Imagine that you want to make something disappear—that unfortunate photograph of you in the sombrero, or the ill-advised iPhone video from your bachelor party, or that part of your seventh-grade diary describing your dream honeymoon with Kirk Cameron. Whatever it is, you want it gone.
You could shred it, but a motivated blackmailer could still piece it back together. You could burn it, but the laws of physics still promise that the information could be reassembled. So you decide to turn to the ultimate destruction: You launch that mortifying evidence right into a black hole and breathe a sigh of relief that now, finally, it is gone for good.
But is it really?
That question is at the heart of a problem that’s been called the black hole information paradox, and some theorists believe that it reveals a deep crack in the foundation of physics as we know it.
The pull of gravity inside a black hole is so strong that nothing can escape its grip. No one can fish your diary out from beyond the lip of the black hole; gravity’s pull on the fishing line would overpower your reel every time. And if a tech-savvy enemy tried to remotely link up with your iPhone and retrieve that embarrassing video, even the electromagnetic waves from your phone would still be trapped inside the black hole. To escape, they would somehow have to travel faster than the speed of light—and that is strictly forbidden by Einstein’s relativity.
But there is a hitch. Quantum mechanics has an equally strong rule that prohibits the loss of information. This principle, called unitarity, is intimately linked with other unbreakable laws of physics, like conservation of energy. “Conservation of information is what holds the world together,” says Steve Giddings, a physicist at the University of California, Santa Barbara.
To emphasize just how important information conservation is, Stanford physicist Leonard Susskind calls it the “minus-first” law of physics—“minus-first because I think it comes before everything else,” said Susskind in an online discussion sponsored by the Kavli Foundation. “If it’s true [that information conservation is violated], we go back to minus-first base.”
Now, you might argue that the information in the black hole isn’t truly lost. It’s just locked up and inaccessible. Theorists contented themselves with this view until 1975, when Stephen Hawking drew a revolutionary conclusion about black holes: Given enough time, a black hole will dematerialize, radiating away through a process we now call Hawking evaporation. And, according to Hawking’s accounting, that radiation would be random, revealing nothing of the black hole’s contents.
As Leonard Susskind wrote in “The Black Hole War,” his 2008 book on the problem of black holes and information loss, “The possibility of hiding information in a vault would hardly be a cause for alarm, but what if when the door was shut, the vault evaporated right in front of your eyes? That’s exactly what Hawking predicted would happen to the black hole.”
There is another possibility, though: Maybe this evaporation isn’t complete. Maybe it leaves behind a tiny ember that contains an enormously compressed version of all the information that ever fell into the black hole. “But this leads to some pretty crazy conclusions, too” says Giddings—specifically, you need to find a way for a single particle to take on infinitely many forms which, Giddings says, “results in equally disastrous consequences.”
This left physicists stuck between a rock and a hard place: Either information could be lost, or somehow something could escape from a black hole. A central tenet of quantum mechanics was pitted against the cornerstone of relativity. One theory, it seemed, had to give.
The debate went public in 1997, when Stephen Hawking and theoretical physicist Kip Thorne made a bet with John Preskill of Caltech that it would ultimately be shown that information was truly lost inside black holes. At stake: one encyclopedia of the winner’s choice, from “which information can be recovered at will.”
At the same time, and out of the media spotlight, string theorists were exploring a remarkable duality in their equations. They found that if you take a mathematical description of a system and add an extra spatial dimension and a negative curvature, you have something that looks very much like quantum fields in a three-dimensional universe without gravity. This observation sounds esoteric, but it gave mathematical chops to an idea called the holographic principle, which puts forward that all the information in our three (spatial) dimensional universe can be “stored” on a two-dimensional surface. In the context of the black hole information paradox, this suggested that information about the stuff in the black hole could somehow be encoded on the surface of the event horizon.
Still, the encyclopedia remained unclaimed as the bet dragged on, until 2004, when Hawking announced that he had changed his mind and was ready to concede. (Preskill’s prize: a baseball encyclopedia.) “My views have evolved,” he told Nature News. He published his results the following year.
To some theorists, Hawking’s concession came many years too late. “Stephen Hawking was like one of those unfortunate soldiers who wander in the jungle for years, not knowing that the hostilities have ended,” wrote Susskind in “The Black Hole War.” But others found Hawking’s explanation unsatisfying and, moreover, were irked at the public perception that one man’s change of heart had truly settled a debate that was still actively raging in the physics community. Still, there was a growing, if uneasy, consensus that this mathematical expression of the holographic principle, dubbed the anti-de Sitter/conformal field theory, or AdS/CFT, duality, pointed the way to a solution to the information problem, even if the roadmap was incomplete.
But now, we’ve hit another roadblock. A team of physicists out of the University of California, Santa Barbara has shown that if information thrown into the black hole is conserved, one of two other “unbreakable” rules of physics must give out. The first rule is that, once you’re a reasonable distance away from a black hole, the laws of physics work as usual. The second is that someone falling into a black hole would experience nothing “special” at the event horizon.
It’s that second rule, the Santa Barbara team argues, that is the weakest link in the chain. To protect the remaining postulate, they are willing to accept that something shocking might exist at the horizon: a “firewall” of broiling radiation that burns all that passes into a fiery crisp. Do they really believe that these firewalls exist? Probably not. But the possibility is enough to light a new fire (pardon the pun) under theorists.
Paradoxes and problems, after all, can drive great new discoveries. And, as science writer Jennifer Ouelette writes, “It comes at a time when theorists are hungry for a new challenge,” thanks to the maddening neatness of the Higgs result and physicists’ inability to spot cracks in the armor of the Standard Model.
So, back to that embarrassing thing you wanted to dump into the nearest black hole. Will it really be lost and gone forever? Will it burn up in a firewall, or be rewritten on a quantum screen at the edge of the universe? With the jury still out, perhaps you’ll do better just to hide it under the mattress.
Author's picks for further reading
arXiv: Black Holes: Complementarity or Firewalls?
The 2012 paper that laid out the firewall problem.
