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d17182
2008-Sep-15, 08:26 PM
A friend and I were watching this TED talk given by Brian Greene in which he briefly explained Einstein's revolutionary idea of gravity being the warping of space time:
http://www.ted.com/index.php/talks/brian_greene_on_string_theory.html

Seemed pretty straightforward enough for me, but my friend asked me a question that I think I know the answer to, but am not eloquent or knowledgeable enough about astrophysics to answer convincingly.
She asked "If Earth is circling around the Sun's gravity funnel, why doesn't it fall into the funnel, like when you put a penny in one of those donation funnels, and it spins around going deeper and deeper until it falls in?"

Seems to me this analogy is confusing the metaphorical funnel of gravity in Greene's illustration with the actual funnel in real life, in which case there is the real physical funnel AND the gravity that acts on the funnel itself and the penny pulling the penny to the Earth. This wasn't good enough for her because she persisted "Yeah, but if gravity is a like funnel, and it has a pull on the Earth, pulling the Earth towards the Sun, why doesn't it pull the Earth into the Sun? Why doesn't the moon spiral down into the Earth?"

At this point, I'm struggling to avoid circular logic, because the best I can come up with is something Zen like "Everything that would have been pulled into the Sun would have been pulled into the Sun by now. Everything that would have gone floating off would have gone floating off by now. That just leaves the things with just the right amount of mass and distance to be in perfect equilibrium to still be hanging around the way they do." And weakly stating "I'm sure it has something to do with angular momentum." :confused:

By the way, I failed physics twice in college and barely passed the third time. Any help yall can lend in explaining this more eloquently would be appreciated. Thanks!

Theophage
2008-Sep-16, 03:40 AM
I think it works like this:

The penny circles the funnel and then eventually falls in, because of the friction of the penny rubbing against the funnel. The friction slows the penny down, so the penny moves to a closer "orbit" of the funnel; essentially "falling" closer because the speed isn't high enough to continue to escape at the old level.

A planet orbiting the sun isn't getting that kind of friction slowing it down, so it's orbit isn't falling inward like the penny.

Assuming I got that right, does that help?

thorkil2
2008-Sep-16, 04:06 AM
I have a fundamental objection to analogies that use gravity to explain gravity.

slang
2008-Sep-16, 08:23 AM
The penny has no gravity interaction with a central object that makes it orbit that object. It just wants to go straight ahead but its trajectory is confined by the funnel.

Jeff Root
2008-Sep-16, 10:23 AM
All analogies are flawed.

That's really important, so I'll repeat: All analogies are flawed.

However, analogies are extremely useful in learning and understanding
relationships between things. The analogy of gravity as a funnel is such
a useful analogy. Just remember that it is only an analogy.

Theophage is correct in saying that friction causes the penny to spiral
into the funnel. If there were no friction between the penny and the
surface it rolls on, of course, it wouldn't roll -- it would slide. A sliding,
perfectly frictionless penny would continue to circle the funnel forever
in the direction it is originally pushed.

Which leads us to the most important idea in this whole thing -- Newton's
first law of motion: A body in motion continues the motion unless and until
a force acts on it to change the motion. Earth would move in a straight
line forever, but gravitational attraction between the Sun and Earth pulls
the Earth out of that straight-line path into an elliptical path around the
Sun. An elliptical orbit.

For a little basic info about orbits, see my web page: Orbital Speed (http://www.freemars.org/jeff/speed/)

-- Jeff, in Minneapolis

trinitree88
2008-Sep-16, 12:52 PM
I have a fundamental objection to analogies that use gravity to explain gravity.

thorik. Which is a good idea. The generation of gravitational waves by binary pulsars is not exclusive to them, causing them to coalesce. All objects in orbit generate them, so eventually they all coalesce....it just takes a very long time. You can look at this as a sort of "friction" of motion in spacetime. pete.

Fiery Phoenix
2008-Sep-16, 01:29 PM
A planet orbiting the sun isn't getting that kind of friction slowing it down, so it's orbit isn't falling inward like the penny.


Doesn't speed also play a role in why a planet doesn't fall into its parent star?

