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-time landscape that Einstein described is like
a painting by Salvador Dalí — seamless, unbroken, geometric. But the quantum particles
that occupy this space are more like something from Georges Seurat: pointillist,
discrete, described by probabilities. At their core, the two descriptions contradict
each other. Yet a bold new strain of thinking suggests that quantum correlations
between specks of impressionist paint actually create not just Dalí’s landscape,
but the canvases that both sit on, as well as the three-dimensional space around
them. And Einstein, as he so often does, sits right in the center of it all, still
turning things upside-down from beyond the grave.
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-Rosen bridges). At the time that Einstein put forth
these ideas — and for most of the eight decades since — they were thought to be entirely
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-page
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
all space-time. Quantum entanglement — the action at a distance that so troubled
Einstein — could be creating the “spatial connectivity” that “sews space together,”
according to Leonard Susskind, a physicist at Stanford University and one of the
idea’s main architects. Without these connections, all of space would “atomize,”
according to Juan Maldacena, a physicist at the Institute for Advanced Study in Princeton,
N.J., who developed the idea together with Susskind. “In other words, the solid and
reliable structure of space-time is due to the ghostly features of entanglement,”
he said. What’s more, ER = EPR has the potential to address how gravity fits together
with quantum mechanics.
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
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-handed
glove, you instantaneously know the other is left-handed. There’s nothing spooky
about that. But in the quantum version, both gloves are actually left- and right-handed
(and everything in between) up until the moment you observe them. Spookier still,
the left-handed glove doesn’t become left until you observe the right-handed one
— at which moment both instantly gain a definite handedness.
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,
short-lived “virtual” particles of matter and anti-matter continually pop into and
out of existence. Hawking realized that if one particle fell into a black hole and
the other escaped, the hole would emit radiation, glowing like a dying ember. Given
enough time, the hole would evaporate into nothing, raising the question of what
happened to the information content of the stuff that fell into it.
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-dimensional event horizon, the surface that marks the point
of no return. The horizon encoded everything inside, like a hologram. It was as if
the bits needed to re-create your house and everything in it could fit on the walls.
The information wasn’t lost — it was scrambled and stored out of reach.
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.
The so-called AMPS paper (after its authors’ initials) presented a doozy of an entanglement
paradox — one so stark it implied that black holes might not, in effect, even have
insides, for a “firewall” just inside the horizon would fry anyone or anything attempting
to find out its secrets.
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-time inside the throats
of black holes can’t exist. A break in the entanglement at the horizon would imply
a discontinuity in space, a pileup of energy: the “firewall.”
David Kaplan explores black hole physics and the problem of quantum gravity in this In
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-time are intimately related.
“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
to half-jokingly as the “octopus picture” — with multiple wormholes leading from
the inside of a black hole to Hawking radiation on the outside.
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-out ones
could be considered one and the same, Maldacena explained — like me, myself and I.
The complex “octopus” wormhole would link the interior of the black hole directly
to particles in the long-departed cloud of Hawking radiation.
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.”
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-authored
a paper describing wormholes with more than two ends. ER = EPR works in very specific
situations, he said, but AMPS argues that the firewall presents a much broader challenge.
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-time,
will appear on Tuesday, April 28.