That Einstein guy was a real bummer for our hopes of a star-hopping, science-fiction-like future. His whole “nothing travels faster than light” rule seems to ensure that exploration of even the local part of our galaxy will be an excruciating slow. But Einstein also gave us a glimmer of hope. He showed us that space and time can be warped — and so the warp drive was conceived. Just recently, a couple of papers contend that this is not pure science fiction.
In 1915, Albert Einstein’s general theory of relativity revealed that the fabric of space and time is mutable and dynamic. It can, in fact, be warped. You can’t travel through space at faster than the speed of light — but there’s no speed limit for the fabric of space itself. This hints at a possibility for faster than light, or superluminal travel.
And only 15 years after general relativity’s debut, the warp drive was invented, propelling humanity on its first journeys to the stars. That “invention” and those journeys were in the 1931 spring edition of Amazing Stories. In Islands of Space, its author John Cambell talks about increasing the cosmic speed limit by curving the fabric of space.
The idea caught on. According to the wonderful historical dictionary of science fiction, the first mention of a “space-warp” as propulsion mechanism was in J. Williamson’s 1936 story The Cometeers. Space-warp drives, time-warp drives, and eventually just warp drives became an increasingly popular way of breaking Einstein’s rules.
The pretend technology really took off with Star Trek in the 60s — and Star Trek inspired the very first real warp field solution to the Einstein field equations. That’s the Alcubierre warp field, derived by Mexican physicist and star trek aficionado Miguel Alcubierre.
Is the warp drive possible?
Now, we covered Alcubierre’s idea 5 years ago. Back then the warp field solution looked like a very rigorous, carefully calculated work of pure fiction — it broke the laws of physics in ways that I’ll come back to. Subsequent studies have tried to fix this. And a very recent paper claims to have solved the worst deal-breaker in Alcubierre’s proposal.
Today we’re going to pull them apart two new papers on warp drives and see if it’s really time to don our Star Trek cosplay and warp out of here. Before we get to the new work, let’s talk light speed and warp drives.
Einstein’s speed limit doesn’t directly say that nothing can travel faster than light. His special theory of relativity just says that it takes infinite energy to accelerate anything with positive mass all the way to light speed. That effectively means that it’s impossible to observe a massive object cross this speed barrier. But there are loopholes.
In certain circumstances, we can think of space itself as moving — and there’s no limit to the relative motion of two patches of space — and so objects in those patches could have superluminal speeds relative to each other.
For example inside black holes where we can think of space as flowing downwards faster than light. Or beyond the cosmic horizon, the expanding universe is carrying galaxies away from us faster than light. The warp drive takes advantage of this loophole by accelerating a patch of space relative to its surroundings. Objects in that warp bubble move with that patch without themselves ever feeling any acceleration.
This is what the Alcubierre drive is supposed to do. It’s a spacetime geometry that is a valid solution to the equation’s general theory of relativity. The Einstein Field Equation. It includes a comfortable, flat region of space surrounded by a region of extreme spatial curvature.
The space behind this bubble is expanded, while the space in front is contracted. The resulting push-pull propels the bubble and any spaceship that it contains. The Alcubierre warp field may be a valid solution to the Einstein field equations, but that doesn’t mean it’s physically possible.
Normally, to solve these equations we input a physically possible distribution of mass, energy, on the right side and it spits out the spacetime geometry on the left. Alcubierre instead solved the equations backward — he figured out the geometry of a warp bubble and then ran it back through the equations to see what distribution of matter and energy would be required.
You can do this with any geometry, but almost every geometry will produce an impossible mass-energy distribution. To combat this, we have a set of energy conditions that go alongside the Einstein field equations that are meant to restrict the allowable energy distributions to what is physically possible Alcubierre’s field breaks all of these energy conditions, and he’s upfront about that fact.
We can summarize this by saying that it requires a negative energy density, which should be impossible except perhaps on the tiniest, quantum scales. Sometimes people talk about the drive requiring negative mass — also called exotic matter — and yeah, that would do the trick too — and is also probably impossible. So that sounds like a dealbreaker.
The other minor hiccup is that Alcubierre’soriginal field required more energy than is contained in all the matter in the visible universe to move a moderate-sized starship. Subsequent studies improved on Alcubierre’s warp design and brought down the energy requirement to less insane levels — although they typically remained somewhat insane.
But the requirement of exotic matter didn’t go away, and in fact, subsequent studies demonstrated that any superluminal warp drive MUST use negative energy densities. Other studies discovered new problems — for example, warp bubbles are un-steerable because the interior of the bubble is causally disconnected from parts of the front warp field.
That inspired Sergey Krasnikov to come up with his Krasnikov tube — a sort of warp conveyor belt, which has the slightly useless property of only working if it’s a full return journey and you don’t get off at the destination. To me, one of the biggest deal-breakers is that any faster-than-light travel can be used to create closed time-like curves, as we discussed previously.