Cosmic Variance: Joe Polchinski on Black Holes, Complementarity, and Firewalls
Go inside the information paradox with physicist Joe Polchinski, one of the authors of the paper that sparked the firewall controversy.
John Preskill: On Hawking’s Concession
The winner of the famous bet on the information problem and the value of scientific wagers.
Physics Today: Black holes, quantum information, and the foundations of physics
In this comprehensive essay, Steve Giddings outlines the information problem and several proposed solutions.
We all dream of finding the one: dependable, motivated, and beautiful, “the one” has it all. But can the search for perfection keep us from appreciating the good thing we already have?
I’m talking, of course, about the search for the one theory of everything, a theory of physics that works in all circumstances no matter how extreme, is motivated by observations, and can be expressed in a few elegant axioms. While some theorists devote their careers to finding the one, others believe that this ideal may be fundamentally unattainable. So we are left with the big question: Is the hope for the one theory of everything realistic, or should we be satisfied to settle down and grow alongside the theories we have?
From the vastness of the cosmos to the inside of an atom: Can one theory accurately describe every layer of the cosmic onion? Image credit: Adapted from Brian Westin, ProLithic 3D & NASA/JPL-Caltech/T. Pyle (SSC/Caltech) by Greg Kestin.
We are currently living with a beautiful theory, a theory that almost
has it all: the Standard Model. It is the most precise theory in human history. The Standard Model can make predictions that match experiments to one part in 10 billion. That is like measuring the width of the United States to the accuracy of a human hair. The Standard Model explains the Sun’s glow, the inner workings of computers, and every atom that makes up our bodies. This theory is in our hands, it’s reliable, and we’re pretty happy...but it isn’t perfect.
There is one huge, glaring omission in the Standard Model: It doesn't explain gravity. Of the four forces in the universe—electromagnetism, the strong force (holding nuclei together), the weak force (governing radioactive decay), and gravity—gravity is the black sheep. Not only is it the runt of the forces, with a strength around a trillion trillion trillion times weaker than the typical strength of the other forces, but our current theory of gravity is completely separate from, and at odds with, the Standard Model. Until we reconcile the two, humanity's understanding of the universe will be incomplete.
Some scientists believe that this reconciliation is just around the corner. Theoretical physicist Garrett Lisi, for one, thinks that extensions of his model, which aims to unify general relativity and the Standard Model within a single framework, can “reproduc[e] all known ﬁelds and dynamics through pure geometry."
“The theory currently evolving from this observation is wonderfully complex and gives me hope that we might be getting close to the full picture,” says Lisi.
Others, like Dartmouth physicist Marcelo Gleiser, argue that we are stuck with at least partial ignorance. “As long as we can't measure all there is in the natural world—and the point is that we simply can't—we can't have a theory of everything. As a consequence, any theory that we may have that purports to explain ‘all’ that we know of the world is also necessarily incomplete.”
Indeed, the more closely we examine the universe, the more levels of complexity we find. Will observing the world more deeply finally lead us to a theory of everything? Or will we be perpetually pulling layers from an infinite onion—a prospect that would make innumerable physicists cry?
If it is the latter, then we must be content with what physicists call an “effective field theory.” The idea is that one should describe the world with the same degree of complexity one wishes to understand. The smaller the details you aim to study, the greater the complexity you should expect from your theory. It is like looking at the sun. If we peer at it just for a moment, it seems to be a smooth, bright, glowing sphere (see on the left, below). This is an “effective theory” of the sun. But if you zoom in (on the right), there is more going on: solar flares, sunspots, and streams of hot plasma shooting into space.
Image credit: NASA/SDO
In the same way, if you peer at a collision between two particles (say, electrons), then the effective theory would describe this as a simple bounce off each other (see on the left, below), but when you zoom in (on the right), there is more complexity: the electrons exchange other particles, causing them to repel each other and "bounce."
By looking closer, you realize your perfect, simple picture of what you may have thought was "the one" correct description is just an approximation of something more complicated and complete.
Indeed, every time we find a theory that seems to “have it all,” a closer look reveals gaps and errors in the theory. Yet from the time of Archimedes, who fathered the idea that we could describe all of nature from just a few axioms, to the modern era of the Standard Model, physicists have kept searching for the one, refusing to settle for “good enough” when flaws and omissions in their theories were revealed.
It is natural to wonder, then, is the Standard Model just an effective theory, the latest in a long line of close-but-not-quite ideas? The consensus among physicist is a resounding “yes,” leaving us with questions: Is there an infinite number of layers, or if we look closely enough can we find that there is one final center onion-core? And how does gravity fit into the onion?
There is reason to believe there may be a “core” to the onion. While historically physicists have peeled back the layers of the onion by “zooming in” on ever-smaller size scales, Heisenberg’s uncertainty principle may limit the ultimate resolution at which we can observe the universe. If you go small enough, then particles don’t have a definite position, you can’t tell where they are, and looking closer could not improve the resolution. The size at which this occurs is called the “Planck scale.”
At the tiny size where we lose particle resolution, gravity, once the runt of the forces, may intensify to a strength similar to that of the forces in the standard model. Gravity would no longer be the weak outcast, giving hope that gravity may “fit in” with the standard model, producing a theory of everything that unites quantum and gravitational phenomena.
Unfortunately, current experiments are far from being able to confirm such a unification. Physicists’ most powerful experiment for examining tiny-distance physics is the Large Hadron Collider (LHC), with its incredible smashing ability that can peel away layers of the onion. But to explore the physics of the Planck scale, we would need a machine more than a million billion times more powerful.
Despite experimental limitations, some of the greatest scientists have searched for a theory of everything. Einstein spent last decades of his life looking for a theory of everything. Unfortunately he passed away before he was able to find it. Stephen Hawking also searched for the theory of everything, before having a change of heart. “Some people will be very disappointed if there is not an ultimate theory that can be formulated as a finite number of principles. I used to belong to that camp, but I have changed my mind,” he has said. Richard Feynman, who is often considered “the best mind since Einstein” once said, “If it turns out there is a simple ultimate law which explains everything, so be it—that would be very nice to discover. If it turns out it's like an onion with millions of layers... then that's the way it is.”
