Black hole.

Might we already be inside a black hole? Is it possible that the viewable universe is spiraling down a black hole? Or for that matter, coming out of a white hole?

I can't seem to pass physics. Just so you know where I am coming from

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But as I recall about a black hole, it stretches you out like spagetti. By that I take it to be increasing the distances between parts of your body or atoms. This leads anyone to conclude that you would die via disintigration. I'm curious how fast this would occur and if there could exist some unseen effect that adjusted for this distance discrepancy allowing you to survive in some manner. Maybe like a funhouse mirror makes you seem disproportionate, yet still whole.

 

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Well, I would think that you would die from the massive g forces (acceleration) before you would even go into "spaghetti" strands, I could be wrong.

To answer the part about being on the "other side" of a black hole:

Would a black hole every really collapse all the way? I hear teachers always talking about them being infinitely small, but would they be? I mean via conservation of angular momentum, as the black hole got small it would spin faster, and thus slow time. Moreover gravity, according to relativity, also slows time. So, since it would take time to collapse and time was going infinitely slow, wouldn’t it "converge" to some lower limit depending on its gravity, and angular velocity before it collapsed?

Food for thought =]

 

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No.

You are speaking about the gravity gradient. I worked it out once for a neutron star, which doesn't have as strong a gravitational field as a black hole. It was something like 400 gravities per meter. When you are in orbit about the neutron star just above the surface, in free fall, you are experiencing zero gravities. However, the top of your head is roughly a meter further from the neutron star than your waist is, and your feet are a meter closer than your waist. If your head masses 10 kilograms, meaning that it weighs 100 newtons on Earth, then in freefall around the neutron star it would be pulling away from your waist with a force of 40,000 newtons. It would feel like you were dangled upsidedown with a two ton mass tied to your head. Your feet would be pulled in the opposite direction.

The effects of a black hole would far more of course. You would be falling at close to the speed of light, and the closer you got to the singularity, the more intense the gravitational gradient would become. One second it might be 10,000 gravities per meter, but a microsecond later it would be a hundred times more.

So how long would you live? High speed cameras might resolve your demise into actual stretching, but the human eye would only see a flash as you were torn apart into your constitutient subatomic particles.

 

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Hey maddad, would you, or anyone else, happen to know why teachers make the assumption that it collapses to an infinitely small size? is there any scientific basis for this? Or is it just an assumption and nothing more?

 

Hum,
The current view is that it can`t get smaller than the Planck length , 10^-35m, that corresponds to the Planck time scale 10^-42 sec

Though, it must be said that the whole black hole theory is still evolving and that there are a few other contending theories out there (ie gravistars)...

 

The assumption is based on the logic that nothing travels faster than light, so if the escape velocity at a radius from a center of gravitational attraction is c, the speed of light (which means that at lower radii the escape velocity is greater than c, if that even has any meaning) then everything must fall to zero radius, to a point not infinitesimally small but rather having no size at all. This is a black hole. The math of general relativity shows that escape velocity at a radius can be c; hence black holes are predicted.

 

@zanket

Hum,
illogical assumption on their part...
how can the event horizon tell the size of a black hole?
Sure it’ll tell the mass, but not about the varying orbits of the photons beyond the event horizon.

Or have i missed something???

 

Technically a black hole has no size. The event horizon is just an imaginary surface where the math says that the escape velocity is exactly c. The circumference of the event horizon is calculable when you know the mass of the black hole. Despite having no size the black hole does have x amount of mass, hence gravitational attraction to keep things in stable orbits around it (provided they orbit far enough away). If the Earth somehow got squeezed to became a black hole (it would need to be squeezed to the size of a grape), the Moon would remain in its orbit.

 

Yes to both. Being “inside a black hole” refers to being inside the event horizon. If we were, reaching the doom of the singularity would be a foregone conclusion but it could be any amount of time in the future. There’s no size limit to a black hole’s event horizon circumference. The larger the circumference, the longer something can survive after crossing the horizon before it reaches the singularity and the more ordinary the crossing will seem. You could have crossed an event horizon while reading this; it’s just an imaginary surface. The stuff outside the horizon would still be visible and the crossing could be undetectable by the finest instruments given a sufficiently large horizon circumference. It could be ten billion years before the singularity is reached.

White holes are more speculative, since the math of general relativity does not predict them.

