Entangled universe: Could wormholes hold the cosmos together?

Weird wormholes through space-time might stitch the cosmos together and make reality real

Weird connections through space-time might make reality real, giving 
us a promising new route to a theory of everything

IT WAS a cryptic email that Juan Maldacena pinged across the US to 
fellow physicist Leonard Susskind back in 2013. At its heart lay a 
single equation: “ER = EPR”. The message clicked with its recipient. 
“I instantly knew what he was getting at,” says Susskind. “We both 
got quite excited.”

Excited, because that one equation promises to forge a connection 
between two very different bits of physics first investigated by 
Albert Einstein almost 80 years ago. Excited, because it could help 
resolve paradoxes swirling around those most befuddling of cosmic 
objects, black holes, and perhaps provide a route to a unified 
theory of physics. Excited, because it might even answer one of the 
most fundamental questions of all: what is reality made of?

The origins of the story lie precisely a century ago. In November 
1915, Einstein presented the final form of his revolutionary theory 
of gravity to the Prussian Academy of Sciences in Berlin. The 
general theory of relativity overturned notions of gravity that 
stretched back as far as Isaac Newton’s day. It said that everything 
that happens in the cosmos at large — be it an apple falling from a 
tree on Earth or the distant whirling of a cluster of galaxies — 
happens because stuff follows invisible contortions in space and 
time that are caused by the presence of other stuff. Gravity follows 
from the geometry of a warped space-time.

In the past century, general relativity has never failed an 
experimental test. Yet the suspicion has grown that it is missing 
something (New Scientist, 10 October, p 29). The theory describes 
space-time as a malleable yet smooth and featureless backdrop to 
reality. Problems start when a great agglomeration of matter folds 
this cosmic fabric so tightly that a black hole singularity arises — 
an object with a gravitational pull so great that nothing can 
escape.

Black holes are a prediction from the earliest days of general 
relativity. But in the 1970s, physicists Jacob Bekenstein and 
Stephen Hawking derived a strange result about them: black holes 
have a temperature, and hence a property called entropy. This takes 
us into the realms of quantum theory where everything, be it forces 
or matter, comes in discrete chunks. Entropy measures how many ways 
you can organise a system’s various constituents — the arrangement 
of atoms in a gas, for example. The greater the number of possible 
configurations, the higher the entropy.

Hole in the theory

But if a black hole is just an extreme scrunching of smooth 
space-time, it should have no substructure, and thus no entropy. For 
Susskind, of Stanford University in California, this contradiction 
points to a hole in Einstein’s theory. “We know that general 
relativity is incomplete,” he says. “Its inability to account for 
the entropy of black holes is probably the most obvious 
incompleteness of the theory.”

That’s a turn up for the books. In his lifetime, Einstein levelled a 
similar charge at quantum theory. In May 1935, the New York Times 
ran a story with the headline “Einstein Attacks Quantum Theory”, 
reporting on a paper Einstein had written with Boris Podolsky and 
Nathan Rosen. It brought to light a weird property of the quantum 
world in which two particles could instantly influence each other, 
even if they were at opposite ends of the universe. In Einstein’s 
view, this “spooky action at a distance” — quantum entanglement, as 
it became known — was preposterous. It was a clear sign there was 
something missing from the quantum description of reality.

But quantum theory has breezed through even more precise 
experimental tests than those devised for general relativity. And it 
is the very property that Einstein discovered — entanglement — that 
continues to expose the contradictions between the two theories. 
Allowing quantum entanglement and general relativity to cohabit in 
the contorted space-time around black holes yields unpleasant and 
unsustainable consequences. For example, information seems to be 
destroyed — an impossibility according to quantum physics — or the 
black hole becomes surrounded by a blazing “firewall” of energetic 
particles (see “Paradox regained”).

So we need some way to square the two schools of thought — to 
quantise space-time and form a quantum theory of gravity. Susskind 
and Maldacena, who works at the Institute of Advanced Studies in 
Princeton, have long been leading lights in perhaps the most 
promising field with this aim: string theory. It replaces the 
point-like particles of current quantum theories with wiggling 
strings of infinitesimal size, and suggests space-time has a grainy 
substructure — you can’t keep chopping it indefinitely into smaller 
and smaller pieces.

