3175x175(CURRENT).thumb.jpg.b05acc060982b36f5891ba728e6d953c.jpg)
You’ve got yourself a fancy new spaceship and you want to start on a five-year tour of the galaxy. But there's a problem: Space is big. Really big. And even at the fastest speeds imaginable, it takes eons of crawling across the interstellar voids to get anywhere interesting.
The solution? It’s time to build a wormhole.
A shortcut. A tunnel. A bridge through spacetime that lets you skip through all that boring space travel and speed to the fun stuff. It’s a staple of science-fiction, and it’s rooted in science-fact. How difficult could it be?
Here’s a hint: incredibly difficult.
Option #1: The Einstein-Rosen Bridge
The first step is to understand that wormholes are totally legit in the mathematics of general relativity (GR). We’re using GR because that’s our language of gravity, and Albert Einstein’s brilliant mathematical engine is relatively straightforward. Einstein realized that while we experience gravity as a force, it's really just the sensation we feel as we’re forced to navigate the bumps, wiggles, and undulations of spacetime. Those same bumps, wiggles, and undulations come from the distribution of matter and energy in that same spacetime.
Matter tells spacetime how to bend; the bending of spacetime tells matter how to move.
If we want to build a tunnel in spacetime—a wormhole—we need to discover some arrangement of matter and/or energy that bends spacetime just so, ensuring that a tunnel will appear. With general relativity as a guide, we need to find a solution to its equations that permits the existence of a wormhole.
And at first glance, we might think that the simplest way to build a wormhole is to build a black hole.
Black holes are regions of spacetime that are cut off from the rest of the Universe. They are punctures in spacetime itself—a point of infinite density known as a singularity, wrapped in a one-way barrier called the event horizon. Once you cross the event horizon, the inrush of gravity is so overwhelming that nothing, not even light, can escape. Indeed, it’s more than a one-way trip; it’s a straight-up highway to a (singular) hell. Once you enter a black hole, you’re guaranteed to reach the singularity—and your doom—in a finite amount of time.
The black hole solution appears in GR as the answer to a very simple question: What happens when you squish matter down to such a high density that no other force is strong enough to counteract it? Boom—black hole.
But black holes are not the only answer to that question. The math of GR permits the complete opposite of a black hole, known affectionately as a white hole. White holes also have a singularity at the center, but their event horizons work in reverse—nothing can enter a white hole, and anything inside the white hole when it forms will quickly find itself flung outward faster than the speed of light.
What does all of this have to do with black holes? Looking at the bare math of GR, when you form a black hole, you automatically get a white hole attached to it. And a connected pair of black and white holes automatically forms a wormhole because of that same baked-in math.
These are called Einstein-Rosen Bridges (or, if you’re feeling fancy, a maximally extended Schwarzschild metric), after Einstein and his collaborator, Nathan Rosen. This solution appears in GR as plain as day.
Black hole blues
There are two small problems with this setup, however.
First, white holes almost certainly don’t exist. They are energetically highly unstable. The problem is the reverse-event horizon, which can never let anything in from the outside but constantly spews things out. Since a white hole is exactly equivalent to a black hole but runs backward in time, an evolving white hole would look like the formation of a black hole but in reverse: the white hole doing its thing until it loses enough mass and spontaneously forms a star.
You can’t spontaneously form a star just because you feel like it, because that would violate the second law of thermodynamics.
So white holes are out.
This means that if you try to leave the clean, sterilized condition of GR’s math behind and attempt to form a black hole in the real world, the white hole never really happens. All the material you would use to form the white hole strangles it in the womb using its own gravitational umbilical cord, cutting off its formation and leaving behind just the black hole.
If you were somehow able to construct a white/black hole pair, you would technically have a wormhole. It just wouldn’t be a very fun one.
The problem with Einstein-Rosen bridges is that the wormhole entrance itself sits within the event horizon of the black hole. You must pass through that one-way barrier to continue on your wormhole journey. But the very nature of the event horizon means you can't leave once you enter, and you will hit the singularity in the center no matter what.
