Here is another kicker about black holes. They spin faster then the speed of light. You might notice a pulsar spining every 1.45 seconds or somtheing. A big star dose not lose energy by shrinking, just like if you are spining around and pull your arms in. Black hole can spin this way because the are infanitly small. So what you might ask, well when something spins this fast it will bend time. If you were sucked into a black hole to some one waching you you wouldn’t ever enter the event horizen. We still don’t know what it would look like to you is you were sucked in.
Light passing a black hole will be bent into it. The curvature of space around the black hole makes light think it’s going straight, but it’s actually bending inwards toward the event horizon.
Suppose a man in a spacesuit was approaching the event horizon. The gravitational pull on his feet would be greater than the pull on his head, so the man would strech out like a rubber band. Well then he would be disassembled so to speak.
Black Holes with no spin are perfectly round. A black hole rotating will get fat around the equator. This fatness will increase as the rate of rotation increases.
I have always wondered about the supermassive black holes at the center of spiral galaxies (as well as others). The massive hydrogen stars that must have formed them.
Black holes do die. They don’t hang around forever.
They recycle matter and space. Then they shrink up and vanish.
*1. The term ‘black hole’ was originally coined to describe what we would ‘see’ if were were looking at one. If a black hole with an event horizon smaller than our sun passed right in front of the sun, it would look like a ‘black hole’ was passsing by. Yes, that’s what a solar eclipse looks like, too. But if we shined a really powerful light on the dark side of the moon during a solar eclipse, we would illuminate the moon and it wouldn’t look black. If we shined a light on a black hole, the black hole would never be illuminated, even if our light were infinitely bright! Black holes are always black, because nothing (except a few instances of exotic particles and radiation) can escape from a black hole, not even light.
- A ‘black hole’ has two parts! This is very important to remember. At the center of the black hole is a definite amount of mass (over 40 times the mass of our sun) crushed into an infinitely small space. This is the first part, the singularity of the black hole. Around that singularity is borderline of gravitational no-return, which is called the event horizon. Anything which passes the event horizon, even light, will never ever get back out past the event horizon. It will eventually be pulled by gravity into the singularity. The event horizon could be a huge distance away from the singularity at the center of it.
3. What happens to a thing as it falls into a black hole from an outside observer’s view?
From an outsider’s frame of reference, it looks like time itself slows down and stops as an object approaches and passes through an event horizon. If you throw a clock at a black hole, you observe its second hand slowing down and then freezing at the point of entrance to the event horizon. That’s because gravity is increasing more and more as you approach an event horizon until it’s so strong at the event horizon, that light can’t even escape. As the clock falls to the event horizon, the light escaping from the clock (that’s how we ‘see’ the clock) has to overcome more and more gravity and it moves slower and takes longer to reach you. The clock ‘appears’ to slow down. (Since it takes more and more energy to try to overcome the high gravity near the event horizon, the image of the clock actually blue shifts (see doppler) to lower and lower energy levels until it just fades from view.)
4. What happens to a thing as it falls into a black hole from the point of view of the person falling in?
Well, imagine that you’re standing on a singularity. Almost instantly, your feet get sucked in and are infinitely compressed. Ouch. Your body and head follow suit in due time (albeit a very short time). Before you get sucked in, you get stretch out. This is called gravitational tidal forces.
The effects of the tidal force actually occur much further away from the singularity. For some certain types of black holes, it’s possible that the tidal forces will rip you apart (quite painfully, I might add) even before you get to the event horizon.
However, there are some black holes where the tidal forces don’t become lethal until you’re well into and past the event horizon. Thus, it is possible to survive and experience falling into a black hole’s event horizon, although, you can never survive the inevitable falling into the singularity.
5. When you fall into an event horizon, do you experience the slowing or stopping of time? Changes in the speed of light or laws of the the universe? See any neat stuff?
Light speed never changes in your frame of reference. Light speed is still measured at the same constant within the event horizon as it is without. Even if you try to shine a flashlight out of the hole (as you’re within in it, falling towards the singularity), and the light just slows down and turns around and comes back chasing you; it is still measured at its usually constant. Why? Because any measuring devices you use will also be bent back inward and wind up giving you the same constant for the speed of light. Imagine tossing a ball up in the air and catching it when it comes back a few seconds later. Then, when you measure the path the ball took, it measures several million miles.
Cool, huh?
You experience no stoppage of time. As you fall through the event horizon, you do not freeze in time. That’s an optical illusion outside observers see. You just whiz by through the event horizon, accelerating toward the singularity. Like freefall. But only into a star over 40 times more massive than the sun. And concentrated into one point.
