If you watched Finding Life Beyond Earth last week, you might be wondering, what's the big deal about carbon-based life? Spock discovered silicon-based life in episode twenty-five of the original Star Trek series. And Star Wars' bounty hunter Zuckuss is ammonium based. How can scientists say these two hegemons of geek culture are wrong? Why does life have to be based on organic molecules?

The answer to this question is actually right in front of you. I'll give you a hint--it's big, colorful, and located directly below this paragraph. It's the periodic table! The information in the periodic table of elements is sufficient to convince scientists that life has to be carbon based--you just have to know how to interpret it.

The answer is in here! Image via Wikipedia .

Let's start with carbon's box on the periodic table. Carbon is element number six, which tells us that it has six negatively charged electrons buzzing around six positively charged protons in the nucleus. That's the information we're given--now let's apply a basic knowledge of chemistry and interpret it.

Atoms house their electrons in concentric energy shells. The first energy shell holds two electrons, the second and third hold eight. (There are more, but let's stop here for now.) Atoms fill their shells sequentially, and they always like to end with a full shell--even if that means giving away electrons. Sodium, for instance, has one electron dangling in its third energy shell and instead of trying to gain seven more electrons, sodium simply gives this electron away. Other atoms steal electrons to fill their outermost shell. Chlorine is one electron short of filling its outer shell, so it plucks an electron from an element, like sodium, and turns into a stable, negatively-charged version of itself.

Imagine for a moment that astronomers have finally discovered an Earth-like planet orbiting another star. After the papers are published, the headlines run, the tweets tweeted, one question will still be burning: Is anyone home?

Discovering a habitable planet is just the first tiny step toward discovering an inhabited planet. So how would astronomers begin to determine whether a newly-discovered Earth-like planet actually harbors life?

First, explains Soren Meibom, we must find out whether the planet has been around long enough for life to take hold. "On Earth it took roughly a billion years for the most primitive microbial forms of life to evolve and another three to three and a half billion years for animals and humans," says Meibom. "Therefore, as we look for life beyond Earth, and in particular beyond our own solar system, the question of time becomes highly relevant."

Because stars and their planets form together, to get at the age of a planet, all you need to know is the age of its star. Sounds easy enough. But in practice, determining a star's age is not so simple. After they are born, stars enter a middle age that continues until shortly before their death; over the course of that sustained middle age, which may last for billions of years, the star's appearance is essentially unchanged. Meibom compares a star to a person who emerges from infanthood as a fully-grown adult, then keeps her smooth skin and brunette hair for eight decades until, finally, she goes grey, gains a dowager's hump, and slips on some orthotics in the time it takes her to finish the early-bird special. (See Meibom's visual interpretation below.)

If people aged as stars age. Image courtesty Soren Meibom.

To pin down stars' ages, Meibom and his colleagues are turning to one property of that does change over time: the rate at which a star spins. "A star's rotation slows down steadily with time, like a top spinning on a table, and can be used as a clock to determine its age," says Meibom. "We can measure a star's rotation period by looking for changes in its brightness caused by dark spots on its surface--the stellar equivalent of sunspots. Any time a spot crosses the star's face, it dims slightly. Once the spot rotates out of view, the star's light brightens again. By watching how long it takes for a spot to rotate into view, across the star and out of view again, we learn how fast the star is spinning." Meibom is now using the Kepler space telescope to do just that.

So if a star and its planets are "of age," then what? It might be possible to look for "biosignatures" in a planet's atmosphere. An alien astronomer looking toward Earth, for instance, would see oxygen produced by plants and bacteria, ozone from the interaction of ultraviolet radiation and oxygen molecules, and nitrous oxide from microbial reactions. If we could pick out similar signatures in the atmosphere of an alien world, we would have strong reason to suspect that life might be present there. As astronomer Lisa Kaltenegger told NOVA back in 2009, "If you have a look at our own Earth, you actually have CO2, you have water, methane, and you have ozone or oxygen...That's the golden fingerprint you're looking for" in the spectrum of a habitable exoplanet.

The most conspicuous biosignatures in Earth's atmosphere are products of tiny microbes, not the plants and animals that loom so large in our notion of "life," points out Harvard astronomer Dimitar Sasselov. Our own planet was transformed by microbial life over the course of more than two billion years; so an exoplanet with lots of oxygen in its atmosphere may not be carpeted in forests, but it may have oceans full of green algae. That algae could be paving the way for more complex, energy-hungry forms of life, says Sasselov.

