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Well, there's the ultimate fate of the core to deal with. It depends on the mass of the core. At about 1.3 times the mass of the Sun, we leave the white dwarf stage and we'll end up with what's called a neutron star. A neutron star is the entire core of the star, with the individual elements (mostly iron) making up the core broken apart on the nuclear level. An atom is mostly empty space. The bohr model serves as a handy shorthand for what an atom is like:  I'm sure you've all seen that sort of image before. It's not quite representative of what an atom is like, but it's in the ballpark. In the atom, the protons and neutrons and electrons are minuscule and very distant from each other: 99.9999999999996% of a hydrogen atom is empty space. If the Earth were an atom, a proton at the center would be about 600 feet across, while the electrons would be around the surface. Everything else is empty space. In a neutron star, the forces that keep things going like that are too strong. This is how we end up with 'degenerate' matter. White dwarfs are electron degenerate matter, but neutron stars are far stronger than that and push far, far further down. 2 or 3 solar masses are compressed down to about the size of manhattan.  Crab Pulsar, a neutron star in the Crab Nebula These guys spin very rapidly, like a ballet dancer that pulls in his or her hands.. All that matter being shrunk to the small size means a very fast spin. Matter behaves very strangely in degeneracy. That's not to say it's all just neutrons stuck together. Here's a diagram describing the theorized interior of a neutron star:  But the key point is that the gravity, pressure, and density overwhelm the forces that keep matter ordered as it is. It butts up against the strong nuclear force: The natural force in the universe that keeps neutrons apart. Incredibly hot, incredibly dense, they even bend light around them: Looking directly at one, you see slightly more of its surface than you would normally (more than half)  (Light deflection by the star: You can see both poles, and each patch represents 30 degrees by 30 degrees.) I hope we get to study these objects in great detail someday, a lot could be learned from them. They're some of the most energetic objects in the sky; their incredibly, millisecond spins in some cases and ability to compress accretion lead to some noisy radio signals, x-rays, and other forms of radiation. (If a magnetically charged pulsar is spinning 642 times a second, that's some pretty crazy electromagnetic emissions.) More massive still, though the exact boundary is not known, and you have enough force and pressure to overcome the neutron degeneracy pressure. Even the strong nuclear force can no longer hold back pressure and gravity of the object. The neutrons themselves collapse, and the density of the object and force of gravity start to curve light toward it so strongly that the light can never escape. Once the stellar remnant collapses to a density that fits under the Schwarzchild radius of the mass, then this is the permanent and bizarre result:  A black hole. Probably the most mysterious and least understood stellar object, though we have excellent observational evidence to confirm their existence and thousands and thousands of pages worth of theory and discussion on them. Light can no longer escape; the laws of space and time break down, relativistic time dilation causes incredibly unusual effects. Cygnus X-1 is the closest known object thought to be a black hole, a powerful x-ray source that can only be explained by the existence of a black hole. (It generates massive energy through the slow accretion of the nearby star in its binary. What doesn't end up past the event horizon is emitted in enormous jets, which traced as path accelerated by the gravity but didn't fall into the singularity itself.)  (Artist's impression of Cygnus X-1) They are incredibly bizarre objects, that seem like division by zero errors of the universe. You wouldn't think the solution to 'the matter is getting too dense to support itself' would just be 'Okay, collapse to almost zero size then, that works out okay by me.' The universe is an odd thing. joke_explainer fucked around with this message at 16:13 on Apr 11, 2015 |
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Apr 10, 2015 21:24
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ron color posted:......what if you built a city planet around a white dwarf star It'd be a great place to put one. The light would last for hundreds of billions of years and be very stable. Of course, when they started to die out, all the others ones would be too, so hopefully you built some really good batteries and have been economical with your energy consumption. Once all the white dwarfs are gone, it's just a matter of using up what energy is left. |
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Apr 10, 2015 21:26
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secdrone I apologize friend but I don't think your thread is going to be gold in the next half hour or so... at least i'm going down with you
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Apr 10, 2015 22:39
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Bwee posted:secdrone I apologize friend but I don't think your thread is going to be gold in the next half hour or so... at least i'm going down with you it's no problem! shame, it was gold yesterday. Who voted 1, so I can know who to hold a grudge against forever? 
