Imagine describing our universe to an alien from an alternate dimension. Where would you start?
You might reasonably begin by explaining that we live in three dimensions of space and one dimension of time. Space and time are so fundamental to our understanding of the universe that they are woven into nearly every equation in physics. They are the words in which we speak the language of nature—so tried, tested, and true that we don’t even know how to talk about the cosmos without engaging space and time in the conversation.
But what if it turns out that space and time are not the fundamental infrastructure of our cosmos—what if they are themselves products of some deeper physics?
This idea is called emergence. We see it in nature, as when fish school or birds flock. If you were only to study an individual fish or bird, you would never predict how they would come together as a group. Yet each one “knows” simple rules that, when combined, create a wide range of agile and elegant behaviors. Could it be that physicists have been studying flocks all along, not realizing that it’s the birds that are truly fundamental?
“There aren’t many things in quantum gravity that everyone agrees on,” says Eleanor Knox, a philosopher at King’s College London who specializes in the philosophy of physics. “Yet the one thing many people seemed to agree on in quantum gravity was that we were going to have to cope with space and time not being fundamental.”
It sounds radical, but physics has a long and proud history of spearheading exactly this kind of coup. “Historically, whenever we thought something was fundamental, it turns out that it is not,” says Nathan Seiberg, a theoretical physicist at the Institute for Advanced Study. Kepler, for instance, believed that the Platonic solids were the fundamental constituents of the universe. Today we know better. In the 17th century, scientists thought that cold was a substance that could flow from one place to another, chilling your doorstep or tip of your nose. Now we understand that heat and cold are just another way of talking about the statistical properties of a collection of molecules. Of course, that doesn’t mean that it feels any less real when you burn your tongue on your hot cocoa.
So why are physicists picking on space? Relativity delivered the first strike. “In relativity, space and time are not rigid. They are dynamic,” says Seiberg. Building all of physics on such a malleable infrastructure is akin to constructing your house on a foundation of Jello.
More alarmingly to theorists, our ability to measure features in space is intrinsically limited. A ruler can’t measure distances smaller than the width of its painted markings; the resolution of a microscope is constrained by the wavelength of the light in which it makes images; even scanning tunneling microscopes are limited by the physical size of their probe tips.
Can’t we just build a better microscope? “It’s not because we don’t have the budget to build a powerful enough machine,” explains Seiberg. If we somehow tried to make an infinitely small measuring device, that device would become so dense that it would warp the fabric of space. The conclusion: “Space itself is ambiguous,” says Seiberg. Strike two.
Space also took a hit from an unlikely foe: the hologram. We think of holograms as the dazzling, silvery images on postcards and credit cards: two-dimensional objects that project three-dimensional pictures. More generally, though, a hologram is anything—even an equation—that encodes an extra dimension’s worth of information. It turns out that you can write equations that describe our universe perfectly well using different combinations of spatial dimensions, creating mathematical holograms that are indistinguishable from reality. Like a book that can be translated into many disparate languages without losing a syllable of meaning, our universe seems to tell a story that is independent of the words in which we have always chosen to express it.
Finally, physicists have known for some time that their descriptions of space start to break down when they’re applied to the strange-but-true environments inside black holes and close to the time of big bang. In such cases, the familiar equations start popping out infinities—nonsense answers that suggest that the equations are missing some essential machinery. “Something else should kick in,” says Seiberg.
But what is that something else? “I don’t think I have an answer to that,” says Seiberg. Knox also leaves the door open to as-yet-unknown possibilities: “Whatever it is that’s fundamental, it’s not the stuff we have a handle on right now.” Morever, Seiberg adds that though theorists have assembled a strong case that space is emergent, time presents a more difficult problem. “In order to understand emergent time, we need a complete revolution in the way we think about physics.”
Letting go of space and time without ready replacements may seem like a surefire way to plunge into the abyss of abstraction. But it may be only by loosening our grip that we can come to grasp what is truly fundamental.
Editor's picks for further reading
Discover Magazine: Newsflash: Space and Time May Not Exist
If time isn't fundamental, what is it?
FQXi: Breaking the Universe's Speed Limit
John Donoghue investigates the possibility that the speed of light is not a constant.