Technology is only now catching up to this ambitious project, though. Measuring an exoplanet atmosphere directly is only practical if the planet is very large and sits far from its parent star--not a great recipe for a habitable world. If an exoplanet happens to pass in front of its star from the vantage point of our telescopes, astronomers can probe its atmosphere indirectly, by comparing starlight that has passed through the planet's atmosphere to starlight that meets our telescopes directly, when the planet is "behind" the star.

Habitable planets will also make juicy targets for SETI--that is, the search for signals from extraterrestrial intelligence. SETI usually means "listening" for radio waves produced, either incidentally or as a beacon, by an alien civilization. (Think Contact.) Some astronomers have also proposed looking for optical signals produced by high-powered lasers.

The discovery of a bona fide habitable world will bring us closer to finding life beyond Earth, but there is still plenty of work to be done before we know whether we have company in the cosmos--or whether we are truly alone among the stars.

For more information on Soren Meibom's work, check out his recent public lecture at the Harvard-Smithsonian Center for Astrophysics, or watch him explain his results at a press conference from last year's meeting of the American Astronomical Society.

Steve Jobs' death at the age of 56 is profoundly sad. His story was the American dream writ large--a California boy with nothing but an idea and a lot of chutzpah, who, with Steve Wozniak, built a computer in his parents' garage and gave birth to the fastest-growing company in American history. Jobs' passing has sparked millions of well-deserved tributes. He's being remembered as a genius. Which he was. A visionary. He was that, too. And he's being called the Thomas Edison of the baby-boom generation. But that, believe it or not, may be selling Steve Jobs a little short.

To watch and read the transcript of the 1990 Steve Jobs interview featured in The Machine That Changed the World, visit the WGBH Open Vault archive.

Finding a comparison to the influence of Steve Jobs might require going back to the 15th century. That's when Johannes Gutenberg, a German blacksmith, introduced the moveable type printing press, which is widely regarded as the most important invention of the modern period. The printing press made books affordable, spread learning to the masses, and laid the material basis for the modern knowledge-based economy.

Unlike Gutenberg, Steve Jobs didn't invent the computer. He didn't even invent the personal computer. But with the introduction of the Macintosh in 1984, with its intuitive graphical interface, a user-friendly device called a mouse, an affordable price, and a brilliant marketing effort, Steve Jobs put the power of the computer into the hands of the masses, ultimately changing forever almost every aspect of the way we live, work, create, communicate, innovate, get and share information, and spend our leisure time.

The printing press was the first machine that changed the world. The computer is arguably the second. That idea was the intellectual underpinning (and the title) of a groundbreaking NOVA mini-series that I was fortunate enough to work on two decades ago. It was the brainchild of Executive Producer Jon Palfreman and overseen by Paula Apsell, NOVA's Senior Executive Producer.

Production on The Machine That Changed the World began in 1989. Everybody in the computer community was excited about the prospect of a television series produced by NOVA that would bring the story of the computer to life; and many luminaries would participate in the series either on camera, as content advisors, or both. There were the founding fathers who most people outside of computer science have never heard of: Presper Eckert, Thomas Watson, Jr., Herman Goldstine, John Backus, Robert Noyce, Gordon Moore, and others. There were the rebels of the 1960s and 1970s: Alan Kay, Doug Englebart, Larry Tessler, Bill Gates, Steve Wozniak, and so on. But there was only one rock star, and that was Steve Jobs.

As we started the series, we were warned time and time again. "You'll never get Steve Jobs on camera," those who knew him told us. But we didn't believe it. How could Steve Jobs, we wondered with no small amount of arrogance, allow a history of the computer to be told without participating in it? But his aversion to the media and the seemingly impenetrable privacy shield with which he surrounded himself were brought home to us when he finally responded to our multiple requests for an interview with an emphatic "No, thank you."

But we had an ace up our sleeve by the name of Robert Noyce. A legend in the computer world as the co-inventor of the microchip and co-founder of Intel, Bob Noyce was a strong supporter of The Machine That Changed the World and served on our advisory board. Like most in Silicon Valley, Steve Jobs revered Bob Noyce, and a one-paragraph letter from Noyce changed Jobs' "no" into a "yes," giving our series one of a limited number of interviews Steve Jobs gave in his short lifetime. (Sadly, it's also likely that our series contains the last interview ever recorded with Bob Noyce, who died in the spring of 1990, about a month after our camera crew recorded his story.)

