#White #holes #candidates #dark #matter
Most people are familiar with black holes. But what are white holes? They are still a highly speculative idea. Carlo Rovelli, shooting star of physics, explains at a public lecture by the ÖAW on May 14th in the Vienna City Hall why there is a lot to this idea and how white holes could be related to another mystery of physics: the mysterious dark matter.
Let’s imagine that we were able to fly a spaceship into a black hole. We would end up in a very long tunnel that gets narrower as we travel closer to the origin of the black hole, the remnants of the star from which it was formed. But what happens when we get there?
Nobody knows that at the moment. But the theoretical physicist and non-fiction author Carlo Rovelli has an idea: “The tunnel would “spring back” and expand again. “That would turn the black hole into a white hole, which would then spit us travelers out again,” says Rovelli, who is considered one of the shooting stars in his field.
On May 14th, Rovelli will speak at the Vienna City Hall at the invitation of the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences (ÖAW).
EINSTEIN WAS RIGHT
They will talk about white holes in Vienna. What is that?
Carlo Rovelli: I will first talk about black holes and explain what we can now say with certainty about these objects, especially their interiors. We can now detect collisions between black holes using gravitational waves and have even taken photos of supermassive representatives using radio telescopes. So we now have solid evidence for the existence of black holes, as Einstein’s equations have predicted since 1915. The general theory of relativity was proven right again. This shows once again that we can trust the theory, even when drawing conclusions about the interior of a black hole.
TRANSFORMATION ARTIST BLACK HOLE
So what does it look like inside a black hole?
Rovelli: First of all, it is larger on the inside than on the outside. Towards the center we would see a very long, thin tunnel, even though from the outside the black hole is a sphere. Once we cross the event horizon and find ourselves in the tunnel, we see the walls shrink: the tunnel becomes steadily longer and narrower as we approach the center. At the end of the tunnel we could see the highly compressed remains of the star from which the black hole once formed. There Einstein’s equations fail and predict a singularity because the tunnel becomes infinitely thin. But that’s wrong and Einstein knew it too: As soon as our tunnel gets narrow enough, quantum effects come into play that are not reflected in Einstein’s equations.
Any black hole would end up as a white hole.
What happens when we reach the center?
Rovelli: This is where our secure knowledge of the universe ends and everything else is currently speculation. To understand the center of a black hole, we need a theory of quantum gravity and unfortunately there is still no consensus among physicists. But we have at least a few very interesting ideas and this is where white holes come into play: They are my favorite idea for getting rid of the singularity inside black holes. For a journey into a black hole, this would mean that the tunnel becomes narrower and longer, but only up to the point where quantum gravity becomes relevant. Then the tunnel would “spring back” and expand again. That would turn the black hole into a white hole, which would then spit us travelers out again. The idea is speculative, but any black hole would end up as a white hole in this scenario.
How long would it take for a black hole to become white?
Rovelli: That depends on the size of the black hole. The large specimens at the centers of galaxies would only become white holes in the extremely distant future. But smaller black holes from the early days of the universe may have already turned into white holes.
Smaller black holes from the early days of the universe may have already transformed into white holes.
WHITE HOLES ARE EVERYWHERE
Could we discover white holes?
Rovelli: This is also speculative, but there is the idea that we have already discovered it, in the form of dark matter. These white holes would be extremely small, about the mass of a human hair, and would spit out tiny amounts of matter. Direct proof would only be possible via the gravitational interaction and would be extremely difficult, but there are ideas about how such tiny forces could be measured.
Is a white hole as heavy as the black hole from which it forms?
Rovelli: No, the mass is not preserved. Before a black hole becomes a white hole, it releases energy in the form of Hawking radiation and slowly loses mass. The transition to the white hole can only take place when the black hole has already become very small. Only tiny black holes can bounce back, which is why all white holes are very small.
How many of these white holes would have to exist to explain dark matter?
Rovelli: Lots of them and they would be everywhere: one of them would fly through every house about every few months. We could then use very fine measuring devices to try to measure the gravity of such a white hole.
These white holes would be extremely small, about the mass of a human hair.
What makes white holes good candidates for dark matter?
Rovelli: I like the idea because white holes are directly predicted by Einstein’s equations, just like black holes. In the 1970s, physicists still thought that black holes were a strange solution to the relativistic equations and too strange to actually exist in the universe. But Einstein was right from the start and I think it’s plausible that he was right about white holes too. For a long time we just had no idea how they could come about. That they emerge from black holes is a new idea that makes real white holes much more plausible.
Has interest in white holes increased because we still haven’t discovered other dark matter candidates?
Rovelli: Dark matter is one of the biggest mysteries in modern physics. We have a lot of evidence that there must be something there and five to six viable hypotheses for an explanation, but none of them have been successful yet. I also like the white hole hypothesis because it doesn’t require any new physics and so far I haven’t heard any serious criticisms.
Einstein was right from the start and I think it’s plausible that he was right about white holes too.
ARE QUANTUM AND RELATIVITY THEORY COMPATIBLE?
Is it conceivable that we can achieve a unification of quantum theory and general relativity without any new physics?
Rovelli: Nature has been telling us for decades that our theories are better than we believe. The standard model, which was actually only intended as a provisional solution, is being confirmed time after time, while new ideas such as string theory, which go beyond existing knowledge, have not yet been confirmed experimentally. Maybe we have excessive expectations of new physics and don’t appreciate what we have. All Nobel Prizes in recent years, the detection of black holes and the Higgs boson, are based on well-known theories. The most recent advances in the field of quantum information were all anticipated by Dirac and Heisenberg in the 1960s and 70s. The only news is that it works. We do not make new discoveries, but rather new applications for existing theories.
Didn’t the incompatibility of quantum theory and general relativity mean that we need a new theory of quantum gravity?
Rovelli: I see no fundamental incompatibility between the theories. If we’re smart enough, we can find a way to unite them without the need for new physics. There are several ways this could work. So I wouldn’t say that we don’t have a theory of quantum gravity today, just that we haven’t agreed on one yet. Now it’s time to compare the candidates with data and observations to see which approach is the right one.
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