But if string theory does hold the answer, it’s well hidden. The 
theory has more than 10⁵⁰⁰ solutions, each describing a different 
sort of universe — making it nigh-on impossible to find the one 
solution that corresponds to the geometrically flat, expanding 
space-time filled with the exact complement of fundamental particles 
we observe around us.

A startling insight from Maldacena in 1997 gave new hope. He 
conjectured that string theory equations describing gravity in some 
volume of space-time were just the same as a set of quantum 
equations describing the surface of that volume. If you could solve 
the surface equations, you could get a viable theory describing 
gravity inside.

This “Maldacena duality” was a bold leap — but physicists found that 
it held. “The funny thing was that it was not proven, and it was 
difficult to even understand why this was happening,” says theorist 
Mark Van Raamsdonk of the University of British Columbia in 
Vancouver, Canada. “It was very mysterious.”

In 2001, Maldacena himself provided an intriguing example, going 
back to a paper written by — you guessed it — Einstein, again with 
Rosen, and again in 1935. This one exposed another peculiarity of 
black holes. It showed how something that looked like two separate 
black holes from the outside might be connected on the inside. This 
interior connection formed a shortcut through space-time, and came 
to be known as an Einstein-Rosen bridge — or in common parlance, a 
wormhole.

Quantum chewing gum

The really odd thing, though, was that Maldacena’s duality showed 
that such a wormhole would only form if the outsides of the black 
holes were quantum-entangled.

By 2009, the underlying mathematics was sufficiently well developed 
for Van Raamsdonk to explore further. Entanglement is not an on/off 
thing — it can exist in varying degrees. So what would happen if you 
were to slowly reduce the amount of entanglement between the black 
holes’ surfaces to nothing? The answer was rather like pulling at 
two ends of a piece of chewing gum. “The two sides get further 
apart, and what’s connecting them is this really thin piece of gum, 
and eventually it snaps,” he says. The wormhole becomes thinner 
until it breaks, and you have two unconnected bits of space-time 
(see diagram). Reverse the process — increase the entanglement — and 
the wormhole starts to form again.

It took a few more years for the penny to finally drop in 
Maldacena’s mind, and for him to make the suggestion laid out in 
that excited email. ER = EPR. ER — the paper Einstein wrote with 
Rosen in 1935 introducing the concept of wormholes. EPR — the paper 
he wrote with Podolsky and Rosen the same year introducing the 
concept of entanglement. What if, asked Maldacena, wormholes and 
entanglement are in fact two sides of the same coin: the same 
physics in two different guises?

The immediate attraction was that the principle seemed to get rid of 
those pesky paradoxes involving firewalls around black holes (see 
“Paradox lost”). But it also provided some form of explanation for 
the phenomenon Van Raamsdonk’s work had exposed, in which space-time 
in the form of wormholes could be created and destroyed simply by 
tweaking the amount of entanglement.

“It’s pointing to a statement that is really quite dramatic,” says 
Van Raamsdonk. “Space-time is really just some geometrical 
manifestation of entanglement.” Maldacena comes to the same 
conclusion. “There is a very close connection between quantum 
mechanics and space-time,” he says. “The continuity of space-time, 
which seems to be something very solid, could come from the ghostly 
properties of entanglement.” Susskind speculates further. Quantum 
entanglement is a form of information, and so “space-time is a 
manifestation of quantum information”, he says.

Heady stuff. But does that really mean that when quantum 
entanglement exists between two particles — as can easily be made to 
happen, say between photons in a lab experiment — they are connected 
by a microscopic wormhole? Or that we live on a backdrop that is 
nothing more than the 1s and 0s of entangled information?

The short answer is we don’t know. One very big caveat is that all 
of the work linking entanglement with space-time so far has been 
done with a space-time that isn’t expanding. Van Raamsdonk and 
others are working to extend the results to the sort of expanding, 
accelerating space-time that makes our cosmos.

But for those involved, this is the most positive lead yet towards a 
theory of quantum gravity that can unify the forces of nature. The 
ER = EPR principle is something “that a theory of quantum gravity 
should obey”, says Maldacena. Susskind thinks so too. “We are sure 
that these things are going to be part of the final story,” he says. 
“But I don’t think we have a clear picture of what that final story 
is yet.”