Steve Jobs, who was 35 when we interviewed him for The Machine That Changed the World, was a vital, vibrant, visionary. His untimely death can only make us wonder what else he would have accomplished if given more time.

Professor Werner Heisenberg is speeding down the highway, when a cop pulls him over. The cop walks up to his car and asks, "Excuse me sir, do you know how fast you were going?" And Heisenberg responds, "No...but I know exactly where I am!"

Watch the full episode. See more NOVA.

If you understood this joke, read no further. However, if you're still a little confused, get ready to dive into some uncensored quantum mechanics. (Just kidding, I've removed the nastiest bits for you!)

This joke is based on the Heisenberg uncertainty principle--one of the fundamental tenets of quantum mechanics. The uncertainty principle can be summed up as: "The more precisely the position of a particle is determined, the less precisely the momentum is known in this instant, and vice versa." But to understand this statement, and hence the joke, it is first necessary to take on some quantum mechanics.

Quantum mechanics describes the behaviors of small particles, like electrons whizzing around an atomic nucleus. Compared to classical mechanics, which explains the behaviors of big objects like baseballs, airplanes, skiers--basically anything you encounter in your daily life--quantum mechanics seems very strange. To understand quantum mechanics, you need to let go of everything you've ever observed on the macro-scale and accept that there is a tiny world governed by its own unique laws.

The first principle of quantum mechanics you need to know in order to understand the joke is this: Subatomic particles are inherently fuzzy. An electron is not a fireball zooming around a chunky nucleus, but a negatively charged blanket that sometimes acts like a sheet crumbled into a ball and sometimes like a quilt spread across a bed. And because of this inherent fuzziness, the properties that describe the behaviors of these particles--such as position and momentum--are inherently fuzzy as well.

But the uncertainty principle is more specific than "particles are fuzzy." It holds that it is possible to determine either the position or the momentum of a particle, but it is impossible to determine both simultaneously. In other words, the more precisely we measure the momentum of a particle, the less precisely we can measure its position, and visa versa.

Are you laughing yet? Or are you beginning to wonder why the act of measurement seems to have such elevated importance? The way Heisenberg saw it, measuring any observable property of a particle actually affects that property. In this view, an electron doesn't even have a finite position and momentum until a scientist attempts to measure it, at which point the electron is forced to choose a state--like an atomic game of musical chairs where each player exists in every chair until the music stops. Not every physicist likes this interpretation of quantum mechanics--it always galled Einstein--but it's the one I'm sticking to for simplicity. After all, we're just trying to understand a joke here!

Still no guffaws? Okay, then it's time for an equation. As perplexing as the uncertainty principle is, it is represented by a very simple one: The product of the uncertainty in position and the uncertainty in momentum will always be greater than or equal to a constant. In other words, these two uncertainties are inversely related--if one increases, the other falls by a proportional amount.

However, the uncertainty principle can only be directly observed on the atomic scale. So sorry, Dr. Heisenberg, but it looks like you're getting a ticket after all.

Here is my list of Top Olde-Timey Things I Will One Day Tell My Incredulous, Hypothetical Grandchildren to prove how very old I am and how much life has changed:

• We used to write letters on paper and put them in metal boxes, from which actual human beings (in uniforms!) gathered them up and helped deliver them to their destinations.
• Telephones used to be bolted to the wall, with dials attached, and they couldn't give you directions or even tell you which sushi place made the best avocado rolls.
• We used to think that that the expansion of the universe was slowing down; that empty space was, basically, empty space; and that we had the majority of the stuff in the universe pretty much figured out.

The discovery that the expansion of the universe is not slowing down--that it is, in fact speeding up, driven by a mysterious "dark energy"--happened when I was in my first year of college. My astronomy professor was so excited about the discovery that we got to have class outside, under a tree, so that we could be closer to this nature thing that was turning out to be so full of baffling surprises.

Now that Saul Perlmutter, Brian Schmidt and Adam Riess have won the Nobel Prize for this discovery, it seems inconceivable that that moment under the tree--the time when we discovered that more than 70% of the universe was a complete mystery--is barely more than a decade old. How amazing that such a profound revelation happened in such recent history.

So as we at NOVA congratulate the winners of this year's Nobel Prize in physics, let's also celebrate living at a wonderful time in the history of science: A time when scientists are still busily mapping out the boundaries of what we don't know, and when the universe is ripe with questions, just waiting to be answered.