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alchemist May29-04 04:35 AM

black holes
 
how is a black hole formed exactly from a neutron star or a white dwarf?? what is it?
is it a star? is it defined as a matter??
and what happens when light gets sucked into it? there ought to be an increase in energy in it right? what happens to this energy?

jcsd May29-04 06:25 AM

When a very large star runs out of fuel it will explode as a type II supernova. It will throw the vast majority of it's mass off during the supernova, reall leaving only the core. The mass left over will collapse into either a neutron star or a black hole.

A black hole is matter as it is massive (i.e. it possesses mass) and it is a region of space from which nothing can escape (i.e. I can't think of a good concise defitnion)

Light that enters the event horizon will goto the infitely dense singulairty at the centre of the black hole. Yes this does increase the nergy of the black hole; as mass and enrgy are equivalent this means that the mass of the black hole will increase.

relativelyslow Jun18-04 07:56 PM

a black hole is formed when a massive star (more massive than a neutron star) reaches its critical circumference (a point where the gravity overwhelms the pressure inside the star) and implodes into a black hole. as things get sucked into the black hole it does become more massive. nothing can escape a black hole; when light "tries" to it is redshifted out of existence.
im reading a book titled black holes and time warps by kip s. thorne. if your interested in them read this. its a good book. i picked it up randomly (out of random interest in black holes) so i had no previous knowledge of physics or black holes (you don't need any to understand this book).

Ygmaince241 Jun23-04 02:25 PM

Cool RS. I was just reading that book. However, I didn't understand something he said. He said there were extremely rare miniscule black holes, around the size of the nucleus of an atom. However, I was just watching a nova, and one of the astromoners says that a "black hole" has to be reasonably massive to be a black hole.

RAD4921 Jun23-04 04:33 PM

Black hole books
 
I read Kip Thorn's book and it was good. I recommend "About Time" by Paul Davies

Nereid Jun23-04 06:09 PM

Quote:

Quote by Ygmaince241
Cool RS. I was just reading that book. However, I didn't understand something he said. He said there were extremely rare miniscule black holes, around the size of the nucleus of an atom. However, I was just watching a nova, and one of the astromoners says that a "black hole" has to be reasonably massive to be a black hole.

GR, the extremely successful physics theory in which BHs are predicted, says nothing about how massive they need to be. However, the rest of science - astronomy and fundamental particle physics in particular - has a hard time coming up with realistic scenarios under BHs could occur (other than those left over from supernovae, or stellar collisions, or at the centre of large galaxies; all these are rather massive BH).

Some folk hypothesise that 'small' BH may have been created early in the history of the universe (all kinds of wild an woolly reasons), and some may still be around. They are called 'primordial black holes' (PBH).

IF (stress on if) they do exist, then some should be evaporating about now, giving off a nice burst of 'Hawking radiation', after Stephen, who first published a paper on why there should be such radiation.

AFAIK, no Hawking radiation has been observed, so PBH must be pretty 'rare'!
:wink: :smile: :tongue2: :surprise:

idontthinkright Jun28-04 07:31 AM

I hear that the planets we are currently seeing in our night-sky is what it looked like millions of years ago.(light takes time to reach into our feeble little eyes)Does that apply to black holes too? They always say that planets are slowly vanishing from our systems,but that means it is what happened millions of years ago!So how do we know that the black holes we see in our telescopes arent actually far but nearer,or vice versa.Someone tell me,are we in imminent danger?

selfAdjoint Jun28-04 09:17 AM

Not planets, but stars and galaxies. The planets of our solar system, the ones orbiting the Sun as the Earth does, are not more than hours away from us as light travels (the Sun is eight minute away).

Stars in our galaxy are a few years up to thousands of years away. Other galaxies are millions up to billions of year away. These are the things you have heard about, where what we see happened thousands or millions or billions of years ago.

So now black holes. We can't see black holes because they absorb the light that falls on them. But we can see the clouds of gas and the fast moving stars that surround them. There seems to be a big one at the center of our galaxy, but that is far away and we are moving around it on big orbit, rather than toward it.

dave19903652 Jul8-04 04:32 PM

With miniscule black holes wouldn't it be possible for humans to do it? Of course we don't have the technology at the mo but if you managed to create the same effect with the right atom. As the bh is smaller than the star then you could get away with something bigger... but it is technically possible

Nereid Jul10-04 07:51 AM

The folk who run Google must have had so many search requests that they've set up a special entry, Science -> Physics -> Relativity -> Observations. Have fun!