This result comes from the same mathematics that permit the existence of the wormhole in the first place, so there’s no getting out of this trap.
Yes, someone could jump in from the other side, perhaps from a distant corner of the Universe. And you could meet and share a brief conversation before hitting the singularity. You could even hold hands as you reach annihilation.
Option #2: The Morris-Thorne Bridge
So if we want to build a usable wormhole, the next step is to place the entrance outside of the event horizon. That way, we can travel down the tunnel (usually called the “throat” in the physics jargon, for unknown reasons) of the wormhole while safely avoiding the mild inconvenience that is the oblivion found at the singularity.
Again, we just have to employ the machinery of GR to tell us what arrangement of matter and energy to cobble together to make this happen. And again, Einstein tells us that it’s perfectly possible to build such a wormhole.
There's one small problem, though: stability.
Wormholes are fantastically unstable. Yes, you could build a tunnel bridging two distant regions in space and time. And yes, you could look upon your creation with wonder and more than a little pride. And the moment anything—even a single photon—traveled down that wormhole, it would instantly pull itself apart like an overstretched rubber band and collapse faster than the speed of light.
Sigh.
So we have to add a second criterion to make a decent wormhole: It has to be stable. It has to allow the passage of massive objects down its throat without collapsing.
And here we are again, with GR telling us exactly what we need to do. The physicists Michael Morris and Kip Thorne discovered the solution in 1988. They found, buried deep in the math of GR, a way to construct a stable, usable, traversable wormhole, one with its entrance above the event horizon.
The matter with negatives
You just need one simple ingredient to build your traversable wormhole: negative matter, sometimes called "exotic matter."
Not antimatter, the opposite-charged twin to normal matter. Not dark matter, the mysterious form of matter that dominates the cosmos. Negative matter.
Matter with negative mass. When tossed into the equations, negative matter has the wonderful property of inflating the wormhole entrance to be big enough, and it also cancels out the destabilizing influence of normal matter.
But what the heck is negative matter?
It’s matter that has negative mass. Imagine picking up a bowling ball and it weighing negative 16 pounds. Or getting a nice juicy steak at the butcher and getting charged for negative two kilograms of meat.
It seems weird and counterintuitive because it is weird and counterintuitive. In fact, we have absolutely no examples of negative matter appearing anywhere in the Universe. And if we did, it would completely upset everything we know about physics.
For example, if I gave you a ball of negative matter and you kicked it, it would travel in the opposite direction of your strike. If you dropped it, it would fly upward. If you took negative matter and placed it next to some positive (i.e., regular) matter, the negative matter would push on the normal matter while the normal matter pulled on the negative matter—they would race off, with no input of momentum, out to infinitely high speeds.
Negative matter would violate laws of momentum and energy conservation simply by existing. And while it’s true that no law of physics is set in stone and new observations can always override existing knowledge, negative matter would be a real stretch.
Option #3: The Exotic Energy Bridge
Negative energy, on the other hand, is where things get juicy.
The Universe allows negative energy to exist, and energy and matter are simply two sides of the same coin (this is most obviously apparent if you remember that the “c” in E=mc2 is merely a constant that tells you how much energy goes into a unit of mass and vice-versa). And the most readily accessible form of negative energy rests in the vacuum of spacetime itself.
Modern physics views the world through the lens of quantum fields, which soak all of spacetime. These quantum fields overlap each other and interact in complicated, interesting ways. For example, pieces of a field can energize and start moving, which we recognize in the everyday world as a traveling particle. Indeed, for every known particle, there exists a corresponding field: a photon field (usually known as the electromagnetic field), an electron field, a top-quark field, and so on.
If you take a piece of spacetime and remove all the particles, giving yourself a complete vacuum, you’re still left with all their corresponding fields. And these fields have a raw amount of energy built into them because the fields are constantly, unceasingly vibrating.
