difference between free fall and zero gravity?

[Schneibster]

The key is observe.................

Paul

That's correct; and that observation is everything. There is one and only one way to measure the curvature of spacetime, and that's by observing the behavior of inertial objects. The definition of flat spacetime is that inertial objects move in straight lines, and the definition of curved spacetime is that they move in curves. There is no observable difference between the curves that inertial objects move in in a gravity field, and the curves that inertial objects move in in an accelerated frame of reference. The inertial object is the only valid reference, because it can be internally measured to be inertial.

 

[Paulhoff]

That's correct; and that observation is everything. There is one and only one way to measure the curvature of spacetime, and that's by observing the behavior of inertial objects. The definition of flat spacetime is that inertial objects move in straight lines, and the definition of curved spacetime is that they move in curves. There is no observable difference between the curves that inertial objects move in in a gravity field, and the curves that inertial objects move in in an accelerated frame of reference. The inertial object is the only valid reference, because it can be internally measured to be inertial.

But, acceleration is still not gravity.........................there is no orbiting....no tides...... it works only on a closed system........

Paul

 

[Schneibster]

That's because it's spherical, which acceleration can't be for fixed size. Not because "gravity is different from acceleration."

ETA: and because its force varies as the distance. Forgot that one, and it is important.

 

[NobbyNobbs]

I'm a little confused, and going to get what some people consider nitpicky, but I think it might be the crux of the misunderstanding. People here keep comparing acceleration with gravity. They are not the same thing, for the same reason that distance and velocity are not the same thing. Gravity is a force, measured in newtons. Acceleration is the effect of a force on a mass, measured in m/s/s. They are not equal, they are not the same. If you want to compare, you need to talk about acceleration due to gravity.

What you're doing is like two people arguing because one insists that the building over there is heavier than the speed of the car. It just doesn't make sense to compare them.

 

[Schneibster]

I think that extreme nitpickiness is required to understand what's being said. So I don't criticize you for it.

Gravity's force is measured in newtons; but we're not talking about the force of gravity, but its essential nature. And I'll point out that ALL forces are measured in newtons; there is no distinguishing between them. You've confused the force of gravity with the phenomenon of gravity; remember that all forces impose accelerations on masses. F=ma and all like that.

The nature of gravity is that it is a curvature of spacetime. But that curvature is indistinguishable on a local level from the curvature of spacetime imposed by ANY acceleration.

Gravity is equivalent to acceleration; that's the postulate of General Relativity. In the most essential sense, gravity and acceleration are the same thing. GR proves it, just as SR proves that all inertial frames are equivalent, and that the speed of light is finite and maximal. It's basic physics.

 

[Paulhoff]

That's because it's spherical, which acceleration can't be for fixed size. Not because "gravity is different from acceleration."

Because it is different. About a year ago there was someone on an other thread trying to say that atoms where getting bigger so that was making gravity by causing acceleration. It did not account for orbiting, it did not account for tides, and gravity waves......... the affect only works for a close system meaning that you can't see outside the box.

Paul

 

[Ziggurat]

Gravity is a force, measured in newtons.

No, it isn't. Even in Newtonian gravity, the FIELD is indeed measured in m/s², ie, the same units as acceleration (GM/r²). The field from a planet, for example, does not depend upon the test mass you use to probe it, but the force on the test mass (GMm/r²) DOES depend upon which test mass you use, indicating that the field is the intrinsic quantity, NOT the force.

 

[69dodge]

The claim is that acceleration due to gravity is indistinguishable from acceleration due to any other force.

What do you mean by "acceleration due to gravity"?

In particular, if you're standing in an elevator that remains motionless at the top floor of a building, are you "accelerating due to gravity" or not? If the cable breaks and the elevator falls with you in it, are you "accelerating due to gravity" or not?

I assume your answers are "no" and "yes", respectively?

If so, your quoted statement is exactly backwards.

Being accelerated by some other kind of force feels the same as not being accelerated by gravity, the same as being in a gravitational field but being prevented from accelerating by, for example, the floor that's in the way.

Conversely, if you're in a gravitational field but nothing prevents you from accelerating in response to it---you're in the falling elevator---that feels the same as if no force is acting on you.

 

[Bodhi Dharma Zen]

Really interesting. If you feel a "resistant force" in a constant direction you are accelerating. Is this correct?

If it is, can we then assume that the changes in the "speed of time" in relation to other inertial frames its because of the actual warping of space due to acceleration. Gravity would be irrelevant, in this sense. Am I right?

 

[Schneibster]

Because it is different.

Not intrinsically. Both are warping of spacetime. The warping is measurable by the curvature of the geodesics of inertial frames.

About a year ago there was someone on an other thread trying to say that atoms where getting bigger so that was making gravity by causing acceleration.

Please don't associate me with someone spouting woo about atoms getting bigger and causing gravity. It's insulting.

It did not account for orbiting, it did not account for tides, and gravity waves......... the affect only works for a close system meaning that you can't see outside the box.

Paul

Basically what you're doing is saying that because the math that you would need to do to describe tides, orbits, and gravity waves as effects of acceleration is very complicated, it's wrong. What you're not getting is that that math is the General Theory of Relativity. So basically what you're doing here is saying GRT is wrong. So now who's a woo?

It's as if you were saying that because the math that describes the forces solid objects exert on one another as manifestations of the electromagnetic force, specifically Van der Waals forces, is so complicated, you don't believe it, and you're insisting that the forces solid objects exert on one another are "different" from electromagnetism. The difference is in the manifestation; at core, they're both electromagnetism.