That leads to paradoxes — to contradictions and inconsistencies. And the appearance of inconsistencies is one of our most powerful indications that our theoretical musings are on the wrong track. So, yes, warp fields became much better studied, but remained implausible — and probably impossible. But I guess the dreams of wannabe starship captains are strong — because the work continued, and just recently two papers have claimed breakthroughs.
Physical Warp Drives
We’ll start with the paper entitled Introducing Physical Warp Drives by Alexey Bobrick and Gianni Martire. These guys propose a general definition for warp fields not tied to a particular solution to the Einstein field equations. To quote the authors: Warp drives are inertially moving shells of positive or negative energy material which enclose a `passenger’ region with a flat metric.
Let’s focus on that “inertially moving” part. Bobrick and Martire are that no past warp solution has an in-built mode of acceleration. All of them, including Alcubierre’s, move with whatever velocity they started at.
This isn’t exactly a new insight — in 2002, Jose Natario demonstrated that the whole expansion and contraction of space was only a side-effect of Alcubierre’s choice of the warp field, and he constructed a warp field without that feature.
Natario, and now Bobrick and Martire, define warp fields as bubbles that slide through space — potentially at superluminal speeds. But all these guys agree that superluminal bubbles are only possible if the warp field uses exotic matter. Bobrick and Martire also point out that there’s still no way to accelerate a warp bubble across the light-speed limit.
A superluminal warp bubble has to have started out superluminal. This seems in conflict with Alcubierre himself, who wrote that the warp bubble “pushes” the spaceship — accelerates it from rest. But this acceleration isn’t actually derived from his warp field solution.
The velocity of the bubble is baked into the equation for the field geometry, but it’s not clear how you change that velocity. This isn’t necessarily a complete downer. As pointed out by Fransisco Lobo and MattVisser, if sub-light-speed or subluminal warp bubbles can be a thing, these might be an example of a “reaction-less” drive — a propulsion method that doesn’t need any propellant- not stuff ejected from behind like in a rocket.
But these researchers also note that even a subluminal reaction-less warp drive would still require negative energy and an enormous amount of it for any decent-sized spaceship.
Breaking the Warp Barrier
OK, let’s move on to the second paper by Erik Lentz: “Breaking the Warp Barrier: Hyper-Fast Solitons in Einstein-Maxwell-Plasma Theory”. This one is a little more star trek friendly than, well, pretty much every prior effort. Lentz claims to have found an actual superluminal warp field solution that does NOT require the impossible negative energy densities.
He does this by exploring a broader family of solutions to the Einstein field equations than previous studies. The details are technical, to put it mildly- but let me try to give you a sense of the difference. We can think of the warp field as a special type of isolated wave moving through space — what we call a soliton.
In previous work, this wave only waved in the direction of motion. That gave them an axial symmetry. For example, in the Alcubierre field, the warp is in front and behind the spaceship, while the exotic matter is in a ring around the direction of motion.
When you calculate the total energy with only this component then it’s always negative. Lentz found that by including components of the wave motion that were perpendicular to the direction of motion he could build a superluminal soliton in which the energy came out positive everywhere.
He came up with this example — here projected onto a 2-D plane. Energy is distributed along these tracks, potentially as a plasma, in some places at some pretty insane temperatures. This would result in the expansion and contraction of space in similar regions. The entire patch of spacetime would travel at superluminal speeds, carrying a spaceship with it. A couple of minor issues persist.
The energy required to carry a 100-meter diameter bubble is about the rest-mass energy of a tenth of our Sun. It’s hard to see how packing that much mass into these strips would NOT create a black hole.
Also, Lentz admits that this warp bubble is inertial just like Bobrick and Martire assert for Alcubierre’s bubble — it can exist at superluminal speeds in theory, but there’s no known way to get it to those speeds in the first place.
Both of these papers are peer-reviewed and published in the reputable journal Classical and Quantum Gravity. Now, that doesn’t mean they’re right — it just means that a couple of respectable physicists refereed the works didn’t find any gigantic blunders — or did, and the blunders were fixed. But much more work is needed to verify anything this speculative.
You can bet that there’ll be plenty of papers critiquing and/or supporting these efforts. So now you’re all up to date with warp technology. Superluminal warp fields exist in theory, and there are very tentative hints that the worst deal-breaker can be resolved — that of requiring a non-existent type of energy.
But other possibly impossible hurdles remain — can the required energy densities be created without creating black holes for any useful-sized warp bubble? And can a warp bubble be created at sub-luminal speeds and then accelerated past the speed of light?
So far there’s no known way to do this, and warp fields may suffer the same strict speed limit as does matter. And of course, we still have the time travel issue — accelerating across the speed of light breaks causality and so probably can’t be a thing. Einstein and the universe appear to be trolling us — alternately inspiring and crushing our hopes for a star-hopping future.
But these studies have made one thing very clear — if warp travel is possible, humanity will figure it out. Scientists are very persistent, especially the ones who are also die-hard science fiction fans. They’ll continue to try to “make it so” by exploring Einstein’s theory — hoping to build a starship, but in the process learning how our universe works. And possibly also building a starship, to propel humanity into the galaxy on waves of warped space-time.
Originally published at https://www.digitalgurugyan.com.