# The Naked Scientists Forum

### Author Topic: Gravity?  (Read 6056 times)

#### Zeig

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##### Gravity?
« on: 13/10/2006 00:19:52 »
Well i'm only in physical science as it is required for all freshman to take but least year i asked my teacher how gravity works. I asked How is there gravity in space if there is nothing but dust at the most inbetween the earth and the moon forsay? How is there a pull and force with nothing inbetween, and then how does rotation make gracity?! Well...he couldn't explain it. He said that gravity is from rotation and the rotation moves the dust and other masses that pull in more masses. Well i asked what is inbetween the dust particals that pulls eachother? and well i never got an answer. can some1 explain hopefully with a visual presentaion or mabey a link to a site that does explain?

#### lightarrow

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##### Re: Gravity?
« Reply #1 on: 13/10/2006 12:38:13 »
I'm sorry I don't really understand what you mean:

"...rotation makes gravity" .
"How is there gravity in space if there is nothing but dust at the most inbetween the earth and the moon forsay".

What do you mean? You have never seen two magnets or two electrically charged objects attract or repel each other from a distance, without anything in between?
Explain better what you want to ask.

If it's the concept of "force at distance" that you don't understand, then you can prefer the concept of exchanging particles, for what concern electromagnetic force, or the concept of curved space-time, for what concern gravity, according to general relativity.

In the first case, the force acts at a distance because charged particles "shoot" each other energy particles (the photons);

In the second case, the force is not present at all, bodies simply move in that way because they must follow the local geometry, like a child that must follow a precise trajectory in a slide.

The local geometry is determined by massive bodies or energy (more precisely: the stress-energy-momentum tensor) present in that place.
« Last Edit: 13/10/2006 12:53:54 by lightarrow »

#### Zeig

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##### Re: Gravity?
« Reply #2 on: 13/10/2006 21:41:47 »
If it's the concept of "force at distance" that you don't understand, then you can prefer the concept of exchanging particles, for what concern electromagnetic force, or the concept of curved space-time, for what concern gravity, according to general relativity.

that lol....

srry, when i said the rotation thing i ment that my teacher said. anything that rotates has gracity. such as earth,the moon, and wait does the sun rotate?

#### lightarrow

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##### Re: Gravity?
« Reply #3 on: 14/10/2006 07:55:45 »
quote:
Originally posted by Zeig

anything that rotates has gracity. such as earth,the moon, and wait does the sun rotate?
What is "gracity"? You mean "gravity"?
What do you mean "anything that rotates has gravity"? I understand it's your teacher who said it but since it has not any meaning for me, you should ask him.

#### chris

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##### Re: Gravity?
« Reply #4 on: 14/10/2006 10:29:54 »
Zeig

I think you need a better teacher!

The only thing that produces gravity is mass. According to Einstein's model, the Universe is a fabric of space-time which may be deformed by mass. Planets therefore sit in a "dip" they create in the space-time fabric, rather like a bowling ball sitting on the surface of a trampoline.

As a result, objects attract each other by falling into each other's dips.

I hope that explains it. It has NOTHING to do with the rotation of objects. This is what is often mis-named "centrifugal" force.

Chris

"I never forget a face, but in your case I'll make an exception"
- Groucho Marx

#### dgdavisjr

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##### Re: Gravity?
« Reply #5 on: 22/10/2006 04:13:08 »
Your teacher cannot explain gravity because no one can. Yet. It is a force that trancends light speed acting instantaneously at all distances. If the sun whent out you would know it on earth in about eight minutes, but you wouldn't know it on pluto for hours. Yet if the sun were removed from this plane of existance, you might still see it's light for a time which varies depending on your distance from it, but you would immediately lose your orbital path, commencing a straight-line flight into interstellar space. Also it is an energy force similar to magnatizim in that it exerts force without consuming energy. It forces you down with a reletively constant and inexhaustable energym uch as two similar magnetic poles resit each other with a constant repulshion no amount of force can diminish. Put them in a vice for a century and they will still repell each other when you release them. Both effects are direct violations of the laws of energy conservation. Somebody call the physics police!

D.G.Davis,Jr.

#### science_guy

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##### Re: Gravity?
« Reply #6 on: 22/10/2006 07:57:49 »
gravity is not a violation of the laws of energy conservation because you need energy in order to lift up any given object from the effects of gravity, and the energy stored in the object while set on high ground is known as potential energy.  When the object falls back down, the potential energy is released through movement.  This is the reason that you cannot make a perpetual motion device based on gravity alone, because if you could, that would be when it violates the laws of energy conservation.

As for magnetism, the magnetic effect is created in an object when electrons in a metal align in a certain way.  Aligning the electrons take energy, and the energy is released slowly while the magnetic force is being exerted.  Therefore, A single magnet could not stay viced together for a century because the electrons would go out of place the longer the magnets are being exerted.

