The concept of black holes?

Question

About two years ago I began learning about black holes in science. To be quite frank, I just don't get it... it doesn't make any sense to me.

Can someone explain the concept of black holes in "caveman" terms because the whole sucking light in, nothing escaping thing has me confused.

Thanks!

Answer 1

Imagine ball falling to Earth from a great height. Way up there, Earth's pull is weak so the acceleration is small. The farther it falls, the faster it goes AND the more gravity pulls too. Eventually it will hit the ground at some speed. For Earth, that final speed is 7 miles per second.

Now imagine another planet with exactly the same mass as earth, but only half the radius (and therefore eight times the density of earth). When the ball reaches Earth's radius, it's going at 7 miles per second (because the gravitational attraction is just the same) -- but it hasn't actually gotten to the surface of the smaller, denser planet yet, so it keeps accelerating to some higher speed before hitting the surface.

These "final" speeds are also called "escape velocity", because that's also the speed you have to travel away from the planet to escape its gravity all together. As you can see, the escape velocity for any celestial object depends on its density.

Now it turns out that if an object is very very dense, it's escape velocity can be greater than the speed of light. That's a black hole. And since nothing can travel faster than light, any matter or energy inside that magic radius -- called the Schwarzchild radius -- can't ever get out.

Oddly, it turns out that not much can get IN, either, since an object falling in would have to reach the speed of light too -- which can't happen.

Answer 2

Keeping it simple: use eelfins's answer.

The escape speed of a body depends on the body's mass and on its radius.

Given two bodies (e.g., planets) with the same radius, the more massive one will have a higher escape velocity.

Given two bodies with the same mass, the smaller one will have a higher escape velocity.

Light has a finite speed (300,000 km/s). If you have a body that is massive enough and small enough that the escape speed is greater than 300,000 km/s, light cannot escape. If someone on the 'surface' shines a flashlight upwards, the photons will not escape pst the event horizon -- we, on the outside, will never see the flashlight.

That ws the original definition of a black hole. That was back in 1796.

Then Einstein came along and, with the theory of Relativity, proposed that nothing in the universe can travel faster than the speed of light. Just before him, others had proposed that strange things happen when anything with mass travels very close to the speed of light (e.g., time slows down).

If nothing can travel faster than light, then if the escape speed of a body is such that light cannot escape, then nothing can escape.
(Hawking's radiation does not escape, it is created just outside the event horizon).

Add to that the effect that such gravitational curvature would have on the space-time continuum (from Einstein), then anything that is inside the event horizon (including the central mass) tends to a singularity (meaning: everything crashes to the exact centre point into an infinite density).


As for the sucking in: a black hole has finite mass (even if it has infinite density at the singularity), therefore the amount of 'sucking' it has on distant object is the same as that of an object with the same mass (but a bigger radius) would have.

For example, if we were to replace our Sun with a black hole that has the same mass as the Earth, our planet would simply continue on the same orbit, because the orbital parameters are determined by the mass of the central object, not its shape or color or...

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Escape speed diminishes as one moves away from the object. For example, escape speed at the surface of the Sun is 617.5 km/s.

However, once you get as far from the Sun as the Earth's orbit, the escape speed from the Sun is down to 42.1 km/s. If Earthling want to shoot something outside the solar system, they have to give it an impulse of at least 42.1 km/s.

The black hole has (in theory) an unbounded escape speed (infinite density and zero radius). However, at some distance away from the singularity, the escape speed will have 'dropped' to the speed of light. That distance is the 'event horizon'. Any object that gets closer than this distance to the black hole can no longer escape.

Answer 3

If you shine a powerful flashlight straight up you can see it shine on the bottom of clouds. The photons in the beam have no trouble moving out against the pull of the earth's gravity, though they do feel the pull. On a black hole the gravity is so strong that the photons can't even move upwards at all. The event horizon around a black hole is the distance above the black hole where the gravity is finally weak enough that photons can get away and head out into space.

