difference between free fall and zero gravity?

[Ziggurat]

You win, if you don't understand that is not my problem.

Understand what? Your position? Quite possibly, and it wouldn't be the first time I've misunderstood another poster. But I understand the equivalence principle just fine.

 

[Paulhoff]

This suggests that gravity is in reality the same as the pseudo force experienced by an observer in an accelerated frame of reference.

But acceleration use to make gravity will someday be a great percentage of light, acceleration is not gravity, nothing can orbit the space ship because of acceleration. Acceleration is not consent, gravity is. It was only a thought experiment.

Paul

 

[Ziggurat]

But acceleration use to make gravity will someday be a great percentage of light,

So? If a uniform gravitational field is large enough (spatially - its strength doesn't need to be large), falling objects will also reach a significant fraction of the speed of light if left to drop for long enough.

acceleration is not gravity, nothing can orbit the space ship because of acceleration.

Orbits are only possible in non-uniform fields. And it is the non-uniformity of real gravitational fields which is the ONLY distinguishing feature compared to acceleration. And it's the only reason one needs to go to GR instead of SR.

 

[Paulhoff]

So? If a uniform gravitational field is large enough (spatially - its strength doesn't need to be large), falling objects will also reach a significant fraction of the speed of light if left to drop for long enough.

No, if you even drop an abject from a light year out, it will only hit the earth at 7 mps no more than escape velocity, the sun would be about 383 mps.

Paul

 

[Ziggurat]

No, if you even drop an abject from a light year out, it will only hit the earth at 7 mps

Because the earth's gravitational field isn't uniform, but falls off with distance. Were the field uniform out that far, your dropped object would indeed reach relativistic speeds by the time it hit the ground.

 

[Paulhoff]

Because the earth's gravitational field isn't uniform, but falls off with distance. Were the field uniform out that far, your dropped object would indeed reach relativistic speeds by the time it hit the ground.

There isn't any uniform field..... geeeeee

Paul

 

[Ziggurat]

There isn't any uniform field..... geeeeee

I've never claimed otherwise, and you'll notice I used the qualifier "uniform" rather consistently when comparing gravity and acceleration.

Are you familiar with the term "tangent space", by any chance?

 

[Paulhoff]

Are you familiar with the term "tangent space", by any chance?

No, never ran into it, looked quickly on the internet, it would that some time to study...........

Paul

 

[Cuddles]

This seems a pretty silly argument. Gravity is not an acceleration, it is a force. Gravity cannot be indisdinguishable from acceleration because it is a completely different thing. However, what you seem to be arguing about is the acceleration due to gravity. Now, the clue there is in the name. The acceleration due to gravity is indistinguishable from an acceleration because it is an acceleration. End of argument.

What you two seem to be arguing about is that Ziggurat says that if you have a force that creates an identical acceleration to that from gravity it is impossible to tell the difference, while Paulhoff says that that could never happen in reality. And you are both right. Now stop arguing.

 

[Ocelot]

This seems a pretty silly argument. Gravity is not an acceleration, it is a force.

That is one point of view.

Another point of view is that gravitry represents a warping of space time that produces a pseudo force in the same way as acceleration does.

Gravity cannot be indisdinguishable from acceleration because it is a completely different thing.

Unless it's not a completely different thing at all.

However, what you seem to be arguing about is the acceleration due to gravity. Now, the clue there is in the name. The acceleration due to gravity is indistinguishable from an acceleration because it is an acceleration. End of argument.

Good point.

What you two seem to be arguing about is that Ziggurat says that if you have a force that creates an identical acceleration to that from gravity it is impossible to tell the difference, while Paulhoff says that that could never happen in reality. And you are both right. Now stop arguing.

Actually IF that's all that Paulhoff is saying then he'd making an assumption that's hard to establish one way or another. If instead he's saying that he doubts it could happen because nothing in human experience suggests a precident for it then that's another matter.

Try proving that the entire universe isn't being uniformly accelerated in one particular direction....

 

[Paulhoff]

There is no assumption, gravity and acceleration may for a time seem the same, but in the long term are not the same. Gravity warps space acceleration does not. Gravity need no input of power, acceleration does. The slowing down of time by gravity is constant, acceleration in not. Gravity needs no movement, well acceleration does.

Paul

 

[Ziggurat]

There is no assumption, gravity and acceleration may for a time seem the same, but in the long term are not the same. Gravity warps space acceleration does not.

Depends on what exactly you mean by that, but much of what we attribute to "warped" space-time does exist for accelerated reference frames as well. For example, did you know that acceleration creates an event horizon? Strange but true. The only difference between gravity and acceleration is the non-uniformity of gravitational fields. With acceleration in a flat space-time, it's possible to choose a single unaccelerated reference frame, whereas with non-uniform gravity, you can only do that locally (which is where tangent spaces comes in).

 

[NobbyNobbs]

There is no assumption, gravity and acceleration may for a time seem the same, but in the long term are not the same. Gravity warps space acceleration does not. Gravity need no input of power, acceleration does. The slowing down of time by gravity is constant, acceleration in not. Gravity needs no movement, well acceleration does.

Paul

I don''t think anyone here is claiming that gravity and acceleration are the same. The claim is that acceleration due to gravity is indistinguishable from acceleration due to any other force. Einstein made this claim, and subsequent experiments have shown it to be true.

 

[Schneibster]

There is no assumption, gravity and acceleration may for a time seem the same, but in the long term are not the same.

