Black Holes and Gravity

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Sirry
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Black Holes and Gravity

One thing I have never understood about black holes is why light can't escape them while gravity can. Photons and gravitons are both massless particles and I know photons at least can interact with each other (can't they?), so why are gravitons immune to other gravitons? Does anyone know if the messenger particles for the strong and weak forces can escape?
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wisnij
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Re: Black Holes and Gravity

Sirry wrote:One thing I have never understood about black holes is why light can't escape them while gravity can. Photons and gravitons are both massless particles and I know photons at least can interact with each other (can't they?), so why are gravitons immune to other gravitons? Does anyone know if the messenger particles for the strong and weak forces can escape?

Well, for starters, gravitons don't exist in general relativity; there gravity is a purely geometric effect. But the same question could be asked about other fields which do exist, like electric charge, so it's not a very satisfying answer.

A better way to think about it is: imagine looking at a black hole as a clock is falling into it. Someone falling alongside that clock will see it cross the event horizon in a finite amount of time (the "proper time"), but from your perspective outside the black hole the clock will never reach the horizon. It will appear to fall more and more slowly, and tick more and more slowly, until it has virtually frozen in place and in time. This is because the light leaving the clock takes longer and longer to reach you the further it falls into the gravity well.

Now imagine watching a star collapse into a black hole. Just like the clock which appears to "freeze" right above the event horizon, the infalling star-stuff also remains visible from your point of view. It appears to hover there, just above the horizon, forever -- where its gravity can still get to you.
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antonfire
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Re: Black Holes and Gravity

There's a lot of really cool (and well-researched) stuff about black holes here: http://casa.colorado.edu/~ajsh/schw.shtml

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tzar1990
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Re: Black Holes and Gravity

IIRC, photons have a very tiny mass (because they have energy and the whole e=mc^2 thing), and that gravitons don't definitely exist.
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Re: Black Holes and Gravity

Sirry wrote:One thing I have never understood about black holes is why light can't escape them while gravity can. Photons and gravitons are both massless particles and I know photons at least can interact with each other (can't they?), so why are gravitons immune to other gravitons? Does anyone know if the messenger particles for the strong and weak forces can escape?

Just for completeness, photons don't interact with each other, while gravitons do.

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LoopQuantumGravity
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Re: Black Holes and Gravity

wisnij wrote:Well, for starters, gravitons don't exist in general relativity; there gravity is a purely geometric effect. But the same question could be asked about other fields which do exist, like electric charge, so it's not a very satisfying answer.

Yeah, the EM field doesn't interact with itself, and neither does the gravitational force. The nuclear forces do, however, and that's why they're short range forces! Photons can interfere with each other (and so, presumably, could gravitons), but they do not interact.

A better way to think about it is: imagine looking at a black hole as a clock is falling into it. Someone falling alongside that clock will see it cross the event horizon in a finite amount of time (the "proper time"), but from your perspective outside the black hole the clock will never reach the horizon. It will appear to fall more and more slowly, and tick more and more slowly, until it has virtually frozen in place and in time. This is because the light leaving the clock takes longer and longer to reach you the further it falls into the gravity well.

Now imagine watching a star collapse into a black hole. Just like the clock which appears to "freeze" right above the event horizon, the infalling star-stuff also remains visible from your point of view. It appears to hover there, just above the horizon, forever -- where its gravity can still get to you.

This is only sort-of true. There will be a last photon to be emitted from the black hole (though, IIRC, you need to include photons being quantized to do this). Not only this, but as time goes on (to the "far-away" reference frame) the light emitted is more and more redshifted, so even ignoring the last part, the energies of emitted photons will be asymptotically small for large times.

edit:
tzar1990 wrote:IIRC, photons have a very tiny mass (because they have energy and the whole e=mc^2 thing), and that gravitons don't definitely exist.

No and no. Photons have no mass, and there's no reason to think that gravitons don't exist.
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antonfire
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Re: Black Holes and Gravity

To elaborate on that a little, tsar1990, physicists rightly don't like to say things like "the mass of an object increases as it speeds up", because that leads to a whole lot of confusion. "The mass" of an object means its rest mass. It does not depend on reference frame. Photons have no mass. They do have energy, which is what determines things like gravitational attraction.

