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More massive the body is more is the gravity.Still often smaller black holes are considered to be very dangerous than the bigger ones,why?
All black holes are dangerous if you get close enough, but smaller ones are more dangerous because they cause stronger tidal effects, so they can cause spaghettification well before you reach the event horizon, whereas you can cross the event horizon of a supermassive black hole without the tidal effects becoming noticeable.
Very small black holes would also be dangerous due to the large amount of Hawking radiation they'd emit. E.g., a 1 million metric ton hole has a luminosity of over 356 terrawatts. However, there's no known mechanism to produce black holes that small, apart from waiting for a large hole to get small by evaporating via Hawking radiation, and that won't even start to happen for a very long time, since the cosmic microwave background is currently around a billion times hotter than even the smallest stellar black holes.
Actually, the main danger in the neighborhood of a black hole is if the hole is active, that is, if it is currently surrounded by matter that it's absorbing. That matter gets accelerated to very high speeds due to the gravitational acceleration and when it collides with other matter large amounts of dangerous radiation is released.
Three reasons why black holes are the scariest things in the UniverseThis simulation of a supermassive black hole shows how it distorts the starry background and captures light, producing black hole silhouettes. Credit: NASA’s Goddard Space Flight Center background, ESA/Gaia/DPAC
H alloween is a time to be haunted by ghosts, goblins and ghouls, but nothing in the Universe is scarier than a black hole.
Black holes – regions in space where gravity is so strong that nothing can escape – are a hot topic in the news these days. Half of the 2020 Nobel Prize in Physics was awarded to Roger Penrose for his mathematical work showing that black holes are an inescapable consequence of Einstein’s theory of gravity. Andrea Ghez and Reinhard Genzel shared the other half for showing that a massive black hole sits at the center of our galaxy.
Black holes are scary for three reasons. If you fell into a black hole left over when a star died, you would be shredded. Also, the massive black holes seen at the center of all galaxies have insatiable appetites. And black holes are places where the laws of physics are obliterated.
I’ve been studying black holes for over 30 years. In particular, I’ve focused on the supermassive black holes that lurk at the center of galaxies. Most of the time they are inactive, but when they are active and eat stars and gas, the region close to the black hole can outshine the entire galaxy that hosts them. Galaxies where the black holes are active are called quasars. With all we’ve learned about black holes over the past few decades, there are still many mysteries to solve.
5 Answers 5
The true answer lies in General Relativity, but we can make a simple Newtonian argument.
From the outside, a uniform sphere attracts test masses exactly as if all of its mass was concentrated in the center (part of the famous Shell theorem).
Gravitational attraction also increases the closer you are to the source of gravitation, but if you go inside the sphere, some of the mass of the sphere will form a shell surrounding you, hence you will experience no gravitational attraction from it, again because of the Shell theorem. This is because while the near side of the shell is pulling you towards it, so is the far side, and the forces cancel out, and the only gravitational forces remaining are from the smaller sphere in front of you.
Once you get near the center of the sphere, you will experience almost no gravitational pull at all, as pretty much all of the mass is pulling you radially away from the center.
This means that if you can get very close to the center of the sphere without going inside the sphere, you will experience much stronger gravitational attraction, as there is no exterior shell of mass to compensate the center of mass attraction. Hence, density plays a role: a relatively small mass concentrated in a very small radius will allow you to get incredibly close to the center and experience incredible gravitational forces, while if the same mass occupies a larger space, to get very close to the center you will have to get inside the mass, and some of the attraction will cancel out.
The conclusion is that a small mass can be a black hole if it is concentrated inside a small enough radius. The largest such radius is called the Schwarzschild radius. As a matter of fact our own Sun would be a black hole if it had a radius of less than $3$ km and the same mass, and the Earth would be a black hole if it had a radius of less than $9$ mm and the same mass.
Stars generate a great deal of energy through fusion at the core. Basically the more massive a star is, the more pressure the core is under (due to the star's own gravity) and the more energy it can generate (somewhat simplified).
