The View Near a Black Hole (April 19, 2009)

[NoelC]

While a thought-provoking and pretty bit of art, one thing about this artist's conception bothers me: The material near the BH is implied to have a doughnut shape, with the material in the foreground appearing to orbit well above the general plane of rotation... Isn't that inaccurate? Would there be a doughnut shape at all?

Yes, I have heard of an "accretion disk". That's a disk, not a doughnut.

I can imagine the general rotation of material around a central galactic black hole in a spiral galaxy to be more or less aligned with the general galactic rotation... I'm having a bit of trouble getting my mind around why a black hole in general (e.g., not at a galactic center) would align material substantially in a disk (or doughnut) however. Are there magnetic influences here as well as gravitational?

http://apod.nasa.gov/apod/ap090419.html

-Noel

[Chris Peterson]

While a thought-provoking and pretty bit of art, one thing about this artist's conception bothers me: The material near the BH is implied to have a doughnut shape, with the material in the foreground appearing to orbit well above the general plane of rotation... Isn't that inaccurate? Would there be a doughnut shape at all?

I don't have a big problem with the donut shape (although I don't know if it's correct). The problem I had with this image is that it appears to show material that isn't symmetric around the plane perpendicular to the rotation axis. It looks like stuff is falling "down" as if the black hole were at the bottom of a whirlpool in the ocean. That makes no sense to me. Maybe I'm seeing the image wrong, or maybe it is symmetric, like a bowtie shape swept around, with the lower part just blocked. But in any case, I find the image confusing.

I can imagine the general rotation of material around a central galactic black hole in a spiral galaxy to be more or less aligned with the general galactic rotation... I'm having a bit of trouble getting my mind around why a black hole in general (e.g., not at a galactic center) would align material substantially in a disk (or doughnut) however. Are there magnetic influences here as well as gravitational?

There are magnetic forces at play, but they aren't responsible for the disc shape (but they are involved in angular momentum transfer that contributes to the dynamics of the disc). The disc happens because a BH only acquires an accretion disc if there's already something in place to provide the material- typically an orbiting companion. Naturally, that material stays in the same plane as the companion orbit (which is also close to the same plane that both rotate on, given the mechanism for how multiple star systems form, and how angular momentum is conserved).

[BMAONE23]

For what is presumed to be a (mathematical) singularity anyway, It seems likely that material in the accretion disk would have thickness whereas the event itself has no thickness and thereby all matter looses thickness as it traverses the event horizon. The accretion disk would gain thickness proportional to its distance from the event horizon. I view this as one side of a mirrored 3 dimentional object.
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[astrolabe]

Hello BMAONE23,

Your visualization is intriguing. The idea being that matter as we know it cannot exist inside the EH as it breaks down into what I assume is pure energy, therefore the ability to exist as a singularity similar perhaps to the possible senario leading to the BB? Good idea really.

[Chris Peterson]

For what is presumed to be a (mathematical) singularity anyway, It seems likely that material in the accretion disk would have thickness whereas the event itself has no thickness and thereby all matter looses thickness as it traverses the event horizon. The accretion disk would gain thickness proportional to its distance from the event horizon. I view this as one side of a mirrored 3 dimentional object.
Similar to this

From the standpoint of the external universe, the black hole is not a singularity. It is a sphere of finite size, and that is what determines how it affects the surrounds. Nobody knows what is inside a black hole, if anything, and it doesn't matter in terms of the external behavior. It is worth noting that in most cases the dynamics of an orbiting system can be treated as if every member were a singularity. We treat the Sun as such when we calculate the orbits of the planets, for instance. And we treat the Earth as one when analyzing satellite orbits. It's only when something gets so close that tidal forces are important that the actual body size matters.

A supermassive black hole is a few light hours across. Small compared with a galaxy, but still large enough to support an accretion disc of considerable thickness. I've always understood (maybe incorrectly) that an accretion disk was thickest near the center, not near the edges.

[aristarchusinexile]

Black holes may or may not exist .. but if space is closed around a black hole, how can anything enter it? (Please don't picture the event horizon as a door because space is said to be "closed" around a black hole.) The more I read about cosmology the more I'm reminded that I'm not the only person with a wild imagination.

[Qev]

I believe they're 'closed' in the sense that, beyond the horizon, spacetime is so warped that there are no possible paths that lead back out of the horizon. So it's more like a Roach Motel™ than a closed door.

[aristarchusinexile]

I believe they're 'closed' in the sense that, beyond the horizon, spacetime is so warped that there are no possible paths that lead back out of the horizon. So it's more like a Roach Motel™ than a closed door.

