12 ms·

Gravitational Machines

217 points by sohkamyung 3y ago

sgt 3y ago
[flagged]
- forgetfreeman 3y ago
  No, no it is not.
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 ago
  Nice 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 ago
      What is the issue with this comment?
      - cwillu 3y ago
        https://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 ago
        If I wanted to know what Bard or ChatGPT had to say about this, I would ask them.
        westurner 3y ago
        But 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 ago
        Given 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
  - floobertoober 3y ago
    If space is expanding, does that also mean that gravitational potential energy is always increasing, too? I'd never thought of that before
    - oopsemist 3y ago
      Yes, 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 ago
      No, 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 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? 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 ago
        Obviously 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 ago
  Dyson was like George Gamow, protean in intellectual power and scope and imagination yet forever just outside the Nobel sphere.
  - robertlagrant 3y ago
    Is that why he invented the Dyson Sphere?
    - samstave 3y ago
      Seems like a well rounded fellow.
    - swader999 3y ago
      Circular logic.
      - mayormcmatt 3y ago
        No, spherical logic.
        TeMPOraL 3y ago
        No, 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 ago
    I think you are being a bit Anglo-centered.
    - bookofjoe 3y ago
      Georgiy 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 ago
  Would Google happen to have a scan of this in their whatever-its-called-book-scanning-archive?
howexactly 3y ago
how exactly is energy extracted from such a system?
- howmayiannoyyou 3y ago
  Can't say exactly, but kinetic energy can slightly alter mass. For example, increasing the speed of an object increases its mass.
- Robotbeat 3y ago
  A large mass is dropped on the right trajectory between the two, the gravitational forces slingshot it back at higher velocity, which can be captured with some sort of electromagnetic regenerative braking mechanism. Just a toy idea for how energy can be extracted.
  - PaulHoule 3y ago
    It's an established technology https://en.wikipedia.org/wiki/Gravity_assist https://en.wikipedia.org/wiki/Gravity_assist there is nothing speculative about it
  - oopsemist 3y ago
    Opposite is true also: it's possible to add energy into three body system and increase distance between them (antigraviation). In principle, spherical gradient of gravitation creates possibility to increase orbit using only energy and interaction between masses even in two body system (if smaller body can change it shape and distibution of mass to simulate three body system). I had idea of such aparatus when I was student 30 years ago, but then I forgot the details.
- mr_mitm 3y ago
  Figure 1 shows one possible mechanism. It's basically a gravitational slingshot using a binary star system. A test mass comes out with more kinetic energy than before and the binary star system's radius decreases, releasing gravity waves at the same time.
- bluepoint 3y ago
lb1lf 3y ago
In a similar vein, John Kraus (Of Antennas... textbook fame) described a gravitational transmitting and receiving system as a fun (?) diversion near the end of the book. It has been a few years, but I seem to recall that the transmitter was a 500T steel bar spun at very close to the maximum RPM the tensile strength of steel allowed; the radiated energy was something like a fraction of an attowatt. (An attowatt = 1*10^-18W) There are more efficient transmitting schemes out there.
- bee_rider 3y ago
  It is kind of wild that we stumbled upon transmitting information via electromagnetic waves so early on. They seem to be hard to beat.
  - thx-2718 3y ago
    Fortunately for us we can see them. Gravity waves are much harder to observe.
    - BurningFrog 3y ago
      You can also argue that we can see them because they are stronger and are evolutionary important.
      - mrkstu 3y ago
        Well, I'd call the inner ear an evolutionary response to gravity...
  - db48x 3y ago
    Not unexpected; electromagnetism is 40 orders of magnitude stronger than gravity!
  - jjk166 3y ago
    It makes sense that the easiest to implement would also be the easiest to learn. It's more shocking just how much harder every other option is.
photochemsyn 3y ago
Dyson was quite the visionary. LIGO / Virgo gravitational wave detectors have confirmed all this (with much more development from people like Caltech's Kip Thorne and many others): > "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... the loss of energy by gravitational radiation will bring the two stars closer with ever-increasing speed, until in the last second of their lives they plunge together and release a gravitational flash at a frequency of about 200 cycles and of unimaginable intensity." The Ligo Lab's youtube channel has lots of great videos on the topic, from the sounds made by a pair of colliding black holes to long talks about how certain elements are mostly made by colliding neutron stars: https://www.youtube.com/@LIGOLabCaltechMIT/playlists https://www.youtube.com/@LIGOLabCaltechMIT/playlists
deleted 3y ago
[deleted]
mr_mitm 3y ago
I always wondered how you might be able to extract energy from the expansion of space. It's particularly interesting because conservation of energy does not hold on such large scales.
- oopsemist 3y ago
  > It's particularly interesting because conservation of energy does not hold on such large scales. IMHO, conservation of energy still applies, so Big Bang model is just wrong. Use Occam's razor when in doubt.
  - mr_mitm 3y ago
    I'm not in doubt on this question. Please refer to https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ https://www.preposterousuniverse.com/blog/2010/02/22/energy-... for a digestible piece on this topic.
    - raattgift 3y ago
