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Physics + MathPhysics & Math

Spooky Action at a Distance

Every age develops its stories or metaphors for how the universe was conceived and structured. According to an ancient Indian creation myth, the universe was created when the gods dismembered the primordial giant Purusa, whose head became the sky, whose feet became the Earth, and whose breath became the wind. To Aristotle, the universe was a collection of 55 concentric crystalline spheres, the outermost being heaven, surrounding those of the planets, Earth and its elements, and finally the seven circles of hell.

ByBrian GreeneNova
In 1947, eight years before his death, Einstein wrote to a friend that he could not seriously believe in quantum mechanics because "physics should represent a reality in time and space, free from spooky actions at a distance." He was referring to quantum entanglement, one of the quantum world's most bizarre attributes.© Bettmann/CORBIS

With Newton and his precise, deterministic mathematical formulation of motion, the description changed again. The universe was likened to an enormous, grand clockwork: After being wound and set into its initial state, the clockwork universe ticks from one moment to the next with complete regularity and predictability.

Einstein's special and general relativity pointed out important subtleties of the clockwork metaphor: There is no single, preferred, universal clock; there is no consensus on what constitutes a moment, what constitutes a now. Even so, you can still tell a clockworklike story about the evolving universe. The clock is your clock. The story is your story. But the universe unfolds with the same regularity and predictability as in the Newtonian framework. If by some means you know the state of the universe right now—if you know where every particle is and how fast and in what direction each is moving—then, Newton and Einstein agree, you can, in principle, use the laws of physics to predict everything about the universe arbitrarily far into the future or to figure out what it was like arbitrarily far into the past.

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Enter quantum weirdness

Quantum mechanics breaks with this tradition. We can't ever know the exact location and exact velocity of even a single particle. We can't predict with total certainty the outcome of even the simplest of experiments, let alone the evolution of the entire cosmos. Quantum mechanics shows that the best we can ever do is predict the probability that an experiment will turn out this way or that. And as quantum mechanics has been verified through decades of fantastically accurate experiments, the Newtonian cosmic clock, even with its Einsteinian updating, is an untenable metaphor; it is demonstrably not how the world works.

Something that happens over here can be entwined with something that happens over there.

But the break with the past is yet more complete. Even though Newton's and Einstein's theories differ sharply on the nature of space and time, they do agree on certain basic facts, certain truths that appear to be self-evident.

two birds
Space, whatever else it is, serves as the medium that separates two distinct things, like two birds, Greene says. But quantum entanglement belies that apparent truth.
© Flavio Vallenari/iStockphoto

How we think of space

If there is space between two objects—if there are two birds in the sky and one is way off to your right and the other is way off to your left—we can and do consider the two objects to be independent. We regard them as separate and distinct entities. Space, whatever it is fundamentally, provides the medium that separates and distinguishes one object from another. That is what space does. Things occupying different locations in space are different things.

Moreover, in order for one object to influence another, it must in some way negotiate the space that separates them. One bird can fly to the other, traversing the space between them, and then peck or nudge its companion. One person can influence another by shooting a slingshot, causing a pebble to traverse the space between them, or by yelling, causing a domino effect of bouncing air molecules, one jostling the next until some bang into the recipient's eardrum.

Being yet more sophisticated, one can exert influence on another by firing a laser, causing an electromagnetic wave—a beam of light—to traverse the intervening space. Or, being more ambitious, one can hypothetically shake or move a massive body (like the moon), sending a gravitational disturbance speeding from one location to another.

To be sure, if we are over here we can influence someone over there, but no matter how we do it, the procedure always involves someone or something traveling from here to there, and only when the someone or something gets there can the influence be exerted.

voodoo totem
Quantum entanglement brings to mind voodoo (here, a voodoo idol from Benin, West Africa). But the scientific evidence that it exists is overwhelming, Greene says.
© Peeter Viisimaa/iStockphoto

Voodoo reality

Physicists call this feature of the universe locality, emphasizing the point that you can directly affect only things that are next to you, that are local. Voodoo contravenes locality, since it involves doing something over here and affecting something over there without the need for anything to travel from here to there, but common experience leads us to think that verifiable, repeatable experiments would confirm locality. And most do.

But a class of experiments performed during the last couple of decades has shown that something we do over here (such as measuring certain properties of a particle) can be subtly entwined with something that happens over there (such as the outcome of measuring certain properties of another distant particle), without anything being sent from here to there.

Intervening space does not ensure that two objects are separate.

While intuitively baffling, this phenomenon fully conforms to the laws of quantum mechanics, and was predicted using quantum mechanics long before the technology existed to do the experiment and observe, remarkably, that the prediction is correct. This sounds like voodoo; Einstein, who was among the first physicists to recognize—and sharply criticize—this possible feature of quantum mechanics, called it "spooky." But the long-distance links these experiments confirm are extremely delicate and are, in a precise sense, fundamentally beyond our ability to control.

star cluster
Quantum connections between two particles can persist even if the two particles are on opposite sides of the universe. (Here, a star cluster within our Milky Way galaxy.)
NASA/JPL-Caltech, D. Figer (Space Telescope Science Institute/Rochester Institute of Technology), E. Churchwell (University of Wisconsin, Madison) and the GLIMPSE Legacy Team

The twain shall meet

Nevertheless, these results, coming from both theoretical and experimental considerations, strongly support the conclusion that the universe admits interconnections that are not local. Something that happens over here can be entwined with something that happens over there even if nothing travels from here to there—and even if there isn't enough time for anything, even light, to travel between the events.

This means that space cannot be thought of as it once was: Intervening space, regardless of how much there is, does not ensure that two objects are separate, since quantum mechanics allows an entanglement, a kind of connection, to exist between them. A particle, like one of the countless number that make up you or me, can run but it can't hide.

According to quantum theory and the many experiments that bear out its predictions, the quantum connection between two particles can persist even if they are on opposite sides of the universe. From the standpoint of their entanglement, notwithstanding the many trillions of miles of space between them, it's as if they are right on top of each other.

Numerous assaults on our conception of reality are emerging from modern physics. But of those that have been experimentally verified, I find none more mind-boggling than this recent realization that our universe is not local.

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