Wikipedia:Reference desk/Archives/Science/2014 September 18

= September 18 =

why all think that the black hole absorbs all the things around and made them to Gravitational singularity
In some of educational films ,scientific simulations and some scientists talks we see that the black hole absorbs all around it and destroys it .how can it happen? In the other hand, for example if the sun change into black hole ,can it absorb earth to its center?--Akbarmohammadzade (talk) 08:31, 18 September 2014 (UTC)
 * For an object outside of a black hole's event horizon, as long as there is no spaghettification, then it will orbit the black hole normally. CS Miller (talk) 10:12, 18 September 2014 (UTC)
 * There will be spaghettification before the event horizon, for a Sun-mass black hole and anything resembling the largeness and material strength of a human. If I recall correctly, it's "the weight of the population of Paris hanging from your feet." Nothing can orbit inside the photon sphere though, and near it will not orbit normally, it will orbit with weird relativity effects like the speed of time and color depending on velocity and distance from the black hole and whether you're the one far away or near the hole. If the black hole had enough stuff in it (millions of Suns) an astronaut could survive past the hole's surface and only feel pulling. With one Sun, I don't know how small or strong something needs to be to avoid spaghettifying before relativity is visible. Relativity is very small at the Sun's surface. Everything in orbit now is further than that and would continue undisturbed, and get very cold. Though rarely a comet that would've been headed towards the Sun will miss it's roughly 100,000 times smaller black hole and extremely rarely one will touch it and therefore be pulled in. Note that due to the scales involved you'd need almost God-like technology to have any solar mass turn into a Sun-mass black hole. It really doesn't want to become one. Sagittarian Milky Way (talk) 14:16, 18 September 2014 (UTC)
 * There are other ideas - see Fuzzball (string theory). Based on the black hole paradox I confess increasing skepticism that anything actually crosses the horizon - from outside, we certainly never see it cross the horizon...) Wnt (talk) 22:23, 19 September 2014 (UTC)
 * The equivalence principle implies that nothing special happens at a black hole event horizon. It's called a horizon because it's the farthest that some observers can see. Nothing happens when you cross a horizon because it's someone else's horizon, not yours. Unless the universe actually revolves around that person, you won't cease to exist when they can't see you. -- BenRG (talk) 03:05, 21 September 2014 (UTC)
 * I take it you're not a fan of Firewall (physics) ideas. I find these (like the point singularity) unaesthetic myself; mostly though I wonder if the Hawking radiation, stripped of virtual particle concepts that people here didn't seem to find very meaningful when I asked about them recently, is somehow the same material that entered the hole due to some sort of uncertainty, or as a result of some higher-order physics as per the fuzzball model.  Certainly I'm no expert, but I don't hear experts being all that confident that nothing unusual happens at the event horizon. Wnt (talk) 14:40, 21 September 2014 (UTC)
 * If you throw away the equivalence principle then I don't think there ever was a black hole information loss problem. You can just assume there's some unknown Planck-scale physics at the surface of the "frozen star" and it's governed by a Hamiltonian and that's that. I suppose that violates Lorentz invariance too, but I think Lorentz invariance is a lost cause anyway at that point—I don't know what it would mean to have gravity and Lorentz invariance without the equivalence principle.
 * The paradox that led to the firewall proposal is interesting and needs a solution, but the firewall solution is like superluminal signaling as a solution to Bell's paradox. It's not logically impossible that it's right, but it looks more like a failure of human imagination. You're right that many experts seem to have no problem with it, but it seems to unavoidably violate the equivalence principle, and they implicitly assume the equivalence principle in everything else that they do. Honestly I don't think they realize how much would change if it turned out to be wrong. Without the equivalence principle, gravity and spacetime geometry are de-unified. What is gravity if it's not geometry? What governs spacetime geometry if it's not gravity? You'd have to rebuild physics from the ground up, I think. -- BenRG (talk) 00:02, 22 September 2014 (UTC)

A difference between a black hole of a certain mass and an ordinary object like a star of the same mass is the way it can grow on the long term by accreting mass. In a simple model we can consider an object of constant density $$\rho$$, if this has a radius of $$r$$, it's mass will be
 * $$M = \frac{4}{3}\pi\rho r^3$$,

The capture cross section is
 * $$A = \pi r^2 = 2\left(\frac{2}{9\pi}\right)^{\frac{1}{3}}\left(\frac{M}{\rho}\right)^{\frac{2}{3}}$$

An average flux of infalling matter of j will thus lead to the mass increasing according to:

$$\frac{dM}{dt} = 2\left(\frac{2}{9\pi\rho^2}\right)^{\frac{1}{3}}j M^{\frac{2}{3}}$$

The mass as a function of time is thus given by:

$$M(t) = M(0)\left[1 + \frac{2}{3}\left(\frac{2}{9\pi\rho^2 M(0)}\right)^{\frac{1}{3}}j t\right]^{3}$$

So, on the long term the mass grows as the third power of time.

