Want to know what you’re waiting for? Here’s a little taste of what’s in store. There’s also much, much more! Quantum mechanics! Time Travel! The Big Bang!

“So, what do you do?”


The life of a physicist can be a lonely one.

Imagine this: You sit down on an airplane, and the person next to you asks you what you do for a living. You reply that you’re a physicist. From here, the conversation can go one of two ways. Nine times out of ten, the first thing out of their mouth is something along these lines:

“Physics? I hated that class!” [1]

You’ll then spend the rest of the trip (or party, or elevator ride, or date) apologizing for the emotional trauma that physics has apparently inflicted on your erstwhile friend. These random encounters often reveal an almost joyful contempt, reserved specifically for the fields of physical science and mathematics. “Oh, I’m terrible at algebra!” for example, is said in an almost boastful tone, in a way that “I barely even know how to read!” never would. But why?

Physics has a somewhat unfair reputation for being hard, impractical and boring. Hard? Perhaps. Impractical? Definitely not. Indeed, when people try to “sell” physics to the public, it is almost always in terms of how it can be used to build bridges or launch rockets – that is, how physics is ultimately the foundation for engineering or chemistry.

But boring? That’s where we really take issue. The problem, as we see it, is that the practical side of physics is almost always put forward at the expense of the interesting side. Even folks with technical focuses like engineering and computer science typically don’t get past mechanics and electro-magnetism to the really fun stuff. And that’s a shame, because quite frankly, there has been very little cutting-edge research done on pulleys in the last few years.

This hostility to physics seems to be ingrained, and makes it difficult to have discussions without jading an audience. In starting a scientific conversation with a “civilian”, we purveyors of physics often feel like we’re trying to force people to eat their vegetables, and rationalize it the same way. We never begin physics discussions with, “It’s fun!” but almost always, “It’s necessary,” which naturally drains all of the fun out of it.

In an era when new technologies are constantly emerging, scientific literacy should be fundamental. On the other hand, it isn’t necessary that you have 4 extra years of college sciences to understand them. You don’t need to have a detailed knowledge of exactly how the physics works to appreciate the revolutions in quantum computing or cosmology. It is important, rather, to understand why these developments are significant, and how they are poised to change technology and our lives.

And it’s not simply that people need to understand a particular theory. Physics is the archetypal inductive science, and by understanding how science proceeds, people are better able to make informed decisions about issues from global warming to “theories” of intelligent design. The hope is that we are more prepared to refute people who disagree with us by offering facts, rather than simply insisting “No.”

The United States, in particular, has an immense problem with science and mathematics education, with high school students performing well below average compared to other developed countries. But we cannot limit ourselves to only blaming teenagers, or their teachers, or, for that matter, programs like No Child Left Behind.

The problem is far reaching, affecting all walks of life. It is most evidently manifested in teenagers because we don’t sit down with people in their 50s and ask science-y questions like, “If you have 10 chickens, and you eat five of them, how much does your cholesterol go up?” Looking at a so-called practical story problem now makes the whole premise of applied math seems absurd. At a very early stage, many children throw up their arms and say, ”When am I ever going to need algebra?” and assume that the sole virtue in studying for the class is getting a good grade.

In an excellent series of books, John Allen Paulos addresses the epidemic of “Innumeracy”, and through a series of lively essays on topics that students normally don’t see, tries to give his readers the ability to think critically about numerical concepts, and tries to show (successfully, in our opinion) that mathematics is interesting above and beyond its practical import in computing the tip on your bill or balancing your checkbook.

As your own experience may suggest, Physics has the same break between the practical and the groundbreaking. While dry mechanics-based classes may drive people away from Physics, they are sometimes drawn back in by science fiction, or newspaper accounts of big discoveries, or the latest pictures from the Hubble Space Telescope.

These accounts, however, rarely feature the latest breakthroughs in inclined plane technology.
Rather, when the public gets excited, it tends to be about the universe, or big experiments like the Large Hadron Collider, or life on other planets. We said before that nine times out of ten, our attempts at discussing Physics at an airport or cocktail party left us with no phone number and a lonely cab ride home, but the rest of the time something wonderful happens. Occasionally, we will actually have conversations instead of confrontations. Sometimes, we’re lucky enough to be seated next to somebody who had a great physics teacher in high school, or whose uncle works for NASA, or who is an engineer and thinks what we’re doing is simply “quaint.”

