I've often heard public lectures describing the degenerate, exotic matter left behind after a star collapses but fails to go supernova; one that has found its way to become a neutron star. The speaker will often describe a tablespoon of the exotic material consisting mainly of neutrons, weighing hundreds if not thousands of tons.  It seems to me what keeps such degenerate material stable is the whole gravitational force of the neutron star, but I realize I may be wrong. Perhaps it's reached a state of stability under its own forces once the force of gravity created this exotic matter?  So hypothetically speaking if one could scoop up a tablespoon of neutron soup from the collapsed star, transport it back to earth, and somehow contain its extreme weight, would the matter remain stable - or would it fly apart?
44334
I've read on NASA's page on neutron star that one teaspoonful of that star would weigh over 20 billion tonnes on Earth. If it was somehow possible to bring it to earth would it:   Burn and disappear during Earth atmosphere entry? Assuming we have 20 billion tonnes of mass occupying the volume of a teaspoon here on Earth, would it fall through the ground under its own weight?
112332
Alright, so we have all seen the movies where someone gets blasted out of the airlock on their starship, or their suit decompresses while on a space walk. The poor schmoe usually either decompresses so violently that blood is oozing out of every orifice in their body, or they freeze instantly.  From this I have two questions:   Would the decompression really be that violent?   Clearly the drastic difference in pressure from a normal "earth" like environment to space would be bad, but would it be that devastating. I vaguely remember that standard atmospheric pressure was something like 15 psi, which doesn't seem like enough to mess you up that bad.  Would you actually freeze instantly in space?   Heat, or lack thereof is a measure of internal energy, but in a vacuum there wouldn't be anything to have internal energy, so does space even have a temperature? Wouldn't some form of matter have to be present in order to cool off? If there were no matter besides yourself and a few stray particles here and there, it seems like it would take a very long time to cool off.
96852
so I had my first approximation to the gravity equation  \begin{equation} F=GmM/r^2 \end{equation} and some questions arose that my teacher couldnt respond: if r approximates to 0 with mM being constant, then the force should tend to infinity, right? so that being true, then if we would travel to the center of the earth, we would be crushed by gravity? if so, how can there be anything right in the center of the earth? wouldnt anything there be compressed and destroyed by the infinite force?  having that in mind, my next question is as gravity force is measured from the center of an object, any part of the same object which is not at the center would be affected by the same object's gravity? if so, how can we differentiate between two objects that are not in a vacuum? my question originated thinking in us standing on the earth, in contact with it, are we, in a gravitational sense, one object? this lead to the last question, if gravity is a property of matter, then how applying the same logic as before, how does it work between two neutrons?(chose neutrons because i assumed it would be more difficult obtaining gravitational force if there was electrostatic forces from charges inbetween) do two newtrons in contact count as 1 gravitational object? and how thats its own gravity affect the newtron from center and out?
80295
I'm a total physics n00b (i.e. I only know the physics as taught in IT grades, and don't remember much of it), and was talking about entropy (initially, not with the physical implications). My friend talked about entropy and gravity and was not sure about the fact that entropy decreases with gravitationally bound systems.  , but "apparently proving" that gravity decreases it. Finally, in the final link where he will show the prove that gravitational systems do not decrease entropy, it shows this text:     So, you're wondering why gravity doesn't violate the 2nd law of thermodynamics, even though the entropy of a gas cloud decreases as it shrinks under its own gravitational pull? The answer is simple, but I'll just give you a hint. We've already seen that as it shrinks, it loses energy. The energy has to go somewhere. Where does it go? If you figure that out, you'll see that the total entropy is not actually decreasing - it's just leaving the gas cloud and going somehere else!   My question is: No, actually I cannot figure if out. Can someone help me to figure it out? (yes, perhaps it's a very simple answer but no, still don't get it).
31637
I've seen in a documentary that when a star collapses and becomes a black hole, it starts to eat the planets around.   But it has the same mass, so how does its gravitational field strength increase?
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I am assuming there are two constituents that obliterate our current model of physics;   that it's infinitely dense that it's infinitely small   Please correct me if I am wrong.
77255
When a star starts to run out of hydrogen to fuse, it begins to collapse due to gravity until the central core temperature rises to $10^8~\text{K}$  Then due the force generated by the fusion of helium, the star expands again and becomes a red giant.  So, why doesn't the expansion cool the core?
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It is known that there are no diproton or dineutron nuclei.  Does this mean that two protons or neutrons are not actually attracted to each other? Even if the attraction was weak, wouldn't it cause bound states anyway?  Related:
4313
In a perfectly symmetrical spherical hollow shell, there is a null net gravitational force according to Newton, since in his theory the force is exactly inversely proportional to the square of the distance.  What is the result of general theory of relativity? Is the spacetime flat inside (given the fact that orbit of Mercury rotates I don't think so)? How is signal from the cavity redshifted to an observer at infinity?
40107
It is commonly known that it is the electron degeneracy pressure that prevents the collapse of a white dwarf into a neutron star, and it is not the electromagnetic force. However, it is also widely accepted that all the forces in the universe are just manifestations of the four elementary forces in the universe. So can the degeneracy pressure, caused due to Pauli's Exclusion principle, be shown to be due to one of these forces? I have read some other questions in SE , , and they do not answer my question fully.  In one of the links, the degeneracy pressure is described as an 'entropic force', but as far as I understand, all entropic forces can be explained by the underlying fundamental forces. If the degeneracy pressure is something that is fundamentally different from the other forces, why isn't it classified as a separate force?
