Thread: Relativity Question (s)

I'm still trying to understand relativity. Particularly its time predictions about gravity are puzzling.

Suppose we have the following situation:

A planet the size of Earth, approaches a super-massive black hole. After entering the event horizon, the planet is sucked in. As it's falling toward the center, it reaches C (probably), and then keeps falling until it gets close enough that its own gravity begins to have a measurable effect on the singularity.

From that moment, and for another C/distance amount of time, the singularity is being pulled by the Earth's gravity as well, right?

Now, since the acceleration of gravity caused by the Earth's mass is the same for all objects, regardless of mass, doesn't that mean that the super massive black hole will actually gain in velocity, toward the direction the Earth is approaching it from?

Since the Earth's mass is negligible, in terms of momentum, compared to the black hole, the impact won't affect the singularity's velocity much, will it?


So my question: Is this an unbalanced action/reaction. I mean, does this violate Newton's 3rd law? ("For every action, there must be an equal and opposite reaction.")

Is the total momentum of the Earth/ Black hole pair greater after they collide than it was before?

I think what I'm really wondering is how relativity accounts for it all. Certainly the Earth sized object will gain relativistic mass as it approaches the black hole, even though it can only be accelerated up to C. That might add to the force of the impact.

Then, there's the time dilation effect of the singularity. Is the acceleration caused by Earth's gravitational field measured relative to the singularity's time reference? (Which would make it almost zero, or maybe even absolutely zero)?

Now, since the acceleration of gravity caused by the Earth's mass is the same for all objects, regardless of mass, doesn't that mean that the super massive black hole will actually gain in velocity, toward the direction the Earth is approaching it from?

Before I answer this, kojax, I just want to know exactly how the Earth got into this. The first part of your post mentions a planet the size of the Earth falling into a black hole. In no way does it mention anytrhing about there being a relation to Earth. Just osmething I was confused by.

Now, I answer this.

As it's falling toward the center, it reaches C (probably), and then keeps falling until it gets close enough that its own gravity begins to have a measurable effect on the singularity.

Firstly, this cannot happen at all. Why? Because even before reaching the singularity, the planet will have been ripped into tiny shreads by the enormous gravity of the singularity. Also, the only noticeable weffect that can be seen is the expansion of the black hole.

Also, the singularity will be attracted, but as long as it has a larger mass, inertia will countereffect much motion in it.

Further, nobody knows how gravitational fields actually interact with each other. This problem, which I've been trying to solve, is present in classical general relativity as well; points of two fields intersect, and there is no real way of finding out the change in gravity. This is the problem that renders quantised general relativity infinite.

And the speed of the planet will never reach the speed of light. That is a given; it can go close to it, but never at it.

So my question: Is this an unbalanced action/reaction

Nope. How can you think so? The same question can be asked for why the Earth does not move towards the moon, while the opposite is true. Answer: the Earth has a larger mass than the moon. The forces felt will be equal, yet the effects will be different.

Is the total momentum of the Earth/ Black hole pair greater after they collide than it was before?

Certainly. But don't expect it to be conserved for eternity; Hawking radiation will eventually kill off the momentum by transferring it to an outgoing particle. Further, there won't be much velocity of the black hole.

I think what I'm really wondering is how relativity accounts for it all. Certainly the Earth sized object will gain relativistic mass as it approaches the black hole, even though it can only be accelerated up to C. That might add to the force of the impact.

Good point. But the momentum will ultimately vanish, and the black hole will destroy Earth before it even manages to go so far as to invade a black hole.

Then, there's the time dilation effect of the singularity. Is the acceleration caused by Earth's gravitational field measured relative to the singularity's time reference? (Which would make it almost zero, or maybe even absolutely zero)?

Acceleration towards whom? The singularity? Remember the singularity is not in motion, so for it, the accelearation will be measuread as very fast, if you're talking of acceleration of Earth, and close to zero, if you're talking of acceleration of the singularity itself.

What I'm trying to figure out is how 2 objects of extremely different mass might interact. I think you're right that I should have left the Earth out of it.


How about if I refine this. 2 black holes, one very small compared to the other, collide.