If there are no paths out, there are no paths in. We cannot just be convenient for the sake of our pet theories.

[Chris Peterson]

If there are no paths out, there are no paths in. We cannot just be convenient for the sake of our pet theories.

I can think of no rational basis for that conclusion. Nature is full of one-way paths, in thermodynamics, classical mechanics, quantum mechanics- everywhere you look.

[aristarchusinexile]

If there are no paths out, there are no paths in. We cannot just be convenient for the sake of our pet theories.

I can think of no rational basis for that conclusion. Nature is full of one-way paths, in thermodynamics, classical mechanics, quantum mechanics- everywhere you look.

One way path in thermodynamics? Not according to the latest. Disorder moves directly to order in some cases. My rationality is that with space closed around the hole, there is no way in as space will direct the energy/matter elsewhere, perhaps 'round and 'round and 'round until it builds enough energy to jet away.

[Chris Peterson]

One way path in thermodynamics? Not according to the latest. Disorder moves directly to order in some cases.

There is nothing in classical thermodynamics that prevents an increase in order. Life would not exist if that were not possible! But entropy in an isolated system never decreases. That's the Second Law, and it sure as heck hasn't been violated experimentally. And there are similar examples to be found in many places.

My rationality is that with space closed around the hole, there is no way in as space will direct the energy/matter elsewhere, perhaps 'round and 'round and 'round until it builds enough energy to jet away.

If you can't put that in a mathematical framework, so it can be evaluated against some theory, there's not much that can be said. There is nothing about "closed space" around black holes that prevents material from falling inwards. The math behind the theories works, and is consistent with observation.

[Markus Schwarz]

I think there are some common misconceptions about black holes which I hope to clarify and to some extend "demystify" black holes.

First of all, black holes are solutions to Einstein's General Theory of Relativity. This means that one can study these solutions and answer some questions.

What are black holes? As said, they are solutions of Einstein's theory of General Relativity, the simplest ones being described by a single parameter M. Depending on this M is another property called the Schwarzschild Radius. Roughly, if an object of mass M is smaller in diameter than its Schwarzschild radius, it is a black hole. For comparison, the Schwarzschild radius of the Earth is just a few millimeters. However, the central mass in the milky way is believed to be distributed within its Schwarzschild radius, and thus is a good candidate for a real black hole. The Schwarzschild radius is also the radius of the event horizon. Once you cross the event horizon you cannot leave it again. In this sense, the event horizon is the boundary of the black hole.

So, does time "freeze" at the event horizon? There is no clear yes or no to this answer, since time is relative in the theory of relativity. What this means is that different observers will experiment different times. In particular, it makes a difference if you are safely far away from a black hole or falling into it.

So, what happens if you fall into a black hole? Imagine that you and your colleague both are far away from a black hole. You shake hands and he departs in a space ship directed into the black hole. Before you depart you also agree that he will always wave back to you at a constant period of, say, 1s. While he falls you
observe him through a telescope. What YOU will see is that the closer he comes to the event horizon the slower he appears to wave. First this is barely noticeable with a period of 1.01s, but starts to grow to 2s, 1h, 1d, ... Finally, he seems to be frozen in time. (You would also have to switch to IR glasses and then to Radio telescopes, since the light wave also become red shifted).

What HE will see is completely different. He just floats in his space ship and waves back at you, always making sure that he waves at a period of 1s. The only thing he notices is that his spectrometer on board measures the star light to become more and more blue shifted. After a finite time, say one hour, he finally reaches the event horizon.

What's the resolution of this apparent paradox? The reason is that time is just a coordinate and its value depends on the observer. Without going into math, the following analogue might be helpful. Consider the South Pole on Earth. Seen from space or from a globe, there is nothing special about it. However, on standard European world map Antarctica is stretched and it would seem to rival Russia in size. But this is just an artifact of the map. It is a mathematical theorem that you cannot map the whole Earth onto a map without distortions. A similar thing happens with time and a black hole. You simply cannot define one time for all observers! This is something which we do not realize in every day circumstances, but these effect are important for GPS systems.

What else will our colleague notice when he falls into the black hole? First, he floats weightlessly inside his space ship. Once he comes closer to the black hole he will slowly be pulled to the part of his space ship which is closer to the black hole. Once he reaches the wall he will feel a gravitational force just as he would on Earth. So far this is indeed not different from astronauts returning from the ISS to Earth. How strong gravity becomes at the event horizon depends on the mass of the black hole. Contra-intuitively: the larger the mass the less its surface gravity. Thus a human being could well reach and pass the event horizon of the black hole in the center of our milky way (if it were not for the hot gas...).

