According to standard physics textbooks, quantum mechanics is the theory of the microscopic world. It describes particles, atoms and molecules but gives way to ordinary classical physics on the macroscopic scales of pears, people and planets. Somewhere between molecules and pears lies a boundary where the strangeness of quantum behavior ends and the familiarity of classical physics begins. The impression that quantum mechanics is limited to the microworld permeates the public understanding of science. For instance, Columbia University physicist Brian Greene writes on the first page of his hugely successful (and otherwise excellent) book The Elegant Universe that quantum mechanics “provides a theoretical framework for understanding the universe on the smallest of scales.” Classical physics, which comprises any theory that is not quantum, including Albert Einstein’s theories of relativity, handles the largest of scales.

Yet this convenient partitioning of the world is a myth. Few modern physicists think that classical physics has equal status with quantum mechanics; it is but a useful approximation of a world that is quantum at all scales. Although quantum effects may be harder to see in the macroworld, the reason has nothing to do with size per se but with the way that quantum systems interact with one another. Until the past decade, experimentalists had not confirmed that quantum behavior persists on a macroscopic scale. Today, however, they routinely do. These effects are more pervasive than anyone ever suspected. They may operate in the cells of our body.

Even those of us who make a career of studying these effects have yet to assimilate what they are telling us about the workings of nature. Quantum behavior eludes visualization and common sense. It forces us to rethink how we look at the universe and accept a new and unfamiliar picture of our world.

A Tangled Tale
To a quantum physicist, classical physics is a black-and-white image of a Technicolor world. Our classical categories fail to capture that world in all its richness. In the old textbook view, the rich hues get washed out with increasing size. Individual particles are quantum; en masse they are classical. But the first clues that size is not the determining factor go back to one of the most famous thought experiments in physics, Schrödinger’s cat.

Erwin Schrödinger came up with his morbid scenario in 1935 to illustrate how the microworld and macroworld couple to each other, preventing arbitrary lines from being drawn between them. Quantum mechanics says that a radioactive atom can be both decayed and not decayed at the same time. If the atom is linked to a bottle of cat poison, so that the cat dies if the atom decays, then the animal gets left in the same quantum limbo as the atom. The weirdness of the one infects the other. Size does not matter. The puzzle was why cat owners only ever see their pets as alive or dead.

In the modern point of view, the world looks classical because the complex interactions that an object has with its surroundings conspire to conceal quantum effects from our view. Information about a cat’s state of health, for example, rapidly leaks into its environment in the form of photons and an exchange of heat. Distinctive quantum phenomena involve combinations of different classical states (such as both dead and alive), and these combinations tend to dissipate. The leakage of information is the essence of a process known as decoherence [see “100 Years of Quantum Mysteries,” by Max Tegmark and John Archibald Wheeler; Scientific American, February 2001].

Larger things tend to be more susceptible to decoherence than smaller ones, which justifies why physicists can usually get away with regarding quantum mechanics as a theory of the mi­cro­world. But in many cases, the information leakage can be slowed or stopped, and then the quantum world reveals itself to us in all its glory. The quintessential quantum effect is entanglement, a term that Schrödinger coined in the same 1935 paper that introduced his cat to the world. Entanglement binds together individual particles into an indivisible whole. A classical system is always divisible, at least in principle; whatever collective properties it has arise from components that themselves have certain properties. But an entangled system cannot be broken down in this way. Entanglement has strange consequences. Even when the entangled particles are far apart, they still behave as a single entity, leading to what Einstein famously called “spooky action at a distance.”

Usually physicists talk about entanglement of pairs of elementary particles such as electrons. Such particles can be thought of, crudely, as small spinning tops that rotate either clockwise or counterclockwise, their axes pointing in any given direction: horizontally, vertically, at 45 degrees, and so on. To measure a particle’s spin, you must choose a direction and then see whether the particle spins in that direction.

Suppose, for argument’s sake, that electrons behaved classically. You might set up one electron to spin in the horizontal clockwise direction and the other in the horizontal counterclockwise direction; that way, their total spin is zero. Their axes remain fixed in space, and when you make a measurement, the outcome depends on whether the direction you choose aligns with the particle’s axis. If you measure both of them horizontally, you see both of them spinning in opposite directions; if you measure them vertically, you detect no spin at all for either.