      Think about harpooning a galaxy at, say, 100 megaparsecs, with a long rope attached to the harpoon. In the Milky Way, loop the rope around the rotor of an electric generator. In the distant galaxy, have the harpoon-end of the rope fall into its central supermassive black hole. Ignoring proper motions (the black hole and the electric generator are likely to move within their host galaxies, and their host galaxies within their galaxy cluster), this gives one about 72 kilometres per second per megaparsec of linear speed on the rope as the space between us and the distant galaxy increases. Of course, you need a lot of rope, for the rope to be indestructible (and ideally of low mass), for lucky aim when harpooning, and for the harpoon to be able to carry rope all the way to the target, and for the target and far end of the rope to be impossible to separate. The more local model for this is to erect a scaffolding well above an object in hydrostatic equilibrium (so anything from a round planet to a supermassive black hole) and fix electric generators to the scaffolding, driven by ropes dropping onto the scaffold-surrounded object. There are a lot of physics questions that can be explored using that model; it's a good exercise in all of them. (Some coursework uses this setting to explore the dominant energy condition of general relativity, since that imposes a maximum tensile strength on non-exotic matter rope or wire or filament: there is a speed limit on the operation of intermolecular/interatomic binding forces; c.f. Bell's rope-spaceship "paradox" in special relativity.) > energy is not conserved Carroll's point is that there is a generalization of conservation of energy in curved Lorentzian spacetimes, where changes in the motion of matter and changes in the spacetime geometry are exactly related. That applies in the harpoon-a-distant-galaxy model as well. The rope (and stresses within it) and power produced by the electric generator are all forms of moving matter, creating a geometrical change which (depending on the properties of the rope) may become non-negligible. A rope that is strong enough (and implicitly having much more mass per cm^3 than empty space) to connect two megaparsec+-separated galaxies (driving a generator at one end for appreciable time and feeding a black hole at the other for appreciable time) forces one into some calculating to answer the question: does the rope slow the metric expansion along its length? Next, how do you get the generator to turn rather than be carried out of our galaxy? (We can sharpen this somewhat by dispensing with a generator, and throwing each end of our megaparsecs-long rope into a megaparsecs-separated galactic centre black hole. What happens if there is a large mass-ratio (heavy:light) between the black holes, or their surrounding galaxies? Does the lighter black hole get pulled out of its galaxy by the heavier? What happens as the mass ratio goes to 1? Carroll's link above, showing \Nabla_{\mu}T^{\mu\nu} = 0 says that as long as we don't introduce further degrees of freedom we can calculate the equations of motion in the systems above. That is, it's fine for an expanding space with nonzero vacuum energy, and for that plus noninteracting (except by gravity) dusts. However, our very long rope cannot be non-interacting (it must be at least self-interacting) and its extra degrees of freedom are liable to become important under extreme tension (e.g., it might get hot and radiate a ~blackbody spectrum), so a somewhat different covariant equation would apply.
abbusfoflouotne 3y ago
Reminds me a lot of the Three Body Problem by Cixin Liu. Lot of space travel concepts explored there similar to this. https://www.amazon.com/Three-Body-Problem-Remembrance-Earths-Past/dp/0765377063 https://www.amazon.com/Three-Body-Problem-Remembrance-Earths...
deleted 3y ago
[deleted]
munchler 3y ago
I think this idea is essentially an example of a "gravitational slingshot". I found it interesting that such a system could be used to "accelerate delicate and fragile objects to a velocity of 2000 km/sec at an acceleration of 10,000 g, without doing any damage to the objects. ... So a large space ship with human passengers and normal mechanical construction could easily survive the 10,000 g acceleration." This seems counterintuitive, but since the object is in freefall the entire time, I guess it makes sense.
- Qem 3y ago
  The 10,000g would be approximately uniform through the entire volume of the ship, so no damage. In a regular rocket, the acceleration would have to be transmitted to passengers and cargo through the normal force, and that would crush you.
  - mrkstu 3y ago
    Makes sense, though if you'd asked me before your explanation, I'd have thought the idea of accelerating at high speed without leaving free fall impossible.
    - perihelions 3y ago
      It's a free fall that leaves the system at a different speed than it arrives!
- z3t4 3y ago
  A gravitational assist/slingshot is just a transfer, think of cogs in a machine - by using axles and cogs you can change the speed and direction of the forces. source: KSP player
HarHarVeryFunny 3y ago
Of course there are also more mundane ways of utilizing gravity such as Tidal and Hydroelectric power, or just walking (controlled falling) for that matter.
- amelius 3y ago
  But tidal energy will make the Moon lose altitude. Better to experiment outside the solar system.
0xAltCunningham 3y ago
[dead]
aldousd666 3y ago
This talks about two bodies, but would the Halo Drive (https://arxiv.org/abs/1903.03423 https://arxiv.org/abs/1903.03423) also be one such thing? I mean, I guess the photons are the other bodies?
abecedarius 3y ago
Does anyone have an idea what sort of design could achieve the proposal near the end: > Clearly the immense loss of energy by gravitational radiation is an obstacle to the efficient use of neutron stars as gravitational machines. It may be that this sets a natural limit of about 108 cm/sec to the velocities that can be handled conveniently in a gravitational technology. However, it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible. From my dim memory of Kip Thorne's popular book, a spinning black hole could be used in this way, which would mean there's at least one solution.