That energy of course radiates outward and heats everything outside the core making it a something like a pressure cooker, with heat creating pressure and the outer regions of the star being kept in place by it's own gravity. Stars would collapse into more dense objects (like white dwarfs and neutron stars and black holes) if this outward heat driven pressure did not exist.
Black holes are created when the fusion process can no longer generate enough energy to produce that pressure to prevent collapse and the star is massive enough so that it's gravitational field can compress itself so far it becomes dense enough to be a black hole.
Roughly speaking, for a star to become a black hole, its physical radius has to become smaller than its Schwarzschild radius. So even the Earth could be a black hole if it shrinks to below 9 milimiters. It is not precise to say that a black hole depends on the density of the object, since a Schwarzschild metric is a vacuum solution of Einstein's field equations.
Or does gravity depend on the density of the object as well?
The problem with this question is that it's rather ambiguous as to what you mean by "gravity". An object doesn't have a single number that is its "gravity". If a ship is near a star, the gravitational force that the ship feels depends on the mass of the star, the mass of the ship, and the distance between them. If we consider the acceleration, rather than the force, then we can divide out by the mass of the ship. So rather than saying "gravity", I will talk about the gravitational acceleration. We can take the mass of the star as being fixed, but that still leaves the variable of the distance between them.
So the question is whether this distance is measured from the center of the object, or from the surface of the object. If the distance is measured from the center, then gravitational acceleration does not depend on the density of the object. If the Sun were to contract and become more dense, the orbit of the Earth would not be affected.
However, the less dense the object is (for a fixed mass), the further the surface will be from the center. So decreasing the density of an object decreases its surface gravitational acceleration. If the Earth were to expand in volume, but not increase in mass, then the gravitational acceleration at its new surface would be lower.
Also, it's more the escape velocity, rather than the gravitational acceleration, that determines whether something is a black hole. However, the escape velocity follows the same pattern as gravitational acceleration: the escape velocity relative to the center of an object does not depend on the density, but the surface escape velocity does. As a star collapses, its surface escape velocity increases, and once the surface escape velocity reaches the speed of light, it is a black hole.
A hungry beast in every galaxy
Over the past 30 years, observations with the Hubble Space Telescope have shown that all galaxies have black holes at their centers. Bigger galaxies have bigger black holes.
Nature knows how to make black holes over a staggering range of masses, from star corpses a few times the mass of the Sun to monsters tens of billions of times more massive. That’s like the difference between an apple and the Great Pyramid of Giza.
Just last year, astronomers published the first-ever picture of a black hole and its event horizon, a 7-billion-solar-mass beast at the center of the M87 elliptical galaxy.
It’s over a thousand times bigger than the black hole in our galaxy, whose discoverers snagged this year’s Nobel Prize. These black holes are dark most of the time, but when their gravity pulls in nearby stars and gas, they flare into intense activity and pump out a huge amount of radiation. Massive black holes are dangerous in two ways. If you get too close, the enormous gravity will suck you in. And if they are in their active quasar phase, you’ll be blasted by high-energy radiation.
How bright is a quasar? Imagine hovering over a large city like Los Angeles at night. The roughly 100 million lights from cars, houses and streets in the city correspond to the stars in a galaxy. In this analogy, the black hole in its active state is like a light source 1 inch in diameter in downtown LA that outshines the city by a factor of hundreds or thousands. Quasars are the brightest objects in the universe.
Supermassive black holes are strange
The biggest black hole discovered so far weighs in at 40 billion times the mass of the Sun, or 20 times the size of the solar system. Whereas the outer planets in our solar system orbit once in 250 years, this much more massive object spins once every three months. Its outer edge moves at half the speed of light. Like all black holes, the huge ones are shielded from view by an event horizon. At their centers is a singularity, a point in space where the density is infinite. We can’t understand the interior of a black hole because the laws of physics break down. Time freezes at the event horizon and gravity becomes infinite at the singularity.
The Event Horizon Telescope (EHT) captured the first direct visual evidence of the supermassive black hole in the centre of Messier 87 and its shadow. Credit: Event Horizon Telescope/ Wikimedia Commons
The good news about massive black holes is that you could survive falling into one. Although their gravity is stronger, the stretching force is weaker than it would be with a small black hole and it would not kill you. The bad news is that the event horizon marks the edge of the abyss. Nothing can escape from inside the event horizon, so you could not escape or report on your experience.