For quantum electrons, however, the situation is astonishingly different. You can set up the particles to have a total spin of zero even when you have not specified what their individual spins are. When you measure one of the particles, you will see it spinning clockwise or counterclockwise at random. It is as though the particle decides which way to spin for itself. Nevertheless, no matter which direction you choose to measure the electrons, providing it is the same for both, they will always spin in opposite ways, one clockwise and the other counterclockwise. How do they know to do so? That remains utterly mysterious. What is more, if you measure one particle horizontally and the other vertically, you will still detect some spin for each; it appears that the particles have no fixed axes of rotation. Therefore, the measurement outcomes match to an extent that classical physics cannot explain.

Acting as One
Most demonstrations of entanglement involve at most a handful of particles. Larger batches are harder to isolate from their surroundings. The particles in them are likelier to become entangled with stray particles, obscuring their original interconnections. In accordance with the language of decoherence, too much information leaks out to the environment, causing the system to behave classically. The difficulty of preserving entanglement is a major challenge for those of us seeking to exploit these novel effects for practical use, such as quantum computers.

A neat experiment in 2003 proved that larger systems, too, can remain entangled when the leakage is reduced or somehow counteracted. Gabriel Aeppli of University College London and his colleagues took a piece of lithium fluoride salt and put it in an external magnetic field. You can think of the atoms in the salt as little spinning magnets that try to align themselves with the external field, a response known as magnetic susceptibility. Forces that the atoms exert on one another act as a kind of peer pressure to bring them into line more quickly. As the researchers varied the strength of the magnetic field, they measured how quickly the atoms became aligned. They found that the atoms responded much faster than the strength of their mutual interactions would suggest. Evidently some additional effect was helping the atoms to act in unison, and the researchers argued that entanglement was the culprit. If so, the 1020 atoms of the salt formed a hugely entangled state.

To avoid the confounding effects of the random motions associated with heat energy, Aeppli’s team did its experiments at extremely low temperatures—a few millikelvins. Since then, however, Alexandre Martins de Souza of the Brazilian Center for Physics Research in Rio de Janeiro and his colleagues have discovered macroscopic entanglement in materials such as copper carboxylate at room temperature and higher. In these systems, the interaction among particle spins is strong enough to resist thermal chaos. In other cases, an external force wards off thermal effects [see “Easy Go, Easy Come,” by George Musser; News Scan, Scientific American, November 2009]. Physicists have seen entanglement in systems of increasing size and temperature, from ions trapped by electromagnetic fields to ultracold atoms in lattices to superconducting quantum bits.

These systems are analogous to Schrödinger’s cat. Consider an atom or ion. Its electrons can exist close to the nucleus or farther away—or both at the same time. Such an electron acts like the radioactive atom that has either decayed or not decayed in Schrödinger’s thought experiment. Independently of what the electron is doing, the entire atom can be moving, say, left or right. This motion plays the role of the dead or alive cat. Using lasers to manipulate the atom, physicists can couple the two properties. If the electron is close to the nucleus, we can make the atom move to the left, whereas if the electron is farther away, the atom moves to the right. So the state of the electron is entangled with the movement of the atom, in the same way that the radioactive decay is entangled with the state of the cat. The feline that is both alive and dead is mimicked by an atom that is moving both to the left and to the right.

Other experiments scale up this basic idea, so that huge numbers of atoms become entangled and enter states that classical physics would deem impossible. And if solids can be entangled even when they are large and warm, it takes only a small leap of imagination to ask whether the same might be true of a very special kind of large, warm system: life.

Schrödinger’s Birds
European robins are crafty little birds. Every year they migrate from Scandinavia to the warm plains of equatorial Africa and return in the spring, when the weather up north becomes more tolerable. The robins navigate this round-trip of some 13,000 kilometers with natural ease.

People have long wondered whether birds and other animals might have some built-in compass. In the 1970s the husband-wife team of Wolfgang and Roswitha Wiltschko of the University of Frankfurt in Germany caught robins that had been migrating to Africa and put them in artificial magnetic fields. Oddly, the robins, they found, were oblivious to a reversal of the magnetic field direction, indicating that they could not tell north from south. The birds did, however, respond to the inclination of the earth’s magnetic field—that is, the angle that the field lines make with the surface. That is all they need to navigate. Interestingly, blindfolded robins did not respond to a magnetic field at all, indicating that they somehow sense the field with their eyes.

In 2000 Thorsten Ritz, a physicist then at the University of Southern Florida who has a passion for migratory birds, and his colleagues proposed that entanglement is the key. In their scenario, which builds on the previous work of Klaus Schulten of the University of Illinois, a bird’s eye has a type of molecule in which two electrons form an entangled pair with zero total spin. Such a situation simply cannot be mimicked with classical physics. When this molecule absorbs visible light, the electrons get enough energy to separate and become susceptible to external influences, including the earth’s magnetic field. If the magnetic field is inclined, it affects the two electrons differently, creating an imbalance that changes the chemical reaction that the molecule undergoes. Chemical pathways in the eye translate this difference into neurological impulses, ultimately creating an image of the magnetic field in the bird’s brain.