AFAIK, there are now >10 X-ray binaries whose masses are sufficiently well determined for us to say that at least one object must be a BH. At the heart of our Milky Way galaxy is an object called Sag A*, which was thought to be massive BH quite some time ago. Gradually all other possible explanations have been ruled out by new observations, including this one, of stars orbiting this BH.

BTW, that site you posted a link to ... the author can't possibly be serious; there's nary a firm prediction on it! Certainly no math, no numbers ... goodness, no matter what we find in the next decade, his stuff is so vague he could (correctly) claim new observations match perfectly. Excuse me, but that's not how I think science is done.

urtalkinstupid Jul10-04 01:29 PM

Hey, I've looked at that google thingy. It only had three sites for observations that are not really persuasive to me.

Quote:

Quote by http://www.owlnet.rice.edu/~spac250/steve/index.html
Black holes, if they truly exist, are very strange objects indeed. These are strange and fascinating objects, truly, but as of yet, they are still considered theoretical.

That doesnt sound too confident to me :rolleyes:

Black holes are not yet proven and are still up for debate, but I have a couple of questions if anyone would like to answer them for me PLEASE. I'm very new to physics, but what I have studied leads me to disbelieve in a lot of well known theories. Now on to my questions about black holes.

1. Has there ever been a black hole observed that did not have a binary star orbiting it?
2. Do you believe in the concept of neutrinos? If so, could it be possible that these orbiting stars are emitting neutrinos.
3. Do Particle and Anti-particle pairs disembark at the event horizon of the black hole leading to one being pulled into the black hole and one being shot out from it? (not to sure if this is even relevant)

Like I said, I'm fairly new to physics. So far, though, what I have read is not very convincing. Heh, I'm only in High School, so please aid me in seeking out the right path in physics, if I'm not on it. :biggrin:

Nereid Jul10-04 09:40 PM

It's quite simple, in principle: from observations of a binary object, estimate the minimum mass of each. From what we know about ordinary (and extraordinary) stars, if an object has a mass >~4 times that mass of the Sun (Msol), then it must be either a very bright star (which would be very visible) or a BH. Why? Because if it isn't emitting copious quantities of photons (esp light, UV, IR), its core won't be hot enough to stop an ordinary matter star from collapsing. And no degenerate matter object* of that mass can avoid being crushed into a BH.

The alternative is that our understanding of physics has a huge hole in it.

The supermassive BH at the heart of galaxies aren't binaries (well, some are, e.g. Arp 220); if they were, they'd merge rather quickly (and create an event that would make a GRB look like a damp squib).

*degenerate electrons, as in a white dwarf, are highly incompressible ... until those at the highest levels start to react with protons to form neutrons (inverse beta decay), and the dwarf collapses to a neutron star, which is like a giant atomic nucleus.

mee Jul10-04 10:13 PM

pbh
 
Would these tiny black holes be dangerous? Would they be powerful enough to eat earth up? (insert higgs boson story here) Are black holes infinitely small or do they have a distinct diameter? If they are infinitely small, why would one have more mass than another? Does the mass of a black hole affect its size or only the distance from the singularity to the event horizon?

phreak Jul10-04 11:02 PM

Quote:

Quote by mee
Would these tiny black holes be dangerous? Would they be powerful enough to eat earth up?

Yes, and yes.

Quote:

Quote by mee
Are black holes infinitely small or do they have a distinct diameter?

The singularity of the black hole is infinitesimal (obviously), but the event horizon has a distinct diameter.

Quote:

Quote by mee
If they are infinitely small, why would one have more mass than another?

They aren't infinitely small; look above. The mass is usually referred to as the radiation/other stuff that is absorbed into the event horizon. Therefore, because the event horizon serves as the barrier of the "mass", the event horizon is usually referred to as the total mass.

(I don't have much info in this area, but from what I've read on black holes, this is accurate. In conclusion, I've probably made things worse. Sorry.)

urtalkinstupid Jul10-04 11:56 PM

Bh
 
i still am in disbelief on the theory of black holes as well as gravity being a pull or even existing. i continue to find more sources that make logical attempts to disprove black holes existences, although they are just theories...they actually make sense and unlike the theories proposed about black holes today have baffled many scientists because of their inaccurate calculations from false mathematical attempts at explanation.

dude check out this site http://www.perceptions.couk.com/uef/nblckhls.html

im sure ill find more sites that take a shot at disproving black holes...and as it takes time to find these sites it also takes time to take an open mind to science as it has no definite way of being operated.