217 points by sohkamyung 3y ago

sgt 3y ago

[flagged]

forgetfreeman 3y ago
No, no it is not.

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 ago
Nice 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 ago
    What is the issue with this comment?
    - cwillu 3y ago
      https://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 ago
        If I wanted to know what Bard or ChatGPT had to say about this, I would ask them.
        westurner 3y ago
        But 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 ago
        Given 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
- floobertoober 3y ago
  If space is expanding, does that also mean that gravitational potential energy is always increasing, too? I'd never thought of that before
  - oopsemist 3y ago
    Yes, 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 ago
    No, 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 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? 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 ago
        Obviously 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 ago
Dyson was like George Gamow, protean in intellectual power and scope and imagination yet forever just outside the Nobel sphere.
- robertlagrant 3y ago
  Is that why he invented the Dyson Sphere?
  - samstave 3y ago
    Seems like a well rounded fellow.
  - swader999 3y ago
    Circular logic.
    - mayormcmatt 3y ago
      No, spherical logic.
      - TeMPOraL 3y ago
        No, 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 ago
  I think you are being a bit Anglo-centered.
  - bookofjoe 3y ago
    Georgiy 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 ago
Would Google happen to have a scan of this in their whatever-its-called-book-scanning-archive?

howexactly 3y ago

how exactly is energy extracted from such a system?

howmayiannoyyou 3y ago
Can't say exactly, but kinetic energy can slightly alter mass. For example, increasing the speed of an object increases its mass.
Robotbeat 3y ago
A large mass is dropped on the right trajectory between the two, the gravitational forces slingshot it back at higher velocity, which can be captured with some sort of electromagnetic regenerative braking mechanism. Just a toy idea for how energy can be extracted.
- PaulHoule 3y ago
  It's an established technology https://en.wikipedia.org/wiki/Gravity_assist https://en.wikipedia.org/wiki/Gravity_assist there is nothing speculative about it
- oopsemist 3y ago
  Opposite is true also: it's possible to add energy into three body system and increase distance between them (antigraviation). In principle, spherical gradient of gravitation creates possibility to increase orbit using only energy and interaction between masses even in two body system (if smaller body can change it shape and distibution of mass to simulate three body system). I had idea of such aparatus when I was student 30 years ago, but then I forgot the details.
mr_mitm 3y ago
Figure 1 shows one possible mechanism. It's basically a gravitational slingshot using a binary star system. A test mass comes out with more kinetic energy than before and the binary star system's radius decreases, releasing gravity waves at the same time.
bluepoint 3y ago

