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 ArghStopItYouStupidDuck
Joined: 4/2/2011
Msg: 197
Black holes...help!Page 9 of 9    (1, 2, 3, 4, 5, 6, 7, 8, 9)
Oh and just realized that I never actually addressed your question of how the black hole can evaporate other than simply stating that it was called Hawking Radiation..

Again, this is way, way out of my league as far as knowledge of the subject goes but I'll throw this out there in hopes that someone more educated on the topic can confirm or deny the accuracy of it..

First of all, remember that mass and energy are different forms of the same thing.

Now, let's take a single particle zooming through space at the black hole, but not quite at an angle that would put it within the event horizon. So it flies around, right outside of the event horizon, speeding up due to the gravitational slingshot effect.

It then rockets out away from the black hole, moving even faster than it was in the first place due to the gravitational boost. The energy required for it to move faster came from somewhere, as energy cannot be created nor destroyed.

Where did the energy come from? The black hole itself. In essence the particle "stole" some of the energy from it and used it for acceleration.

Since the black hole no longer possesses this energy, it is the same as if you took a tiny, insignificant amount of mass away from it. When this happens, it's own gravitational field weakens a minute amount, allowing other particles to escape (ones that were previously trapped in orbits around the hole). These particles also "steal" some of the hole's energy in the process of escaping, repeating the process. Eventually enough mass/energy is stolen that the field degrades to the point that the event horizon shrinks, allowing the particles that are actually "part of" the black hole to escape.

Obviously, the black hole has an incredible amount of gravitational force, so the amount of radiation escaping is very tiny, but the process is continuous and ultimately fatal for the black hole.


Again, I'm putting this out here as much for my own edification as to provide an explanation, but hopefully if I'm off somewhere a more knowledgeable poster can clarify what really happens..

Jack
 abelian
Joined: 1/12/2008
Msg: 198
Black holes...help!
Posted: 5/15/2011 6:22:58 AM
Can black holes explode at some point?

No, but black holes can evaporate. They will evaportate faster as they get smaller and the smaller they are, the hotter they are.

Also, can black speres have "galaxies" and "universes" even in its sphere?

In principle, yes, although the size of the black hole would have to be enormous. What you are calling ``its sphere'' is the region of spacetime bounded by the event horizon. The event horizon is the boundary such that once anything crosses it, it must move toward the singularity and cannot return to any point outside the horizon. However, there is nothing special about the horizon other than that. The gravitational field at the horizon could be the same as the earth's gravitational field if the black hole was large enough. (Off the top of my head, I think this is about 10^42 solar masses - a 10 with 42 zeroes behind.) If you were to fall into one that large, as you crossed the horizon, you would feel nothing any different than you would feel free falling on earth.

Black holes are not dense. There is plenty of space inside. The matter all ends up at the singularity.
Can someone explain just how can a black hole evaporate if it is so dense after consuming matter for centuries (it could be for thousands of years too) ...

That is actually quite difficult to explain in a simple way. To do so, requires relativistic quantum field theory. However, a somewhat oversimplified explanation is the following:

In a region of spacetime where there is no matter, no gravitational field, you have nothing but empty space. The space is not really ``empty.'' Particles and antiparticles are created in pairs and anihilate with each other all the time. This is called the quantum vacuum state. These particle pairs are created and anihilated very quickly, so that they have no effect on anything. They can't without violating conservation of energy. (They do play a role in quantum field theory, but for this purpose, that is not germane.)

