How can black holes have infinite density?

Discussion in 'Strategic Forces' started by ajay_ijn, Oct 4, 2009.

  1. ajay_ijn

    ajay_ijn Regular Member

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    ppl talk about singularity, infinite density, gravition so strong that even light cannot escape, spacetime twisted so much that some laws of physics don't apply there.

    to an average person who thinks laws of physics apply everywhere, this is very very confusing. one cannot comprehend black holes so easily.

    What do scientists mean by nothing can return after event horizon? where does that mass go once its crosses event horizon?, it reaches singularity but what after that?

    there are too many questions and not many answers.

    and yet black holes are everywhere, every galaxy probably has supermassive black holes at centre running the whole show.

    i keep reading black holes gobbles up stars, but where does the mass go once its reaches inside black hole is all a mystery to us till date.
     
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  3. LETHALFORCE

    LETHALFORCE Moderator Moderator

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    Supermassive Black Holes Bringing Universe Closer to Death | Wired Science | Wired.com

    Supermassive Black Holes Bringing Universe Closer to Death

    For all its tumult — erupting stars, colliding galaxies, collapsing black holes — the cosmos is a surprisingly orderly place. Theoretical calculations have long shown that the entropy of the universe — a measure of its disorder — is but a tiny fraction of the maximum allowable amount.


    A new calculation of entropy upholds that general result but suggests that the universe is messier than scientists had thought — and slightly further along on its gradual journey to death, two Australian cosmologists conclude.

    An analysis by Chas Egan of the Australian National University in Canberra and Charles Lineweaver of the University of New South Wales in Sydney indicates that the collective entropy of all the supermassive black holes at the centers of galaxies is about 100 times higher than previously calculated. Because supermassive black holes are the largest contributor to cosmic entropy, the finding suggests that the entropy of the universe is also about 100 times larger than previous estimates, the researchers reported online September 23 at arXiv.org.

    Entropy quantifies the number of different microscopic states that a physical system can have while looking the same on a large scale. For instance, an omelet has higher entropy than an egg because there are more ways for the molecules of an omelet to rearrange themselves and still remain an omelet than for an egg, notes cosmologist Sean Carroll of the California Institute of Technology in Pasadena.


    A black hole is the entropy champ because there are myriad ways for all the material that has fallen into it to be arranged microscopically while the black hole retains the same numerical values for its observable properties — charge, mass and spin.

    Researchers who previously calculated the cosmic sum of black hole entropy had assumed that, on average, each galaxy houses a 10 million solar-mass black hole at its center. Under this assumption, researchers had determined that supermassive black holes contribute an entropy of about 10102, in units derived from a quantity known as Boltzmann’s constant.

    In contrast, Egan and Lineweaver relied on new data that included a fuller range of the masses of supermassive black holes rather than just using the average mass. “The upshot was that much more entropy is contributed by a smaller population of much larger, 1-billion-solar-mass black holes,” Egan says.

    Carroll says that the team’s calculation looks sensible. “I see no reason to doubt their numbers,” he says.

    Having a more reliable entropy estimate is important, says Egan, because for life or other complex phenomena to exist, the entropy of the universe must be less than the maximum possible value. Consider, he notes, when hot water is poured into a cold bath. Initially the hot and cold water are separate and the system is orderly — it has low entropy. But once the hot and cold water are thoroughly mixed, the entropy is maximized and no further heat flow is possible.

    In the case of the universe, Egan says, “we’d like to know [when and] if the entropy will eventually reach a maximum value, marking the end of all dissipative processes, including life.” Physicists have dubbed that maximum entropy “heat death.”

    Egan and Lineweaver’s new value for the entropy of the universe is still a billionth of a billionth the maximum possible entropy that researchers have estimated. Nonetheless, the new value “indicates that that the universe is a bit closer to the heat death than previously computed,” comments theorist Paul Davies of Arizona State University in Tempe.

    Not everyone agrees that the higher entropy contributed by supermassive black holes puts the universe closer to heat death. Theorist Ned Wright of the University of California, Los Angeles says that because the extra entropy is locked inside the black holes, the rest of the universe should have lower entropy and be further away from heat death.

    The new entropy calculation also highlights a cosmic puzzle, Carroll says. The entropy was relatively small in the early universe (1088), bigger now (10104), but still falls far short of the maximum (10122). No known physical principle can explain why the cosmic entropy is so low. But it’s a good thing because the low value “is responsible for everything we experience about the [unidirectional] flow of time — breaking eggs, growing older and dying, remembering the past but not the future,” notes Carroll. “The universe is incredibly more orderly than it has any right to be. Egan and Lineweaver have shown that it’s just a bit more disorderly than we thought.”

    Image: A supermassive black hole “blows” gas bubbles in the bottom left. NASA.