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Apr 10, 2015 22:39
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*starts modifying the probation clock into an automated probation machine when viewed by a mod* |
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Apr 10, 2015 22:40
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1.0: Vendor Trash 1.0: Koishi Komeiji 1.0: Throwdini 1.0: Crap 1.0: cute anime girl
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Apr 10, 2015 22:44
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JUUULLLLLIOOOO!!!!! |
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Apr 10, 2015 22:45
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Oh, please don't gas the thread though, I might add to it later if anybody has questions or astrophysics stuff they want to talk about. (USER WAS PUT ON PROBATION FOR THIS POST) |
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Apr 10, 2015 22:53
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of course not. bonne nuit mon ami (USER WAS PUT ON PROBATION FOR THIS POST)
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Apr 10, 2015 22:55
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owned |
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Apr 10, 2015 22:57
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on the upside, that's an additional 20 cents to animals in need |
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Apr 10, 2015 23:06
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joke_explainer posted:JUUULLLLLIOOOO!!!!! I made my intentions clear at the beginning of the month. (USER WAS PUT ON PROBATION FOR THIS POST) |
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Apr 10, 2015 23:29
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Bwee posted:1.0: Vendor Trash Jesus. |
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Apr 10, 2015 23:47
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ulvir posted:on the upside, that's an additional 20 cents to animals in need Actually it's like $1.60 i think |
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Apr 10, 2015 23:49
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who the hell is throwdini |
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Apr 10, 2015 23:55
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While security dron is probed let me take this opportunity to tell you all that the truth lies in the luminiforous æther and also the terracentric aristotelian cosmology
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Apr 10, 2015 23:57
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Alright. Now that that's over with. Alnilam, anything I got way wrong about nucleosynthesis? Let's hear it. Questions about black holes? If I can't answer it, a bunch of people here probably can. |
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Apr 11, 2015 06:04
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what's the skinny on worm holes op?
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Apr 11, 2015 06:40
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alnilam posted:Actually it's like $1.60 i think even better |
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Apr 11, 2015 08:37
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Isn't it possible that we don't know enough about space and the beginning of the universe that what we postulate about the big bang and the end of the universe is actually meaningless? Kinda like the aether or earth being the center of the universe. It matches everything we know now, but we might not actually know any of the important things. |
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Apr 11, 2015 13:10
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alnilam posted:While security dron is probed let me take this opportunity to tell you all that the truth lies in the luminiforous æther and also the terracentric aristotelian cosmology This. |
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Apr 11, 2015 13:10
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joke_explainer posted:Alright. Now that that's over with. Alnilam, anything I got way wrong about nucleosynthesis? Let's hear it. Questions about black holes? If I can't answer it, a bunch of people here probably can. No it's all more or less correct except that the real truth is everything was put here by God our Lord to give us an earth of plenty in His kingdom |
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Apr 11, 2015 13:41
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Apr 11, 2015 13:49
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Is it still a theory that black holes lead thru a wormhole to a white hole that ejects all the stuff sucked into the black hole? That never made a ton of sense to me. like i understand the idea of black holes mathematically being a singularity in spacetime (a hole), but on the other hand, they're "just" an object that has become so sense that it passes the threshold where light can escape. To me all that means is, well, that there's still a super dense object in there collecting all the mass it's sucking in. I get why the math suggests an actual hole in spacetime, but to me, without evidence to the contrary, i have to assume it's a failure of the math (it is a very extreme case after all) rather than a real rip in spacetime. In short, I think black holes are just dense objects (smaller than the Schwarzchild radius or w/e) getting heavier and heavier as they pull in more stuff and they're NOT a rip in spacetime that leads to a wormhole or something. Is there any evidence besides pure theory/math to suggest otherwise? |
