**Physics of Reality – 6**

**Quantum Entanglement, Wormholes, and the Firewall Problem**

**by Charles Phelan**

*Intro about Charles at*Part 4]

Part 1

In the previous two articles, we reviewed Leonard Susskind's
mind-blowing theory of Black Hole Complementarity (BHC), which tells us that
two completely contradictory observations are both true for their respective
observers. We also noted that BHC is purely theoretical, not yet settled
science. In this article, we'll examine a recent challenge to the theory of BHC
called the “firewall paradox,” as well as Susskind's brilliant response to it.

Let's start with a quick review on Black Hole Complementarity. At
the center of a black hole is a singularity, where matter has been squeezed to
an infinitesimally small point. The gravitational force of a singularity is so
strong that nothing can escape it, not even light itself. Hence the phrase
“black hole,” originally coined by physicist John Wheeler. The point of no
return where an object falling toward the black hole can no longer escape is
called the event horizon.

According to BHC as well as Einstein's Equivalence Principle, our
intrepid explorer Alice will experience nothing unusual as she passes through
the event horizon. She will only become human “spaghetti” when she hits the
singularity later, which could take anywhere from less than a second to many
years, depending on the size of the black hole.

Meanwhile, Alice's partner Bob has remained behind to observe what
happens to her, and he sees a completely different picture. Bob sees Alice move
toward the event horizon but never reach it due to the time dilation effect
described by Einstein's Theory of Relativity. Both stories are true for their
respective observers, with the catch being that they cannot communicate with
one another (i.e. pass information back and forth).

This lands us squarely in an observer-dependent universe, and
leads to reality-bending ambiguities in the bargain. For example, a particle
may be either a microscopically small object or a smeared out probability
distribution that covers the entire event horizon of an enormous black hole,
with both views being “true” depending on the observer's frame of reference!

Clearly, if BHC is valid, then it forces us to change our thinking
about an objective reality “out there” that is reliably the same for all
observers. While Susskind's BHC proposal was initially not widely accepted,
after advances in String Theory and other related subfields of physics, it did
come to be accepted by most physicists. There remain vocal critics, of course,
and papers challenging the math and logic behind Susskind's theory.

The most significant attack came in 2012, when four physicists
(Almheiri, Marolf, Polchinski, and Sully) published a paper proposing a
solution to a perceived inconsistency in BHC theory. The solution proposed was
called the “firewall phenomenon,” and the proposal became known as the “AMPS
firewall,” AMPS being an acronym for the authors' last names.

In order to understand the AMPS firewall, we first need to discuss
the concept of

*quantum entanglement*. When pairs of particles are entangled, there can be no independent description of the separate particles. Essentially, quantum entanglement tells us that such particle pairs are a*whole*and must be described as a*system*, rather than as separate entities. While this may not seem like such a big deal, it leads to some astonishing conclusions that completely defy logic. For example, measurements on entangled particle pairs are always correlated, no matter how far apart physically the pairs have been separated. If we measure the “spin” property of a particle, it's counterpart will always measure with an opposite spin, even if the entangled particles are split to opposite sides of the universe before they are measured!
At first glance, quantum entanglement appears to violate
Einstein's discovery that nothing whatsoever can exceed the speed of light. If information
is somehow transmitted between a pair of entangled particles instantaneously,
that would constitute a violation of General Relativity, a theory that has been
proven over and over again via astronomical and other physical observations. In
fact, Einstein, together with his colleagues Boris Podolosky and Nathan Rosen,
published a paper critical of QM theory that became known as the EPR paradox.
The popular phrase used by Einstein to describe such instantaneous transmission
was “spooky action at a distance.”

Another term for this is nonlocality, where particles are
considered as a whole rather than a discrete system of two separate entities.
It's a proven fact of physics that Einstein was wrong about this issue.
Nonlocality has been successfully demonstrated in entangled particle pairs,
beginning with the ground-breaking experimental work of Alain Aspect in 1982,
and continuing with many independent confirmations since. At least within the
context of quantum particle pairs, the universe is indeed nonlocal.

