8 ms·
It's an article from 1962: "After the detection of the gravitational wave GW170817, Jason T. Wright (Physics Today, 72, 5, 12, 2019) reminded the community tha
by valyagolev 3y ago
It's an article from 1962:
"After the detection of the gravitational wave GW170817, Jason T. Wright (Physics Today, 72, 5, 12, 2019) reminded
the community that many of its features had been predicted by Dyson more than half a century earlier. Dyson’s article was
published only once, in Cameron’s long out of print collection, though a scan may be found at the web site of the Gravity
Research Foundation (https://www.gravityresearchfoundation.org https://www.gravityresearchfoundation.org). Dyson thought it had been reprinted (in his Selected
Papers, AMS Press, 1996, forward by Elliot H. Lieb) but it was not. Hoping to make the article easier to find, I wrote Dyson
for his permission to post it at the arXiv"
It's about using two big bodies, A and B, to accelerate objects: "The energy source of the machine is the gravitational potential between the stars A and B. As the machine continues to operate, the stars A and B will gradually be drawn closer together, their negative potential energy will increase and their orbital velocity V will also increase."
- ur-whale 3y agoNice idea. I wonder if one could calculate an upper bound of the available potential gravitational energy available in the entire universe by estimating how far every massive point (baryon) is from all the others.
- westurner 3y ago[flagged]
- westurner 3y agoWhat is the issue with this comment?
- cwillu 3y agohttps://chat.openai.com/share/39580cd0-b5b2-4bec-89d4-09297c89481a https://chat.openai.com/share/39580cd0-b5b2-4bec-89d4-09297c... “[…] 5. Perceived Irrelevance or Lack of Relevance: Depending on the context of the Hacker News post, copy-pasting AI-generated content might be seen as irrelevant or not contributing to the topic at hand. This can be frustrating for users who are looking for meaningful and relevant insights from other community members.”
- throwway120385 3y agoIf I wanted to know what Bard or ChatGPT had to say about this, I would ask them.
- westurner 3y agoBut you don't know to ask: that's the problem; They're all trying new QG and dark energy correction factors. One can't solve 3-body problems without Superfluid Quantum Gravity, thus this that I took the time to add is very relevant. Not worth helping then.
- westurner 3y agoGiven such an anti- AI-with-citation policy, researchers preparing comment content with citations for the platform have a counterproductive incentive to parallel construct after using e.g. search engines that use AI (Google, Bing,), and also an incentive to not cite their sources?
- deleted 3y ago[deleted]
- westurner 3y ago> One can't solve 3-body problems without Superfluid Quantum Gravity The article (from 1962) does not predict 3-body accelerations of large masses or particles using current methods (probably because they had not yet been developed at the time). (Ironically, in context to the rejection of AI methods to summarize superfluid Quantum Gravity for the thread's benefit (and not my own),) N-body gravity problems are probably best solved by AI methods; it is well understood that there are no known closed-form solutions for n-body gravity problems. Three body problem: https://en.wikipedia.org/wiki/Three-body_problem https://en.wikipedia.org/wiki/Three-body_problem
- thumbuddy 3y agoI'd love to see a chatbot try lol. That's all I'll say about that...
- ThrowawayR2 3y agoIn a past thread whose topic was "Ask HN: Should HN ban ChatGPT/generated responses?" (https://news.ycombinator.com/item?id=33945628 https://news.ycombinator.com/item?id=33945628), dang has said "They're already banned—HN has never allowed bots or generated responses. If we have to, we'll add that explicitly to https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html, but I'd say it already follows from the rules that are in there."
- floobertoober 3y agoIf space is expanding, does that also mean that gravitational potential energy is always increasing, too? I'd never thought of that before
- oopsemist 3y agoYes, it should increase on paper, but no source of energy to power that expansion is found yet. Big Shrink can power itself, so I'm voting in favor of https://en.wikipedia.org/wiki/Shapley_Attractor https://en.wikipedia.org/wiki/Shapley_Attractor
- pdonis 3y agoNo, it doesn't, because the concept of "gravitational potential energy" is not meaningful for an expanding universe considered as a whole. It's only meaningful for isolated systems within the universe.
- thx-2718 3y agoWouldn't there be meaning in saying it would take this much energy to push all the matter in the universe to one place? Which that amount should be increasing as space-time expands. Thought experiment, if you could place a mass of an arbitrary amount at any one point in space, how much mass would you need such that all the mass of the universe is now falling towards it. Or could you bend space-time to a point that all mass falls into it.