This is why they are not violations of Physics in any way.  And a peice of logic:  If these forces are violations of physics, what are they doing existing?

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#### syhprum

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##### Re: Gravity?
« Reply #7 on: 22/10/2006 09:15:26 »
The force of gravity propagates as either as a wave (well proven) or it can be visulised as being mediated by particles (as yet hypothetical) gravitons.
the speed of propagation of this wave has been measured and found to be equal to 'c' while even if you took the view that gravitons were involved their speed would be limited to the same speed even if they have no rest mass

syhprum

#### lightarrow

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##### Re: Gravity?
« Reply #8 on: 22/10/2006 10:40:03 »
quote:
Originally posted by syhprum

...while even if you took the view that gravitons were involved their speed would be limited to the same speed even if they have no rest mass
I wouldn't say:" even if they have no rest mass", I would say: just because the have no rest mass!

I have understood what you wanted to mean, but, however, we have to be precise and say that nothing with non zero rest mass can be accelerated to the speed of light, so, only particles with no rest mass can travel at the speed of light.
(This was a little clarification, the rest you said it's ok!)
« Last Edit: 22/10/2006 10:44:55 by lightarrow »

#### syhprum

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##### Re: Gravity?
« Reply #9 on: 22/10/2006 12:59:48 »
I will be more careful with my lanuage in future

syhprum

#### Atomic-S

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##### Re: Gravity?
« Reply #10 on: 23/10/2006 07:19:53 »
quote:
if the sun were removed from this plane of existance, you might still see it's light for a time which varies depending on your distance from it, but you would immediately lose your orbital path, commencing a straight-line flight into interstellar space.
No, I believe this is incorrect. Were the sun suddenly removed, a gravitational wave would be generated which would propagate from the site at the speed of light, and only when it reached the orbit would the change occur. Of course, it is necessary to specify also that to remove the sun in a phyically admissible way, it can't siomply vanish (violates conservation of mass-energy), but must go someplace. The best way to do that is to suddenly yank it away via superstrong tether tied to a massive object a great distance away, causing the sun to experience a violent acceleration which accelerates it to near the speed of light. Such violent jerking creates, according to general relatity, the gravitational wave, but the earth will not feel any change till it reaches the earth.

#### dgdavisjr

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##### Re: Gravity?
« Reply #11 on: 25/10/2006 06:29:12 »
In fact, I believe there are Two forms of gravity, not one. Static: imagine everything in the universe floating at zero buoyancy in a giant - REALLY giant - aquarium full of water, or in this case gravity, and Active: imagine now that everything has a magnetic charge (representing gravitational attraction) and is moving at orbital and super orbital speed. If something could be rotated 90' out of the current existence plane, the event would be felt at the farthest reaches of the tank at once, even though the shock wave of spacial collapse would naturally take quite some time to propagate across our imaginary universal "pond".

D.G.Davis,Jr.

#### daveshorts

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##### Re: Gravity?
« Reply #12 on: 25/10/2006 08:50:09 »
The speed of gravity was measure in 2002 when a quasar went behind jupiter and jupiter's gravity distorted it. The measurement wasn't very accurate but was consitent with the speed of light and less than twice the speed of light. So odds are it is probably the speed of light.

#### syhprum

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##### Re: Gravity?
« Reply #13 on: 25/10/2006 09:35:27 »
Unless the speed of gravity waves is infinite it can be shown that the orbits of the planets are inherently unstable.
It is now accepted that due to the well established finite speed of propagation of gravity waves there is a degree chaos in these orbits but ejection of the Earth or its collision with the Sun is not going to happen tomorrow!.
I believe the energy disipated by Jupiter in the form of gravitational waves is about 20 watts and if you consider the amount of energy stored up due to its rotation about the sun you can see that this points to a long time scale.

syhprum

#### Zeig

• Jr. Member
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##### Re: Gravity?
« Reply #14 on: 25/10/2006 23:08:59 »
hmm...well this is a good lot to think about. lol well he's not my teacher anymore so thats okay. I have heard of the time/space sheet thing and thanks for explaining it. lol my friend who was taking quantum mechanics (iff thats is spelled right) tryed to explain it but it didn't work out so well.

to say my opinion...i don't like that theory at all. granted i may not knw much about this but if it were on a sheet for say then howcome we can position satilites all around it. a sheet that masses would be on would need to be on would have to have some sort of energy to keep them down correct? Also this is what i hate with wht i have recently learned about photons being massless yet being energy (i know energy=mass) but you have to have mass to exist don't you?

yea well knock ur heads on that lol.... [O8)] yay fro...

#### The Naked Scientists Forum

##### Re: Gravity?
« Reply #14 on: 25/10/2006 23:08:59 »