FQXi: Melting Spacetime
Joanna Karczemarek investigates how space and time could emerge from deeper physics.
There is a moment in a dream when you realize that things don’t add up: You know that geese don’t speak English, and yet there you are chatting away with one about the price of gasoline. You’re sure that you never went to flight school, so why are you piloting this Cessna over Dubuque?
Then you wake up.
But sometimes, you don’t. Sometimes, as you confront two seemingly unassailable, clashing truths, you realize that you’re not dreaming at all—you’ve just encountered a paradox, and it’s a wake-up of an entirely different kind.
For centuries, paradoxes have helped physicists and philosophers challenge and deepen their understanding of how our world works. Paradoxes reveal assumptions and prejudices we never knew we had and open hidden hatches into new physics.
“There’s no getting around it: The universe is really strange, and paradoxes hit you with that,” says Anthony Aguirre, a physicist at the University of California, Santa Cruz.
For Aguirre, that’s a good thing: “That feeling of mystery is really what’s exciting in physics. You know there is something fun, interesting, and potentially important to be gained by going down that road.”
“Sometimes I consider that my knowledge is broken up into tectonic plates of understanding on the Earth of my total knowledge--a small part of the total universe of possible understanding,” says physicist Robert Nemiroff, who gives a special lecture on paradoxes to his students at Michigan Technological University. “Sometimes, I learn something that demands that two plates collide--both plates cannot be used to understand this new thought. This new thought can frequently be coined as a paradox. If resolved, these plates can lock into a larger plate of greater understanding, if I am lucky.”
“Paradoxes heighten what’s at stake conceptually,” says MIT science historian David Kaiser, who adds that physicists like Niels Bohr, John Wheeler, and Albert Einstein all deployed paradoxes strategically to underline mathematical contradictions that others deemed inconsequential. “Paradoxes are one way of grappling with what the equations really say.”
Here are the stories of three paradoxes from far-flung times and places in the history of physics and math. Though they have all been resolved, they remind us just how weird the universe really is. It is a dream from which we will never wake up. But who would want to?
How is it that anyone or anything ever moves from point A to point B? This simple question is the crux of a paradox first posed by the ancient Greek philosopher Zeno, and it has made generations of math students question the nature of reality every time they walk across a room.
Here’s one rendition of the paradox: Say you want to walk down the hallway from your bedroom to your bathroom. First, you have to cover half the distance between the rooms. Next, you’ll need to cross half the remaining distance. As you continue down the hall, you will always have half the previous distance left to cross. Though you will move ever closer to the threshold of the bathroom door, you will never actually reach it.
Obviously, we don’t spend our entire lives stranded in hallways. Why not? The answer is at the heart of calculus: It turns out that infinitely long sequences of numbers can actually have finite sums. This means that even though we must cross an infinite number of progressively smaller “chunks” of space on our way to the bathroom, the time it takes to do so is finite. That’s why we eventually get there.
Yet Zeno’s paradox also reflects one of the biggest questions in physics today: Is space—or spacetime—continuous, or is it broken up into discrete chunks? In Zeno’s world, space was continuous: It could be subdivided into smaller pieces on and on into infinity. Yet we know this isn’t how matter works. If you split a cookie in half over and over again (as many guilty sweet-tooths have no doubt tried at home) you will eventually be left with the indivisible components (electrons and quarks) of one atom. Eat them or don’t, but you cannot divide them in half.
This is also the moral of the story of quantum mechanics: The energy contained in all the particles that make up the universe is quantized. Why should spacetime be any different? In fact, some of the leading theories of quantum gravity predict that, on the tiniest scales, spacetime should break down into discrete chunks. Like a pointillist painting, spacetime may look perfectly smooth from afar, but up close it dissolves into pixels. There are currently experiments in the works to test this prediction.
Why is the sky dark at night? Before you say that it’s because the sun has set, consider that it took the greatest minds in science more than three hundred years to resolve this paradox. Everyone from Einstein to Edgar Allan Poe was swept up by this apparently simple puzzle.
