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June 18, 2011

"Living in a Quantum World" — Vlatko Vedral


Above in the headline, the title of a fantastic article in the June 2011 issue of Scientific American.

Long story short: "Quantum mechanics is not just about teeny particles. It applies to things of all sizes: birds, plants, maybe even people."

Below, a summary of the article.


Below, the first two paragraphs.


More excerpts follow.


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.

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.

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."

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.

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 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.


Any theory that considers space, time and gravity as secondary and emergent rather than fundamental is a theory I can conjure with.

Sign me up.



June 18, 2011 at 12:01 PM | Permalink


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Shakespeare said it and I've always agreed. "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy." One never knows.

Posted by: Tamra | Jun 18, 2011 1:57:40 PM

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