In these cases, the conversation goes quite differently. It seems that every so often, we run into someone who has been holding a question about how the universe works in reserve for some time, but couldn’t figure out the keywords to plug into Wikipedia. Maybe the latest NOVA special only hinted at a topic, and they were eager know more. Some recent questions have included:

  • I heard the Large Hadron Collider is going to create mini Black Holes that will destroy the universe. Is this true? (Providing yet more evidence, as if any were needed, that Physicists are perceived as nothing more than mad scientists who would love nothing more than to destroy the earth.)
  • Is time travel possible?
  • Are there other parallel universes?
  • If the universe is expanding, what’s it expanding into?
  • What happens if I’m traveling at the speed of light and I try to look at myself in the mirror?

These are the sorts of questions that are what got us excited about physics in the first place. Indeed, the last question on the list above was one that Einstein himself posed, and was one of the main motivations for his development of Special Relativity. In other words, when we talk to people about what we do, we find that some people, however rare they may be, are excited about exactly the same aspects of physics as we are.

The most obvious approach is to make the subjects more approachable through available mathematics and science teaching materials. In response to this, most textbook authors try to make physics exciting by putting pictures of volcanoes, locomotives, and lightning bolts on the covers.[2] The desired response, presumably, is that students will look at the book and say, “Cool! Physics is really coming alive for me!” Our own experience is that students aren’t fooled by these ploys. If they are, they end up looking for the ‘How to Make Your Own Lightning’ chapter, and are even more disappointed when they fail to locate it.

We’d like to note in passing that we don’t take that approach in this book. You won’t see any cool graphics[3] , or anything else likely to increase the publication costs of the book. Rather, our approach will be quite simple: the physics itself is interesting. No, really! And, if you need further persuasion, we solemnly promise to deliver no fewer than five bad jokes per chapter (including groaners, puns, and facile cartoons). To give you an idea of the sort of family-friendly humor that you’re in for, consider the following:

Q: What did the photon do at the ballpark?
A: The lightwave!

With that in mind, each chapter of this book will start with a cartoon featuring an inexcusably terrible pun, and a question about how the universe works. By way of answering the question, we’re going to take you on a tour of the physics surrounding it, and by the end of the chapter, it’s our hope that the mystery surrounding the question will become clear, and that given the opportunity to re-examine it, the cartoon will suddenly appear hilarious. We will do so in exactly the way that you’d expect from scientists — very circuitously.

That is not to say that you must be a physics guru to understand; quite the contrary. Our aim is to find some middle ground between those who appreciate the underlying majesty of the physics foundation, and those who would rather gag themselves with a spoon than be caught dead within 100 yards of a protractor.

Without equations, many science writers usually resort to analogies, but the problem is that it isn’t always clear to the reader that what’s being written is an analogy, rather than a literal description of a problem. Without using math, it’s clear that there will be some crucial element of the physics missing. What we’d like to convey is how you would want to think about the problem, even if you don’t have the equations to set it up. In other words, once you understand what’s really going on, doing the math is just, well, math.

This description raises the question: What exactly do you eggheads expect from me? In writing this book, we make no presumptions. Every bit of evidence we present is constructed from the basics. It is not our intention to scare you with mathematics or daunting equations. In fact, why don’t we get all of the equations out of the way right now?


That’s it. That didn’t hurt too badly, did it?

“Is there life on other planets?”


Physicists really do deal with some mind-numbingly difficult questions. Already, we’ve talked about the beginning of time, the end of time, and all of the time in between. We tackled the vast stretches of space, and what makes matter. In our discussion of quantum mechanics, we rubbed up uncomfortably with what is destined to be the biggest question ever: free will versus determinism. A constant stream of weirdness pervades the field, and it’s sometimes the safest scientific policy to just lower your head, plug through the calculation, and peek at the answer afterwards . [1]

At the same time, there tends to be some sort of public perception that by thinking about physics on the scale of the universe, we might have some sort of special insight into the true nature of reality, or whether we’re alone in the universe. Things that, when asked, make a guy blush and remember that he has a calculation to plug through. The big esoteric questions aren’t easy to brush off. Newton, famously, was both the greatest physicist of his (or, arguably any other) generation, and was a devout Christian. In between inventing physics and calculus, he still had enough time to ponder how many angels could dance on the head of a pin. Applying physics to unphysical questions has a proud heritage, which means that when someone asks us if we believe in aliens, it’s not enough to feign ignorance. We’ll feign knowledge instead.