The history of Olbers’ paradox goes back at least as far as Johannes Kepler, who posed it in 1610. If the universe is infinite, argued Kepler, containing an infinite number of stars distributed evenly across the sky, then every point on the night sky should be illuminated by starlight. The brightness of any individual star, as seen from Earth, fades in proportion to the square of its distance from Earth, but the number of stars at a given distance from Earth increases in proportion to the square of the distance from Earth, so it is a wash. The night sky, therefore, should be just as bright as the daylight sky. To Kepler, this meant that the universe must not be infinite after all.
In 1823, Heinrich Olbers drew up a different solution to the paradox that now bears his name. Olbers argued that as the light from each star makes its way toward Earth, it runs into interstellar dust and gas that absorb some its energy. Stars that are sufficiently far enough away from Earth would therefore be “cut off” from us.
The problem with Olbers’ logic is that dust and gas must spit back out the energy that they have absorbed, leaving us with the same problem we started with.
This is where Poe enters the picture. In his prose poem “Eureka,” published in 1848, he inserted time into the equation: What if light from distant stars just hasn’t had enough time to reach us yet? Poe wrote:
...yet so far removed from us are some of the "nebulae" that even light, speeding with this velocity, could not and does not reach us, from those mysterious regions, in less than 3 millions of years.
He may have had the details wrong, but the idea was right: Because stars have not been shining forever, and because it takes time for light to travel from here to there, there is a certain horizon from beyond which light has not yet reached us. Today, we know that the universe had a beginning (the Big Bang) and that the universe is expanding, causing light from distant stars to get stretched out (“redshifted”) beyond the visible part of the spectrum and into the infrared and radio bands, compounding the dark-sky effect.
“Olbers’ paradox is based on such a mind-numbingly simple observation,” says Aguirre. “What impresses me is the sheer amount of time that went by as people came up with one complicated and wrong solution after the next.”
The Twin Paradox
When Einstein proposed that time and space were elastic, it was weird enough. But the twin paradox challenges our understanding of Einstein’s rules even further. Imagine two twins, one a space traveler and the other an avowed homebody. The traveler sets out on a mission to a distant planet in a newfangled rocket that zips along at close to the speed of light. It’s a round-trip journey, so when she gets back, she is eager for a reunion with her twin. She wants to share the amazing stories of her voyage, of course, but she’s also looking forward to gloating about one favorable side effect of life in the cosmic fast lane: Because time passes more slowly for objects traveling close to the speed of light, the traveling twin anticipates that though she has hardly aged a bit, her stay-at-home sister will be sporting many years’ worth of new wrinkles. (Both twins are a bit vain.)
The stay-at-home sister is just as excited to see her twin. She knows her special relativity, too, and reasons that from her point of view, she was traveling close to light speed aboard spaceship Earth, while her sister sat complacently aboard her stationary vessel. Therefore, she will be the young-looking one, and will have fun counting up all her sister’s gray hairs.
And here is the paradox: Einstein tells us that no observer is “more correct” than any other, but the sisters can’t both be younger than each other. Still, Einstein wasn’t wrong. The solution to the paradox is that the sisters’ journeys were not actually identical. The traveling twin did not keep up a constant velocity throughout her entire trip. She accelerated to get up to speed, and then she changed directions—another kind of acceleration—before decelerating to get back into orbit around Earth. So the traveler, not her stay-at-home sister, gets the anti-aging benefits of time dilation.
Paradox solved? In one sense, yes. But in another sense, says Aguirre, the twin paradox reveals a deeper conundrum within the laws of relativity. The crux of the resolution, he says, is that even while velocity is relative, acceleration is absolute. “Where does this absolute non-accelerated reference frame come from? Einstein tried to do away with it in general relativity, but even a century later the question is largely open.”
Will today’s paradoxes be tomorrow’s truisms? Paradoxes arise when equations clash with our intuition about reality, says Kaiser. But intuition can change. “Newtonian mechanics did not look or sound ‘intuitive’ in the 17th century,” Kaiser points out. Today, we take it for granted that Newton’s conceptions of speed, gravity, and mass are in harmony with human intuition, but perhaps intuition itself has been reshaped by Newton’s view of the world. Will the apparent paradoxes of quantum theory and relativity one day feel just as natural as Newton’s laws? Is it possible to “grow out” of a paradox?
