The information is preserved in a daughter universe that splits off from ours—a nice idea, that unfortunately does not yet have a matching physical theory.
Instead of in spatial dimensions, the information is preserved in temporal correlations. The idea can be worked out well within current physics, but contradicts the idea of nature as an entity developing in time.
The information encoded in the three-dimensional space within the black hole is also saved on its two-dimensional boundary surface. This ‘holographic principle’ developed by the physicist Juan Malcadena assumes our universe consists both of a three-dimensional structure and a 2D element. In the 3D universe strings, gravitation and black holes follow the theory of relativity, while in the flat part, elementary particles and their fields obey the laws of quantum physics. All the information in one part is also encoded in the other one, but for a member of one structure the other structure is inaccessible. Thus information evaporating with the black hole would not be lost but is preserved in the 2D structure of the universe.
A so-called firewall develops at the event horizon. This assumes that the particles appearing out of nowhere are entangled. If one of the partners falls into the black hole, the entanglement breaks and energy is released. The information is preserved in this firewall in the form of an entanglement of all the particles. However, this scenario contradicts the general theory of relativity, according to which all gravitational fields are principally identical. Why should something different happen during the fall of an object into a black hole than when a ball falls to the ground?
Wormholes. The firewall around the black hole, which violates the general theory of relativity, could be dispensed with if the black hole itself and all the particles of its Hawking radiation were connected by wormholes—shortcuts through space-time. This idea would allow for particles and anti-particles to remain entangled. The entanglement not only preserves the information—in theory, this information could also be recovered.
The evaporation of the black hole will reach an end at some point. This idea was developed by the Cambridge University physicist George Ellis. A black hole distorts space-time through its gravitation. It is not alone in doing so. The cosmic background radiation, under the influence of the gravitational field, also causes singularities to develop shortly behind the event horizon, which change the structure of space-time. If the virtual particles of the Hawking radiation form close to such a distortion, both slip behind the event horizon. The more a black hole shrinks, the more likely this process becomes—until it happens more often than its opposite and the mass of the black hole stabilizes. Ellis cannot tell exactly why this happens—but regardless of it the black hole would not completely evaporate but approach a certain mass and thus preserve the information inside it.
Recently, the concept of ‘soft hairs’ has been discussed. This is what I used in The Hole. Shortly before the surface of a black hole there would be zero-energy intermediate particles—photons, gravitons, etc., collectively called soft hairs—that, like a matrix, absorbed the information that would otherwise disappear behind the event horizon. The supporters of this theory have shown at least for light particles (photons) that there are an infinite number of storage locations on the surface of a black hole and that the approaching particles can actually excite these soft hairs. For the theory to be completely accepted, they would need evidence that this also works for ‘gravitons,’ whose existence has not yet been proven. Furthermore, this concept cannot explain how the information might return to our reality. Therefore, this part of the solution in The Hole is speculative, but still fits with current scientific discussions.
Would it be so bad if information got lost in a black hole? Haven’t we forgotten things without the universe coming to an end? This is the problem: Information corresponds to order, and a loss of information therefore equates to disorder or entropy. In thermodynamics, an increase in entropy corresponds to a rise in temperature. If black holes destroyed information, they would have to heat up by quintillions of degrees in a short time.
A Journey into a Black Hole
How would it feel to approach a black hole? Science knows surprisingly much about this in theory, even though we are centuries away from making it reality.
It is easiest to show this for an external, immobile observer who comfortably watches someone else undertake this dangerous voyage. The traveler simply becomes slower and slower while nearing the black hole. The light he emits is shifted increasingly toward the red end of the spectrum, making him first look brown and then almost black. From the perspective of the observer he never reaches the event horizon. No matter your patience, you will never be able to watch someone entering the event horizon.
Okay, that was boring! So now let’s get a bit more courageous. We take our spaceship and enter into an orbit around a black hole that is as heavy as possible. We circle it at a constant distance. At first we see a round, black spot surrounded by a strangely distorted disk. The latter is created by the black hole redirecting light coming from behind it. If we take a closer look we can see a narrow strip at the edge of the black spot, where the complete sky is endlessly repeated—as in the case of two mirrors aligned so you see yourself reflected over and over, ad infinitum.
Now we decrease our orbital radius. In doing so, the ship has to move faster and faster so it won’t fall into the black hole. The black spot increases in size. Initially, it just fills the foreground, and if you turn your head right or left you still see normal sky. But a little bit later—for a stellar hole at a distance of 45 kilometers, meaning 15 kilometers from the event horizon—the blackness almost fills your field of vision. If you glance to the right or left, you see the back of your own head! Only behind you is there still a normal universe, that you watch as if you could see infrared radiation. In actuality, the infrared is shifted into the range of visible light.
You have reached the ergosphere—in other metrics called the ‘photon sphere’—where only photons survive. The light emitted by the back of your head has circled the black hole and arrived here again. The closer you get to the event horizon, the smaller the field of view into space, until it shrinks to a tiny spot.