Now, however, our colleague has a problem. If he decides to come back to you he can no longer do that. No matter how hard he fires his thrusters, he can no longer pass the horizon again. Even the SOS signal he sends to you will not come out. Gravity is just too strong behind the horizon. Everything inevitably falls into the center of the black hole. AT THE CENTER of the black hole your knowledge breaks down. Einstein's equation just yield infinities. Many physicists believe that a new, yet unknown, theory takes over before that.

To summarize: black holes are solutions to Einstein's equations. They have an event horizon, from which nothing can escape. If the mass of a black hole is large enough, a human could reach the event horizon safely and within a finite amount of time. But once he crosses it, he can no longer turn back. Only at the center of a black hole do our equations break down.

[aristarchusinexile]

There is nothing in classical thermodynamics that prevents an increase in order. Life would not exist if that were not possible! But entropy in an isolated system never decreases. That's the Second Law, and it sure as heck hasn't been violated experimentally. And there are similar examples to be found in many places.

Read more.

If you can't put that in a mathematical framework, so it can be evaluated against some theory, there's not much that can be said.

Tell that to Michael Faraday.

[Markus Schwarz]

True, at least to my knowledge, Faraday did not have much of a mathematical education. But he published a lot of experimental results that other people could reproduce. And based on his findings other people finally found the mathematical framework that Faraday could not. So, if you know of some experiment which violates the Second Law of Thermodynamics in a closed system then give us the reference! Otherwise a further discussion on that topic seems pointless to me.

[The Code]

Markus brilliant read thanks.

If our ''space time'' universe, can stretch for 47 billion light years. And it is possible to go forward in time via speed of light..Is it also possible that time can go the other way (back in time past the event horizon) and that the moment after t=0 all time was created in one instant? thanks

mark

[bystander]

If our ''space time'' universe, can stretch for 47 billion light years. And it is possible to go forward in time via speed of light..Is it also possible that time can go the other way (back in time past the event horizon) and that the moment after t=0 all time was created in one instant? thanks

For the most part, current theory allows time to flow in one direction only (arrow of time). There was no time before t=0. Time came into existance with the rest of the physical universe.

[aristarchusinexile]

True, at least to my knowledge, Faraday did not have much of a mathematical education. But he published a lot of experimental results that other people could reproduce. And based on his findings other people finally found the mathematical framework that Faraday could not. So, if you know of some experiment which violates the Second Law of Thermodynamics in a closed system then give us the reference! Otherwise a further discussion on that topic seems pointless to me.

Discussion on any topic can point to important knowledge, for instance, that even though Faraday couldn't do the calculus his peers could (and for that he was derided) his name became the most important in his era of science, and one of the most important in ours, perhaps most important because of his discovery of Fields. Math is of no doubt important; but for someone to value math too highly shows a narrow range of opportunity for that person. (!) I would discuss thermodynamics more, but I don't make notes of most of my reading.

[aristarchusinexile]

For the most part, current theory allows time to flow in one direction only (arrow of time). There was no time before t=0. Time came into existance with the rest of the physical universe.

I'm glad you said "for the most part..." Bystander, because some PHDs say we absolutely can travel back in time (Turock, I believe, is one of those) and that time does have a reverse flow (Moffat). Also, I find the phrase "the time before the Big Bnag" repeated fairly often in my reading of PHD written material.

[Chris Peterson]

I'm glad you said "for the most part..." Bystander, because some PHDs say we absolutely can travel back in time

Nobody who understands anything at all about science would make such a claim. That's because nobody who understands anything about science would use "absolutely" to describe how possible something might be.

There are weak theories (that is, theories that don't produce testable predictions, or which are extremely difficult to test) that allow for the possibility of "time travel". That's all.

[Chris Peterson]

There is nothing in classical thermodynamics that prevents an increase in order. Life would not exist if that were not possible! But entropy in an isolated system never decreases. That's the Second Law, and it sure as heck hasn't been violated experimentally. And there are similar examples to be found in many places.

Read more.

Fine. Tell me where to read about the Second Law of Thermodynamics being violated.

If you can't put that in a mathematical framework, so it can be evaluated against some theory, there's not much that can be said.

Tell that to Michael Faraday.

Faraday is a great example of how science progresses. His work was quite rapidly put into a testable, rigorous, mathematical framework, which is why it was (and is) accepted.

