freefall=no force?

[dstebbins]

I read somewhere that when an object accelerates to a source of gravity, it is actually following a constant velocity due to no net force, but only appears to be accelerating because the gravity is stretching the space and time around it so that a cubic meter of space labeled X would be smaller than a cubic meter of space labeled Y directly in between it and a gravity source (I'm sorry if that just confuses you, but that's the best I can do). Call me stupid, but that doesn't make a bit of sense.

 

If only space and time are affected by gravity, and not matter, then why do objects have weight? Weight is a force resulting from gravity, but if freefalling objects are traveling at a constant velocity, then no forces are acting on it, yet an object has the force of weight acting on it, so if gravity has no affect on matter, then that can only mean that weight is coming from some other force.

 

What the hell is going on, here? Am I the only one who sees this paradox?

[Klaynos]

It is still accelerating, the reason for this acceleration is a little complex in general relativity. But it is still accelerating, if you measure the affect you would measure the acceleration.

[foodchain]

I read somewhere that when an object accelerates to a source of gravity, it is actually following a constant velocity due to no net force, but only appears to be accelerating because the gravity is stretching the space and time around it so that a cubic meter of space labeled X would be smaller than a cubic meter of space labeled Y directly in between it and a gravity source (I'm sorry if that just confuses you, but that's the best I can do). Call me stupid, but that doesn't make a bit of sense.

 

If only space and time are affected by gravity, and not matter, then why do objects have weight? Weight is a force resulting from gravity, but if freefalling objects are traveling at a constant velocity, then no forces are acting on it, yet an object has the force of weight acting on it, so if gravity has no affect on matter, then that can only mean that weight is coming from some other force.

 

What the hell is going on, here? Am I the only one who sees this paradox?

 

 

would that mean falling is a different type of motion then also?

[dstebbins]

It is still accelerating, the reason for this acceleration is a little complex in general relativity. But it is still accelerating, if you measure the affect you would measure the acceleration.

 

OOOOOH, something about science I don't know yet! Sounds sexy! *pops a boner!*

 

Tell me about it, cuz in case you haven't gathered it yet, science is better than porn!

[Klaynos]

My general relativity is not really up to scratch (it's pretty much non-existant) so I'm going to have to point to the experts and hide under the table...

[wilgory]

dstebbins,

 

In general relativity (GR), it is mass that warps space-time, not gravity. Gravity is the result. Perhaps you should reread the article or better yet go to:

 

http://en.wikipedia.org/wiki/Introduction_to_general_relativity

 

In a vacuum,on earth, an object "apparently" accelerates in freefall at 22 ft per second/per second. In GR motion is relative and when an object is being held up by a table or something it is "actually" accelerating with respect to freefall. Freefall is the natural state (following the curvature of space-time caused by the mass of the earth).

 

As Klaynos said GR is complex and I am not an expert, just an interested layman. The link above is a good place to start but you should not accept it as the final authority.The web has many sites dealing with GR to varying degrees.

[dstebbins]

Thanks for the article, but I'm not good at understanding things with just words alone. I usually need an illustration, sometimes even a moving illustration, to understand science. I'm a visual kind of person. Right down to scientific formulas. I need to see the formula in writing on the blackboard, because if my teacher were to orally say it, I'd be lost. Do you know any webpages that offer illustrations to help explain the lessons?

[theCPE]

An easy illustration of general relativity:

 

Place a few balls (ping pong, golf, tennis, marbles even) in random spots on your bed.

 

Stand in the center of your bed.

 

The surface of your bed is spacetime being bent by mass (you).

 

The balls will roll to you. (gravity)

 

If the balls had energy or motion they would not roll directly to you, but instead would orbit or curve their path due to you.

 

That is general relativity and gravity.

 

The spacetime curvature makes objects accelerate.

 

Newtonian physics calls gravity a force between massive objects.

 

Either way, an acceleration occurs due to mass.

