Is it possible to create a science fiction-type force field?

SDStaff Karen Lingel replies:

James, please. What kind of force field? A protective shield, such as you might put around a starship? A tractor beam, to pull things in? Or the invisible barriers that keep Star Trek aliens in the brig? One wants the right tool for the right job.

Let’s see, how could you create a force field? There are four known forces (or five or three or one, depending on how you count) — gravity, electromagnetism, weak, and strong. In the olden days, electricity and magnetism were counted as two separate forces (for a total of five), until it was discovered they were two aspects of the same thing (still charges and moving charges), and could be described mathematically in one theory, namely Maxwell’s equations. Particle theorists have since combined the weak force with electromagnetism into a single theory called electroweak, so if you count that way there are three forces. Of course, particle theorists are gung-ho about combining all the forces into one huge Grand Unification Theory, which will give us just one force. But for the present, most physicists count four.

The strong and weak forces keep the quarks tightly bound inside protons and neutrons, and keep the protons and neutrons from disintegrating. The problem with the weak and strong forces is that they are very short distance forces. We’re talking about distances much smaller than the protons and neutrons inside nuclei. Alas, for regular matter, the nuclei are well protected at relatively large distances by electron clouds, so even if you could generate a macroscopic weak or strong force field, it can never be brought close enough to ordinary matter to actually exert any force. So until you come up with some evil aliens composed of quark-gluon plasma (nucleonic matter with no electrons), let’s give up on weak and strong.

How about gravity? Two problems. First of all it is exceptionally weak — the weakest of all the forces by many orders of magnitude. If you assign the strong force a strength of 1, then electromagnetic has a strength of 10^-2, weak has a strength of 10^-5, and gravity has a strength of 10^-38! Second, gravitational forces are attractive, not repulsive. So you couldn’t generate a force field to keep people out, but you could generate one to keep people in — it could be a tractor beam! Gravitational fields are generated by mass, and you’ll need a huge pile of it, since gravity is so weak. For instance, the mass of the earth is generating a nice tractor beam that prevents us humans from leaping into outer space. So let’s say the evil extraterrestrials have a spaceship that has the same mass as a 747, and it goes at a speed of Warp 9, or nine times the speed of light. Here’s where the calculation gets a little dicey, since physics doesn’t permit speeds greater than the speed of light. So let’s throttle down to just under the speed of light. To generate a sufficient tractor beam, you would basically need a black hole. I don’t know about you, but I wouldn’t keep a black hole in my trunk.

That leaves electromagnetism. The problem is that electromagnetic forces work on charged objects, and most of your humans and aliens are electrically neutral. For example, we move around in the magnetic field of the earth with no problem. However, at sufficiently small distances — atomic distances — our neutral charge is arranged in positively charged nuclei surrounded by negatively charged electron clouds. So at the very surface, we are slightly negatively charged. This gives us something to work with. Even better, we are composed of atoms, and electrons of the same quantum state are not allowed to occupy the same space due to the Pauli exclusion principle. You see where I’m going with this? If we could get, say, an immobile sheet of electrons, in approximately the same quantum states as our own electrons, then we could not pass through because the electrons in our atoms would be both electrically repelled by the charge, and forbidden to occupy the same space due to Pauli. This kind of force field would keep Star Trek criminals in the brig! The easiest way to generate an immobile sheet of electrons is to build a wall. The electrons on the surface of this wall would generate a short-distance electric field that would repel (at atomic distances) other matter. Ta-da, a force field!

If it needs to be invisible, try plexiglass.

SDStaff Karen Lingel, Straight Dope Science Advisory Board

Send questions to Cecil via cecil@straightdope.com.

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