A tip from an expert: Choose the heaviest black hole you can find in order to experience this exciting journey with an acceleration you can stand.
This also applies to our third attempt. Now you are going to be particularly brave, leave the ship in a spacesuit and let yourself fall. Of course you were careful to choose a hole without an accretion disk, as otherwise it might get unpleasantly hot.
Let’s start again far to the outside. Gravitation will make you move faster and faster so that you soon reach 99 percent of the speed of light. With a small black hole there would be the problem of ‘spaghettification,’ sometimes referred to as the ‘noodle effect.’ Forces of different strength are working on your legs, closer to the hole, and your head, which is farther away—your body would be painfully stretched. With a supermassive black hole the forces are greater, but they increase more slowly, so this fate would only occur after you had time to enjoy the view.
Unlike in the previous experiment, the black disk you are hurtling toward remains relatively small. The phenomenon is called ‘aberration.’ You know it from driving a car while snow is falling. The snow seems to come toward you, even though it falls vertically. Here it works the same way. Beams of light that come from above for an observer at rest, will arrive for you from ahead and at an angle. And indeed you will get the impression that the black spot is still innocuously small when you cross the event horizon.
But then it is too late, and there is no turning back. By the way, in free fall you would see the light coming from ahead shifted towards blue, so infrared would become visible light, while from behind red-shifted light reaches you, and you can perceive ultraviolet as visible light. The moment you cross the event horizon you will see a flash. It contains photons from the entire history of the universe, but shifted towards blue. If you had more tim
e, you could record the entire past. However, you would have no chance to tell anyone about it.
No one knows what the inside of a black hole looks like, but your view might be distorted by the strong forces and relativistic effects anyway. Everything you see at first has a kidney shape, then that of a donut wrapped around your hips, and finally that of a narrow cone. At the same time, your presence here causes enormous disturbances of the equilibrium. You will be constantly compressed and stretched. Not even the particles of your body remain, as the four fundamental forces—gravity, light/electromagnetism, weak nuclear force, and strong nuclear force—unite again, and you turn into a kind of primeval soup. Yummy!
The form of the singularity is determined by the type of our black hole. Because it certainly rotates, the singularity has the shape of a ring—otherwise it would be a point. It whirls in front of your eyes like a one-dimensional hoop through the surrounding quantum foam. You peer into it and detect, at unattainable distances, numerous other universes that are so very different from ours.
Now, that last sentence was pure speculation. ‘Detect’ isn’t the appropriate word, because inside the singularity there is neither light, nor gravitation, nor the strong and weak interaction. Everything is like it was shortly before the Big Bang inside the singularity from which our universe developed.
A Universe Inside a Black Hole?
Simple answers—humans have always been looking for them, particularly in difficult times. Whether they are ‘42,’ or ‘God,’ or ‘It’s the government’s fault,’ they have a certain allure that even physicists are not immune to. And one has to admit that physicists have all the right in the world to look for easy answers. Ultimately, their science is getting more complex year by year.
The more that methodical physicists—and the author of this book is one—look at our world, the more dimensions they need to describe it perfectly. First there were three dimensions, while the general theory of relativity already needed four. This sufficed for a few years, until it became clear that neither relativity nor quantum theory could categorize the universe completely.
One way out was the concept of string theories, which originally used 10, then 11, and finally up to 21 dimensions. Of course it is hard to imagine that the additional dimensions are so tightly folded that humans cannot perceive them. There are some mathematical arguments in their favor, though. The fact that quantum theory could be used for calculations had led some people to ignore the drawbacks from imagining things, and they began doing exactly that. ‘Theories’ is the word for such educated imaginings—to be followed by calculating out each of them and then measuring whether prognosis and reality match. The motto in these cases is ‘shut up and calculate.’
But perhaps the reality we can comprehend is only a complex illusion—a holographic projection of a much simpler reality. Scientists have been trying to use this refreshing concept in holographic cosmology, and it plays a role in The Hole. Take a look at the hologram you might have on your credit card. It seems three-dimensional, yet it is flat, stored in two dimensions. If you use VR goggles to walk though impressive virtual landscapes, you see a three-dimensional space, yet it is created in front of your eyes by two flat screens. Generally the ‘holographic principle’ refers to a connection between a spatial structure and its equivalent on a surface.
The principle can be used, for example, to solve the information paradox of black holes mentioned above. If we physicists had a complete description of all the properties of an object at a certain point in time, we would—being physicists—want to find out how it had behaved shortly before that point. However, if this information had been destroyed, finding it would no longer be possible. Unless, that is, the information could somehow have been encoded on the surface of the event horizon. In that case, the prior spatial representations were never anything but holograms.