Lepton
2008-Sep-16, 01:36 PM
A friend and I were watching this TED talk given by Brian Greene in which he briefly explained Einstein's revolutionary idea of gravity being the warping of space time:
http://www.ted.com/index.php/talks/brian_greene_on_string_theory.html

Seemed pretty straightforward enough for me, but my friend asked me a question that I think I know the answer to, but am not eloquent or knowledgeable enough about astrophysics to answer convincingly.
She asked "If Earth is circling around the Sun's gravity funnel, why doesn't it fall into the funnel, like when you put a penny in one of those donation funnels, and it spins around going deeper and deeper until it falls in?"

Seems to me this analogy is confusing the metaphorical funnel of gravity in Greene's illustration with the actual funnel in real life, in which case there is the real physical funnel AND the gravity that acts on the funnel itself and the penny pulling the penny to the Earth. This wasn't good enough for her because she persisted "Yeah, but if gravity is a like funnel, and it has a pull on the Earth, pulling the Earth towards the Sun, why doesn't it pull the Earth into the Sun? Why doesn't the moon spiral down into the Earth?"

At this point, I'm struggling to avoid circular logic, because the best I can come up with is something Zen like "Everything that would have been pulled into the Sun would have been pulled into the Sun by now. Everything that would have gone floating off would have gone floating off by now. That just leaves the things with just the right amount of mass and distance to be in perfect equilibrium to still be hanging around the way they do." And weakly stating "I'm sure it has something to do with angular momentum." :confused:

By the way, I failed physics twice in college and barely passed the third time. Any help yall can lend in explaining this more eloquently would be appreciated. Thanks!

Earth is falling towards the sun. You can illustrate it by throwing a ball a few feet and watching it gently (or not so gently) curving to the ground. then throw a ball a few feet farther and once again you notice the curve to the ground. Throw it further again and again it curves (falls) to the ground. There is a speed that the ball can be thrown where the curve to the ground never meets the ground yet continues around the globe and that is orbit. Although there is orbit the ball is still in freefall. Since there is nothing to slow the Earth's fall in space, the orbit continues.

Cougar
2008-Sep-16, 02:56 PM
The penny circles the funnel and then eventually falls in, because of the friction of the penny rubbing against the funnel.

I agree.

What good is this analogy? Well, the idea is to enable the visualization of space, which is usually kind of hard. It is space that is curved because of the mass of the Sun (or whatever). Of course this analogy is flawed, but its main point is to give some idea of the curvature of space. The idea that gravity is a force of a central object that pulls things toward it is fairly firm in the collective consciousness, but with General Relativity, one must ignore this long-held idea and view things very differently.

Lepton
2008-Sep-16, 03:00 PM
IThe idea that gravity is a force of a central object that pulls things toward it is fairly firm in the collective consciousness, but with General Relativity, one must ignore this long-held idea and view things very differently.

I'm not following that last statement, you mean gravity doesn't attract things towards itself? It is just the attracted objects are falling along geodesics which are curved by space-time.

Hornblower
2008-Sep-16, 03:51 PM
I'm not following that last statement, you mean gravity doesn't attract things towards itself? It is just the attracted objects are falling along geodesics which are curved by space-time.
I follow Cougar's last statement with relative ease. As I think I understand general relativity, an object in what we call gravitational free fall is in unforced motion along a geodesic in warped spacetime. No force pushing or pulling it in this line of thought.

mugaliens
2008-Sep-16, 06:51 PM
...why doesn't it pull the Earth into the Sun? Why doesn't the moon spiral down into the Earth?"

The friction/inertia ratio of a penny rolling in a funnel is very high. The friction/inertia ratio of a planet orbiting a star is very low.

thorkil2
2008-Sep-17, 06:21 PM
thorik. Which is a good idea. The generation of gravitational waves by binary pulsars is not exclusive to them, causing them to coalesce. All objects in orbit generate them, so eventually they all coalesce....it just takes a very long time. You can look at this as a sort of "friction" of motion in spacetime. pete.