One neat thing you would see is that the field of stars around you would start to contract into an ever diminishing circle above you. If you think about it, once inside the event horizon, the light from the outside stars to the ‘side’ of you will never make it to you. They get bent into the singularity below you. You can only see the light from the stars above as those photons chase after you into the hole.
6. If the singularity is just a point, and the event horizon is just a boundary of no return; how can black holes ‘spin’?
Stars spin. As a spinning star collapses into a singularity, it spins faster (like a spinning skater pulling in their arms). All that spinning mass yanks space itself along with it. Though a singularity is a point in space, and points can’t spin (see geometry); it is space itself which continues to rotate around the singularity. Angular momentum is conserved!
And now a really crude and oversimplified explanation of how energy can escape a black hole: Just like a spinning top loses angular momentum (spin) due to friction, so too, the spinning space of a spinning black hole can loose some of it’s energy to friction with space around the hole.
Someone said that a spinning black hole bulges. My memory says (not 100% sure) they don’t bulge.
7. Any ways to detect a black hole?
Call nature crazy, but it loves to randomly create out of nothing a sub-atomic particle and its corresponding anti-particle and have them immediately merge and cancel each other out and disappear. However, when nature does this at an event horizon, one particle may get sucked in, while the other escapes. The escaping particles may be detected one day as a tell tale sign of a black hole.
Also, as mention, accelerating mater approaching an event horizon (while it’s still outside) can emit tell tale radiation patterns. Especially when the matter is circling and spiralling in on the event horizon – it can reach near light speed velocity. Sometimes, such matter can emit a maser (microwave laser). I believe such masers have been detetected.
Peace.
Check out the Black Holes FAQ at: http://cfpa.berkeley.edu/BHfaq.html
Great post moriah, but there’s one mistake:
Of course, you meant to say that it red-shifts to lower energy (and hence frequency) levels. Blue-shifting would happen to light you were watching falling in.
Also liked your post, moriah, but another quibble
My understanding is that black holes come in any size they damn well please (YOU gonna tell a black hole they ain’t the right size?). Current theory is that the centers of galaxies are mongo massive black holes with masses greater than the sun by several orders of magnitude. Also, micro black holes (less than solar mass)existed in great quantities in the good old days when the Universe was in short pants, but have mostly “evaporated” via Hawking radiation.
If anyone wants a read a great combination of fact, speculation and WAG’s about black holes, I cannot recommend strongly enough “Earth” by David Brin. The man writes the hardest of hard science fiction. In particular, I recommend his essay at the end of the book in which he explains where his ideas, speculation, etc. came from.
a site about the orbits of boodies around black holes. This is fun! annd stuff makes a homer simpson d’oh when it falls onto the black hole
From North America
http://www.fourmilab.to/gravitation/orbits/
From Sweeden
http://www.fourmilab.ch/gravitation/orbits/
Jeff_42, I’m flattered. Yes, you’re right, and Olentzero, in this case at least, is wrong. Energy (specifically, in this case, light energy) is, in fact, affected by gravity, but light is affected a bit differently than matter. If a material particle tries to escape from a gravitational field, it’ll slow down and possibly stop and return. If light tries to escape from a gravitational field, it can’t slow down, so it loses energy by redshifting, instead. In the case of a black hole, any material object, no matter how fast it’s thrown, will fall back, and light that tries to escape will be redshifted to infinite wavelength (and hence zero energy).
As for light having mass, photons or anything else that travels at c have no rest mass, but you can talk meaningfully about the mass equivalent of the energy that they do have. It’s a subtle distinction, and often glossed over.
Actually, if you went in feet first, your feet and lower legs would be pulled off from your body.
Since the speed of the escape is faster than the speed of light, then your feet would also be travelling at that speed. But since the rest of your body is not at that speed yet, then off go your feet, since they want to go fast, and your body doesn’t.
Ah, yes, you are quite right. A little bit of poor wording on my part. By definite, I meant measurable. Many people confuse infinite density with infinite mass. There is a definite (not infinite) amount of matter in the singularity.
But in the ‘usual’ formation of a black hole, it takes a star of at least 40x the mass of our sun to have enough mass to actually collapse into a black hole. By acquiring more mass when things ‘fall into it,’ black holes can have an obscenely huge amount of mass compared to our sun. And, through Hawking radiation, a black hole can loose its mass to less than 40xSol without re-expanding. Although… that loss can’t go on forever. At some point, the gravity of that diminishing mass will be too little to support infinite compression, and then, ka-boom. You can’t have an infinitely light black hole.
Peace.
As to my other mistakes that people found… bite me. No, wait, I mean ‘quite right.’
