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Scientists May Get Best View Yet of a Black Hole in Action

wired.com

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Re: Scientists May Get Best View Yet of a Black Hole in Action

#21

Post by pdonis »

higherpurpose wrote: post #14
pdonis wrote: post #11

One small nitpick about a statement in the article; it says: Though we think of them as cosmic vacuum cleaners, black holes are actually just like any other massive body, such as a star. This means other objects can safely orbit them, until they get within a particular distance and pass what’s known as the event horizon, after which there is no escaping being sucked in. Technically, you can't "safely orbit" a black h…

Technically there may be no "safe distance" at all, depending on how much time you're talking about. The whole galaxy seems to be caught in the spiral of a super massive blackhole, and it's just a matter of time before everything falls into it, although most stars will probably die out before they do. So I guess you could consider that "safe", since the time it takes to fall into the blackhole is longer than the time…

The whole galaxy seems to be caught in the spiral of a super massive blackhole, and it's just a matter of time before everything falls into it

That's not necessarily true. From far away, a black hole is just like any other object with the same mass; there's no extra ingredient to a black hole's gravity that makes things more likely to fall into it from far away.

Re: Scientists May Get Best View Yet of a Black Hole in Action

#22

Post by millstone »

higherpurpose wrote: post #14
pdonis wrote: post #11

One small nitpick about a statement in the article; it says: Though we think of them as cosmic vacuum cleaners, black holes are actually just like any other massive body, such as a star. This means other objects can safely orbit them, until they get within a particular distance and pass what’s known as the event horizon, after which there is no escaping being sucked in. Technically, you can't "safely orbit" a black h…

Technically there may be no "safe distance" at all, depending on how much time you're talking about. The whole galaxy seems to be caught in the spiral of a super massive blackhole, and it's just a matter of time before everything falls into it, although most stars will probably die out before they do. So I guess you could consider that "safe", since the time it takes to fall into the blackhole is longer than the time…

The central supermassive black hole does consume the galaxy, but in fact the rate of consumption is so excruciatingly slow that the galaxy will have "evaporated" long before then. Random processes like collisions with other galaxies will eventually eject nearly all stellar remnants into intergalactic space. The central black hole will only end up consuming about 1% of the galaxy's stellar remnants (as you say, all stars will have died trillions of years before the galaxy evaporates).

The Milky Way is fated for just such a collision with Andromeda in a few billion years, and there's a chance our solar system will be ejected!

Re: Scientists May Get Best View Yet of a Black Hole in Action

#23

Post by Ygg2 »

IanCal wrote: post #4
sillysaurus3 wrote: post #3

A question for the physicists on HN: I've heard that black holes emit radiation. But if nothing can go faster than the speed of light, and a black hole's gravity is so strong that not even light can escape, then how can a black hole emit anything? Is there a kind of black hole which is so massive that not even that radiation can escape, or do all black holes emit some kind of radiation? (In fact, do they emit radiati…

Armchair Physicist here, not a real one. There are two reasons why you'd say black holes emit radiation. An interesting one and a very interesting one (warning, other peoples scales may be calibrated differently to mine). 1) Black holes accelerate things massively , so they're travelling at an astonishing speed before they "enter". This can result in huge amounts of x-rays being emitted due to heating things to milli…

Also some particles might travel faster than speed of light by tunneling or similar quantum mechanisms.

Re: Scientists May Get Best View Yet of a Black Hole in Action

#24

Post by Ygg2 »

shire wrote: post #18

this might sound like a stupid question but what happens when you get sucked up by a black hole? what's in the other side of it.

To be completely honest - There is no straight answer. One modern answer was that particles falling into a black hole would appear to be destroyed for outside observer (in a vortex of fire) while the observer would be scrambled into a holographic representation (keep in mind ashes would be a highly energetic representation of same observer).

See

http://en.wikipedia.org/wiki/Firewall_%28physics%29

https://www.youtube.com/watch?v=2DIl3Hfh9tY

Re: Scientists May Get Best View Yet of a Black Hole in Action

#26

Post by millstone »

fargolime wrote: post #8

If only black holes weren't illogical! [1] Any decent software developer (i.e. highly logical thinker) who knows basic info about black holes can look at the picture there to quickly see that black holes are inconsistent with the core postulate of general relativity, the equivalence principle. They're a bug! (which Einstein spent a decade trying to find, or some other way that nature could prevent black holes he thou…

The simplest way to show the essential physical error is via this picture, from the article: http://finbot.files.wordpress.com/2008/03/t9.png

This picture shows the event horizon as having constant position in a free-falling frame. But the event horizon accelerates in all nearby inertial frames, exactly like how the surface of the earth accelerates (towards you) in every inertial frame near the earth. So the line representing the event horizon cannot be straight: it must curve. It must be hyperbolic. That is the key feature that the article gets wrong.

> Then law J shows that a locally inertial frame relative to which the particle is at rest can’t even in principle extend below the horizon

The horizon forms a hyperbola in any such inertial frame. Because the horizon accelerates, whether a spatial point is below the horizon depends on time.

If you have a test particle moving with constant acceleration, can you reach it with a signal at some point after it leaves? In Newtonian physics you can: just send the signal fast enough. But in relativity, test particles moving with constant acceleration can be "uncatchable" even though they never reach c. See Rindler coordinates for more.

This illustrates why an outgoing test particle cannot cross the event horizon. Picture the event horizon as a hyperbola, and the test particle as an asymptote: it can never intersect it.

> a test of the laws of physics can distinguish X from an inertial frame in an idealized, gravity-free universe

Well duh. There are no inertial frames that cross the horizon. Technically any two separated points in our frame will be accelerating with respect to each other. We approximate this acceleration as zero, but this approximation can break down near the speed of light, as things tend to do.

