A Physicist Grudgingly Admits That Jericho Got EMPs Right

Since I grumbled about how Jericho dealt with fallout and nuclear weapons, I should be happy that the CBS show mostly got electromagnetic pulses correct. Right? Right?

You must be new here. As long-time readers know, I’ll lecture at a drop of the hat. Jericho may have gotten EMPs right, but that’s not enough — you must know why their treatment of EMPs is right.

Yes, I used to get paid for teaching physics. Why do you ask?

Electromagnetic Radiation

To start with, let’s discuss electromagnetic waves, since they’re at the heart of how electromagnetic pulses work. Those of you who remember this stuff from high school or college physics classes may skip down to the next big heading.

Anything that has an electrical charge produces an electric field. Anything that is magnetic produces a magnetic field. Broadly speaking, electricity and magnetism are related. For example, electricity flowing through a wire causes a magnetic field, which is how electromagnets work.

You know how, if you take a rope, tie one end to something, and wiggle the other end up and down, you’ll make waves? You can make waves in the electric field and the magnetic field. The easiest way to do this is wiggle something charged up and down. If you take an electron, which is negatively charged, and move it up and down, up and down, it will make waves in both the electric and magnetic field, just like wiggling the end of the rope makes waves along the rope. In fact, you can just move the electron in a straight line and it will make waves in the electromagnetic field. Those waves are called electromagnetic waves or electromagnetic radiation.

Even if you don’t know it, you’re familiar with EM radiation. The light coming from your computer screen is a type of EM radiation. So are the microwaves that your microwave oven puts out. There’s a whole bunch of those waves. What kind of wave they are depends on how fast the wave is wiggling. If the wave is wiggling slowly, you get radio waves that you can listen to in your car. If you wiggle a little faster, you get microwaves to cook your food. Faster still and you’re into infrared that the Predators use, then visible light, then the ultraviolet light that gives you your tan, and on and on through x-rays and eventually to gamma rays. The faster the wave is wiggling, the more energy the wave has. We call this rate of wiggling the wave’s frequency, and we measure the frequency in Hertz. One hertz means one wiggle per second.

(Note that I’m talking about how fast the wave wiggles back and forth, not how fast the wave is moving. All EM waves move at the speed of light.)

Nuclear Weapons Make EM Radiation

When an atomic bomb goes off, you get EM radiation. The most dangerous are the gamma rays. That’s the radiation that will fry you dead.

The gamma rays also do something else: they knock electrons off of atoms in the air through a process known as Compton scattering. The gamma rays have enough energy that, if they smack an electron, they rip the electron out of the loving embrace of its atom’s nucleus. The end result is a bunch of electrons running free in about the same direction as the gamma rays were originally going. Since the electrons are moving, they make electromagnetic waves. But they don’t just move in a straight line: they spiral around some because they’re in the Earth’s magnetic field. They look kind of like those seeds that helicopter around as they fall. What you end up with is a big pulse of electromagnetic radiation that’s moving out spherically from the nuclear bomb, and that radiation has all kinds of different frequencies. It’s mostly radio waves, which is why the EMP doesn’t really do anything to people.

Why is this a problem, then? Because all of this EM radiation is carrying energy, and it whacks into electronics very very fast. The EM radiation causes voltage spikes that happen in a millionth of a second or faster. Those voltage spikes do very bad things to unshielded electronic components, like heat them up. Semiconductors are toast. Transistors and diodes melt across their junctions. Capacitors explode like popcorn. You can even fry resistors.

Location, Location, Location

So why didn’t this happen to Jericho when the first bomb went off? As in real estate, it’s all about location. If you’re near the ground — 200 meters up or lower — a lot of your gamma rays go into the ground. You get an intense EMP up to around 5 kilometers from the bomb’s location, and less EMP out to 10+ kilometers.

Go higher, up to around 40 km, and you get a different story. I mentioned that the EMP’s radiation travels out from the bomb in roughly a straight line, which is why you have to have direct line-of-sight to the bomb itself to suffer EMP effects. Up higher, you can see it from further away.

But if you really want a strong EMP, you need to get your bomb 40 km high or higher. That’s the high-altitude burst region. The atmosphere is thin, so gamma rays travel far before they start smacking electrons around. The gamma rays that are headed down produce the EMP starting at around 40 km high and continuing down to a height of 20 km. You can blanket a lot more of the ground with EMP if you’re that high, and the EM radiation is very strong.

If you were a mad scientist bent on screwing the US over, you’d need to go really really high. A big hydrogen bomb detonated about 400 km above Kansas would blanket the continental US with an EMP. Of course, the International Space Station orbits some 360 km up, so it’s not the kind of thing small terrorist cells are going to be able to do.

Electrical Shielding

At one point in the episode, Hawkins says that his laptop’s okay because it’s ruggedized. He probably meant that it is shielded, since ruggedization is what you do to mil spec equipment so that it doesn’t break when grunts drag it through mud and over rocks. Of course, Hawkins also said that the EMP would fry anything with a wire, which also isn’t true — your 1966 Ford Fairlane is probably going to be just fine, though you may have to replace some fuses.

Anyway, how do you shield electrical equipment? Mainly with Faraday cages. You make a mesh out of metal that is a good conductor, and the EM radiation is kept outside through the magic of physics. That’s probably how Hawkins’s computer is shielded, though it looked a little thin to be well-shielded.

Nowadays the government is more worried about people spying on their computers by monitoring the EM radiation that it gives off. You can eavesdrop on the radio waves that a computer gives off and reconstruct things like the data being displayed. If you shield a system from emitting such signals, a process known as TEMPEST shielding, you can shield it from EMP damage with a minimal amount of additional work.

Notes for the Picky

Some technical notes for the super-nitpickers. The type of EMP I’ve described above is called HEMP. There’s a secondary effect called magnetohydrodynamic EMP, or MHD-EMP. MHD-EMP occurs because the bomb’s plasma and scattered gamma radiation take the Earth’s magnetic field and gives it a shove, like shaking a taut sheet on a bed. The shove sets up EM waves over two time scales: around ten seconds long and around a thousand seconds long. You’ll sometimes hear that referred to as heave waves or the heave effect, because the Earth’s magnetic field is heaving like a drunken sailor in a storm.

Technical notes for super-super-nitpickers: HEMP isn’t just caused by Compton scattering. The photoelectric effect and pair production also contribute, but Compton scattering is far and away the major source of HEMP.

Heh, heh, I said “hemp”.

6 Comments