- pdonis 3y ago> Wouldn't there be meaning in saying it would take this much energy to push all the matter in the universe to one place? No. The universe is not an isolated system that we can operate on from the outside. You can't treat it as though it is. So your thought experiments aren't meaningful.
- thx-2718 3y agoObviously the thought experiment requires energy that doesn't 'exist' or doesn't have meaning in the sense that it could happen literally. It's a what-if and that does have a number and that does have meaning. So there is meaning to the previous persons question which is what the thought experiments were meant to show but obviously that's something you can't imagine.
- raattgift 3y ago> estimating how far every ... baryon ... is from all the others The true metric of the universe, sourced by every massive object (and every massless one, and all self-energies and interactions and fluxes of momentum-energy), is fantastically complicated. The standard cosmology is tractable only because it coarse-grains all this into a model where at every point in space (having taken a particular slicing of the whole spacetime into spatial volumes ("space") indexed by the scale factor) has a total energy-density. The total is a sum that includes the energy-density from baryons, radiation, dark matter, and so forth. The energy-density in any given slice is modelled as the same at every point in the spatial slice, leading to the observed large-scale isotropy and homogeneity that is central to the model, and allows for a cosmological frame to be picked out. In the cosmological frame, where coordinates expand with the metric expansion of space, we can talk about the energy-density at a given point in space. In principle we can make measurements at a large number of points and produce an average energy-density. Finally, we can talk about the consequences of the average value: https://www.astronomy.swin.edu.au/cosmos/C/Critical+Density https://www.astronomy.swin.edu.au/cosmos/C/Critical+Density So, in a sense, cosmologists do talk about whether the entire universe will recollapse or expand forever (or fall into some steady state), and (average) baryon-density is an important factor. Thus there is some upper bound for a local quantity not a million miles from gravitational potential energy. (Below your question there are others pointing out that there is no such global quantity available. This mainly means that while one can imagine increasing the average baryon or DM density leading to a collapse of the whole universe, this is still firmly in the FLRW universe, and definitely not converting from FLRW to an asymptotically flat spacetime around a central collapsing mass. But see below.) Going further, we can evolve the (average) baryon-density in a space along the scale factor. In the timelike (scale factor) direction away from baryogenesis, the energy-density of baryons decreases. At a finer-grained level that means clouds of neutral atoms and molecules tend to thin out. (So does radiation, so does dark matter, so do relic neutrinos, and so forth; pretty much everything but the cosmological constant (which is constant, after all) falls to zero on average far enough from the formation of the cosmic microwave background). This coarse-grained picture can be refined in several ways, by e.g. introducing inhomogeneities: overdensities or underdensities of baryons, for example, which evolve into large black holes and voids respectively. One can then ask questions in an inhomogeneous cosmology about the (average) density of black holes of various masses, and compare that to the energy-densities of baryons and the rest. The question is, for a given spatial slice, what fraction of baryons have fallen into black holes, and what fraction has not? Then evolve that question along the scale factor (e.g., in the future, are most baryons in black holes, or are they mostly spread out in wispy filaments around the edges of large voids?). One can do some headstands and try to understand the fraction of all baryons not yet fallen into black holes in that sort of cosmology as relating to gravitational potential energy, however I think that it's bound to be more useful to think, like above, of the (averaged) energy-densities and how that averaged (and thus coarse-grained) picture generates a metric comparable to FLRW. Instead, interpreting your question somewhat, you might start with something that ultimately must be nonuniformities in a contraction of the Riemann curvature tensor (see <https://en.wikipedia.org/wiki/Scalar_curvature https://en.wikipedia.org/wiki/Scalar_curvature>) then dusting that geometry with objects whose trajectories you'd study so that in some set of coordinates you could carve out (for each of them) from their intrinsic mass a kinetic energy and potential energy. I don't think that approach would be fundamentally wrong. It is essentially along the lines of Lagrangian mechanics (L = T - V, where V is a potential energy), although in a general-relativistic setting this gets hard, see <https://en.wikipedia.org/wiki/Relativistic_Lagrangian_mechanics#Lagrangian_formulation_in_general_relativity https://en.wikipedia.org/wiki/Relativistic_Lagrangian_mechan...>. Inevitably you would have to do some coarse-graining for tractability, and would want to make sure your coarse-graining procedure is not unphysical. Finally, apologies for not expanding a number of acronyms, and for wandering off-track a bit. Yours was an interesting question especially in light of some of the more technical replies below, and I was torn about what audience-expertise to write for (and settled on probably satisfying nobody).
