What If? Page 6

  But solar panels aren’t used just in space.

  Solar power

  Emergency call boxes, often found along the side of the road in remote locations, are frequently solar-powered. They usually have lights on them, which provide illumination every night.

  Like wind turbines, they’re hard to service, so they’re built to last for a long time. As long as they’re kept free of dust and debris, solar panels will generally last as long as the electronics connected to them.

  A solar panel’s wires and circuits will eventually succumb to corrosion, but solar panels in a dry place, with well-built electronics, could easily continue providing power for a century if they’re kept free of dust by occasional breezes or rain on the exposed panels.

  If we follow a strict definition of lighting, solar-powered lights in remote locations could conceivably be the last surviving human light source.3

  But there’s another contender, and it’s a weird one.

  Cherenkov radiation

  Radioactivity isn’t usually visible.

  Watch dials used to be coated in radium, which made them glow. However, this glow didn’t come from the radioactivity itself. It came from the phosphorescent paint on top of the radium, which glowed when it was irradiated. Over the years, the paint has broken down. Although the watch dials are still radioactive, they no longer glow.

  Watch dials, however, are not our only radioactive light source.

  When radioactive particles travel through materials like water or glass, they can emit light through a sort of optical sonic boom. This light is called Cherenkov radiation, and it’s seen in the distinctive blue glow of nuclear reactor cores.

  Some of our radioactive waste products, such as cesium-137, are melted and mixed with glass, then cooled into a solid block that can be wrapped in more shielding so they can be safely transported and stored.

  In the dark, these glass blocks glow blue.

  Cesium-137 has a half-life of thirty years, which means that two centuries later, they’ll still be glowing with 1 percent of their original radioactivity. Since the color of the light depends only on the decay energy, and not the amount of radiation, it will fade in brightness over time but keep the same blue color.

  And thus, we arrive at our answer: Centuries from now, deep in concrete vaults, the light from our most toxic waste will still be shining.

  1When Enrico Fermi built the first nuclear reactor, he suspended the control rods from a rope tied to a balcony railing. In case something went wrong, next to the railing was stationed a distinguished physicist with an axe. This led to the probably apocryphal story that SCRAM stands for “Safety Control Rod Axe Man.”

  2The purpose of the crash was to safely incinerate the probe so it wouldn’t accidentally contaminate the nearby moons, such as the watery Europa, with Earth bacteria.

  3The USSR built some lighthouses powered by radioactive decay, but none are still in operation.

  Machine-Gun Jetpack

  Q. Is it possible to build a jetpack using downward-firing machine guns?

  —Rob B

  A. I was sort of surprised to find that the answer was yes! But to really do it right, you’ll want to talk to the Russians.

  The principle here is pretty simple. If you fire a bullet forward, the recoil pushes you back. So if you fire downward, the recoil should push you up.

  The first question we have to answer is “can a gun even lift its own weight?” If a machine gun weighs 10 pounds but produces only 8 pounds of recoil when firing, it won’t be able to lift itself off the ground, let alone lift itself plus a person.

  In the engineering world, the ratio between a craft’s thrust and the weight is called, appropriately, thrust-to-weight ratio. If it’s less than 1, the vehicle can’t lift off. The Saturn V had a takeoff thrust-to-weight ratio of about 1.5.

  Despite growing up in the South, I’m not really a firearms expert, so to help answer this question, I got in touch with an acquaintance in Texas.1

  Note: Please, PLEASE do not try this at home.

  As it turns out, the AK-47 has a thrust-to-weight ratio of around 2. This means if you stood it on end and somehow taped down the trigger, it would rise into the air while firing.

  This isn’t true of all machine guns. The M60, for example, probably can’t produce enough recoil to lift itself off the ground.

  The amount of thrust created by a rocket (or firing machine gun) depends on (1) how much mass it’s throwing out behind it, and (2) how fast it’s throwing it. Thrust is the product of these two amounts:

  If an AK-47 fires ten 8-gram bullets per second at 715 meters per second, its thrust is:

  Since the AK-47 weighs only 10.5 pounds when loaded, it should be able to take off and accelerate upward.

  In practice, the actual thrust would turn out to be up to around 30 percent higher. The reason for this is that the gun isn’t spitting out just bullets—it’s also spitting out hot gas and explosive debris. The amount of extra force this adds varies by gun and cartridge.

  The overall efficiency also depends on whether you eject the shell casings out of the vehicle or carry them with you. I asked my Texan acquaintances if they could weigh some shell casings for my calculations. When they had trouble finding a scale, I helpfully suggested that given the size of their arsenal, really they just need to find someone else who owned a scale.2

  So what does all this mean for our jetpack?

  Well, the AK-47 could take off, but it doesn’t have enough spare thrust to lift anything weighing much more than a squirrel.

  We can try using multiple guns. If you fire two guns at the ground, it creates twice the thrust. If each gun can lift 5 pounds more than its own weight, two can lift 10.

