There are a lot of things that pull folks to science fiction, but probably the biggest draw comes because it makes impossible things seem possible. Who wouldn’t want to travel to distant stars or back in time? Aren’t there times when being invisible would be handy? Given a choice, I’d teleport instead of mucking about with the average commute.

But the interesting thing about some of the best science fiction is that the science in it doesn’t stay fiction for long. Remember Jules Verne and the Nautilus or the communicators on Star Trek? These fictional devices are part of our everyday world because science doesn’t stand still. It makes you wonder which of today’s science fiction tropes are tomorrow’s reality.

I had the opportunity to talk to theoretical physicist Michio Kaku about his book Physics of the Impossible, in which he breaks the impossible down into three useful categories. Class I—Technologies that are impossible today, but do not violate the known laws of physics. They might be possible in a few decades or in this century. Class II—Technologies that sit at the edge of our understanding. They are centuries to millennia away from realization. Class III—These break the known laws of physics.

The handy thing about thinking of the impossible in this way is that it helps when planning science fiction. If you can make a guess about whether an invention might have occurred by the time your story takes place, then you can build more believable futures.

Take, for instance, force fields. These invisible barriers have been a part of science fiction since at least 1912 with William Hope Hodgson’s The Night Land, but have often seemed to be pure fantasy. Much to my surprise, it turns out that not only are force fields mere Class I impossibilities, the elements to make one are already in development.

In 1995, physicist Ady Herschcovitch invented something called a “plasma window” at Brookhaven National Laboratory in Long Island, NY. Plasma is essentially super-heated gas made of ionized atoms. Using electric and magnetic fields to shape plasma, Dr. Herschcovitch can create a “window” capable of preventing air from entering or leaving a space. You can, in fact, use it to contain a vacuum. The downside is that the plasma is heated to 12,000º F, which is hot enough to vaporize metal. Standing anywhere near the glowing blue window would be unbearably hot, which makes it a little questionable for daily use, but handy as a protective field.

Dr. Kaku theorizes that one could make a more traditional force field by using layers of technology—all of it almost within reach. Beginning with a plasma window, he posits adding a criss-crossing curtain of high-energy lasers, also capable of vaporizing projectiles, and finishing with a lattice of nanotubes.

Lasers have been around for a while, but the prototypes of carbon nano sheets also exist. When those nanosheets are perfected, they will be one molecule thick, stronger than Kevlar and invisible. By using the three layers, a force field will be able to repel most attacks.

Is that exactly the way force fields are described in fiction now? No, not really. But in most ways, it fulfills the need for a force field by creating an invisible, impenetrable barrier.

Impenetrable, that is, to everything except lasers. If something is transparent, then light can pass through it. Someday, Dr. Kaku says, one might add a layer of “photochromatics” to the force field. You know what those are, right? That’s the same technology that causes sunglasses to darken outside. They still won’t stop a laser. Yet.

Speaking of lasers, let’s take a look at another stand-by of science fiction, the ray-gun. We’re all familiar with the discrete bolts of energy that fly out of blasters and phasers alike. The problems with this are pretty elementary. Light is invisible unless it goes into the eye.

What? Yes, light is invisible. It’s like this. Lasers are coherent light, so all the particles are vibrating in unison and in the same direction. You really only see it when it bounces off something toward your eye. Picture a focused flashlight in a dark room. You can see things reflecting the light, but unless there are dust particles in the air, you can’t see the beam of light itself. A laser is just coherent light.

The way they make lasers visible in the movies is to use animation or to put dust in the air to cause a portion of the laser to bounce toward the camera. If the light has nothing to bounce off, you only see where it hits.

So all those ray-gun battles would involve invisible rays, especially in space. As a writer, I have to wonder how you dodge something you can’t see? One pictures personal foggers, designed to show the enemy’s laser lines.

So, if lasers exist and can be both deadly and invisible, why don’t we have ray-guns today? The main reason comes down to power. They require tremendous power to operate and no one has yet come up with a portable battery that’s strong enough. Can you imagine Han Solo hauling an extension cord around the halls of the Death Star?

It’s amazing the way one piece of technology can cascade out to affect the rest of a society. Knowing that for ray-guns to exist in the future, in any sort of practical sense, there would have to be batteries with enough power to fuel a small city also tells you that this is a future where energy problems have been largely solved.

That’s the nice thing about Class I impossibilities; you can look at them and see what technology hurdles have to be met in order to have that piece of tech in your future. It makes extrapolation easier.

Class II impossibilities, on the other hand, are a little trickier, because they are on the edge of our understanding.

Time travel is a pretty standard trope of science fiction. The first recorded time travel comes from Samuel Madden’s 1733 story called “Memoirs of the Twentieth Century.” But for all that, we’ve not seen any evidence of time travelers actually occurring.

The science fiction community was abuzz last year when Professor Ronald Mallet published a theory claiming that time travel was possible. The catch, he said, was that you couldn’t travel any farther back than the moment when the first time machine was turned on. So we haven’t seen any time travelers because they can’t get here until someone invents a working time machine.

I asked Dr. Kaku what he thought of this theory. He said, “You need fabulous amounts of matter and energy to bend time into a pretzel, on the scale of a black hole, and a tiny laser beam simply won’t work. But there are reputable physicists who are working on this.”