Hmmm. I think you might not quite get the point. The appearance of the universe to one who is accelerating is central to Einstein's understanding of the nature of gravity.

Gravity warps space acceleration does not.

Actually, this is incorrect. Acceleration warps space in exactly the same way that someone who is motionless within a gravitational field at a point which experiences the same acceleration sees it.

Gravity need no input of power, acceleration does.

Resisting the pull of gravity, which leads to acceleration, does. In a gravity field, to stay in the same place, you have to continuously accelerate. (Stop thinking about the surface of a planet, and imagine yourself in a gravity field in open space, perhaps suspended 200 miles above the surface of the Earth, for example. If you wish to remain in the same place, you must exert thrust. If you do, you will experience acceleration- without moving. Perhaps that will make it clearer for you.)

The slowing down of time by gravity is constant, acceleration in not.

According to whom? The only valid observer is the inertial observer- and in a gravity field, that observer is falling, and therefore moving deeper in the gravity field, and from that observer's point of view, the slowing of time for the object that stays motionless and experiences acceleration is not constant.

Gravity needs no movement, well acceleration does.

Gravity is a warping of spacetime in such a manner that an inertial observer must necessarily be moving relative to an inertial observer outside the field (or so much farther away that the effect of the field is unmeasurable). On the other hand, inertial observers in a gravity field report the same results as inertial observers outside one, for experiments that do not look at gravity.

 

[Schneibster]

One last point. Special Relativity says there is no absolute motion. But that doesn't mean there is no absolute acceleration; in fact, as far as we can tell, acceleration is absolute. You can always tell if you're accelerating by performing a local experiment. What you can't tell is whether that acceleration is due to acceleration by gravity, or by another force.

 

[Paulhoff]

Actually, this is incorrect. Acceleration warps space in exactly the same way that someone who is motionless within a gravitational field at a point which experiences the same acceleration sees it.

Acceleration does not wrap space..............

Paul

 

[Paulhoff]

One last point. Special Relativity says there is no absolute motion. But that doesn't mean there is no absolute acceleration; in fact, as far as we can tell, acceleration is absolute. You can always tell if you're accelerating by performing a local experiment. What you can't tell is whether that acceleration is due to acceleration by gravity, or by another force.

Absolute motion has nothing to do with gravity. And once again I will state that if one does not see outside the ship that is true, but once one see outside the ship, one knows it is acceleration and not gravity holding them to the floor.

Paul

 

[Schneibster]

Acceleration does not wrap space..............

Paul

Ummm, well, actually it does.

Acceleration^WP: "With changing velocity, accelerated objects exist in warped space (as do those that reside in a gravitational field). Therefore, frames of reference must include a description of their local spacetime curvature to qualify as complete."

Let's say you don't like Wikipedia. Fine. Here's the deal: We define local spacetime curvature in an inertial frame as "flat." Objects moving at a constant velocity are inertial. To an observer who is in an accelerated frame of reference, such objects move not in a straight line, but in a curve. It is in fact the nature of accelerated frames of reference that all inertial objects will move in curves with respect to them. Since we have already defined inertial motion as being in a straight line, and space in an inertial frame as flat, it follows that an accelerated frame must be a curved frame; otherwise, straight lines would still be straight, and they are not, they are curved. Observers in accelerated frames therefore observe spacetime curvature, because inertial objects move in curves. The degree of curvature of the frame's spacetime can be directly measured, by measuring the curvature of the motion of inertial objects in that frame.

But hang on a minute: to an observer motionless in a gravity field, all inertial motion is curved. Thus, it directly follows that for such an observer, spacetime is curved; and that curvature is called "gravity."

There is no difference between gravity and acceleration that cannot be explained by the fact that a gravity field is spherical. Locally, acceleration and gravity are identical in every respect. That is the meaning of the equivalence principle.

 

[Schneibster]

Absolute motion has nothing to do with gravity.

I never said it did. What I said is that absolute acceleration has to do with gravity. Please read what I say if you want to have a meaningful conversation.

And once again I will state that if one does not see outside the ship that is true, but once one see outside the ship, one knows it is acceleration and not gravity holding them to the floor.

Paul

Incorrect. You are postulating some "absolute frame of reference" with respect to which motion can be defined. There is none. And if you can't determine your absolute velocity, how then will you determine that you are accelerating? The ONLY way is to determine the observed behavior of inertial objects. If they are moving in straight lines, then you are not accelerating. If they are moving in curves, then you are accelerating. It's as simple as that. Inertial objects are the key to absolute acceleration. And absolute acceleration is the proof of absolute spacetime. That's relativity.

Gravity and acceleration are equivalent; to stay motionless in a gravity field with respect to inertial objects outside it, you must constantly accelerate. Gravity curves space and makes this necessary. Inertial objects in a gravity field move in curves, not in straight lines; in other words, geodesics in a gravity field are curved, whereas outside one they are straight.

 

[Paulhoff]

To an observer who is in an accelerated frame of reference, such objects move not in a straight line, but in a curve. It is in fact the nature of accelerated frames of reference that all inertial objects will move in curves with respect to them. Since we have already defined inertial motion as being in a straight line, and space in an inertial frame as flat, it follows that an accelerated frame must be a curved frame; otherwise, straight lines would still be straight, and they are not, they are curved. Observers in accelerated frames therefore observe spacetime curvature, because inertial objects move in curves. The degree of curvature of the frame's spacetime can be directly measured, by measuring the curvature of the motion of inertial objects in that frame.

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

Paul