JEROME - Black holes do not exist

[MattusMaximus]

Make believe!!!

Dark energy is the make believe that is used to justify the theory in spite of the data.

If nothing else, you are predictable Jerome.

 

[JEROME DA GNOME]

Because there is some kind of vacuum energy ('dark energy') stretching it.

Do we have any evidence of dark energy outside of the fact that it is needed to justify the currently accepted theories?

 

[Powa]

Make believe!!!

Dark energy is the make believe that is used to justify the theory in spite of the data.

No, dark energy is used to explain the increasing rate of expansion of the universe. It's just a name for the phenomenon that's causing the acceleration. As of yet no one knows what it is.

 

[Herzblut]

The reason why the astronauts aboard the ISS appear weightless is because they are in a state of perpetual free-fall around the Earth. They are falling towards the Earth at precisely the same rate as the Earth is curving away from beneath them, thus they never hit the ground - they just keep going around and around...

Mattus, I hate you!

 

[Powa]

Do we have any evidence of dark energy outside of the fact that it is needed to justify the currently accepted theories?

JEROME, do you EVER read the links supplied to you? I linked to the Wikipedia article on dark matter just a few posts ago.

ETA: Look under 'Evidence for dark energy' in the article.

 

[MattusMaximus]

Mattus, I hate you!

You're welcome - I think

 

[Herzblut]

You're welcome - I think

Man, I was so curious to see the answers of some 'experts' here.

But the science board seems the wrong place for this trial...

BTW, I wouldn't say they just appear weightless, they actually are. Ideally, strictly running along a geodesic, there's no difference between the two. I think the point is they appear to be on zero gravity level, and that is a totally false perception. Interestingly, didn't Einstein's Principle of Equivalence state that falling freely in a windowless, isolated elevator cannot be, by no way, distinguished from floating in zero-gravity space.

 

[MattusMaximus]

BTW, I wouldn't say they just appear weightless, they actually are. Ideally, strictly running along a geodesic, there's no difference between the two.

Ah, I see your point. Duly noted. It is really a question of the observer's frame of reference - from the FoR inside the ISS they observe weightlessness, but from the FoR on the Earth we observe them in free-fall. See my comments below.

I think the point is they appear to be on zero gravity level, and that is a totally false perception. Interestingly, didn't Einstein's Principle of Equivalence state that falling freely in a windowless, isolated elevator cannot be, by no way, distinguished from floating in zero-gravity space.

This is correct, if you are inside of a FoR that you unable to see beyond (hence the term, "isolated"). For all practical purposes, in this case the two are equivalent. However, since we have the capacity to observe the ISS from outside its FoR, we can clearly see that gravity is still acting upon it. If gravity were not acting on it, then we on Earth would observe the ISS to fly off into interplanetary space due to its inertia.

There are other ways to verify this as well. For example, gravitational time dilation - the clocks on the ISS will run at a slightly different rate compared to the clocks on the Earth due to the fact that they're inside the gravity well of the Earth. But then, this would mean being able to make comparative measurements between the two FoR, which would be impossible in a completely isolated system.

 

[robinson]

Man, I was so curious to see the answers of some 'experts' here.

But the science board seems the wrong place for this trial...

BTW, I wouldn't say they just appear weightless, they actually are. Ideally, strictly running along a geodesic, there's no difference between the two. I think the point is they appear to be on zero gravity level, and that is a totally false perception. Interestingly, didn't Einstein's Principle of Equivalence state that falling freely in a windowless, isolated elevator cannot be, by no way, distinguished from floating in zero-gravity space.

Good points. What is the difference between being in the ISS far from earth, and being in an earth orbit? As far as gravity is concerned. If the windows were all darked out, could anybody inside tell the difference?

As for the trip to the moon, what happens to the scale as you make the trip? Is there a point in the trip where the scale reads no gravity?

Which was my sly attempt to get to this same question. If there is no gravity, what happened to it?

If you go with GR, there is no force of gravity. There is only warped spacetime and acceleration and stuff, but no force of gravity.

Which takes us back to our mythical spaceship, between Galaxies, on which the descendants have never been in a gravity field. How would they know about gravity? Like we do?

 

[robinson]

In case it is hard to wrap your mind around the concepts. The ISS is not falling towards the earth. If it were falling, due to gravity, it would be accelerating. Which is very much is not.

That explanation is used for schoolkids to try and explain orbital dynamics, but that is NOT what is happening, anymore than the Apollo missions were "falling" towards the moon, or in reverse, towards the earth.

 

[robinson]

And, drifting slightly off topic, the Earth is not "falling" towards the sun either.

 

[Herzblut]

However, since we have the capacity to observe the ISS from outside its FoR, we can clearly see that gravity is still acting upon it. If gravity were not acting on it, then we on Earth would observe the ISS to fly off into interplanetary space due to its inertia.

That should truely make the observers scratch their head.

There are other ways to verify this as well. For example, gravitational time dilation - the clocks on the ISS will run at a slightly different rate compared to the clocks on the Earth due to the fact that they're inside the gravity well of the Earth. But then, this would mean being able to make comparative measurements between the two FoR, which would be impossible in a completely isolated system.

Yep. But I wonder...

Let's assume an elevator is on Earth's ground, static, equipped with an atomic clock. Another one, identical construction, is in space accelerating with exactly 1g. They are totally equivalent, right?

I'd assume the two clocks would not get out of sync due to gravitation. The acceleration of the elevator basically creates an identical gravitational field for the objects inside of it, when gravitational and inertial mass are really equivalent. I'd actually assume experiments of that kind (roughly, very roughly) are used to empirically test that equivalence.