 

The tidal force, the difference in acceleration at the various altitudes of you, is the spaghetti strands. Other than that, gravitational acceleration is not felt when you’re free-falling. We're free-falling at some significant rate now, toward some other galaxy or cluster of galaxies. We're accelerating toward it, but we don't feel it because the difference in acceleration at the different altitudes (relative to the attracting galaxy) of us is negligible.

The slowing is just from the perspective of an observer fixed at a higher altitude. The local rate of time is always 100% = normal. Suppose you fell across the horizon while I remained at a fixed altitude above it. You’d fall across normally. Say you reached the horizon tomorrow at noon on your watch. I’d never see you reach the horizon; I'd see you ever more slowly approach it. I’d see the time displayed on your watch ever more slowly approach but never reach tomorrow at noon.

 

I take this as a very limited view of reality. Indeed one can look at gravitational potentials but the feet are not physically bound to the gravity well. Your physical structure would undergo extreme stress but your body parts would be dragged along faster than free fall from the lower gravity well by body parts in a deeper gravity well.

The force would be one of F=ma at the head minus the acceleration provided by the black hole acceleration at the feet. All linked by the tensil strength of the body parts.

 

but wouldn't that mean that from our perspective the black hole would stop, and thus never collapse all the way(in our view)?

I think I might understand part of it. at any place below the E.H. the escape velocity is greater than c, but how does that mean that it has no size? does it mean that the size is just undefined? please explain it to me =]

 

Or is this nothing more than an assumption *Thinking* Hmmm. It is our best guess.

It does not. It tells the point of no return. There is no physical dimension to the event horizon because it is an abstract concept. zanket's got it.

 

< off topic>

Hum,
zanket's got what?
Once inside >> "<i>then everything must fall to zero radius, to a point not infinitesimally small but rather having no size at all.</i>"

???

Er, i could illustrate it with an example of a planet, Earth, and lets suppose we change the speed of light to 11 km/sec (er, its hypothetical)...in such a scenarioan event horizon would envelope the planet, due to the light not exceeding the gravity well of the earth; so it seems illogical to assume that the planet suddenly becomes a finite point like `black hole`. (or grapefruit sized, or the size of manhattan, or <i>please insert your own choise</i>)

We don`t really know the physics of space-time beyond the event horizon to make accurate predictions....

(see cato `s last post)

< /off topic>

 

Yes. An alternative name for a black hole is a “frozen star,” implying that the collapse of the star slowed (due to gravitational time dilation) such that its surface ever more slowly approaches but never reaches the event horizon, from the perspective of a faraway observer. The frozen star still appears black because the radiation that still emanates from its surface that is still above the horizon—from the perspective of a faraway observer—is redshifted to approach infinite wavelengths that are all but undetectable.

“No size” is from the perspective of an observer free-falling into the black hole. You’d free-fall toward the mass, find empty space at the event horizon, and find empty space all the way to the singularity at radius zero. Other objects could be in the space around you but they’d be falling as well—the space is empty of anything staying fixed at a given radius or rising above a given radius, for when the escape velocity is > c all objects must fall, even photons.

It can difficult to visualize a black hole from these different perspectives. One trick is to think of gravitational time dilation in a time travel sort of way. An observer free-falling into a black hole is falling into the future—the future becomes this observer’s “now.” A faraway observer stays in the free-falling observer’s past, and so does not witness the free-falling observer cross the horizon. Some books will tell you that a free-falling observer reaches the infinite future at the singularity, so that whatever end befalls the universe at large will be experienced at the moment the singularity is reached.

 

Let's make the speed of light almost Earth's escape velocity. In this hypothetical case the Earth would quickly—but not instantly—become point-sized, which is to say no-sized. When the escape velocity is greater than the speed of light then everything including photons must fall to the lowest radius, which is zero. There is no known physical mechanism that can prevent such fall.

Physics can make predictions below the event horizon, but the accuracy of these predictions cannot be tested in a way that can be communicated to those above the event horizon. What physics cannot make predictions for is what happens at the singularity—infinities arise in the results of the equations there.

It is good to keep in mind that general relativity, the theory that predicts black holes, could be wrong. But general relativity makes predictions that match observations more closely than do other theories to date. The theory has not been well-tested for the existence of black holes and probably will not be in our lifetimes.

 

Hum,
>><i>"Let's make the speed of light almost Earth's escape velocity"</i>.
Ok, we’ll make it 11.1 km/sec, so light cannot escape...

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In my <b>new</b> scenario the earth doesn’t shrink at all...

From the `outside` the black hole looks just the same as your black hole...