The small black hole will be accelerated to very nearly C (but not quite) and travel at that speed for the rest of its trip. At a certain point, the two black holes will be close enough to each other that the smaller black hole's gravity begins to have a noticeable effect on the larger black hole.

Since the acceleration of gravity on an object is independent of its mass, does this mean the very large black hole will be noticeably accelerated by the small one? (Because for C/distance amount of time, it was near the gravitational field of the small black hole.)


Suppose the two black holes are of astronomically different sizes. Say the large one is a billion times more massive than the small one, or something like that.

I would imagine that the smaller singularity will form a decaying orbit around the big one until they merge if its trajectory is not dead on, but I would also imagine that the distance between them can only get smaller. As for the acceleration: It would be the same as an object being dropped on earth. The small singularity will accelerate at the big one at the same rate as a hypothetical super planet that would not break up. The rate at which the big singularity falls towards the small one will be greater than the planet, but if you say the big one is a billion times as massive as the small one, then the difference will be very minute because of its great inertia. The two singularities would probably be discs if they are spinning and the mutual attraction might warp the two dimensional disks, but other than that I don't know of any weird things (that doesn't say much though) that would happen to two singularities.

The small black hole will be accelerated to very nearly C (but not quite) and travel at that speed for the rest of its trip.At a certain point, the two black holes will be close enough to each other that the smaller black hole's gravity begins to have a noticeable effect on the larger black hole.

I just want to mention there are limits to how far this can go,. The event horizon bars us from ever finding out what happened after a black hole went past it.

Since the acceleration of gravity on an object is independent of its mass, does this mean the very large black hole will be noticeably accelerated by the small one?

The acceleration of gravity is not independent of mass, my friend. It is simply independent of a mass entering the graviational field where it operates. It is determined by



where G is the graviaitonal constant, M the mass of the object causing the field and d is obviously distance.

Also, remember that the acceleration of the black hole will not be very huge, if we're talking about two different masses. So, no, the black hole will not be noticably accelerated by the first black hole.

I was thinking it was GMm/R^2

Where M is one mass, and m is the other. In any pair of bodies, then, the acceleration of the second, due to the gravity of the first, is independent of the second's mass.

Why?

Because A = F/M (M is the accelerated object's mass, F is the force applied.)

F = GMm/R^2 = M * Gm/R^2 (Where M is the accelerated object's mass, and m is the mass of the object exerting the force of acceleration)

So, A = ( M * Gm/R^2) / M = Gm/R^2

So, A = Gm/R^2

Notice that M is missing in the final equation for acceleration. This is Newtonian, of course. Relativity's version might vary somewhat.


This is where my concern comes from. Theoretically, at least according to this, the more massive black hole is still getting accelerated by the smaller one, even though the smaller one appears to have a negligibly small mass by comparison to it.

Theoretically, at least according to this, the more massive black hole is still getting accelerated by the smaller one, even though the smaller one appears to have a negligibly small mass by comparison to it.

It will accelerate, there is no doubt about it. However, its mass sufficiently renders the acceleration to be almost unobservable.

Inertia will prevent the second black hole from undergoing a noticeable acceleration.

That's my point. Apparently inertia plays no role whatsoever in deciding how much a nearby gravitational body will accelerate you.

If you're moving rapidly away or toward the other body (or if it's moving toward/away from you), then you will be spending less/more time inside its gravitational field, which will lessen the amount of acceleration imparted, but your mass does not impede the acceleration from gravity.


The only way the mass of the object being accelerated plays a role is if the gravitational force from that object's mass is moving the object that is accelerating it. (So it spends less or more time inside the accelerating object's gravity)


This is why this question puzzles me. The gravity of the super-massive black hole can't accelerate the less massive black hole to move faster than C. This would mean that the mass of the smaller black hole matters more than the mass of the larger black hole, because the smaller black hole is only going to be sucked in slightly faster (smaller and smaller increments approaching C) by a black hole a trillion times larger than it, as compared with a black hole only a billion times larger.


On the other hand, if the smaller black hole's mass were double, then the acceleration it imparted on the larger black hole would be double.

kojax,

hello : ), sorry I don't really understand what you are trying to do, but will try to add a thought.

I think you have gone wrong saying, or thinking, earth as a whole would approach the galaxies black hole. It will be crushed to very tiny pieces by the black hole, as the black hole was coming closer.