Oh, I understand the theory and the principles very well. My problem is with graphic depictions of space-distorting gravity wells, in which the circling object eventually falls into the well. It implies gravity to explain gravity. Not acceptable, even in a world in which all analogies are imperfect.

Grashtel
2008-Sep-17, 08:18 PM
Oh, I understand the theory and the principles very well. My problem is with graphic depictions of space-distorting gravity wells, in which the circling object eventually falls into the well. It implies gravity to explain gravity. Not acceptable, even in a world in which all analogies are imperfect.
Have you got any suggestions for a better alternative?

Neverfly
2008-Sep-17, 08:40 PM
Oh, I understand the theory and the principles very well. My problem is with graphic depictions of space-distorting gravity wells, in which the circling object eventually falls into the well. It implies gravity to explain gravity. Not acceptable, even in a world in which all analogies are imperfect.
I think most of us can agree on this.
All analogies are flawed by nature. And these flaws can lead to misconceptions. But as Grashtel asks, does anyone know a better way?

tommac
2008-Sep-17, 09:22 PM
Its at equilibrium. It is moving in a straight line away from the sun. But the gravity curvature from sun ( at the distance of the earth from the sun ) is exactly enough to keep it in orbit.



A friend and I were watching this TED talk given by Brian Greene in which he briefly explained Einstein's revolutionary idea of gravity being the warping of space time:
http://www.ted.com/index.php/talks/brian_greene_on_string_theory.html

Seemed pretty straightforward enough for me, but my friend asked me a question that I think I know the answer to, but am not eloquent or knowledgeable enough about astrophysics to answer convincingly.
She asked "If Earth is circling around the Sun's gravity funnel, why doesn't it fall into the funnel, like when you put a penny in one of those donation funnels, and it spins around going deeper and deeper until it falls in?"

Seems to me this analogy is confusing the metaphorical funnel of gravity in Greene's illustration with the actual funnel in real life, in which case there is the real physical funnel AND the gravity that acts on the funnel itself and the penny pulling the penny to the Earth. This wasn't good enough for her because she persisted "Yeah, but if gravity is a like funnel, and it has a pull on the Earth, pulling the Earth towards the Sun, why doesn't it pull the Earth into the Sun? Why doesn't the moon spiral down into the Earth?"

At this point, I'm struggling to avoid circular logic, because the best I can come up with is something Zen like "Everything that would have been pulled into the Sun would have been pulled into the Sun by now. Everything that would have gone floating off would have gone floating off by now. That just leaves the things with just the right amount of mass and distance to be in perfect equilibrium to still be hanging around the way they do." And weakly stating "I'm sure it has something to do with angular momentum." :confused:

By the way, I failed physics twice in college and barely passed the third time. Any help yall can lend in explaining this more eloquently would be appreciated. Thanks!

Sime99
2008-Sep-18, 04:38 PM
Originally Posted by thorkil2
Oh, I understand the theory and the principles very well. My problem is with graphic depictions of space-distorting gravity wells, in which the circling object eventually falls into the well. It implies gravity to explain gravity. Not acceptable, even in a world in which all analogies are imperfect.

Is the penny/funnel not just a simple model of the effects of gravity in 2-spatial dimentions rather than 3? I don't think it explains anything other than showing roughly what would happen in a 2d system. And does relativity not just describe the effects of gravity rather than explain it?

The funnel/penny analogy is probably more like a LEO satellite in a decaying orbit because of the atmosphere than the earth-sun system. As was stated earlier, its the friction that does it. Take away the atmosphere and the satellite will continue on its merry way rather than crash and burn.

mugaliens
2008-Sep-18, 07:06 PM
Is the penny/funnel not just a simple model of the effects of gravity in 2-spatial dimentions rather than 3? I don't think it explains anything other than showing roughly what would happen in a 2d system. And does relativity not just describe the effects of gravity rather than explain it?

It's a very elegant way of demonstrating gravity, provided one keeps in mind the limitations, such as friction (http://www.bautforum.com/questions-answers/78884-seeking-gravitational-warping-explanation.html#post1324992).


The funnel/penny analogy is probably more like a LEO satellite in a decaying orbit because of the atmosphere than the earth-sun system.

