A Frequently Asked Introduction Everyone has questions It's an inherent part of being human. As a species, we may not agree on much: politics, favorite sports team, best place to get a taco at twelve a.m. But one thing brings us all together: a need to know. We all wonder, and deep down we all have the same questions. Why can't I travel back in time? Is there another version of me out there? Where did the universe come from? How long are humans going to be around? And who eats tacos at twelve a.m. anyway? Fortunately, we have answers. Science has made incredible progress over the last few hundred years, and there's a lot we can say about some very fundamental questions about the universe. There are, of course, still huge mysteries (see our previous book, We Have No Idea: A Guide to the Unknown Universe), but for our species, things in the understanding-the-universe department seem to be going in the right direction. So much so that we felt it was time that somebody compiled a list of easy-to-read, cartoon-laden answers to some of humanity's most frequently asked questions. In this book, we'll explore answers to some of the deepest and most existential questions that people can ask about themselves, the planet, and the nature of reality itself. Have you ever wondered why aliens haven't visited us (assuming they haven't)? Or whether you are truly unique, or just a preprogrammed simulation in some alien video game? Do you stay up at night, wondering if life after death is possible? In your hands are answers to all of these questions. Each chapter covers a frequently asked question, hopefully revealing in the process some mind-blowing truth about our amazing universe. Think of this book as a primer for your next cocktail party, or as a quick, fascinating read while you sit on the toilet (thankfully, we made each chapter fairly short). You might wonder what makes us qualified to answer these questions. Rest assured that we have that utmost of qualifications to be authorities on a given topic: we have a podcast. In our humbly titled, twice-weekly audio program, Daniel and Jorge Explain the Universe, we cover topics ranging from microwaves to intergalactic phenomena to hypothetical fundamental particles. But it's answering questions from listeners that really inspired us to write this book. For us, that's one of the most exciting parts of having a podcast. Nothing brightens our day more than opening our in-box and reading a thoughtful question from a curious listener. And questions we definitely get! The question askers vary in age (nine to ninety-nine), occupation, and location. You might be surprised at the amazing questions that a nine-year-old from Devonshire can have about the observable universe. It seems that asking questions and the desire to know is in our hearts. Many would say that wondering about the nature of our cosmos and our place in it is one of the joys of being alive. Of course, it might be frustrating to not know the answers right away, or to only end up with more questions (as in some of the answers in this book), but there's power in just asking the questions. You see, asking questions supposes that it's possible to find the answers, which we believe is an act of hope. What could be more hopeful than believing that the universe and all its wondrous mysteries can one day be unraveled and understood? So join us as we plug into the collective curiosity of your fellow humans and take a dive into the questions that frequently stump them. The answers will sometimes be surprising, and they might challenge your view of the universe. Other times the answers will be agonizingly incomplete because they push up against the edge of human knowledge. In all cases, just remember that most of the fun is in asking the question. Enjoy! PS: Don't forget to flush. Why Can't I Travel Back in Time? Actually, who said you can't travel back in time? It's a very common wish to be able to travel back in time. Who among us wouldn't want to go back and talk to famous figures in history, or watch important moments happen in person? You could figure out who really killed JFK or what ended the dinosaurs. More practically, it would be great to go back in time for smaller stuff, like fixing a mistake you made. If you spilled coffee on your pants, you could go back in time and . . . not spill it. If you said something to your boss you now regret, just go back and not say it. If you ordered a pizza with pineapple on it and then realized that it's actually gross, you could go back and order a real pizza. It'd be like having an undo button (the equivalent of Ctrl+Z, or Command+Z for Mac snobs) for the universe. And yet, so far, scientists have not built such a device. The past remains unchangeable. Time is still our great enemy, and it seems we are doomed to live forever in regret about our past mistakes. There are no do-overs in this universe. But why is that? Why does it seem like we can change the future but not the past? Is there a deep law of physics that makes time travel impossible, or is it just a matter of solving a hard technical problem? And what's the difference anyway? Well, you might be pleasantly surprised to learn that time travel has not actually been ruled out by physicists. It actually is technically possible to go back in time. It doesn't work the way you've seen in movies, but it might not be impossible to build a rewind button. In fact, at the end of this chapter we describe a brand-new, physicist-approved idea for time travel. So strap on your time machine goggles, prepare your hoverboard and DeLoreans, because we are about to answer that timeless question: Why can't I travel back in time . . . yet? Practical Versus Possible Versus Not Impossible First, let's clarify what we mean when we ask if something is "possible." It depends on who you ask. If you ask an engineer whether something like time travel is possible, they will say yes if they think they can build a time machine for less than a trillion dollars and do it in less than a decade. But if you ask a physicist whether something is possible, they look at the question differently. A physicist will say something is possible if they don't know of a law of physics that prevents it. For example: Since this is a book about physics and the universe, we take the physicist's point of view. That means that our goal in this chapter is to figure out whether time travel breaks any laws of the universe, not whether it would take 14.7 bazillion dollars and hundreds of years to make it real. We trust that once the physicists declare it possible, the engineers will eventually figure out a way to make it practical. Then the next step is for them to hand it off to the software people, who can code an app for it ("Siri, unspill