Mar 24, 2021 05:32 PM
Has the black hole information paradox evaporated?
https://www.symmetrymagazine.org/article...evaporated
INTRO: If there’s one misconception people have about black holes, it’s that nothing ever escapes them. As physicist Stephen Hawking and colleagues showed back in the 1970s, black holes actually emit a faint glow of light. There’s a funny consequence to this glow: It carries energy away from the black hole. Eventually this drip, drip, drip of radiation drains a black hole completely and causes it to disappear. All that remains is the light.
In the 1970s, scientists’ calculations suggested that this light contained almost no information. Black holes seemed to be destroyers not just of the objects that sank into them but also of any information about what those objects had been in the first place.
The problem is: According to quantum mechanics, that’s impossible. A core tenet of quantum mechanics, the study of particle behavior on the subatomic level, is this: If you know the current state of any system, then you know everything there is to know about its past and its future.
Somehow, black holes seemed to be destroying information that, according to quantum physics, cannot be destroyed. This problem, today known as the black hole information paradox, has befuddled physicists for decades.
But over the last several years, theoretical physicists have identified key pieces that Hawking’s original calculation overlooked. Calculations completed in 2019 gave scientists insight into how that information might stick around.
Those developments could mean more than just solving the information paradox—they could also provide clues that could help finally solve the mystery of how gravity works at the subatomic level, says MIT physicist Netta Engelhardt, whose work with Institute for Advanced Study physicist Ahmed Almheiri, along with similar work by University of California, Berkeley physicist Geoff Penington and colleauges, pointed the way toward the latest results. The research was supported in part by the US Department of Energy’s Office of Science.
“This,” she says, “is where we need to look to understand quantum gravity better.” (MORE)
The crisis of quantum gravity
https://iai.tv/articles/the-crisis-of-qu..._auid=2020
EXCERPTS: The fact that a theory of quantum gravity hasn't been found reflects a crisis in physics. But it is not obvious what the crisis is. How might we "diagnose" it? [...] The reason that physicists want a theory of quantum gravity is not that there are any anomalous observations or unexplained experimental results that indicate a need to replace general relativity. Indeed, until recently, there were no observational results agreed to be evidence of quantum gravitational effects, and none of the attempts at a theory made any testable predictions. ... some physicists are starting to dissent.
Dissenting physicists argue that there are flaws in the formalism and methodology of some of the approaches to quantum gravity, particularly string theory. The underlying causes of the crisis, according to these physicists, range from psychosocial factors such as academic groupthink on the one hand to philosophical and scientific factors on the other.Here, I would like to present [...] the reason we haven't found a theory is because we don't know what we are looking for.
So, what is quantum gravity? Quantum gravity is whatever satisfies the constraints, or criteria of acceptance, we take to define quantum gravity. A constraint is a principle, or feature, that physicists assume the new theory must satisfy. Physicists impose these in their search in order to narrow down the space of possible theories so that they're not left groping around completely in the dark. [...] The constraints on quantum gravity are both empirical (coming from observational and experimental data) and non-empirical.
[...] Although we have no theory of quantum gravity, we have several approaches to finding one. These have different starting points, different methodologies, and different non-empirical constraints. In other words, the different approaches to quantum gravity have different goals: they are looking for different things. There is currently no single answer to the question of "what is quantum gravity?"
[...The...] driving motivation is often framed as the non-empirical constraint of unification: we require a theory that describes quantum and gravitational effects as originating from the same source, e.g., from some basic entity or interaction. But unification is not necessary in order to satisfy the requirement of describing those particular domains.
[...] Not all constraints adopted are done so explicitly. Many are assumed without being noticed by physicists or are so ingrained that no one questions them. It may be that they are indeed necessary or useful but we do not know that until we have examined them. One is the cherished principle of generalised correspondence (physicists also call this reduction). The new theory must "link up" with the theory it's replacing in the domains where the older theory is known to be successful, via various mathematical relations that connect the two theories.
Correspondence is taken as an unquestionable constraint for empirical reasons: it's supposed to be a shortcut for showing that the new theory does not conflict with currently known observational data. It's an invaluable shortcut because there is so much existing data that doing all the necessary calculations to check it using the new theory simply isn't feasible. But correspondence is a mathematical relation between two theories, and, as such, is certainly a non-empirical constraint.