  At this point, it’s clear where we’re headed:

  You will not go to space today.

  If we add enough rifles, the weight of the passenger becomes irrelevant; it’s spread over so many guns that each one barely notices. As the number of rifles increases, since the contraption is effectively many individual rifles flying in parallel, the craft’s thrust-to-weight ratio approaches that of a single, unburdened rifle:

  But there’s a problem: ammunition.

  An AK-47 magazine holds 30 rounds. At 10 rounds per second, this would provide a measly three seconds of acceleration.

  We can improve this with a larger magazine—but only up to a point. It turns out there’s no advantage to carrying more than about 250 rounds of ammunition. The reason for this is a fundamental and central problem in rocket science: Fuel makes you heavier.

  Each bullet weighs 8 grams, and the cartridge (the “whole bullet”) weighs over 16 grams. If we added more than about 250 rounds, the AK-47 would be too heavy to take off.

  This suggests our optimal craft would comprise a large number of AK-47s (a minimum of 25 but ideally at least 300) carrying 250 rounds of ammunition each. The largest versions of this craft could accelerate upward to vertical speeds approaching 100 meters per second, climbing over half a kilometer into the air.

  So we’ve answered Rob’s question. With enough machine guns, you could fly.

  But our AK-47 rig is clearly not a practical jetpack. Can we do better?

  My Texas friends suggested a series of machine guns, and I ran the numbers on each one. Some did pretty well; the MG-42, a heavier machine gun, had a marginally higher thrust-to-weight ratio than the AK-47.

  Then we went bigger.

  The GAU-8 Avenger fires up to 60 1-pound bullets a second. It produces almost 5 tons of recoil force, which is crazy considering that it’s mounted in a type of plane (the A-10 “Warthog”) whose two engines produce only 4 tons of thrust each. If you put two of them in one aircraft, and fired both guns forward while opening up the thrott
le, the guns would win and you’d accelerate backward.

  To put it another way: If I mounted a GAU-8 on my car, put the car in neutral, and started firing backward from a standstill, I would be breaking the interstate speed limit in less than three seconds.

  “Actually, what I’m confused about is how.”

  As good as this gun would be as a rocket pack engine, the Russians built one that would work even better. The Gryazev-Shipunov GSh-6-30 weighs half as much as the GAU-8 and has an even higher fire rate. Its thrust-to-weight ratio approaches 40, which means if you pointed one at the ground and fired, not only would it take off in a rapidly expanding spray of deadly metal fragments, but you would experience 40 gees of acceleration.

  This is way too much. In fact, even when it was firmly mounted in an aircraft, the acceleration was a problem:

  [T]he recoil . . . still had a tendency to inflict damage on the aircraft. The rate of fire was reduced to 4,000 rounds a minute but it didn’t help much. Landing lights almost always broke after firing . . . Firing more than about 30 rounds in a burst was asking for trouble from overheating . . .

  — Greg Goebel, airvectors.net

  But if you somehow braced the human rider, made the craft strong enough to survive the acceleration, wrapped the GSh-6-30 in an aerodynamic shell, and made sure it was adequately cooled . . .

  . . . you could jump mountains.

  1Judging by the amount of ammunition they had lying around their house ready to measure and weigh for me, Texas has apparently become some kind of Mad Max–esque post-apocalyptic war zone.

  2Ideally someone with less ammo.

  Rising Steadily

  Q. If you suddenly began rising steadily at 1 foot per second, how exactly would you die? Would you freeze or suffocate first? Or something else?

  —Rebecca B

  A. Did you bring a coat?

  A foot per second isn’t that fast; it’s substantially slower than a typical elevator. It would take you 5-7 seconds to rise out of arm’s reach, depending how tall your friends are.

  After 30 seconds, you’d be 30 feet—9 meters—off the ground. If you skip ahead to page 168, you’ll learn that this is your last chance for a friend to throw you a sandwich or water bottle or something.1

  After a minute or two you would be above the trees. For the most part, you’d still be about as comfortable as you were on the ground. If it’s a breezy day, it would probably get chillier thanks to the steadier wind above the tree line.2

  After 10 minutes you would be above all but the tallest skyscrapers, and after 25 minutes you’d pass the spire of the Empire State Building.

  The air at these heights is about 3 percent thinner than it is at the surface. Fortunately, your body handles air pressure changes like that all the time. Your ears might pop, but you wouldn’t really notice anything else.

  Air pressure changes quickly with height. Surprisingly, when you’re standing on the ground, air pressure changes measurably within just a few feet. If your phone has a barometer in it, as a lot of modern phones do, you can download an app and actually see the pressure difference between your head and your feet.

  A foot per second is pretty close to a kilometer per hour, so after an hour, you’ll be about a kilometer off the ground. At this point, you definitely start to get chilly. If you have a coat, you’ll still be OK, though you might also notice the wind picking up.