Steve

Moderator note:
Certian posts in this thread have been split off to a thread in pseudoscience.

So, to generalize:

1)

The rate gravity accelerates you does not depend on your mass/inertia. 2 objects of very different weights, dropped at the same time, will generally hit the ground at the same time (unless their air resistance is different)

2)

In theory this is true even of objects that are very large. If you put a pebble out in orbit at the same distance from the Earth as the Moon (but far from the Moon, so as not to be influenced by the Moon's gravity), it will experience the same amount of acceleration as the Moon is experiencing.

3)

Presumably, we could expect that even an object the size of a super massive black hole would obey this rule.

4)

It seems like, for this to happen, there would be an uneven exchange of momentum in some extreme cases.

The smaller object cannot be sucked in at a speed greater than C. So, the larger object will be under the influence of the smaller object's gravity for an amount of time related to distance/C.

Suppose the super massive black hole is accelerated by .0005 meters/second in that amount of time. When the smaller object hits it, we'll say the collision only changes its velocity by .0002 meters/second, because the smaller object is so much smaller.

That would mean the larger object and smaller object are both moving at .0003 meters/second in one direction. So, maybe the forces didn't balance out to be equal and opposite?

I'm pretty sure relativity has some kind of rules that prevent this from happening, but I'm not sure I understand what they are.

Originally Posted by kojax

I'm still trying to understand relativity. Particularly its time predictions about gravity are puzzling.

Suppose we have the following situation:

A planet the size of Earth, approaches a super-massive black hole. After entering the event horizon, the planet is sucked in. As it's falling toward the center, it reaches C (probably), and then keeps falling until it gets close enough that its own gravity begins to have a measurable effect on the singularity.

From that moment, and for another C/distance amount of time, the singularity is being pulled by the Earth's gravity as well, right?

Now, since the acceleration of gravity caused by the Earth's mass is the same for all objects, regardless of mass, doesn't that mean that the super massive black hole will actually gain in velocity, toward the direction the Earth is approaching it from?

Since the Earth's mass is negligible, in terms of momentum, compared to the black hole, the impact won't affect the singularity's velocity much, will it?


So my question: Is this an unbalanced action/reaction. I mean, does this violate Newton's 3rd law? ("For every action, there must be an equal and opposite reaction.")

Is the total momentum of the Earth/ Black hole pair greater after they collide than it was before?

I think what I'm really wondering is how relativity accounts for it all. Certainly the Earth sized object will gain relativistic mass as it approaches the black hole, even though it can only be accelerated up to C. That might add to the force of the impact.

Then, there's the time dilation effect of the singularity. Is the acceleration caused by Earth's gravitational field measured relative to the singularity's time reference? (Which would make it almost zero, or maybe even absolutely zero)?

Read the following, this should help I wrote it a while ago and it explains relative points of view:

Gravity has a strange effect on light. It will couple with light, and bend it around large masses. The idea arose from Einstein, and on the 9th of November in 1919, light was seen to bend around the sun during an eclipse. The times reported on the discovery, 'space would no longer be looked at as extending indefinitely in all directions. Straight lines would not exist in Einstien’s space. They would all be curved and if they traveled far enough they would return to their starting point.'

Finding that our universe was not a Euclidean flat spacetime was indeed a marvel of physics. It showed that space was highly twisted and curved into time, and that gravity itself was a product of these bends in time and space through the presence of matter. It was these types of distortions that led the way for a new prediction in Einstien’s 'theory of relativity.' It predicted a black hole - a whopping gravitational body, unto which nothing can escape its grasp. The center of a black hole has perfect infinite curvature; and it is here that the distortions of space and time become so highly stressed it can actually rip a hole in the fabric of spacetime itself. This is the singularity at the center of the black hole - but it wasn't the same as the singularity of the big bang.

A black hole has this strength because it is a dense concentration of mass. Actually, this mass is so dense, it actually drags space and time around with it, and the curvature it produces is fantastic. For a space shuttle to leave earth's gravitational pull, it needs to have a speed that is strong enough to make the 'escape velocity.' You can imagine the escape velocity is stronger the closer you are to the earth's core. To leave earth, you need a constant speed of something like .