“Even once you know the golden thread that unravels the seeming contradiction, paradoxes are still appealing,” says Kaiser, who thinks of them as a kind of mental bodybuilding. “Paradoxes are like a much more satisfying version of Sudoku or a crossword puzzle.” The best part: “In the process we might really learn something about how the universe works.”
Editor's picks for further reading
arXiv: Paradox in Physics, the Consistency of Inconsistency
In this article, Dragoljub A. Cucic classifies paradoxes in physics and reviews their utility.
Edge: The Paradox
In this essay, Anthony Aguirre argues that a better understanding of paradoxes would improve our cognitive toolkit.
FQXi: Black Holes: Paradox Regained
Stephen Hawking conceded his bet on the black hole information paradox, but the debate continues.
To celebrate Thanksgiving, we've asked some of our contributors and friends to tell us what physics they are most thankful for. We hope you'll join the conversation by sharing your own thoughts in the comments section. On Thanksgiving day, we'll have more thoughts from physicists, plus some of our favorite reader comments, on our Twitter feed, @novaphysics. Just look for the #thanksphysics hashtag. Wishing a safe, happy, and inspiring holiday to all!
Frank Wilczek: I'm thankful that the world gives us puzzles we can solve, but not too easily.
James Stein: I’m thankful that physics both intrigues the intellect and is a major driver of technological improvement. While this is true of science in general, energy is the ultimate coin of the universe, and physics is the means by which we discover how it is produced, how it is transformed, and how we can use it to better our lives.
Delia Schwartz-Perlov: I'm grateful to be living at this moment in history, when dark energy hasn’t been dominating our universe for too long, and we are therefore still able to see our magnificent universe. Creatures living billions of years in the future will not be able to see all of this.
Jim Gates: Physics is the only piece of magic I've ever seen. I'm grateful for real magic.
Sean Carroll: I'm thankful for the arrow of time, pointing from the past to the future. Without that, every moment would look the same.
Clifford Johnson: I'm thankful for the "Hoyle resonance" of carbon 12. It is an excited state of carbon that allows it to be produced in stars from helium collisions. Hoyle realized that this is the only way that the carbon we are all made of could be produced, and so reasoned the fact that he (a human, made of 18% carbon) was around to puzzle over the problem was a prediction of the existence of this state. The resonance was later discovered by nuclear physicists, with exactly the properties he said it should have.
James Stein: I’m thankful for Michael Faraday’s discovery of the principle of electromagnetic induction. It made it possible to use electricity to advance the human condition, and I think it is the single most productive discovery in the history of physics.
Delia Schwartz-Perlov: I’m eight months pregnant, so I am grateful for the physics that enables ultrasound. It is pretty amazing and exciting to see what’s going on in there.
Edward Farhi: As the physicist Ron Johnson once said, I'm grateful to quantum mechanics for an interesting life.
You think you know what holograms are? Think again. Once restricted to credit cards, postcards, and the occasional magazine cover, holograms are taking a great cosmic leap thanks to a new hypothesis called the holographic principle.
The holographic principle, simply put, is the idea that our three-dimensional reality is a projection of information stored on a distant, two-dimensional surface. Like the emblem on your credit card, the two-dimensional surface holds all the information you need to describe a three-dimensional object—in this case, our universe. Only when it is illuminated does it reveal a three-dimensional image.
This raises a number of questions: If our universe is a holographic projection, then where is the two-dimensional surface containing all the information that describes it? What “illuminates” that surface? Is it more or less real than our universe? And what would motivate physicists to believe something so strange? That answer to the final question has to do with black holes, which turn out to be the universe's ultimate information-storage devices. But to understand why, we will have to take a journey to the very edge of a black hole.
It doesn’t matter which black hole we choose, because each one looks essentially the same. Only a handful of qualities distinguish them: mass, electric charge, and angular momentum. Once an observer knows these three things about a black hole, he or she knows all that can be known. Whether the black hole contains the remains of a thousand dead stars, or all the lost socks from every Laundromat in the galaxy; whether it is a billion years old or was born yesterday; all of this information is lost and inaccessible in a black hole. No matter what is inside a black hole or how those innards are arranged, a black hole will “look” just the same.