He cites Richard Gott, who proposes that it’s possible to travel back in time by using a collision of giant cosmic strings, which have yet to be discovered. Though theoretically possible, it would only work for a short time and even that would require “more than half the mass-energy of an entire galaxy.”

In fact, all of the time travel theories require an enormous amount of energy. Why? Dr. Kaku says that the basic ingredient needed to create the most likely form of time travel—a transversable wormhole—is negative energy. To generate this requires the energy of a star.

But that doesn’t mean all hope of time travel is lost. There’s also, he says, a loophole in Einstein’s theory of relativity. In school, they initially tell students that “Gravity sucks” evenly.

Later, they say, “We lied. Gravity does not suck. Space pushes.” Dr. Kaku explains that one possibility is that we don’t go back in time, the past comes to you. If you compress time, then it’s a matter of stepping across. In theory that could happen faster than the speed of light.

Which brings us to FTL . . .

“The sad thing for hard SF writers is that the theory of relativity prohibits Faster Than Light travel. It says that an object’s mass increases as it approaches the speed of light; the more its mass increases and the more energy you’d need to move it. To reach light speed, you’d require infinite energy. Most science fiction writers who use FTL just power it with handwavium and move on. Sure, we know it’s not really possible, but it lets us do stories that are so darn cool.”

However, there are viable FTL options out there. Have you ever heard of an Alcubierre Drive?

In this theoretical engine, space gets compressed in front of the ship and dragged out behind it. Miguel Alcubierre, who came up with the idea using Einstein’s theory of gravity, says, “People in Star Trek kept talking about warp drive, the concept that you’re warping space. We already had a theory about how space can or cannot be distorted, and that is the general theory of relativity. I thought there should be a way of using these concepts to see how a warp drive would work.”

Dr. Kaku describes it as placing a football on a fishnet tablecloth. The “waist” of the football has negative matter, but the tips have positive energy. As you push the football forward, it causes the tablecloth to bunch up in front of it. Because you can pass across the compressed space faster, it creates the illusion of traveling faster than light, without breaking any laws.

There are two downsides. First, it requires a series of exotic matter generators along the path of the engine. Sort of like subway stops. So, we’d still need slower than light travel to place the generators. Second, it needs that pesky negative energy to power it.

FTL falls into the Class II category quite neatly. It’s theoretically possible but involves so many technological hurdles that it’s still in the distant future.

Unfortunately, so is FTL communication. In Orson Scott Card’s Enderverse he uses Alain Aspect’s discovery that “under certain circumstances subatomic particles such as electrons are able to instantaneously communicate with each other” to create his ansible. This fictional device takes quantum entangled electrons and separates them across the galaxy. When one moves, the other vibrates, allowing faster than light communication.

I asked Dr. Kaku what he thought about this notion. He explained that Aspect’s work actually referred to an experiment Einstein created, commonly called the EPR experiment, or Einstein–Podolsky–Rosen paradox, after the physicists who proposed it. To begin with, it helps to know that if two electrons are vibrating in unison, and one electron is polarized up, then its partner electron is polarized down. So if you have a source that can emit a pair of electrons with opposite spins and then separate those electrons, you’d still know the state of the electron that wasn’t with you, because it would be doing the opposite of the one that was with you. It’s like knowing that Rob always wears one red sock and one green one but switches which foot they are on. If you see the red sock on his right foot, you instantly know that the left one has a green sock. If you’re looking at an electron, you’d know the state of its twin faster than light. But it’s random information, so you can’t use it to send binary or Morse code.

Any loophole that would allow the quantum entanglement to work as communication would use a theory that depends on Quantum theory and Relative theory. Anything on that border has to be slightly suspect. Dr. Kaku suggests that if there is a viable loophole it would have to be in string theory, which is his specialty.

Class III impossibilities cover things that really seem to be impossible with known physics but there are vanishingly few of them. In fact, in his book, Dr. Kaku only lists two: Perpetual motion machines and precognition.

Out of all the seemingly impossible things, that’s not a bad ratio for physics. Granted there are some things in pure mathematics that are truly impossible, but in terms of our understanding of physics at this time, that leaves a lot of room for exploration. And what better medium to suggest new ground to explore than science fiction?

While a lot of physicists used to mask their interest in science fiction, a lot of them now refer to their early influences as coming from science fiction. Hubble read Jules Verne and had a career change. Albucierre reacted similarly to Star Trek. Dr. Kaku says that Asimov’s Foundation Trilogy is partly responsible for sending him into theoretical physics. “It forces you to think about what physics will look like fifty thousand years from now. Most physicists only think of the future in terms of twenty to thirty years. When I read Asimov as a child, it gave me a new world outlook. That really shook me up. Impossibilities are in degrees.”

With that in mind, I asked him to tell me what future technologies from current theoretical possibilities SF writers should be predicting in their fiction.

Without even having to think about the question, he said, “Nano-tech is coming very fast. Invisibility is coming very fast. It exists on a bacterial level now, but moving fast. Teleportation is moving slowly.”

Looking at the range of things that are almost possible, it looks like science fiction is going to have to push even harder to create the next generation of impossibilities.