Now, take the speed of a photon (light) - the fastest particle known. The speed of light is very hard to grasp - saying that it travels miles a second isn't always easy to reconcile; just remember, the sun is million km away, and it takes a photon a little over minutes to reach us!

Now imagine a massive body in space with such a high concentration of mass, it is actually able to stop light itself - this is a black hole - and this must mean it has an escape velocity of light! A photon, traveling quite happily will be abruptly slowed down until it reached zero-speed. All Luxens (that is particles with a speed of light v=c) and obviously all Bradyons (particles with a velocity under the speed of light v<c) would inexorably be trapped by the intense pull of the black hole... only a hypothetical particle called the 'Tachyon' could escape its pull, quite easily actually. A Tachyon is a particle that moves faster than light .

The idea that an object with a large concentration of dense mass goes right back to the 18th centaury - just after Einstein developed his important relativity theory. It was a physicist Karl Schwarzschild (that is were the black hole gets the name, ‘Schwarzschild radius’ from) who discovered a mathematical solution to the equations of the theory that described such an exotic object. It was only later in the 1930's that theorists Oppenheimer, Volkoff and Snyder took the theory seriously.

Certain stars that cannot support itself against its own gravitational field have a special destiny ahead of them - a star that does this will collapse and form into a black hole. It was John A. Wheeler that coined the term 'black hole' - before that, it had been called 'frozen stars.' Our star, as big as it is, will not collapse until another 5-6 billion years. Altogether, our sun will have lived a total of 11 billion years, and this is quite a good lifespan. Other stars will not be so lucky. They would collapse into a spherical black hole in half that time.

Let's consider a star that is 666,000 times that of the mass of planet earth - this star will have a lifespan of about 5.5 billion years. And there will be much heavier stars out there. You can imagine, stars with a lesser life span with 5.5 billion years as a lifespan would not have given earth enough time to develop life properly; in fact, if science is correct, there wouldn't have been enough time to allow human life to form, considering science informs us that human life did not appear until only about 100,000 years ago, and the earth being 6 billion-odd years old itself. This is another factor that makes human life on earth rather extraordinary.

Physicist Stephen Hawkings, arguably the best mind in the world, has spent much of his time working on the theory of Black Holes. His contribution into the hypothetical black hole is astounding, and if you want more information on his work, i advise you to read his book, 'A Brief History of Time.'
A black hole has something called 'the event horizon' - the event horizon is the spherical surface, or boundary of the black hole. This is the point, that if anything passes it, nothing can escape (apart from a Tachyon mentioned earlier), or unless an object began its journey from the interior; this is because of a strange rule: You cannot pass the surface twice.

It was this reason it was called the event horizon, just like a sunset horizon - you can travel towards it but never quite reach it, or at least, this is what it would be like for an observer sitting comfortably away, watching me traveling towards the black hole... It would seem to take an infinitely long amount of time, and it would look like as if the closer i got to it, the slower i would be in momentum, until it looked as if i had stopped completely. This is because time becomes highly dilated between the traveler and the observer who is a bit away - this is the bizarre effect of relativity. We must take these facts into consideration, when one moves closer to the weird singularity. If our calculations are singular, this means that aspects, like a time interval, or space itself take on infinite values. If this is hard to imagine or a little tedious on the mind - do not fret - anything you don't understand just move on and tackle it later.

If one passed the event horizon, you will inevitably move closer and closer to the singularity in its center, moving faster and faster because space is dragging you closer to the speed of light.

To an observer who is sitting comfortably far away from the event horizon, the hole itself appears static. However, if we moved a little closer to the boundary, it would become visible that the hole itself has a remarkable velocity - in fact, a black hole spins with a velocity of the speed of light. Once inside of the black hole, spacetime are distorted to such a degree, that space and time switch roles (more on this in next part). We could not jump into a nonrotating black hole - the force of the black hole would rip matter apart!

How big can a black hole be?