lb1lf 3y ago

In a similar vein, John Kraus (Of Antennas... textbook fame) described a gravitational transmitting and receiving system as a fun (?) diversion near the end of the book. It has been a few years, but I seem to recall that the transmitter was a 500T steel bar spun at very close to the maximum RPM the tensile strength of steel allowed; the radiated energy was something like a fraction of an attowatt. (An attowatt = 1*10^-18W) There are more efficient transmitting schemes out there.

bee_rider 3y ago
It is kind of wild that we stumbled upon transmitting information via electromagnetic waves so early on. They seem to be hard to beat.
- thx-2718 3y ago
  Fortunately for us we can see them. Gravity waves are much harder to observe.
  - BurningFrog 3y ago
    You can also argue that we can see them because they are stronger and are evolutionary important.
    - mrkstu 3y ago
      Well, I'd call the inner ear an evolutionary response to gravity...
- db48x 3y ago
  Not unexpected; electromagnetism is 40 orders of magnitude stronger than gravity!
- jjk166 3y ago
  It makes sense that the easiest to implement would also be the easiest to learn. It's more shocking just how much harder every other option is.

photochemsyn 3y ago

Dyson was quite the visionary. LIGO / Virgo gravitational wave detectors have confirmed all this (with much more development from people like Caltech's Kip Thorne and many others): > "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... the loss of energy by gravitational radiation will bring the two stars closer with ever-increasing speed, until in the last second of their lives they plunge together and release a gravitational flash at a frequency of about 200 cycles and of unimaginable intensity." The Ligo Lab's youtube channel has lots of great videos on the topic, from the sounds made by a pair of colliding black holes to long talks about how certain elements are mostly made by colliding neutron stars: https://www.youtube.com/@LIGOLabCaltechMIT/playlists https://www.youtube.com/@LIGOLabCaltechMIT/playlists

deleted 3y ago

[deleted]

mr_mitm 3y ago

I always wondered how you might be able to extract energy from the expansion of space. It's particularly interesting because conservation of energy does not hold on such large scales.

