CHAPTER 1: ENTANGLED WORMHOLES
Wormholes Untangle a Black Hole Paradox
A bold new idea aims to link two famously discordant descriptions of nature. In doing
so, it may also reveal how space-
By: K.C. Cole
April 24, 2015
One hundred years after Albert Einstein developed his general theory of relativity,
physicists are still stuck with perhaps the biggest incompatibility problem in the
universe. The smoothly warped space-
Like initials carved in a tree, ER = EPR, as the new idea is known, is a shorthand
that joins two ideas proposed by Einstein in 1935. One involved the paradox implied
by what he called “spooky action at a distance” between quantum particles (the EPR
paradox, named for its authors, Einstein, Boris Podolsky and Nathan Rosen). The other
showed how two black holes could be connected through far reaches of space through
“wormholes” (ER, for Einstein-
When Einstein, Podolsky and Rosen published their seminal paper pointing out puzzling
features of what we now call entanglement, The New York Times treated it as front-
But if ER = EPR is correct, the ideas aren’t disconnected — they’re two manifestations
of the same thing. And this underlying connectedness would form the foundation of
Not everyone’s buying it, of course (nor should they; the idea is in “its infancy,” said Susskind). Joe Polchinski, a researcher at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, whose own stunning paradox about firewalls in the throats of black holes triggered the latest advances, is cautious, but intrigued. “I don’t know where it’s going,” he said, “but it’s a fun time right now.”
The Black Hole Wars
The road that led to ER = EPR is a Möbius strip of tangled twists and turns that folds back on itself, like a drawing by M.C. Escher.
A fair place to start might be quantum entanglement. If two quantum particles are entangled, they become, in effect, two parts of a single unit. What happens to one entangled particle happens to the other, no matter how far apart they are.
Juan Maldacena at the Institute for Advanced Study in Princeton, N.J.
Maldacena sometimes uses a pair of gloves as an analogy: If you come upon the right-
Entanglement played a key role in Stephen Hawking’s 1974 discovery that black holes
could evaporate. This, too, involved entangled pairs of particles. Throughout space,
But the rules of quantum mechanics forbid the complete destruction of information. (Hopelessly scrambling information is another story, which is why documents can be burned and hard drives smashed. There’s nothing in the laws of physics that prevents the information lost in a book’s smoke and ashes from being reconstructed, at least in principle.) So the question became: Would the information that originally went into the black hole just get scrambled? Or would it be truly lost? The arguments set off what Susskind called the “black hole wars,” which have generated enough stories to fill many books. (Susskind’s was subtitled “My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics.”)
Eventually Susskind — in a discovery that shocked even him — realized (with Gerard
’t Hooft) that all the information that fell down the hole was actually trapped on
the black hole’s two-
Susskind continued to work on the idea with Maldacena, whom Susskind calls “the master,” and others. Holography began to be used not just to understand black holes, but any region of space that can be described by its boundary. Over the past decade or so, the seemingly crazy idea that space is a kind of hologram has become rather humdrum, a tool of modern physics used in everything from cosmology to condensed matter. “One of the things that happen to scientific ideas is they often go from wild conjecture to reasonable conjecture to working tools,” Susskind said. “It’s gotten routine.”
Holography was concerned with what happens on boundaries, including black hole horizons. That left open the question of what goes on in the interiors, said Susskind, and answers to that “were all over the map.” After all, since no information could ever escape from inside a black hole’s horizon, the laws of physics prevented scientists from ever directly testing what was going on inside.
Then in 2012 Polchinski, along with Ahmed Almheiri, Donald Marolf andJames Sully, all of them at the time at Santa Barbara, came up with an insight so startling it basically said to physicists: Hold everything. We know nothing.
Scaling the Firewall
Here’s the heart of their argument: If a black hole’s event horizon is a smooth, seemingly ordinary place, as relativity predicts (the authors call this the “no drama” condition), the particles coming out of the black hole must be entangled with particles falling into the black hole. Yet for information not to be lost, the particles coming out of the black hole must also be entangled with particles that left long ago and are now scattered about in a fog of Hawking radiation. That’s one too many kinds of entanglements, the AMPS authors realized. One of them would have to go.
The reason is that maximum entanglements have to be monogamous, existing between
just two particles. Two maximum entanglements at once — quantum polygamy — simply
cannot happen, which suggests that the smooth, continuous space-
The AMPS paper became a “real trigger,” saidStephen Shenker, a physicist at Stanford, and “cast in sharp relief” just how much was not understood. Of course, physicists love such paradoxes, because they’re fertile ground for discovery.
Both Susskind and Maldacena got on it immediately. They’d been thinking about entanglement
and wormholes, and both were inspired by the work of Mark Van Raamsdonk, a physicist
at the University of British Columbia in Vancouver, who had conducted a pivotal thought
experiment suggesting that entanglement and space-
“Then one day,” said Susskind, “Juan sent me a very cryptic message that contained the equation ER = EPR. I instantly saw what he was getting at, and from there we went back and forth expanding the idea.”
Their investigations, which they presented in a 2013 paper, “Cool Horizons for Entangled
Black Holes,” argued for a kind of entanglement they said the AMPS authors had overlooked
— the one that “hooks space together,” according to Susskind. AMPS assumed that the
parts of space inside and outside of the event horizon were independent. But Susskind
and Maldacena suggest that, in fact, particles on either side of the border could
be connected by a wormhole. The ER = EPR entanglement could “kind of get around the
apparent paradox,” said Van Raamsdonk. The paper contained a graphic that some refer
The ER = EPR idea posits that entangled particles inside and outside of a black hole’s event horizon are connected via wormholes.
In other words, there was no need for an entanglement that would create a kink in
the smooth surface of the black hole’s throat. The particles still inside the hole
would be directly connected to particles that left long ago. No need to pass through
the horizon, no need to pass Go. The particles on the inside and the far-
Holes in the Wormhole
No one is sure yet whether ER = EPR will solve the firewall problem. John Preskill, a physicist at the California Institute of Technology in Pasadena, reminded readers of Quantum Frontiers, the blog for Caltech’s Institute for Quantum Information and Matter, that sometimes physicists rely on their “sense of smell” to sniff out which theories have promise. “At first whiff,” he wrote, “ER = EPR may smell fresh and sweet, but it will have to ripen on the shelf for a while.”
Whatever happens, the correspondence between entangled quantum particles and the
geometry of smoothly warped space-
To be sure, ER = EPR does not yet apply to just any kind of space, or any kind of
entanglement. It takes a special type of entanglement and a special type of wormhole.
“Lenny and Juan are completely aware of this,” said Marolf, who recently co-
Like Polchinski and others, Marolf worries that ER = EPR modifies standard quantum mechanics. “A lot of people are really interested in the ER = EPR conjecture,” said Marolf. “But there’s a sense that no one but Lenny and Juan really understand what it is.” Still, “it’s an interesting time to be in the field.”
Clarification on April 27, 2015: The article has been altered to clarify that only maximally entangled particles have to have monogamous entanglements.
Part two of this series, exploring the details of how entanglement could construct space-