Most black holes will have formed from supernovae, so it is expected that they will be as big as a standard candle (usually depicted as bright white dwarfs - the remnant of stars) and much bigger, and if Stephen Hawkings is correct, each supergalaxy has a supermassive black hole at their centers. And if theory is correct, the universe itself has a supermassive black hole at its center, where all matter orbits over billion upon billions of years. And there is even a theory suggesting our universe is a black hole itself, based on the fact that our universe has a lot of mass, but isn't too dense. And if black holes do exist, Stephen Hawking believes we might be able to detect a small black hole, as it will radiate a glow... a natural lantern in space. I presume that black holes would also be more visible nearer stars. Light reflects off natural objects and creates the ability to see them. A black hole would absorb light, and it would become visible as a hole.
The attention black holes have received over the years is truly mind-blowing... let us just hope that the work does not go in vain, and that black holes do indeed exist. They should exist... after all, Cosmology and Relativity Theories predict them as real 'things out there'. Whether or not they are indeed portals into other universes is another thing... Though, if theory is right, a lot of physicists will be proven wrong; it would seem to indicate a universe without the collapse of the wave function, as we shall see later in part three.

Falling into a Black Hole

If black holes do actually exist, there is some debate as to whether a human could endure a trip into one – the reason why is because anything that falls into Black Holes get’s shredded into spaghetti. Why would we even want to jump into a black hole? Well, theory says that 'wormholes' which are topological openings inside the black hole might lead to other universes! This is the theory of parallel universes, and we shall see more on this theory in next part. It was John A. Wheeler who named these openings as wormholes. The problem is, if one does not enter a wormhole in the correct way, there is the chance that the object will be stretched apart.

It was in 1935, Albert Einstein and Nathan Rosen predicted that black holes themselves where natural bridges into another possible universe. This bridge from one world into another, came to be known as the 'Einstein-Rosen Bridge,' and most of the developments of this theory came from several physicists - some being Arthur Eddington, John Wheeler and Martin Kruskal.

So let's imagine i decided to jump into a spinning black hole inside a space ship... what would i see? Well, before i entered, i would see nothing spectacular. I would just see a big ball of darkness. I wouldn't even see it rotate at first - neither do i feel anything - i am in what is called a state of 'free-fall'.

Free-fall is when all the atoms and molecules i am made of are all being pulled at the same rate. Even my ship is being pulled at the same pace towards the black hole. A good way to compare this is with astronauts that orbit our earth - they too are in a state of free-fall.

Now i begin to pass the event horizon (remember that is the first boundary, or surface). Now something quite remarkable happens. The space coordinates switches roles with the time coordinate. What does this mean? Well, we move through space freely, back and forth without any problems, and when we consider time, that imaginary dimension, we tend to think we sweep along with it without recourse. Once i pass the event horizon space begins to drag me and my ship, and i begin to move in one direction only - that being forward - however, i begin to move through time backwards and forwards, just as easily as i had moved through the space dimension. In this case, we say that space has become 'timelike', and time has a 'spacelike' character - they are thus interchangeable given the correct conditions.

As i move closer and closer to the black hole, the force of gravity becomes stronger and stronger. Now, suppose my legs are closer to the dreaded center of the black hole, i will begin to feel as if my body was being stretched. A greater force will be pulling at my feet, than that of the force pulling at my head. This is called the 'gravitational tidal effect' - thus called because it is similar to the tidal effect on earth caused by the moon.

If i looked out of a window towards the singularity, i would see something rather interesting. The center will look like a dark sphere, with a halo of light surrounding it. This light is coming from another universe. And, if i looked back out of the event horizon, i might be fortunate enough to see the universe, and all of its history and future flash past me as if it took no time at all. I would see all the stars die out... most of them forming black holes, but they would not be visible to the naked eye. I might even see the universe undergo an 'omega point' (the end), as a 'Big Crunch' were everything is drawn back, or quite possibly by a 'Big Rip', were everything physical is ripped apart by the universal pressure of acceleration, (note however, someone outside of the black hole cannot see you).

Now i have crashed into the dreaded singularity, and i will no longer exist. Here, just like the Big Bang singularity mentioned in part one of chapter one, everything takes on infinite attributes - the laws of physics become invalid. However, you might not crash into the center. It is possible you can fall into the 'inner horizon' - this horizon is adjacent to the singular region. Here, space and time flows the correct way. In theory, you can float around in the inner horizon without ever crashing into the dreaded center.