Quote:

Quote by NEREID
Excuse me, but that's not how I think science is done.

yea thats how science works...that man is questioning. that's how science works if im not mistaken
heheh :smile:

but if you can could you provide me with links on GR and SR...and answer my question about particle/anti-particle interatcions at this phenomenons event horizon?

Labguy Jul11-04 09:38 AM

Quote:

Quote by urtalkinstupid
but if you can could you provide me with links on GR and SR...and answer my question about particle/anti-particle interatcions at this phenomenons event horizon?

Here is just a part of an older post about virtual particle production at a BH event horizon. Both Hawking radiation and magnetic "quantum tunelling". It shows that virtual particles are produced and released at the EH by two methods. This has nothing to do with matter and energy release from an accretion disk.
________________________________________________________________
One thing that I have noticed, after reading many pages of info on "classical" Hawking Radiation (HR), is that it was conceived and most often described as in its original form as applying to a static, non-rotating, non-accreting and chargless black hole (BH) in the original Schwarzchild configuration as a simple mass-only expression of the Schwarzchild Radius (Rs) where Rs = 2GM / c^2. All of the virtual particle pair production scenarios are based on this and require one particle to "fall back" with the other escaping as a real particle causing a mass loss of the BH.

However, many other research sites and past papers have noted that it is almost impossible to form a BH with no angular momentum (spin). Even the "no hair" statement by Hawking was that a BH has only three observable properties; (1) mass, (2) angular momentum and (3) charge (usually net zero). But a lot of recent discoveries (and older theories) have added one new property that is (4) magnetic field. At first, it was thought that a magnetic field would only surround a BH that was accreting matter, but Ramon Khanna and Yakov Zeldovich have shown that all black holes will have a magnetic field. There are also the terms "Hawking Process" and “Hawking Effect” appearing, which include/combine the original HR work with work of others such as Thorn and especially Kerr (for spin) and Newman (for charge). The "Kerr-Newman" BH. (A source quote:) “David Finkelstein's Black Hole, which shows how Mass curves SpaceTime by Gravity, can be generalized to deal with Spin and Electric Charge. The generalization, called a Kerr-Newman Black Hole, was developed by Kerr (who generalized to add angular momentum J to mass M in 1963) and by Newman (who generalized to add charge e in 1965), according to the book General Relativity, by Robert Wald (Chicago 1984).

In his paper Generation and Evolution of Magnetic Fields in the Gravitomagnetic Field of a Kerr Black Hole, Ramon Khanna says: "... a rotating black hole can generate magnetic fields in an initially un-magnetized plasma. In axisymmetry a plasma battery can only generate a toroidal magnetic field, but then the coupling of the gravitomagnetic potential with toroidal magnetic fields generates poloidal magnetic fields. Even an axisymmetric self-excited dynamo is theoretically possible, i.e. Cowling's theorem does not hold close to a Kerr black hole. Due to the joint action of gravitomagnetic battery and gravitomagnetic dynamo source term, a rotating black hole will always be surrounded by poloidal and toroidal magnetic fields (probably of low field strength though). The gravitomagnetic dynamo source may generate closed poloidal magnetic field structures around the hole, which will influence the efficiency of the Blandford-Znajek mechanism whereby coupling of the gravitomagnetic potential with a magnetic field results in an electromotive force that drives currents that may extract rotational energy from a black hole.”

In June of 1971 Zeldovich announced a spinning black hole must radiate ... “a spinning metal sphere emits electromagnetic radiation ... The radiation is so weak ... that nobody has ever observed it, nor predicted it before. However, it must occur. The metal sphere will radiate when electromagnetic vacuum fluctuations tickle it. Zeldovich's mechanism by which vacuum fluctuations cause a spinning body to radiate showed a wave flowing toward a spinning object, skimming around its surface for a while, and then flowing away. The wave might be electromagnetic and the spinning body a metal sphere ... or the wave might be gravitational and the body a black hole. The incoming wave is not a "real" wave ... but rather a vacuum fluctuation. ... the wave's outer parts are in the "radiation zone" while the inner parts are in the "near zone" ... the wave's outer parts move at the speed of light ... its inner parts move more slowly than the body's surface is spinning ... the rapidly spinning body will ... accelerate ...[the inner parts of the incoming wave] ... <and this> acceleration feeds some of the body's spin energy into the wave, amplifying it. The new, amplified portion of the wave is a "real wave" with positive total energy, while the original, unamplified portion remains a vacuum fluctuation with zero total energy. Zeldovich proved that a spinning metal sphere radiates in this way; his proof was based on the laws of quantum electrodynamics.”