I. Where is everybody?

Let’s begin with the obvious. Just because something isn’t a physics question doesn’t mean that we don’t have something interesting to add to the conversation. For instance, “Have we ever made extraterrestrial contact?”

The simplest answer is that since we’re not conspiracy theorists, and therefore don’t believe in Area 51, we’re pretty sure that UFO’s have never crash-landed on earth. Sure, we want to believe, but even so, we would be extremely surprised if we’ve ever been visited.

Humans have only been broadcasting signals out into space for about 60 years. Aliens wouldn’t want to visit us unless they had detected suspicious looking signals coming from earth and subsequently wanted to check out where they were coming from (although, this desire would probably fade if they actually watched the television they were receiving). Assuming that they set off as soon as they saw the signals, the fastest that they could get here would be just shy of the speed of light.

If any aliens were to visit us, they’d have to be within about 30 light years of earth or so. There are about 400 such stars within the requisite distance, but so far, we haven’t seen direct indications that any of them contain earth-like planets, let alone life, and certainly not intelligent life. What’s more, since our signals are extremely weak, it’s unlikely that an alien civilization would have detected us even if they could be bothered to look for us in the first place.

However, it’s a big enough universe that it feels as though there must be some other civilizations out there. Enrico Fermi, one of the greatest physicists of the 20th century, encapsulated the basic problem as follows: consider the enormous number of stars out there. Unless there is something staggeringly special about us, odds are that some of those stars – perhaps many of them – might have eventually developed intelligent life. Then, and this is critical, many of those intelligent civilizations will have spread to other planets. If our experience here on earth is any indication, people (or people-like aliens) can spread to every habitable nook very, very quickly. Since the universe is so very old, it seems like it should be jam-packed full of intelligent creatures, and it’s likely that we should have been contacted many times over. As Fermi put it, “Where is everybody?”

Fermi played a bit fast and loose with his numbers, and was perhaps a bit unduly optimistic about the prospects of developing faster than light travel or colonizing other galaxies. Still, Fermi’s paradox sets the stage for trying to use our knowledge of Astronomy and Physics to figure out the odds that there are aliens out there with a ticket for earth. Given what we know about our galaxy, what are the odds that there are other intelligent beings in it?

The simplest approach would be to take long, repeated observations of many, many nearby stars. In principle, a super-civilization who wanted to advertize their existence to the outside world would send out radio signals with recognizable numerical patterns in them so that other intelligent civilizations could detect them. We, being somewhat less advanced, could only receive the signal; the power necessary to transmit over interstellar distances is well beyond our capabilities. If this scenario seems somehow familiar, it should. It’s the basic premise of Contact, written by Carl Sagan in 1985, and later made into a perfectly watchable movie starring Jodie Foster . [2]

While the part about us actually making contact is wishful thinking, the science behind the search is not. Since the 1960’s, there has been a very active collaboration known as the Search for Extra-Terrestrial Intelligence (SETI) whose aims are pretty much spelled out in the name . [3] Not to spoil the surprise, but so far the search for E.T.’s hasn’t produced anything to phone home about.

Could aliens come to visit (if they wanted to)?

Imagine that eventually SETI discovers an alien civilization, and – what luck! – they’re virtually in our backyard. Suppose we then wanted to mount a manned expedition to visit them on Alpha Centauri, about 4 (actually 4.3, but who’s counting) light years from earth. Is this the sort of thing we could actually do? Practically speaking, no, but it can’t hurt to speculate what a little sci-fi grade engineering might be able to do for us.

We can’t use warp drives to travel faster than light, because that’s just nuts, and don’t even get us started on how impractical it would be to set up a wormhole. We also can’t just accelerate instantly up to 99% the speed of light, even if we had the technology – the g-forces would kill us! Let’s say our spaceship has only 1 g of acceleration. We’d be riding in cool comfort. For the first half of the trip, we’d be thrown toward the back of the ship, but because our rate of acceleration, artificial gravity would feel like earth-normal. For the second half, during which we slow down, the front of the ship would become “down.” There’s also the issue of energy. Even if our “spaceship” consisted of a single pod with just enough room for a single human [4], it would still take as much energy as is currently consumed by the entire United States in a 3-month period to get up to speed.

