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JEROME - Black holes do not exist

Herzblut said:
Right.

What I say is that first of all gravitation is not a local effect, but a large scale phenomena with infinite range. Secondly, I tried to say that matter creates space, without matter there's no space. If matter in Universe was pulled together on a cosmical scale, space would not be expanding but contracting, following matter.
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General Relativity disagrees: Space exists independently of matter. There are solutions of GR that have no matter (De Sitter universe). The curvature of space and matter though are intimately connected.

The effect of gravity is to cause matter that is close together to stay together regardless of the expansion of space. The amount of space between galaxies means that the force of gravity is weak enough so that the expansion of space dominates. We thus observe the cosmological redshift for galaxies.
 
Reality Check said:
General Relativity disagrees: Space exists independently of matter. There are solutions of GR that have no matter (De Sitter universe). The curvature of space and matter though are intimately connected.
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What do you mean by "exists"? A mathematical solution of the field equations does not necessarily describe a universe that could ever come into existence. It's first of all a mathematical model.

An absolute space without any objects in it does not make any physical sense to me. You can define a metric such that the geometrical distance between two points can be calculated, but it can never be measured because there is nothing there to measure.

Reality Check said:
The effect of gravity is to cause matter that is close together to stay together regardless of the expansion of space. The amount of space between galaxies means that the force of gravity is weak enough so that the expansion of space dominates. We thus observe the cosmological redshift for galaxies.
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What do you mean by "close" together? The largest scales of structures are billions of light years.

My point is, again, that if matter would be coalescing on largest scales, physical space could not expand.
 
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Herzblut is not so far off the mark. If the only stuff in the universe were ordinary matter and radiation, the rate of expansion would indeed be slowing. If might or might not stop the expansion entirely and begin contracting, depending on the initial "velocity" of the big bang (the equations are more or less the same as those that describe an object fired vertically from the surface of the earth - it either reaches a maximum height and then falls back, or it started with more than escape velocity and goes forever).

The observation that the expansion is accelerating indicates that there is some non-standard form of energy around - that's what's called dark energy. It must have the property that it pushes things apart rather than pulling them together. The simplest possibility - and something that was present in general relativity from the very beginning, because it is the unique term one can add that's consistent with general covariance and important at long distances - is a positive cosmological constant.

EDIT - by the way, if dark energy is in fact a positive cosmo. const., the future of our universe is de Sitter space. So it probably does exist even in your strict sense :).
 
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robinson said:
In water, in orbit, in outer space, you are not accelerating, but there is no gravity.
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Robinson, it is trivial to show that you are accelerating in orbit, all you need is Newton's law of gravitation and his second law.

So (ignoring the signs)

F = GMmr-2 (1)

F = ma (2)

In circular motion, assuming no bits are falling off your spacecraft or the body you're orbiting, F must be constant since GMm and, by definition, r are all constant. So obviously if F is non-zero and constant then a must also be non-zero and constant. Subbing (2) in to one and cancelling the m's we get

a=GMr-2 (3)

(Aside: insert the mass of the Earth for M and its radius for r. And what pops out? a = 9.81 ms-2.)

(3) is independent of velocity, thus the instantaneous acceleration of a satellite in orbit at r is identical to that of an object with 0 tangential velocity at r. The latter would obviously be in free fall. Therefore a body in orbit is constantly accelerating and is in free fall.
 
robinson, it occurs to me that you might be confused about the difference between "speed" and "velocity" and what acceleration means.

Speed is a scalar measurement, meaning it is just a number like temperature. A car can be traveling 50 mph west and another car can be traveling 50 mph north and they both are traveling at a speed of 50 mph.

Velocity is a vector measurement, meaning it is composed of both a speed (a scalar amount) and a specified direction. The cars in the example above may have a common speed, but they have different velocities because they are traveling in different directions.

Acceleration, as noted above, is the rate of change of velocity. There are two ways you can change the velocity of an object, you can change it's speed or you can change its direction. To use the car analogy, you are accelerating whenever you push the gas or break peddle or whenever you turn the steering wheel. So if the north-bound car were to make a right turn while remaining at a speed of 50 mph, it is still said to have accelerated because a force has been applied to it that changed its velocity. In this case, the force of friction of the tires against the road accelerated the 50 mph north velocity into a 50 mph east velocity.

Back to the orbit: As we know from old Newton's laws of motion, an object in motion tends to stay in motion unless acted upon by a force. If there were no force (i.e. acceleration) acting upon the ISS, it would continue along in a straight line on its current vector. However, because the Earth's gravity is constantly accelerating the ISS toward the Earth's center, the ISS is continually changing vectors over time, even though its speed may remain constant.

Check out the vector diagrams shown in this article about uniform circular motion.

(of course, in the real world, there are many more forces at play than just gravity and momentum (the velocity that that is perpendicular to the velocity due to gravity), but momentum and gravity are the two largest velocities in play.)
 
Herzblut said:
Space expands together with spreading out matter. Space cannot expand without matter dispersing.
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Locally, gravity is stronger than whatever force is causing the universe to expand. So things clump together.

This implies that whatever force is causing expansion is stronger over great distances than gravity...perhaps it works in reverse of other forces and gets stronger as the distance increases.
 
sol invictus said:
Herzblut is not so far off the mark. If the only stuff in the universe were ordinary matter and radiation, the rate of expansion would indeed be slowing. If might or might not stop the expansion entirely and begin contracting, depending on the initial "velocity" of the big bang (the equations are more or less the same as those that describe an object fired vertically from the surface of the earth - it either reaches a maximum height and then falls back, or it started with more than escape velocity and goes forever).

