Why The Field Of Gravity Is Negative?

[Nicholas Kang]

An article about the findings of swirls of wave patterns in The BICEP 2 in South Antarctica has shown that the physics of inflation is true. Alan Guth of MIT must be happy. The findings are published in a section in BBC Knowledge`s magazine Asia Edition. The book introduce quantum fluctuation and the fact that gravitational field is negative. I understand quantum fluctuation but why the field of gravity is negative. Here is a brief description from the book

 

Imagine ions/atoms forming the sun spread over distance. Given the fact that gravity follows inverse square law, they would have zero gravitational force. When they eventually mix up and fall together and jostle between one another, releasing heat and light. The atoms start up with zero energy, so now they have negative energy. A simple calculation shows that the mass-energy of the atoms can simply offset the negative gravitational energy, so basically I have blots of star materials but out of nothing at all.

 

Shouldn`t the gravity between particles increase as they get closer to each other? Why the magazine seemed to contradict this fact?

 

Please shed light on this issue. Thanks.

 

Serving the scientific community,

 

Nicholas Kang

[swansont]

The force is attractive, so the potential energy is negative.

 

You start with zero at long distances. As they attract they speed up, meaning kinetic energy increases, and that's positive. This had to come from the PE, so the PE must be negative.

 

You are free to define the PE at infinite separation to be something other than zero, but that's probably only going to complicate matters. Zero makes for a simple formula.

[Nicholas Kang]

Ah, you mean Potential energy. The article didn`t mention anything about PE. So, the inverse square law is true for both PE and gravity?

[imatfaal]

Ah, you mean Potential energy. The article didn`t mention anything about PE. So, the inverse square law is true for both PE and gravity?

 

NO. Inverse square works for the force. The potential energy varies with the inverse of distance. This makes sense if you consider that work done is force times distance.

[latex]F= -\frac{GMm}{r^2}[/latex]

 

[latex]U= -\frac{GMm}{r}[/latex]

 

Work = Change in Energy = Force x Distance

[Nicholas Kang]

Ok, get it. So, the further something is from something, the lesser the potential energy, the higher the attractive force. And what the article mentioned is actually using mass-energy of the object to offset its negative potential energy, not the gravity?

I have found a link of Nature`s article in youtube:

Edited September 10, 2014 by Nicholas Kang

[imatfaal]

Ok, get it. So, the further something is from something, the lesser the potential energy, the higher the attractive force. And what the article mentioned is actually using mass-energy of the object to offset its negative potential energy, not the gravity?

 

Note what SwansonT wrote

 

"You start with zero at long distances. As they attract they speed up, meaning kinetic energy increases, and that's positive. This had to come from the PE, so the PE must be negative."

 

Max potential gravitational energy is defined as zero. Your post is a bit mangled at present - you have things flipped. Have a read of Hyperphysics

 

http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html#ui

 

It is essential you get your directions correct in your head - I think this page is one of the best to get it sorted correctly at the begining.

[Nicholas Kang]

Ok, the gravitational energy is referred to as gravitational potential energy. So, should I just substitute the term "gravitational wave energy" into every energy that I had mentioned before and then I can get the correct answer and it still implies the same meaning?

[imatfaal]

Ok, the gravitational energy is referred to as gravitational potential energy. So, should I just substitute the term "gravitational wave energy" into every energy that I had mentioned before and then I can get the correct answer and it still implies the same meaning?

 

Gravitational Potential Energy DOES NOT EQUAL Gravitational Wave Energy

 

Gravitational Potential energy is well defined, I have given the formula for calculating it, and a webpage that talks through the idea; it can be referred to as gravitational energy - but it is well to keep the potential in the phrase.

 

Gravitational Waves are a prediction of General Relativity - that are yet to be directly measured - and are not the same thing as GPE. Gravitational waves do carry energy (as well as momentum and angular momentum). They are in effect the propagation of a change in the curvature of space time - they move outwards from a varying gravitational source at the speed of light.

 

Get your knowledge of Gravitational Potential Energy - which can be discussed at a classical and Newtonian level - straight before moving on to General relativity and its ramifications; this directly relates back to your question elsewhere about why we teach things in certain orders.

 

Importantly - "every energy that I had mentioned before and then I can get the correct answer" No - as I mentioned your post 5 is incorrect.

[swansont]

You have to be especially careful of terminology here, and you can't rely on a pop-sci article to get it all correct. For an additional example, gravity waves and gravitational waves are not the same thing.