Others are less convinced. Joe Polchinski and Don Marolf are 
physicists at the University of California, Santa Barbara, and part 
of the team that exposed the black hole firewall paradox. Polchinski 
is concerned that the ER = EPR idea will end up modifying a central 
principle of quantum theory, known as superposition. Exemplified by 
Schrödinger’s cat, this principle explains that a quantum system can 
exist in two different states at the same time. When quantum objects 
become entangled, they also enter a superposition.

At first glance, the ER = EPR hypothesis would mean quantum systems 
that become entangled, and therefore enter a superposition, suddenly 
gain a wormhole — a conjuring trick the superposition principle 
doesn’t obviously allow. That’s problematic, says Polchinski. 
“Quantum mechanics is weird, but it works,” he says. “When you give 
up superposition, it’s just weird.”

Still, he remains open to the eventuality. “In the history of 
science, things that seemed absolute in many important cases have 
turned out to be not absolute,” he says — Newton’s law of 
gravitation, for example. “Maybe superposition is one of them.” 
Maldacena says that it’s too early to say if their work is 
threatening the superposition principle, because the mathematics 
hasn’t been worked out in detail.

Marolf for his part isn’t convinced the ER = EPR equality works in 
all circumstances: Susskind and Maldacena have shown how to avoid 
the firewall only for a particular entangled state of black holes. 
“You might think that it shows how to get out of the [firewall] 
paradox for any highly entangled state, but that’s not true,” says 
Marolf.

Given that Einstein developed the ideas of both wormholes and 
entanglement, one can only wonder what he would have made of it all. 
“My guess is that the old Einstein would have said poppycock,” says 
Susskind — after all, Einstein spent much of his later years arguing 
for a hidden reality that wasn’t subject to the vagaries of quantum 
mechanics. “But the young Einstein apparently had a much more 
flexible mind. My guess [is] that the young Einstein would have 
embraced these ideas, loved them.”

Paradox regained: The black hole problem

In the 1970s, Stephen Hawking showed that black holes emit 
radiation. The mechanism has to do with quantum mechanics, which 
allows pairs of quantum-entangled particles to spontaneously pop 
into existence. When this happens near a black hole’s event horizon, 
one particle may travel outwards, while the other goes towards the 
black hole. The result is a steady stream of outgoing particles, 
called Hawking radiation.

If no new matter falls into the black hole, this emission means the 
black hole will eventually evaporate. But matter is information, and 
in quantum theory information is sacrosanct: it can never be 
destroyed. So if a black hole evaporates, what happens to the 
matter, and therefore information, that fell into it?

One possible solution to this “black hole information loss paradox” 
is the idea that information escapes with the Hawking radiation. But 
in 2012, Joseph Polchinski and Don Marolf of the University of 
California, Santa Barbara, and colleagues showed this option creates 
other problems. General relativity demands that the space-time 
around a black hole’s horizon should be smooth and featureless. It 
turns out that for this to be the case and for information not to be 
lost, a Hawking particle on its way in would have to be entangled 
with all other Hawking particles that left the black hole at all 
earlier times, rather than just its partner outside the horizon.

This offends a fundamental quantum rule known as “monogamy of 
entanglement” — that a quantum particle can only ever be fully 
entangled with one particle at a time. But if you break the 
polyamorous entanglement of Hawking particles, an energetic 
“firewall” of radiation forms at the event horizon. That, 
unfortunately, goes against the tenets of general relativity. 
Paradox preserved.

Paradox lost: the black hole solution

The main problem with the black hole paradox is the idea of quantum 
monogamy — a particle can only be entangled with one other particle 
at a time (see “Paradox regained”). This means that three quantum 
systems — say a particle inside a black hole’s event horizon, a 
particle outside it, and a third far, far away — can’t all be 
entangled at the same time.

But physicists Juan Maldacena and Leonard Susskind argue that this 
can be resolved if the particles just inside the horizon and the 
particles far away are connected via a wormhole. When a wormhole 
connects two objects, one must lie to the future of the other.

So, although these two particles might be entangled, their 
entanglement doesn’t necessarily conflict with the entanglement of 
the particle inside the horizon and its immediate partner outside 
the horizon — because they aren’t all happening at the same time. 
Uncomfortable apparitions such as blazing firewalls at the event 
horizon disappear. Paradox removed.

This article appeared in print under the headline “Entangled 
universe”

By Anil Ananthaswamy

Anil Ananthaswamy is a consultant for New Scientist