This strange quality give black holes something that physicists call maximal entropy. Entropy describes the number of different ways you can rearrange the components of something—“a system”—and still have it look essentially the same. The pages of a novel, as Brian Greene points out, have very low entropy, because as soon as one page is out of place, you have a different book. The alphabet has low entropy, too: Move one letter and any four-year-old can tell something is wrong. A bucket of sand, on the other hand, has high entropy. Switch this grain for that grain and no one would ever know the difference. Black holes, which look the same no matter what you put in them or how you move it about, have the highest entropy of all.
Entropy is also a measure of the amount of information it would take to describe a system completely. The entropy of ordinary objects—people, sand buckets, containers of gas—is proportional to their volume. Double the volume of a helium balloon, for instance, and its entropy will increase by a factor of eight. But in the 1970s, Stephen Hawking and Jacob Bekenstein discovered that the entropy of a black hole obeys a different scaling rule. It is proportional not to the black hole's three-dimensional volume but to its two-dimensional surface area, defined here as the area of the invisible boundary called the event horizon. Therefore, while the actual entropy of an ordinary object—say, a hamburger—scales with its volume, the maximum entropy that could theoretically be contained in the space occupied by the hamburger depends not on the volume of the hamburger but on the size of its surface area. Physics prevents the entropy of the hamburger from ever exceeding that maximum: If one somehow tried to pack so much entropy into the hamburger that it reached that limit, the hamburger would collapse into a black hole.
The inescapable conclusion is that all the information it takes to describe a three-dimensional object—a black hole, a hamburger, or a whole universe—can be expressed in two dimensions. This suggests to physicists that the deepest description of our universe and its parts—the ultimate theory of physics—must be crafted in two spatial dimensions, not three. Which brings us back to the hologram.
Theorists were intrigued by the idea that a parallel set of physical laws, operating in fewer dimensions, might be able to fully describe our universe. But probing that idea mathematically for our own universe was too daunting, so physicists began with a "toy" universe that is much simpler than the universe we live in: a universe with four spatial dimensions plus time, curved into the shape of a saddle. In 1997, the theoretical physicist Juan Maldacena showed that the mathematical description of this universe was identical to the description of a different kind of universe, one with three spatial dimensions, one time dimension, and no gravity. Maldacena’s discovery was the first concrete realization of the holographic principle, and it also made work easier for theorists, who now had two approaches available for every tricky math problem: They could choose to express the problem in the mathematics of the five-dimensional, gravitating universe, or they could opt for the four-dimensional, gravity-free version.
None of this adds up to "proof" that we are living in a hologram, but it does contribute to a body of circumstantial evidence suggesting that the laws of physics may in fact be written in fewer dimensions than we experience. That, combined with the mathematical utility of the holographic principle, is motivation enough for many physicists. The other questions with which we began this journey—Where is the surface on which our universe is inscribed? What illuminates it? Is one version of the universe more "real" than the other?—are still unresolved. But if the holographic principle is right, we may have to confront the notion that our universe is a kind of cosmic phantom—that the real action is happening elsewhere, on a boundary that we have not yet begun to map.
Editor's picks for further reading
FQXi: The Holographic Universe
Alex Maloney investigates the holographic principle.
Scientific American: The Holographic Principle
A brief introduction to the holographic principle.
University of California Television: The World as a Hologram
In this video, theoretical physicist Raphael Bousso provides an introduction to the holographic principle.
Why is quantum mechanics like cricket?
Because for me, no matter how many times the rules are explained, I can’t seem to get my head around what the game is actually about.
Is quantum theory a system of equations? A description of the behavior of invisible particles? A philosophy for the post-post-modern age?
And how strange is it that we even have to ask? Unlike other scientific theories, quantum physics is so slippery that its formalism—the equations that add up to a mathematical representation of what we humans call reality—is divorced from its physical interpretation. Sure, we can solve the Schrödinger equation for the case of a particle stuck in a box, but what is that telling us about how the natural world really works?