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Apr 11, 2015 13:50
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bog pixie posted:Isn't it possible that we don't know enough about space and the beginning of the universe that what we postulate about the big bang and the end of the universe is actually meaningless? Kinda like the aether or earth being the center of the universe. It matches everything we know now, but we might not actually know any of the important things. Of course, but any explanation we find in the future has to explain the temperature, wavelength and existence of the background radiation, immature shapes of distant galaxies, the apparent age of white dwarves, the abundance and ratio of hydrogen and helium in the universe, and explain why the universe is expanding (and why that expansion seems to match Hubble's law), and why all those things line up and support each other observationally. Earth centricism didn't have a wealth of information that supported it, nor did the concept of the luminous aether. The whole point in the explanation is there probably is no explanation other than the Big Bang; whatever changes to it will be refinements, not complete redefinitions. Kind of like Newton's laws of motion. It's not like anything ever came along and was like, nope, f=ma is wrong!!! Einstein's theories were just refinements on Newton's, which were well observationally supported (but still had some unanswered questions). The whole point of explaining all that was to demonstrate how well the observational evidence came together. We very clearly do have the information we need to come to those conclusions. Not flying blind here. joke_explainer fucked around with this message at 14:28 on Apr 11, 2015 |
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Apr 11, 2015 13:56
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do you think it's possible for life to develop on a world orbiting a white dwarf? such a world wouldn't be on the 5 billion (or much less) countdown to destruction that Earth is on, it would be safe from star death for hundreds of billions of years right?
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Apr 11, 2015 16:38
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Yeah but the star itself will make fun of anything smaller than it, can something really grow when it's burned every day, all day?
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Apr 11, 2015 16:40
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smoobles posted:what's the skinny on worm holes op? Wormholes are the realm of pure theory - absolutely no observational evidence has been observed that supports the idea they exist. They are general solutions to some conditions in general relativity. Like black holes, they are a mathematical solution to Einstein's field equations, ten equations that define how matter can interact with space and time in general relativity. Unlike black holes, we've found no objects in the sky that can only be explained by the existence of them. The conditions in our universe that might allow for them are a lot more particular than making a black hole, but may (or may not) be physically possible. Unfortunately Underpinnigs So in 1905, it had been measured in the Michelson-Morley experiments that the speed of light was always exactly the same. This was an unusual result, and a lecture by Lord Kelvin claimed it was a fundamentally rule-breaking discovery that had no explanation in current physics. Einstein, in 1905, was just a guy working in the patent office in Bern, and he had this year where he submitted a series of papers that would solve this problem to a degree. Electromagnetic waves had been observed, and the idea of the luminiferous ether as being a medium for the transmission of these waves had taken hold, but observational evidence completely contradicted this. The Earth appeared to be stationary to the ether, and no real evidence for the ether could be found. The very basic gist of it was that the speed of light was invariant regardless of where it was observed, and that the speed of light was a constant in the universe regardless of where or how it was observed. This was possible because time and space were variant based on the reference frame of the observer; Two parties travelling in different ways can never experience a truly simultaneous event. The exact effect of this can be a challenge to understand, so I'll try a couple different ways. First, let's compare Euclidean space.  