Although the evidence for Ritz’s mechanism is circumstantial, Christopher T. Rogers and Kiminori Maeda of the University of Oxford have studied molecules similar to Ritz’s in the laboratory (as opposed to inside living animals) and shown that these molecules are indeed sensitive to magnetic fields because of electron entanglement. According to calculations that my colleagues and I have done, quantum effects persist in a bird’s eye for around 100 microseconds—which, in this context, is a long time. The record for an artificially engineered electron-spin system is about 50 microseconds. We do not yet know how a natural system could preserve quantum effects for so long, but the answer could give us ideas for how to protect quantum computers from decoherence.

Another biological process where entanglement may operate is photosynthesis, the process whereby plants convert sunlight into chemical energy. Incident light ejects electrons inside plant cells, and these electrons all need to find their way to the same place: the chemical reaction center where they can deposit their energy and set off the reactions that fuel plant cells. Classical physics fails to explain the near-perfect efficiency with which they do so.

Experiments by several groups, such as Graham R. Fleming, Mohan Sarovar and their colleagues at the University of California, Berkeley, and Gregory D. Scholes of the University of Toronto, suggest that quantum mechanics accounts for the high efficiency of the process. In a quantum world, a particle does not just have to take one path at a time; it can take all of them simultaneously. The electromagnetic fields within plant cells can cause some of these paths to cancel one another and others to reinforce mutually, thereby reducing the chance the electron will take a wasteful detour and increasing the chance it will be steered straight to the reaction center.

The entanglement would last only a fraction of a second and would involve molecules that have no more than about 100,000 atoms. Do any instances of larger and more persistent entanglement exist in nature? We do not know, but the question is exciting enough to stimulate an emerging discipline: quantum biology.

The Meaning of It All
To Schrödinger, the prospect of cats that were both alive and dead was an absurdity; any theory that made such a prediction must surely be flawed. Generations of physicists shared this discomfort and thought that quantum mechanics would cease to apply at a still larger scale. In the 1980s Roger Penrose of Oxford suggested that gravity might cause quantum mechanics to give way to classical physics for objects more massive than 20 micrograms, and a trio of Italian physicists—GianCarlo Ghirardi and Tomaso Weber of the University of Trieste and Alberto Rimini of the University of Pavia—proposed that large numbers of particles spontaneously behave classically. But experiments now leave very little room for such processes to operate. The division between the quantum and classical worlds appears not to be fundamental. It is just a question of experimental ingenuity, and few physicists now think that classical physics will ever really make a comeback at any scale. If anything, the general belief is that if a deeper theory ever supersedes quantum physics, it will show the world to be even more counterintuitive than anything we have seen so far.

Thus, the fact that quantum mechanics applies on all scales forces us to confront the theory’s deepest mysteries. We cannot simply write them off as mere details that matter only on the very smallest scales. For instance, space and time are two of the most fundamental classical concepts, but according to quantum mechanics they are secondary. The entanglements are primary. They interconnect quantum systems without reference to space and time. If there were a dividing line between the quantum and the classical worlds, we could use the space and time of the classical world to provide a framework for describing quantum processes. But without such a dividing line—and, indeed, without a truly classical world—we lose this framework. We must explain space and time as somehow emerging from fundamentally spaceless and timeless physics.

That insight, in turn, may help us reconcile quantum physics with that other great pillar of physics, Einstein’s general theory of relativity, which describes the force of gravity in terms of the geometry of spacetime. General relativity assumes that objects have well-defined positions and never reside in more than one place at the same time—in direct contradiction with quantum physics. Many physicists, such as Stephen Hawking of the University of Cambridge, think that relativity theory must give way to a deeper theory in which space and time do not exist. Classical spacetime emerges out of quantum entanglements through the process of decoherence.

An even more interesting possibility is that gravity is not a force in its own right but the residual noise emerging from the quantum fuzziness of the other forces in the universe. This idea of “induced gravity” goes back to the nuclear physicist and Soviet dissident Andrei Sakharov in the 1960s. If true, it would not only demote gravity from the status of a fundamental force but also suggest that efforts to “quantize” gravity are misguided. Gravity may not even exist at the quantum level.

The implications of macroscopic objects such as us being in quantum limbo is mind-blowing enough that we physicists are still in an entangled state of confusion and wonderment.