According to Stephen Hawking, black holes are slowly evaporating. In the far future of the universe, long after all stars have died and galaxies have been wrenched from view by the accelerating cosmic expansion, black holes will be the last surviving objects.
The most massive black holes will take an unimaginable number of years to evaporate, estimated at 10 to the 100th power, or 10 with 100 zeroes after it. The scariest objects in the universe are almost eternal.
Chris Impey, University Distinguished Professor of Astronomy, University of Arizona
This article is republished from The Conversation under a Creative Commons license. Read the original article.
5 facts about black holes explained!
We may still be learning about these space marvels, but read on to find out what we do know about black holes.
1. Black holes used to be stars
The core of a star has collapsed, forming a black hole inside. Within a few seconds there is a gamma-ray burst of matter away from the balck hole. ( Image courtesy NASA/SkyWorks Digital)
Want to know how a black hole is born?
Stars live a very long time, but just like us, they don&rsquot live forever. They slowly burn through the fuel that keeps them shining.
When they run out, one of three things happens, mainly depending on its mass (which is how much matter something contains). The star will either transform into a white dwarf, a neutron star or a black hole.
If the star is big enough (like 10 or 15 times as big and heavy as the sun), it will explode when it reaches its end! The explosion causes the star to cave in on itself, making it much smaller. Because the size of the star gets smaller but the mass of it does not, the gravity surrounding the star becomes so strong it absorbs everything around it, including light. That's how a black hole is born.
2. There are 3 types of black holes
An artist's concept depicts a supermassive black hole at the center of a galaxy. (Image courtesy NASA/JPL-Caltech)
Want to learn the different kinds of black holes? Here they are:
- Primordial: These are considered to be tiny! They range from the size of a single atom to a mountain. Mountains probably don&rsquot seem tiny to you, but space is a mighty big place!
- Stellar: These are the ones found most often. They are about 20 times bigger than the Sun!
- Supermassive: The black hole discovered in 2015 falls into this category. To be in this group, the hole has to be more than a million times heavier than the Sun!
3. You can&rsquot see them with the naked eye
An artist's concept shows a supermassive black hole at the center of a far-away galaxy absorbing the last pieces of a star. (Image courtesy NASA/JPL-Caltech)
No matter how hard you stare, you won't be able to spot a black hole all on your own!
The reason black holes are so black is because they consume everything around them, including light! That's because the gravitational pull at their centre is super strong.
But with no reflection, we have nothing that can detect the hole directly. So instead, scientists look for the traditional effects a black hole has on its surroundings.
When a star is being pulled into the hole, it breaks apart and becomes distorted. As it&rsquos sucked in, the bits of matter from the star move faster, create intense heat and throw off a glare of X-rays. That&rsquos what astronomers can use to identify a hole.
4. Black holes helped create galaxies
An artist's concept shows a primitive supermassive black hole (central black dot) at the core of a young, star-filled galaxy. (Image courtesy NASA/JPL-Caltech)
Astronomers aren&rsquot entirely sure yet what part the black holes played in the creation of galaxies. But one theory is that a large star exploded, a black hole formed and the rest of the galaxy was created around it!
5. There&rsquos a black hole in the Milky Way
An artist's concept shows the activity at the core of our Milky Way galaxy where there is a supermassive black hole in the region known as Sagittarius A*, or Sgr A*, with a mass of about four million times that of our sun. (Image couresy ESA&ndashC. Carreau)
Did you know our very own galaxy has a black hole in the centre known as Sagittarius A*?
In fact, it's believed by scientists that there&rsquos a supermassive black hole in the middle of almost every galaxy!
Because the pull of a black hole is so strong, you might wonder whether Earth is in any danger of being sucked into one of the supermassive varieties.
Well, worry not! Sagittarius A*, the Milky Way&rsquos black hole, is 26,000 light years from Earth. That's too far away for it to affect us! Phew!
