World Science: Signs of dark matter found?

freyar said:

(I believe x-rays for the solar-mass type ones, radio waves for the one at the center of the galaxy --- because that one is so much more massive, light loses more energy escaping the gravitational pull).

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If I recall correctly, quite the opposite. The larger the black hole, the less pull you feel at the event horizon - for really big ones, you can cross the event horizon without knowing it, as it is almost flat outside the hole. That means stuff falling is is getting heated less, so it only emits in radio waves.

You're absolutely right, that's what I get for posting when I'm thinking about something else. The tidal forces that make things heat up are much less for the bigger ones...

Uhm... did I read right that black holes do emit particles because of temperature? Wouldn't they need to be faster than light to escape its gravitation? (i did read something a few years ago about researchers measuring radio waves randomly faster than c)

Atanatotatos said:

Uhm... did I read right that black holes do emit particles because of temperature? Wouldn't they need to be faster than light to escape its gravitation? (i did read something a few years ago about researchers measuring radio waves randomly faster than c)

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Well there is the Hawking Radiation theory, which says that BH's actually diminish as time goes on by radiating heat or somesuch. I'm not quite educated in this matter. Maybe some of our greater minds corrects me. However this theory is still being disputed and if I remember correctly even Hawking himself, who originally made the theory, has by now rebutted it.

For more details on Hawking radiation see wikipedia

Atanatotatos said:

Uhm... did I read right that black holes do emit particles because of temperature? Wouldn't they need to be faster than light to escape its gravitation? (i did read something a few years ago about researchers measuring radio waves randomly faster than c)

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Basically, the radiation doesn't come from the black hole, but is created at its event horizon.

They are a result of the uncertainity priniciple, basically. Just as we can't determine position and speed of a particle both at the same time with 100 % precision (because the act of measurement changes them), we also can't determine the number of particles and the rate of change in the number of particles at the same time. So even in vacuum, there can't be no particles and no change of the number of particles at the same time. This leads to the theory that there are actually "virtual" pairs of elemental particle (one regular, one anti-particle) that annihalte each other in a very short time frame - but nearby black holes, one partner in this pairing can fall into the event horizon while the other barely escapes, so the particle escapes as a radiation. The energy inherent to these two particles can't come from nothing, so the black hole must be losing it.
The result is a radiation (that gets stronger the smaller the black hole is, interestingly) that makes the black hole smaller. Black Holes as they result from novas or in the center of galaxies are so large that they gain more energy from the cosmic background radiation then from this radiation, so the effect is neglible.

Blackrat said:

Well there is the Hawking Radiation theory, which says that BH's actually diminish as time goes on by radiating heat or somesuch. I'm not quite educated in this matter. Maybe some of our greater minds corrects me. However this theory is still being disputed and if I remember correctly even Hawking himself, who originally made the theory, has by now rebutted it.

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I don't know about any rebuttals. I always thought the problem was that the black holes we have identified as such would just not emit any noticeable amounts of radiation.

This is very interesting. Is there a theory (or... observations of sorts) on the interaction of anti-matter with black holes?
Also... while there is a "limit" theorized for speed (that is, c), has such a limit ever been applied to a force. With all this talking of black holes, the thought came to me, is it theoretically possible for a black hole to attain "infinite" gravitational force? And if this is the case, would it be sufficient for the "Big Crunch" (if i remember the name correctly) to take effect.
I don't know if I've expressed correctly and I hope my questions aren't "infinitely" dumb I'm facing two language barriers here... ^_^

I'm facing two language barriers here... ^_^

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Atanatotatos said:

This is very interesting. Is there a theory (or... observations of sorts) on the interaction of anti-matter with black holes?

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Eaten up just like regular matter. If it happens to collide with matter inside, matter and anti-matter would annihalate each other as usual, but that doesn't change anything for the outside observer. Keep in mind: While it is called anti-matter, it doesn't have "anti-mass" or "anti-energy" (or negative mass or negative energy). While the theories about the expansion of the universe speak of effects that work against the attractive gravitational force, this is not the role or a property of anti-matter.

With all this talking of black holes, the thought came to me, is it theoretically possible for a black hole to attain "infinite" gravitational force? And if this is the case, would it be sufficient for the "Big Crunch" (if i remember the name correctly) to take effect.
I don't know if I've expressed correctly and I hope my questions aren't "infinitely" dumb

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The more interesting question is if there is no "Big Crunch" nor a "Big Rip" (the big rip is the opposite extreme of the big crunch - instead of the mass in the universe causing its expanision to slow down and reverse, other effects - basically the aforementioned counterforce to gravity - would grow stronger and increase the speed of the expansion until even atoms and quarks are split apart), what will happen to black holes. If the universe exists infinitely without blowing up or crushing down, the end would only see a lot of black holes. If the universe grows big enough, the cosmic radiation would be too low to sustain black holes, and they would "bleed out" via the hawking radiation, leaving a homogenous level of radiation in an otherwise empty universe...

What happens at the "extremes" of a black hole is probably one of the biggest mysteries. To get answers here, we would need the unified theory of relativity and quantum physics, because both matters are concerned.