Perhaps we could imagine the universe as the inside of a giant black hole, on the plane of whose event horizon reality happens. The rest is illusion. In that case, however, there must be a projection mechanism. It has to be possible to turn multidimensional theories into lower-dimensional ones, or vice versa, without losing anything. For a long time this was proven only for negatively curved spaces, but since 2015 it has been known to be possible in our almost flat universe. In 2017, a study in Physical Review Letters went further—a step beyond: Its authors applied various holographic quantum field theories three-dimensionally—i.e. reduced by one dimension—to a simulation of the early universe shortly after the Big Bang, and compared which parameters created which features in the cosmos. And in fact some of the holographic theories work as well in describing reality as the standard model of cosmology, ‘ΛCDM,’ Lambda-Cold Dark Matter. The theories that work well are also able to predict specific phenomena like measured anomalies in the cosmic background radiation.
Of course this is no proof we are living in a holographic universe. For that, researchers would have to prove that the tiny imponderables of the underlying quantum theories are also manifested in space. This means space itself would have to become diffuse in the very smallest dimensions. This is what the holometer at Fermilab is supposed to show through its measurements. See holometer.fnal.gov.
The Big Bang itself might give us clues about the holographic nature of our own universe. The Big Bang, which is generally considered the beginning of space and time, literally has a little problem. It must have happened in an extremely tiny space. The closer you get to it, the more densely the complete energy of the cosmos was compressed in a unit of space, until everything was concentrated in a point of infinite density.
This state cannot be comprehended with the help of the general theory of relativity. Therefore physicists are forced to consider the Big Bang with a theory that unites both big (the cosmos) and small (the quantum world). So far, there is no agreement on this, just a few competing candidates.
There is the ‘string theory,’ for example, that sees space as consisting of tiny objects resembling piano strings. These strings are one-dimensional and they each vibrate with a specific frequency to which energy can be assigned. By now, physicists have expanded this idea and reached an ‘M-theory’ by adding other structures—point particles and most of all membranes—’branes’—which can have up to nine dimensions. In order to reach the elementary particles and the laws of nature we know, the extra dimensions must be ‘curled up’ in a specific way, as researchers call this procedure. There are very different ways of curling up membranes and strings, and, depending on which one you choose, a different kind of universe emerges. Overall, 10100 different universes—a google’s worth—are possible, and many of these could exist simultaneously without the inhabitants of one having any clue about the creatures living in the other universes. However, if two three-dimensional worlds got too close to each other while moving through an additional dimension, they could collide—and give birth to our universe in the Big Bang.
A competitor of the string theory called ‘loop quantum gravity’ yields even better results. According to it, the universe only seems to be continuous. In reality, though, absolutely everything is quantized, or divided into small bits—even gravity. Space is no longer the container for the universe, but a part of it, and it is also fragmented and takes on the form of a network of lines and knots. Then elementary particles form different types of knots and between the lines and knots there is nothing. The theory of loop quantum gravity leads to some strange-sounding conclusions, but in turn it is better at describing some interesting phenomena than other theories.
Its consequences for the Big Bang were first simulated in 2004 by the German physicist Martin Bojowald. First of all, it avoids the concept of a singularity, because the loop quantum universe has a specific minimum structural size it cannot go below. If you calculate conditions increasingly closer to the Big Bang, you get a new, different, or also predecessor universe, in which all directions, including that of time, are reversed. This universe before the Big Bang is shrinking in the direc
tion of the Big Bang. If space has contracted extremely under the influence of gravitation, the quantum loop fabric of space-time fractures at some point—and through the effect of this ‘quantum recoil,’ gravity turns into a strong repulsive force that drives the universe apart again.
But perhaps we don’t need all these nice new theories to explain the Big Bang. Three Canadian physicists noticed that the singularity underlying the beginning of the universe had one peculiarity. Unlike all singularities known so far, which are within black holes, it could not have been surrounded by an event horizon.
In an article, the researchers described the conjecture that they developed from this: Maybe we do not see this event horizon because we are a part of it. The universe as we know it would then be the three-dimensional event horizon of a black hole with four spatial dimensions, which was created by the collapse of an also four-dimensional star. This sounds even more plausible because the physicists’ simulation showed that such a three-dimensional event horizon would have to expand continually—a process we notice as the expansion of the universe.
This process would also explain why the universe is so markedly homogenous. Until now, cosmologists have had to use a very fast expansion phase shortly after the Big Bang to account for this, the so-called inflation phase. According to this, when the universe was between 10-38 and 10-35 seconds old, it expanded by a factor between 1030 and 1050. While it was initially the size of a proton, it afterwards had the dimensions of a soccer ball. In order to arrive at a reasonable explanation, which would fit into the previous cosmological model, this inflation required the so-called ‘inflatons.’ These particles—which, strangely enough, never show up again afterwards—are not attracted to each other by gravity, but instead repulse each other. The fictive four-dimensional universe, though, in which the 4D-star must have collapsed, would have had plenty of opportunity to achieve a homogeneous structure in the course of its comparably eternal existence.
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