This principle may seem natural, or even obvious, but it's also a bit strange—we are seeking a replacement for a particular theory because we believe that theory is incorrect. Yet, we are relying on this older theory as an essential means of confirming the newer theory, as a substitute for direct empirical testing. In the case of quantum gravity, this means showing how both quantum field theory and general relativity, in their respective domains, are entailed by any new theory of quantum gravity. The newer theory (through the correspondence relations) is supposed to explain why the older theories were successful, in spite of their being incorrect. We know that general relativity works to describe our massive bodies as they fall towards the ground; quantum gravity cannot make this untrue, but it should offer deeper insight into why.
[...] There are, however, other reasons for imposing correspondence as a constraint: for instance, if we define quantum gravity as a theory that is more fundamental than general relativity and quantum field theory, then correspondence is a way of demonstrating this.
Quantum gravity is understood as a theory that is not only more fundamental than our current theories but [...] that there is no deeper theory beyond. This is another constraint that we can question. Must quantum gravity be fundamental? Why should we assume it is? Could it be detrimental to the search for the theory to assume that it must be fundamental?
[...] For a theory to be fundamental in this sense, it [...] should give predictions at all short distance scales, or, equivalently, all possible high energy scales. ... General relativity, like Newtonian mechanics before it ... gives predictions at all distance scales. Yet, we know that Newtonian mechanics is not correct at short distance scales, since it gets replaced by quantum theory. And now we are assuming that general relativity is not correct at all short distance scales—though there is no empirical evidence that suggests general relativity fails here—we are assuming that it gets replaced by quantum gravity. [...Such...]has standardly been taken as a constraint on quantum gravity: but again, we can, and should, question this...
[...] I have suggested one way of framing the crisis of quantum gravity is ... we do not know the constraints that help to define the theory we are looking for. And we do not know which non-empirical guides can help us find it.
The radical action I advocate involves critically examining all of the principles we use, or could use, in searching for the theory—no matter how essential, fruitful, or convenient these might appear. [...] If we have a clearer understanding of the question “what is quantum gravity”, we will be better equipped to find our answer. The aim is not to do away with our non-empirical constraints, but rather to establish their validity and to ensure we have a firm foundation for future inquiry... (MORE - details)
https://www.symmetrymagazine.org/article...evaporated
INTRO: If there’s one misconception people have about black holes, it’s that nothing ever escapes them. As physicist Stephen Hawking and colleagues showed back in the 1970s, black holes actually emit a faint glow of light. There’s a funny consequence to this glow: It carries energy away from the black hole. Eventually this drip, drip, drip of radiation drains a black hole completely and causes it to disappear. All that remains is the light.
In the 1970s, scientists’ calculations suggested that this light contained almost no information. Black holes seemed to be destroyers not just of the objects that sank into them but also of any information about what those objects had been in the first place.
The problem is: According to quantum mechanics, that’s impossible. A core tenet of quantum mechanics, the study of particle behavior on the subatomic level, is this: If you know the current state of any system, then you know everything there is to know about its past and its future.
Somehow, black holes seemed to be destroying information that, according to quantum physics, cannot be destroyed. This problem, today known as the black hole information paradox, has befuddled physicists for decades.
But over the last several years, theoretical physicists have identified key pieces that Hawking’s original calculation overlooked. Calculations completed in 2019 gave scientists insight into how that information might stick around.
Those developments could mean more than just solving the information paradox—they could also provide clues that could help finally solve the mystery of how gravity works at the subatomic level, says MIT physicist Netta Engelhardt, whose work with Institute for Advanced Study physicist Ahmed Almheiri, along with similar work by University of California, Berkeley physicist Geoff Penington and colleauges, pointed the way toward the latest results. The research was supported in part by the US Department of Energy’s Office of Science.
“This,” she says, “is where we need to look to understand quantum gravity better.” (MORE)
The crisis of quantum gravity
https://iai.tv/articles/the-crisis-of-qu..._auid=2020
EXCERPTS: The fact that a theory of quantum gravity hasn't been found reflects a crisis in physics. But it is not obvious what the crisis is. How might we "diagnose" it? [...] The reason that physicists want a theory of quantum gravity is not that there are any anomalous observations or unexplained experimental results that indicate a need to replace general relativity. Indeed, until recently, there were no observational results agreed to be evidence of quantum gravitational effects, and none of the attempts at a theory made any testable predictions. ... some physicists are starting to dissent.