< I’m actually a believer of singularities>

But, lets suppose that we were to travel <i>beyond</i> a real black holes` event horizon; how can we be sure that there are even still 3 spatial dimensions?

we may find that at some distance in towards the `centre`, that other dimensions can unfold/expand.

What would <i>that</i> do to our physics?

< /I’m actually a believer of singularities>

 

Not sure why you say that. It will shrink as I explained above.

There’s just no reason to believe otherwise. For a similar reason it is widely accepted by cosmologists that the part of the universe we cannot see is similar to the part that we can see—we have no reason to believe differently.

Anything is possible, but it’s academic absent some reasoning to show that the idea has merit.

There is the string theory that has other dimensions. I’ve read that it’s all but untestable. If so, expect that its acceptance by the physics community at large will be limited. An untestable theory is akin to a fantasy.

 

Hum,
<b>Well there are problems…</b>
You may recall the story about the three cosmologists who made a famous bet as to whether <a href="http://www.carm.org/evolution_archive/singularities.htm">information</a> that enters a black hole ceases to exist (<i>ie. whether the interior of a black hole is changed at all by the characteristics of particles that enter it</i>)

Stephen Hawking (originally) proposed that the particles have no effect whatsoever. But his theory violated the laws of quantum mechanics and created a contradiction known as the “information paradox.”…

So, if the Earth collapsed into a singularity, its event horizon would measure approximately 1 centimetre. As to what lies in the region between a singularity and its event horizon, was anyone’s guess. No matter what type of material formed the singularity, the area inside the event horizon was supposed to be devoid of any structure or measurable characteristics.

And that is a problem.
<b>The information is lost</b>…

The problem with your classical theory is that you could use <b>any</b> combination of particles to make the black hole - protons, electrons, stars, planets, whatever - and it would make no difference; the <i>final state</i> of the system is always the same.
That kind of uniformity violates the quantum mechanical law of reversibility.
(<i>We must be able to trace the end product of any process, including the process that makes a black hole, back to the conditions that created it. If all black holes are the same, then no black hole can be traced back to its unique beginning, so any information/(entropy) about the particles that created it is lost forever at the moment the hole forms.</i>)

Theory holds that a black hole should have a billion, billion times more entropy sometimes referred to as states, than the star it formed from.

However, according to <b>string theory</b> that information is retained.
These strings, tiny as they are, can form black holes through a phenomenon called fractional tension. (<i>er,Strings are stretchable, but each carries a certain amount of tension, as does a guitar string. With fractional tension, the tension decreases as the string gets longer.</i>)

Just as a long guitar string is easier to pluck than a short guitar string, a long strand of quantum mechanical strings joined together is easier to stretch than a single string. So when great many strings join together, as they would in order to form the many particles necessary for a very massive object like a black hole, the combined ball of string is very stretchy, and expands to a wide diameter.
I believe that a derived formula for the diameter of a `<i>fuzzy black hole</i>` made of strings is found to match the diameter of the black hole event horizon suggested by the classical model.

<a href="http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BlackHoleThermo/BlackHoleThermo.html">Black Hole Has No Hair theorem</a>

So as soon as you entered the event horizon you would actually be in a 10 dimensional space-time...

(<i>The conjecture suggests that strings continue to exist inside the black hole, and the nature of the strings depends on the particles that made up the original source material, then each black hole is as unique as are the stars, planets, or galaxy that formed it. The strings from any subsequent material that enters the black hole would remain traceable as well. That means a black hole can be traced back to its original conditions, and information survives...</i>)

An <b>alternative</b> to this type of fuzzy black hole singularities is a `<b>gravastar</b>`; ie, matter that is transformed into a spherical void surrounded by an extremely durable form of matter , something akin to the Bose-Einstein condensate, (<i>er , something akin to `the hollow earth`…</i>)
The matter inside a <a href="http://web.abqtrib.com/archives/news03/060903_news_bright.shtml">Gravistars </a> would exist in a vacuum, surrounded by an ultra-thin, ultra-cold, ultra-dark bubble, hence the name <b>gra</b> (<i>vitational</i>) <b>va</b> (<i>cuum</i>) <b>star</b>,(gravastar)...

This would get around the problem of the tremendous entropy, or information, that a black hole would contain…
Gravastars do not have this problem, as their entropy is very low, and they don’t have those pesky singularities....

 

thanks for the in-depth explanation of most of what were talking about.=]
however, I am still a bit fuzzy as to why they teach it as an "infinitely small" thing. it is my opinion that it either has some finite size or its size is undefined