[wilgory]

dstebbins,

 

Here are some links to some good sites. Due to the size, use the search feature on the NASA site to find the subject you want. As far as visuals go, you will find some on these sites, but relativity is better understood using math. My math isn't that good so I do the best I can by reading.There are some visuals on these pages and links to more.You can google something like "general relativity visual tutorial" and find many more.

 

http://www.math.ucr.edu/home/baez/RelWWW/

 

http://www.astro.ucla.edu/~wright/cosmolog.htm

 

http://www.nasa.gov/

 

http://archive.ncsa.uiuc.edu/Cyberia/Expo/cosmos_nav.html

[foodchain]

An easy illustration of general relativity:

 

Place a few balls (ping pong, golf, tennis, marbles even) in random spots on your bed.

 

Stand in the center of your bed.

 

The surface of your bed is spacetime being bent by mass (you).

 

The balls will roll to you. (gravity)

 

If the balls had energy or motion they would not roll directly to you, but instead would orbit or curve their path due to you.

 

That is general relativity and gravity.

 

The spacetime curvature makes objects accelerate.

 

Newtonian physics calls gravity a force between massive objects.

 

Either way, an acceleration occurs due to mass.

 

Right, but then everything should always go planar in regards to geometry then right? IN space or elsewhere, or am I getting something wrong?

[theCPE]

Well the bed illustration is not 3 dimensional as actual spacetime curvature would be.

 

Planets and stars aren't actually sitting on spacetime like you would be standing on your bed.

[foodchain]

Well the bed illustration is not 3 dimensional as actual spacetime curvature would be.

 

Planets and stars aren't actually sitting on spacetime like you would be standing on your bed.

 

well its a sound analogy really just that I don’t understand then why differentiation would take hold overall in regards to say rings around a planet for instance. Such as the orientation they take can be different or not always planar, though I don’t know if this a fringe of a solar system effect, or if the effect of gravity on spacetime will simply vary in each situation it comes to bear in.

[Farsight]

A body in freefall is not accelerating. If you say it is, you're mixing reference frames. You're looking at it from some reference frame out somewhere in space rather than from the reference frame of the body in freefall. The principle of equivalence between gravity and an accelerating frame applies when you're standing on the planet, not when you're in freefall. There really is no force acting on that freefalling body. You can't feel any, you can't see any, you can't measure any. Because there is none. And if you look out of the window to see the moon going backwards you're mixing frames again. This is why gravity is not technically a force. Einstein called it a pseudoforce. Note that gravity is not spacetime curvature. A change in gravity is, though even that is open to new interpretation.

 

I could say more, but people who don't know what they're talking about give me sneering abuse, say I'm talking crackpot nonsense, and kick me into pseudoscience. Not good.

[insane_alien]

ahh so when something is in free fall the entire universe accelerates past it. yeah that makes so much more sense.

 

go jump out a plane(parachute optional) and TELL me that you didn't accelerate.

[Farsight]

Go learn some relativity.

[theCPE]

A body is initially accelerated by the curvature of spacetime; imagine a body traveling a specific path at an unchanging pace. If it encounters a spacetime curvature, the body is accelerated and its path, and possibly pace changes.

[Royston]

Go learn some relativity.

 

Farsight, the equivalence principle would be applicable if the person free falling was in a box, but there are two frames of reference here...the accelerating frame, and earth. The two frames are distinguishable, so you can conclude the free falling body is accelerating.

[Farsight]

Not when you're in the box.

 

It's simple. You're in the box. There are no windows. You're in freefall. You chuck a ball across the inside of the box and it goes straight as a die, bounces off the side, and back into your hand. You are in an inertial reference frame. You can feel no gravitational force, and you can detect no gravitational force acting upon the ball.

 

Now one hour later you're still in the box, but you landed safely on earth. You chuck a ball across the inside of the box and... it falls in a parabola and you miss it. Because you're in a non-inertial reference frame. You can feel the gravitational force as weight, and you can detect it acting on the ball.

 

The second situation is "equivalent" to this:

 

You're in the box, and it's sitting on top of a rocket powering through space accelerating at 9.8m/s/s. You chuck a ball across the inside of the box and it falls to the floor. Et cetera.