When a very large star runs out of fuel it will explode as a type II supernova. It will throw the vast majority of it's mass off during the supernova, reall leaving only the core. The mass left over will collapse into either a neutron star or a black hole.
A black hole is matter as it is massive (i.e. it possesses mass) and it is a region of space from which nothing can escape (i.e. I can't think of a good concise defitnion)
Light that enters the event horizon will goto the infitely dense singulairty at the centre of the black hole. Yes this does increase the nergy of the black hole as mass and enrgy are equivalent this means that the mass of the black hole will increase.
I read Kip Thorn's book and it was good. I recommend "About Time" by Paul Davies
GR, the extremely successful physics theory in which BHs are predicted, says nothing about how massive they need to be. However, the rest of science - astronomy and fundamental particle physics in particular - has a hard time coming up with realistic scenarios under BHs could occur (other than those left over from supernovae, or stellar collisions, or at the centre of large galaxies all these are rather massive BH).
Some folk hypothesise that 'small' BH may have been created early in the history of the universe (all kinds of wild an woolly reasons), and some may still be around. They are called 'primordial black holes' (PBH).
IF (stress on if) they do exist, then some should be evaporating about now, giving off a nice burst of 'Hawking radiation', after Stephen, who first published a paper on why there should be such radiation.
AFAIK, no Hawking radiation has been observed, so PBH must be pretty 'rare'!
Not planets, but stars and galaxies. The planets of our solar system, the ones orbiting the Sun as the Earth does, are not more than hours away from us as light travels (the Sun is eight minute away).
Stars in our galaxy are a few years up to thousands of years away. Other galaxies are millions up to billions of year away. These are the things you have heard about, where what we see happened thousands or millions or billions of years ago.
So now black holes. We can't see black holes because they absorb the light that falls on them. But we can see the clouds of gas and the fast moving stars that surround them. There seems to be a big one at the center of our galaxy, but that is far away and we are moving around it on big orbit, rather than toward it.
The folk who run Google must have had so many search requests that they've set up a special entry, http://directory.google.com/Top/Science/Physics/Relativity/Black_Holes/Observations/ [Broken]. Have fun!
AFAIK, there are now >10 X-ray binaries whose masses are sufficiently well determined for us to say that at least one object must be a BH. At the heart of our Milky Way galaxy is an object called Sag A*, which was thought to be massive BH quite some time ago. Gradually all other possible explanations have been ruled out by new observations, including http://curious.astro.cornell.edu/blackholes.php [Broken], of stars orbiting this BH.
BTW, that site you posted a link to . the author can't possibly be serious there's nary a firm prediction on it! Certainly no math, no numbers . goodness, no matter what we find in the next decade, his stuff is so vague he could (correctly) claim new observations match perfectly. Excuse me, but that's not how I think science is done.
Hey, I've looked at that google thingy. It only had three sites for observations that are not really persuasive to me.
spac250/steve/index.html]Black[/PLAIN] [Broken] holes, if they truly exist, are very strange objects indeed. These are strange and fascinating objects, truly, but as of yet, they are still considered theoretical.
That doesnt sound too confident to me
Black holes are not yet proven and are still up for debate, but I have a couple of questions if anyone would like to answer them for me PLEASE. I'm very new to physics, but what I have studied leads me to disbelieve in a lot of well known theories. Now on to my questions about black holes.
1. Has there ever been a black hole observed that did not have a binary star orbiting it?
2. Do you believe in the concept of neutrinos? If so, could it be possible that these orbiting stars are emitting neutrinos.
3. Do Particle and Anti-particle pairs disembark at the event horizon of the black hole leading to one being pulled into the black hole and one being shot out from it? (not to sure if this is even relevant)
Like I said, I'm fairly new to physics. So far, though, what I have read is not very convincing. Heh, I'm only in High School, so please aid me in seeking out the right path in physics, if I'm not on it.
4 times that mass of the Sun (Msol), then it must be either a very bright star (which would be very visible) or a BH. Why? Because if it isn't emitting copious quantities of photons (esp light, UV, IR), its core won't be hot enough to stop an ordinary matter star from collapsing. And no degenerate matter object* of that mass can avoid being crushed into a BH.