Anyway, back to the original question: here's another link which seems to offer an explanation that's different from what's been said: http://www.faqs.org/faqs/astronomy/faq/ ... on-11.html
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btilly
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Re: Black Holes and Gravity

antonfire wrote:To elaborate on that a little, tsar1990, physicists rightly don't like to say things like "the mass of an object increases as it speeds up", because that leads to a whole lot of confusion. "The mass" of an object means its rest mass. It does not depend on reference frame. Photons have no mass. They do have energy, which is what determines things like gravitational attraction.

To complicate that a little, most physicists don't like to say that because of convenience. However a few do because it leads to conceptual elegance. See, for example, The Feynman Lectures on Physics for an example of a treatment where mass is taken to mean something other than rest mass.
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Re: Black Holes and Gravity

On a related note, why are photons pulled by gravity if they have no mass?

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Re: Black Holes and Gravity

One answer is the equivalence principle. If you were in an accelerating elevator, and shone a flashlight on the opposite wall, you'd see the beam bend, if you weren't so preoccupied by being a pile of chunky salsa on the floor. Being in an accelerating frame is (locally) indistinguishable from being in a gravitational field.

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Re: Black Holes and Gravity

Sirry wrote:One thing I have never understood about black holes is why light can't escape them while gravity can. Photons and gravitons are both massless particles and I know photons at least can interact with each other (can't they?), so why are gravitons immune to other gravitons? Does anyone know if the messenger particles for the strong and weak forces can escape?

You're comparing real photons to virtual gravitons. Virtual force particles are the particles that mediate the field. Real force particles are the quantizations of waves. Virtual photons can escape black holes as evidenced by black holes that have charges. My understanding is that real gravitons cannot escape a black hole. If you created a gravitational wave inside a black hole, it would not propagate past the event horizon.

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Re: Black Holes and Gravity

J Spade wrote:On a related note, why are photons pulled by gravity if they have no mass?

The GR answer is that they aren't "pulled" by gravity. Photons will always travel in a "straight line". It's just that gravity curves space and so changes what a straight line is.

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Re: Black Holes and Gravity

LoopQuantumGravity wrote:
wisnij wrote:Now imagine watching a star collapse into a black hole. Just like the clock which appears to "freeze" right above the event horizon, the infalling star-stuff also remains visible from your point of view. It appears to hover there, just above the horizon, forever -- where its gravity can still get to you.

This is only sort-of true. There will be a last photon to be emitted from the black hole (though, IIRC, you need to include photons being quantized to do this). Not only this, but as time goes on (to the "far-away" reference frame) the light emitted is more and more redshifted, so even ignoring the last part, the energies of emitted photons will be asymptotically small for large times.

Sure, which is why the black hole is black (modulo some Hawking radiation). I only meant "visible" in the "still perceivable from your reference frame" sense; a distant observer will never measure it crossing the event horizon. But fields don't get redshifted.
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LoopQuantumGravity
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Re: Black Holes and Gravity

wisnij wrote:
LoopQuantumGravity wrote:
wisnij wrote:Now imagine watching a star collapse into a black hole. Just like the clock which appears to "freeze" right above the event horizon, the infalling star-stuff also remains visible from your point of view. It appears to hover there, just above the horizon, forever -- where its gravity can still get to you.

This is only sort-of true. There will be a last photon to be emitted from the black hole (though, IIRC, you need to include photons being quantized to do this). Not only this, but as time goes on (to the "far-away" reference frame) the light emitted is more and more redshifted, so even ignoring the last part, the energies of emitted photons will be asymptotically small for large times.

Sure, which is why the black hole is black (modulo some Hawking radiation). I only meant "visible" in the "still perceivable from your reference frame" sense; a distant observer will never measure it crossing the event horizon. But fields don't get redshifted.

Yeah, but the last photon still leaves in a finite amount of time (for any observer). IIRC, in a "reasonable" amount of (far-away) time, but I don't recall how to calculate it offhand, and don't really feel like searching through MTW or something to find a formula...

