Friday, January 22, 2016

Physics of Reality – 6: Entanglement, Wormholes, and Firewalls by Charles Phelan

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:


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.

Einstein and his colleagues did not link the two concepts, but Susskind and Maldacena did. The idea is that entangled particles (EPR) are connected by wormhole bridges with their counterparts outside the event horizon (ER). One of the their key arguments was that the AMPS authors had assumed there could be no connection between space inside and outside the event horizon. The ER = EPR solution provides for particles inside the event horizon to remain entangled with their counterparts in the cloud of Hawking radiation particles previously leaving the black hole. Thus there is no need to break entanglement at the event horizon, and therefore no firewall. This is why Susskind and Maldacena used the phrase “cool horizons” in the title of their paper. ER = EPR preserves the “No Drama” interpretation of BHC, Alice feels nothing at the event horizon, and she still turns into spaghetti when she hits the singularity later.

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! 

( QM, 2015)

(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).