In case of a black hole, the mass is related to the Schwarzschild radius R by

$$R = \frac{2G}{c^2}M$$

The effective capture cross section is given by:

$$A =\pi R^2 = \frac{4\pi G^2}{c^4}M^2$$

The same average flux of j then leads to a mass increase according to:

$$\frac{dM}{dt} = \frac{4\pi G^2}{c^4}j M^2$$

The mass as a function of time is thus given by:

$$M(t) = \frac{M(0)}{1-\frac{4\pi G^2}{c^4}j t}$$

So, the mass would grow infinitely large after a finite time of

$$t = \frac{c^4}{4\pi G^2 j}$$

if the flux j could be sustained indefinitely. In reality the black hole will depleat the nearby matter sources leading to a reduction of the flux. Count Iblis (talk) 18:52, 21 September 2014 (UTC)

Is it possible that they say we found an exo planet with 3 Jupiter mass ?
In fact can any planet be exist with 3 Jupiter mass?--Akbarmohammadzade (talk) 09:07, 18 September 2014 (UTC)


 * Brown dwarf stars are in the range 13-80 Jupiter masses (with some debate to the exact range). The is a sortable list of exoplanets at http://exoplanet.eu/catalog/, it has several well into the brown dwarf range. CS Miller (talk) 10:09, 18 September 2014 (UTC)


 * So yes, 3 Jupiter masses would still be a planet, not a star. Hot Jupiter describes some such exo-planets.  (They are "hot" because our current detection methods favor those which are close to their stars.) StuRat (talk) 14:28, 18 September 2014 (UTC)
 * Astronomical categories depend allot on definition ofcourse. A category of "huge concentrated gas objects" would contain the socalled "gas giants" like Jupiter or Neptune as well as almost all socalled "suns". You will need astrophysic basic knownledge to understand the distinction or why that distinction is infact arbitrary. --Kharon (talk) 05:23, 20 September 2014 (UTC)

Why did a magnetic fields are always been considered weak?
Why did a magnetic fields are always been considered weaker than electromagnetic fields, because a magnetic fields can not create an electrical charges and electric current and also dynamics of electric current, is it been so?--Alex Sazonov (talk) 11:39, 18 September 2014 (UTC)


 * There are no magnetic monopoles (they can exist according to theory, but they have zero abundance in the universe). The magnetic fields we observe are due to relativistic effects which makes the effects of the magnetic fields weak in practice. You can see this clearly in CGS units where magnetic fileds and electric fields are measured in the same units. The velocity dependent part of the Lorentz force is then given by the cross product of the velocity and the mangnetic field divided by the speed of light. Then in SI units we don't have the 1/c factor in there, but that's because we absorb that into the definition of the magnetic field in SI units. Count Iblis (talk) 17:01, 18 September 2014 (UTC)


 * Why did, then in the conditions of the natural magnetic field of the planet Earth are always really had been a phenomena of electric charges and electric current and also dynamics of the electric current, if a magnetic fields are not been powerful fields of dynamics and statics of magnetism as well as of dynamics and statics of magnetic inductive (magnetic induction)?--Alex Sazonov (talk) 19:08, 18 September 2014 (UTC)


 * Did I understood correctly, that all powerful magnetic fields could always been created only by electromagnetic fields in such case as electromagnetic induction, or natural magnetic field of the planet Earth is always been creates a backward electromagnetic induction which is always been capable of creating an electric charges and electric current and also dynamics of the electric current?--Alex Sazonov (talk) 17:25, 19 September 2014 (UTC)


 * Could the magnetic field been doing (done) the work of an electric current and also the work of dynamics of the electric current?--Alex Sazonov (talk) 09:56, 21 September 2014 (UTC)


 * Obviously, that the dynamic and static magnetic fields of the electric current and dynamics of the electric current are always been only powerful magnetic fields, so that as statics as well as dynamics of electric charges are always been created only powerful magnetic fields.--Alex Sazonov (talk) 16:53, 22 September 2014 (UTC)

- Note: As did an electromagnetic induction, and also as did a backward electromagnetic induction are always could been done the work of electric current and also the work of dynamics of the electric current!--Alex Sazonov (talk) 19:03, 19 September 2014 (UTC)
 * An electric current is always been able to done the work of the electromagnetic field as well as the electromagnetic field is always been able to done the work of an electric current, so that is never been doing rule out the scientific fact that the backward electromagnetic induction is always been able to done the work of an electric current and also the work of dynamics of the electric current.--Alex Sazonov (talk) 09:16, 21 September 2014 (UTC)

Microwave-made steel and UPSteel
Way back in 2009 engineers from Michigan Tech industrialized [making steel in microwaves http://www.mtu.edu/news/stories/2009/september/researchers-receive-15-million-make-steel-syngas-using-microwaves.html] with syngas as a byproduct, starting the company U.P.Steel. The company's website is down or nonexistent, and it seems that nobody is using the microwave method. Does anybody know what happened to the method or the company? SamuelRiv (talk) 19:56, 18 September 2014 (UTC)
 * Hard to tell. You could try emailing the listed contact person at Michigan Tech: . --Amble (talk) 20:30, 18 September 2014 (UTC)
 * Perhaps a patent search is useful; at least, it indicates activity in 2014. (Note the recent Chinese patent.  Perhaps it's ignorant, but I'm thinking "duh, nobody actually makes shit in the U.S., they just get the research money here...) Wnt (talk) 22:21, 19 September 2014 (UTC)
 * They ran into allot of trouble i guess because a molten metal mass would rather reflect microwaves instead of absorb the energy. Infact household microwaves can only be used securely because of their steel (reflecting) cage. You can only heat up powdered or granular (sintering) metals. Also i read the degree of absorbtion changes dramatically with temperature. On top i highly doubt it can generally be patented as microwave heating is already well known technology for some time.
 * There are always prommising manufacturing methodes alike, for example thixocasting, that look ideal in theory but fail in practice for reasons like processual reliability in sense of failsavety when these processes rely on a narrow set of conditions to work because in practice you end up to often producing lots of junk instead of what you want. --Kharon (talk) 05:01, 20 September 2014 (UTC)