Re: Scientists May Get Best View Yet of a Black Hole in Action

#27

Post by sigterm »

ChuckMcM wrote: post #19
shire wrote: post #18

this might sound like a stupid question but what happens when you get sucked up by a black hole? what's in the other side of it.

If you read Stephen Hawking's book "A Brief History of Time" you will see that a variant on this question is what started him on the path of reasoning about black holes in the first place. The question does not currently have a definitive answer. Although current mathematical analysis has in falling matter being dismantled at the sub-atomic level as it undergoes the tidal stresses associated with gravity. Basically i…

> Basically if you were standing at the event horizon the pull on your feet would be several billion times the pull on your head.

This is not always the case. If the black hole is massive enough there won't be strong tidal force at the event horizon to disintegrate objects.

I don't know GR enough to judge your other statements. However, when discussing the time slowing down close by a black hole I suspect one must take into account of the strong gravity field and not just the velocity of the falling object.

Re: Scientists May Get Best View Yet of a Black Hole in Action

#28

Post by fargolime »

pdonis wrote: post #20
fargolime wrote: post #12

Earlier quoted context omitted.

You've linked to that before, but there's nothing there that refutes the blog (not my argument). If there's a flaw it should be a sentence or two of logic, not multiple paragraphs that summarize as "it's not that simple". The blog shows that inertial frames falling through a horizon are that simple (after all, the equivalence principle demands that), and Taylor and Wheeler agree there. Extraordinary evidence is not a…

there's nothing there that refutes the blog I understand that you don't agree that what I posted refutes the blog, but I'm not trying to convince you, and I'm not going to rehash the argument here. I'm just linking to that discussion for the record, so others reading this thread will understand that your claims and the blog's claims about black holes are not undisputed. (not my argument). You may not have written the…

What sentence in that picture's caption is wrong, or what doesn't follow from its premises? Asking for extraordinary evidence isn't a scientific thing. Valid logic is all the evidence needed.

Yes it's my argument in that way. I'll challenge a refutation or accept it, but that's difficult when the counter argument is a vague wall of text. If I were to try to summarize what you think is wrong I couldn't do it. I can't decipher your points to see how they attempt to refute any particular sentence in the blog. Please be way more clear and short and to the point, and I'll agree, or disagree with my reasoning. (For example check out Millstone's reasoning above. He/she disagrees that an inertial frame can cross the horizon. One short sentence is enough to summarize!)

> the blog post's claims about what the theory of relativity actually says are not correct

Anyone can say that; be more specific. What the blog says in that way sources from experts in relativity. Its statement of the equivalence principle, for example, is Kip Thorne's.

> Agree with what?

They agree that frame X in the blog post is validly defined and validly used in the thought experiment. Their quote in the blog makes that clear.

Re: Scientists May Get Best View Yet of a Black Hole in Action

#29

Post by fargolime »

millstone wrote: post #26
fargolime wrote: post #8

If only black holes weren't illogical! [1] Any decent software developer (i.e. highly logical thinker) who knows basic info about black holes can look at the picture there to quickly see that black holes are inconsistent with the core postulate of general relativity, the equivalence principle. They're a bug! (which Einstein spent a decade trying to find, or some other way that nature could prevent black holes he thou…

The simplest way to show the essential physical error is via this picture, from the article: http://finbot.files.wordpress.com/2008/03/t9.png This picture shows the event horizon as having constant position in a free-falling frame. But the event horizon accelerates in all nearby inertial frames, exactly like how the surface of the earth accelerates (towards you) in every inertial frame near the earth. So the line rep…

> This picture shows the event horizon as having constant position in a free-falling frame. But the event horizon accelerates in all nearby inertial frames

The line represents the horizon at a single moment in the life of frame X, during which it has a specific position relative to X and is not accelerating relative to X.

> If you have a test particle moving with constant acceleration, ...

The test particles are defined as freely falling. The frame X is defined as inertial. No free particle can accelerate in an inertial frame (at least not measurably), including in relativity. Check out Taylor and Wheeler's quote at the bottom of the blog post: "Keys, coins, and coffee cups continue to move in straight lines with constant speed in such a local free-float frame." According to you, those things would not move at constant speed; rather they'd be "moving with constant acceleration".

> Well duh. There are no inertial frames that cross the horizon.

This again disagrees with Taylor and Wheeler's quote. They are clear that not only can an inertial frame cross a horizon, but also that frame is "a free-float frame like any other".

I didn't respond to everything you said because it hinges on the same disagreement. Before we could get further in the discussion you would need to accept Taylor and Wheeler there, or show how I'm misunderstanding the conflict between what you said and their quote that seems to explicitly disagree with you.

Re: Scientists May Get Best View Yet of a Black Hole in Action

#30

Post by jjoonathan »

IanCal wrote: post #4
sillysaurus3 wrote: post #3

A question for the physicists on HN: I've heard that black holes emit radiation. But if nothing can go faster than the speed of light, and a black hole's gravity is so strong that not even light can escape, then how can a black hole emit anything? Is there a kind of black hole which is so massive that not even that radiation can escape, or do all black holes emit some kind of radiation? (In fact, do they emit radiati…

Armchair Physicist here, not a real one. There are two reasons why you'd say black holes emit radiation. An interesting one and a very interesting one (warning, other peoples scales may be calibrated differently to mine). 1) Black holes accelerate things massively , so they're travelling at an astonishing speed before they "enter". This can result in huge amounts of x-rays being emitted due to heating things to milli…

> one of the particles to fall in and the other to escape. This results in a loss of mass of the black hole (told you it was weird)

How does the energy to create the virtual particles come from the black hole (which it has to in order for the accounting to work: -2+1=-1)? Is it a "Quantum Field Theory doesn't care about the event horizon" type thing?

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