    Tags: black holes, entropy
     
  4. Martian

    Martian Respected Member Senior Member

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    Shouldn't we worry more about the death of the Sun as it becomes a red giant in a few billion years? I think the heat death of the universe is unimaginably far into the future on the order of hundreds of billions or trillions of years.

    Regarding black holes and infinite density, I was taught that if your equation gives you a nonsense result like infinity then it probably means that the model behind your equation breaks down.
     
  5. ajay_ijn

    ajay_ijn Regular Member

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    talking about death of universe, they say that dark energy would influence the whole universe more than any other kind of force and that universe would be simply ripped apart as matter farther away at increasing velocities.
     
  6. LETHALFORCE

    LETHALFORCE Moderator Moderator

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    Authors like Robert Zubrin speculates if a civilization is advanced enough they can reingnite nuclear fusion in dying suns and maybe even in brown dwarfs and even more advanced civilizations can possible even create suns?? We are nowhere near that hopefully humans will have advanced to a point to do something about our sun becoming a red drwaf nothing so dramatic, but possibly building ships or moving on into other solar systems.
     
  7. LETHALFORCE

    LETHALFORCE Moderator Moderator

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    The End of the World - Supernovae, Asteroids, Gamma Rays and Magnetars | The Planets of the Solar System | SpaceTravel.org

    The End of the World - Supernovae, Asteroids, Gamma Rays and Magnetars
    Thursday, 29 March 2007 12:07


    There are about a hundred billion stars in our galaxy. At any one time thousands of these are dying. Mostly there deaths are unspectacular and fairly non-eventful. However, when one of the largest stars runs out of fuel and explodes this is definitely spectacular. Supernovae may only occur about once in every 30 years but present a definite threat to our solar system as it moves through the Milky Way.



    The supernova occurs in less than a second as the star collapses in on itself, matter is crushed so densely that quantum forces create a massive explosion. Radioactive matter, beta particles, X-rays and gamma rays spew out and would be a substantial threat to life on Earth from up to 200 light years away. Of course, we can't predict if we'll encounter a supernova as we journey around the galaxy. However, as our Solar System orbits faster than the spiral arms of the galaxy we have to pass through them as we orbit. It is for the few million years or so that we pass through an arm that we are at more risk because of the higher density of stars.

    Recent studies have shown that cold periods in Earth's climate have coincided with high levels of cosmic rays, these rays could be coming from nearby supernovae.

    The variations in the flux of cosmic rays were measured by studying the ages of meteorite surfaces. Cosmic ray levels were then compared with climate records from ice cores and tree rings. It was found that warmer periods matched with periods of low cosmic ray activity and colder periods are linked to high cosmic ray activity. Whenever the Earth travels through a spiral arm of the Milky Way, roughly every 150 million years, there is a corresponding cold period with more Ice Ages.

    Exactly how cosmic rays may cause climate change is not known, but it seems most likely that their ionising effect on our atmosphere may lead to increased cloud formation. More clouds mean more sunlight reflected back into space and therefore a colder climate.
    According to the spiral-arm theorists, Earth may be heading for a cold snap as we pass through the Orion arm, possibly a short ice age. However, after a few million years we should see a greenhouse period of around 50 to 60 million years, before we pass through the Perseus arm, when we may see a return to dramatic ice-house conditions.

    Giant Molecular Clouds

    As if Supernovae aren't bad enough, also waiting for little Earth in the spiral arms of the Milky Way are huge, thick clouds of hydrogen gas. It is believed that passing through such a cloud could lead to total glaciation of the planet.

    We are normally protected from cosmic dust by the solar wind. But the densest clouds would choke the solar wind and flood the atmosphere with dust, blocking out the sunlight and causing rapid cooling of the planet. It might take 200,000 years for Earth to travel through the cloud. An estimated 1 in 30 of these clouds are dense enough to overcome the protective solar wind, and it is predicted that the Earth encounters one roughly once in a billion years.
    While the Earth has completely frozen over twice in its lifetime of 4.6 billion years, no one has come up with a convincing explanation for these freeze-overs around 600 million and 750 million years ago. But some scientists believe giant molecular clouds may be the explanation - a dense cloud would be sudden, intense and lasting.

    Unfortunately it doesn't end there, even a moderately dense cloud could pose a threat. Fast moving hydrogen nuclei within the cloud may act as cosmic rays. We are normally protected from cosmic rays by the Earth's magnetic field. However the magnetic field reverses every 200,000 years during which time it is much weaker. So if a cloud collision occurred at the same time as a reversal the hydrogen nuclei ‘cosmic rays' would flood our upper atmosphere and break down ozone. As a cloud collision could last around a million years it is likely that there would also be a magnetic field reversal during the collision. This combination of cloud collision and reversal would cause a predicted drop in ozone levels by 40 per cent - enough to cause mass extinction. The statistics show we probably hit about eight moderately dense clouds every 250 million years.


