John Baez's post explains that the common notion of black holes shrinking due to Hawking radiation is a simplification. While Hawking radiation exists, it's emitted by the hot "quantum atmosphere" surrounding the black hole, not the singularity itself. This atmosphere is formed from infalling matter interacting with intense spacetime curvature. The black hole's mass, contained within its event horizon, only decreases because this infalling matter effectively loses some energy as Hawking radiation before crossing the horizon. Therefore, it's more accurate to say the black hole's atmosphere radiates and shrinks as it loses energy, indirectly causing the enclosed black hole to shrink over vast timescales.
David Tong's webpage provides a collection of freely available lecture notes covering various topics in theoretical physics, aimed primarily at advanced undergraduate and beginning graduate students. The subjects range from classical mechanics and electromagnetism to quantum field theory, general relativity, string theory, and solitons, each offering a comprehensive and pedagogical approach. The notes are based on lectures Tong has delivered at the University of Cambridge and are continually updated. They include problem sets, further reading suggestions, and, in some cases, accompanying video lectures.
Hacker News users generally praised David Tong's lecture notes for their clarity, comprehensiveness, and accessibility. Several commenters highlighted the notes as an excellent resource for both students and those seeking to refresh their knowledge of physics. Some pointed out specific strengths, such as the clear explanations of challenging concepts like quantum field theory and general relativity. A few users mentioned minor typos or areas for improvement, but the overall sentiment was overwhelmingly positive, with many thanking Tong for making these resources freely available. Several commenters also discussed their own experiences using the notes, either as students or for self-study, further reinforcing their value.
Multi-messenger astronomy, combining observations of photons, neutrinos, and gravitational waves, offers a richer understanding of the universe. While electromagnetic radiation (photons) has long been the cornerstone of astronomy, neutrinos and gravitational waves provide unique, complementary information. Neutrinos, weakly interacting particles, escape dense environments where photons are trapped, offering insights into core-collapse supernovae and other extreme events. Gravitational waves, ripples in spacetime caused by accelerating massive objects, reveal information about mergers of black holes and neutron stars, inaccessible through electromagnetic observations. The combined detection of these messengers from the same source allows for a more complete picture of these energetic phenomena, providing crucial insights into their underlying physics.
HN users discuss the limitations of traditional electromagnetic astronomy and the potential of gravitational wave astronomy to reveal new information about the universe, particularly events involving black holes and neutron stars. Some highlight the technical challenges of detecting gravitational waves due to their incredibly faint signals. The discussion also touches upon the different information carried by photons, neutrinos, and gravitational waves, emphasizing that combining these "messengers" provides a more complete picture of cosmic events. Several commenters appreciate the linked lecture notes for being a clear and concise introduction to the topic. There's a brief discussion of the history and development of gravitational wave detectors, and some users express excitement about future discoveries in this emerging field.
The Euclid telescope has captured a remarkably clear image of a complete "Einstein Ring" in the galaxy NGC 6505. This phenomenon, predicted by Einstein's theory of general relativity, occurs when light from a distant background galaxy is bent and magnified by the gravity of a massive foreground galaxy, creating a ring-like distortion. This observation showcases Euclid's impressive imaging capabilities and its potential to study dark matter and the distribution of galaxies throughout the universe by analyzing such gravitational lensing effects. The sharp image of the Einstein Ring in NGC 6505 allows astronomers to study the properties of both the lensing and lensed galaxies in greater detail.
HN commenters generally express awe at the image and the science behind it, with several remarking on the elegance and strangeness of gravitational lensing. Some discuss the technical aspects of Euclid's capabilities and its potential for future discoveries, highlighting its wide field of view and infrared instruments. One commenter questions the described "completeness" of the ring, pointing out a seemingly incomplete section, leading to a discussion of image artifacts versus true features of the lensed galaxy. A few commenters offer additional resources and context, linking to other examples of Einstein rings and explaining redshift. There's also a brief thread about the naming of astronomical objects and the preference for descriptive over eponymous designations.
