract spacetime to make your path shorter, therefore effectively allowing you to travel faster than the speed of light while not violating Relativity or causality. One way to imagine it is to imagine a cup across the table from you on a mat out of reach. You could get up and walk over to pick it up, or you could just pull the mat towards you and reach out to get the cup. A warp drive would work kind of like that. Physicist Dr. Harold “Sonny” White gives another example:
It’s like using a “travelator,” those horizontal conveyor belts at major airports. Normally, you walk along at about three miles an hour going from one gate to another. But in some locations, you have these horizontal ‘travelators,’ and you step on top of them. So you’re still walking at three miles an hour, but the belt is moving as well. Conceptually speaking, the belt is contracting space in front of you and expanding space behind you, so that it augments your apparent speed. But locally, you’re still going at the same speed.
According to Alcubierre, quantum field theory allows for the existence of regions of spacetime that have negative energy densities, known as the Casimir Effect. He theorized that if a “ring” of negative mass could be created around a spacecraft, then spacetime could perhaps be contracted in front of the ship and expanded behind the ship, allowing the spacecraft to effectively travel faster than the speed of light without violating Einstein’s theory because it is merely riding a wave generated by the expansion and contraction of local spacetime. The problem is, just like with the wormholes, such a warp drive would require huge amounts of negative energy or mass to make it work, and although he had ideas on how to generate negative mass, it was deemed to require too much regular mass and energy to make it feasible. To put it into perspective, in 1999, physicist Chris Van Den Broeck proposed a way to lower energy requirements for such a warp drive by expanding the volume inside the bubble but keeping the surface area constant, but this would still require mass equivalent to that of the sun, making it hardly practical. However, there have been researchers working on rectifying that.
One of these is the team of physicists Alexey Bobrick and Gianni Martire, who came up with a model for a warp drive that could conceivably cut way back on energy consumption by using positive energy (i.e. “normal” energy) or from a mixture of negative and positive energy combined with a massive gravitational force, but it still would require compressing a planet-sized mass to a manageable spacecraft-module size in order to use its gravity, putting it out of our current means to build. Another researcher working on the problem is physicist Erik Lentz from Göttingen University in Germany, who believes he has found a solution to bring warp drives closer to reality without the need for negative energy and using conventional energy sources.
In a 2021 paper, Lentz proposed that a functioning warp drive could be made without negative energy by using a new class of hyper-fast solitons, which are waves that maintain their shape and energy while moving at a constant velocity. With enough energy these solitons could be hypothetically put into configurations that could essentially function as a “warp bubble”, capable of contracting space in front of it and expanding space behind to go faster than lights and enable an object to pass through space-time while shielded from extreme tidal forces, all without using exotic materials and obeying the theory of relativity. However, there are some hurdles. The problem with Lentz’ method is that it would require extremely vast amounts of energy to operate, beyond anything we could practically produce at the moment, although Lentz believes there could be ways. He explains of it:
The energy required for this drive traveling at light speed encompassing a spacecraft of 100 meters in radius is on the order of hundreds of times of the mass of the planet Jupiter. The energy savings would need to be drastic, of approximately 30 orders of magnitude to be in range of modern nuclear fission reactors. This work has moved the problem of faster-than-light travel one step away from theoretical research in fundamental physics and closer to engineering. The next step is to figure out how to bring down the astronomical amount of energy needed to within the range of today's technologies, such as a large modern nuclear fission power plant. Then we can talk about building the first prototypes.
For now, faster than light travel still remains in the domain of science fiction, but it has started to crawl out of the shadows as something that could hypothetically at least be possible. Researchers such as these have demystified the concept to a considerable degree and laid the foundations for possibly using this sort of technology in the future. Will we ever be able to go past the confines of our solar system and push into new realms beyond the stars? There is no way to be sure, but for now it does not seem to be off the table, and while it might not happen in our lifetime, perhaps our grandchildren or great granchildren will be taking this technology for granted and living on other worlds.