And at core, gravity is acceleration. There's no "sort of" in the Equivalence Principle.

 

[Schneibster]

Really interesting. If you feel a "resistant force" in a constant direction you are accelerating. Is this correct?

Yes, and also if you feel it in a changing direction. Acceleration need not be constant to remain acceleration. Consider rotational motion.

If it is, can we then assume that the changes in the "speed of time" in relation to other inertial frames its because of the actual warping of space due to acceleration. Gravity would be irrelevant, in this sense. Am I right?

Hmm, well, not exactly.

First, both gravity and acceleration warp both space and time, or more properly, spacetime; so it's not just changes in the "speed of time."

Second, most physicists don't see time as something that passes, they rather see time as a dimension, in which events have particular positions. Because of the way we experience the world, we can be said to either "see time passing," or "see ourselves moving through time." Both of these are illusions, because of the way our physical brains operate; we actually exist at all the times we occupy, in the spaces we occupy at those times; a long pink worm of "meness" stretching from a womb to a grave through spacetime.

Third, you said "other inertial frames," and the speed of time or even the ordering of time events need not be the same for inertial frames that do not share the same velocity.

Fourth, it's warping of spacetime, see above, not warping of space.

Fifth, gravity would not be irrelevant; the effects are gravity.

 

[Paulhoff]

Gravity is not acceleration, or we would call to that. I can see you have no idea of where I am coming from. Acceleration gives the illusion of gravity, it is not gravity.

Also I mention the other about atoms because I was not sure if you were coming in the back door on that idea.

Paul

 

[Paulhoff]

Fourth, it's warping of spacetime, see above, not warping of space.

No, space is warped. What do you think they are trying to detect now, gravity waves, they warp space.

Paul

 

[Schneibster]

Gravity is not acceleration, or we would call to that. I can see you have no idea of where I am coming from. Acceleration gives the illusion of gravity, it is not gravity.

Look, if you want to argue with the person whose picture you use as your avatar, go ahead, but don't ask me not to respond with what the textbooks say. It's simple, direct, straightforward, and unambiguous, it's a postulate of General Relativity, and it's called the Equivalence Principle. You can look it up in any physics textbook from the last 80 years or so.

I've offered quotes from good sources, and demonstrations of the reasoning behind it. The fact that you're incapable of understanding them doesn't make them false. You have offered not one single quote of your own, and you just keep saying the same thing over and over like it's going to mean something it didn't when it got refuted the last time. I'll leave it to others to decide where they've seen that kind of behavior before. If you have some references that support your view, please offer them. Otherwise, I won't bother responding other than to parrot the standard textbook view of relativity so that others aren't confused by what you say.

Also I mention the other about atoms because I was not sure if you were coming in the back door on that idea.

Paul

I'm not coming in the backdoor on anything. I'm quoting the standard 80-plus-year-old physics textbook understanding of the meaning of the Theory of General Relativity. I'm quoting the well-known Equivalence Principle, and stating that it is not merely an analogy, or misstatement, or whatever it is you're claiming (which, by the way, you have not ever made clear), but a well-known and well-proven theory of the nature of gravity: that gravity and acceleration are interchangeable and equivalent, and that the only differentiation between them is that gravity is a force generated by matter, which because it is so makes a gravity field spherical and obedient to the inverse-square law. Other than that, there is absolutely no difference between gravity and acceleration. And on the local level, you cannot perform any experiment that will show a difference.

If you want to argue with 80 years worth of textbooks and every spacetime physicist out there in the real world, several of whom hold Nobel Prizes in physics, go ahead, but try not to take it too badly when I point out that you haven't a clue what you're talking about.

 

[Paulhoff]

Sorry but I'm done...........

Paul

 

[Schneibster]

No, space is warped.

First, previously you contended that space cannot be warped. So which is it?

Second, you are just plain flat wrong, and anyone who knows anything about black hole physics will tell you that gravity warps time as well as space.

Third, the warping of time happens for GPS satellites, and must be compensated for in order for a GPS receiver on the ground to give an accurate position reading.

Go look it up for yourself; I've given references and I'm tired of giving them and having you ignore them. If anyone else is bothering, speak up and I'll provide references from the websites of reputable and well-known institutions, including those who put GPS up there, for you to examine.

What do you think they are trying to detect now, gravity waves, they warp space.

Paul

So, the fact that they warp space means they don't warp time? Listen, did it ever occur to you that SR says that, "Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality." That, by the way, is a quote from Hermann Minkowski, the inventor of the concept of spacetime. In other words, you can't warp space without there existing some observer (and an inertial observer, at that) who will see at least part of that warping as a warping of time in their frame.

This is easy, obvious stuff. Please go read some textbooks and figure out for yourself how it works.

 

[Paulhoff]

I know how it works, bye.

Paul

 

[Schneibster]

Sorry but I'm done...........

Paul

Sorry, but you're woo. You have all the signs:
1. Argues against a theory that they don't show deep understanding of.
2. Doesn't present any references to support their views.
3. Combative when challenged.
4. Asks the same question over and over after it's been answered over and over, like nothing previous was said.
5. Denies the correctness of a well-known theory.

That's woo. I'd suggest if you want not to be seen that way that you go figure out how it really works; there are enough people here who know that you were bound to get in trouble on this at some point.

 

[Schneibster]

I know how it works, bye.

Paul

No, you don't, and you're spreading misinformation. That's the problem. You're denying the Theory of General Relativity.

 

[Paulhoff]

Whatever you what to believe, I'm OK with it....

Paul



There are many audio woo-woo's out there too, feel free to call me anything you want, remember I didn't name call you.