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The Edges of Our Universe
- echelon 6y agoThis is such a cool paper! One of the neat ideas it communicates is that the longer we wait, the less of the universe we can access: Delay - Galaxies lost 1 million years ~ 0.02% 10 million years ~ 0.2% 100 million years ~ 2% 1 billion years ~ 20% 10 billion years ~ 80% 150 billion years ~ 99.9999997% Or this, "For example, dropping matter into black holes via their accretion disks converts 5.7% of its rest mass into light, which far exceeds the efficiency of the thermonuclear reactions which power stars, and might be a much more efficient way to gain access to the mass-energy stars contain (though it clearly comes with its own engineering obstacles)." "A related question is not how much matter (or energy) could be reached, but what is the greatest amount a civilisation could secure in one gravitationally bound location, so that it could be used for some unified long-lasting project during the time of isolation" Maybe we can pinpoint where advanced alien civilizations would be based on an understanding of their energy needs. Spoiler alert on Hubble Extreme Deep Field photo: "This image thus spans the edge of our affectable universe, with most of the places it shows being forever beyond our reach. " I find all of this incredibly beautiful.
- hn_throwaway_99 6y agoI haven't read the paper, but knowing a little about cosmic expansion, even if we "don't wait" and manage to launch 2 different teams to 2 different galaxies, at some point those galaxies will become unobservable from each other.
- deleted 6y ago[deleted]
- throwaway_kufu 6y agoIt always seems we have a certain bias when it comes to imagining alien civilizations more advanced than our own and that their energy needs would somehow be the same, similar, or greater than our own. Ignoring gravitational differences of various potential origin planets and the energy requirement differences to get into orbit based on a given planet’s gravity fields...what if the beings themselves were many orders of magnitude smaller or larger (or both for that matter). Even here on earth imagine some other beings with all the capabilities as us but could essentially stretch their arms upward and place little old us into orbit without the need for propulsion, or other beings so microscopic we could never be capable of detecting them even if they had the capability of building craft that could escape Earth’s gravity field.
- Larrikin 6y agoAre you referring to Dragons egg or just imagining the plot independently?
- Sharlin 6y agoThe only assumption made here is ambition, and it is explicitly stated. An ambitious civilization will figure out a use for whatever amount of harvestable energy is available. Computation, if nothing else. The paper is about upper bounds; whether any civilization can (or wants to) actually reach anywhere near them is another question for another paper.
- chriswarbo 6y agoThis sounds a lot like the topics discussed by SFIA https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g For example the 'civilisations at the end of time' series https://www.youtube.com/playlist?list=PLIIOUpOge0LvHsTP5fm8oxB1qPS54sTMk https://www.youtube.com/playlist?list=PLIIOUpOge0LvHsTP5fm8o...
- throwaway_kufu 6y agoI don’t think I’ll ever understand the astrophysics use of the verbiage “causal” and “observable” in lieu of just using “light/ERM” and “spacetime”. The only argument I could see for the need of these terms in addition to just light/ERM would be the ability to detect the existence of something beyond the furthest edges of the universe from where light reaches us through phenomena such as gravitational pull from beyond the edges on the bodies at the edges.
- isaacg 6y agoIn the early universe, light could not travel freely, because the ambient temperature was hot enough that the universe was filled with plasma. The end of this period is called "recombination", and the light from that point in time is called the Cosmic Microwave Background. That light is the earliest light we can observe, but it is not the earliest event to have had casual effects on us, nor is the earliest event we can observe. In particular, that light was emitted about 370,000 years after the Big Bang, while events within the first second after the Big Bang have had casual effects on us. So there is an important difference between "light" and "casual" in physics.
- throwaway_kufu 6y agoNice, my initial impression of your comment still fits squarely within the door I left open for using causal...except when I was thinking phenomena outside the edges physical space where light reaches us you expand it to outside the edges of time where light still reaches us. Unfortunately I’m now just sort of stuck thinking about causality, determinism and free will...not exactly knowing where you are on Earth I guess it’s a possibility you have never been and never will in my light cone, yet my comment was the causality of your reply in some sense. And in maybe an HN 1st you’ve made me reconsider my position, and Recognize I need to more deeply consider causality as having good use outside phenomena from outside light at the edges of space and time and spacetime.
- panda-giddiness 6y agoOf course they've been in your lightcone -- otherwise it would've been impossible for you two to have a back-and-forth exchange!