On the other hand, at the horizon of a black hole, things are different. If a particle anti-particle pair is created at the horizon, one member of the pair may escape the gravitational field, in which case, the other must fall into the black hole. To conserve energy, if the outgoing particle has a positive enery (which it must), the ingoing particle must have a negative energy. That negative energy decreases the mass of the black hole. (One might equally well ask why the negative energy particle in the pair always falls inside. The answer is that the explanation is that isn't quite how it works and a detailed explanation would fix this minor defect, but then making clear what's really happening would get mathematically complicated.) Suffice it to say that if you treat the particles and anti-particles correctly in a curved spacetime, it doesn't matter which falls in. All that matters is that one member of the pair crosses the horizon and one does not. )

Since photons are massless, photons will be the most likely types of particles produced, in which case, the blcak hole evaprorates by producing ordinary electromagnetic radiation. It's actually a perfect blackbody and produces a blackbody radiation spectrum. However, the temperatures of even a pinhead sized black hole are so low that, the radiation would extremely difficult to observe and the time required for it to evaprorate would exceed the lifetime of the universe.

Once "evaporated" what is the contents of the "molecules" once evaporated?

Photons. In principle, other particles could be produced, but the heavier the particle, the less likely it is to produce them. There are some other factors that would surpress heavier particles even more.
 ArghStopItYouStupidDuck
Joined: 4/2/2011
Msg: 199
Black holes...help!
Posted: 5/15/2011 6:58:00 AM
^

Woohoo! It's one of the aforementioned way out of my league posters!!


But he never answered my questions...

Jack
 chrono1985
Joined: 11/20/2004
Msg: 200
Black holes...help!
Posted: 5/15/2011 7:44:32 AM
One concept of a black hole you rarely hear about, which is yet another function of them that media like to ignore. Even space falls into a black hole. This is why light can not escape, because the space it's falling toward is falling at the same rate. It's like if you jumped off your chair and all of a sudden the ground started to fall, so that you were always 1ft above the ground.

The disc you see on a black hole with spin is formed by that key component of a black hole. It occupies the area called the ergosphere, an area where space is pull along with the black hole. Anything falling through that space now must travel the curvature of space formed by the rotation of the black hole. Keep in mind though that not all black holes spin.
 ArghStopItYouStupidDuck
Joined: 4/2/2011
Msg: 201
Black holes...help!
Posted: 5/15/2011 8:34:10 AM
Thanks for mentioning that!

Reading about the ergosphere, it appears that at the points that the jet streams are emitted, the ergosphere perfectly matches the boundary of the event horizon.. Does this mean that, assuming something could power through the jet stream (impossible I know, but hypothetically), it would be able to reach the event horizon without being pulled down into the accretion disk?

It would seem to follow that a the event horizon of a rotating black hole would be shaped more like a sideways figure eight if you could take a cross section of it and see it from the side (though still round when looking straight into it), is this actually the case? If not, why would the light reflected from particles directly above or below the hole (in the pie piece shaped sections that differentiate a figure 8 and an oval) not be able to escape? Or is it the case that light COULD be reflected except that there are no particles actually present there due to either being pushed out by the jet stream or pulled down into the accretion disk?

Sorry for my zillion questions here, but I've wondered about these things for a long time and want to take advantage of the brilliant people available here to answer some of them if yall would be so kind! =)


Thanks!
Jack



P.S. Also just found out that what I ignorantly described as "Hawking radiation" is actually called the "Penrose process"... Curious about why there's a 29% limit to the energy lost through this process though...
 sum1reel
Joined: 6/5/2005
Msg: 202
Black holes...help!
Posted: 5/15/2011 12:48:43 PM
^
Pick up the June issue of 'Discover' magazine. The cover story is about new insights on Black-holes. Its title is 'beyond the event Horizon". The writer speaks about Andrew hamilton's computer model descritions of what may be going on inside.... its very interesting.
 ArghStopItYouStupidDuck
Joined: 4/2/2011
Msg: 204
Black holes...help!
Posted: 5/15/2011 5:37:45 PM
I'm not really one of the learned people on here, so no worries about annoying me! =D

Yup, the place that stars are created are referred to as "stellar nurseries" and are also what you may have heard referred to as "nebulae". There's some incredible pictures of nebulae out there with points of light emanating from them where gas has pulled itself together and ignited from the pressure:

http://en.wikipedia.org/wiki/Star_formation

As you said, stars eventually end up as white dwarves, neutron stars, or black holes depending on how large they are and how much mass is expelled by their supernova/hypernova.