  At about two hours and two kilometers, the temperature would drop below freezing. The wind would also, most likely, be picking up. If you have any exposed skin, this is where frostbite would start to become a concern.

  At this point, the air pressure would fall below what you’d experience in an airliner cabin,3 and the effects would start to become more significant. However, unless you had a warm coat, the temperature would be a bigger problem.

  Over the next two hours, the air would drop to below-zero temperatures.4,5 Assuming for a moment that you survived the oxygen deprivation, at some point you’d succumb to hypothermia. But when?

  The scholarly authorities on freezing to death seem to be, unsurprisingly, Canadians. The most widely used model for human survival in cold air was developed by Peter Tikuisis and John Frim for the Defence and Civil Institute of Environmental Medicine in Ontario.

  According to their model, the main factor in the cause of death would be your clothes. If you were nude, you’d probably succumb to hypothermia somewhere around the five-hour mark, before your oxygen ran out.6 If you were bundled up, you may be frostbitten, but you would probably survive . . .

  . . . long enough to reach the Death Zone.

  Above 8000 meters—above the tops of all but the highest mountains—the oxygen content in the air is too low to support human life. Near this zone, you would experience a range of symptoms, possibly including confusion, dizziness, clumsiness, impaired vision, and nausea.

  As you approach the Death Zone, your blood oxygen content would plummet. Your veins are supposed to bring low-oxygen blood back to your lungs to be refilled with oxygen. But in the Death Zone, there’s so little oxygen in the air that your veins lose oxygen to the air instead of gaining it.

  The result would be a rapid loss of consciousness and death. This would happen around the seven-hour mark; the chances are very slim that you would make it to eight.

  She died as she lived—rising at a foot per second. I mean, as she lived for the last few hours.

  And two million years later, your frozen body, still moving along steadily at a foot per second, would pass through the heliopause into interstellar space.

  Clyde Tombaugh, the astronomer who discovered Pluto, died in 1997. A portion of his remains were placed on the New Horizons spacecraft, which will fly past Pluto and then continue out of the solar system.

  It’s true that your hypothetical foot-per-second trip would be cold, unpleasant, and rapidly fatal. But when the Sun becomes a red giant in four billion years and consumes the Earth, you and Clyde would be the only ones to escape.

  So there’s that.

  1It won’t help you survive, but . . .

  2For this answer, I’m going to assume a typical atmosphere temperature profile. It can, of course, vary quite a bit.

  3. . . which are typically kept pressurized at about 70 percent to 80 percent of sea level pressure, judging from the barometer in my phone.

  4Either unit.

  5Not Kelvin, though.

  6And frankly, this “nude” scenario raises more questions than it answers.

  weird (and worrying) questions from the what if? INBOX, #3

  Q. Given humanity’s current knowledge and capabilities, is it possible to build a new star?

  —Jeff Gordon

  Q. What sort of logistic anomalies would you encounter in trying to raise an army of apes?

  —Kevin

  Q. If people had wheels and could fly, how would we differentiate them from airplanes?

  —Anonymous

  Orbital Submarine

  Q. How long could a nuclear submarine last in orbit?

  —Jason Lathbury

  A. The submarine would be fine, but the crew would be in trouble.

  The submarine wouldn’t burst. Submarine hulls are strong enough to withstand 50 to 80 atmospheres of external pressure from water, so they’d have no problem containing 1 atmosphere of internal pressure from air.

  The hull would likely be airtight. Although watertight seals don’t necessarily hold back air, the fact that water can’t find a way through the hull under 50 atmospheres of pressure suggests that air won’t escape quickly. There may be a few specialized one-way valves that would let air out, but in all likelihood, the submarine would remain sealed.


  The big problem the crew would face would be the obvious one: air.

  Nuclear submarines use electricity to extract oxygen from water. In space, there’s no water,[citation needed ] so they wouldn’t be able to manufacture more air. They carry enough oxygen in reserve to survive for a few days, at least, but eventually they’d be in trouble.

  To stay warm, they could run their reactor, but they’d have to be very careful how much they ran it—because the ocean is colder than space.

  Technically, that’s not really true. Everyone knows that space is very cold. The reason spacecraft can overheat is that space isn’t as thermally conductive as water, so heat builds up more quickly in spacecraft than in boats.

  But if you’re even more pedantic, it is true. The ocean is colder than space.

  Interstellar space is very cold, but space near the Sun—and near Earth—is actually incredibly hot! The reason it doesn’t seem that way is that in space, the definition of “temperature” breaks down a little bit. Space seems cold because it’s so empty.

  Temperature is a measure of the average kinetic energy of a collection of particles. In space, individual molecules have a high average kinetic energy, but there are so few of them that they don’t affect you.

  When I was a kid, my dad had a machine shop in our basement, and I remember watching him use a metal grinder. Whenever metal touched the grinding wheel, sparks flew everywhere, showering his hands and clothes. I couldn’t understand why they didn’t hurt him—after all, the glowing sparks were several thousand degrees.