Black holes are predicted to form from the collapsed states of certain large stars, about several times larger
than our star. They do so, because of gravitational acceleration, given by the formula;



Remember, a free falling object will have the force of gravity totally cancelled out as it’s that weak.

We know that from Newton’s Force Equation is derived as , where this also shows an inertial system to derive the acceleration due to gravity. So the gravitational acceleration is the mass of a gravitationally warped object M, and the distance d from it.

We use the same method to work out the mass of the earth. The is Newtons universal gravitational constant (6.7×10-11 m3/(kg sec2). We find the Earth's mass kilograms kilograms.

The smallest black hole need to be of Planck Mass at smallest size . The Compton Wavelength given as

~ of a black hole is proportional to its Schwartzchild Radius;

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– which leads to the solution of -

;

Very small black holes are very hot. This is because the decrease in size and magnification of density makes these little things extremely hot. A typical micro black hole would have a temperature of , which is [tex]200 GeV[tex], or about million times hotter than the sun!

We can measure the density, and radius of a black hole in a series of proportionalities. The radius of a black hole, even a micro black hole is directly proportional to its mass . And the density of a black
hole is found to be given by its mass divided by its volume .

If our universe is indeed a black hole, you might imagine we exist in the inner horizon. In fact, our universe may as well be a black hole. Now, if one passes by the singularity, we might be able to move out of the inner horizon and pass through a second inner horizon, and then by finally passing another outer horizon, we will have entered another universe - but i had better be careful. There is a very good chance that this universe is made up mostly of antimatter. If i come into contact with antimatter, me and my ship will explode in a flash of light.

I would like it known to my readers that Hawkings has changed his mind on the theory of Black Holes, as he no longer believes that it is possible for a spacetime traveler to jump into one and move into other universes… This was proposed because of a fundamental problem involving information. If information moved into a Black Hole, it would suggest that the information would be lost, but here lies the paradox, because information can never totally be lost. Thus instead, he now believes that information is ‘’mangled’’ and returned back into this universe through quantum tunneling. In fact, a more recent research into mathematics shows us that there actually needs not be any Black Holes at all! If any do exist, then they would have formed at the very beginning of time. But to keep things not too complicated, I will continue with the idea that it is all still possible, and this is based on one well-known fact: That is, that our mathematics could be formulating a lie, instead of the truth. Thus, as much as I like the idea that no one can travel into other universes, because I protest against the multiverse theory, I must admit still that we may have it all very wrong, because mathematics may be pointing to the wrong conclusions… Who knows but God? We will certainly never achieve any unification, as I believe. Such knowledge must be left to God alone > Thus, for the sake of it, let us imagine we have got it wrong, and that universal spacetime traveling is possible…

One small nitpick. Black holes have been shown to exist. They're not considered just theoretical anymore.

Originally Posted by MagiMaster

One small nitpick. Black holes have been shown to exist. They're not considered just theoretical anymore.

No. Wrong.

We have never observed (or for better terms directly wittnessed) the existence of a black hole, as far as we can tell. The electron could be a black hole, as predicted by Dr Greene, but i think that's the best you will get out of it. We speculate however cosmological structures may have them at their centers, however, again, we have not detected a single black hole, not even a gravitational wave.

Gravitational waves aren't the only means of detecting a black hole. Here's a question for you. If not a black hole, what's at the center of the galaxy? There's something there weighing as much as a million suns, but it isn't a million times as big. That much is certain. It's been carefully measured multiple times.

Also, what would you call something that can't be seen directly, but otherwise behaves exactly as a black hole is predicted to behave?

I would call it something that is very likely a black hole

I'm pretty sure a nobel prize was given out a few years ago (in the 90s I think) for the indirect observation of a black hole.

Acceleration towards whom? The singularity? Remember the singularity is not in motion, so for it, the acceleration will be measuread as very fast, if you're talking of acceleration of Earth, and close to zero, if you're talking of acceleration of the singularity itself.

I beg to differ; the singularity is, in fact, moving, just as the universe is expanding, which could effect the mass transfer of the planet (pieces of planet) to some extent because as the singularity moves it is subsequently forced to interact with the background 3.7K (3.7K?) vacuum of space. That sort of friction might be the cause of various phenomena, of which i have no inclination towards what they could be. Maybe it is similar to accelerating a rock under water, which in the case of singularity, would cause ripples in the fabric of space-time.