The Sun will eventually grow to be a red giant, consuming the earth in the process, then shrink down to become a quiet little white dwarf. You can see how large the Sun will become as a red giant here:

http://en.wikipedia.org/wiki/Red_giant

Though "little" is a bit of an understatement, as other than black holes and neutron stars, white dwarves are composed of the densest known matter in the universe. Eventually the white dwarf will burn off all of its remaining energy and become a black dwarf, though none of these are assumed to be in existence yet as the universe is too young.

You may also hear about a brown dwarf, which is essentially a failed star that is like a giant Jupiter, extremely large but not large enough to start the gravity-driven fusion process and ignite.

The sun isn't anywhere close to large enough to form a black hole, but even if it somehow did, it wouldn't suck in all of the planets as the characteristic of being a black hole doesn't actually bestow additional gravity onto a celestial object, if something is in a stable orbit around a star it will maintain that same orbit around an equal mass black hole. They would, of course, be much colder though as the black hole doesn't radiate energy in all directions the way that a star does (except in the path of the jets, which are perpendicular to the orbital plane of the hole which is where planets would be rotating).

One thing that I've found to be particularly thought-provoking is the story of PSR B1257+12, the star that hosts the first extra-solar planets ever to be confirmed.. It's a pulsar (a neutron star that spins around spraying out deadly gamma rays many, many times a second) that is orbited by three planets, two of which are Super Earths (rocky planets basically like Earth but larger). These planets almost certainly don't host any life, as if they did it would have died a long, long time ago in such an incredibly harsh environment.

http://en.wikipedia.org/wiki/PSR_B1257%2B12

But, just think... a dead star that blew up millions of years ago, spraying out radiation at two dead Earthlike planets... Somehow it's incredibly creepy, but wonderous at the same time... It makes you wonder if life was there before, if that life moved on before the star exploded, if it could be out there now..

There's so many amazing things out in the universe, a good way to start looking into it is just to plug something in wikipedia (like black holes), then keep clicking link after link until you've found out everything you want to know about, well, everything! =)

That and asking the learned people here of course (though you might have to ignore scrubs like me piping up every once in a while)!


Jack
 ArghStopItYouStupidDuck
Joined: 4/2/2011
Msg: 205
Black holes...help!
Posted: 5/15/2011 6:13:31 PM
Oh and thank you for mentioning that article sum1reel, I'll definitely check it out!

I love that magazine anyway, I should really get a subscription to it...


Thanks!
Jack
 abelian
Joined: 1/12/2008
Msg: 206
Black holes...help!
Posted: 5/17/2011 7:23:40 AM
A star is born. It burns itself out and collapsing on itself.

A star dies...

Although this explains the process by which a black hole is formed and the conditions required for it to happen, it really only explains black holes as one possibilty for the end of a star's life. It really doesn't explain what a black hole is. Gravity is what holds the earth together. The pressure is very large as you go closer to the eath's center. As the earth formed, the pressure increased and the pressure from gravity heated the center faster than the heat could escape, so the earth gets hotter as you go toward the center.

If the earth were much larger (i.e., more massive), that pressure would be much higher and the earth would be hotter. If the earth was massive enough, the pressure would be large enough and themperature hot enough that molecules could not form (except perhaps, close to the surface). (Put a diamond in a fire and you'll vaporize it, for example.) If the earth is larger and more massive still, the atoms will be stripped of their electrons and nuclei will collide and fuse, giving off a great deal of heat. Now you have a star. The intense heat creates a great deal of pressure and that pressure prevents the star from being compressed into cold lump of matter. But, eventually, the the lighter elements have all fused into heavier elements and the heaviest element that can be formed by fusion while still producing energy, is iron. If the star merely burned itself out (and some do), you you'ld have a cold lump of matter, held together by gravity. The only thing that prevents the lump of matter from collapsing into something smaller is the stability of matter that requires quantum mechanics to explain. (cf the Pauli exclusion principle.)