The alternative is that our understanding of physics has a huge hole in it.
The supermassive BH at the heart of galaxies aren't binaries (well, some are, e.g. Arp 220) if they were, they'd merge rather quickly (and create an event that would make a GRB look like a damp squib).
*degenerate electrons, as in a white dwarf, are highly incompressible . until those at the highest levels start to react with protons to form neutrons (inverse beta decay), and the dwarf collapses to a neutron star, which is like a giant atomic nucleus.
Would these tiny black holes be dangerous? Would they be powerful enough to eat earth up? (insert higgs boson story here) Are black holes infinitely small or do they have a distinct diameter? If they are infinitely small, why would one have more mass than another? Does the mass of a black hole affect its size or only the distance from the singularity to the event horizon?
The singularity of the black hole is infinitesimal (obviously), but the event horizon has a distinct diameter.
They aren't infinitely small look above. The mass is usually referred to as the radiation/other stuff that is absorbed into the event horizon. Therefore, because the event horizon serves as the barrier of the "mass", the event horizon is usually referred to as the total mass.
(I don't have much info in this area, but from what I've read on black holes, this is accurate. In conclusion, I've probably made things worse. Sorry.)
i still am in disbelief on the theory of black holes as well as gravity being a pull or even existing. i continue to find more sources that make logical attempts to disprove black holes existences, although they are just theories. they actually make sense and unlike the theories proposed about black holes today have baffled many scientists because of their inaccurate calculations from false mathematical attempts at explanation.
im sure ill find more sites that take a shot at disproving black holes. and as it takes time to find these sites it also takes time to take an open mind to science as it has no definite way of being operated.
yea thats how science works. that man is questioning. that's how science works if im not mistaken
but if you can could you provide me with links on GR and SR. and answer my question about particle/anti-particle interatcions at this phenomenons event horizon?
Here is just a part of an older post about virtual particle production at a BH event horizon. Both Hawking radiation and magnetic "quantum tunelling". It shows that virtual particles are produced and released at the EH by two methods. This has nothing to do with matter and energy release from an accretion disk.
One thing that I have noticed, after reading many pages of info on "classical" Hawking Radiation (HR), is that it was conceived and most often described as in its original form as applying to a static, non-rotating, non-accreting and chargless black hole (BH) in the original Schwarzchild configuration as a simple mass-only expression of the Schwarzchild Radius (Rs) where Rs = 2GM / c^2. All of the virtual particle pair production scenarios are based on this and require one particle to "fall back" with the other escaping as a real particle causing a mass loss of the BH.
However, many other research sites and past papers have noted that it is almost impossible to form a BH with no angular momentum (spin). Even the "no hair" statement by Hawking was that a BH has only three observable properties (1) mass, (2) angular momentum and (3) charge (usually net zero). But a lot of recent discoveries (and older theories) have added one new property that is (4) magnetic field. At first, it was thought that a magnetic field would only surround a BH that was accreting matter, but Ramon Khanna and Yakov Zeldovich have shown that all black holes will have a magnetic field. There are also the terms "Hawking Process" and “Hawking Effect” appearing, which include/combine the original HR work with work of others such as Thorn and especially Kerr (for spin) and Newman (for charge). The "Kerr-Newman" BH. (A source quote:) “David Finkelstein's Black Hole, which shows how Mass curves SpaceTime by Gravity, can be generalized to deal with Spin and Electric Charge. The generalization, called a Kerr-Newman Black Hole, was developed by Kerr (who generalized to add angular momentum J to mass M in 1963) and by Newman (who generalized to add charge e in 1965), according to the book General Relativity, by Robert Wald (Chicago 1984).
