The universe is bigger than you think.
This means any deep-space future awaiting humanity outside our solar system will remain beyond the span of a single life until we develop a means of propulsion that outclasses conventional rockets. And, when three studies rocked the world earlier this year, it felt like a dream come true: Warp drive was no longer science fiction, potentially unlocking a theoretical basis to build faster-than-light warp drive engines that could cut a trip to Mars down to minutes.
However, a recent study shared in a preprint journal cast doubt on the theory, pointing to a gap in the math that could put the viability of a physical warp drive back into the realm of speculation.
The question, then, is raised: Do warp drive engines violate the laws of physics?
Warp drives must satisfy several energy conditions
For decades, research into faster-than-light (superluminal) travel couldn't escape the need for unconscionable amounts of hypothetical particles, in addition to matter exhibiting "exotic" physical properties, like negative energy density. These either can't be found in the universe, or require a level of technological prowess far beyond ours. Erik Lentz, a physicist and author of the second major warp-drive study of this year, sought to circumvent this apparent dead-end by experimenting with Einstein's field equations to find a new configuration of space-time curvature, which is a volume of space-time whose internal properties are "warped" in comparison with the external structure of space-time.
This method has appeal in the hunt for superluminal travel, since a soliton or warp bubble avoids breaking the speed limit that Einstein's theory of general relativity places on all matter in the universe (the speed of light). Since physical matter can't be accelerated from sub- to superluminal speeds without violating the laws of physics, we might instead try creating a soliton "bubble" around a spaceship that moves the very fabric of space-time at superluminal speeds. In theory, this could result in faster-than-light velocities without forcing the ship itself to undergo unconscionable levels of acceleration, safe and secure inside the inner region of the soliton.
However, while this avoids moving matter past the speed limit (and breaking the laws of physics), any viable warp drive still has to satisfy a set of energy conditions, one of which is the weak energy condition (WEC). "The weak energy condition imposes that the energy any physical observer sees is always positive," explained Physicist and co-author Jessica Santiago of the recent study in a video interview with IE. But, in Lentz's June 2020 preprint study, he "claimed one observer sees positive energy, but [he] hadn’t shown that for all observers," added Santiago.
'All Natário warp drives' violate the weak energy condition
Surprisingly, Lentz agreed, but with a different conclusion. In his initial work with warp drive solitons, he wasn't trying to pass the WEC, opting instead to widen the scope of his analysis for the final version of his study, which was published in the journal Classical and Quantum Gravity. "In the published version, my analysis was expanded to look at all timelike frames," he said to IE. According to him, Santiago and her colleagues had only looked at his unpublished preprint, eluding the updates added for his final, published version. "When I did that, I found that one could still find a class of solitons that satisfied the full WEC — that every timelike reference would satisfy the 'no negative energy density' condition." Lentz also argued that his final paper did consider all timelike observers "and found that the energy was non-negative everywhere." In essence, Lentz suggested that Santiago and her colleagues had only proved that the Natário class of warp drives had a negative energy density (violating the WEC), instead of the specific drive Lentz used for his final study. But Santiago and her co-authors Matt Visser, of the University of Wellington, and Sebastian Schuster, at Charles University of Prague, disagreed.
"Proving weak energy condition violations (WEC violations) was done in our paper without any extra requirements," explained Santiago, in response to Lentz's counterargument. "It is simple and valid for all generic Natário warp drives, therefore proving whatever Lentz has to say [on the subject of Natário warp drives with positive energy densities] wrong." In other words, the schism between Santiago and her colleagues and Lentz came down to logic. By analogy, we might say: If all Tesla cars sink in the ocean, but yours has a polka-dot paint job, that doesn't make it an exception to every other metal object placed in the ocean without sufficient buoyancy.
Subluminal warp drives could still revolutionize space travel
And the Natário warp drive presented in Lentz's study also runs into issues with other energy conditions, namely, the dominant energy condition (DEC). "Erik Lentz's [...] warp metrics in the superluminal regime need superluminal matter," explained Alexey Bobrick, a Belarusian scientist, astrophysicist at Lund University, and co-author of an initial published study on warp drives, in a separate interview with IE. In the study, Bobrick and his colleague, Gianni Martire, developed a formalism for fully general warp drives, and provided the first model of a physical (subluminal) warp drive that is fully consistent with all the energy conditions. To Bobrick, any warp drive formed using the Natário drive from Lentz's study would call for matter that moves faster than the speed of light, which violates the DEC. "This is equivalent to saying that they violate the dominant energy condition in the superluminal regime. To the best of our knowledge, superluminal matter probably doesn't exist."
However, all hope for warp drives is not lost. It's hard to overstate how wide the range of speed is between the velocities of chemical rockets — the fastest of which is the Parker Solar Probe, which used a gravity assist to strafe by the sun at 330,000 mph (531,083 km/h) — and the speed of light, which is 186,000 miles per second (300,000 km/s), according to NASA. Even at a quarter of lightspeed, a spacecraft would move at more than 167 million mph. According to Alexey Bobrick and his colleague and Co-Founder at Applied Physics Gianni Martire, if solitons tend to violate physics at superluminal velocities, we might have better luck looking for one that works at more comparatively modest and subluminal speeds.
"We should explore the full diversity of warp drive spacetimes," suggested Bobrick. "This includes morphologies, the gravity they generate outside, and their effects on spacetimes inside" like the rate of time experienced inside warp drive bubbles, which accelerates for some classes, like Natário. "Natário drives are a very specific subclass of all possible warp drives," and the problems discussed in Santiago's work are resolved, "at least in the subluminal case, once one considers more general metrics." While there's still hope for warp drive engines beyond science fiction, we may have to put faster-than-light velocities on the back-burner while the physics of forming a soliton in general is further explored. And to do that, we'll have to look at a wider variety of solitons of many other classes besides the Natário. In short, the pursuit of a viable warp drive may require us to "crawl" at sublight speeds before we can run at superluminal ones.
Editor's note: This article has been updated to clarify the order of publication of warp drive studies, and Bobrick et al.'s emphasis on advancing subluminal drives.