The quantum mechanical description of the vacuum allows for the creation of the particle/antiparticle pairs, and the electric field tends to separate the charges. If the field is strong enough, the particles tunnel through the quantum barrier and materialize as real particles. The field necessary to accomplish this feat is achieved when the work done to separated the charges by a Compton wavelength equals the energy necessary to create the particles. It should be noted that conservation of energy is not violated, as the energy it took to create the particles would be precisely equal to the decrease in the energy of the weakened electric field." .. (LABGUY NOTE: not necessarily just BH mass loss as with Hawking radiation).

Sources:
Carrol, Bradley W. and Ostlie, Dale A. An Introduction to Modern Astrophysics. Reading: Addison-Wesley, 1996.
Wald, Robert M. General Relativity. Chicago: University of Chicago, 1984.
Eisberg, R. and Resnick, R. Quantum Physics. New York: John Wiley & Sons, 1985.
Narlikar, J.V. Introduction to Cosmology. Cambridge: Cambridge University Press, 1993.
Hawking, S.W. Hawking on the Big Bang and Black Holes. New Jersey: World Scientific Publishing Co., 1993.
Hawking, S.W. A Brief History of Time. New York: Bantam Books, 1988.
Shapiro, S. and Teukolsky, S. Black Holes, White Dwarfs, and Neutron Stars - The Physics of Compact Objects. New York: John Wiley & Sons, 1983.
Thorne, Price, and Macdonald, eds. Black Holes: The Membrane Paradigm. New Haven: Yale University Press, 1986.
Wald, Robert M. General Relativity. Chicago: University of Chicago, 1984.


Web sources:
http://www.mpifr-bonn.mpg.de/gcnews/...dsgr.abs.shtml
http://xxx.lanl.gov/abs/astro-ph/9903091
http://www.physics.gatech.edu/people...nkelstein.html
http://www.innerx.net/personal/tsmith/d4d5e6hist.html
http://www.innerx.net/personal/tsmith/cdomain.html
http://www.innerx.net/personal/tsmit...tml#KerrNewman
http://www.physics.hmc.edu/student_p...radiation.html

mee Jul11-04 02:01 PM

tiny black holes (again)
 
How could a black hole with an event horizon the size of an atomic nucleus contain enough mass to not let light escape. It seems to me that it would only have the mass of a softball and would thus not be any more dangerous than a softball.

Nereid Jul11-04 05:42 PM

Quote:

Quote by urtalkinstupid
i still am in disbelief on the theory of black holes as well as gravity being a pull or even existing. i continue to find more sources that make logical attempts to disprove black holes existences, although they are just theories...they actually make sense and unlike the theories proposed about black holes today have baffled many scientists because of their inaccurate calculations from false mathematical attempts at explanation.

dude check out this site http://www.perceptions.couk.com/uef/nblckhls.html

im sure ill find more sites that take a shot at disproving black holes...and as it takes time to find these sites it also takes time to take an open mind to science as it has no definite way of being operated.
Quote:

Quote by Nereid
Excuse me, but that's not how I think science is done.

yea thats how science works...that man is questioning. that's how science works if im not mistaken
heheh :smile:

but if you can could you provide me with links on GR and SR...and answer my question about particle/anti-particle interatcions at this phenomenons event horizon?

When there's two sets of material on one topic - for example, the sorts of websites you've posted links to vs 'textbook physics' on BHs - one way to make a decision as to which is worthwhile spending time studying and extending is the degree to which the two quantify things, and make specific, concrete, testable predictions.

In the present case, IMHO, it's pretty black & white - those websites are little more than handwaving and nice words; the 'textbook physics' is detailed, specific, and concrete, with plenty of testable, quantitative predictions.

Sure, those websites have lots of questions; but how many of them deal with specific, quantatitive data? You say they are 'logical attempts to disprove black holes existences''; that's certainly not science! Science is about experimental/observational support (or not) for hypotheses. The extent to which 'logic' plays a role is limited to consistency of deriving expected results, or consistency with other, well-established parts of science; none of the websites you posted are engaged in these kinds of logical exercises.

To make an absurd example: how scientific is the following (as a reason why the medicine won't work): "This medicine can't work because it's yellow"


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