Named for the ancient Greek mathematician and founder of geometry. In Euclid's universe, you move up, you go up. You move down, you go down. Left or right, you go left or right. There are three spatial dimensions, and time is invariant in Euclid's universe, its not a consideration at all. All variance in spatial position is the application of a linear map on coordinates in three dimensional space. It's an appealing notion that resonates with the universe we see around us; if you toss a beer bottle forward, it goes forward, comes to rest over there. Newton's laws mesh cleanly with Euclid space. If you toss a tennis ball forward off a moving train, it retains the energy of both the train and the toss, even if it strikes an observer. Position and speed are related to your velocity, your vector, your acceleration -- the line you trace through space follows simple Newtonian mechanics. You might already see the problem, however. Light emitted from the train should follow the same principle. Shine a flashlight forward and light would be going train speed + light speed. Shine it backward and it should be going light speed - train speed. This would mean we'd see light from distant, speedy objects heading toward us as going nearly twice the speed of light, or more. Or if a train was going fast enough, you could walk back through the cars of the train and the subtraction of the train's velocity from the speed of the light would mean you could outpace the light and sit down before the light from your own travel reached you, so you'd see yourself come over and sit down where you were now sitting. Slow light falling out the back of these trains would startle bystanders hours later. Observationally, these things weren't the case. Light in a vacuum was invariant no matter what source you measured it from. So the easiest way to understand this involves calculus, which I'll skip because I think BYOB's eyes would glaze over if I bust out formulas this early, but basically someone named Minkowski in 1908 found every equation and consequence of special relativity could be mapped on what came to be called Minkowski space. Euclidean space's lines were a product of the x, y, and z coordinate of each path, but Minkowski saw if you added an additional, well, coordinate, X0, that would enter into the derivation of any transformation along Minkowski space.  This diagram shows what happens to simple geometry within the addition of this additional factor. The left is the rotation of lines in Euclidean space, while the right shows the rotation in Minkowski space. t denotes time while x is spatial coordinate location in any given spatial dimension within Minkowski space. The space is hyperbolic instead of flat: the further the events approach the 'corners', the more hyperbolic the events are. The red lines denote worldlines for light-like objects -- fixed on certain four-vectors for all instances. Points instead become events in Minkowski space, with coordinates (x, y, z, t) instead of (x, y and z), though it's better not to think of them as coordinates on some fixed origin space but as four-vectors, with the distance between any two events being the spacetime interval. If four-vectors are your direction through four dimensional space, a worldline is the path traced by the four-vectors, and we end up with three basic ways to describe the spacetime interval (distance between two events): Spacelike, timelike, or lightlike (null). To describe what those mean, we have to define a light cone. Any given event in Minkowski space is the spacetime interval of every wordline-linked event in the past to any worldline-linked event in the future. Here's the most common depiction, a representation of a light cone:  What defines whether or not an event is linked? It's solely whether or not light could travel from event A in the past to event B in the future. When is an event in the future related? When light from this event B could reach event C, in the future. This is both practically and theoretically the case for any given event. Example: Let's say Alpha Centauri were to catastrophically explode right now, sending out a wash of deadly radiation in our direction. Describe the current state of the earth as event A. While the past light cone of A includes plenty of light from Alpha Centauri, the light of its explosion cannot reach Earth for another 4.4 years or so. Therefore the event of Alpha Centauri's catastrophic explosion is outside of the light cone for the Earth at A: They are not causally related from any observer's perspective, yet. In four years, they become causally related, once the light from the event reaches Earth. The interval of the light from this explosion is a light-like traversal of Minkowski space. The future light cone of the explosion does include the Earth. event A's future light cone and the explosion event's light cone will share a section where the worldlines 'intersect' (though they only do so on the timelike curve; they don't quite intersect on the spacelike curve for this, as they don't occupy the same space, they just bump into each other). When you have two objects separated by space, then you have a spacelike interval between them. Like two tennis balls on opposite sides of the room. When you have a tennis ball in one place, and you remove it, then put one down later in the same place, those two tennis balls were separated by a timelike interval. Ultimately what this describes is just the invariance of physical laws from any particular observer: The observer's own event of observations will result in a the universe's laws remaining constant regardless of the position or velocity in space and time of the observer (or the observer's light cone). So what does this mean? To jump to the consequences without understanding the theoretical underpinnings can just be confusing, so if you wanted to skip that entire previous section, this might seem completely out of nowhere. But