All this qualifying of reference frames reminds me of the horror of taking tests in relativity from someone whose native language is not English! At least include a diagram or something....
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Re: Black Holes and Gravity

As I have stated before, I am uneducated, and as such am having a little trouble understanding how gravity works.

I had a strange idea, and would like someone to explain it to me: Since all astral bodies are assumed to be formed from gas left over from stars forming. Could there be a direct relation between black holes and the birth of astral bodies (as opposed to the death of a star and the birth of a black hole).
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Re: Black Holes and Gravity

You're associating one phenomena in one part of the universe with another, and while there is most certainly a statistical relationship, density of matter within a galaxy is one factor that comes to mind, there is no reason to suggest that every time a black hole is formed a star is formed, or something like that. All phenomena are local until proven otherwise, so there's no cosmic equivalence of "every time a bell rings, an angel gets its wings." Correlation yes, but not causation.
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LoopQuantumGravity
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Re: Black Holes and Gravity

Anpheus wrote:You're associating one phenomena in one part of the universe with another, and while there is most certainly a statistical relationship, density of matter within a galaxy is one factor that comes to mind, there is no reason to suggest that every time a black hole is formed a star is formed, or something like that. All phenomena are local until proven otherwise, so there's no cosmic equivalence of "every time a bell rings, an angel gets its wings." Correlation yes, but not causation.

Although it is the case that events that trigger star formation could trigger the formation of very large stars that could collapse into black holes, leading to areas of high star formation having more black holes than you might expect. I believe this has been observed, but it is not always the case, and the size of stars formed during various processes that trigger formation isn't fully understood.
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aoanla
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Re: Black Holes and Gravity

Of course, black holes can't actually form in a finite time from the perspective of an external observer, so we don't really have to worry about any of this.

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Re: Black Holes and Gravity

Ok, so we assume that Earth was formed from swirling cloud remnants of the sun's creation, if I recall the theory correctly. But I seem to recall the possibility of black holes existing virtually anywhere in various sizes. My question is, could a black hole be the catalyst in the creation of an astral body?

Is it feasible that black holes create stars and not the other way around?
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Re: Black Holes and Gravity

ekzrated wrote:I had a strange idea, and would like someone to explain it to me: Since all astral bodies are assumed to be formed from gas left over from stars forming. Could there be a direct relation between black holes and the birth of astral bodies (as opposed to the death of a star and the birth of a black hole).

when stars die, all the matter that makes it up is ejected and forms a nebula. within that nebula, all that mass can form into new planets/stars.
ekzrated wrote:Is it feasible that black holes create stars and not the other way around?

its a cycle.

also, addressing this to the general population, how does gravity curve space exactly?
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Re: Black Holes and Gravity

I can only use the commonly used illustration by saying, in the same way an orange would bend stratched out netting or fabric when placed on it, creating a curved funnel shape. That's not very technical but a bit easier to visualise.

Or are you asking how it works?

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Re: Black Holes and Gravity

So, is Earth's gravity generated at it's center, or is it generated at it's outer shell? It seems to me that it must be generated at it's center, and expands outwards.
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Re: Black Holes and Gravity

It's generated by every particle cumulatively.
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Re: Black Holes and Gravity

I think I need a crash course on gravity. Keep the info coming, please!
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Re: Black Holes and Gravity

Each individual particle possesses its own properties like we said, and as is known now, particles are merely an approximation, but they're very useful and we'll continue along that idea so I don't confuse you.

Each particle has states like its spin, its momentum, it's velocity, electric charge, and more descriptors than that actually. Its mass is a peculiar one we haven't really narrowed down, that is, we don't have a really good idea for -why- things have the mass they do.

It's also interesting to note that Gravity is the weakest force. Strong and weak nuclear forces cancel each other out at very short distances so you don't observe them. But even at the large scale, electromagnetic forces are much, much stronger than gravity. A tiny magnet is able to stick to your fridge or anything in particular for thousands of years and oppose the force of gravity, the force of the whole earth pulling on it. That magnet stays put because electromagnetic forces are much stronger. An earth-sized magnet would be unimaginably powerful, and would be able to attract an earth-sized magnet of opposite charge from light years away.