But what would you mean by "infinite" gravitational force? The only time this can happen is if you actually have an infinitely sized mass.

Also... while there is a "limit" theorized for speed (that is, c), has such a limit ever been applied to a force.

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Light itself is considered a force - the electromagnetic force. So the speed limit also applies to gravity as a force. That means yes, if someone would pop a infinite mass 2 light years away from ous, we would feel its infinite force in 2 years (but then we probably wouldn't notice much, just existing in one moment and then stopping existence in the next. )

This was a concept I took some time to grasp. Even describing gravity as "bumps" or warping in the fourdimensional spacetime doesn't change this, apparently. (And in fact, the unifying models like string theory describe forces the same way, just using more dimensions then 4 of spacetime)

Atanatotatos said:

This is very interesting. Is there a theory (or... observations of sorts) on the interaction of anti-matter with black holes?

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Black holes eat everything. They don't care about the differences between types of particles. It's all food

Also... while there is a "limit" theorized for speed (that is, c), has such a limit ever been applied to a force.

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Yes. Every known force in the universe can be seen as being transmitted by a particle (photons for electromagnetism, gravitons for gravity, and so on). As particles, they are limited in speed.

With all this talking of black holes, the thought came to me, is it theoretically possible for a black hole to attain "infinite" gravitational force?

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Um, yes and no.

The force of gravity one feels from a given object is dependent on two things - the mass of the object, and the distance from the object. Specifically, the force you feel goes like 1/d^2. So, if the object is really small, you can get really close to it, and the force you feel gets really, really big.

This is what happens to black holes. When the star runs out of fuel, the pressure of the outflowing energy drops off, and stops holding the mass of a star up, so it begins to collapse. If the star has enough mass, there is no force in the universe that'll stop that collapse. Ever. It collapses down to to the point where at the surface of the star, the force of gravity is so strong that even light cannot escape from the surface. The force isn't infinite, just very strong.

But the collapse doesn't stop there. Nothing stops it. It just keeps going. We can't see it, but it just keeps on collapsing down to a mathematical point - at that point, technically the force of gravity on any object would be infinite.

And if this is the case, would it be sufficient for the "Big Crunch" (if i remember the name correctly) to take effect.

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No. But that requires some explanation.

As far as we can see, the Universe was born with some total amount of energy. No new energy has ever been created - it just swaps back and forth between being energy and various forms of matter. Whether or not the Universe will experience a "Big Crunch" depends upon that total, not on local conditions.

Black holes are just one place into which some of the mass/energy falls. It doesn't create "more gravity" by doing so. At large distances, gravitationally a black hole and a star of the same mass look the same. It is only when you get close to the hole that things get weird. So, the universe as a whole doesn't care if you have a black hole or a bunch of stars.

I see, I see. Actually I know some of that stuff 'cause I studied it, albeit superficially, but it's the details that usually escape curious non-specialist, I guess.
Like, I never figured out why hypotheses have been made that "on the other side" of a black hole there might be an opposite reflex (something like a "white hole", I think I've read).
I mean, science-fictional parallel dimension out of holes in the time-space, ok, but, scientifically speaking, if all we know is that a black hole is an object of enourmous mass esercising enormous attraction, why should it even have "another side"?
Or maybe I just misinterpreted something, or even just read fluff out of bad articles?

Mustrum's explanations of Hawking radiation and speed limits are pretty good!

Blackrat said:

Well there is the Hawking Radiation theory, which says that BH's actually diminish as time goes on by radiating heat or somesuch. I'm not quite educated in this matter. Maybe some of our greater minds corrects me. However this theory is still being disputed and if I remember correctly even Hawking himself, who originally made the theory, has by now rebutted it.

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Mustrum_Ridcully said:

I don't know about any rebuttals. I always thought the problem was that the black holes we have identified as such would just not emit any noticeable amounts of radiation.

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This is the issue with Hawking radiation: As it was first proposed, Hawking radiation was exactly thermal. The thing is, that contradicts quantum mechanics since it represents a loss of information. If the radiation coming out is thermal, it tells you nothing about what went into the black hole. (On the other hand, if you could analyze all the light, smoke, etc, coming out of a fire, you could tell everything about what was burning, down to the relative positions of atoms, etc.) So this meant either (1) the calculation of Hawking radiation was missing something or (2) quantum mechanics was wrong.

For a long time, gravity theorists (who started out studying general relativity) believed #2, while particle physicists (who approached quantum gravity the same was as electromagnetism and other forces) believed #1. Hawking himself believed #2. Over the years, more and more evidence has accumulated, including from string theory, that option #1 was correct -- Hawking's original calculation wasn't quite complete (in a very subtle way), so quantum mechanics is right. About 3 or 4 years ago, Hawking completed a new calculation and convinced himself also of option #1 (he was among the last few major scientists who thought quantum mechanics was wrong). This actually made the news, since he'd made a bet about it. This was kind of amusing for me, since I was working down the hall from the winner of the bet (a professor at CalTech) at the time.