The observation that the expansion is accelerating indicates that there is some non-standard form of energy around - that's what's called dark energy. It must have the property that it pushes things apart rather than pulling them together. The simplest possibility - and something that was present in general relativity from the very beginning, because it is the unique term one can add that's consistent with general covariance and important at long distances - is a positive cosmological constant.

EDIT - by the way, if dark energy is in fact a positive cosmo. const., the future of our universe is de Sitter space. So it probably does exist even in your strict sense :).
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This may be a stupid question, but is it possible that there are an infinite number of forces, each of which is weaker than the one below it but acts over a longer distance?

For instance, the strong nuclear force is much stronger than the electromagnetic force, but is not felt at all out where the electrons live.

The electromagnetic force is much stronger than gravity, but only has an effect when you are extremely close.

Whatever force is making the universe expand is clearly weaker than gravity at the galaxy level, but is stronger than gravity over billions of light years.

Can it go on like this forever?
 
Tubbythin said:
(Aside: insert the mass of the Earth for M and its radius for r. And what pops out? a = 9.81 ms-2.)
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Yep.

To estimate gravitational force there on the ISS assume Earth's radius R=6.600km and orbit height h=330km above sea level. The ISS is just a 5% further away from Earth's center than we are here on ground. Hence, gravity there is still 90% of g, if I'm not totally mistaken.*

Gravity here and there are almost equal.

* g*(1/1,05)^2.
 
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Herzblut said:
What do you mean by "exists"? A mathematical solution of the field equations does not necessarily describe a universe that could ever come into existence. It's first of all a mathematical model.

An absolute space without any objects in it does not make any physical sense to me. You can define a metric such that the geometrical distance between two points can be calculated, but it can never be measured because there is nothing there to measure.


What do you mean by "close" together? The largest scales of structures are billions of light years.

My point is, again, that if matter would be coalescing on largest scales, physical space could not expand.
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Close together means on a galactic scale.
There is no limit on the force of gravity. It is just that on a galactic scale it dominates the expansion of space.
On a scale larger than galactic gravity does not stop matter from showing the effects of the expansion of space.
Gravity is also responsible for the largest scales of structures.
 
Robinson.

It's really very simple to show that an object in orbit is accelerating. You've been given data on equations which show that circular motion is a constant acceleration, but you've contended that that isn't an acceleration. So let's assume, for a moment, that you're right, and circular motion does not require an acceleration.

Let's look at the orbital data of the ISS;

ISS orbital parameters.

And NASA's version.

The important number here is the eccentricity, E. This is the relationship between the long and short axes of the elliptical path that the ISS follows.

Now, let's look at Kepler's laws of orbital motion. (I know it says planets, but it applies to all orbits)

From Wikipedia's article on Keplerian orbits.
1. The orbit of every planet is an ellipse with the sun at one of the foci.
2. A line joining a planet and the sun sweeps out equal areas during equal intervals of time as the planet travels along its orbit.
3. The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axes (the "half-length" of the ellipse) of their orbits.
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Now, consider the second law, because that is the important one in this case. In order for the ISS's orbital vector to sweep out equal areas in equal time intervals, and maintain an elliptical orbit, it must slow down when it is further away from the Earth, and speed up when it is closer. This means that its angular speed must be constantly changing. So, even if you maintain that an object in circular orbit isn't accelerating (which is just plain wrong), the ISS is in an elliptical orbit, and must, therefore, be accelerating and decelerating in terms of its angular speed.

In other words, there are times when the ISS is getting closer to the Earth, and is being accelerated by its gravity. In even simpler words - there are times when the ISS is falling towards the Earth.

Orbits are are particular case of free fall, and they are subject to acceleration due to gravity.

I'm a professional astrophysicist. I've studied orbital mechanics, and work with people who do cutting edge research on the subject. So I can say the following with some authority.

You are wrong. Plain and simple. Wrong.
 
wollery said:
In other words, there are times when the ISS is getting closer to the Earth, and is being accelerated by its gravity. In even simpler words - there are times when the ISS is falling towards the Earth.

Orbits are are particular case of free fall, and they are subject to acceleration due to gravity.

I'm a professional astrophysicist. I've studied orbital mechanics, and work with people who do cutting edge research on the subject. So I can say the following with some authority.

You are wrong. Plain and simple. Wrong.
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So, an orbit is both a free fall and... what?

If the object is sometimes getting closer to the Earth, it is at times moving away from the Earth, yes?


:gnome:
 
Upchurch said:
momentum perpendicular to the pull of gravity.
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When the object is moving away from the Earth it is moving perpendicular?

We are talking about an ellipse are we not?

Is there not a point in an ellipse in which the object is moving away from the center?


:gnome:
 
JEROME DA GNOME said:
When the object is moving away from the Earth it is moving perpendicular?

We are talking about an ellipse are we not?

Is there not a point in an ellipse in which the object is moving away from the center?


:gnome:
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An ellipse has 2 centers (an ellipse is a path such that the sum of the distances to two fixed points is a constant). An orbiting body orbits the 2 centers and does not go directly away from either.
 
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