my coffee"). To figure out if time travel can be physicist-approved, first we need to think about time the way physicists do. Time is a very slippery subject, one that has confused and baffled people for a long . . . well, time. Basically, physics thinks about time as the thing that allows the universe to change. It's the flow, the motion, the way that then turns into now. It's what orders and organizes a series of still photos into a smooth movie. Because the universe does seem to flow smoothly. It doesn't just jump wildly from one moment to a dramatically different moment. You're not on your couch reading this book and then all of a sudden sitting on the beach. That's because the past puts limits on what can happen in the present. If you were sipping coffee a moment ago, then the possibilities for the present include that you enjoy the coffee or spill it on your pants. It doesn't include that you suddenly transform into a blue dragon drinking fermented celery juice. The past controls the kinds of futures we can have. That's called "cause and effect," and it's at the core of how physics tries to make logical sense of this crazy, bonkers, coffee-stained universe and how it changes. Those changes happen smoothly and require time. Nothing in this universe is instantaneous. Events are connected to one another. When you want to make a pizza, there's a process. You can't just snap your fingers and turn some flour, tomatoes, and cheese instantly into a pizza. The universe requires you to go through the motions: you have to mix the ingredients, knead the dough, cook the tomatoes, drink wine, bake, and so on. There are steps you have to follow to change from one configuration (raw ingredients) to another (hot pizza). Time is what connects those steps, and without it, the universe just doesn't make sense. With that understanding of time, let's think about some possibilities for time travel. You Can't Go Back to the Future One of the most tempting reasons to want to time-travel is to jump to the past and change something, hoping to influence the future. Like not spilling your coffee, or buying shares in Netflix instead of Blockbuster Video (RIP). You'd like to make a change in the past and then jump back to the present and enjoy the fruits of your manipulations. There's one big problem with this concept. Quite simply, it just doesn't make sense. Thinking about time as how the universe flows (or how the pizza bakes), we can see easily that changing the past is nonsense. Let's say you wake up one morning at eight a.m. and you make yourself some coffee. The only problem is that the coffee is bad. So you decide to hop into your time machine, go back to eight a.m. today, and make tea instead of coffee. This makes sense if you're watching this happen in a movie, but it doesn't make sense from a physics point of view. From a physics point of view, a configuration of the universe exists (the one in which bad coffee was created) that is not connected to past configurations of the universe. If you made tea instead, how did the bad coffee get made? To a physicist, this breaks the law of cause and effect: there's an effect (bad coffee) but no cause (you made tea instead). In other words, it's like you made a pizza without ever mixing the ingredients. Unfortunately, this makes changing the past impossible. Breaking the law of cause and effect means that the universe is not consistent with itself, which is a big no-no for physicists. Now, you might be thinking, But what about split timelines! Alternate histories! I saw this happen in the Avengers movies! Unfortunately for Doc Brown (and Iron Man), this also doesn't make sense. How can you change a timeline, or create a new one, when the very idea of change depends on time itself? Timelines represent change, so they can't themselves change. And while the concept of a multiverse is something scientists seriously consider, the possibility that we can move or choose between alternate universes is not. So there are lots of reasons why physics says you can't suddenly jump to another time and change things, which means your dream of manipulating the stock market and getting rich off of physics just went up in a cloud of smoke. Where There's a Physicist, There's a Way Does being strict about cause and effect mean that time travel is impossible? Actually, no! It just means that changing the past is impossible. What if we wanted to go to the past without changing anything? That might actually work. Let's say you wanted to see the dinosaurs, or skip ahead and see what the future is like. Is that possible? According to our current understanding of physics, that is totally possible (just don't ask the engineers if it's possible yet, though). To understand how this could work, you have to get used to thinking about space as more than just space. Physicists like to think of space and time together as something called (not very imaginatively) "space-time." We are used to moving through space near the surface of the Earth, where things are simple. You throw a ball up, it comes down. You walk sideways, you go sideways. Time is equally simple here on Earth: the clock ticks forward and clocks around the world agree with one another. But physics tells us that in some parts of the universe, space gets really weird. And in those cases, it's best to think of it as being joined together with time. To a physicist, we aren't just moving through space in time; we are moving through one thing called space-time. And space-time is weird. It does things that are hard for our minds to imagine, like it can bend. And fold on itself. It can even loop around. Let's explore a couple of ways in which this weirdness of space-time could allow time travel. Infinitely Long Cylinders of Dust According to Einstein, space-time bends whenever you have something massive around. That's his idea of gravity: it's a distortion of space and time instead of a force. For example, the moon goes around Earth not because our gravity is pulling on it but because it's coasting around a funnel of space-time bent by Earth's mass, like a race car doing laps on a curved track. But mass doesn't just bend space; it also stretches and squeezes time. And weird configurations of mass can do super-strange things with time. For example, if you make an infinitely long cylinder of spinning dust, you might be able to do something amazing: near that weird column of spinning dust, time and space would bend in a way that lets you move in a loop through time. That means that an object could potentially travel on a path that takes it back to where-and when-it started. Excerpted from Frequently Asked Questions about the Universe by Jorge Cham, Daniel Whiteson All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.