This article was previously published in two parts on the Beyond Advaita blog maintained by our Dr. Ramesam Vemuri, in continuation of an ongoing series of articles exploring the relevance to Advaita of some of the latest research in theoretical physics. Science is converging to a view that no description of reality can be complete without the observer, and that so-called “objective reality” is really more of a holographic illusion than anything truly solid or substantial. Today’s scientists are busy trying to tease apart Maya’s tricks to see how this illusion works. Leonard Susskind’s theory of Black Hole Complementarity (BHC) — the topic of this article — provides a good example of this driving curiosity in action.

BHC is another breakthrough that forces us to dispense with any view of a single objectively real universe, and demonstrates yet again that “reality” is observer-dependent. Susskind developed BHC during his decades-long battle with Stephen Hawking. The disagreement was about a quantum loophole identified by Hawking, which became known as the paradox of information loss in black holes, and Susskind’s theory was his proposed solution to the paradox.

Before we get further into the physics of black holes and information loss, let’s briefly touch on a few points from Advaita. The Vedas speak of in terms of vast cosmological time scales, immense epochs (kalpas — each but one day in the life of the creator Brahma), and the entire cycle of creation, preservation, and dissolution, sRiShTi-sthiti-laya.

If we are to take seriously the Advaita teachings on the accrual of puNya and pApa (i.e., karmic merit or demerit) to the jIva, then we can legitimately ask: What happens to the karmic “information” during the period of dissolution between kalpas? Does it somehow get “recorded” and carried over to the next cycle? Or does it get destroyed in the pralaya phase? Asking such questions is essentially no different from asking whether information is conserved or destroyed when it enters a black hole.

Just what is a black hole anyway? When a star collapses at the end of its life, completely spent of fuel and no longer able to produce fusion, it may shrink by orders of magnitude and become a white or brown dwarf or a neutron star, depending on its original size. Given sufficiently large mass, a star will collapse all the way to what is called a singularity, a point where the equations of physics break down and begin producing infinities. (Perhaps we can think of pralaya as a form of singularity?)

Physicists call this singularity a black hole, simply because its gravitational force is so strong that even light cannot escape. No form of matter or energy that falls into the clutches of a black hole can ever get free again. To get out of Earth’s gravity well and into orbit, one must reach velocities exceeding 40,000 kilometers per hour, something we do routinely with chemical rocketry. With a black hole, even the speed of light is insufficient. For all practical purposes, the escape velocity of a black hole is infinite. There are no rockets, chemical, nuclear, or otherwise, that can possibly escape a black hole. (Sorry, Star Trek fans!)

The point at which an object falling toward the singularity passes the point of no return is called the event horizon. Anything passing through the event horizon is doomed to eventually hit the singularity, where the force of gravity is so strong that a human being gets stretched into a piece of spaghetti thousands of miles long, most certainly not an enjoyable experience!

Dr. John Wheeler, a key 20th century figure in theoretical physics, and mentioned previously in this series, was also a pioneer in the study of black holes. In fact, he is the physicist who originally coined that term. One of Wheeler’s early quips was, “Black holes have no hair.” By “no hair,” he meant they are completely smooth and featureless, without any apparent irregularities, essentially all the same as one another except for size. This, of course, was just Wheeler’s poetic phrasing for what the equations of General Relativity were telling him about the structure of black holes.

Along came Stephen Hawking, who proved that black holes are not entirely bald after all. Hawking discovered that there was more going on with black holes than had previously been assumed, and through a rigorous mathematical analysis he showed that they gradually evaporate and fade away to nothing. The reason for this evaporation has to do with the quantum entanglement of virtual particle pairs, with one part of the entangled pair falling inside the event horizon and the other outside, i.e., “hair.” Theoretically, via this quantum mechanical process, photons are emitted as Hawking radiation, causing the black hole to eventually evaporate and then completely vanish.

Hawking’s analysis was rigorous and solid, and it left physicists like Leonard Susskind scratching their heads. If Hawking was correct, then objects falling into the black hole would carry information beyond the event horizon and into the singularity where it could never be recovered. That, of and by itself, does not represent a problem for physics. However, if the black hole were to fully evaporate later, then the information would be lost forever. This is a gross violation of the most fundamental understanding of physics, which firmly denies the possibility of any such information loss. It would be the equivalent of taking a safe and locking some valuables inside it, only to then watch the safe evaporate and vanish, along with the valuables. It seemed more a magic trick than science!

Many physicists were intuitively convinced there was something wrong with Hawking’s approach, but a solution remained elusive for decades. What it took to resolve the paradox of information loss was a series of advances in the physics of black hole entropy and String Theory. Combining several such breakthroughs, Leonard Susskind’s proposed solution was Black Hole Complementarity.

What BHC states is that information falling into a black hole is reflected off a “stretched” hot horizon, and could theoretically be recovered from the Hawking radiation, AND that information also passes the event horizon and is eventually destroyed when it reaches the singularity. The catch is that both observations cannot be made at the same time, meaning that an observer outside the event horizon could confirm that information is reflected off the event horizon, and an observer inside could confirm information loss, but never both at the same time.

Stephen Hawking ultimately conceded the bets he had made about information loss in black holes. It had been proven to his satisfaction that the information going into a black hole could come back out via the evaporative Hawking radiation itself, rather than being lost permanently as he originally proposed. By 2008, Leonard Susskind published his book, “The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics.” The battle was over, but Hawking’s brilliant challenge had stimulated an entire new wave of research leading to some truly astounding results.

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.

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 Quantum Mechanics.) 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.

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 Black Hole Complementarity make essentially the same point some Advaita thinkers had already been making for centuries.