# Why two mass attracts each other?

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#### hansda

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Newton, in his Law of Gravitation explains that two mass attracts each other and developed an equation for this gravitational force of attraction between two mass.

Two electrical charges of opposite polarity also attracts each other.

Two magnets of opposite polarity also attracts each other.

Why two mass attract each other and not repel each other?

Does a mass also have some kind of polarity?

Mass does have some kind of polarity, but only one kind, not two. And it is attractive.

Mass does have some kind of polarity, but only one kind, not two. And it is attractive.

We know that, "Like pole repels". If the polarity is one kind, that means it is "like pole". So, two mass should repel but why they are not repelling rather attracting?

The why of gravity is still one of the big questions that's being explored in today's physics.

We know that, "Like pole repels". If the polarity is one kind, that means it is "like pole". So, two mass should repel but why they are not repelling rather attracting?
Like charges also repel. You ask:

So, two mass should repel but why they are not repelling rather attracting?
I ask: Why should masses follow the same principles as charges and magnets? Why can't they be the opposite?

There may be a reason behind your question, or there may be no reason, it may just be fundamentally so.

Like charges also repel. You ask:

I ask: Why should masses follow the same principles as charges and magnets? Why can't they be the opposite?

There may be a reason behind your question, or there may be no reason, it may just be fundamentally so.

Even if it is fundamentally so, there has to be some reason.

Even if it is fundamentally so, there has to be some reason.

No, if it is fundamentally so, then by definition there is no reason.

Whatever the reason is, no one currently knows for sure.

It depends what your starting place is for the fundamentals of a model. For example general relativity describes gravity in terms of a metric, a way of measuring distances. This metric defines distances along a path by $$d = \int_{\textrm{path}} \sqrt{g_{ab}\dot{x}^{a}\dot{x}^{b}}dt$$. The metric is the object $$g_{ab}$$ which basically generalises Pythagoras's theorem to allow for curved space. There are 2 indices on this object. Electromagnetism describes the electromagnetic field in terms of a gauge potential, $$A = A_{a}dx^{a}$$. This has 1 index. It is possible to show in quantum field theory that if you quantise such formulations you find that the force due to the 1 index objects can both repel and attract but for those forces due to 2 index objects only attraction is possible. This doesn't require us to explicitly formulate quantum gravity, it is based on more general methods.

We can go a little further in justifying/explaining these things. If a force is described by a mathematical object with 0, 1 or 2 indices then it is known as spin 0, 1 or 2. For spinor fields (ie matter like electrons etc) the spin is half integer, so the electron is spin 1/2, while there's a hypothetical extension which includes spin 3/2. But what about spin 5/2 and above? Well you can again show from relatively high level arguments in quantum field theory that if you obtain spin 5/2 particles you are going to get inconsistent dynamics (causality violation and all that). So we've now narrowed down a reason for why we don't see other, weirder, forces.

Using some pretty general arguments about the structure of quantum field theories we can explain why the photon is massless and doesn't interact with itself, assuming the electron obeys a particular symmetry. We can explain why in the absence of any knowledge of the weak force we can still predict its existence. We can also motivate why, despite being allowed by the spin argument I just mentioned precluding 5/2 and above, we don't see spin 3/2 particles. Using arguments of gauge invariance, Lorentz invariance, renormalisation, causality and a hint of Occam's razor you are lead pretty much to the form of the Standard Model without needing to do many experiments. Experiments then amount to trying to find the values of the coefficients which are not fixed by these arguments.

It is possible to show in quantum field theory that if you quantise such formulations you find that the force due to the 1 index objects can both repel and attract but for those forces due to 2 index objects only attraction is possible. This doesn't require us to explicitly formulate quantum gravity, it is based on more general methods.

This may raise the question on why something has a certain no. of indices. From a reductionist standpoint we can reduce things to the fundamentals, though we can never be sure if it is truly fundamental.

As per GR, space-time curves around a mass to generate force.

Is it possible that, curvature of space-time around two mass will form a loop and because of this looping of space-time; two mass will attract but not repel.

Imagine two observers, each one standing at a different point along the earth's equator. Now both observers start walking north - what happens ? The further north they get, the more they approach each other, until, at the north pole, they finally meet. There is no force between the two, their approach is purely due to the geometric properties of the earth's surface.