    Comets and Asteroids

    The Oort cloud is made up of icy lumps orbiting the Sun far out beyond Pluto. When the gravitational balance of the Oort cloud is disturbed some of these lumps may fall as comets towards the inner solar system, where they may hit Earth.

    There are many small rocky asteroids orbiting closer to the Sun. If their orbits coincide with Earth's they can land with devastating impact. A crater on the Yucatan Peninsula in Mexico is dated at 65 million years ago, matching the extinction of the dinosaurs. If this impact occurred today it would without doubt destroy our civilisation.

    An impact the size of the Yucatan Peninsula occurs once in 100 million years. It also hit the right rocks, carbonate and sulphate to do so much damage. Fortunately these rocks only account for 2 per cent of the Earth. Another major impact crater is the 100km wide Popigai in Siberia, dated at 35 million years ago. There was no major extinction after Popigai because it landed on less volatile rocks.

    As we move around the galaxy we move up and down in the plane of the galaxy, in a period of about 30 million years. This causes disturbance for the Oort cloud causing more comets and more risk of impact for Earth.

    Travelling through a giant dust cloud may also cause a redistribution of comets. Although asteroids present the most likely threat as current statistics show they are responsible for 99 per cent of impacts on Earth whereas comets account for the remaining 1 per cent.


    Gamma-Ray Bursts

    So it's a risky business travelling through the spiral arms of the Milky Way, but there are also risks in the space between.
    Since the late 1960s short, intense bursts of gamma-rays have been detected from outer space. The cause of these gamma ray emissions is still not known. But astronomers hypothesise that they may be due to an especially massive supernova. The gamma-ray burst is 100 times brighter than an ordinary supernova with tightly focused jets of gamma-rays and cosmic rays pouring out of the collapsing star. The bursts are also much shorter, lasting less than a minute.

    Gamma-ray bursts can be observed right up to the edge of the known universe and astronomers spot around one a day, compared to one supernova every few decades in our galaxy. Ordinary supernovae are far more common in the universe as a whole but gamma-ray bursts may present a greater threat to life on Earth. It is believed that if a burst occurred within 6000 light years of our planet it would destroy 35 per cent of the ozone layer and life would fry under the high levels of ultraviolet B.

    The gamma-rays would break nitrogen bonds in the upper atmosphere and break down the ozone, a burst of only a few seconds would cause devastation for many years. Ozone depletion would persist for over five years and devastate life on Earth. Ultraviolet B is damaging to DNA. More bad news for Earth and its inhabitants would be the cosmic rays from the burst that could cause severe radiation damage to the Earth's surface.

    There is no direct evidence to suggest that Earth has been hit by a gamma-ray burst, but it is believed that the mass extinction 443 million years ago may have been caused by gamma-rays. Over half the species of animals and plants were lost. The fact that fauna afterwards descended mainly from deep-water organisms points to UVB as a likely candidate for the extinction. However, gamma-ray bursts can't account for all mass extinction events on Earth because close range bursts just aren't frequent enough.
    A gamma-ray burst close enough to damage the Earth may only be likely to occur around once every billion years.


    Magnetars

    Magnetars were discovered 10 years ago and are thought to be remnants of supernovae. They appear similar to a neutron star with an incredibly strong magnetic field - 10 gigatesla, that's about 100 trillion times stronger than Earth's - stronger than anything else in the known universe. Magnetars are observed to give out a flash of gamma and x-rays, lasting about half a second. But these flashes can give out as much energy as our Sun does in a whole year.
    A magnetar pulse within 10 light years of Earth would significantly deplete the ozone layer, leading to the inevitable on the planet's surface.

    Magnetars are hard to find and difficult to study, so it is hard to calculate any risk to Earth. But they do occur more frequently than gamma-ray bursts and are more likely to occur close to the Earth. So even though magnetars are less energetic than a gamma-ray burst they may be a greater threat to our planet. So far two magnetars have been identified within our galaxy. Scientists can only speculate on the risk to our planet, and there is no certainty as to how often giant magnetar flares occur.
     
  8. prahladh

    prahladh Respected Member

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    There is no hole to sink matter, but matter (an entire star/stars) is compressed into a marble. It's black because light doesn't reflect back and hence nothing can be seen. Thats my take on this.
     
  9. AkhandBharat

    AkhandBharat Regular Member

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    There is a theoritical discussion on the existence of antimatter, but it has been said that the known universe is composed of almost entirely matter. Maybe the black hole is a portal that leads to matter-antimatter annihilation leaving nothing behind.
     

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