But I might be wrong.

 

[Terry]

In case it is hard to wrap your mind around the concepts. The ISS is not falling towards the earth. If it were falling, due to gravity, it would be accelerating. Which is very much is not.

That explanation is used for schoolkids to try and explain orbital dynamics, but that is NOT what is happening, anymore than the Apollo missions were "falling" towards the moon, or in reverse, towards the earth.

The ISS is accelerating. it's velocity vector is constantly changing.

 

[robinson]

The ISS is actually losing velocity, and "falling" towards the earth each orbit, by a very small amount. It has to be bumped a little now and then, to prevent the orbit from decaying. Usually by a shuttle.

But once an object reaches escape velocity, it is not bound by gravity, in the sense it is not falling back to earth. Like a bullet does.

The moon is in orbit around the earth. It is actually moving away from the earth with each orbit. It is not "falling" towards the earth.

The ISS is not falling, it is in an orbit.

Just like the Earth is not falling towards the sun. It is in an orbit around the sun.

 

[robinson]

And this tremendous monster of a thread takes yet another turn.

 

[Herzblut]

Good points. What is the difference between being in the ISS far from earth, and being in an earth orbit? As far as gravity is concerned. If the windows were all darked out, could anybody inside tell the difference?

Let us neglect things like friction of the atmosphere, tidal effects, gravitational impact of moon and sun, then - no!

Free fall motion* is indistiguishable from gravitation-free inertial (not accelerated) motion. Both is a "straight line" motion in curved spacetime, a geodesic. (I still assume an isolated system of course which cannot exchange signals with the outside world.)

*Edit: In a gravitational field assumed homogeneous across the moving body, of course.

 

[Herzblut]

The ISS is not falling, it is in an orbit.

I think this is a communication problem. What is meant by "free fall" is a motion under exclusive impact of the gravitational force of a heavy mass.

Insofar, orbiting around that mass is a free fall, per definition. Perse, this says nothing about the change of distance to the mass.

 

[Acleron]

The ISS is actually losing velocity, and "falling" towards the earth each orbit, by a very small amount. It has to be bumped a little now and then, to prevent the orbit from decaying. Usually by a shuttle.

The ISS is actually hitting small objects, these tend to reduce the ISS's velocity and cause its orbital average radius to shorten.

But once an object reaches escape velocity, it is not bound by gravity, in the sense it is not falling back to earth. Like a bullet does.

If a bullet reached escaped velocity then it wouldn't fall back to earth, unless of course, it continued to hit the same small objects that the ISS hits.

The moon is in orbit around the earth. It is actually moving away from the earth with each orbit. It is not "falling" towards the earth.

Oh yes it is, the amount it moves away is far smaller by magnitudes, than the amount it falls towards the Earth each orbit.

The ISS is not falling, it is in an orbit.

eh?

Just like the Earth is not falling towards the sun. It is in an orbit around the sun.

Eh? Eh?

 

[robinson]

Well, we run into that eternal problem of precision in writing. What does falling mean, in this context?

In the Amazing Zero Gravity Xperience

the lucky few are indeed falling, as the plane makes a parabolic path to give a brief free fall. The craft and occupants all fall at the same rate, so it feels like zero G, but in this case, there is indeed acceleration at some point. First there is loss of velocity, then a moment of no falling, then everybody is falling. At the point where the velocity of falling becomes faster than the jet can dive, "gravity" returns to the people onboard.

In orbiting bodies, the ISS as well as the shuttle, satellites, the moon, the earth, it isn't like that at all. Speed is almost constant, and the gravity one "feels" is either tidal forces, or from your own mass.

Using the ISS as an example, the orbit is not circular, as some assume, but an ellipse, and the distance from the center of the earth varies, same as with the moon. When the moon is at apogee the velocity also is different than perigee. (Keppler's Laws still work for that)

In other words, the distance from the body being orbited may actually be increasing, so to say something in orbit is "falling" is absurd. Falling means something, and Newton's Law of Gravity tells us that the same force that causes the apple to fall, keeps an orbiting body from flying off into space.

But this does not mean the moon is falling.

Consider the Earth. It has an elliptical orbit around the sun. It is not falling into the sun. To say it is falling is absurd. As an astute observor said, to the orbiting body, it is traveling in a straight line, spacetime is curved, due to gravity,. To an object in motion, there is no curve. If there were a curved path, people on the ISS would ffeel motion, be stuck to the side of the station or something.

All this is most difficult to explain without delving into technical writing. And after a wild Fourth of July, I am not up to it.

Let us go for a ride on Voyager, which is leaving the solar system. We feel no gravity, free fall, zero G, because we are have achieved escape velocity from the Sun. And yet, we are still in orbit around the center of the Galaxy. So is the entire solar system.

But what one experiences as gravity depends on the nearest body that influences us. Even in deep space, something is effecting us. If we escaped the Galaxy itslef, the nearest super cluster would be pulling us in! But we don't experience it as gravity. Because everything is moving at the same rate, it is zero G.

Even the language to describe stuff makes it clear we don't understand eberything about gravity. Curved spacetime indeed.

 

[sol invictus]

Even the language to describe stuff makes it clear we don't understand eberything about gravity.

Let me fix that for you:

Even the language I use to describe stuff makes it clear I don't understand anything about gravity.

In fact, the "stuff" you are failing to describe has been perfectly well understood for at least 400 years. While there are certainly deep mysteries associated with gravity, they have nothing to do with basic orbital dynamics.