What if quasars are actually white holes
Star Stories Episode 430: White Holes
I've already told a lot about black holes in the star stories. And when you deal with the subject, sooner or later the term "white hole" appears. So what is a white hole? The simple answer: a white hole is like a black hole, only the other way around. Thank you for listening, that's it for today.
No, of course the simple explanation is not enough. It's not entirely wrong, but it's also of little help. The problem with a longer explanation, however, is the problem that always comes up when you talk about cosmology, black holes, and the like. It's about phenomena that have absolutely nothing to do with our everyday life. It's about phenomena that take place far beyond what our brain has learned to understand in the course of evolution. For example, if you want to imagine the universe as a whole, then you have to imagine a four-dimensional object. We simply cannot do that. We have to imagine a three-dimensional curved space, which is theoretically possible, but not easy, which is why we are mostly content with imagining the usual rubber blanket with a bowling ball in it. Which is a lot simpler, but just a two-dimensional object that curves in a three-dimensional space. This is not how the real curvature of space in the universe works; nor is it in a higher dimensional space into which it curves or into which it expands. But we just can't imagine that.
In this episode, however, it should not be about what we cannot imagine about the expanding universe, but about what we cannot imagine about black and white holes. And that's almost all. We can certainly research such extreme objects scientifically. But the way we think about it meaningfully takes math. We can describe a black hole mathematically, we can explore the mathematics by which it is described and thus come to new knowledge. But it is abstract mathematics, which is abstract precisely because it cannot be imagined intuitively. I explain it all so precisely because it is exactly the same with the white holes; maybe even a little worse. Everything I explain in the following is actually an attempt to illustrate something that cannot be illustrated. Since I don't just want to recite mathematical equations in this podcast, I'll try anyway.
Let's start again with the black holes. I talked about it in detail in episodes 40 and 41; in episode 238 I went into even more detail. The most important concept to understand here is the event horizon. If there is mass lying around somewhere in the universe, it exerts a gravitational force. How strong this force is depends on the one hand on the amount of mass. And on the other hand, how close you get to this crowd. But you can't get as close as you want to normal mass. We humans are, for example, as close as we can to the entire mass of the earth. We walk around directly on their surface, it doesn't get any closer. The attraction that we feel from the earth is already the maximum that we can feel. But it could be bigger. Because even if the ground under our feet is directly below us, a lot more mass of the earth is further away. After all, the ground extends 6371 kilometers to the center of the earth and then again the same distance to the other side of the earth. The mass there is therefore over 12,000 kilometers away from us and its attraction is correspondingly weaker. If we could compress the earth onto a sphere 10 meters in diameter, then we would be much closer to the mass on the surface of this sphere. The other side would now only be 10 meters away. We would feel the gravitational pull of the earth's mass much more strongly! And it would be much more difficult for us to leave the earth. Which is already not easy; we have to reach the so-called escape speed of 11.2 kilometers per second if we want to leave the earth permanently and that is only possible with rockets. On the shrunken earth we would have to become significantly faster if we want to escape the much stronger gravitational pull.
If we were to squeeze the mass of the earth to less than a centimeter in diameter, something strange would happen. Then the necessary escape speed to escape from this small but very massive ball would be greater than the speed of light. Which means something like: We would not be able to escape. Because nothing can move faster than light. We would be trapped on this sphere forever. This is exactly the event horizon: the distance from a mass at which the escape speed becomes greater than the speed of light. So it is clear that you only get an event horizon if you compress enough mass to a sufficient extent. This is not the case with most objects in the universe. A star like our sun, for example, would have to be squeezed to less than 6 kilometers in diameter to get an event horizon. Such forces do not usually exist. But it can still happen with very large stars. If at the end of their life they can no longer do nuclear fusion, then the whole thing collapses under its own weight. The outward pressure of the radiation disappears and everything is compressed more and more. If the mass of the star is large enough, the mass is compressed so much that an event horizon is created. Then we get a real black hole; for example in the case of Cygnus X-1, which I explained in more detail in episode 406.
We know that black holes can form when very large stars die. Or rather, we know that after their death, large stars can collapse to such an extent that an event horizon forms around them. We do not know what is beyond the event horizon. Our theories predict that all of the mass will unite into a single point called a singularity. But that's just a sign that the theories no longer work in these extreme cases. There can be no "points" in the real universe. The star's mass behind the event horizon will already assume some state - but we have no idea what it looks like.