the end result of the formulation of space as spacetime instead of just space with an invariant time are a few fundamental results: No two events can be absolutely simultaneously Since an event's observed position in spacetime is dependent on their velocity (their reference frame vs relative velocity, at least), you cannot always agree on the simultaneity of events. All events occur non-simultaneously on some level, though practically we don't run into this problem as our velocities don't approach relativistic in our day to day life. I like to use this video to describe this: https://www.youtube.com/watch?v=wteiuxyqtoM Those guys really need some hearing protection or something. Dude on the platform is definitely looking at the train like, 'drat... didn't even stop for me, but I guess I don't want to get on that train getting truck by lighting all the time anyway...' Time dilation: Time lapsed between events is not invariant from reference frame to reference frame The speed with which time passes is not the exact same for all events, a central consequence of the Minkowski space detailed above. Since all events are coordinates in spacetime, and any motion in any four-vector effects the others in spacetime, the distance traveled in time is different for different events, and it's relative to each person's reference frame for those events. This one is easily calculated through this (algebraic) formula:  where Δt is the time passed between two events as measured by an first non-comoving observer (taken as if he was watching a perfect stopwatch while observing, 'proper time' from his reference frame, Δt′ is the time passed as measured by another observer moving with velocity v relative to the first observer, and c is the speed of light. (γ is the Lorentz factor, 1 divided by the square root of 1 - velocity squared divided by the speed of light squared, through which a lot of these effects can be calculated; you can see how as v approaches c, the divisor gets closer and closer to 0.) Practically this means less time passes for a fast-moving observer, even though they never notice any difference from their frame of reference. This leads to the 'twin paradox', which isn't really a paradox, but a situation where one twin gets into a rocket, flies around the solar system at a huge percentage of the speed of light, returns in one year and finds his twin has aged 40 years while he's only experienced one year. For the vast majority of low velocities, time dilation is practically nil. While you experience some level of it just driving around town, you would need an incredibly accurate set of clocks to track this (though, we have experimentally confirmed it in this way.) Like most relativistic effects, it stays almost imperceptible, until you get close to the speed of light when it goes crazy hyperbolic (a consequence of the hyperbolic planes of Minkowski space):  (the degree of effect the closer you get to the speed of light). This effect is a practical implication of relativity: We have to compensate for it in our satellites. GPS would not work if the onboard clocks didn't factor in the velocity of the satellite relative to the ground, as pictured here:  Length contraction: The measured size of an object is not static with regards to all observers The length of an object contracts the closer it is toward the speed of light, as measured by an observer from a different reference frame. So you can fit a 12 foot ladder in a 6 foot wide garage if it's traveling near the speed of light. (But only very, very briefly, and only for a non-comoving observer with the garage. All of these events are relative, so from the perspective of someone riding the ladder, it's the garage that is contracted, not the ladder. The apparent paradox is resolved by the lack of simultaneity; the ladder ending the garage and leaving the garage does not happen at the same time for all parties) This contraction takes place along the vector of motion, so the ladder would have to be pointing lengthwise and not hurtling sideways. Adding velocities You can't just simply add velocities. Firing a 200 km/h projectile off of a boat moving 40 km/h does not exactly result in a projectile moving 240 km/h. This is almost completely irrelevant in most cases, but comes into effect drastically when you have two travelers, both with relative velocity >0.5 the speed of light to each other. The relative velocity measured between them is never over the speed of light, no matter how fast they go. What they'll measure is dependent on their position and velocity. So the thing you probably have noticed about special relativity by now: It doesn't describe gravity whatsoever. It wasn't until 1915 when general relativity was introduced that gravity would enter the mix. General relativity builds on the outline of spacetime in special relativity so I had to start here to get to general relativity, but black holes in particular were a theorized consequence of general relativity that we ended up finding observational evidence of as a confirmation of the theory. Wormholes are another theorized consequence of general relativity, though not one we are likely to find naturally. I'll go into more detail after brunch if people are still interested, and I hope this wasn't too confusing / misleading to anyone. alnilam posted:Is it still a theory that black holes lead thru a