Quantum mechanics allows one to calculate how much pressure is required to overcome If the star is larger still then gravitational pressure will crush the matter still further and if the star is large enough, nothing can stop gravity from causing the matter to collapse completely to a simgle point. This is a black hole. The event horizon is the region where nothing can escape the gravitational field of the matter that collapsed to form the black hole. In principle, black holes could form in other ways, (like high energy collisions of particles) but the death of massive stars is the only simple way to get some matter close enough together for gravity to be strong enough to overcome everything else.

I am still trying to figure out that when you look at the sky it is like reading yesterday's newpaper... Well, not really yesterday's but millions of yesterdays ago...

As a first approximation, this is straight forward to understand. Light proagates at a finite speed, so it takes time for light to reach you, even if you're looking at your hand. It's not a long time in that case, but you're still seeing your hand a few nano seconds in the past. If you're looking at something that is so far away that it took a year for the light to reach you, what you are seeing happened a year ago, not right now.

However, this inevitably leads one to think that the speed at which light travels is relevant and that there really is a non-ambiguous way to define ``right now'' such that if one could travel faster than light, one wouldn't be constrained to observing what is in the past. The real explanation is that there is no unambiguous definition of ``right now'' because in special and general relativity, the entire geometry of spacetime is not what you think it would be based on the kind of geometry one learns in high school. This is not hard to explain to someone in a one-on-one setting with a pen and paper available to draw pictures, but translating the pictures to text makes it seem incredibly difficult unless one already understands enough geometry and algebra to not be intimidated by equations and one has some ability to see how those equations relate to physical things.

In particular, most people are familiar with the pythagorean theorem for a right triangle, c^2 = a^2 + b^2. That works ok for objects in space, but once you throw time into the picture and treat time as just another direction in space (i.e., spacetime), the pythagorean theorem is different if one of the ``legs'' of a triangle is along an exis in time, i.e., if the distance in time is b, then it becomes c^2 = a^2 - b^2. The best not-all-that-technical book that will make this clearer is ``Spacetime Physics,'' by Taylor and Wheeler.

Fascinating really...

Yep. I became a physicist because it is fascinating enough that I'd not trade what I've learned for anything, even sex and I'd say that sex has to be number two on the list.

What if our sun becomes a black hole. All the planets, gases and dust would be sucked in it right? Would that be possible?

Apart from no longer receiving light and heat from the sun, nothing, at least not at distances far away from the sun (meaning any distance further than the radius of the sun as it is now. There's nothing special about black holes (apart from what happens inside the horizon). If the sun were to become a black hole, the black hole woud have a radius of 1.5 km (a bit less than a mile, or just under 2 miles in diameter). The gravitational field of the sun on the earth and planets would be exactly the same because the graviational field of any object at a distace greater than the size of that object is exactly what it would be if all of the mass were concentrated at a point.


For anyone interested inthe jets emitted by accretion discs surrounding massive rotating objects, here's a good explanation of the best model that exists, at this point in time:

http://www.physics.purdue.edu/~mlister/articles/meier2001_sci_291_84.pdf
 abelian
Joined: 1/12/2008
Msg: 207
Black holes...help!
Posted: 5/17/2011 7:53:18 AM
Reading about the ergosphere, it appears that at the points that the jet streams are emitted, the ergosphere perfectly matches the boundary of the event horizon..

The reason for that is the speed of rotation at the points along the rotation axis is zero. Spin a globe and you'll notice that the speed at which points on the equator move is faster than points nearer to the poles. The ergosphere is a consequence of the rotation, so at the two points where the poles occur, there is no ergosphere. The ergosphere is largest at the equator and becomes smaller as you move toward north or south the poles.

There are actually two horizons in a rotating black hole. The outer horizon is similar to the horizon of a non rotating black hole. The inner one is much more interesting. Look up Kerr black hole, or Kerr metric.

Does this mean that, assuming something could power through the jet stream (impossible I know, but hypothetically), it would be able to reach the event horizon without being pulled down into the accretion disk?