So, while searching for a theory of everything is exciting, we may be well advised to take time to appreciate what we already have.
Author's suggestions for further reading
American Museum of Natural History: Isaac Asimov Memorial Debate: Theory of Everything
A panel of acclaimed physicists, including Lee Smolin, Brian Greene, and Janna Levin, debates whether it is possible to explain the universe with a single, unifying theory.
Godel and the End of Physics
In this lecture, Stephen Hawking asks whether it is possible to find a complete set of laws of nature.
The Island of Knowledge: The Limits of Science and the Search for Meaning
In his forthcoming book, Marcelo Gleiser asks if there are fundamental limits to how much science can explain.
NOVA: A Theory of Everything
In this essay, Brian Greene explores how string theory could unite quantum mechanics and general relativity.
Another chapter has unfolded in the dramatic saga of the Higgs boson. On the morning of October 8, 2013, the Swedish Academy of Science made an announcement that had been widely anticipated by the blogosphere: Francois Englert and Peter Higgs shared the 2013 Nobel Prize for physics for the prediction of a new physics mechanism to which Higgs (unwillingly) lent his name.
Event recorded with the CMS detector. Image credit: CERN
The Standard Model of particle physics is a stunningly successful theory that describes the matter of the universe. It was developed in the 1960s and has been extensively validated in the intervening decades. However, the theory had one striking weakness. It did not explain why the smallest and most fundamental particles had mass, instead of being massless, which seemed to be a more natural state of affairs.
In 1964, Belgian physicists Robert Brout and Francois Englert published a paper describing a way to modify a class of so-called Yang-Mills theories. By adding a new field of energy to the existing theories, they found, they could give subatomic particles mass.
British physicist Peter Higgs independently developed the idea and his treatment was published a couple of weeks later. A third treatment of the problem by the American physicists Gerald Guralnik, Carl Hagen, and Tom Kibble appeared shortly thereafter. All three papers were named Milestone Papers by the American Physical Society in its 50th anniversary issue. A fourth paper, written by Peter Higgs, made the crucial observation that if this modification was true, it predicted a new particle. Over the intervening years, the energy field has come to be called the Higgs field and the predicted particle the Higgs boson.
While the ideas described in these papers from 1964 were possible explanations for the origins of the mass of fundamental particles, the ideas could have been wrong. In order to test the theory, scientists began a search for the Higgs boson.
On July 4, 2012, after nearly 50 years of searching, researchers using the Large Hadron Collider (LHC) at the CERN laboratory in Europe announced that they had found a new particle that was “consistent with being” a Higgs boson. In science, the term “consistent with being” has a technical meaning. It means that some of the predicted properties had been tested and verified but not all. It also means that no observations disagreed with the theory. By way of an analogy, if scientists had discovered a fruit that was consistent with being an apple, they might have touched and looked at the fruit and confirmed that it was apple-like, but they had not smelled or tasted it yet. Because of these residual uncertainties, awarding a Nobel Prize for the successful prediction of the Higgs boson in 2012 would have been premature.
In March of 2013, researchers updated their results, using two and a half times as much data as they used in July of 2012. With the extra data and more refined analysis techniques, the scientists were able to confirm that the newly-discovered particles had even more properties that were identical to those the Higgs boson was predicted to have. The case supporting the Higgs discovery was firming up.
There remains some possibility that the newly-discovered-particle is not the Higgs boson. For instance, the theories of 1964 predicted that a single variety of Higgs boson exists. Given that scientists have found only one variety, this is great news for the prediction. However it could be that there are other varieties of Higgs bosons that have not yet been discovered. Being absolutely sure will require more data taken at the LHC when it resumes operations in 2015.
So why award the Nobel Prize before this additional confirmation? First, the observed particle has many properties that are identical to the predictions of 1964. Those predictions seem to be part of the story. Second, time is a real concern. The prize cannot be awarded posthumously, and both Higgs and Englert are in their 80s. (Brout died in 2011.)
Thanks to the near-synchronicity of the milestone Higgs papers, narrowing the field of Nobel candidates must have been difficult. While the details of the selection process are private, it appears that the Swedish Academy of Science acknowledged that Englert and Brout got there first, while Higgs was the first to associate the new field with a particle. The decision was no doubt a difficult one, but is consistent with how the prize has been awarded in the past.
Our ongoing study of the rules that govern the universe is not complete with the observation of the Higgs boson, but the discovery is a tremendous step forward. The 2013 physics Nobel Prize is an affirmation of the importance of those ideas, first conceived of nearly half a century ago.
Author's suggestions for further reading
The Quantum Frontier: The Large Hadron Collider
Author Don Lincoln's look inside the LHC and the physics it explores.
Massive: the Missing Particle that Sparked the Greatest Hunt in Science
Science writer Ian Sample's examination of the quest for the Higgs.
Higgs: The Invention and Discovery of the God Particle
Science writer Jim Baggott on the history and implications of the Higgs discovery.
The Particle at the End of the Universe
Physicist Sean Carroll goes behind the scenes at the LHC to explore the story of the search for the Higgs.
Albert Einstein during a lecture in Vienna in 1921, the year of his Nobel Prize. Image credit: Ferdinand Schmutzer, via Wikimedia
It's 5 a.m. and you're sitting by the phone, hoping for that "magic call" from Stockholm that bears the news you've waited so long for: You've won the Nobel Prize in Physics! Whom will you tell first? How will you celebrate? And what color Bugatti should you buy with your prize money?1 But while you're mentally debating the relative merits of Obsidian Black and Italian Red, you realize that the sun has come up and the phone still sits silent: You've been passed over once again. How can you turn things around in 2013? With the announcement of the 2013 Nobel Prize in Physics expected to come on Tuesday, October 8, we consulted with winners and watchers of the Nobel Prize to prepare this helpful guide to nabbing your very own physics Nobel.
