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Science
by Pat Murphy & Paul Doherty

DEATH RAYS AND OTHER EXPERIMENTS TO TRY AT HOME

Science fiction writers began writing about lasers years before they actually existed. OK, so we called them blasters or ray guns or death rays. But we had the basic idea down: a device that delivered an amazing burst of energy in the form of light.

Theodore Maiman at Hughes Aircraft Company made the first real laser in 1960. Since that time, lasers have become much smaller and cheaper. Today, you can buy a solid state laser pointer at a supermarket checkout for less than $10.

Despite the advances in laser technology, this method of delivering energy with light hasn't yielded a death ray weapon just yet. Instead, lasers have proven useful for communication and measurement, for cutting cloth and steel, for serving as scalpels in delicate surgery, and, of course, for making bright red dots on a screen during lectures. The US military has spent tens of billions of dollars researching laser weapons and has succeeded in knocking down a few missiles using lasers. There are also laser-guided bombs and missiles, but so far the military uses of lasers run a far distant second to other uses.

In this column, we'll tell you a little about lasers and their uses, we'll provide some interesting experiments you can do with a laser pointer, and we'll tell you how you could, with existing technology, create a death ray. (Trust us: we really will!)

START WITH A NEON LIGHT

Send an arc of electricity through neon gas, and the gas glows. Stop there, and you've got a neon light. (One look at Las Vegas reveals how offensive a neon light can be, but it's not a weapon--unless you count the power of capitalism as a weapon).

Here's what's happening in the neon light. The electric current flowing through the neon gas is made of electrons, which are moving at a fair clip. Those high-speed electrons crash into neon atoms and the atoms absorb energy. Then the atoms give up that energy and emit light.

But the atoms don't just absorb and emit random amounts of energy. The electrons in an atom occupy various discrete energy levels When an electron crashes into an atom, the energy that the atom absorbs boosts an electron up to a higher energy level. When the electron drops down to a lower energy level, it gives off a particle known as a photon--a particle of light. This photon has an energy equal to the energy difference between the high and low energy states, producing red light.

Before we go on, we'd better say a word about photons. We've talked before about the dual nature of light--it's both a particle and a wave. Photons are particles because they are created entirely or not at all and they disappear the same way. They are waves because they travel with a wavelength and a frequency.

THEY DO IT WITH MIRRORS

To make a neon light into a laser beam, you need to produce a bunch of photons that are all traveling in the same direction, with the same frequency or color, the same polarization, and the same phase. The wave crests of one photon have to be side by side with the wave crests of all the others.

How do you manage that? Well, Einstein came up with the operating principle behind the laser. He knew that an atom in an excited state decays into a lower energy state by emitting a photon. Einstein figured out what would happen if you hit an atom in an excited state with a photon that had exactly the same energy and frequency as the one the atom was going to emit. You would stimulate the atom to emit that photon. This is called stimulated emission, and it's the key to creating a laser.

Einstein also figured out that the newly emitted photon would be the identical twin of the stimulating photon. It would be going in the same direction with the same phase. So to make a laser, you stimulate atoms to emit photons, then keep all those photons going in the same direction with the same phase. When you have a beam with a few billion billion identical photons, you focus the beam on a tiny spot and deliver all of the energy in the photons to that spot. Presto! It's a laser.

The first laser was a pulsed solid state laser made from a ruby rod with mirrors on both ends. A flash lamp shone through the sides of the rod, soaking the atoms in light energy. The atoms absorbed the light and became excited. The atoms in the ruby decayed, emitting photons in random directions. Once a random photon started bouncing back and forth between the mirrors, it triggered other atoms to discharge. Photons would bounce back and forth between the parallel mirrors on the ends of the laser, triggering all of the excited atoms in the rod to release their energy into the laser beam. It takes several back-and-forth bounces of the photons to deplete all of the excited atoms of their energy but even so, the atoms are quickly drained of their energy, resulting in a very short pulse of bright laser light.

TO THE MOON AND BACK

Pulsed solid state lasers similar to the first one have done great things. One very cool project has used laser light to measure the distance to the moon to one centimeter accuracy.

To measure the distance to the moon, scientists bounced light off a reflector left behind by the Apollo 11 astronauts. The laser beams are projected from pulsed neodymium YAG (yttrium aluminum garnet) lasers at the McDonald observatory in Texas. Only a laser beam will stay together long enough to reach the moon and return with enough energy to be detected.