The easiest way to imagine gravity is that you've got this infinitely large rubber sheet, and to figure out the effect of adding particles, you just add a ball bearing of the right mass to the right spot on the sheet until it curves. That sheet is "spacetime" and if you roll objects on it, you'll see that they tend to fall into areas where there's a lot of mass: that's what we mean by spacetime being curved. The idea is that the ball is traveling in a straight line the whole time, but the course that line takes depends on the velocity of the ball. A really slow moving ball will tend to fall into those curved areas much more easily, and a really fast moving ball has more inertia and will travel along a route that we would consider to be "more straight" from our perspective. The straightest lines of all come from photons, which travel very very fast, and the more energetic they are, the less they will stray from what we consider a straight line.

That picture shows an object (the earth, for example) orbiting the sun. The object is traveling in a straight line, except the curvature keeps pulling it in. As a result, it follows a circular or elliptic (oval-shaped) path around the sun because as it's rolling it keeps wanting to roll in what we observe as a straight line, but the curvature is pulling it toward the sun at nearly the same rate. The idea is that the only true straight lines are the ones that objects actually travel along for any given velocity, and that no universal ruler exists with which to measure a straight line. (And no universal clock exists either, to measure distance over time arbitrarily.)
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ekzrated
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Re: Black Holes and Gravity

So why does Earth's magnetic polarity change? And what relation does it have with the rest of the astral bodies in our solar system?

I have a feeling we're just tapping the edge on this...
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Re: Black Holes and Gravity

Sure, the earth has experienced several recorded magnetic polarity changes. It's an extremely complex process that is not fully understood by physicists / geologists at this time.

Edit: Oh, no, the Earth is very close to electrically neutral even though it has a magnetic field. The magnetic field or dynamo is the result of moving charged particles, but the whole of the earth is relatively close to neutral. Most matter in the universe is very close to electrically neutral because of the way electric charges worked, if there were, for whatever reason, a very large positively charged region, then that region would attract negative charged particles toward it until it became very close to neutral.
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Re: Black Holes and Gravity

Anpheus wrote:Sure, the earth has experienced several recorded magnetic polarity changes. It's an extremely complex process that is not fully understood by physicists / geologists at this time.

Edit: Oh, no, the Earth is very close to electrically neutral even though it has a magnetic field. The magnetic field or dynamo is the result of moving charged particles, but the whole of the earth is relatively close to neutral. Most matter in the universe is very close to electrically neutral because of the way electric charges worked, if there were, for whatever reason, a very large positively charged region, then that region would attract negative charged particles toward it until it became very close to neutral.

So, does the Earth spin because of it's core spin or because it's being influenced by other astral bodies? And what does that mean for the moon? I was sitting in an oceanography class, and had a bunch of thoughts pop into my head as the instructor was showing slides illustrating continental drift. One, being the thought that black holes are at the center of astral bodies. Another thought was the idea of the multiverse, if we were to grow large enough to see planets as molecules, what would the rest of the universe look like? Then I wondered if all the major sciences compared and correlated data regularly. By the way, I'm insane.
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Re: Black Holes and Gravity

Conservation of momentum. The solar system started out spinning, and so when the matter condensed into the sun and the planets they began spinning as well relative to the rest of the solar system. Over billions of years tidal locking caused the moon to always 'face' us, it's a complex gravitational process in which a closely bound two-body system results in the removal of rotation of one body relative to the other (there will be a plainly defined obverse and reverse face of one body.)

Don't consider yourself insane, you're just exploring your thoughts. We can say with relative certainty that there is no black hole at the center of most astral bodies, but cannot empirically determine it. The reason is such a black hole would have to be so tiny as to gain mass, and thus radius, so slowly that it wouldn't affect any of our observations. Such a small black hole would grow very slowly, and so we know we can estimate that the black hole must have been small enough when the earth formed that it would not affect our observations over the course of four billion years.
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Re: Black Holes and Gravity

Anpheus wrote:Conservation of momentum. The solar system started out spinning, and so when the matter condensed into the sun and the planets they began spinning as well relative to the rest of the solar system.

What is the solar system's spin in relation to the rest of the galaxy?