- Think big: What kind of discovery is most likely to earn Nobel laurels? Prize-winning work "runs the gamut" from basic to applied science and from lone-wolf labor to cast-of-thousands collaborations, says Adam Riess, who, along with Saul Perlmutter and Brian Schmidt, received the award in 2011 for the discovery of the accelerating expansion of the universe. Says Riess: "I think the key is its importance must be fundamental, generally involving new physics."
- Do an experiment: Physicists are often divided into two camps: theorists, who need nothing more than paper, pencil, and their prodigious brains to do their work, and experimentalists, who toil and tinker with arcane equipment in their attempts to prove (or disprove) the ideas thought up by the theorists. So, which group bags more Nobels? In The Nobel Prize: A History of Genius, Controversy, and Prestige, science historian Burton Feldman lands firmly on the side of experiment. Tallying up the winners from 1901 through 1999, he finds that experimentalists scooped up 87 awards while the theorists made do with a measly 51. Even Einstein, despite a bushel of nominations, was rejected year after year for the Nobel because his relativity theories were just that—theories. (He eventually won the 1921 prize, for his work on the photoelectric effect.) Experimentalists have continued to dominate in the last decade, with a few notable exceptions, like the 2004 award, which went to David Gross, David Politzer and Frank Wilczek for developments in the theory of the strong force, one of the fundamental forces of physics.
- Keep it in the family: Marie Curie shared the prize with her husband, Pierre Curie, and five father-son pairs have won the award (though only William and Lawrence Bragg won it in the same year for work done collaboratively). As David Kaiser, a physicist and science historian at MIT, puts it: To win the award, "one should select one's parents carefully."
- You can’t choose your family, but you can choose your thesis advisor: “As the great sociologist of science Harriet Zuckerman demonstrated years ago, among all the Nobel laureates who conducted their prize-winning research in the United States (at least up through 1972), more than half had been mentored early in their careers by other Nobel laureates,” reports Kaiser. “The proportion was highest—nearly 2/3—among Nobel Prize-winners in physics.”
- Be a man—and be eligible for the AARP: Of the 193 winners of the Nobel Prize in Physics, only two (Marie Curie and Maria Goeppert-Mayer) were female. Average age: 55.
- Get lucky: “They key to winning the Prize, I believe, is to be extremely lucky,” says Riess. Of course, as any fortune cookie can tell you, good luck alone isn’t enough: it has to be combined with the day-in, day-out hard work that often obscures the serendipitous path to the breakthrough. But it is possible to “court serendipity” by being open to surprising and unexpected new findings. The Institute of Physics has compiled a list of just such lucky breaks. There’s Jocelyn Bell’s “accidental” discovery of pulsars—radio signals so uncannily regular that she momentarily thought they might be beacons from an alien civilization. (They weren’t. Incidentally, Bell didn’t get the prize; it went to her supervisor, Anthony Hewish. See #5, above.) And then there’s the first detection of the radio buzz we now know as the cosmic microwave background radiation, which future Nobel laureates Arno Penzias and Robert Wilson chalked up to pigeon droppings before they realized it was actually an electromagnetic echo of the Big Bang.
- Be patient: The Nobel committee is not much for instant gratification. Though some Nobel prizes come quick on the heels of the work that they honor—the 2010 award, for instance, went to Andre Geim and Konstantin Novoselov for their work on graphene, just six years after the material was discovered—the prize more often comes a decade or two (or five) after the discoveries are first made. Subramanyan Chandrasekhar, for one, had to wait more than 40 years, and 53 years passed before Ernst Ruska was honored for building the first electron microscope that could out-magnify a traditional optical scope.
- Be prepared for life after Nobel: “What happens now to the rest of my life? What comes after this?” said Tsung-Dao Lee, who received the physics prize in 1957, when he was just 31. Indeed, some laureates, particularly those who receive the award early in their careers, founder after making the trip to Stockholm. As Mitchell Wilson put it in a 1969 essay in The Atlantic, "If, before winning the prize, the man has received very few, if any, of the signs of the scientific world’s recognition of the worth of his work, the sudden rise to stardom can completely distort the pattern of the rest of his life." But Nobel laureate Frank Wilczek, asked to weigh in on how to up your Nobel odds, has a more spirited outlook: "I'm more confident giving this tip, about what to do immediately after winning a Nobel Prize. And that is, you should take some dancing lessons. They'll pay off handsomely during the festivities."
1Just kidding; I'm not aware of any Nobel laureates who plunked down their prize money on a supercar. Nobel winners typically spend the purse on serious and practical things, like charitable donations or their children's college fund. TIME put this impolite question to a handful of winners back in 2009; here are their answers.
Author's suggestions for further reading
The Atlantic: How Nobel Prizewinners Get That Way
In this 1969 essay, physicist-turned-novelist Mitchell Wilson profiles prominent winners of the physics Nobel, including his onetime boss, laureate Enrico Fermi.
The Nobel Prize: A History of Genius, Controversy, and Prestige
The late science historian Burton Feldman's comprehensive history of the prize.
Scientific Elite: Nobel Laureates in the United States
Published in 1977, this book by sociologist Harriet Zuckerman looks at the life and career trajectories of the scientific super-elite.
Could the birth of a four-dimensional black hole have created our three-dimensional universe?
That’s the idea put forth by the authors of a new paper on the arXiv preprint server1. Traditional Big Bang cosmology aligns well with many of today’s precision astrophysical measurements, they write, but it still leaves some important questions unanswered: In particular, what happened at the infinitely dense point, or singularity, from which the Big Bang sprung?
Artist's impression of a growing supermassive black hole. Image credit: NASA/CXC/A.Hobart
As Niayesh Afshordi, an astrophysicist at the Perimeter Institute for Theoretical Physics and one of the paper’s authors, told Zeeya Merali for Nature News, “For all physicists know, dragons could have come flying out of the singularity.”