The laser sends out 1017 photons in a pulse through its 0.8 meter diameter telescope. By the time it reaches the moon, the beam has spread to 4 kilometers in diameter. Of those 1017 photons, a mere billion photons hit the 0.4 meter square retroreflector and bounce back toward the earth. By the time this light arrives back at the telescope of the McDonald observatory (just three seconds after its departure), there is only one photon left from the original batch.

The round trip travel time of this photon gives the precise distance from the telescope to the retroreflector and back again. Decades of these measurements have given scientists a much better understanding of the orbit of the moon. For example, they know that the moon's orbit is moving away from the earth at an average of 38 cm per year. The distance to the moon changes in a way that shows that the moon has a liquid core. ("Crunchy on the outside; chewy on the inside," says Paul.)

A COUPLE OF MIRRORS AND SOMETHING EXCITABLE

Most lasers, like the solid state pulse laser, have a mirror on each end and bounce light back and forth. The material sandwiched between the mirrors determines the type of laser. In the original laser, the material was a ruby rod.

Soon after the invention of the solid state laser, Ali Javan at MIT managed to coax a gas into emitting laser light. In the classic gas laser, a mixture of helium-neon gas is excited by an electrical discharge, like a neon light.

In gas lasers, the mirror at one end of the tube is made as perfect as possible. The other is designed to allow 2 percent of the light to leak through. That means that the gas laser can to emit a continuous beam of light, rather than a short pulse. It also means that the laser light inside a gas laser is fifty times brighter than the beam that leaks out of the laser.

Carbon dioxide gas lasers (emitting invisible infrared light) create beams with hundreds of watts of power focused into a small pencil of light. These are the lasers that have been used to cut out cloth, slicing through many pieces of cloth at the same time to make pieces that are exactly the same size. They've also been used to cut steel.

The military developed their first "death rays" using gas lasers. These experiments culminated with an air force jet shooting down a few anti-aircraft missiles using a gas laser. The details of this experiment show why laser weapons haven't arrived quite yet. The gas laser filled the belly of an entire, huge C-135, a four jet engine transport plane. The experiment showed that you could actually shoot down an antiaircraft missile, but it also proved that the laser you needed to do this was too big to be useful on the battlefield.

LED LASERS

The hope for future laser weapons can be seen in the ten dollar laser pointer you can buy at your grocery store check-out line. This is the laser diode based on light emitting diode LED technology.

In the LED laser, a semiconductor is excited by an electric current passing through the diode. You can think of the semiconductor in an LED laser as a giant manmade atom, designed to absorb energy and release photons . Instead of the many energy levels that an atom has, the semiconductor laser has just two levels called "bands"--the higher energy conduction band and the lower-energy valence band. Rather than boosting atoms to higher energy levels, the electons flowing through a diode can drop into what physicists call "holes," gaps suited to receiving electrons. When an electrons falls into a hole, a photon rushes away with the energy. (This is how a light emitting diode, LED, works.) But a passing photon can stimulate an electron to drop into a hole and emit a photon. Sound familiar? Once again, we have stimulated emission--which turns the LED into a laser.

The mirrors at either end of a semiconductor laser can be terrible--it doesn't take much reflection to make this laser work. The electric current through the laser delivers energy so fast it can keep up with the light leaking out the ends. The mirror at the output end of the LED laser is just the flat face of the semiconductor crystal. When light inside a ten dollar laser pointer hits this face, 70% of it is reflected. The remaining 30% blasts out to make the spot on the slide screen.

Those supermarket laser pointers are amazing. With just three small batteries, they produce a bright beam for a long time. This beam is created by exciting the atoms in a solid, and solids are about a thousand times denser than gasses. So, there are many more atoms per unit volume to soak up the excitation energy. A death ray based on a solid state laser can be a thousand times smaller volume than one using a gas.

If you read the side of a laser pointer, you'll find that it produces a beam with about a milliwatt of power--not much power at all. That power is concentrated in a narrow beam. Look at the size of the beam as it hits a nearby piece of paper--the spot of light is about a millimeter wide and a millimeter high.

The damage done by light depends on its intensity--that is, its power per unit area. Even a milliwatt can have a high intensity if it is focused on a small enough area. Because we know the power of the beam and the area of the beam, we can easily calculate the power per unit area in the beam: 1 milliwatt divided by 1 millimeter times 1 millimeter = 1000 Watts per meter squared.

This number is very close to "the solar constant," the intensity of sunlight at the surface of the earth. If you shine your laser on a sunlit white wall, you'll see that the laser spot vanishes in the sunlight, demonstrating that it is less intense than the sun. The laser light shining on your skin is no more intense than sunlight on your skin--but that doesn't mean it can't be dangerous.

