# Matter creates gravity

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How?

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Not just mass, energy also.

Matter and energy bend spacetime, and in spacetime, objects take the shortest path in spacetime, which because of the its curvature move in curved trajectories. We experience this as gravity.

When you are new to this topic, this article might be a good introduction.

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!

Moderator Note

Moved to relativity

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3 hours ago, Lan Todak said:

How?

As Eise says, mass/energy bends space, and that is what we perceive as gravity. We don't know why mass/energy does this. We know that it does, and have a well-supported theory on what happens as a result.

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I know... but empty space is alive

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2 minutes ago, Lan Todak said:

I know... but empty space is alive

Not by the definition used in science. What do YOU mean by "alive"?

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3 hours ago, Lan Todak said:

I know... but empty space is alive

?

Gravity is spacetime geometry.

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10 hours ago, Phi for All said:

Not by the definition used in science. What do YOU mean by "alive"?

I mean empty space has unique properties and account to  97% total energy of our universe. matter can not simply bend empty space. 99.9999999% energy of a matter resides in empty space too.

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OK let's clarify something here. First consider the following definitions.

Mass is resistance to inertia change

Energy the ability to perform work.

Spacetime a geometric model system with 3 spatial dimensions with 1 time dimension.  In physics dimension is an independent variable or value that can change without affecting any other mathematical object.

So how does mass curve spacetime. Well GR models bodies in free fall that is without any force applied. Time is given units of length and can be called an interval. This is done by setting c which is constant to all observers and adding a unit of time. So the time coordinate is given units of length by ct.

$(t,x,y,z)=(ct,x,y,z)=(x^1,x^2x^3x,x^4)$ the last is in four momentum form for convenience as its useful to model a particle trajectory along the x axis.

Now what is spacetime curvature. Well space is just volume... (Very important ) it isn't a stretchable bendable fabric...

Those are just analogy descriptive.

What spacetime truly means by curvature is the worldline paths for light it us the null geodesic.

If you shoot two laser beams in flat spacetime those beams stay parallel. If spacetime is curved then the beams converge for positive curvature and spread apart for negative curvature.

This is a consequence of how the mass term affects the time it takes for a particle to go from emitter to observer. That whole resistance to inertia.

So let's drop two objects toward a planet. You have the usual Centre of mass. As the objects free fall they do not stay parallel. They will converge upon one another as they approach the center of mass.

That what is really meant by curvature the free fall paths are curved. Not the volume of space.

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15 hours ago, Mordred said:

OK let's clarify something here. First consider the following definitions.

Mass is resistance to inertia change

Energy the ability to perform work.

Spacetime a geometric model system with 3 spatial dimensions with 1 time dimension.  In physics dimension is an independent variable or value that can change without affecting any other mathematical object.

So how does mass curve spacetime. Well GR models bodies in free fall that is without any force applied. Time is given units of length and can be called an interval. This is done by setting c which is constant to all observers and adding a unit of time. So the time coordinate is given units of length by ct.

(t,x,y,z)=(ct,x,y,z)=(x1,x2x3x,x4) the last is in four momentum form for convenience as its useful to model a particle trajectory along the x axis.

Now what is spacetime curvature. Well space is just volume... (Very important ) it isn't a stretchable bendable fabric...

Those are just analogy descriptive.

What spacetime truly means by curvature is the worldline paths for light it us the null geodesic.

If you shoot two laser beams in flat spacetime those beams stay parallel. If spacetime is curved then the beams converge for positive curvature and spread apart for negative curvature.

This is a consequence of how the mass term affects the time it takes for a particle to go from emitter to observer. That whole resistance to inertia.

So let's drop two objects toward a planet. You have the usual Centre of mass. As the objects free fall they do not stay parallel. They will converge upon one another as they approach the center of mass.

That what is really meant by curvature the free fall paths are curved. Not the volume of space.

Do you mean in 4 dimensional space?... in 3d, space distorted whenever object passing by like when black hole is passing over galaxy. At least we can clearly see distortion in 3d. A mass can really curve space... i am just asking how... in quantum level.