Originally Posted by MagiMaster

Gravitational waves aren't the only means of detecting a black hole.

Indeed, but i never said that either. However it is the most likely, since, if you take two black hole collisions, they ripple the very fabric of spacetime and they send out gravitational waves. If we haven't observed a gravitational wave, the either the wave does not exist, or the black holes are in small numbers. Of course, you could also say that there are no black holes at all, and that is why we have never observed a gravtitational wave, which is a preference i dare to take.

Here's a question for you. If not a black hole, what's at the center of the galaxy?

A bit of a ''red-herring'' by the way. At the center of any collection of large enough particles, it is attracted inwards. Since we have never been able to reconcile all the needed or required matter for gravitational effects in the universe, it seems very likely that General Relativity breaks down when long distances are taken into effect.

There's something there weighing as much as a million suns, but it isn't a million times as big. That much is certain. It's been carefully measured multiple times.

Hence, indeed the question of what causes the relevent issue, but does not address the motive or origin if you like, which would be a cause without a means. It's quite simple. Relativity cannot work on long distances but works well within local area's, just as it's creator had intended the theory too.

Also, what would you call something that can't be seen directly, but otherwise behaves exactly as a black hole is predicted to behave?

I cannot call it anything really. Just as much as the observation of galactic centers have provided a curvature (or radiation of gravity) very similar to the so-far non-observed (or of confirmed) origin as such a strange structure of a black hole.

It all seems twaddle to me.

So basically you're saying that you'll never accept that black holes exist because, by nature, they can't be observed directly? Well, believe what you want, but I'll stick with science. Indirect observations count too.

BTW, the current best theory of galaxy formation does provide a reason for there to be a black hole at the center of every galaxy.

Originally Posted by Manynames

Gravity has a strange effect on light. It will couple with light, and bend it around large masses. The idea arose from Einstein, and on the 9th of November in 1919, light was seen to bend around the sun during an eclipse. The times reported on the discovery, 'space would no longer be looked at as extending indefinitely in all directions. Straight lines would not exist in Einstien’s space. They would all be curved and if they traveled far enough they would return to their starting point.'

That would make the concept of "direction" almost impossible to determine with accuracy, wouldn't it? How do we know any distant light source is really located in the direction it appears to be at? I'm assuming we don't, at least not with absolute certainty.


One interesting interpretation of relativity I've been considering is the case of a spaceship moving at nearly C, with 2 telescopes located on the outer tips of its wings (say it's a 10 km wide space ship) and on it's way toward say... Proxima Centauri (my favorite destination for spaceships moving near C)

The telescope on the right would observe the telescope on the left to be further away from the goal than it was, making the ship appear to be aligned somewhat sideways with respect to the direction of motion. Because the non-zero amount of time it takes light to travel from one side of the ship to the other technically means you're seeing the other telescope where it was a split second ago.... behind where it is now.

I don't know how that relates to gravity warping light paths, or especially to black holes warping our notion of direction..... but it seems like it should relate somehow.

Now i begin to pass the event horizon (remember that is the first boundary, or surface). Now something quite remarkable happens. The space coordinates switches roles with the time coordinate. What does this mean? Well, we move through space freely, back and forth without any problems, and when we consider time, that imaginary dimension, we tend to think we sweep along with it without recourse. Once i pass the event horizon space begins to drag me and my ship, and i begin to move in one direction only - that being forward - however, i begin to move through time backwards and forwards, just as easily as i had moved through the space dimension. In this case, we say that space has become 'timelike', and time has a 'spacelike' character - they are thus interchangeable given the correct conditions.

I wonder if the switch of roles would make life different for a life form native to such an environment, or if the switch is complete, so that time is space to them, and space is time.

Now i have crashed into the dreaded singularity, and i will no longer exist. Here, just like the Big Bang singularity mentioned in part one of chapter one, everything takes on infinite attributes - the laws of physics become invalid.However, you might not crash into the center. It is possible you can fall into the 'inner horizon' - this horizon is adjacent to the singular region. Here, space and time flows the correct way. In theory, you can float around in the inner horizon without ever crashing into the dreaded center.

"the laws of physics become invalid."