“In the current best theories that we have, we know that we don’t know,” says Sean Carroll, a theoretical physicist at Caltech who was not part of the team that published the new paper. “We have theories of the universe that work really, really well, but they just don’t say anything about the Big Bang. They fail to give an opinion.” And when the equations of general relativity are applied to the Big Bang singularity, they pop out infinite answers. “What that really means is that the equations are breaking down,” explains Carroll.
Many physicists can stomach that breakdown as long as the singularity is quarantined behind an event horizon, an invisible boundary beyond which no information can pass to an outside observer. “If they [singularities] are ‘hidden’ behind event horizons, they do not affect our predictions, and so we can still use laws of standard physics,” says Afshordi. But the Big Bang singularity is not shielded in this way; instead it is what physicists call “naked.”
“’Naked’ singularities are not hidden, and thus anything to the causal future of ‘naked’ singularities will be affected by laws beyond standard physics,” says Afshordi.
Searching for a way to avoid the naked singularity at the Big Bang—and perhaps explain other vexing properties of our universe in the process—the authors of the new paper turned to a model of the cosmos called the “braneworld.” In the braneworld, our observable, three-dimensional universe actually lives inside another universe which has extra spatial dimensions. To use a two-dimensional analogy, our universe is like the skimmable membrane (“brane”) of fat on top of the pea soup of the universe.
We can’t detect this soup, called “the bulk,” directly, but it could explain some bizarre quirks of physics, like why gravity is so much weaker than the other fundamental forces. Yet physicists have not had much to say about what kinds of objects might live in the bulk and how they might affect us here on the brane.
The new paper analyzes what would happen if a black hole formed within the bulk. Unlike a regular old three-dimensional black hole, which is surrounded by a two-dimensional event horizon, a four-dimensional black hole would have a three-dimensional event horizon. And that event horizon would be constantly expanding. Sound like any universe you know?
The paper’s authors argue that this picture could address other mysteries of Big Bang physics, like how the universe settled down to such a uniform temperature so quickly. Physicists typically explain this problem using a phenomenon called cosmic inflation, which is believed to have caused the universe to swell up rapidly soon after the Big Bang. This swift, early expansion means that parts of the universe that seem disconnected today—that is, they are so far apart that they can’t exchange photons—could have “touched” in the distant past.
Inflation has passed nearly every test we’ve put it to. It even matches up nicely with the latest data from the Planck space observatory, which made the most precise map ever of the cosmic background radiation. The new black hole model doesn’t agree as closely with the Planck data. Plus, physicist Paul Halpern points out, “The authors have constructed something that needs to be closely manipulated and tweaked to get the parameters that inflation gets very naturally.”
Halpern also isn’t convinced that the problems the new model sets out to solve are truly so dire. The naked singularity that plagues the equations of general relativity might melt away once we have a theory that combines general relativity with quantum mechanics, says Halpern. The Big Bang singularity “doesn’t really trouble people who think there will eventually be a theory of quantum gravity.” Plus, Halpern points out, there are practical limits on our ability to observe the naked singularity at the beginning of the universe. “As we go back in time, it's harder and harder to make observations.”
It doesn’t seem likely that the Big Bang is going to be dethroned by the Big Black Hole anytime soon. But, says Halpern, “It’s important to be generous in terms of allowing for a wide range of theoretical models. You never know which will help us, ultimately.”
1The paper has been prepared for submission to the Journal of Cosmology and Astroparticle Physics, though it has not yet been peer-reviewed and published.
Author's picks for further reading
arXiv: 4D Gravity on a Brane in 5D Minkowski Space
In this academic paper, physicists Gia Dvali, Gregory Gabadadze, Massimo Porrati propose the brane world scenario on which the new paper is based.
The Nature of Reality: A Journey Into Extra Dimensions
In this video pencast, theoretical physicist Delia Schwartz-Perlov explains what physicists talk about when they talk about extra dimensions.
The Question of Cosmic Censorship
Physicist Roger Penrose on the problem of naked singularities and the possibility that physics may prevent them through “cosmic censorship."
How would it feel to fall into a black hole? Would you cruise pleasantly toward your doom, only feeling the deadly tug-and-crunch long past the point of no return? Or would you slam headlong into a scorching wall of fire at the black hole's event horizon? It might sound the like the plot of the latest summer blockbuster, but this question turns out to have profound implications for physics.
On Wednesday, September 25 at 3 p.m. ET/noon PT, the Kavli Foundation will be hosting a live webcast on this question, known as the firewall paradox, with four of the top researchers in the field, Raphael Bousso (U.C. Berkeley), Juan Maldacena (Institute for Advanced Study), Joseph Polchinski (U.C. Santa Barbara) and Leonard Susskind (Stanford University). Members of the public are invited to submit questions via email, and you can watch the webcast here or on the Kavli Foundation site.
Want to read up in advance of the event? Try the following primers:
Nature News: Fire in the hole!
The New York Times: A Black Hole Mystery Wrapped in a Firewall Paradox
Slate: New York Times Wants to Fight Einstein, Einstein Declines
It’s not unusual for me to receive mail questioning quantum mechanics and special relativity. I’ll admit, these ideas can sound a bit crazy. For some people, these ideas are simply too counterintuitive to accept. Occasionally, I can convince a correspondent that they accurately describe the universe. But I have some bad news for my pen pals: physicists no longer think about the universe in these simple terms. Our experiments have long shown the subatomic realm to be far more mind-blowing than those modestly-perplexing ideas. It has been nearly a century after all. In the words of my teenage daughter, those ideas are soooooooo 1920s.
Quantum mechanics tells us that an electron is both a particle and a wave and you can never be certain what it will do. Relativity tells us that clocks aren’t absolute, distances depend on the observer, and that energy can be converted into matter and back again. These ideas are still correct, but they’re just the tip of the iceberg.
Physicists now use a class of theories called quantum field theories, or QFTs, which were first postulated in the late 1920s and developed over the following decades. QFTs are intriguing, but they take some getting used to. To start, let’s think only about electrons. Everywhere in the universe there is a field called the electron field. A physical electron isn’t the field, but rather a localized vibration in the field. In fact, every electron in the universe is a similar localized vibration of that single field.