28622
I'm assuming that the answer to the question in the title is a resounding yes. Since Baryonic matter and dark matter interact via gravitational forces.  If this is the case how is information not lost if, as is presumed, dark matter does not interact via the electromagnetic force?
90380
If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the Pauli exclusion principle: It's made entirely of degenerate matter, each particle of which cannot occupy the same quantum state of any other.  So these objects are so massive that they gravitationally lens light. If you make them more massive, they bend the light more. Keep going and going until they bend the light so much that light passing near the surface can barely escape. It's still a neutron star. Add a bit more mass, just enough that light passing just over the surface cannot escape. Now it's a black hole with an event horizon (I think?). Does this mean the neutron star has become a singularity? Isn't it still just a neutron star just beneath the event horizon?  Why are black holes treated as having a singularity instead of just an incredibly massive neutron star at its center? Does something happen when an event horizon is "created?"
1587
Is it possible to make a solid rigid evacuated "balloon" out of Beryllium or other elements or alloys?  The critical buckling pressure at which an evacuated sphere is given as $$ P_1=\frac{2E\cdot\left(h/R\right)^2}{\sqrt{3\left(1-\nu^2\right)}} $$ where $E$ is , $h$ the shell's thickness, $R$ the radius to the midsurface shell, and $\nu$ is .  Of course this is theoretical and experimental and literature values for submersibles hover around 1/4 of the theoretical pressure.  Beryllium has the following properties:   Density=1.85 g/cm$^3$ Young's Modulus=287*10$^9$ Pa Poisson Ratio=0.032   Calculating the upward force, buoyancy at 101.3 kPa atmospheric pressure and Radius of 10m gives 4.24*10$^8$ N. The weight of this sphere with shell thickness 1mm has 2.32*10$^7$ N downward force of Beryllium material, so it will theoretically float at ground level.   Calculating the critical buckling stress of this beryllium sphere is 331.57 kPa, which is 3.27 times the atmospheric pressure of 101.3 kPa, so theoretically it won't buckle. But in experimentation done in earlier experiments as stated before, due to construction/material non-ideal situations, the critical buckling formula predicts about 4 times stronger than actual, so it would need about 400 kPa, not 331 kPa as Beryllium sphere with this specification would hold. Increasing the strength capability also increases weight and therefore lowers it's ability to float, but Beryllium is the closest element with this situation I have calculated that could work if ideal (I have only been using periodic table elements and household materials)  Is it possible with an alloy of elements? Using the equations for buckling and buoyancy, the following is needed. What alloys/materials come to your mind?  Material considerations:   Material with larger Poisson ratio Material with larger Young's Modulus Less dense material Construction (is it possible)   Material considerations affect on lift   Thickness can be larger, increases strength by the squared. But also increases weight. If a material with lower density with similar structural properties as Beryllium is used, it would help. Radius can be lower for same reason, but also lowers buoyancy (volume)   My other designs (don't worry about these just yet, just if you are curious is a vacuum sphere with a pressurized "outer tube" with spokes in tension (carbon fiber nanotubes maybe) to keep the 2 spheres in position. Another is a spherical truss out of carbon fiber nanotubes with a light membrane around. The truss prevents the outer membrane from collapsing due to the atmospheric pressure  The "point" of this if you ask, as to why not use a traditional balloon is it can be used as a boat by filling the sphere with air, and then used as a aircraft at will by evacuating the sphere. An alternative mode of transportation for sea vessels etc.
91404
We know from big bang nucleosynthesis that baryonic matter accounts for about 5% of the universe's total mass-energy density. What is the current best estimate of how much of this is in the form of stars? I'm guessing that this would be known only very roughly. It seems like you would just have to survey large volumes of space for stars and larger structures made out of stars. Although galaxies may pretty efficiently cycle their hydrogen and helium through stars, I would assume that there is a lot of hydrogen out there in the spaces between superclusters that has never had a chance to form a star and never will.  related:
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I know that the gravitational interaction of antimatter is expected to be the same as normal matter.  But my question is, has it ever been experimentally validated?  I think it would not be a trivial experiment, because electromagnetic effects have to be eliminated, so neutral particles would be needed. Maybe diamagnetically trapped antihidrogen atoms could be examined as to which direction they fall?
47589
Most of the books which I looked at give approximately 10 km as the radius of a neutron star. Just yesterday I looked at a book by Dave Goldberg titled The Universe In the Rearview Mirror (2013) which says that they're "only about 5 km in radius" [p.225]. Is this true, [i.e., is there some recent evidence for this], or did he make a mistake here?
88920
Some scenarios describing the fate of the matter vs  tug of war on the universe involve the acceleration of the universe increasing to the point that it ends up ripping apart even atoms. This is called the .  This makes no sense to me. It looks like all of these   (GR) models of the universe assume it has a uniform isotropic distribution of matter and energy. This works great at long scales, but it is also clearly wrong even at the length scale of the separation of galaxies.  The density of atomic nuclei remains the same even though the universe is expanding even as we speak. I don't think the "scale" in the  can be interpreted so literally. Or maybe said better, it's got to break down when the cosmological horizon distance gets to the scale where the isotropic assumption breaks down, right?  How can scientists be claiming that runaway expansion will rip apart atoms?