I've already said quite a bit without saying anything about the white holes. But we have to sort out a few things about black holes before we can start. Black holes are real astronomical objects. But we just haven't really understood them well. The theory with which we currently want to understand it is Albert Einstein's general theory of relativity. It works really well, but only if the size of the objects is not too small. When we are dealing with something as heavy as a whole star but compressed so much that it is only the size of a tiny particle, then the theory of relativity no longer knows what to do. Then one would actually need quantum mechanics, which is specialized in the description of the smallest particles. But it cannot deal with gravitational forces for this. But if we now just follow what the relativity theory tells us, then its mathematics tells us that when a mass collapses, a singularity will eventually arise that is surrounded by an event horizon. However, the mathematical equations are symmetrical in time. Which means something like: If I simply change the sign of the variable for the time in the formulas, then I get a solution that is just as mathematically valid as the other. In other words: if one of my solutions describes a black hole, then I always have a second solution that describes a white hole. So you can think of a white hole as the reversal in time of a black hole. But what you shouldn't do, because then everything will only get more confusing.
Let's stay with what is still reasonably easy to understand. A white hole, like a black hole, is marked by an event horizon. When you approach the event horizon of a black hole, the attraction becomes stronger and stronger and at some point I can't get away. But if you get closer to the event horizon of a white hole, then at some point it becomes more and more difficult to get closer. And you will find that you would have to move faster than light to cross it. Nothing comes out of a black hole that is once inside. And nothing can penetrate into a white hole; from there only comes out what was already inside.
The way I have described it, you can imagine a white hole almost like a real object. Just as we can imagine the dark nothingness of the black hole, we just imagine a bright white something. In fact, a white hole would look exactly like this. Since nothing comes in but only stuff or radiation comes out, it would shine brightly. But this idea is wrong because we have left the path of pure mathematics. What I said earlier about the time symmetry of the equations applies in a universe in which there is no star at all from which a black (or white) hole could form. That sounds strange, but it works. Mathematics doesn't have to know how a black hole is created. Once there is a singularity, it's in the equations; then there is no need for a real process to create something like this. To think of how the purely mathematically described object can arise in the real universe is a matter of astronomy, not mathematics. And the mathematical formulas are much simpler when there is no disruptive mass lying around. So one can mathematically describe a universe that contains event horizons. Both black and white holes. If one were to include something like stars in this mathematical description, then the white holes would disappear. They would then no longer be reasonable solutions to the equation. And that doesn't even have to be a star; any mass would suffice, even if it was just a single atom.
It is impossible to imagine why this is so without looking closely at the math. But it follows from this: We would only have white holes in the universe if the universe had had white holes right from the start. There is no reason why that should have been the case, however. And then a lot of mass was created in the real universe. If there had been white holes, they would have been gone long ago. In other words: even if white holes can be described purely mathematically with the equations of general relativity, that does not mean that they actually have to exist. There is no known mechanism by which they could arise and there is neither a reason nor any indication that a mechanism as yet unknown to us exists. We use mathematics because it also produces a lot of reasonable solutions with which we can describe real observations, such as the black holes that actually exist in space. But we also know that the math we are using is not absolutely correct math. It is a very good approximation of correct mathematics, otherwise it would not be able to describe so many observations so precisely. But because we know that we need a combination of the theory of relativity and quantum mechanics for a complete description, and we do not have this combination, it follows that our mathematics is incomplete. So it's not surprising that we don't know what to do with all of the solutions it produces.
From what we currently know, white holes are just a mathematical curiosity. We have not made observations anywhere in space that can only be explained by the existence of white holes, nor can we even identify any mechanism that leads to the formation of white holes. It used to be thought that quasars might be white holes. I already talked about these things in detail in episode 52. They actually shine absurdly bright and when you discovered them in the 1960s, you had no idea what it was about. But now we know very well that these are the centers of distant galaxies, in which there is a BLACK hole and with its gravitational force accelerates the matter there so quickly that it begins to glow. And in contrast to the white holes, the black holes not only have mechanisms such as the collapse of a star that produces an event horizon, but we have also repeatedly made observations that could only be explained by the presence of a lot of mass in a very small space. Black holes are fascinating. Certainly white holes too. But unlike the black holes, the white holes are not real.
Research continues to do this because it's a great way to better understand the math behind it all. And the white holes are sure to continue to spark people's imaginations. As are wormholes, which, by the way, are closely related to the white holes (and almost certainly not real for the same reasons as the white holes, but that would be a topic for another episode). At some point we might get better math to describe the universe. It is very likely that the white holes will simply disappear from our theories. Or maybe we're learning something completely new that we don't even know we can learn now. That's exactly why we do science!
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