Exactly.


As was stated earlier, its the friction that does it. Take away the atmosphere and the satellite will continue on its merry way rather than crash and burn.

Precisamundo.

loglo
2008-Sep-20, 05:42 PM
It makes a good model of a migrating hot Jupiter too! :)

Neverfly
2008-Sep-20, 08:48 PM
It makes a good model of a migrating hot Jupiter too! :)

Eh... You liked that picture eh?



Onward...:whistle:

Torg
2008-Sep-20, 09:13 PM
I've found out something recently: the whole " gravity warping space like a rubber sheet" thing actually DOES have a utility beyond showing how heavy things warp it more. It can be said that a heavy object changes the value of pi for a circle drawn all the way around it. Those diagrams that show gravity wells as downwards pointing cones successfully illustrate that in the vicinity of a very heavy object the diameter is larger than circumference/(2 pi). In the analogy its because the 2d rubber sheet space goes downwards and the pitch of the cone comes into account, in the real world its because the space becomes "positively" curved and non-euclidean. You can sort of consider precession of orbits that happens from relativistic effects as the planet sort of ending up further around than it should have because the distance all the way around is more than a full circumference cause the circumference shrinks relative to the radius in positively curved space by a miniscule amount (under normal circumstaces).

undidly
2008-Sep-20, 11:45 PM
Oh, I understand the theory and the principles very well. My problem is with graphic depictions of space-distorting gravity wells, in which the circling object eventually falls into the well. It implies gravity to explain gravity. Not acceptable, even in a world in which all analogies are imperfect.

"It implies gravity to explain gravity. Not acceptable, even in a world in which all analogies are imperfect."

I agree but you only see part of the analogy.
It does not need gravity ,just acceleration.
If the cone accelerates upwards then the coin feels a force just like gravity.
Will the cone go faster and faster?.In a straight line yes ,in a circle no.
Imagine the funfair "stick on the wall centrifuge" made of rubber and far from Earth.Put in a mass and spin it up.The mass makes a dent (cone ) in the wall.
Other masses fall into this dent or roll around it(orbit)if they have a side movement.

This is the cone that the coin rolls in.I behaves as though there is gravity but it is centrifugal force.
The dent ,at right angles to the 2D surface,in the wall is the equivalent of the dent ,at right angles to the 3D "surface" of the universe we live in.
The spin of the universe provides the centrifugal force for the 3D analogy.

A 4D person outside the universe would see these dents as we see the 3D dents
in flat rubber sheet.

Jeff Root
2008-Sep-20, 11:56 PM
I see the fact that the analogy uses gravity to explain gravity as amusing,
but not a problem. But I can imagine that some people might be confused
by it. I'd be interested to learn of actual cases of that confusion, which
would take some effort to learn from the confused people.

-- Jeff, in Minneapolis

mugaliens
2008-Sep-21, 02:37 AM
But I can imagine that some people might be confused
by it. I'd be interested to learn of actual cases of that confusion, which
would take some effort to learn from the confused people.

I'm confused...

loglo
2008-Sep-21, 03:33 PM
Eh... You liked that picture eh?



Onward...:whistle:

I used to call him No 2! It seemed more in line with his acting ability.

thorkil2
2008-Sep-21, 09:55 PM
But as Grashtel asks, does anyone know a better way?

Actually, yes. If you take a 2-dimensional grid, then place a circle (representing a sphere in 2-dimensions) in the grid so that it distorts the grid lines around it, you have a set of curved geodesics. If you define linear motion through space as being always at right angles to some arbitrary set of the grid lines (they are an arbitrary description of the space, so any motion in empty space is by definition perpendicular relative to some set), and define motion as always maintaining that perpendicularity to that set, then as a moving object encounters the distortion, and maintains the perpendicular relationship, the straight line of its motion is bent toward the distorting body. Also not a perfect analogy, but it doesn't require something to fall into a hole to demonstrate why something falls into a hole. BTW, this was not related to the penny in the chute example so much as the textbook graphics of planes distorted by gravity well depressions in the surface, which imply a "falling in."