This isn’t a question you’d even think to ask about classical mechanics. Remember Newton’s Second Law, the one relating force to mass and acceleration? Its formalism is F=ma, and its interpretation is pretty simple: If you want to know the force an object is exerting, just multiply its mass by its acceleration.
That’s F=ma. But what about:
“Quantum mechanics needs an explanation worse than other theories do because others always had a physical picture that guided the formulation of the mathematics,” explains John Cramer, a physicist at the University of Washington who also happens to be the author of his own interpretation of quantum mechanics—more on that later. Newton had his (possibly apocryphal) apples, his inclined planes, his cannonballs. Werner Heisenberg, one of the “fathers” of quantum mechanics, by contrast, had some elegant mathematics, a vision more akin to numerology than to a picture of the physical world, in Cramer's view.
"The Copenhagen interpretation is like a religious text," says MIT physicist Max Tegmark. "It leaves a lot open to interpretation."
Yet Heisenberg, like his colleague Niels Bohr, felt that quantum mechanics needed no further interpretation. This view, which is now known as the Copenhagen interpretation, holds that there is no “objective reality” lurking beneath the formalism. If the equations say that I have a 50% chance of measuring a particle in a certain state—say, spin up—and then I go ahead and measure it in that state, what more is there to say? To guess at what the particle was doing before I made the measurement would be worse than speculation; nothing can be said about the particle except in the context of a measurement. “Reality” is no more and no less than what our instruments and senses reveal it to be. The Copenhagen interpretation may give you a headache, but according to Anton Zeilinger, the University of Vienna physicist most famous for his teleportation experiments, "It works, is useful to understand our experiments, and makes no unnecessary assumptions."
Still, many physicists find this notion unsatisfying. “Quantum mechanics is full of strange things that cry out for an interpretation,” says Cramer. There’s the problem of “spooky action at a distance,” the apparent connection between “entangled” particles that seems to violate the finite speed of light; and there’s Einstein’s famous discomfort with the idea that no reality exists outside of our own perceptions. As Einstein put it: “Do you really think the moon isn't there if you aren't looking at it?"
There’s also a niggling problem with exactly what defines “looking at it”—or, in quantum-speak, what defines a “measurement.” If we truly cannot say anything definite about a particle until after we’ve measured its state, then the act of measuring it must be pretty special. But why? What happens in that moment? Physicists often talk about it as the “collapse of the wavefunction”—that is, the moment when all of the possible particle states represented in the probability equation called the wavefunction collapse into a single, measured state. The instantaneous collapse of an entity that wasn’t physically real to start with is weird in itself. But physicist Steven Weinberg pointed to another weak link in this interpretation in a 2005 article in Physics Today: “The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe.”
If not Copenhagen, then what? Let’s take a quick tour of a handful of the (many!) competing interpretations of quantum mechanics.
- Copenhagen interpretation: This is the interpretation we’ve just met, and the one you’ll see in most physics books—though even Heisenberg and Bohr didn’t always agree on the particulars. To put it in terms of our cricket analogy, let’s say that you’re following a cricket match on your cell phone. Actually let’s make it a baseball game because, as I've already confessed, I don't understand cricket. So you’re using one of those apps that updates the box score every time you press “refresh,” but you can’t actually see the game in progress. According to the Copenhagen interpretation, there is no game—just the results you get when you ping the server. So it’s no use talking about whether the batter is getting into the pitcher’s head, or the appearance of the rally squirrel, or even the trajectory the ball takes on its way into the first baseman’s glove. The box score is real; the game isn’t.
- Consistent histories: The Copenhagen interpretation applies to a situation in which an observer (the baseball fan) makes a measurement (checks the score) on some external system. But what happens when the observer is himself part of the system—say, the shortstop? That's the problem that a special breed of physicists called quantum cosmologists encounter when they attempt to study the entire universe as a single quantum system. The Copenhagen interpretation falls short in this case, but the consistent histories interpretation, developed in the 1980s and early 1990s, does away with external "observers" and "measurements"—they are treated as part of one big system.