Likewise in GR. Substitute "observers" by "masses", and "north" by "the future". As time passes, the two masses propagate through space-time, slowly approaching one another due to the presence of curvature, Eventually they collide. There are no forces involved, this is a purely geometric phenomenon, and it is quantified in the so-called Rhaychaudhuri Equation.

Imagine two observers, each one standing at a different point along the earth's equator. Now both observers start walking north - what happens ? The further north they get, the more they approach each other, until, at the north pole, they finally meet. There is no force between the two, their approach is purely due to the geometric properties of the earth's surface.

What drives both the observers to walk towards NORTH? Why one of the observer does not go towards SOUTH?

Likewise in GR. Substitute "observers" by "masses", and "north" by "the future". As time passes, the two masses propagate through space-time, slowly approaching one another due to the presence of curvature, Eventually they collide. There are no forces involved, this is a purely geometric phenomenon, and it is quantified in the so-called Rhaychaudhuri Equation.

In that case all the mass of our Universe should collide or our Universe should "shrink" but instead our universe is expanding.

The why of gravity is still one of the big questions that's being explored in today's physics.

For sure. In fact I am convinced we have explained things well. We are told the reason planets do not crash into each other is the gravitational pull of the sun is greater than the gravitational pull of one planet to another. Okay, so why aren't all the planets sucked into the sun?

In that case all the mass of our Universe should collide or our Universe should "shrink" but instead our universe is expanding.

Exactly

Actually here are two theories http://news.nationalgeographic.com/...iverse-expansion-what-is-dark-energy-science/ But considering such things are not known for sure, it is humorous the some science folks are so sure there is no God.

For sure. In fact I am convinced we have explained things well. We are told the reason planets do not crash into each other is the gravitational pull of the sun is greater than the gravitational pull of one planet to another. Okay, so why aren't all the planets sucked into the sun?

In the Solar system, the planets are not merging with the Sun because they are in their respective orbits and orbiting around the Sun.

Had all the Planets merge with the Sun, perhaps it will create a Black-Hole.

We are told the reason planets do not crash into each other is the gravitational pull of the sun is greater than the gravitational pull of one planet to another. Okay, so why aren't all the planets sucked into the sun?

I think, all the planets that could've been sucked have been sucked. We are what's left.

You can play with this cool tool:

http://www.nowykurier.com/toys/gravity/gravity.html

For sure. In fact I am convinced we have explained things well. We are told the reason planets do not crash into each other is the gravitational pull of the sun is greater than the gravitational pull of one planet to another. Okay, so why aren't all the planets sucked into the sun?

Imagine that you were burning a ball of gasoline in space. It was a burning sphere, transforming mass into heat. The sphere is forcing energy away from it. The hotter the sphere the more force in the outward direction. There is a reason that you can't get too close to the sun without burning up. Even if you didn't burn up, you would have to FORCE your way to the sun, and the closer you got to the sun the more force you would have to use to get closer, and the closer you got the more force you would need.

Now think opposite. The sun and its extreme power is forcing the planets away. The planets are orbiting the sun and getting farther away because the sun is FORCING THEM AWAY!

For sure. In fact I am convinced we have explained things well. We are told the reason planets do not crash into each other is the gravitational pull of the sun is greater than the gravitational pull of one planet to another. Okay, so why aren't all the planets sucked into the sun?

The sun keeps trying. But the planet's momentum means that they always miss the sun and "fall" around it instead.

Imagine that you were burning a ball of gasoline in space. It was a burning sphere, transforming mass into heat. The sphere is forcing energy away from it. The hotter the sphere the more force in the outward direction. There is a reason that you can't get to close to the sun without burning up. Even if you didn't burn up, you would have to FORCE your way to the sun, and the closer you got to the sun the more force you would have to use to get closer, and the closer you got the more force you would need.

Now think opposite. The sun and its extreme power is forcing the planets away. The planets are orbiting the sun and getting farther away because the sun is FORCING THEM AWAY!

Haha, nope. The Sun ain't "burning" like a ball of gasoline. Not the same.

Secondly, whatever force there is is negligible.

Haha, nope. The Sun ain't "burning" like a ball of gasoline. Not the same.

Secondly, whatever force there is is negligible.

So the force of the heat coming from the sun is negligible? (rolls eyes indeed!)

Do you deny that the burning sun exerts a force on the earth?

So the force of the heat coming from the sun is negligible? (rolls eyes indeed!)

Do you deny that the burning sun exerts a force on the earth?
Let me ask you this: do you have to lean forward when walking west just before sunset?

Sometimes I wonder if you are serious.

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