Technically, they have an infinite amount of energy built into them.
That means that the vacuum of spacetime is buzzing with an incredibly high amount of energy. As you might imagine, this presents several headaches for those trying to develop a theory based on these fields, and the entirety of modern physics is based on clever mathematical techniques to work around those infinities and make predictions for the behaviors of particles (which, by the way, largely work).
This overabundance of energy means that you can concoct clever scenarios for locally reducing the amount of energy—all you have to do is get anything other than an infinite amount of energy in a local patch, and there you have it: negative energy.
One bit of physics that produces this is called the Casimir effect, named for the Dutch physicist Hendrik Casimir. If you take two parallel metal plates and put them extremely close together, you limit the kinds of vibrations that can exist between the plates. It’s still an infinite amount, but it’s less infinite than the vibrations outside the plates. Again, through careful mathematical tricks, you can subtract the two infinities and discover a negative energy, which manifests as an attractive force between the plates.
Look to the vacuum
The Casimir effect is real, and it has been measured (it’s actually a nuisance for building nano-scale machines, but that’s a different story). Negative energy is a reality in our Universe.
And where there’s negative energy, there’s the potential for building stable, traversable wormholes. There’s just one problem: We’ll need to solve the biggest outstanding problem in physics to have a hope of realizing this potential.
Physicists are confident that the ultimate answer to building wormholes lies in the unknown territory of quantum gravity, the marriage of quantum mechanics and general relativity. GR tells us that wormholes may exist, but only if the right conditions are allowed (i.e., negative energy). And quantum mechanics—as expressed through quantum fields—tells us how to make negative energy. But we’re not sure how those two puzzle pieces fit together. We have no theory of quantum gravity.
For example, it’s not clear if the negative energy found in situations like the Casimir effect is the right kind of negative energy. It’s negative relative to the rest of the Universe, which may be enough to create and stabilize a wormhole, but maybe not. We might need negative energy in the absolute sense, which could be just as fantastical as negative matter.
The negative energy found in the Casimir effect is also incredibly weak and small-scale. Sure, you can point to the microscopic separation between two parallel metal plates and confidently say that negative energy exists there, but we don’t know how to scale that effect up into a macroscopic object.
We might be able to build wormholes with more exotic structures. For example, cosmic strings are the theoretical fractures left in spacetime from when the four forces of nature split off from each other in the very early universe. It might be possible to thread these cosmic strings through the open throat of a wormhole, “anchoring” the ends like the cables holding up a suspension bridge, thereby stabilizing the wormhole for transit. But while most cosmologists are confident that cosmic strings exist, no such strings have been found.
Theoretical physicists have also discovered that some theories of modified gravity, originally designed to explain the phenomenon of dark energy, may allow the presence of stable wormholes without any exotic forms of matter or energy. But those theories of modified gravity also predict that the speed of gravity is slower than the speed of light, which is difficult to reconcile with the 2017 observation of gravitational waves from a kilonova (a merger of two neutron stars), which showed that gravity and light travel at nearly the same speed.
String theory hopes to become a solution to the problem of quantum gravity by replacing the point-like particles in physics with extended objects, known variously as strings and branes. And indeed, some theorists have discovered that string theory may allow the existence of stable wormholes. Alas, string theory is not complete and so far has failed to provide a solvable theory of physics.
Investigations into the nature of quantum fields near the event horizons around black holes have found that it might—might—be possible to build a stable wormhole by contorting its shape. But those wormholes must be incredibly tiny, no bigger than about 10^-35 meters across, which is… less than useful. And those same mathematics rely on a host of simplifying assumptions about the nature of quantum gravity that may not hold up.
This is where modern wormhole research sits. Physicists are fascinated by wormholes because they provide a powerful laboratory for studying quantum gravity. Also, they're really cool. So while I don’t recommend planning a trip to the Andromeda galaxy quite yet, I can’t quite rule it out as a possibility, either.
Recommended Comments
There are no comments to display.
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.