But forgoing those minor technical difficulties, could we make it to Alpha Centauri in our lifetime?
Easily. Crunching through the numbers, it will only take about 1.7 years to travel the first light year, and only another 1.1 years to travel the next. By the time we’re halfway there, we’d be traveling 94% the speed of light. Of course, at that point, we’d have to start decelerating at 1 g, lest we crash into our destination at nearly the speed of light. Adding it up, the entire trip would take about 5.6 years. It wouldn’t make for particularly riveting sci-fi [5], but it’s definitely feasible.

There’s a complication—the times above are the times as measured by our friends back on earth. As we saw in Chapter 1, time seems to slow when we travel a sizable fraction of the speed of light. According to someone on the ship, the entire trip would take only 3.6 years – less than the 4 year “minimum” that we’d expect given that Alpha Centauri is 4 light years away. Make no mistake, we’d still be traveling at less than the speed of light, but our enormous speed distorts both space and time. Because of this time-dilation effect, we could, in principle, visit even more distant stars in our lifetime. The problem is that time passes normally for everybody else, and they might not care to wait for us.

Should we be optimistic? Clearly Fermi thought so, but he figured the aliens could come from any galaxy in the universe. It might be somewhat more realistic to think in terms of just our own galaxy. We can use a little of that statistical inference we’ve been touting to try to figure out the odds of detecting an extraterrestrial civilization. Frank Drake, one of the founders of SETI, formulated a probabilistic approach in the 1960’s to whether intelligent aliens live among us in our galaxy.

Though it’s been rewritten in a number of ways, The Drake Equation in its simplest form allows us to multiply together all of the limitations to developing an intelligent civilization:

  • How many stars are there in the galaxy?
  • What fraction has planets?
  • What fraction of the planets can support life?
  • What fraction of these planets does support life at some point?
  • If they develop life, what’s the probability that it ultimately involves into intelligent life?
  • What’s the probability that intelligent life will broadcast their existence into space?
  • How long do we expect these civilizations to last?

The first few questions can be answered with a fair amount of precision, but by the time we reach the bottom, your guess is (almost) as good as anyone’s. Still, by plying the tools of our trade, we might be able to make some pretty decent estimates.

Let’s start with the easiest question, the one at the top. On the face of it, we might expect that there are a lot of intelligent E.T.’s out there. After all, there are about 10 billion stars in our Galaxy (The Milky Way), and the typical star lasts tens of billions of years. On average, over the age of our Galaxy, 10 new stars form every year, and each star is a new opportunity for a new civilization. What this doesn’t tell us, however, is how many of these stars might give rise to solar systems like our own. And if there’s one thing we’re pretty sure of, it’s that life needs a planet to call home.

II. How many habitable planets are there?

When SETI was first established, exactly 9 planets were known, all within our solar system. Since Pluto has since been demoted to a “dwarf planet” and all of the others are either too hot, too cold and/or made of gas, we might be tempted to say that prospects were pretty dim for either finding another civilization or (should we finally trash this planet entirely) finding another place to colonize. It’s not that we thought there weren’t any planets around other stars. We just hadn’t found them yet. [6]

This all changed in the late 1980’s and early 1990’s when planets started being discovered left and right. Typically, we find new planets by looking at their star; planets orbit the sun, and the sun technically orbits its planets as well, albeit very slightly. If the planet is massive enough and close enough to its parent star, then the star will appear to wobble a little bit on every orbit, a measurable effect that can be used to infer the mass of the orbiting planets.

We’ve even started to be able to see some of the newly discovered planets directly. In 2008, groups from U.C. Berkeley and The Hertzberg Institute in British Columbia took direct images of planetary systems known as Fomalhaut b, and the HR 8799, respectively. Don’t expect to see images of lush beaches and urban skylines. Each of the photos is only one pixel across. Besides, these exoplanets aren’t the sort of places you’d like to vacation anyway. They’re all much more massive than Jupiter and almost certainly made of gas.