wormhole to a white hole that ejects all the stuff sucked into the black hole? No, absolutely not. Naturally occuring wormholes are incredibly unlikely, absolutely bonkers imo. alnilam posted:That never made a ton of sense to me. like i understand the idea of black holes mathematically being a singularity in spacetime (a hole), but on the other hand, they're "just" an object that has become so sense that it passes the threshold where light can escape. To me all that means is, well, that there's still a super dense object in there collecting all the mass it's sucking in. I get why the math suggests an actual hole in spacetime, but to me, without evidence to the contrary, i have to assume it's a failure of the math (it is a very extreme case after all) rather than a real rip in spacetime. I'll come back to this one down the line, but no one really thinks that black holes all go to white holes or anything like that. The general consequence of falling into a black hole is immediate and rapid destruction as far as anyone can tell, but black holes are realistically a deep inescapable well, or a hole in spacetime, not just a 'hyper dense object', though the two are meaningfully the same as GR will demonstrate relative to space as described in my post above. The observational evidence for black holes we have confirms this, as only something that conforms to our idea of GR and has the properties of a black hole could explain what the observed objects are. smoobles posted:do you think it's possible for life to develop on a world orbiting a white dwarf? such a world wouldn't be on the 5 billion (or much less) countdown to destruction that Earth is on, it would be safe from star death for hundreds of billions of years right? Yeah, I think it's definitely possible. I have no idea about the deeper factors concerning its probability, we only have one to go on. But they would be an excellent, long-term stable source of light for at least a hundred billion years and continue to radiate for many hundred trillion years. |
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Apr 11, 2015 17:08
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good post, very interesting |
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Apr 11, 2015 17:22
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man imagine if a white dwarf's planet had a civilization that lasted 100 billion years, then invented the technology to live in orbit of the white dwarf itself once it cooled, surviving trillions of years it's cool to imagine maybe an extremely advanced humanity could move further out into the solar system when our sun goes pop, then move back inward ---------------- |
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Apr 11, 2015 17:29
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smoobles posted:man imagine if a white dwarf's planet had a civilization that lasted 100 billion years, then invented the technology to live in orbit of the white dwarf itself once it cooled, surviving trillions of years This is a central part of the Fermi paradox really. Humanity has done a lot in the 6000 years or so of recorded history. Arguably we tottered around for 200,000 years or so with relatively the same genetic equipment we have today. If humanity survives for another 6000 years even, it's hard to imagine the sort of understanding of the fundamental forces we'll have by then. A million year civilization? Putting a sun out for safekeeping or circumsolar habitats don't seem completely ludicrous. I'm on phone atm so I can't really look stuff up, but there was a classification system for civilizations coined a while back called the Kardashev scale. A type 1 civilization harnesses all the available energy on a global scale: The equivalent of an entire Earth made of solar panels for whatever purposes. We obviously aren't type 1 yet. A type 2 civilization would harness the complete output of a star, through a Dyson swarm or some unimaginable way even more complex. That's an unbelievable amount of energy that would already be an unimaginably powerful civilization. A type 3 would harness their entire galaxy. At type 2 it's just s matter of repeating the trick a few billion times. Long term sustainability would require any civilization to start conservation efforts to minimize entropy in order to keep the universe capable of supporting intelligent life for as long as possible so it's imaginable they would bother to do something like that. But some billion year type 2, harnessing the radiation of a white dwarf, it'd be trivially easy for them to visit or send probes everywhere in the galaxy. But as far as we see they haven't, and don't really bother chatting if they exist at all. |
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Apr 11, 2015 17:55
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Apr 11, 2015 17:58
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Don't fret dear Smoobles! It's always possible we're just the first and not that all life is doomed |
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Apr 11, 2015 18:00
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FUTURE LIGHT CONE
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Apr 11, 2015 20:24
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HYPERSURFACE OF THE PRESENT
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Apr 11, 2015 20:25