We can say with relative certainty that there is no black hole at the center of most astral bodies, but cannot empirically determine it. The reason is such a black hole would have to be so tiny as to gain mass, and thus radius, so slowly that it wouldn't affect any of our observations. Such a small black hole would grow very slowly, and so we know we can estimate that the black hole must have been small enough when the earth formed that it would not affect our observations over the course of four billion years.

I'm assuming you mean what happens at the event horizon, right? The idea that matter would seem to be suspended in time even though it's being sucked into the hole, right? So, do we know if black holes only increase in size? Is it possible that since they're bending space, that this could become a "pocket" and fill up with mass? If that were the case, anything not sucked up into the even horizon would be left spinning, and compressing, evenutally gathering enough mass to create an astral body?

Anpheus wrote:Don't consider yourself insane, you're just exploring your thoughts.

Thanks, but I am insane. I tend to think that all things can be possible, including that science is wrong at times, and needs to be revisited.
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Re: Black Holes and Gravity

Anpheus wrote:Conservation of momentum. The solar system started out spinning, and so when the matter condensed into the sun and the planets they began spinning as well relative to the rest of the solar system. Over billions of years tidal locking caused the moon to always 'face' us, it's a complex gravitational process in which a closely bound two-body system results in the removal of rotation of one body relative to the other (there will be a plainly defined obverse and reverse face of one body.)

Minor correction, conservation of angular momentum, not momentum.

Otherwise correct. And it explains why virtually everything rotates in the same direction.
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Re: Black Holes and Gravity

It's not insane to believe we should continue to test our hypothesis about how the universe works.

The solar system's rotation occurred in a manner similar to the individual stars: when the solar system condensed out of a nebula, the conservation of angular momentum (thanks for the correction, btw) caused the solar system to begin rotating.

There is a curiosity (that some consider evidence for dark matter) that the galaxy does not seem to have as much mass as we observe. That is, it's as though there's this huge, relatively evenly distributed mass throughout the galaxy, the origin of which we can't detect. So we certainly don't know -everything- yet.
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Re: Black Holes and Gravity

I was talking to my oceanography teacher today after his discussion on tectonic plates. I imagine the planet as a huge mass of different elements, all turning, and churning within each layer, with the more dense of these, of course, closer to the center. Wouldn't it make sense that something with a very strong gravitic pull be at the center? Maybe not something like a star-sized black hole, but maybe something similar and smaller. Even taking into account the direction of rotation, wouldn't that account for how celestial bodies are formed? We know that black holes radiate energy as do most planets. As for what kind of energy is being radiated I imagine depends on what materials it is composed of (or in the case of the black hole, which materials have been sucked in). This could be why the Earth's surface is constantly at motion. Does this seem like it's possible?
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Re: Black Holes and Gravity

Anpheus wrote:There is a curiosity (that some consider evidence for dark matter) that the galaxy does not seem to have as much mass as we observe. That is, it's as though there's this huge, relatively evenly distributed mass throughout the galaxy, the origin of which we can't detect.

I've been thinking about this too. Space cannot be empty. In my mind, it seems like everything is floating in something else. Even space has to be made up of something. Kind of like walking through a room in your house. You're actually walking through quite a lot of stuff. Air, dust particles, bacteria, etc. When you swim, you're still going through something, water, minerals, dust, fish poop particles, microbes, etc. I imagine that space is the same way. Didn't the first Astronauts encounter space particles when they left the Earth's atmosphere? I can't recall the names for them, but I can't beleive that space is a vaccum in the sense that there's nothing there. That's the easiest way I can explain it.
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EricH
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Re: Black Holes and Gravity

ekzrated wrote:I was talking to my oceanography teacher today after his discussion on tectonic plates. I imagine the planet as a huge mass of different elements, all turning, and churning within each layer, with the more dense of these, of course, closer to the center. Wouldn't it make sense that something with a very strong gravitic pull be at the center?

Actually, the gravitational pull decreases, as you move deeper into the planet; what keeps increasing is the pressure, from all the material that's now above you. So if imagine yourself in a cave that's only a few miles from the center of the Earth (a cave that's somehow strong enough not to collapse under the pressure of thousands of miles of rock), you would find it difficult to tell which direction is down; it's only microgravity at that point.

ekzrated wrote:Maybe not something like a star-sized black hole, but maybe something similar and smaller. Even taking into account the direction of rotation, wouldn't that account for how celestial bodies are formed?

