Gigantic Black Holes at the Center of Galaxies
Carlo Rovelli
Theoretical physicist, Centre de Physique Théorique, Aix-Marseille University, Marseille, France; author, Seven Brief Lessons on Physics
Evidence has recently piled up that there is a gigantic black hole, Sagittarius A*, with a mass 4 million times that of our Sun, at the center of our galaxy. Similar black holes appear to exist at the center of most galaxies. Some have masses billions of times that of our Sun. Can you imagine a black hole a billion times the size of our Sun?
The existence of these giants changes, once again, our picture of the universe. Clearly these monsters must have played a major role in the history of the cosmos but we do not know what it was. Astronomers are building an “instrument” as large as the Earth, connecting many existing radio telescopes to see Sagittarius A* directly.
But these immense black holes are also the boundary of our current knowledge: We see matter falling into them, but we have no idea what ultimately happens to it. Space and time appear to come to an end inside. Or, better said, to morph into something we do not yet know. The universe is still full of mystery.
The Universe Is Infinite
Rudy Rucker
Mathematician, computer scientist, cyberpunk pioneer; co-author (with Bruce Sterling), Transreal Cyberpunk
Many cosmologists now think our spatial universe is infinite. That’s news. It was only this year that I heard about it. I don’t get out as much as I used to.
Thirty years ago, it was widely believed that our spatial universe is the finite 3D hypersurface of a 4D hypersphere—analogous to the finite 2D surface of a 3D sphere. Our underlying hypersphere was supposedly born, and began expanding, at the Big Bang. And eventually our hypersphere was to run out of momentum and collapse back into a Big Crunch—which might possibly serve as the seed for a new Big Bang. No yawning void of infinity and no real necessity for a troublesome initial point in time. Our own Big Bang itself may have been seeded by a prior Big Crunch. Indeed, we could imagine an endless pearl-string of successive hyperspherical universes. A tidy theory.
But then experimental cosmologists found ways to estimate the curvature of our space, and it seems to be flat, like an endless plane, not curved like the hypersurface of a hypersphere. At most, our space might be “negatively curved,” like a hyperbolic saddle shape, but that’s probably infinite as well.
If you’re afraid of infinity, you might say something like this: “So, OK, maybe we’re in a vast infinite space, but it’s mostly empty. Our universe is just a finite number of galaxies rushing away from each other inside this empty infinite space—like a solitary skyrocket exploding and sending out a doomed shower of sparks.” But many cosmologists say, “No, there are an infinite number of galaxies in our infinite space.”
Where did all those galaxies come from? The merry cosmologists deploy a slick argument involving the relativity of simultaneity and the inflationary theory of cosmic inflation—and they conclude that in the past there was a Big Bang explosion at every single point of our infinite space. Flaaash! An infinite space with infinitely many galaxies!
Note that I’m not talking about some shoddy “many universes” theory here. I hate those things. I’m talking about our good old planets-and-suns single universe. And they’re telling us it goes on forever in space, and on forever into the future, and it has infinitely many worlds. We aren’t ever going to see more than a few of these planets, but it’s nice to know they’re out there.
So, OK, how does this affect me in the home?
You get a sense of psychic expansion if you begin thinking in terms of an infinite universe. A feeling of freedom, and perhaps a feeling that whatever we do here does not, ultimately, matter that much. You’d do best to take this in a “Relax!” kind of way, rather than in an “It’s all pointless” kind of way.
Our infinite universe’s inhabited planets are like dandelions in an endless meadow. Each of them is beautiful and to be cherished—especially by the little critters who live on them. We cherish our Earth because we’re part of it, even though it’s nothing special. It’s like the way you might cherish your family. It’s not unique, but it’s yours. And maybe that’s enough.
I know some of you are going to want more. Well, as far as I can see, we’re living in one of those times when cosmologists have no clear idea of what’s going on. They don’t understand the start of the cosmos, nor cosmic inflation, nor dark energy, nor dark matter. You might say they don’t know jack.
Not knowing jack is a good place to be, because it means we’re ready to discover something really cool and different. Maybe next year, maybe in ten, or maybe in twenty years. Endless free energy? Antigravity? Teleportation? Who can say. The possibilities are infinite and the future is bright.
It’s good to be an infinite world.
Advanced LIGO and Advanced Virgo
Paul Davies
Theoretical physicist, cosmologist, astrobiologist, Arizona State University; author, The Eerie Silence: Renewing Our Search for Alien Intelligence
The end of 2015 coincided with the centennial of Einstein’s general theory of relativity, which the great man presented to the Prussian Academy of Sciences in a series of four lectures in the midst of World War I. Widely regarded as the pinnacle of human intellectual achievement, general relativity took many years to be well tested observationally. But after decades of thorough investigation, physicists have yet to find any flaw with the theory.
Nevertheless, one key test remains incomplete. Shortly after Einstein published his famous gravitational field equations, he came up with an intriguing solution of them. It describes ripples in the geometry of spacetime itself, representing waves that travel across the universe at the speed of light. The detection of these gravitational waves has been an outstanding challenge to experimental physics for several decades. Now, in early 2016, the long search seems to be nearing its culmination.