- astatine 6y agoOff-topic, but does anyone else feel claustrophobic when reading about the vastness of the universe? I enjoy astronomy related topics but when reading articles about the scale or the expansion of the universe, etc I start feeling physically uncomfortable and need to take a break.
- R0b0t1 6y agoYou're looking for agoraphobic.
- neetrain 6y agoWe're forever stuck inside a thin layer of the Earth surface. It makes me kind of claustrophobic.
- d_silin 6y agoDefinitely not forever.
- neetrain 6y agoIt's only my perceptions. Yes, several hundreds of people had already been in space. Space travel could be the norm in the next few decades. Man could terraform Mars in the next millennium, or conquer the milky way galaxy five million years later. However, I still feel like "we're stuck forever".
- astatine 6y agoThe sense of smallness of the self is probably what causes the claustrophobia. The last time I read of agoraphobia was in an Ian Fleming novel!
- crancher 6y agoTry and "step back" and observe the universe observing itself through your mind. Your "you" is local chunk of universe coded with a capacity to consider the smallest and largest scales of itself. Your mind can easily encompass the universe in thought, summing up everything known as easily as you take a breath. It's an incredible ability, cherish it!
- wideareanetwork 6y agoThe interesting thing about the universe is that it appears, as far as can be measured, to be flat. This implies that it’s either infinite or so large that if it’s curved then we can’t detect the curve over 90 billion light years. Either way, the universe is unbelievably, vastly bigger even than the observable 90 billion light years we live in.
- EMM_386 6y agoWhat my mind fails to understand is, if the universe was infinite, how does that work with the "big bang" theory? Apparently the universe originally (somehow) began as a point of infinite density and expanded outward from this point. During this process, it would seem to need a defined size in order to be expanding at all. Even with the "blowing up a balloon" way of thinking of the universe, while the individual galaxies are growing further apart, the surface area is measurable (not infinite). I am likely thinking of this incorrectly but I can't fit "infinite size" in with "expanding universe" in my head. Unless these theories are incompatible, and to get infinite size you need the steady-state theory.
- yazantapuz 6y agoI think you should read this article "Misconceptions about the Big Bang": http://pages.erau.edu/~reynodb2/LineweaverDavis_BigBang_SciAm_March05p36.pdf http://pages.erau.edu/~reynodb2/LineweaverDavis_BigBang_SciA... The big bang was an "explosion" of space itself. There was not previously empty space.
- EMM_386 6y ago> The big bang was an "explosion" of space itself. There was not previously empty space. I am very familiar with all of this. My question was merely around the opposing concepts of "expansion" and "infinite". Not infinite size AT the big bang. Infinite size of the current universe.
- simonh 6y ago
- gorgoiler 6y agoOur galaxy is huge but we don’t really notice because it is so bright, and moreover has been bright for a very long time. The light we see is old. It’s of interest for studying the science of bright old things but for anything contemporary it is useless. If you ignore our own star and count out the ancient starlight, the sky is completely dark to us. It is as if we are searching for someone to talk to by rummaging through rocks, looking for fossils. It applies to humans too. Anyone we sent to another star would be gone for years. They would be physically gone, but fundamentally we just wouldn’t be able to talk to them at all for years on end. Unless something about the speed of light changes, Earth and her inhabitants will always be completely and fundamentally alone.
- simonh 6y agoOur galaxy is "only" 50,000 lightyears across. 50k years is an eyeblink in cosmological terms. Behaviourally modern humans have been around about that long and it's too short a time for fossils to even form, except in extraordinary conditions. It's a thousandth of the time to the dinosaurs. I think that's reasonably up to date in evolutionary terms. Andromeda at 2.5m ly is a bit more of a stretch, but it's still a fairly short period in the history of life on Earth. It all depends on how long we think technological civilizations last on average. If they can reach sustainability it could be at least as long as their solar systems remains habitable, maybe hundreds of millions of years, even without interstellar travel.
- vardump 6y ago"The light we see is old. It’s of interest for studying the science of bright old things but for anything contemporary it is useless. If you ignore our own star and count out the ancient starlight, the sky is completely dark to us." Even sunlight is really old. The photons from our own star, the sun, were produced 100,000 years ago. That's how long they take to make their way to the surface from center of the sun where the nuclear fusion occurs.
- chrisweekly 6y agoTrue, but then it's just 8 minutes from the sun's surface to the earth's.
- forgotpwd16 6y agoHaven't seen a non-TeX paper that looked that good (on arXiv) for a while.