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In QFT treatment through the principle of least action. You can work through the Eueler Langrene equations that correlate the kinetic energy of the particle to the chosen path probability with the fields potential energy. Feymann has a good starting beginner's lecture.

The definitions above apply in all physics treatments so set them to memory.

Though mass may be replaced by coupling constants in different field treatments.

The SM models 18 parameters are all various coupling constants. 10 are Yukawa couplings 2 are the Higgs. I'd have to double check the others

Edited by Mordred

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On 8/9/2019 at 12:11 AM, Mordred said:

OK let's clarify something here. First consider the following definitions.

Mass is resistance to inertia change

Energy the ability to perform work.

Spacetime a geometric model system with 3 spatial dimensions with 1 time dimension.  In physics dimension is an independent variable or value that can change without affecting any other mathematical object.

So how does mass curve spacetime. Well GR models bodies in free fall that is without any force applied. Time is given units of length and can be called an interval. This is done by setting c which is constant to all observers and adding a unit of time. So the time coordinate is given units of length by ct.

(t,x,y,z)=(ct,x,y,z)=(x1,x2x3x,x4) the last is in four momentum form for convenience as its useful to model a particle trajectory along the x axis.

Now what is spacetime curvature. Well space is just volume... (Very important ) it isn't a stretchable bendable fabric...

Those are just analogy descriptive.

What spacetime truly means by curvature is the worldline paths for light it us the null geodesic.

If you shoot two laser beams in flat spacetime those beams stay parallel. If spacetime is curved then the beams converge for positive curvature and spread apart for negative curvature.

This is a consequence of how the mass term affects the time it takes for a particle to go from emitter to observer. That whole resistance to inertia.

So let's drop two objects toward a planet. You have the usual Centre of mass. As the objects free fall they do not stay parallel. They will converge upon one another as they approach the center of mass.

That what is really meant by curvature the free fall paths are curved. Not the volume of space.

Damn good post, +1

Thanks

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The way I'm picturing it at the moment, is that massive bodies have an effect on inertial reference frames in their vicinity. At any point in space, an inertial reference frame is one in which an object that is experiencing no force is at rest, or moving at a constant velocity.

At the surface of the Earth, an object that experiences no force, accelerates downwards at 32 ft/sec/sec, or 9.8 m/s2. So inertial reference frames have to be accelerating downwards at that rate.

The Equivalence principle of General Relativity sheds light on it for me https://en.wikipedia.org/wiki/Equivalence_principle

First paragraph :  In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.

So the force on you, as you stand on the Earth's surface, is the pseudo-force caused by being in an accelerating inertial frame of reference, that's accelerating downwards at 9.8 m/s2.

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12 minutes ago, mistermack said:

The way I'm picturing it at the moment, is that massive bodies have an effect on inertial reference frames in their vicinity. At any point in space, an inertial reference frame is one in which an object that is experiencing no force is at rest, or moving at a constant velocity.

At the surface of the Earth, an object that experiences no force, accelerates downwards at 32 ft/sec/sec, or 9.8 m/s2. So inertial reference frames have to be accelerating downwards at that rate.

You can approximate curved space-time as a series of inertial frames of reference.

You can model these as accelerating towards the mass and, in fact, Painleve-Gullstrand coordinates make this explicit:

Someone in free fall with those frames of reference is actually not accelerating - which your description makes clear: they are always in an inertial frame of reference. Someone in free-fall feels no force acting on them (unlike someone undergoing acceleration).

That is a good insight

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On 8/29/2019 at 10:01 AM, Bufofrog said:

Damn good post, +1

Absolutely.

Occasionally there are posts on here that lead to immediate "aha" and my connection with science kicks up a gear. This is one.

Thanks Mordred

Edited by druS

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Glad to help, it's nice to see that post help cut through some of the common difficulties that ppl often have in understanding gravity in terms of spacetime and mass.

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