New research is mapping the chaotic interior of charged black holes, revealing a surprisingly complex structure. Using sophisticated computational techniques, physicists are exploring the turbulent dynamics within, driven by the black hole's electric charge. This inner turmoil generates an infinite number of nested, distorted "horizons," each with its own singularity, creating a fractal-like structure. These findings challenge existing assumptions about black hole interiors and provide new theoretical tools to probe the fundamental nature of spacetime within these extreme environments.
Several commenters on Hacker News expressed excitement about the advancements in understanding black hole interiors, with some highlighting the counterintuitive nature of maximal entropy being linked to chaos. One commenter questioned the visual representation's accuracy, pointing out the difficulty of depicting a 4D spacetime. There was discussion about the computational challenges involved in such simulations and the limitations of current models. A few users also delved into the theoretical physics behind the research, touching upon topics like string theory and the holographic principle. Some comments offered additional resources, including links to relevant papers and talks. Overall, the comments reflected a mix of awe, curiosity, and healthy skepticism about the complexities of black hole physics.
The blog post "Open and Closed Universes" explores the concept of universe curvature and its implications for the universe's ultimate fate. It explains how a "closed" universe, with positive curvature like a sphere, would eventually collapse back on itself in a "Big Crunch," while an "open" universe, with negative curvature like a saddle, would expand indefinitely. A "flat" universe, with zero curvature, represents a critical point between these two scenarios, also expanding forever but at a decelerating rate. The post uses the analogy of a ball thrown upwards to illustrate these concepts, where the ball's trajectory depends on its initial velocity relative to escape velocity. It concludes by mentioning the current scientific consensus, based on observations, which favors a flat or very slightly open universe, destined for continuous expansion and eventual heat death.
HN commenters largely discuss the difficulty of truly comprehending the vastness and complexity of the universe, with some pointing out the limitations of human intuition and the challenges of visualizing higher dimensions. Several express fascination with the concept of a closed universe and its implications for the finite yet unbounded nature of space. Some debated the philosophical implications, touching upon the potential for simulated universes and questioning the nature of reality if our universe is indeed closed. A few comments also delve into more technical aspects, like the role of dark energy and the expansion of the universe in determining its ultimate fate. One commenter suggests looking at the problem through the lens of information theory and entropy, proposing that the universe might be both open and closed simultaneously depending on the observer's perspective.
Noether's theorem, proven by mathematician Emmy Noether in 1915, reveals a profound connection between symmetries in nature and conservation laws. It states that every continuous symmetry in a physical system corresponds to a conserved quantity. For example, the symmetry of physical laws over time leads to the conservation of energy, and the symmetry of laws across space leads to the conservation of momentum. This theorem has become a cornerstone of modern physics, providing a powerful tool for understanding and predicting the behavior of physical systems, from classical mechanics and electromagnetism to quantum field theory and general relativity. It unified seemingly disparate concepts and drastically simplified the search for new laws of physics.
HN commenters generally praised the Quanta article for its clear explanation of Noether's theorem, with several sharing personal anecdotes about learning it. Some discussed the theorem's implications, highlighting its connection to symmetries in physics and its importance in modern theories like quantum field theory and general relativity. A few commenters delved into more technical details, mentioning Lagrangian and Hamiltonian mechanics, gauge theories, and the relationship between conservation laws and symmetries. One commenter pointed out the importance of differentiating between global and local symmetries, while others appreciated the article's accessibility even for those without a deep physics background. The overall sentiment was one of appreciation for both Noether's work and the article's elucidation of it.
Cosmologists are exploring a new method to determine the universe's shape – whether it's flat, spherical, or saddle-shaped – by analyzing pairings of gravitational lenses. Traditional methods rely on the cosmic microwave background, but this new technique uses the subtle distortions of light from distant galaxies bent around massive foreground objects. By examining the statistical correlations in the shapes and orientations of these lensed images, researchers can glean information about the curvature of spacetime, potentially providing an independent confirmation of the currently favored flat universe model, or revealing a surprising deviation. This method offers a potential advantage by probing a different cosmic epoch than the CMB, and could help resolve tensions between existing measurements.