Answer 4

Some of the preeminent Physicists of our time explain the black hole phenomena in layman's terms:

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David Brin
Physicist and Author

A black hole is one of these folds in space that is so deep that things spiral down into it and they can't get out. Whether that hole leads someplace else or is completely self-contained, and whether that matter and energy and information is going to come back into our universe someday—these are subjects of argumentation. But the best way to look at it is that the fabric has been pulled in such a way that if you stick your finger down in there, you ain't getting it back.
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Roger Blandford
Astrophysicist
Stanford University

Well, what it is really is a region of space and time where the properties are very different from those we encounter in the space and time around us. It has lots of weird effects that it can exhibit—and then those we think we can understand as physicists. But right at the heart of a black hole is the center nugget, which we call the singularity, and there we don't know what goes on. There I can't answer your question, I'm afraid.
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Steve Ritz
Astrophysicist
NASA Goddard Space Flight Center & University of Maryland

A black hole is one of the most interesting and unusual ideas in all of science. It's enigmatic, because it represents the laws of physics in the most extreme environments that we can imagine. It's outside of our experience. We don't make black holes, at least yet, here on Earth, so we don't really have all the words for describing them. One way you can think about it is that it's an incredibly simple object. Looking from outside, there are really only a very limited number of properties that describe a black hole. It has a total mass. The event horizon has a certain size, or there is a certain curvature associated with the hole. We know that it can have a spin, just like the Earth has a spin. And in principle it can also have an electric charge. So you can think of it as a fundamental particle which has a mass, a spin, and a charge. And that's it.
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Kip Thorne
Physicist
California Institute of Technology

When a black hole forms by the implosion of a star, which is how most black holes probably form, the matter implodes, and as it gets more and more compact its mass generates the warping of space and time around it; its spin generates the whirling motion of space around it. But the matter continues to shrink smaller and smaller and smaller, shrinks down to the very center where it gets destroyed in a singularity, a region of infinite warped space and time. And it's gone. When it's gone there is nothing left except the warped space and the warped time. So the common idea that a black hole is just made of very compacted matter—it's wrong. It is just simply wrong. It may have been created from very compacted matter, but the matter is gone. It's been completely destroyed; it no longer exists. And all that's left behind is the warped space and the warped time, and this little nugget of a singularity at the center of the black hole that we don't understand.
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Answer 5

K so star begins to die. Depending on how "big" it is, it'll do one of 3 things, I really forgot the two unimportant ones, but the important one is that it starts to shrink.

Except by conservation of momentum (I think it's conservation of angular momentum....name is unimportant) as it loses mass (size), it must increase it's angular velocity, it's "Spinning speed"

Soon, it compresses itself into such a small space, it is impossible to measure, and it's spinning so fast, that the escape velocity is greater than that of the speed of light.

Escape velocity: required speed of an object to "escape" the gravitational pull of said object.

therefore light cannot get out

I believe this is all generally correct...I took an AP physics B class two years ago, but this section was after the ap test so I kinda only slightly paid attention.

Edit: eelfins, just for statistical correctness, the escape velocity is only 7 miles a second upward if you ignore air resistance and all other factors...

-bv (or -bv^2) is quite powerful.

Answer 6

Let us do this as you request, in simple terms. And we will address this concerning an existing black hole.

What is a black hole? It is a certain distance from a singularity. That certain distance is the point where the escape velocity of the singularity is the speed of light.

The escape velocity is that speed at which an unpowered projectile (a photon, a bullet, a ball) must travel to continue moving indefinitely away from an object (usually the object is a planet, moon, star, etc.). It is important to note that the limitation of escape velocity applies only to unpowered projectiles. Once the photon is moving at its speed or the gun had fired the bullet or the ball has been thrown, the projectile in question can receive no more power.

A powered projectile, such as a rocket, does not need to reach escape velocity in order to pull away from the Earth. It just uses more power to dig its way out.

Therefore, in simplest terms, no unpowered projectiles can leave the inside of a blackhole due to gravity because the surface of a black hole is where the escape velocity is the speed of light. And no thing can travel faster than the speed of light (according to our best knowledge today).

A singularity, which creates the black hole, is a collection of mass that has (for whatever reason) generated so much gravity that the force of its gravity is greater than any of the other forces that exist and so no force stops it from contracting to the smallest possible size.

It is at this point that there is a difference of opinion. Some scientists feel that the collection of mass shrinks to zero size while others feel that there is a lower limit to the size. I am of the latter persuasion and will continue on that vein.

As a side note: it is often said that the gravity of a black hole is infinite; this is wrong. It is properly considered to be the highest source of gravitational power and it is AS THOUGH its power were infinite because the fastest thing in the universe, light, cannot escape from it.

So the proper picture of a black hole is that there is a singularity in existence that exerts great gravitational force. There is a distance from the singularity at which the escape velocity is the speed of light, which is also called the event horizon, which is the "surface" of the black hole.

There are also distances from the singularity at which the escape velocity is substantially less than the speed of light and distances at which the escape velocity is a leisurely stroll.

