Suppose you are getting ready to take a physics test. Everything is set—but wait! Your calculator battery died. What do you do? If you're extra crafty, you could grab an LED (light-emitting diode) and use it to get your calculator to function again. I know this seems crazy, but it's true. In fact, I did indeed run a calculator using some LEDs, which I will show you below.

Of course, to really understand how this works we need to look at what an LED actually is. I'm sure you have a few in the smartphone in your pocket. Many video displays use LEDs. It's very possible you've got one screwed into your ceiling light. They are everywhere.

Let's start off with just a diode. A diode is a device that is made from two types of semiconductors that are connected together. In one of the semiconductors, there are extra electrons (negative charges) that can move around to make the material a conductor. We call this an n-type semiconductor (the n stands for negative). The other type of material is called a p-type semiconductor. I bet you can guess what the p stands for—yup, positive charges. In the p-type there are actually atoms with missing electrons. These are called positive holes because an electron should be there. But these holes essentially behave like a positive charge.

When you put a p-type together with an n-type, you get a diode. If a current of negative electrons (which is the way most electrical currents work) enters the n-type side of the diode, everything works fine. The negative electrons can move through the n-type part of the diode with no problems. When these charges get to the p-type side, they combine with a positive hole (they fill in the holes). This makes it look as if a positive hole is moving in the opposite direction as the negative charge, such that there is a constant current across the diode.

If you switch the direction of the electric current, something different happens. To do that, you have to change the direction of the electric field inside the diode. This field then pushes the negative charges in the n-type and the positive holes in the p-type farther apart. Now it is much harder for the n's and p's to combine, so you essentially get no current.

That's the essence of a diode. Current can go one way through it, but not the other way. But wait! What about the light part? It turns out that a negative charge in the n-type side has a greater energy than the positive holes in the p-type side. So when a negative charge combines with a hole, there is a decrease in energy for the charge. Since energy has to be conserved, that energy has to go somewhere. It does. It makes light.

It's actually even crazier that that. It turns out that the frequency of the light produced is proportional to the change in energy. Yes, this is from quantum mechanics, but it is still real. Here is that relationship:

In this expression, ΔE is the change in energy of the electron and f is the frequency of light. The h is Planck's constant—it's kind of a big deal in quantum mechanics. But that is your LED, the light-emitting diode. I use them. You use them. Everyone uses them. They are great for lights because they mostly just create light and don't get very hot like incandescent or fluorescent bulbs.

Now let's get super crazy. What if you take an LED and you don't connect it to a battery? Instead you connect the LED to a voltmeter and measure the electric potential across the leads of the LED? Check this out.

Notice that by connecting the LED to the voltmeter, you get a voltage right away. This LED comes from an overhead light. When I cover up the LED, the voltage drops. Shining a bright light increases the voltage quite a bit. But why? Essentially, the diode is acting like a solar panel. OK, it IS a solar panel. The light gives energy to the electrons in the n-type material so that it has enough energy to move to the p-type side. This movement of charges builds up a potential difference (it's essentially acting like a capacitor here) so that you get voltage.

In case you can't tell, I think this is awesome. The LED is a two-way device. Run a current through and you get light. Shine light on it and you can get an electric current (if you connect it to something). OK. Game on. Can I use some LEDs to power something? In fact, YES. Check this out. Here are a bunch of LEDs connected in parallel such that the current from each LED adds to the total current. This LED bank is connected to a solar power calculator with the solar cell removed.

It works. OK, I had plans for something bigger. I wanted to have this run some tiny electric motor, but I couldn't get it to work. The calculator is pretty low power so it's perfect for this job.

But wait. If an LED can be both a light and a solar panel, can a solar panel also be a light? Apparently, yes. I didn't get this to work, but I've been told that if you connect a solar panel to a power supply, it will glow. Oh, you can't see it—it glows in the near infrared (like your TV remote). This means you will need a camera without an infrared filter to see it. I'm going to keep trying with this.

Let me tell you my real plan (since it didn't work). I was going to connect a motor to an LED and shine a light on the LED to run the motor. Then I was going to turn the motor really fast so that it acts like a generator and lights up the LED. That would be pretty cool.