HN commenters discuss the challenges of measuring the universe's shape, questioning the article's clarity on the new method using gravitational waves. Several express skepticism about definitively determining a "shape" at all, given our limited observational vantage point. Some debate the practical implications of a closed universe, with some suggesting it doesn't preclude infinite size. Others highlight the mind-boggling concept of a potentially finite yet unbounded universe, comparing it to the surface of a sphere. A few commenters point out potential issues with relying on specific models or assumptions about the early universe. The discussion also touches upon the limitations of our current understanding of cosmology and the constant evolution of scientific theories.
This paper explores the implications of closed timelike curves (CTCs) for the existence of life. It argues against the common assumption that CTCs would prevent life, instead proposing that stable and complex life could exist within them. The authors demonstrate, using a simple model based on Conway's Game of Life, how self-consistent, non-trivial evolution can occur on a spacetime containing CTCs. They suggest that the apparent paradoxes associated with time travel, such as the grandfather paradox, are avoided not by preventing changes to the past, but by the universe's dynamics naturally converging to self-consistent states. This implies that observers on a CTC would not perceive anything unusual, and their experience of causality would remain intact, despite the closed timelike nature of their spacetime.
HN commenters discuss the implications and paradoxes of closed timelike curves (CTCs), referencing Deutsch's approach to resolving the grandfather paradox through quantum mechanics and many-worlds interpretations. Some express skepticism about the practicality of CTCs due to the immense energy requirements, while others debate the philosophical implications of free will and determinism in a universe with time travel. The connection between CTCs and computational complexity is also raised, with the possibility that CTCs could enable the efficient solution of NP-complete problems. Several commenters question the validity of the paper's approach, particularly its reliance on density matrices and the interpretation of results. A few more technically inclined comments delve into the specifics of the physics involved, mentioning the Cauchy problem and the nature of time itself. Finally, some commenters simply find the idea of time travel fascinating, regardless of the theoretical complexities.
Summary of Comments ( 79 )
https://news.ycombinator.com/item?id=44015872
Hacker News users discuss the surprising assertion that dead stars don't radiate, focusing on the definition of "dead" in this context. Several point out that even black dwarfs, the theoretical endpoint of stellar evolution, would still emit some radiation due to Hawking radiation, though at an incredibly low temperature and over vast timescales. The discussion also touches on the complexities of heat death, the challenges of simulating such long-term processes, and the limitations of current scientific understanding regarding proton decay and other long-term phenomena. Some users highlight the immense timescales involved, emphasizing the difference between theoretical predictions and observable reality. Others express fascination with the concept and appreciate the thought-provoking nature of the article.
The Hacker News post titled "Dead Stars Don’t Radiate" generated a moderate amount of discussion, with several commenters engaging with the premise and its implications.
One of the most compelling threads revolved around the definition of "dead" in the context of stars. A commenter pointed out that even black holes, often considered the ultimate dead stars, emit Hawking radiation, albeit at incredibly low levels. This sparked a discussion about the timescale over which this radiation would lead to complete evaporation, highlighting the immense durations involved in cosmological processes. Another user added to this by explaining that while a true black dwarf (a hypothetical, fully cooled white dwarf) wouldn't radiate, the universe isn't old enough for any to have formed yet. This clarification helped contextualize the article's claim within the current understanding of stellar evolution.
Several comments focused on the idea of heat death, the theoretical state where the universe reaches maximum entropy. Users discussed the implications of a universe where no temperature differences exist, and whether this truly represents a "dead" state. One comment explored the possibility of quantum fluctuations allowing for localized pockets of energy even in a heat-dead universe, leaving a glimmer of potential for activity even in such an extreme scenario.
A more technical discussion branched off, delving into the thermodynamics of black holes. Users debated the nature of the information paradox and the role of entropy in black hole evaporation. This conversation touched on complex theoretical concepts, showcasing the depth of understanding some commenters brought to the discussion.
A few commenters expressed their appreciation for the article's clarity and its ability to explain complex physics concepts in an accessible manner. This sentiment highlighted the value of clear scientific communication and the article's success in achieving this goal.
Finally, a couple of comments offered additional resources, such as links to relevant Wikipedia articles and other scientific papers, further enriching the conversation and providing avenues for deeper exploration of the topic.