Dissenting physicists argue that there are flaws in the formalism and methodology of some of the approaches to quantum gravity, particularly string theory. The underlying causes of the crisis, according to these physicists, range from psychosocial factors such as academic groupthink on the one hand to philosophical and scientific factors on the other.Here, I would like to present [...] the reason we haven't found a theory is because we don't know what we are looking for.
So, what is quantum gravity? Quantum gravity is whatever satisfies the constraints, or criteria of acceptance, we take to define quantum gravity. A constraint is a principle, or feature, that physicists assume the new theory must satisfy. Physicists impose these in their search in order to narrow down the space of possible theories so that they're not left groping around completely in the dark. [...] The constraints on quantum gravity are both empirical (coming from observational and experimental data) and non-empirical.
[...] Although we have no theory of quantum gravity, we have several approaches to finding one. These have different starting points, different methodologies, and different non-empirical constraints. In other words, the different approaches to quantum gravity have different goals: they are looking for different things. There is currently no single answer to the question of "what is quantum gravity?"
[...The...] driving motivation is often framed as the non-empirical constraint of unification: we require a theory that describes quantum and gravitational effects as originating from the same source, e.g., from some basic entity or interaction. But unification is not necessary in order to satisfy the requirement of describing those particular domains.
[...] Not all constraints adopted are done so explicitly. Many are assumed without being noticed by physicists or are so ingrained that no one questions them. It may be that they are indeed necessary or useful but we do not know that until we have examined them. One is the cherished principle of generalised correspondence (physicists also call this reduction). The new theory must "link up" with the theory it's replacing in the domains where the older theory is known to be successful, via various mathematical relations that connect the two theories.
Correspondence is taken as an unquestionable constraint for empirical reasons: it's supposed to be a shortcut for showing that the new theory does not conflict with currently known observational data. It's an invaluable shortcut because there is so much existing data that doing all the necessary calculations to check it using the new theory simply isn't feasible. But correspondence is a mathematical relation between two theories, and, as such, is certainly a non-empirical constraint.
This principle may seem natural, or even obvious, but it's also a bit strange—we are seeking a replacement for a particular theory because we believe that theory is incorrect. Yet, we are relying on this older theory as an essential means of confirming the newer theory, as a substitute for direct empirical testing. In the case of quantum gravity, this means showing how both quantum field theory and general relativity, in their respective domains, are entailed by any new theory of quantum gravity. The newer theory (through the correspondence relations) is supposed to explain why the older theories were successful, in spite of their being incorrect. We know that general relativity works to describe our massive bodies as they fall towards the ground; quantum gravity cannot make this untrue, but it should offer deeper insight into why.
[...] There are, however, other reasons for imposing correspondence as a constraint: for instance, if we define quantum gravity as a theory that is more fundamental than general relativity and quantum field theory, then correspondence is a way of demonstrating this.
Quantum gravity is understood as a theory that is not only more fundamental than our current theories but [...] that there is no deeper theory beyond. This is another constraint that we can question. Must quantum gravity be fundamental? Why should we assume it is? Could it be detrimental to the search for the theory to assume that it must be fundamental?
[...] For a theory to be fundamental in this sense, it [...] should give predictions at all short distance scales, or, equivalently, all possible high energy scales. ... General relativity, like Newtonian mechanics before it ... gives predictions at all distance scales. Yet, we know that Newtonian mechanics is not correct at short distance scales, since it gets replaced by quantum theory. And now we are assuming that general relativity is not correct at all short distance scales—though there is no empirical evidence that suggests general relativity fails here—we are assuming that it gets replaced by quantum gravity. [...Such...]has standardly been taken as a constraint on quantum gravity: but again, we can, and should, question this...
[...] I have suggested one way of framing the crisis of quantum gravity is ... we do not know the constraints that help to define the theory we are looking for. And we do not know which non-empirical guides can help us find it.
The radical action I advocate involves critically examining all of the principles we use, or could use, in searching for the theory—no matter how essential, fruitful, or convenient these might appear. [...] If we have a clearer understanding of the question “what is quantum gravity”, we will be better equipped to find our answer. The aim is not to do away with our non-empirical constraints, but rather to establish their validity and to ensure we have a firm foundation for future inquiry... (MORE - details)