The reason that there is so much hoopla about a black hole which is only a singularity and a certain amount of space surrounding it are the events which occur at the event horizon and those that occur closer to the singularity (where, for simplicity's sake, the escape velocity is some given figure above the speed of light, also ignoring that nothing of which we know can go faster than the speed of light.)

For the sake of simplicity, we will ignore the fact that the gravity at the surface of a black hole is so great that anything would be instantly flattened/destroyed/and so forth. We will simply assume that a person can stand on the surface of a black hole for our experiment and let it go at that.

I have been leading up to what is so fantastic about black holes (and the following is just the tip of the iceberg) and here it is.

Einstein put forth the idea that time does not pass at a constant rate everywhere in the universe. All our normal lives, we have accepted the notion that time passes at the same rate everywhere because that is what we experience.

A pair of twins are used in this experiment. They are (essentially) the same age; that is, they are both a certain number of years old. One stays at home and the other takes a trip around the world. When the traveling twin gets back home, will she be a different age than her twin brother?

Of course not. That is a ridiculous question.

What if she flies in a plane for a hundred years going around the world non-stop all that time? Then would she be a different age than her twin brother when she gets back?

Of course not. Are you taking the drugs or something? This is all common sense, you know.

And so, it was all common sense for thousands of years until Einstein thought about traveling at the speed of light in his thought experiments. Suffice it to say that he eventually came to understand that if one were to travel at very close to the speed of light, time for the traveler would pass at a different rate for the traveler than for the brother at home.

It is now accepted by science that if this twin girl were to travel at near the speed of light, when she returned she would be much younger than her stay-at-home brother. Simply, time passed more slowly for her.

But what does this have to do with the fantastic gravity machine that we call a black hole?

It turns out that the same thing that happens with time at the speed of light ALSO happens in gravitational fields. Simply put, the more gravity an object experiences, the slower time passes for that object.

So if you were standing on the surface of a black hole, an outside observer looking at you would see that you stand perfectly still for all time. AND you would see all of time pass in the very shortest time. Your stay-at-home brother would age billions of years (given that he stayed alive) while you do not age but one tiny bit.

As I said, this is only the tip of the iceberg when it comes to the fantastic things that occur in relation to a black hole.

The next step is to take this thought experiment one step further. If time slows to the slowest possible speed at the event horizon of a black hole, what happens whe you move into a stronger gravitational field to increase the effect of time dilation?

The answer is: we don't know yet. Everything up to this point has been worked out mathematically. However, to calculate what happens further into a black hole requires math that doesn't exist... yet. The math would have to allow you to divide by zero and by non-imaginary, non-negative, non fractional numbers less than zero.

So would you be so kind as to take this challenge and invent such a math? Someone has to do it eventually and it surely won't be me; I am too tired.

Answer 7

Imagine a gigantic star that is far, far away. Imagine its gravity to be billion or trillions of times greater than the gravity of our sun. At the end of this star's life cycle, it dies down and all matter that compose the star starts collapsing toward the center of the star (because of its gravity). In the end, this star will have totally collapsed into itself, resulting in a single very, very dense matter whose size is no larger than a golf ball but whose gravitational pull is still trillions of times than that of the sun. Because of its extremely high gravity, even light rays that pass near it will be totally attracted to it, unable to escape from its gravity. This makes this matter invisible to observers on earth, as no light from the matter ever escape to reach the observers. Any observer will simply see total darkness on the part of the sky where this matter exist. No planets, stars, or anything exist in the vicinity, as all of them had already been absorbed by this matter due to its extreme gravity. For this reason, these spots in the heavens are called black-holes. But the gravitational effects of these black-holes can be observed by looking at the galaxies near to them, as if some invisible hand are influencing these galaxies' behaviors.

Answer 8

The earth has an escape velocity of 7 miles per second. That means you have to shoot a bullet (for example) that fast for it not fall back to earth again. An escape velocity of the speed of light (186,000 miles per second) would require a lot more mass than the earth, since mass produces gravity. Therefore, any object of sufficient mass would produce the gravity necessary to create an escape velocity the speed of light. Since nothing can travel faster than light (according to Einstein's special theory of relativity, another subject), then nothing can escape the gravity of a black hole.

Answer 9

It's a celestial phenomenon caused by the collapse of a neutron star. The gravity well generated by the star’s collapse becomes so great that neither matter nor light can escape...Did you get that, kinda sorta? :]

*Edit: Okay, people. She said "caveman terms".