Electrons aren’t the only particles to consist of localized vibrations of a field; all particles do. There is a photon field, an up quark field, a gluon field, a muon field; indeed there is a field for every known particle. And, for all of them, the thing that we visualize as a particle is just a localized vibration of that field. Even the recently discovered Higgs boson is like this. The Higgs field interacts with particles and gives them their mass, but it is hard to observe this field directly. Instead, we supply energy to the field in particle collisions and cause it to vibrate. When we say “we’ve discovered the Higgs boson,” you should think “we’ve caused the Higgs field to vibrate and observed the vibrations.”
This idea gives an entirely different view of how the subatomic world works. Spanning all of space are a great variety of different fields that exist everywhere, just like how a certain spot can simultaneously have a smell, a sound, and a color. What we think of as a particle is simply a vibration of its associated field.
This has significant consequences on how we think about how particles interact. For instance, consider a simple process whereby two electrons are fired at one another and are scattered. In the quasi-classical view of scattering, one electron emits a photon and then recoils. The photon travels to the other electron, which also recoils. This is like having two people in boats and having one of them throw a sack to the other—the thrower’s boat moves in response to the mass of the sack, as does the catcher’s boat.
A traditional Feynman diagram (top) and the same subatomic process using quantum field thinking (bottom). On the left, a photon field is vibrating and the quark and gluon fields are quiescent. When the photon makes a quark and antiquark pair, the quark field is vibrating while the other two fields have no excitation. Finally, when the quark and antiquark combine to make a gluon, only the gluon field has a vibration.
In the QFT approach, a vibration in the electron field induces a vibration in the photon field. The photon field vibration transports energy and momentum to another electron vibration and is absorbed.
In the well-known process where a photon converts into an electron and an antimatter electron, the photon field vibrations are transferred to the electron field and two sets of vibrations are set up—one consistent with an electron vibration and the other consistent with the antimatter electron.
This idea of fields and vibrations explains how the universe works at a deep and fundamental level. These fields span all of space. Some fields can “see” other fields, while being blind to others. The photon field can interact with the fields of charged particles but cannot see gluon or neutrino fields. On the other hand, a photon can interact indirectly with the gluon field, first by making quark vibrations which then make gluon vibrations. It’s kind of like when two quarrelling siblings use a third to pass messages.
Quantum fields are really a mind-bending way of thinking. Everything—and I mean everything—is just a consequence of many infinitely-large fields vibrating. The entire universe is made of fields playing a vast, subatomic symphony. Physicsts are trying to understand the melody.
Author's picks for further reading
QED. Richard P. Feynman.
The Particle at the End of the Universe. Sean Carroll.
On March 28, 1949, at 6:30 in the evening, astrophysicist Fred Hoyle gave one of his authoritative radio lectures on The Third Programme, a cultural broadcast on the BBC’s that featured such intellectuals as philosopher Bertrand Russell and playwright Samuel Beckett. At one point, as he was trying to contrast his own scenario—one of continuous creation of matter in the universe—with the opposing theory, which claimed that the universe had a distinct and definite beginning, Hoyle made what was to become a controversial statement:
We now come to the question of applying the observational tests to earlier theories. These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past [emphasis added]. It now turns out that in some respect or other all such theories are in conflict with the observational requirements.
This lecture marked the birth of the term “big bang,” which has since been inextricably attached to the initial event from which our universe sprouted. Contrary to popular belief, Hoyle did not use the term in a derogatory manner. Rather, he was simply attempting to create a mental picture for his listeners.
Hoyle’s most enduring works were in the areas of nuclear astrophysics and stellar evolution. Yet most of those who remember him from his popular books and prominent radio programs know him as a cosmologist and co-originator of the idea of a steady state universe. (The steady state model predicted that galaxies that are billions of light-years away should look, statistically speaking, just like nearby galaxies, even though we see the former as they were billions of years ago because of the time it takes their light to reach us.)
He started from the observational fact that the universe is expanding. This immediately raised a question: If galaxies are continuously rushing away from each other, does that mean that space is becoming more and more empty? Hoyle answered with a categorical no. Instead, he proposed, matter is continually being created throughout space so that new galaxies and clusters of galaxies are constantly being formed at a rate that compensates precisely for the dilution caused by the cosmic expansion. In this way, Hoyle reasoned, the universe is preserved in a steady state. He once commented wittily, “Things are the way they are because they were the way they were.”
The idea of matter being continuously created out of nothing may appear crazy at first. However, as Hoyle was quick to point out, no one knew where matter had appeared from in the big bang cosmology, either. The only difference, he explained, was that in the big bang scenario all the matter was created in one explosive beginning, while in the steady state model matter has been created at a constant rate throughout an infinite time and is still being created at the same rate today. Hoyle contended that the concept of continuous creation of matter (when put in the context of a specific theory) was much more attractive than creation of the universe in the remote past, since the latter implied that observable effects had arisen from “causes unknown to science.”
The big bang and steady state models made distinctly different predictions about the distant universe. When we observe galaxies that are billions of light-years away, we get a picture of those galaxies as they were billions of years ago. In a continuously evolving universe (the big bang model), this means that we observe that particular part of the universe when it was younger and therefore different. In the steady state model, on the other hand, the universe has always existed in the same state. Consequently, the remote parts of the universe are expected to have precisely the same appearance as the local cosmic environment.
The first signs of trouble for the steady state model came not from optical telescopes but from radio astronomy. One of the pioneers in this endeavor was a physicist from the Cavendish Laboratory at Cambridge: Martin Ryle.
Unlike Hoyle, whose father was a wool and textiles merchant, Ryle came from a privileged background—his father was physician to King George VI—and he had received the best of what private education could offer. After some pioneering radio observations of the Sun in the late 1940s, Ryle and his group embarked on an ambitious program to detect radio sources beyond the solar system. Following some impressive improvements to the observational techniques that allowed them to discard background radiation from the Milky Way, Ryle and his colleagues discovered several dozen “radio stars” distributed more or less isotropically across the sky. Unfortunately, since most of the sources did not have visible counterparts, there was no way to determine their distances precisely.
