Physics of Reality – 5: Black Hole Complementarity by Charles Phelan

Physics of Reality – 5: Black Hole Complementarity 
by Charles Phelan

[Intro about Charles at Part - 4]

Part 1    Part 4

We can better understand just how astounding Black Hole Complementarity is by taking a trip with Bob and Alice to a black hole. Bob and Alice are the favorite veterans of many thought experiments pertaining to extreme physics, and they are also stand-ins for a quantum entangled pair of particles. When they near the black hole, Alice (usually the more brash of the pair) ignores Bob's warnings and jumps down toward the event horizon. Bob remains behind and watches Alice's progress from a safe vantage point far outside the event horizon. 

What do Alice and Bob observe respectively? Alice observes nothing much at all. She crosses the event horizon without even noticing it or feeling anything special. We're in Einstein territory here, with the Equivalence Principle coming into play. (The Equivalence Principle tells us there is no difference between how gravity "feels" compared to an identical force that has nothing to do with gravity, such as constant acceleration. And in the context of freefall, the laws of physics operate as though there is no gravity at all, which directly brings in Special Relativity and its strange effects.) Alice is in free fall, and free fall in a vacuum doesn't feel like motion at all. So she feels nothing special when she moves across the event horizon, passing the point of no return on her way toward the singularity.

Alice most definitely will feel something when she approaches the singularity! However, depending on the mass of the black hole, which determines its radius and therefore the distance between horizon and singularity, that could be anywhere from seconds to days, perhaps even a lifetime for a super massive black hole.

Spaghettification

What does Bob observe? As Alice falls toward the event horizon of the black hole, her voice sounds deeper and deeper until it drops below his range of hearing. Due to the relativistic effects of time dilation, the gap between her signals grows longer and longer. Eventually it takes years for her signals to arrive. Bob could literally wait an infinite time and still not observe Alice actually reach the event horizon! 

Which story is true? Does Alice drop through the event horizon and feel nothing until she hits the singularity later? Or is she trapped in space-time, moving ever more slowly toward the event horizon, taking an infinite amount of time to reach it? Black Hole Complementarity says that both stories are true! For Alice, it's true that she falls through the event horizon and goes on to become human spaghetti later on, and for Bob, it's true that Alice never reaches the event horizon. 

While this seems impossible at first glance, there is an all important catch. They cannot communicate back and forth to confirm both observations at the same time. (There is a technical reason for this restriction, the No Cloning rule of QM.) So we are forced to choose the perspective of one observer or the other, with each story being true relative to its own observer perspective. 

With this background in mind we can more easily understand why Susskind named it Black Hole Complementarity. This is a direct reference to the complementarity proposed by Niels Bohr relative to the paradox of wave-particle duality. He said these two descriptions were the complement of one another, and that both must therefore be accepted as applicable within their respective domains. 

BHC is another kind of complementarity, one that forces us to abandon any notion of a single objective reality that holds valid for both observers at the same time. If BHC is correct, then “reality,” at least as it pertains in the extreme environment of black holes, is observer-dependent. It also forces some very strange conclusions about particles. Here is Susskind himself on how bizarre BHC is:

“To most physicists, especially those who specialized in the General theory of Relativity, Black Hole Complementarity seemed too crazy to be true. It was not that they were uncomfortable with quantum ambiguity; ambiguity at the Planck scale was entirely acceptable. But Black Hole Complementarity was proposing something far more radical. Depending on the state of motion of the observer, an atom might remain a tiny microscopic object, or it might spread out over the entire horizon of an enormous black hole. This degree of ambiguity was too much to swallow. It seemed strange even to me.” Leonard Susskind, The Black Hole War, p. 354.

Recalling the discussion in Part 3 of the series on QBism and the work of Dr. Christopher Fuchs, we saw that some of the deepest paradoxes of Quantum Mechanics may only be resolved by jettisoning any notion of a single objective reality shared by all. The wave function describes the observer, not an objective universe as had been supposed. So it seems we have a convergence to this understanding from multiple researchers at the cutting edge of modern physics. And the consequences are dizzying.

It appears there is no longer any solid ground to stand upon anywhere. Ghostly virtual particles pop into and out of existence on a foam of quantum probabilities, while photons behave as either waves or particles depending on how we measure them. Matter is composed mostly of empty space, yet at the tiniest possible interstices of that “empty space” there may still be information in the form of vibrating one-dimensional strings. It seems the more we learn about our universe, the farther away we get from our everyday view of reality. 

Yet is not this the essence of what Advaita has told us all along? That the measurable objective universe we think we see is actually an illusion born of Ignorance? What can be more mithyA than two contradictory stories being simultaneously true at the same time? According to Advaita, what do we see but nAmarUpa, name and form? What is name and form if not information? The very word “information” breaks down to in-form-ation. If nAmarUpa = information, then physics is describing the apparent “creation.” Science began from the position that it was possible to separate the observer from the observed, but that view has receded and now we see highly suggestive confirmation that “objective reality” is actually personal rather than truly objective. 

Further, the commonsense view of reality is that the creation/universe was there first, and then we came along to see it. In Advaita terms, this would be sRRiShTi dRRiShTi vAda, meaning creation first and then the perception of it. But this theory is later sublated and replaced with dRRiShTI sRRiShTi vAda, giving perception the priority. There is only an apparent creation there because we are perceiving it, and more, that perception still doesn't make that apparent creation real (anymore than dream perceptions make the dream-world real). 

It seems to this writer that a theory like BHC points clearly to dRRiShTI sRRiShTi vAda rather than sRRiShTi dRRiShTi vAda, i.e., perception creating the apparent world rather than the other way around. But we must note that BHC is not settled science yet, and may never be. After all, it is impossible to directly conduct experiments on black holes, so the entire discussion must take place via mathematics and thought experiments. 

Further, it is all theoretical -- simply because no actual existing black hole will even start to evaporate until the universe is vastly older than it is today. All extant black holes are still growing and will continue to grow for hundreds of billions, perhaps trillions of years to come. So we are clearly in speculative territory here. Still, it is fascinating to see a theory like BHC make essentially the same point some Advaita thinkers had already been making for centuries. 

(To Continue …….. Physics of Reality – 6)

In the next article, we'll review a recent important challenge to the validity of Black Hole Complementarity and Susskind's amazing response to it, and also explore some other fascinating new ideas at the frontier of physics. 

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