 

But they aren't quite equivalent, because when you're on earth you are in a "proper" gravitational field rather than a "uniform" gravitational field. You can detect the slight change in gravity between the top and the bottom of the box via a Pound-Rebka experiment wherein there is a measurable photon redshift. The gravitational field diminishes with distance from the earth.

[Klaynos]

Just wondering, you are accelerating through a magnetic field (the earths), so if you hold out a wire, can you measure a charge on it?

 

Secondly accelerating charges ALWAYS radiate, so surely there would be some radiation from that coming from you?

[dstebbins]

An easy illustration of general relativity:

 

Place a few balls (ping pong, golf, tennis, marbles even) in random spots on your bed.

 

Stand in the center of your bed.

 

The surface of your bed is spacetime being bent by mass (you).

 

The balls will roll to you. (gravity)

 

If the balls had energy or motion they would not roll directly to you, but instead would orbit or curve their path due to you.

 

That is general relativity and gravity.

 

The spacetime curvature makes objects accelerate.

 

Newtonian physics calls gravity a force between massive objects.

 

Either way, an acceleration occurs due to mass.

 

Okay, let me try and put these into my own words.

 

A massive object pulls on spacetime around it, causing a ripple in spacetime. The spacetime around it that hasn't been affected yet then swarm around this "nothingness" to fill it up, just like gas molecules do in a vacuum, and when spacetime shifts to fill up the nothingness, so does that mass and energy in said spacetime.

 

Is that in the ball park?

[theCPE]

Er.

 

Not exactly.

 

The bodies that cause curvature of spacetime don't just suddenly appear causing the shifts.

 

Also, it isn't like molecules filling gaps either. The best way to look at it is that the actual dimension has been distorted.

[dstebbins]

Okay, so if the demensions were like a power line, with the electricity going through it representing the matter, then if you suddenly bend and twist the power line, the electricity will go in the new direction that the power line is headed, but we see it as changing direction. Is that a closer paraphrase?

[wilgory]

dstebbins,

 

In General Relativity, gravity is nothing more than the warping of space-time. No other particles or energies or forces involved.

 

The bowling ball on a trampoline analogy is 2 dimensional, with a fabric being distorted. Gravity is a 4 dimensional warping of space-time without any fabric involved.

 

General Relativity is not easily comprehended without knowing some non-euclidean geometry (my understanding is limited because I do not know more than the basics), but if you want to understand gravity you need to learn enough to understand General Relativity.

 

This is a link to a site with some diagrams that may help.

 

http://www.astro.ucla.edu/~wright/relatvty.htm

[dstebbins]

dstebbins,

 

In General Relativity, gravity is nothing more than the warping of space-time. No other particles or energies or forces involved.

 

The bowling ball on a trampoline analogy is 2 dimensional, with a fabric being distorted. Gravity is a 4 dimensional warping of space-time without any fabric involved.

 

General Relativity is not easily comprehended without knowing some non-euclidean geometry (my understanding is limited because I do not know more than the basics), but if you want to understand gravity you need to learn enough to understand General Relativity.

 

This is a link to a site with some diagrams that may help.

 

http://www.astro.ucla.edu/~wright/relatvty.htm

 

........Okay, could you put that link in laymen's terms?

[wilgory]

I don't think I can do any better than this.

 

http://math.ucr.edu/home/baez/physics/Relativity/GR/gravity.html

 

You are trying to visualize something than might be impossible unless you have a mind similar to Einstein's. Try to visualize this. In non-euclidean geometry the closest distance between 2 points is not a straight line.

 

Then consider that you have to visualize a 4 dimensional warp, when all you know is 3 dimensional.

 

That's why the only way to understand gravity is to learn the math.

 

Do not be discouraged. There are many people on this forum that are much better than I at explaining these things. Hopefully some will help you grasp these idea's. It took me years to get where I'm at in understanding what we "know" about the universe. The Internet helped me immensely. For years I had to rely on books where it was impossible to ask questions.

 

All the luck on your search for understanding,

 

Wilgory