A microscopic black hole, in a cloud of interstellar dust or gas, would act as a 'seed', drawing material toward itself slowly; if the gas is sufficiently dense compared to the size of the black hole, material would be piling up around the black hole faster than it could be swallowed. I can't say it's impossible, but it's not necessary to explain the universe as we see it--which means if you want to move the idea from the realm of idle speculation to scientific hypothesis, you have to actually do work; that is, gather some kind of evidence....

ekzrated wrote:We know that black holes radiate energy as do most planets. As for what kind of energy is being radiated I imagine depends on what materials it is composed of (or in the case of the black hole, which materials have been sucked in). This could be why the Earth's surface is constantly at motion. Does this seem like it's possible?

That the internal heat of Earth comes from Hawking radiation of a microscopic black hole? There's too much of it. A black hole big enough to radiate that kind of heat would have swallowed the planet a long time ago.

ekzrated wrote:I've been thinking about this too. Space cannot be empty. In my mind, it seems like everything is floating in something else. Even space has to be made up of something. Kind of like walking through a room in your house. You're actually walking through quite a lot of stuff. Air, dust particles, bacteria, etc. When you swim, you're still going through something, water, minerals, dust, fish poop particles, microbes, etc. I imagine that space is the same way. Didn't the first Astronauts encounter space particles when they left the Earth's atmosphere? I can't recall the names for them, but I can't believe that space is a vacuum in the sense that there's nothing there. That's the easiest way I can explain it.

Technically, you're right, it's not a complete vacuum; there are particles everywhere in space, they're just spaced farther apart than we're used to on Earth--on the order of millimeters, instead of nanometers. And that's just the Newtonian model--quantum mechanics adds a host of virtual particles.

In the same breath that I tell you you've got hold of a germ of truth, I have to add a caution, though--you've got a tendency to trust your intuition more than evidence. Very Aristotelian, but he was operating without the benefit of the scientific method. Doubt what you're told, but doubt what you guess, as well. Sometimes, the way things really work doesn't seem to make a lot of sense, and the solution isn't to just disbelieve the facts that you disagree with, it's to come up with a new explanation that incorporates both new facts and old ones. Experienced scientists still struggle with this sometimes, because there's a tendency to think that 'the explanation I've been using' is one of the facts that has to be included, when it's actually just an opinion, and probably one that's getting in the way.
Pseudomammal wrote:Biology is funny. Not "ha-ha" funny, "lowest bidder engineering" funny.

ekzrated
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Re: Black Holes and Gravity

Thanks. Again, I'm uneducated, and I dont' think I'll ever make it as far as being a physicist, or mathematician.
EricH wrote:That the internal heat of Earth comes from Hawking radiation of a microscopic black hole? There's too much of it. A black hole big enough to radiate that kind of heat would have swallowed the planet a long time ago.

I didn't mean that the radiation came from the black hole. I should have stated that more clearly. I meant it as the starting point for the radiation. But definitely thank you. I always welcome knowledge.
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EricH
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Re: Black Holes and Gravity

ekzrated wrote:Thanks. Again, I'm uneducated, and I dont' think I'll ever make it as far as being a physicist, or mathematician.

That needn't stop you from thinking like one.... You've already got the 'come up with a crazy new idea' part down pat.
Pseudomammal wrote:Biology is funny. Not "ha-ha" funny, "lowest bidder engineering" funny.

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Re: Black Holes and Gravity

Thanks. I'll keep on thinking. And I suppose, one of these days I'll actually read the forum rules. I can get kind of... explosive... in other forums, that is.
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Re: Black Holes and Gravity

Anpheus wrote:The straightest lines of all come from photons, which travel very very fast, and the more energetic they are, the less they will stray from what we consider a straight line.

I'm pretty sure this is wrong. All photons will follow a geodesic regardless of their energy.

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Re: Black Holes and Gravity

EricH wrote:Actually, the gravitational pull decreases, as you move deeper into the planet; what keeps increasing is the pressure, from all the material that's now above you