Your eye can focus both the laser beam or light from the sun to a tiny spot on your retina. The beam is focused to a spot 10-4m across. So the intensity becomes a whopping 100 Kilowatts per meter squared. Just as looking directly at the sun can burn your retina (which is why you should never do it), shining the milliwatt beam of a ten dollar laser in your eye theoretically could burn your retina. (Paul says that your blink reflex is fast enough to save you from any damage from the laser pointer, but it's an experiment we recommend against under any circumstances.)

NOW ABOUT THAT DEATH RAY

To qualify as a death ray, we figure a laser needs to do something dramatic--like destroy a missile. To destroy a missile, you're probably going to need 10 kilowatts or so concentrated on a small spot. Now here's what you've been waiting for. We're going to tell you how to do it.

Simple. Just find ten million friends who all have 1 milliwatt supermarket lasers. Now convince them all to shine their lasers on exactly the same spot--somewhere on the missile you want to destroy.

There you go--10 kilowatts all on the same spot. Death ray--sure enough!

You say you don't have ten million friends with lasers? Well, you could find one million friends who have ten lasers each. Or ten friends with a million lasers each.

Hey, we said it was simple. We didn't say it would be easy.

If you want to experiment with lasers but don't have 10 million of them, we suggest the easier experiments below.

EXPERIMENTS TO TRY AT HOME

Red Jell-O

Jell-O has been one of Pat's favorite scientific materials--ever since she figured out how to make lenses from it by molding lemon Jell-O in measuring spoons and plopping them out on clear deli lids. Red Jell-O is one of the best materials for laser experimentation. We make it at extra-strength, using 1/4 of the water called for in the recipe. The red laser beam scatters wonderfully from the red gelatin (It also scatters from clear or lemon or other flavors, but red is really the best.)

You can use the Jell-O to trace the path of the laser beam. Shine the beam into a blob of Jell-O, watch it bend and reflect inside the Jell-O. Make Jell-O lenses and see how they change the path of a beam of laser light. And when you are done with your experiments, you can snack on the Jell-O. It's dessert and it's a science experiment--two treats in one!

Fun with Mirrors

If you have a small, square mirror, you can do some of Paul's favorite laser experiments.

Put some white paper down on a table top and prop up the mirror so it stands vertically over the paper. Make a dot of laser light on the paper in front of the mirror. Move the spot of light toward the mirror in a straight line. When the dot hits the mirror, keep moving the laser, and the dot will seems to bounce off and travel back out along the white paper.

This is a great way to study reflection. Bring the dot straight into the mirror and it bounces straight back. Bring it into the mirror at an angle and it bounces off at the same angle. (A physicist would say: the angle of incidence equals the angle of reflection.) It's as if you are in control of the speed of light, bring the dot in slowly and it bounces off slowly, sweep it in quickly and you'll see a line of light bounce off the mirror.

If you have another mirror, you can study what happens in the laser itself. Put the mirrors face to face about a fist width apart. Shine the laser over the top of the mirrors onto the paper below. Move the spot of light toward one of the mirrors, notice that the spot bounces off the first mirror, then hits the second and bounces off it, then back off the first. Back and forth. That's what's going on inside the laser.

You can also use the mirrors to make an infinity box. Just look over the top of the two mirrors standing face to face and stick a finger in from the side. Notice the array of finger images running off to infinity.

Check out the details. Each finger image faces opposite the direction of its neighbor images, fingerprints alternating with fingernails. How many reflected fingers can you see? Some light is lost on each bounce so the images get dimmer and dimmer after multiple reflections. If you wear glasses take them off, and you may notice that you see fewer images. Tilt one or both mirrors away from perfect parallelness. Notice how the images sweep off in curves.

Welcome to infinity.

Laser kaleidoscope

Face the two mirrors toward each other and then bring two mirror edges together. This makes the mirrors into an open book. You can use the laser to study how light bounces off the mirrors. Make a dot of light on the paper and move the dot toward one of the mirrors. Notice how the light bounces off one mirror, then the other and eventually comes back out from between the two mirrors. Change the angle between the two mirrors until it is 90 degrees, a right angle. When the laser bounces off both mirrors it comes out along a path parallel to the ingoing path. The arrangement is a retroreflector (like the one Apollo astronauts left on the moon).

Note: For more about Pat Murphy's and Paul Doherty's work, check out their web sites at: www.brazenhussies.net/murphy and www.exo.net/~pauld.

===THE END===

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