The accretion disc is nothing but matter that is spiralling into a massive, rotating body because it's velocity isn't great enough to allow it to stay in a stable orbit, like a satellite that falls back to the earth. If you want to know why particles are pushed down into a disc, again, picture a globe except this time, picture a stiff wire that rund from the north to the south pole along a meridian and on this wire there is a bead which can slide back and forth between the poles. Start with the bead at either pole and spin the globe. The bead will slide toward the equator.

What is orbiting black holes and neutron stars are charged particles. Moving charges produce electrical currents which produce magnetic fields. Here are some pictures:

http://www.physics.purdue.edu/~mlister/articles/meier2001_sci_291_84.pdf

Notice the currents around the rotating object are ``tightest'' at the equator, like a solenoid that is wound tightly on a small diameter at the center and more loosely with larger and larger radii as one moves toward the poles. Unlike the stiff wire on the globe which constrains the bead, charged particles follow field lines (look up Lorentz force), hence if the particles move fast enough, they will follow the field lines in the directions ofthe poles but since the field lines bend away from the poles instead of toward the poles, the particles will escape in directions along the rotation axis.

Keep in mind though that not all black holes spin.

All physically realistic black holes spin. The solution for a non-spinning blackhole is obtained by assuming that the spacetime is static, i.e., it looks the same wheter you go forward or backward in time, so a Scwarzchild black hole is eternal - it has always existed and always will exist. It can only be an approximation to a real black hole, however it's an exact solution to the field equations and it is also a really good approximation to more realistic black holes outside the horizon, so one typically uses it to obtain the basic features of any black hole. The difference only matters if you want to know really detailed information about regions where the gravitational fieds are very strong.



 abelian
Joined: 1/12/2008
Msg: 209
Black holes...help!
Posted: 5/19/2011 4:35:17 PM
Now, does a beam of light slow down if it does not bounce off of anything?

No, but neither does a rock if you throw it in outer space.

I understand the concept of speed of light but can a light beam travel at the speed of light forever?

Light can only propagate at the speed of light. (It travels slower in a medium, like a crystal, but I assume you mean in free space. Why it travels slower in a medium isn't really relevant to the question you're asking.)

Does a beam of life "die" off after a period of time?

That depends on what you mean by a ``beam'' of light. However, let's cover the possibilities. First, consider all of the light emitted from the surface of the sun. Picture an imaginary spherical surface drawn completely around the sun. All of the light that crosses that spherical surface at a given instant will have some total intensity. Call it I. Now picure a much much bigger sphere drawn around the sun. All of the light that crossed that first surface will, at some later time, also cross the surface of that much larger sphere (unless something in beween is blocking it which we will assume there is not.) The total intensity must therefore be the same as the intensity of light that crossed the smaller spheere. However, the larger sphere has a much larger surface area, so the light is more spread out. In that sense, if you take a piece of paper that's 1 ft by 1 foot square, the intensity of the light that lands on that 1 square foot of paper if you are a foot away will a lot greater than if you are 100 feet away.

So, if you were to create a light beam by putting up a big screen with a hole in it, the beam would diverge, but the total intensity of the beam would be the same no matter how far it travels. It would just be spread out over a larger area. A laser on the other hand is slightly different. Laser light it produced in such a way that the beam doesn't diverge very much. If you picture a laser which could produce a perfect beam of light that didn't diverge, the intensity would be the same no matter how far the beam traveled.

If a beam of light does not slow down then speed of light is sort of like a constant.

Well, it's not sort of like a constant. It is exactly a constant. In fact, it's known to a constant with such great precision that a distance of 1 meter can be defined to much greater precision by measuring how far light travels in a specific time interval with much greater precision than using the platinum bar that used to define the meter.


Can a burst of light from a sun slow down at some point?

No.

Can a burst of light penetrate any sphere in rotation in a direct line sort of speak?

I don't understand this question.

Can a burst of light lose its strength the longer it is travelling without bouncing off of any space matter?

