The last couple of weeks, I’ve been discussing the potentially destructive implications of Hawking radiation, the mechanism by which black holes slowly decay. One of the most pressing implications is the information paradox. A core tenet of quantum physics, the conservation of quantum information, demands that quantum information is not ever destroyed or created. But Hawking radiation seems to defy this rule. Information that enters a black hole becomes irretrievable, but it’s not destroyed. But, when a black hole evaporates into random thermal radiation, what happens to that information? One of the most popular theories involves all the information inside a black hole being encoded on the event horizon surface. Some quality of this information may be imparted onto the Hawking radiation leaving the black hole, thus returning the information back into the accessible universe. The shocking implication of this theory is that the entire volume of the black hole (a 3-dimensional object) is encoded onto a 2-dimensional surface—like a hologram. If this is truly how information behaves inside a black hole, then it could mean that the universe itself is also a hologram.
When you cross the event horizon of a black hole, reality fragments in two separate perspectives. From your perspective, your body continues into the volume of the black hole, toward the singularity at the center. But from an outsider’s perspective, your body is smeared over the surface of the black hole’s event horizon. It looks as if the black hole is a physical black sphere, and when you impact it, the strong gravity flattens you onto its surface—like a squashed bug. This would explain why the outside observer witnesses your body stretched and distorted over the surface.
Over time, the image of your body fades, as all of the light reflecting off it cannot escape the black hole to reach the observer. But does that actually mean that your body doesn’t exist on the surface anymore? You may think that the answer to that question is obvious. You are inside the black hole, so it only stands to reason that you cannot also be at the surface of the black hole. In quantum field theory, this concept is expressed by the no-cloning theorem. The no-cloning theorem is an extension of the conservation of quantum information. If information is duplicated, then the universe as a whole has increased in total information, which can’t happen. The physics of this is tricky, but it really just boils down to this: you can’t exist in two places at the same time. (Note this doesn’t actually preclude physical cloning. You could still clone yourself but, if you did, your clone wouldn’t really be you—it would be a physical recreation of you made from other molecules that already existed in the universe.)
So how is it possible that two seemingly disconnected versions of you exist both inside and outside the black hole? Does this break the no-cloning theorem? Or is the version of you on the surface simply a mirage? The key to answering this question turns out to be time. Above, when I explained the no-cloning theorem, I said that it is impossible for you to exist in two different places at the same time. But it turns out that the exterior and the interior of the black hole exist on different timescales. The surface version of you and the interior version of you existed at the same time and place at the moment you crossed the event horizon—they were both you. But, after crossing this boundary, these two versions split into two different timelines.
Outside the black hole, time moves forward normally. Inside the black hole, time is essentially frozen. The two timelines are separated by the surface of the event horizon. Time freezes the moment you cross the event horizon, but for an outside observer, that moment is always in the future. For you, inside the black hole, the moment you crossed the event horizon is now—but it’s also in the past and it’s also in the future (trippy, I know, but stay with me). Inside a black hole, time and space switch places, meaning space always moves forward (towards the singularity) but you have free rein to move any direction in time. It doesn’t matter if you go a hundred years in the past or a million years in the future, you will always be moving towards the singularity at a fixed speed. This timeline is completely disconnected from the timeline on the surface, where you are currently still frozen on the lip of the horizon (as far as any observer can comprehend).
This concept, called black hole complementarity, was first suggested by Leonard Susskind, a string theory physicist from Stanford University. Black hole complementarity states that the interior and exterior states of a black hole cannot both be known simultaneously. From the outside of the black hole, the observer can see you frozen and smeared on the surface of the event horizon. Over time, the image of you (created from the light reflecting off your body) passes out of the visible spectrum, but the information is still there—printed on the surface. Physicists believe that Hawking radiation carries bits of the black hole’s surface information (including the information from your body) away from the black hole and back into the universe. Thus, information is conserved.
Physicists still don’t know for sure how information is encoded in Hawking radiation, but most physicists agree that the information must escape with the radiation in some way. But, if it’s true that the information inside a black hole is encoded on the surface, then that leads to an even more startling conclusion: the holographic principle. The holographic principle is a concept, also introduced by Leonard Suskind, that explains how the 3D volume of a black hole can be encoded on a 2D surface (the same way that a 3D Princess Leia hologram is encoded on the 2D surface of R2-D2’s projector). If the holographic principle actually describes the behavior of information inside a black hole, then it might imply that our entire 3D universe (and everything in it) is projected off of some distant 2D boundary. And mathematically, we wouldn’t necessarily be able to determine which universe we are in . . .
Check back next week, when I’ll delve deeper into the mysteries and mathematics of the holographic principle. For now, comment below or email me at contact@anyonecanscience.com to let me know what you think about this week’s blog post and tell me what sorts of topics you want me to cover in the future. And subscribe below for weekly science posts sent straight to your email!