oopsemist 3y ago
> It's particularly interesting because conservation of energy does not hold on such large scales. IMHO, conservation of energy still applies, so Big Bang model is just wrong. Use Occam's razor when in doubt.
- mr_mitm 3y ago
  I'm not in doubt on this question. Please refer to https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ https://www.preposterousuniverse.com/blog/2010/02/22/energy-... for a digestible piece on this topic.
  - raattgift 3y ago
    Think about harpooning a galaxy at, say, 100 megaparsecs, with a long rope attached to the harpoon. In the Milky Way, loop the rope around the rotor of an electric generator. In the distant galaxy, have the harpoon-end of the rope fall into its central supermassive black hole. Ignoring proper motions (the black hole and the electric generator are likely to move within their host galaxies, and their host galaxies within their galaxy cluster), this gives one about 72 kilometres per second per megaparsec of linear speed on the rope as the space between us and the distant galaxy increases. Of course, you need a lot of rope, for the rope to be indestructible (and ideally of low mass), for lucky aim when harpooning, and for the harpoon to be able to carry rope all the way to the target, and for the target and far end of the rope to be impossible to separate. The more local model for this is to erect a scaffolding well above an object in hydrostatic equilibrium (so anything from a round planet to a supermassive black hole) and fix electric generators to the scaffolding, driven by ropes dropping onto the scaffold-surrounded object. There are a lot of physics questions that can be explored using that model; it's a good exercise in all of them. (Some coursework uses this setting to explore the dominant energy condition of general relativity, since that imposes a maximum tensile strength on non-exotic matter rope or wire or filament: there is a speed limit on the operation of intermolecular/interatomic binding forces; c.f. Bell's rope-spaceship "paradox" in special relativity.) > energy is not conserved Carroll's point is that there is a generalization of conservation of energy in curved Lorentzian spacetimes, where changes in the motion of matter and changes in the spacetime geometry are exactly related. That applies in the harpoon-a-distant-galaxy model as well. The rope (and stresses within it) and power produced by the electric generator are all forms of moving matter, creating a geometrical change which (depending on the properties of the rope) may become non-negligible. A rope that is strong enough (and implicitly having much more mass per cm^3 than empty space) to connect two megaparsec+-separated galaxies (driving a generator at one end for appreciable time and feeding a black hole at the other for appreciable time) forces one into some calculating to answer the question: does the rope slow the metric expansion along its length? Next, how do you get the generator to turn rather than be carried out of our galaxy? (We can sharpen this somewhat by dispensing with a generator, and throwing each end of our megaparsecs-long rope into a megaparsecs-separated galactic centre black hole. What happens if there is a large mass-ratio (heavy:light) between the black holes, or their surrounding galaxies? Does the lighter black hole get pulled out of its galaxy by the heavier? What happens as the mass ratio goes to 1? Carroll's link above, showing \Nabla_{\mu}T^{\mu\nu} = 0 says that as long as we don't introduce further degrees of freedom we can calculate the equations of motion in the systems above. That is, it's fine for an expanding space with nonzero vacuum energy, and for that plus noninteracting (except by gravity) dusts. However, our very long rope cannot be non-interacting (it must be at least self-interacting) and its extra degrees of freedom are liable to become important under extreme tension (e.g., it might get hot and radiate a ~blackbody spectrum), so a somewhat different covariant equation would apply.

abbusfoflouotne 3y ago

Reminds me a lot of the Three Body Problem by Cixin Liu. Lot of space travel concepts explored there similar to this. https://www.amazon.com/Three-Body-Problem-Remembrance-Earths-Past/dp/0765377063 https://www.amazon.com/Three-Body-Problem-Remembrance-Earths...

deleted 3y ago

[deleted]

munchler 3y ago

I think this idea is essentially an example of a "gravitational slingshot". I found it interesting that such a system could be used to "accelerate delicate and fragile objects to a velocity of 2000 km/sec at an acceleration of 10,000 g, without doing any damage to the objects. ... So a large space ship with human passengers and normal mechanical construction could easily survive the 10,000 g acceleration." This seems counterintuitive, but since the object is in freefall the entire time, I guess it makes sense.

Qem 3y ago
The 10,000g would be approximately uniform through the entire volume of the ship, so no damage. In a regular rocket, the acceleration would have to be transmitted to passengers and cargo through the normal force, and that would crush you.
- mrkstu 3y ago
  Makes sense, though if you'd asked me before your explanation, I'd have thought the idea of accelerating at high speed without leaving free fall impossible.
  - perihelions 3y ago
    It's a free fall that leaves the system at a different speed than it arrives!
z3t4 3y ago
A gravitational assist/slingshot is just a transfer, think of cogs in a machine - by using axles and cogs you can change the speed and direction of the forces. source: KSP player

HarHarVeryFunny 3y ago

Of course there are also more mundane ways of utilizing gravity such as Tidal and Hydroelectric power, or just walking (controlled falling) for that matter.

amelius 3y ago
But tidal energy will make the Moon lose altitude. Better to experiment outside the solar system.

0xAltCunningham 3y ago

[dead]

aldousd666 3y ago

This talks about two bodies, but would the Halo Drive (https://arxiv.org/abs/1903.03423 https://arxiv.org/abs/1903.03423) also be one such thing? I mean, I guess the photons are the other bodies?

abecedarius 3y ago

Does anyone have an idea what sort of design could achieve the proposal near the end: > Clearly the immense loss of energy by gravitational radiation is an obstacle to the efficient use of neutron stars as gravitational machines. It may be that this sets a natural limit of about 108 cm/sec to the velocities that can be handled conveniently in a gravitational technology. However, it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible. From my dim memory of Kip Thorne's popular book, a spinning black hole could be used in this way, which would mean there's at least one solution.