No. It can't even slow down by bouncing off something. It either gets absorbed or it bounces off and propagates at the same speed, perhaps in a different direction.

Do all "unconsummed" beams of light end up being sucked up by a black hole ?

No. Light which falls into a black hole cannot escape. Light passing by a black hole may get pulled in by the gravitational field or it might just get bent around the black hole (like light would be bent by a focussing lens. However, light aimed directly at the black hole will disappear into the black hole.

Is it just me or the more I learn, the less I seem to understand.

Unfortunately, relativity and black holes are always described by the media in a way that tries to appeal to what people are familiar with and what they gain with that initial simplicity, they lose in the ability to explain the weirder things in a way that makes sense. Understanding the basic features of black holes is not really that difficult and it's actually pretty intuitive for the most part, but the price you have to pay on the front end is understanding a little more geometry and getting comfortable thinking about time as just another dimension instead of something intrinsically different from space. However, it does not require a lot of mathematics. Just good pictures with decent explanations.
 FrankNStein902
Joined: 12/26/2009
Msg: 211
Black holes...help!
Posted: 5/24/2011 2:02:21 PM

Black holes...help!


Astronomers snap black hole murder in graphic detail (video)


We tend to imagine a black hole sucking everything around it straight into oblivion. The truth, however, is even more gruesome. Astronomers have just captured an ultra hi-res image of our neighbouring galaxy, Centaurus A, and it helps to reveal what actually happens. Matter is yanked helplessly towards a black hole at the galaxy's core, but it refuses to die quietly. For some unknown reason, it erupts as it falls, spewing out vast plumes of particles -- like blood from celestial murder. These death throes emit radio waves, allowing us to witness them using radio telescopes even though we are 12 million light-years away. If only we were closer; if only we could intervene. Alas, all we can do is watch the video after the break and hit the source links for a fuller explanation -- though, admittedly, none of those sound like awful options.

http://www.engadget.com/2011/05/22/astronomers-snap-black-hole-murder-in-graphic-detail-video/
 ArghStopItYouStupidDuck
Joined: 4/2/2011
Msg: 212
Black holes...help!
Posted: 5/26/2011 4:39:15 PM
Thanks for the replies, everyone!

I had a long post typed up, but I just realized a way to summarize it:

Stars are nature's way to produce omnidirectional energy via nuclear fusion. Is it possible that black holes are the way for nature to produce directed energy via fission?

Jack
 R_O_U_S
Joined: 3/20/2011
Msg: 213
Black holes...help!
Posted: 8/10/2011 7:39:21 PM
a black holes gensis is based on density and what some belive to be relativity, but i beleve to be dark matter collected on density causing enuogh bridge for the large density to brace from 3rd to higher dimensions to time collapse. i opine paul sayers sighning off.
 abelian
Joined: 1/12/2008
Msg: 214
Black holes...help!
Posted: 8/11/2011 5:01:27 AM

a black holes gensis is based on density and what some belive to be relativity,

A black hole is a consequence of general relativity. Without general relativity, you have no theory to tell you what a black hole is supposed to be or how to identify one. Without general relativity, you might be able to define something like a black hole in general relativity, but first you need a theory to tell what it is.
 PragmaticDoc
Joined: 8/4/2009
Msg: 215
Black holes...help!
Posted: 8/11/2011 1:32:24 PM
Ok I just finished "The Universe in a Nutshell" and have a question pertaining to black holes and the law of conservation of energy. A black hole is pulling eveything into itself, Light, mass, energy, everything. Since the total amount of energy always remains the same, as per the law states, does this mean all energy, as well as the light and mass are being moved to another dimension or simply being stored awaiting that last blast?
 swingarm1966
Joined: 3/27/2011
Msg: 216
Black holes...help!
Posted: 8/11/2011 5:46:18 PM
dosnt a black hole punch a hole through the space time fabric? If this is true there is something else going on other than just a "dark sphere" no? Sorry have not read the hole thread.
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