A laser system designed to pick up the passage of gravitational waves emanating from violent astronomical events has recently been upgraded, and rumors abound that it has already seen something. The system, called Advanced LIGO (for Laser Interferometer Gravitational-wave Observatory), uses laser beams to spot almost inconceivably minute gravitational effects. In Europe, its counterpart, Advanced Virgo, is also limbering up. Advanced LIGO and Advanced Virgo are refinements of existing systems that proved the technology but lacked the sensitivity to detect bursts of gravitational waves from supernovae or colliding neutron stars on a routine basis. The stage is now set to move to that phase.
The detection of gravitational waves would not merely provide a definitive test of Einstein’s century-old theory; it would serve to open up a whole new window on the universe. Existing conventional telescopes range across the entire electromagnetic spectrum, from radio to gamma rays. LIGO and Virgo would open up an entirely new spectrum and with it an entirely new branch of astronomy, enabling observations of black-hole collisions and other cosmic exotica.
Each time a new piece of technology has been used to study the universe, astronomers have been surprised. Once gravitational astronomy is finally born, the exploration of the universe through gravitational eyes will undoubtedly provide newsworthy discoveries for decades to come.
The News Is Not the News
Frank Wilczek
Theoretical physicist, MIT; recipient, 2004 Nobel Prize in physics; author, A Beautiful Question: Finding Nature’s Deep Design
On the ice-capped heights of Labrador, through winter, snow falls. With the coming of spring, much of it melts. Sometimes more falls than melts, and the ice grows; sometimes more melts than has fallen, and the ice shrinks. It is a delicate balance. The result varies from year to year, by many inches. But let the balance tip ever so slightly, so that amid much larger fluctuations one inch, on average, survives, and Earth is transformed. Great glaciers grow and cover North America in ice. If corresponding processes in Greenland or Antarctica tip the other way,
melting more than is frozen, then oceans will swell and drown North America’s coasts.
Episodes of both sorts have happened repeatedly in Earth’s history, on timescales of a few tens of thousands of years. They are probably controlled by small, long-period changes in Earth’s orbit. Today we are living in a relatively rare interglacial period, expected to last another 50,000 years. Notoriously, over the last few decades human activity has tipped the balance toward melting, threatening catastrophe.
These mighty stories derive from systematic trends that can be hard to discern within the tumult of much larger but ephemeral noise. The news is not the news.
So it is with the grandest of human stories: the steady increase, powered by science, of our ability to control the physical world. Richard Feynman memorably expressed a related thought: “From a long view of the history of mankind, seen from, say, ten thousand years from now, there can be little doubt that the most significant event of the 19th century will be judged as Maxwell’s discovery of the laws of electrodynamics.”
In that spirit, the most significant event of the 20th century is the discovery of the laws of matter in general. That discovery has three components: the frameworks of relativity and quantum mechanics, and the specific forces and laws embodied in our core theory, often called the Standard Model. For purposes of chemistry and engineering—plausibly, for all practical purposes—we’ve learned what nature has on offer.
I venture to guess that the most significant event of the 21st century will be a steady accumulation of new discoveries, based on deeper use of quantum physics, which harness the physical world. In the 21st century, we will learn how to harvest energy from the Sun and store it efficiently. We will learn how to make much stronger, much lighter materials. We will learn how to make more powerful and more versatile illuminators, sensors, communication devices, and computers.
We know the rules. Aided by our own creations, in a virtuous cycle, we’ll learn how to play the game.
We Know All the Particles and Forces We’re Made Of
Sean Carroll
Theoretical physicist, Caltech; author, The Big Picture: On the Origins of Life, Meaning, and the Universe Itself
Sometimes news creeps up on us slowly. The discovery of the electron by J. J. Thomson in 1897 marked the first step in constructing the Standard Model of particle physics, an endeavor that culminated in the discovery of the Higgs boson in 2012. The Standard Model is a boring name for a breathtaking theory describing quarks, leptons, and the bosons that hold them all together to make material objects. Together with gravity, captured by Einstein’s general theory of relativity, we have what Nobel Laureate Frank Wilczek has dubbed the core theory: a complete description of all the particles and forces that make up you and me, as well as the Sun, Moon, and stars, and everything we’ve directly seen in every experiment performed here on Earth.
There is a lot we don’t understand in physics: the nature of dark matter and dark energy, what happens at the Big Bang or inside a black hole, why the particles and forces have the characteristics they do. We certainly don’t know even a fraction of what there is to learn about how the elementary particles and forces come together to make complex structures, from molecules to nation-states. But there are some things we do know—and that includes the identity and behavior of all of the pieces underlying the world of our everyday experience.
Could there be particles and forces we haven’t yet discovered? Of course—there almost certainly are. But the rules of quantum field theory assure us that if new particles and forces interacted strongly enough with the ones we know about to play any role in the behavior of the everyday world, we would have been able to produce them in experiments. We’ve looked, and they’re not there. Any new particles must be too heavy to be created, or too short-lived to be detected; any new forces must be too short-range to be noticed, or too feeble to push around the particles we see. Particle physics is nowhere near complete, but future discoveries in that field won’t play a role in understanding human beings or their environment.