[aristarchusinexile]

Faraday is a great example of how science progresses. His work was quite rapidly put into a testable, rigorous, mathematical framework, which is why it was (and is) accepted.

Faraday's work was totally accepted before it was put into math because he demonstrated it clearly without the math. If the math had not supported the demonstrations, would the demonstration be wrong? At least one biographer has written that if Faraday had been able to do calculus he would never have been able to think imaginatively enough to conceive of what he saw clearly.

[Chris Peterson]

Faraday's work was totally accepted before it was put into math because he demonstrated it clearly without the math.

No, the observations were accepted as being of good quality. It didn't become usable theory until the observations could be described mathematically, and that math could be used to predict other observations.

If the math had not supported the demonstrations, would the demonstration be wrong?

No, of course not. It would only mean that the theory was wrong, and people would have kept looking until they found a theory that worked. That happens all the time. Somebody makes an observation, they or somebody else attempts to define the underlying rule, and that is tested. If it works, the rule gains support; if it doesn't, the rule loses support, or is completely rejected. That, in a nutshell, is how modern science works.

At least one biographer has written that if Faraday had been able to do calculus he would never have been able to think imaginatively enough to conceive of what he saw clearly.

That strikes me as a pretty silly observation. Math is just a tool- it has nothing to do one way or the other with how imaginatively somebody can think. If he had been able to do more of his own analysis, it's likely the work would have entered the mainstream faster.

[The Code]

http://en.wikipedia.org/wiki/Poincar%C3 ... ce_theorem

I do not think of time like most. if a black hole event horizon can stop physics dead. The BB was how many times more power full? millions? billions? Black holes do explode but they do not create another universe? If a black hole distorts time which they believe, what did the big bang do to time? Created it? All of it, past and future in one bang. All time is one room that can be accessed by all powerful?. This is my view and does not represent main stream physics theories.

Mark

[aristarchusinexile]

No, the observations were accepted as being of good quality. It didn't become usable theory until the observations could be described mathematically, and that math could be used to predict other observations.

Theory is not needed for a Dad to teach his Son how to hit a nail with a hammer .. demonstration is needed.

No, of course not. It would only mean that the theory was wrong, and people would have kept looking until they found a theory that worked. That happens all the time. Somebody makes an observation, they or somebody else attempts to define the underlying rule, and that is tested. If it works, the rule gains support; if it doesn't, the rule loses support, or is completely rejected. That, in a nutshell, is how modern science works.

Science is not all theory. There are items known as 'facts' and items known as 'demonstrations'. Theory is wonderful from a theoretical point of view .. I have great respect for mine, even if some scientists don't classify them as theories.

At least one biographer has written that if Faraday had been able to do calculus he would never have been able to think imaginatively enough to conceive of what he saw clearly.

That strikes me as a pretty silly observation. Math is just a tool- it has nothing to do one way or the other with how imaginatively somebody can think. If he had been able to do more of his own analysis, it's likely the work would have entered the mainstream faster.[/quote]

Math is great stuff; but how come Faraday's peers with their excellent educations in math couldn't conceive of what he conceived, and derided him for his conceptions? Because their heads were locked on a blackboard instead of roaming space and time. 1 + 1 = 2 doesn't leave room for imagination. Einstein also was said to be poor in math, working outside the normal parametres of scientific thought. Entering the mainstream of science, Chris, means getting around the high, hardened wall of the pride of consensus .. a formidable task even for the mathematically gifted.

[Chris Peterson]

Theory is not needed for a Dad to teach his Son how to hit a nail with a hammer .. demonstration is needed.

Hitting a nail with a hammer is a procedure, it isn't an observation from nature that requires explanation. Your example has nothing at all to do with science.

Science is not all theory. There are items known as 'facts' and items known as 'demonstrations'.

The entire purpose of science is to explain nature, and that is done through the development of theories. Without theory, you don't have science. There are no facts in science. There are observations, and there are theories that attempt to explain those observations. Demonstrations (or experiments) are simply tools to connect the theories and observations.

Math is great stuff; but how come Faraday's peers with their excellent educations in math couldn't conceive of what he conceived, and derided him for his conceptions?

Because until his ideas were converted to a symbolic language so that everybody could work with them, there was no way to evaluate them. Once his observations were converted to formal theory, which could be tested, acceptance followed quickly.

Faraday was a smart guy, and his insight is what led to his discoveries. Math doesn't automatically give you that kind of insight, but neither does it limit insight. As I said before, it's just a tool. IMO if Faraday had been more mathematically capable, his progress would have been faster, and his ideas would have gained acceptance sooner.