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←------------------------------------------SPACE→
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Apr 11, 2015 20:26
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General Relativity So Einstein in 1905 realized the relationship between energy and mass, and the relationship between mass and gravity was seemingly well understood, so there still needed to be an explanation for the effect gravity would have in a Minkowski spacetime. An invariant force that remained immutable despite an observer's condition's didn't fit into the framework and it needed some kind of reconciliation with special relativity. He'd start working on this in 1907, and by 1915 have a fully working theory of gravitation which is still our understanding of gravity today. The foundation of the concept was a simple thought experiment: How is gravity any different from acceleration? If you were standing in an elevator with the doors closed, and not moving, what test could you perform to determine you were on the Earth versus secretly being in a rocket constantly accelerating at 9.8m/s^2 (Earth standard gravity)? Einstein though, there is no difference at all. Gravity is identical to acceleration. This was called the equivalence principle.  einstein and newton observe the same effects here If it held up, the idea was acceleration was completely identical to gravitation in every way. The force of gravitation was a not due to an invisible party pushing on the reference frame, but due to something else. What he ended up solving was that all spacetime is curved by the energy, inertia and momentum within a certain section of space and time. This means the effect of gravity is not, actually, a person curving toward the ground. The 'straight line' through that region of space is just curved; to a non-inertial frame observing the party falling victim to gravity, they are being accelerated, but from the reference frame of the person being accelerated, they feel no force or acceleration. Any object that travels through this curvature of space ends up changing its path in some way. This is still a "straight line", but it's a straight line applied to a curved, four-dimensional surface (a geodesic) instead of in 3-space. This explains things like orbits without added a fictitious force from nothing propagated by a massive body: The spacecraft or satellite orbiting is just following the geodesic around the object.  A geodesic is a projection of a line on a curved surface, like the lines of this triangle in this graphic It's worldline is being changed in every instant; The light cone is being shifted toward the massive conglomeration of energy that is any chunk of matter. The worldline's path, it's spacetime interval, is changed as it appears to accelerate to an observing non-inertial party.  (A common simplified graphic showing the modified path through curved space. Obviously, this is a 2D graphic depicting a 2D universe where the curvature of space is happening across the third dimension instead of what we actually see, which is the same effect but on a Minkowski-like spacetime) What defines the curvature of space? The total amount of energy within an area, which changes the path worldlines trace around objects. This is called the stress-energy tensor. Tensors are just ways to describe the linear motion of vectors without needing to define special cases or preferred frames. That's basically what the entire theory is about -- gravity had to affect mass as we observe, but it couldn't result in the creation of preferred frames or special relativity was wrong. Light is also affected by gravity (something that Newton's laws did say should happen), but the actual effect is about twice what would be predicted by newtonian mechanics. The worldline of light as described by general relativity gives an answer much more in line with expected  How light can end up returning toward its point of origin Light ended up being discovered 'lensed' through all sorts of deformations as it passed around a massive body:  Einstein's cross, four images of the same object, gravitational lens G2237 + 0305  An Einstein's ring around LRG 3-757. These images are a way astronomers can actually get a distant perspective from across the universe, and are valuable tools wherever they are found for observational astronomy Newton's idea of gravity as a propagation of force from a massive body worked extremely well, but there were a few minor discrepancies. The orbits of all the planets had a slight quirk where the furthest point away from the sun in their orbits would get slightly further ahead of itself on every consecutive orbit. Newtonian mechanics could not explain this, but Einstein's general relativity did. The deflection of light as mentioned before was another, and relativistic effects were required to accurately calculate Mercury's orbit. Other effects of GR: Gravity travels at the speed of light in gravitational waves, which are ripples in spacetime. A series of probes are set to observationally confirm gravitational waves. It's thought they will likely find them. The existence of gravitational time dilation. Light leaving a massive body is redshifted; light being emitted, or going 'downwell' toward a massive body is blueshifted. Time will move more slowly for an object within a massive body than outside of it. This is due to the equivalence of gravity's effect to acceleration. This is experimentally verified and consistent with GR  And GR predicted the existence of black holes. At the event horizon of a black hole, no worldline can be traced out of the spacetime regardless of acceleration. Every possible path leads toward the singularity -- spacetime is so curved that no potential path forward, no part of the 'future light cone' of anything that has traveled past the event horizon can have any possible path out.  (simulation, black hole passing in front of a galactic background) Outside of a black hole's event horizon, a particle can move in whatever direction: It is only limited by the speed of light. It can always escape gravity, though it is affected by it.  As we get closer, there are more paths toward the black hole than away from it:  Finally within the event horizon, there are no paths that lead away from the center. Every possible direction you or any particle can move will take you closer to the center of the black hole, the singularity.  No way out  (A 4,400 light year jet of relativistic-speed material being jettisoned by a supermassive black hole at the center of a galaxy, just a cool hubble photo, messier 87) They're very literally places where the curve of spacetime forms a hole, as you cannot move back across the surface of spacetime that sinks into it. Anything fallen into that well is gone forever. In the context of general relativity which most accurately describes our universe so far, black holes are absolutely broken spacetime, at least as far as 'broken' means 'matter is unable to traverse', to answer your question alnilam. Alright, so what does all that have to do with wormholes? Well, we get back to topology that we discussed earlier. John Wheeler famously described the relationship between mass and space with the saying, "Mass tells space-time how to curve, and space-time tells mass how to move.". What wormholes describe are accurate solutions to particular cases in general relativity where you end up with spacetime that is not simply connected. A space is simply connected when you can contract any arbitrary loop on its surface into a point. Think of a sphere:  vs, say, a torus.  No matter how you tighten those loops you never get a point. Normal spacetime is simply connected, but solutions exist for holes in spacetime. One of them was discussed earlier. In the spacetime interval after crossing an event horizon, you are leaving simply connected space.  stay out of these things they're dangerous  In perfect, mathematic, maximal solutions to the Schwarzchild metric and Einstein's field equations, you can see the Einstein-Rosen bridge, which is basically the exact same thing as a black hole, except minus all the infalling stellar matter, and + a very particular organization that results in a quantum interaction that prevents the formation of a singularity. The interaction would be complex, and while it's a solution for GR, there's good reason to think that quantum interactions and GR do not fully play well together at the moment and it probably contradicts even the possible existence of the metric. Even within the math of GR, the result is instantly unstable and it's predict that not even one photon could traverse it before the bridge broke. Everything else in wormholes relies on exotic matter with negative energy density, or matter that would have the opposite effect on spacetime that normal, positive energy density does. (i.e. all matter creating the stress-energy tensor). This exotic matter would be pretty cool stuff as it would allow us to make spacetime our plaything in several ways, but it's a completely theoretical thing (other than some results through the Casimir effect, which probably do not produce negative-gravity perturbations in spacetime you'd see from the stress-energy tensor anyway so aren't useful for that)  the visualizations are incredibly rad though As you can probably tell, we're hitting the limit on what I understand at all (and even this is fairly questionable... there's definitely some out there stuff here, mostly the quantum stuff, that I don't get). I can give it a shot if you have any questions, but if they're about the more complicated quantum effects and their effect on general relativity and gravity, I can't really tell you anything about that as no one knows for sure: there is no unified theory of QFE / general relativity, quantum gravity. They aren't DIRECTLY at odds with each other but there's a lot of solutions that would really need them to work together better than they currently do. |
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Apr 11, 2015 23:14
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Just here to say this has been one of the most enjoyable threads I've ever read in BYOB. |
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Apr 11, 2015 23:21
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GUISSEPPE PIZZAPIE posted:Just here to say this has been one of the most enjoyable threads I've ever read in BYOB.
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Apr 12, 2015 00:46
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I am going to goldmine this when you are done regardless of the contest
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Apr 12, 2015 00:46
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