To clear, however, a new set of laws does replace them, doesn't it?

If our universe is indeed a black hole, you might imagine we exist in the inner horizon. In fact, our universe may as well be a black hole. Now, if one passes by the singularity, we might be able to move out of the inner horizon and pass through a second inner horizon, and then by finally passing another outer horizon, we will have entered another universe - but i had better be careful. There is a very good chance that this universe is made up mostly of antimatter. If i come into contact with antimatter, me and my ship will explode in a flash of light.

That would be one way to rationalize the idea of it having a finite size.

How well would it explain Hubble Redshift, if light from all distant stars is really light emitted from different parts of the interior of a singularity?

Originally Posted by kojax

One interesting interpretation of relativity I've been considering is the case of a spaceship moving at nearly C, with 2 telescopes located on the outer tips of its wings (say it's a 10 km wide space ship) and on it's way toward say... Proxima Centauri (my favorite destination for spaceships moving near C)

The telescope on the right would observe the telescope on the left to be further away from the goal than it was, making the ship appear to be aligned somewhat sideways with respect to the direction of motion. Because the non-zero amount of time it takes light to travel from one side of the ship to the other technically means you're seeing the other telescope where it was a split second ago.... behind where it is now.

No. Remember, as far as the spaceship is concerned , it is motionless and it is Alpha C that is rushing towards it at near c. You will see nothing odd about the alignment of the ship with respect to the line of realtive motion.

What you will see is stellar aberration. where the where the stars of the galaxy tend to shift forward and cluster more tightly around Alpha C from your viewpoint.

Here's my confusion, then.

Code:

telescope 1                                             telescope 2
                    || -----------[]-------------|| 
                                 main ship

________________________________________________

At time 0 telescope 1 emits light. 



                     ||  )   ------[]-------------|| 
                   
________________________________________________

If the light wave expands in all directions, and the ship keeps moving 
forward, then we have to calculate the intersection point. 

Ship location at
Time 1                    || -----------[]-------------||
                                                      
                                                      _
                                                      /|

                                                _
                                                /|
                                            _
                                            /|
                            ^         _ 
                            |         /|
 
                            ^    (trying to draw arrows)
                            |   _
Ship location at                /|
Time 0                     ||  ->  ->      []-------------||

It seems, then, that the light from telescope 1 will appear to be arriving from an angle oriented behind telescope 2, rather than from the side.

Does length contraction solve this?

Originally Posted by kojax

Here's my confusion, then.

Code:

telescope 1                                             telescope 2
                    || -----------[]-------------|| 
                                 main ship

________________________________________________

At time 0 telescope 1 emits light. 



                     ||  )   ------[]-------------|| 
                   
________________________________________________

If the light wave expands in all directions, and the ship keeps moving 
forward, then we have to calculate the intersection point. 

Ship location at
Time 1                    || -----------[]-------------||
                                                      
                                                      _
                                                      /|

                                                _
                                                /|
                                            _
                                            /|
                            ^         _ 
                            |         /|
 
                            ^    (trying to draw arrows)
                            |   _
Ship location at                /|
Time 0                     ||  ->  ->      []-------------||

It seems, then, that the light from telescope 1 will appear to be arriving from an angle oriented behind telescope 2, rather than from the side.

Does length contraction solve this?

Remember, there is no test that the crew of the spaceship can perform that will tell them if they are moving or at rest. The drawing you made is what it would appear to someone the ship is moving relative to, but not how it would appear to someone in the ship.

As an analogy, imagine you are on a train with a friend, passing a ball back and forth across the train. From the point of view of someone standing on the embankment watching the train go by, the ball follows a zig-zag path. To you in the train, the ball just goes back and forth. (when you catch the ball it doesn't seem to come from some point closer to the rear of the train.

How would we reconcile the two views, then? The view of the third person observer can't be fully incompatible with the view of a person aboard the ship, can it?


It seems like the part of the light waves emitted by the telescope on the left should be hitting the detector on the telescope on the right from a rearward angle, a little bit less steep than 45 degrees.

I'm taking into account the fact that light waves expand as a sphere. It seems like the part of that sphere that actually makes contact with the detecting telescope's detectors would be a different part if the detecting telescope is moving before the light can reach it.