In his paper Generation and Evolution of Magnetic Fields in the Gravitomagnetic Field of a Kerr Black Hole, Ramon Khanna says: ". a rotating black hole can generate magnetic fields in an initially un-magnetized plasma. In axisymmetry a plasma battery can only generate a toroidal magnetic field, but then the coupling of the gravitomagnetic potential with toroidal magnetic fields generates poloidal magnetic fields. Even an axisymmetric self-excited dynamo is theoretically possible, i.e. Cowling's theorem does not hold close to a Kerr black hole. Due to the joint action of gravitomagnetic battery and gravitomagnetic dynamo source term, a rotating black hole will always be surrounded by poloidal and toroidal magnetic fields (probably of low field strength though). The gravitomagnetic dynamo source may generate closed poloidal magnetic field structures around the hole, which will influence the efficiency of the Blandford-Znajek mechanism whereby coupling of the gravitomagnetic potential with a magnetic field results in an electromotive force that drives currents that may extract rotational energy from a black hole.”
In June of 1971 Zeldovich announced a spinning black hole must radiate . “a spinning metal sphere emits electromagnetic radiation . The radiation is so weak . that nobody has ever observed it, nor predicted it before. However, it must occur. The metal sphere will radiate when electromagnetic vacuum fluctuations tickle it. Zeldovich's mechanism by which vacuum fluctuations cause a spinning body to radiate showed a wave flowing toward a spinning object, skimming around its surface for a while, and then flowing away. The wave might be electromagnetic and the spinning body a metal sphere . or the wave might be gravitational and the body a black hole. The incoming wave is not a "real" wave . but rather a vacuum fluctuation. . the wave's outer parts are in the "radiation zone" while the inner parts are in the "near zone" . the wave's outer parts move at the speed of light . its inner parts move more slowly than the body's surface is spinning . the rapidly spinning body will . accelerate . [the inner parts of the incoming wave] . <and this> acceleration feeds some of the body's spin energy into the wave, amplifying it. The new, amplified portion of the wave is a "real wave" with positive total energy, while the original, unamplified portion remains a vacuum fluctuation with zero total energy. Zeldovich proved that a spinning metal sphere radiates in this way his proof was based on the laws of quantum electrodynamics.”
The quantum mechanical description of the vacuum allows for the creation of the particle/antiparticle pairs, and the electric field tends to separate the charges. If the field is strong enough, the particles tunnel through the quantum barrier and materialize as real particles. The field necessary to accomplish this feat is achieved when the work done to separated the charges by a Compton wavelength equals the energy necessary to create the particles. It should be noted that conservation of energy is not violated, as the energy it took to create the particles would be precisely equal to the decrease in the energy of the weakened electric field." .. (LABGUY NOTE: not necessarily just BH mass loss as with Hawking radiation).
Can Black Holes Expel Matter?
Yes! As a matter of fact, they do expel out the matter by what is known as Hawking Radiation. It takes millions of years, but black holes spit out information. Just like a star, the bigger it is, the faster it expels matter out.
Black holes seem to take in everything without releasing anything. The second law of thermodynamics states that the entropy of an isolated system never decreases with time. This non-decreasing behaviour of black holes was similar to entropy, which is the degree of disorder of a system.
But there was one flaw to this suggestion. If a body has entropy, then it must also have some temperature. But a body with non-zero temperature would emit some form of energy, or radiation, which went against what was previously assumed of black holes. This meant that black holes do indeed emit particles!
So, if one manages to capture each particle as it was expelled, one might be able to reconstruct a person from the spaghetti!
1. Why haven't we been able to photograph a black hole until now?
No single telescope on Earth is powerful enough. A research team known as the Event Horizon Telescope project linked together a network of eight telescopes around the world in April 2017. The telescopes collected radio waves from the black hole, and the data was combined to produce the first images ever taken of a black hole in space.
M87's black hole was chosen for images because it was predicted to be one of the largest black holes that is viewable from Earth.
Flare of X-rays around black hole
This diagram shows how a shifting feature, called a corona, can create a flare of X-rays around a black hole. The corona (represented in purplish colors) gathers inward (left), becoming brighter, before shooting away from the black hole (middle and right). Astronomers don't know why the coronas shift, but they have learned that this process leads to a brightening of X-ray light that can be observed by telescopes.
In 2014, NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, and Swift space telescopes witnessed an X-flare from the supermassive black hole in a distant galaxy called Markarian 335. The observations allowed astronomers to link a shifting corona to an X-ray flare for the first time.