Is nonlocality not suggestive of nonduality as conceived by
Advaita? To be precise, we must note that “quantum entanglement” refers
specifically to the description of a theoretical particle-pair, and we should
not leap to conclusions that “QM proves nonduality.” That caveat
notwithstanding, nonlocality does look like one of Maya's magic tricks. Measure
the spin of a particle, and its companion will collapse to the other spin
state, even if the two particles are in a condition of space-like separation
that would entail transmission of information faster than light-speed. Due to
QM being based on probabilities rather than certainties, all this happens
without violating General Relativity. Nothing can travel faster than light, and
yet particle pairs are apparently linked across vast stretches of spacetime.
Such phenomena appear to show us the "footprints" of nonduality,
stamped squarely on such paradoxes as wave-particle duality and quantum
nonlocality.

Returning to the AMPS paper's rebuttal to Black Hole
Complementarity, the authors showed that there is a potential flaw in the
theory. To understand this criticism, we first need to know that highly
entangled particle pairs may only be party to “monogamous” entanglements. This
means that one particle is entangled with another, and cannot be the subject of
multiple entanglements. Yet there are at least two potential entanglements if
all the assumptions inherent in BHC are maintained.

Recalling our previous discussion on information loss in black
holes, we know information is reflected off the event horizon in the form of
Hawking radiation. So we have information falling into the black hole entangled
with information coming out of it, and also entangled with prior Hawking
radiation particles, leaving one entanglement too many. Their solution
demonstrated that a break of entanglement at the event horizon would lead to a
buildup of energy, hence the term “firewall.”

The AMPS paper skillfully argued that some key assumption must
give way to resolve the inconsistency of multiple entanglements. Joe
Polchinski, the “P” in AMPS, argued that the Equivalence Principle must fail at
the event horizon, and that instead of “No Drama” scenario experienced by
Alice, she will burn up when she hits the wall of energy there. The AMPS paper,
if correct, would resolve the apparent inconsistency in BHC where Bob and Alice
experience contradictory stories that are both true. And it would do so by
breaking the entanglement of particles at the event horizon. In effect, the
authors were saying there is no “inside” to a black hole, as nothing can get
past the event horizon.

How did Susskind respond? Working with Juan Maldacena, a fellow
physicist from the Institute for Advanced Studies at Princeton, Susskind
published a paper titled, “Cool Horizons for Entangled Black Holes,” in which
he proposed an answer to the firewall problem as proposed by AMPS. The short
version of their response was:

ER = EPR

This means that entangled pairs are linked through

*wormholes*that tunnel*outside*the event horizon, thus resolving the apparent identification of one too many entanglements by AMPS.
The above explanation is very compact, so let's break it down and
elaborate a bit further. We've already touched on EPR above, the
Einstein-Podolsky-Rosen paradox of non-locality, or “spooky action at a
distance.” The “ER” in ER=EPR stands for Einstein-Rosen bridges, and refers to
a paper the two men published about wormholes connecting black holes across
distant regions of space.

As so often happens with cutting edge research, however,
this proposal implies much more significant possibilities than a resolution to
the AMPS firewall paradox. The ER = EPR theory points to a deep connectedness
underlying all of spacetime. We have already seen this with nonlocal
entanglement, and also with the wormhole idea. This approach says that ER and
EPR are two sides of the same phenomenon, the same thing viewed from different
angles. This is precisely why there is an equals sign in the equation! And the
“spooky action at a distance” that gave Einstein fits may actually be what
literally stitches together spacetime. As Maldacena put it, “... the solid and
reliable structure of spacetime is due to the ghostly features of
entanglement.” Further, the long-term physics project of uniting the conflicting
theories of gravity between General Relativity and Quantum Mechanics may be
facilitated by approaches such as ER = EPR.

As we have seen over and over again in these articles,
modern physics is converging to an understanding of reality as
observer-dependent. It seems we are coming closer to a scientific view that
supports the ancient Advaita perspective that “perception creates the world.”
We will never be able to say that science or physics has “proved” nonduality
conclusively, simply because any such knowledge or proof must always be less
than the Whole. But it certainly appears that we are starting to understanding
some of Maya's tricks. Look out to the extremes of the universe, at both macro
and micro levels, and it's possible to notice some of the rough edges. Quantum
foam that allows something to come from nothing, entangled particles,
wormholes, and black holes are some of those edges.

It seems there is no limit to the bizarreness of some
of the new proposals in physics. Could science actually prove that the entire
empirical universe is nothing more than a snake-in-the-rope illusion, as Adi
Shankara advised us 1,200 years ago? Perhaps so!

(To Continue .... Physics of Reality - 7

In the next article, we'll
talk about the Holographic Principle, which says that we are living in a
hologram).

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