Ryle began picking apart the steady state model by evaluating one of its testable predictions—that distant parts of the universe should look exactly the same as the local cosmic environment. He started to collect a large sample of radio sources, and to count how many of them there were at different intensity intervals. Since he had no way of knowing the actual distances to most sources (they were beyond the detection range of optical telescopes), Ryle made the simplest assumption: namely, that the observed weaker radio sources were, on average, more distant than the sources of the strong signals. He found that there were dramatically more weak sources than strong ones. In other words, it seemed that the density of sources at distances of billions of light-years (and therefore representing the universe billions of years ago) was much higher than the current density nearby. This was clearly at odds with a model of a never-changing universe, but it could be made consistent with a cosmos evolving from a big bang, if one assumed (correctly, as we now recognize) that galaxies were more prone to emit intense radio signals in their youth than at present, in their older age.
By the early 1960s, Ryle’s group had at its disposal even an entirely new radio observatory, funded by the Mullard electronics company. By then, Ryle and Hoyle had become engaged in a series of intellectual skirmishes, culminating in one particularly unpleasant incident. Hoyle later described this traumatic experience in his autobiographical book Home Is Where the Wind Blows. It all started with what appeared to be an innocent phone call from the Mullard company in early 1961. The person at the other end of the line invited Hoyle and his wife to attend a press conference at which Ryle was expected to present new results that were supposed to be of great interest to Hoyle. When they arrived at the Mullard headquarters in London, Hoyle’s wife, Barbara, was escorted to a seat in the front now, while Hoyle was led to a chair on stage, facing the media. He had no doubt that the announcement would be related to the counting of radio sources according to their intensity, but he couldn’t believe that he would have been invited if the results were to contradict the steady state theory.
Unfortunately, what Hoyle found utterly unthinkable did happen. When Ryle appeared, rather than making a brief announcement, as advertised, he launched into a technical, jargon-filled lecture on the results of his larger, fourth survey. He finished by claiming confidently that the results now showed unambiguously a higher density of radio sources in the past, therefore proving the steady state theory wrong. The shocked Hoyle was merely asked to comment on the results. Incredulous and humiliated, he barely mumbled a few sentences and rushed away from the event. The media frenzy that followed in the subsequent days disgusted Hoyle to the point that he avoided phone calls for a week and was absent even from the following Royal Astronomical Society meeting on February 10. Even Ryle realized that the press conference had crossed the border of common decency. He called Hoyle to apologize, adding that when he agreed to the Mullard event, he “had no idea how bad it would be.”
On the purely scientific front, however, despite these disturbing failures in etiquette, Ryle’s arguments grew increasingly compelling, and by the mid-1960s, the vast majority of the astronomical community agreed that the proponents of the steady state theory had lost the battle.
The discovery of extremely active galaxies, in which the accretion of mass onto central, supermassive black holes releases sufficient radiation to outshine the entire galaxy, cemented the evidence against a steady state universe. These objects, known as quasars, were luminous enough to be observed by optical telescopes. The observations allowed astronomers to use Hubble’s law to determine the distance to these sources, and to show convincingly that quasars were indeed more common in the past than at present. There was no escape from the conclusion that the universe was evolving and that it had been denser in the past. At that point, the floodgates opened, and the challenges to the steady state model kept pouring in.
In spite of Hoyle’s valiant efforts, beginning in the mid-1960s most scientists stopped paying attention to the steady state theory. Hoyle’s continuing attempts to demonstrate that all the confrontations between the theory and emerging observations could be explained away looked increasingly contrived and implausible. Worse yet, he seemed to have lost that “fine judgment” that he had once advocated, which was supposed to distinguish him from “merely becoming a crackpot.” Even as late as the year 2000, at the age of 85, he published a book entitled A Different Approach to Cosmology: From a Static Universe Through the Big Bang Towards Reality, in which he and his collaborators, Jayant Narlikar and Geoff Burbidge, explained the details of the quasi–steady state theory and their objections to the big bang. To express their contemptuous opinion of the scientific establishment, they presented in one of the book’s pages a photograph of a flock of geese walking on a dirt road with the caption, “This is our view of the conformist approach to the standard (hot big bang) cosmology. We have resisted the temptation to name some of the leading geese.” Perhaps the best thing said about the book appeared in the review by Britain’s Sunday Telegraph, and it referred not so much to the contents of the book as to Hoyle’s fiery personality: “Hoyle systematically reviews the evidence for the Big Bang theory, and gives it a good kicking . . . it’s hard not to be impressed with the audacity of the demolition job . . . I can only hope that I possess one- thousandth of Hoyle’s fighting spirit when I, like him, have reached my 85th year.”
Hoyle’s blunder was in his apparently pigheaded, almost infuriating refusal to acknowledge the theory’s demise even as it was being smothered by accumulating contradictory evidence, and in his use of asymmetrical criteria of judgment with respect to the big bang and steady state theories. What was it that caused this intransigent behavior?
A few statements made by Hoyle himself provide the best evidence. In Home Is Where the Wind Blows, he wrote the following striking paragraph:
The problem with the scientific establishment goes back to the small hunting parties of prehistory. It must then have been the case that, for a hunt to be successful, the entire party was needed. With the direction of prey uncertain, as the direction of the correct theory in science is initially uncertain, the party had to make a decision about which way to go, and then they all had to stick to the decision, even if it was merely made at random. The dissident who argued that the correct direction was precisely opposite from the chosen direction had to be thrown out of the group, just as the scientist today who takes a view different from the consensus finds his papers rejected by journals and his applications for research grants summarily dismissed by state agencies. Life must have been hard in pre-history, for the more a hunting party found no prey in its chosen direction, the more it had to continue in that direction, for to stop and argue would be to create uncertainty and to risk differences of opinion breaking out, with the group then splitting disastrously apart. This is why the first priority among scientists is not to be correct but for everybody to think the same way. It is this perhaps instinctive primitive motivation that creates the establishment.