As I underatand it it depends on the size of the collapsing star. In some case you woud be stretched out like a piece of spaghetti (or just plain ripped apart).
I also thought that in a sufficiently large black hole you may not even notice you passed the event horizon (or standing on a sufficiently large star collapsing to a black hole). You could quite happily sail on…alive…till it’s far too late for you to escape.
(This of course presumes you are superhuman enough to actually stand on a star in the first place)
Black holes formed by stellar collapse do not shrink due to Hawking radiation. Hawking radiation is thermal in nature. The temperature (due to Hawking radiation) of a black hole with a mass a few times the sun is one-ten-millionth of a degree. The is much lower than the 2.7 degree background radiation. It will take quite a long time for the universe to cool enough for Hawking radiation to make normal black holes shrink; and even then it will take much, much longer than the present age of the universe to disappear completely.
The only black holes that could shrink due to Hawking radiation now are primordial black holes.
A minor correction: a star can be much less than 40 stellar masses to form a black hole. The Chandrasekhar limit is one and one half the mass of the sun. A cold star with a mass greater than Chandrasekhar limit must collapse into a black hole. If the mass is less, it will settle down to a white dwarf star.
I will also mention the “Black hole has no hair” theorem. It says that the only properties that a black hole has are mass, spin, and electric charge. For example, you can’t tell wheither a hole is made of matter, anti-matter or energy.
One more thing I forgot to say in my previous post:
moriah, I’m impressed!!
The link that emarkp gave us led me to:
http://cfpa.berkeley.edu/BHfaq.html#q4
This link seems to imply that light actually does slow down coming from just outside the event horizon. As in, it would take a long time for light to reach you coming from near a black hole. Is this guy pulling my leg? I thought the speed of light in a vaccum was a constant.
Boris B
You are confusing the special relativity and general relativity.
The speed of light in a vacuum is constant in special relativity (SR). SR only deals with inertial reference frames. General relativity deals with any reference frame allowing acceleration and gravity to be considered. The speed of light is not constant in General Relativity. In GR (as in SR), nothing can go faster that the local speed of light, but (unlike in SR) light can speed up and slow down. At the event horizon light going outward is suspended. Light speed is zero outward. Since matter cannot go faster than zero outward, it must fall into the hole.
The speed of light in a vacuum is constant only in an inertial reference frame. An inertial reference frame is one that experiences no acceleration or gravity. If it is in a gravitational field, it must be in free fall to transform away the effects of gravity. Near a black hole (or any massive body) an inertial reference frame must be in freefall.
A minor correction to your minor correction: there are actually two Chandrasekhar limits. The orginal one (about 1.4 solar masses) is the dividing line between white dwarfs and neutron stars. There’s another one that’s about 2 solar masses that’s a dividing line between neutron stars and black holes.
However, note that stars lose lots of matter during their lifetimes and especially at the end of their lives. It’s thought that any star starting with less than about 8 solar masses will actually end up as a white dwarf. I’m not exactly sure (and I think astronomers aren’t really either) about the beginning stellar mass required to end as a black hole. From what I remember, the 40 solar masses mentioned earlier is in the right ballpark.
dtilque
Thank you for your information. Moriah’s treatise seemed too carefully written to be that far off.
I did say cold star. (Although I have no idea what the hell a cold star might be.)
Dr. Matrix–
A white dwarf or neutron star, or a burnt-out red dwarf, can be arbitrarily cold, provided it’s old enough. The result is called a black dwarf. The Universe isn’t old enough for any formerly-red black dwarfs to exist, but it’s believed that some white dwarfs or neutron stars might have cooled off enough.
As for the concept of light being slowed trying to escape from the vicinity of a black hole, this does give the same results for a body just falling in, but if youtake an object (a really powerful rocket, say), and bring it near a black hole and then away again, you’ll need to use gravitational time dilation to explain the results-- slow light won’t cut it.
Of course, when we say “light slows down,” we’re talking ‘apparently.’ Either time or space is dilating (or in the case of space, severely bending, twisting, or spinning) which gives the appearance that light is slowing down. Since we don’t ‘see’ time or space distorting, it appears that the speed of light has changed. However, if we tried to measure the space and time that the ‘slow light’ passed through, our measuring tools will be dilated or distorted and thus render the ‘normal’ constant of the true speed of light.
It’s a universal conspiracy. The speed of light, made of photons, can never be changed – it’s a rock hard granite defining base of all reality. And, at the same time, the speed and location of a photon can never be pinned down – it’s the ultimate random probablistic and fuzzy ambivalence of all reality.
Peace.
Actually, I think that that should be momentum and location. Werner’s Uncertainty Principle doesn’t set any limits on the precision with which we can determine c, does it?