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Mar
30
comment How can you tell if a critical energy density is actually a black hole?
Hopefully this isn't the case, but I believe currently there is no known simple answer. What you essentially want is a full proof of the hoop conjecture which there is none that I know of. While it makes intuitive sense that if we shove enough mass into a region a black hole forms, it is difficult to discuss precisely. It is even difficult to define what we mean by mass contained in a local region in a completely general case en.wikipedia.org/wiki/Mass_in_general_relativity
Mar
30
comment Is there an energy density limit in GR?
As a clarification for John, what Lawrence is essentially referring to here is the hoop conjecture ( en.wikipedia.org/wiki/Hoop_Conjecture ). However the mass referred to here can't simply be related to the energy density, otherwise the hoop conjecture is trivially wrong. We can always change coordinates to add as much kinetic energy as we want and it clearly doesn't change whether a black hole forms or not. So if speaking of only the energy density -- no there is not a limit. Mass in general relativity can be a tricky concept en.wikipedia.org/wiki/Mass_in_general_relativity
Mar
30
revised Is there an energy density limit in GR?
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Mar
30
answered Is there an energy density limit in GR?
Mar
27
revised Does change of coordinate system require acceleration?
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Mar
27
answered Does change of coordinate system require acceleration?
Mar
24
accepted How can there be net linear momentum in a static electromagnetic field (not propagating)?
Mar
24
accepted Is GR vacuum equation unique?
Mar
23
comment What is the electric field generated by a spinning magnet?
@John A dipole is symmetric about the axis just like a cylinder. The dipole actually has the same symmetry as a finite cylinder.
Mar
22
comment Does the math work out for there being enough time for the formation of the heavier elements and their distribution as seen in today's universe?
I've heard iron before as well, but the wiki link says "The only other major competing process for producing elements heavier than iron is the s-process in large, old red giant stars, which produces these elements much more slowly, and which cannot produce elements heavier than lead." The wikipedia s-process article also lists Pb as the limit. I skimmed the article it cited on the claim and didn't see anything on lead. I'm not very knowledgeable on this, so it may be a wiki error.
Mar
22
comment Does the math work out for there being enough time for the formation of the heavier elements and their distribution as seen in today's universe?
Non-super nova stars create elements heavier than lithium. Red giants can apparently produce up to Lead en.wikipedia.org/wiki/Supernova#Source_of_heavy_elements .
Mar
22
comment Can a car get better mileage driving over hills?
@Anna You seem to be claiming that a car can not go up a hill running at peak performance, period. That is nonsense. Your extreme case merely comments on the limits of available gears. You cannot extrapolate this to claim a car engine can never be ran at its efficiency peak while going up a hill. For the engine it is mechanically equivalent to push a car against air friction or against gravity up a hill. Are you claiming the mechanical output of turning the wheels is fundamentally different in each case? If so START A NEW QUESTION. Discussing here is not very productive.
Mar
22
comment Can a car get better mileage driving over hills?
@Anna We would only be off the performance peak if you insist on maintaining the same speed. You seem stuck on this idea that we must move at a constant speed. That is not required by the question, so please stop trying to impose a constant speed. It is true that to remain at the performance peak the car would move slower up a hill than it would go on a flat road ... but that is the whole point! Less energy is wasted in air friction. If you are still confused, please ask a separate question about what is bothering you, for lengthy discussions in comments are not very useful.
Mar
22
answered How do anti-lock brakes know when to brake?
Mar
22
revised Can a car get better mileage driving over hills?
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Mar
22
awarded  Commentator
Mar
22
comment Can a car get better mileage driving over hills?
@Master of Disaster: The answer you provided and the comment here, make it sound like we are disagreeing on basic physics. Can you please update your answer to more clearly state your case, or at least comment here specifically what you feel is incorrect in my answer? Thank you.
Mar
22
revised Can a car get better mileage driving over hills?
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Mar
22
revised Can a car get better mileage driving over hills?
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Mar
22
revised Can a car get better mileage driving over hills?
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