One can hardly imagine a stronger advocacy for dissent from mainstream science. Hoyle echoes here the words of the influential second-century physician Galen of Pergamum: “From my very youth I despised the opinion of the multitude and longed for truth and knowledge, believing that there was for man no possession more noble or divine.” However, as Martin Rees, Astronomer Royal for Britain, has pointed out, isolation has its price. Science progresses not in a straight line from A to B but in a zigzag path shaped by critical reevaluation and faultfinding interaction. The continuous evaluation provided by the scientific establishment that Hoyle so despised is what creates the checks and balances that keep scientists from straying too far in the wrong direction. By imposing upon himself academic isolation, Hoyle denied himself these corrective forces.
I have noted several times that the idea of a steady state universe was brilliant at the time it was proposed. In retrospect, the steady state universe, with its continuous creation of matter, shares many features with currently fashionable models of an inflationary universe: the conjecture that the cosmos experienced a faster-than-light growth spurt when it was a fraction of a second old. In some respects, the steady state universe is simply a universe in which inflation always occurs.
Hoyle’s brilliance was also revealed in the fact that he belonged to that small group of scientists capable of investigating two mutually inconsistent theories in parallel. In spite of continuing to hold out against the big bang for his entire life, Hoyle actually contributed important studies to big bang nucleosyntheses, in particular concerning the cosmic helium abundance and the synthesis of elements at very high temperatures. Hoyle’s theories, even when eventually proven wrong, were always dynamizing, and they unfailingly energized entire fields and catalyzed new ideas.
From Brilliant Blunders by Mario Livio. Copyright ©2013 by Mario Livio. Repreinted by permission of Simon & Schuster, Inc.
What does symmetry mean to physicists?
There is geometrical symmetry. The human body, for instance, has one kind of geometrical symmetry: The left and right side of our bodies are pretty much the same. A typical starfish has a five-fold symmetry, meaning that if you rotate its body through 1/5 of a circle, it looks like it wasn’t rotated at all. A circle has even deeper symmetry: No matter how much you rotate it, the rotated circle looks just like the old one.
There are many kinds of symmetries, from the left-right symmetry of a face (left), to the five-fold symmetry of a starfish (center), to the complete symmetry of a circle (right). These sorts of symmetries are also seen in physics theories, leading some physicists to describe the equations as beautiful.
Symmetry also has an aesthetic meaning, although this is harder to define; artistic symmetry is beauty found in a pleasing and regular form.
Both of these definitions of symmetry have some place in the meaning used by physicists. Equations are geometrically symmetric if they can be “flip flopped” without changing their meaning. For instance, take the simple sum 3 + 4. If we swap the order, we get 4 + 3. Both of these equations equal 7 and we can thus say that addition is symmetric in this case. Of course, not all equations are symmetric when the order is swapped. For example, in subtraction 4 – 3 isn’t the same as 3 – 4.
These simple symmetries give us an insight into more complex symmetries. These more complex symmetries have a huge impact on theoretical physics. To understand how that is true, we must turn to a physicist who may not be a household name, but should be.
Emmy Noether has been called the most influential woman in mathematics. In an era when women were often expressly forbidden from the academic world, she won the highest respect of leading scientists and mathematicians, including Albert Einstein and David Hilbert.
Before Noether, scientists noticed that certain things, like energy and electrical charge, were “conserved.” That is, the amount of energy in a system is the same before and after an event like a collision. Similarly, electrical charge might move around, but the total charge remains the same. (Note that this only works in “closed” systems, which aren’t gaining or losing energy or charge to external sources.) Exactly why these things were conserved wasn’t understood, but these conservation laws were (and are) taught in all introductory physics classes.
Noether connected these conservation laws with mathematical symmetries that could be expressed in equations. She saw that each symmetry implied a physically conserved quantity. If an equation was unchanged if you swapped it from one point in time to a different point in time, this meant that energy was conserved. If an equation was unchanged if you changed a position with a different position, momentum was conserved.
This observation was a brilliant revelation. Conservation laws weren’t an unexplained phenomenon. They were the measurable manifestation of symmetries in the laws governing the universe. The beauty of the cosmos was the beauty of symmetry.
Noether’s theorem led theoretical physicists to explore the idea of symmetry in natural law more fully, leading to a deeper appreciation of the role of symmetry in the rules that govern the cosmos. Now the symmetry of a particular theory is among the first things physicists consider as they evaluate its merit.
If you talk to a physicist—especially a theoretical physicist—about modern theories and why they are the way they are, the scientist may well wax lyrical about the beauty and simplicity of the equations. Symmetry is the basis for this aesthetic judgment. You need not be a physicist to see the beauty of the stars glittering in a dark midnight sky, the allure of a shimmering rainbow and the delicacy of a snowflake, yet they, too, are inscribed in the symmetry of written formulas, there for all to see—once you know how.
Author's picks for further reading
Deep Down Things: The Breathtaking Beauty of Particle Physics
In this book, experimental particle physicist Bruce Schumm connects abstract mathematics with the elegance of the Standard Model of physics.
The Particle at the End of the Universe
Physicist Sean Carroll goes inside the hunt for the Higgs boson.
In this video pencast, theorist Delia Schwartz-Perlov explains what physicists are really talking about when they talk about extra dimensions of space. Could our universe actually contain unseen dimensions, and could these extra dimensions help unify quantum theory and gravity?
Editor's picks for further reading
Cosmos: Carl Sagan: The 4th Dimension
In this scene from the classic "Cosmos" series, Carl Sagan imagines what happens when a three-dimensional character enters a two-dimensional world.
FQXi: Taking on String Theory’s 10-D Universe with 8-D Math
In this article, discover how theorists Tevian Dray and Corinne Manogue are using ten-dimensional math to describe subatomic particles.
NOVA: Imagining Other Dimensions
Journey from a two-dimensional "flatland" to the ten- (or more) dimensional world of superstring theory in this illustrated essay.