We’ll continue to push deeper. There’s a very good chance that “particles and forces moving through spacetime” isn’t the most fundamental way of thinking about the universe. Just as we realized in the 19th century that air and water are fluids made of atoms and molecules, we could discover that there is a layer of reality more comprehensive than anything we currently imagine. But air and water didn’t stop being fluids just because we discovered atoms and molecules; we still give weather reports in terms of temperature and pressure and wind speed, not by listing what each individual molecule in the atmosphere is doing. Similarly, 1,000 and 1,000,000 years from now we’ll still find the concepts of the core theory to be a useful way of talking about what we’re made of.
Could we be wrong in thinking that the core theory describes all of the particles and forces that go into making human beings and their environments? Sure, we could always be wrong. The Sun might not rise tomorrow, we could be brains living in vats, or the universe could have been created last Thursday. Science is an empirical enterprise, and we should always be willing to change our minds when new evidence comes in. But quantum field theory is a special kind of framework. It’s the unique way of accommodating the requirements of quantum mechanics, relativity, and locality. Finding that it was violated in our everyday world would be one of the most surprising discoveries in the history of science. It could happen—but the smart money is against it.
The discovery of the Higgs boson at the Large Hadron Collider in 2012 verified that the basic structure of the core theory is consistent and correct. It stands as one of the greatest accomplishments in human intellectual history. We know the basic building blocks of which we are made. Figuring out how those simple pieces work together to create our complex world will be the work of many generations to come.
Computational Complexity and the Nature of Reality
Amanda Gefter
Science writer; author, Trespassing on Einstein’s Lawn
Physicists have spent the last 100 years attempting to reconcile Einstein’s theory of general relativity, which describes the large-scale geometry of spacetime, with quantum mechanics, which deals with the small-scale behavior of particles. It’s been slow going for a century, but now, suddenly, things are happening.
First, there’s ER=EPR. More idea than equation, it’s the brainchild of physicists Juan Maldacena and Leonard Susskind. On the lefthand side is an Einstein-Rosen bridge, a kind of geometric tunnel connecting distant points in space, otherwise known as a wormhole. On the right are Einstein, Rosen, and Podolsky, the three physicists who first pointed out the spooky nature of quantum entanglement, wherein the quantum state of two remote particles straddles the distance between them. Then there’s that “equals” sign in the middle. Boldly it declares that spacetime geometry and the links between entangled particles are two descriptions of the same physical situation. ER = EPR appears brief and unassuming, but it’s a daring step toward uniting general relativity and quantum mechanics—with radical implications.
Intuitively, the connection is clear. Both wormholes and entanglement flout the constraints of space. They’re shortcuts. One can enter a wormhole on one side of the universe and emerge from it on the other without having to traverse the space in between. Likewise, measuring one particle will instantaneously determine the state of its entangled partner, even if the two are separated by galaxies.
The connection becomes more intriguing when viewed in terms of information. For maximally entangled particles, the information they carry resides simultaneously in both particles but in neither alone; informationally speaking, no space separates one from the other. For particles that are slightly less than maximally entangled, we might say there is some space between them. As particles become less and less entangled, information becomes more and more localized, words like “here” and “there” begin to apply and ordinary space emerges.
One hundred years ago, Einstein gave us a new way to think
about space—not as the static backdrop of the world but as a dynamic ingredient. Now ER=EPR gives us yet a newer interpretation: What we call space is nothing more than a way to keep track of quantum information. And what about time? Time, physicists are beginning to suspect, may be a barometer of computational complexity.
Computational complexity measures how difficult it is to carry out a given computation—how many logical steps it takes, or how the resources needed to solve a problem scale with its size. Historically, it’s not something physicists thought much about. Computational complexity was a matter of engineering—nothing profound. But all that has changed, thanks to what’s known as the black-hole firewall paradox—an infuriating dilemma that has theoretical physicists pulling out their hair.
As a black hole radiates away its mass, all the information that ever fell in must emanate back out into the universe, scrambled among the Hawking radiation; if it doesn’t, quantum mechanics is violated. The very same information must reside deep in the black hole’s interior; if it doesn’t, general relativity is violated. And the laws of physics decree that information can’t be duplicated. The firewall paradox arises when we consider an observer, Alice, who decodes the information scrambled among the Hawking radiation, then jumps into the black hole where she will find, by various accounts, an illegal information clone or an inexplicable wall of fire. Either way, it’s not good.
But Alice’s fate recently took a turn when two physicists, Patrick Hayden and Daniel Harlow, wondered how long it would take her to decode the information in the radiation. Applying a computational-complexity analysis, they discovered that the decoding time would rise exponentially with each additional particle of radiation. In other words, by the time Alice decodes the information, the black hole will have long ago evaporated and vanished, taking any firewalls or violations of physics with it. Computational complexity allows general relativity and quantum mechanics to peacefully coexist.
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