- Many worlds: We talked earlier about the problem of the collapsing wavefunction. But what if the wavefunction never actually collapses? What if every possibility it represents really does happen in its own universe? With every measurement, each universe branches off into countless others, each of which in turn branches into ever more universes. The many worlds interpretation was first proposed in the 1950s by the young physicist Hugh Everett, and though it never gained much traction at the time, its star is now ascending: In the film Parallel Worlds, Parallel Lives, Tegmark called the many worlds interpretation “one of the most important discoveries of all time in science,” and he and his colleagues recently posited that Everett's parallel universes might be congruent with the parallel universes proposed by cosmologists. Of course, plenty of physicists can't stomach the idea of a multiplicity of fundamentally unobservable universes. Yet—back to baseball for a moment—there is something appealing about an interpretation that insists upon the existence of a universe in which the baseball rolls squarely into Buckner’s glove; an interpretation that guarantees that every heartbreaker in our universe is shadowed by a heroic comeback in another; an interpretation in which the Red Sox and the Yankees win, year after year after year.
- Transactional interpretation: The transactional interpretation might solve some of quantum theory’s biggest quandaries, if you can get your head around the idea of a wave with negative energy that travels back in time. The transactional interpretation was first proposed in the 1980s by John Cramer, and suggests that the wavefunction includes not just one but two probability waves—the familiar one that travels forward in time, plus an exotic twin that travels backward. When they meet, they exchange a “handshake” across space-time, says Cramer; at other points, they cancel each other out completely, removing any telltale traces of the journey backward in time.
So, is there any way to know which interpretation is right or wrong? "Unless you can catch an interpretation deviating from the mathematics, you can't rule it out," says Cramer. And though some experiments could maybe, possibly tip the scales in favor of one interpretation or another, there is no consensus that any of the contenders above have been favored or nixed by experiment. Perhaps, some physicists argue, the pursuit of an interpretation is a flawed endeavor. "There is no logical necessity of a realistic worldview to always be obtainable," wrote Christopher Fuchs and Asher Peres in a Physics Today opinion piece titled, transparently, "Quantum Theory Needs No 'Interpretation'." "If the world is such that we can never identify a reality independent of our experimental activity, then we must be prepared for that, too." Perhaps the interpretation problem isn't a problem of quantum physics at all, but a problem of human beings.
When I was seven, I understood relativity perfectly. I got it. At least I thought I did. And it was glorious.
It all started with The Flight of the Navigator, that movie about the kid who accidentally stumbles upon a space ship and has a grand old afternoon traversing the galaxy in the company of its cargo of friendly creatures. The problem? When he returns to Earth he finds that eight years have elapsed. He has been declared missing; when he rings the bell at his old house, a stranger answers the door.
The Flight of the Navigator is a story about time dilation. The ship was traveling close to the speed of light, so time passed significantly more slowly for its passengers than it did for those back on Earth. That’s special relativity for you. It seemed easy, elegant, amazing.
For the next few years, that was my relationship with relativity. Then came the equations, and suddenly relatively didn’t seem so easy any more. Elegant, maybe. But that first spark of intuition got lost in symbols, equations, calculations and miscalculations. So I wondered: Why can’t we have it both ways? Is it possible to be both wide-eyed and rigorous, methodical and amazed?
That's a balance even physicists struggle with. As MIT theorist Max Tegmark told NOVA, "When I was in high school, physics was my most boring subject....It was only later that I realized that physics isn’t at all about just solving some equations. It’s about figuring out what reality is all about."
That’s what we're doing on this blog: taking on the biggest questions of all, the ones that get at the very nature of reality. What happens to information inside a black hole? What are we really talking about when we talk about teleportation? What if the laws of physics can't be unified? Every week, we’ll have a new post from a scientist or science writer--look for James Stein, Sean Carroll, and Frank Wilczek in the coming weeks and months--with plenty of room for you to join the discussion with your own questions and ideas.
We hope you'll let us know what subjects in fundamental physics you want to hear about—the stranger, the better. Can neutrinos really go faster than the speed of light? What is this spacetime thing anyway? Is our universe just one of countless others in a multiverse? Join us as we dive deep and discover how physics is changing what we thought we knew about the nature of reality.