In early 2009, NASA launched the Kepler satellite. This instrument will continuously monitor about 100,000 stars, and look for signals that planets are eclipsing their host. As the planet passes in front of its star, the light of the star dims by a small amount. Since this effect is periodic, we use the dimming to figure out the length of the planet’s year, it’s physical size, the distance from the star, and other key properties.

So far, we’ve discovered over 300 planets outside our solar system, and doing a rough estimate, it seems that at least 15% of stars have planets, many of them more than one. However, the vast majority of those so far discovered are much more like Jupiter than Earth, and, unless swimming through a giant gaseous sphere of hydrogen appeals to you, aren’t the sorts of places we’d expect life to flourish.

What we’d really like to do is find a rocky planet – “terrestrial,” as they say in the biz. This is very tough to do. Since terrestrial planets are much less massive than gas giants, they produce much less wobble in their parent star and so they are much, much harder to detect than their bulkier Jovian brothers and sisters. Still, we’re working on it. The hope is that the Kepler satellite will find lots of earths – we just don’t know how many. It’s designed in such a way that a lucky alien civilization who’d built their own version of Kepler would be able to detect the earth.

But why wait for Kepler to start sending back results? In Chapter 6, we talked about a phenomenon known as “gravitational lensing” in which the light of a distant galaxy is magnified and distorted by the gravitational field of a galaxy or cluster of galaxies sitting between the two of us. Any mass can magnify background light, and so for several decades, astronomers have been monitoring “microlensing” events in which a star or some other object happens to pass between the earth and a distant star. The distant star gets brighter over the course of a few days or weeks, and then dimmer again. Because of this, we can detect any sort of mass, including planets, but we have to be very lucky to do so. In 2005, the Optical Gravitational Lens Experiment (so named on the basis of the crude acronym) saw a tiny extra signal when it was looking at a star. It detected the most earth-like planet ever seen outside our solar system, with a mass only about 5 1/2 times that of the earth. As it happens, though, we couldn’t live on OGLE-2005-BLG-390Lb (as it’s been dubbed), since it’s about -370 degrees Fahrenheit on the surface.

We’re making the assumption that life has to evolve on rocky planets with liquid water because that’s the sort of environment that gave birth to us. Maybe that’s fair, since we really don’t know what sort of other life might be out there. It’s entirely possible that the life grows up not on the planet, but on one of the moons surrounding it. There has been a lot of conjecture that the Jupiter’s moon, Europa, might have liquid water under the surface. Perhaps life could arise there, or some place very like it in our galaxy? The only thing we can say is that life doesn’t seem to have evolved on the moon or any of the other planets in the solar system. Besides, even if it’s possible that life could have evolved on a wider range of planets than we’re assuming, it doesn’t change the basic picture. We’d still expect life to be relatively rare.

Even within our solar system, just being a rocky planet doesn’t guarantee that the planet is “class M,” as Captain Kirk and the gang used to say. Mercury and Venus are by far too hot, while Mars has no atmosphere. Only the Earth falls into the Goldilocks zone – just right. Note, too, that all of the planets in our own solar system have nearly circular orbits around the sun, which means that they don’t vary widely in temperature over a year. However, most of the 300 planets discovered outside the solar system have very elliptical orbits, meaning you’d be roasted part of the year, and freezing the other part. None of these are very conducive to life.

We have prospects, though. In 2007, a planet known as Gliese 581d was discovered. While it’s about 8 times the mass of the earth, it’s almost close enough to its central star to allow water to melt. Though we don’t know whether there is any water to melt, or whether there are any greenhouse gases to warm the planet up, Gliese 581d remains the current record holder for the best prospect of a life-supportable planet yet found.

So while we’ve gotten relatively good at finding extra-solar planets, we haven’t yet found one that could support life. Going on to question 4 of the Drake equation, we have to shrug when we read, “What fraction of planets do support life?” Since we only know of one planet that could support life, and that planet does support life [7], it’s hard for us to say anything definitive.

There’s plenty of reason for optimism. Consider that the earth is about 4.6 billion years old, and that life seems to have gotten started after a mere 800 million years. In other words, in the one case we can look at, life seems to have gotten started almost as soon as it could have.

And so on. But much like those 1-900 lines, you’ll have to buy the book to see more.

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