- bookofjoe 3y agoDyson was like George Gamow, protean in intellectual power and scope and imagination yet forever just outside the Nobel sphere.
- robertlagrant 3y agoIs that why he invented the Dyson Sphere?
- samstave 3y agoSeems like a well rounded fellow.
- swader999 3y agoCircular logic.
- mayormcmatt 3y agoNo, spherical logic.
- TeMPOraL 3y agoNo, distributed swarm logic, as apparently he later admitted that what he thought about was what we now call a Dyson swarm, and it's just the public that took the "sphere" bit literally and run with it.
- heywhatupboys 3y agoI think you are being a bit Anglo-centered.
- bookofjoe 3y agoGeorgiy Antonovich Gamov (Ukrainian: Георгій Антонович Гамов, Russian: Георгий Антонович Гамов) was born on March 4, 1904 in Odessa, Russian Empire (now Ukraine). His father taught Russian language and literature in high school, and his mother taught geography and history at a school for girls. In addition to Russian, Gamow learned to speak some French from his mother and German from a tutor. Gamow learned English in his college years and became fluent. Most of his early publications were in German or Russian, but he later used English for both technical papers and for the lay audience. He was educated at the Institute of Physics and Mathematics in Odessa[2] (1922–23) and at the University of Leningrad (1923–1929). Gamow studied under Alexander Friedmann in Leningrad, until Friedmann's early death in 1925, which required him to change dissertation advisors. At the university, Gamow made friends with three other students of theoretical physics, Lev Landau, Dmitri Ivanenko, and Matvey Bronshtein. The four formed a group they called the Three Musketeers, which met to discuss and analyze the ground-breaking papers on quantum mechanics published during those years. He later used the same phrase to describe the Alpher, Herman, and Gamow group. Upon graduation, he worked on quantum theory in Göttingen, where his research into the atomic nucleus provided the basis for his doctorate. He then worked at the Theoretical Physics Institute of the University of Copenhagen from 1928 to 1931, with a break to work with Ernest Rutherford at the Cavendish Laboratory in Cambridge. He continued to study the atomic nucleus (proposing the "liquid drop" model), but also worked on stellar physics with Robert Atkinson and Fritz Houtermans. In 1931, Gamow was elected a corresponding member of the Academy of Sciences of the USSR at age 28 – one of the youngest in its history. During the period 1931–1933, Gamow worked in the Physical Department of the Radium Institute (Leningrad) headed by Vitaly Khlopin [ru]. Europe's first cyclotron was designed under the guidance and direct participation of Igor Kurchatov, Lev Mysovskii and Gamow. In 1932, Gamow and Mysovskii submitted a draft design for consideration by the Academic Council of the Radium Institute, which approved it. The cyclotron was not completed until 1937. Defection Gamow worked at a number of Soviet establishments before deciding to flee the Soviet Union because of increased oppression. In 1931, he was officially denied permission to attend a scientific conference in Italy. Also in 1931, he married Lyubov Vokhmintseva (Russian: Любовь Вохминцева), another physicist in Soviet Union, whom he nicknamed "Rho" after the Greek letter. Gamow and his new wife spent much of the next two years trying to leave the Soviet Union, with or without official permission. Niels Bohr and other friends invited Gamow to visit during this period, but Gamow could not get permission to leave. Gamow later said that his first two attempts to defect with his wife were in 1932 and involved trying to kayak: first a planned 250-kilometer paddle over the Black Sea to Turkey, and another attempt from Murmansk to Norway. Poor weather foiled both attempts, but they had not been noticed by the authorities. In 1933, Gamow was suddenly granted permission to attend the 7th Solvay Conference on physics, in Brussels. He insisted on having his wife accompany him, even saying that he would not go alone. Eventually the Soviet authorities relented and issued passports for the couple. The two attended and arranged to extend their stay, with the help of Marie Curie and other physicists. Over the next year, Gamow obtained temporary work at the Curie Institute, University of London, and the University of Michigan. https://en.wikipedia.org/wiki/George_Gamow https://en.wikipedia.org/wiki/George_Gamow
- samstave 3y agoWould Google happen to have a scan of this in their whatever-its-called-book-scanning-archive?