Yes, an electric motor and an electric generator are the same thing. If you run current through it, it spins. If you spin it, you can get a current. Boom. Double duty. There are other devices that go both ways. What about the speaker? If you connect a speaker to the audio input on your computer, it acts as a microphone. Also, there is the TEG (thermoelectric generator). This is a device that is essentially just two different metals connected together. If you heat up one of the metals you can create an electric current. This sort of device is used with spacecraft (and a radioactive source for heat) to provide deep-space power. However, if you take this same device and run current through it, one side gets hot and one side gets cold. It's an electric cooler with zero moving parts.

So, now I'm adding the LED to this list of dual-purpose devices. Now I just need to figure out how to build an LED solar panel from scratch. That will be fun.

Friday morning began with delays at New York’s LaGuardia Airport. That’s not unusual—New York’s airports are famously balky. But this time, the cause wasn't something prosaic, like a blizzard. It was staffing. Because of the federal government shutdown, the airport didn’t have enough Transportation Security Administration agents and air traffic controllers; things slowed to a ground stop.

Then it started to spread—Newark, Philadelphia, even the key hub of Atlanta all began to wind down. And that’s terrifying. Airports are nodes on a global network, and the science that guides how that network behaves means that if one node has a problem, that problem will spread. The international air travel network exists on something of a knife-edge. It doesn’t take much to knock it out of optimal flow.

Basically, the delay problem is one of "connected resources"; planes land and have to get turned around to perform other flights, and some of the passengers on them are getting onto other flights, too. If you’ve ever flown, you know all that, but what it means in practice is that small mistakes or delays at one airport get magnified as they move down the line, propagating and sometimes intensifying. “The systems operating these queues are very close to capacity,” says Hamsa Balakrishnan, an aerospace engineer at MIT who studies the air transport network. “Both LaGuardia and Newark had wind-related delays today. With full staffing you might have been able to manage, but with a decrease in staffing as well you have delays, which then end up spreading to other airports as well, because of connectivity.”

Typically you might expect that the biggest airports in the world—the ones with the most flights in and out, say, or that move the most people—would have the biggest effect on overall movement across the network. But in fact, an airport’s “delay propagation multiplier” varies depending on all kinds of things, from how an airport is scheduled to its overall capacity, and even the weather. By one calculation, a minute of delay causes an average of 30 seconds of slowdown elsewhere in the network. But some airports are more resilient than others. The time it takes to get from one to the other has an effect. It’s so complicated that it daunts even the most intrepid network modelers.

Airlines try to account for all this by building slack into the schedule. They calculate the amount of time a given flight should take—the “scheduled block time”—and the amount of time the plane should have to spend on the ground, the “scheduled turnaround time.” But then they have a choice. “They insert buffer time in their schedules and ground operations,” says Bo Zou, a transportation engineer at the University of Illinois. “They still encounter delays, and a newly formed delay for one flight will propagate to the second and third flights. Part of it will be absorbed by the buffer, but not all of it.” Build too small a buffer, and the delays propagate further. Build too large a buffer and you’re not using your fleet efficiently, and losing money. “One side is efficiency, the other is robustness,” Zou says.

And it changes all the time, depending on changing conditions—some are predictable, like winter storms, and some are not, like government shutdowns and informal sick-outs. That’s called a “dynamical complex network.” It has to adapt, constantly.

Because if it doesn’t? According to one study, flight delays cost the US economy over $30 billion a year. It’s not just lost time or flight expenses; it’s whatever the people on those flights were planning on doing when they arrived. “A prolonged shutdown, or even slow down, would likely affect all kinds of unforeseen things,” says Luís Bettencourt, a network scientist at the University of Chicago. “The reliability of time­-sensitive logistics will degrade, and the hub character of some of these cities will have to be bypassed, at least temporarily. A prolonged slow down would be most disastrous to large cities, their influence, and their economies.”

Having shut down the shutdown, the government can now get its TSA agents and air traffic controllers back on station. That’ll build some resilience back into the airports just in time for a big-ass snowstorm due to hit the midwest next week. But the overall health of the air travel network will still be precarious.

That’s why researchers are working on accumulating more and more data on how it all works (or doesn’t). If humans can’t schedule all these flights in an efficient and robust way, maybe an algorithm can. Balakrishnan has even cofounded a startup that’s trying to make it happen. “There are so many moving pieces that it’s hard for a human being to come up with all possible solutions,” she says. “But that’s something we know how to get computers to do.” If you enjoy flying on an intractable and incomprehensible network now, wait until it’s run by an intractable, incomprehensible robot.