How does Gravity exist?

[JonathanPerregaux]

Because if the Cosmos didn't suck, it would blow.

[YNM]

11 hours ago, p1t1o said:

Dunno, but find me a chemistry equation that includes the gravitational constant and Ill try and figure it out

The equation of not dropping and breaking your glass apparatus.

 

All I'm learning on only ought to care for gravity, as long as we're on Earth, that it makes weight, with a factor of ~10. Job done.

Edited September 8, 2017 by YNM

[JonathanPerregaux]

"Spacetime tells matter how to move; matter tells spacetime how to curve."

—John Archibald Wheeler

[softweir]

On 9/7/2017 at 9:48 PM, wumpus said:

Lighting a match/candle in zero g is said to be pretty weird.  The flame/plasma is spherical, and after it consumes the immediate air around it, it suffocates (no gravity, no convection and no new oxygen).

Depends. A candle, yes, that is true - the wax has such low melting & vapourisation points that the plasma loses heat very rapidly. The candle also conducts away a lot of heat. You end up with a spherical puffball of vapourised wax!

A match, however, acts differently. If there are no air currents to mess things up and the match is held perfectly still, then the plasma can't lose heat except by radiation, and plasma is not a very efficient black-body radiator! The wood is not a good conductor of heat, so the combustion doesn't lose heat that way. The centre of the sphere of plasma is almost perfectly insulated by layers of decreasingly hot air that can't conduct the heat away.

However, combustion continues: oxygen molecules diffuse in to the plasma, and combustion products diffuse out, exchanging heat with the oxygen molecules as they go so the zone of combustion barely loses any. As the gasses diffuse, so the oxygen mixes with superheated combustants and reacts with them, generating more heat. You end up with a concentration gradient of superheated combustants at the centre and cold air outside. The "fire" is perfectly spherical, intensely hot, emits no visible light, and slowly consumes the entire match. If disturbed by an air current then there is a very sharp "pop" as the plasma mixes with fresh air and combusts, and then the fire goes out as heat is carried away by the moving air.

Such fires are hard to detect.  No smoke is generated, as any particulates formed would diffuse more slowly than the gasses, and so would stay in the hottest zone until they evaporated and combusted.  Though IR is emitted, it is at a shorter wavelength than is normal for gravity-sourced fires, so beyond the range of most IR detectors. Combustants are perfectly oxidised, so CO monitors would fail to pick them up.

This would be a serious danger on on any space station that is less than perfectly-maintained. A short-circuit could create combustion that would be almost undetectable. The danger is that the sphere of combustion would grow large enough that it could become supercritical, expanding rapidly until it turned into an explosion, or at least caused critical equipment failure.

(Sorry, very off-topic!)

[wumpus]

11 hours ago, softweir said:

This would be a serious danger on on any space station that is less than perfectly-maintained. A short-circuit could create combustion that would be almost undetectable. The danger is that the sphere of combustion would grow large enough that it could become supercritical, expanding rapidly until it turned into an explosion, or at least caused critical equipment failure.

(Sorry, very off-topic!)

My understanding that during the last days of Mir, fire was a constant threat (and cosmonauts spent a great deal of time fighting the fires).  No idea if this was a large part of the problem.

[p1t1o]

11 hours ago, softweir said:

oxygen molecules diffuse in to the plasma, and combustion products diffuse out, exchanging heat with the oxygen molecules as they go so the zone of combustion barely loses any.

I dunno, here you appear to be describing a Maxwell's Demon. All diffusion in either direction works to drop the temperature of the combustion zone, not conserve it.

Also, some of what you say directly disagrees with NASA research on the topic. For example, the slow spread of the flame and limited diffusion promotes CO production.

And ummmm...thats not how explosions work.

 

[monophonic]

To turn back towards the topic a little, have I got this right? In a nutshell gravity is that having mass sucks?

[Green Baron]

Or the other way round ? Having mass in a nutshell ... sucking a nutshell on mars ...

Gravity is property of mass. It keeps things in "place". Otherwise (no restmass) they'd be condemned to the limiting speed, aka c, forever (our ever, not theirs), and have no perception of the life, the universe and all. 42 had no meaning then.

But things have mass, thus exchange gravity over a still to determine medium, which makes sentences like "I am here, where are you ?" possible and meaningful. Or questions like why does gravity exist.

Sadly there are more questions than answers. Wit the exception of 42, that answer has no question i have heard.

:-)

 

Edited September 11, 2017 by Green Baron

[Sun-Guardian]

I can't wait till mankind finds out how a black hole truly works, because if mass has gravity, and nothing escapes the gravitational pull of a black hole... that's some serious mass!

[YNM]

1 hour ago, Sun-Guardian said:

I can't wait till mankind finds out how a black hole truly works, because if mass has gravity, and nothing escapes the gravitational pull of a black hole... that's some serious mass!

You're pulling the Earth as strong as the Earth pulls you. The same applies to black holes. It's all a matter of coordinate.

[kerbiloid]

9 hours ago, Green Baron said:

Gravity is property of mass.

What is mass?

[Green Baron]

7 hours ago, kerbiloid said:

What is mass?

That what, if subject to a change of velocity over time right above your foot, will hurt you, if your foot isn't a prosthesis.

Will you ask me now what is "above" ? :-)

Edit: oh, and a base si-unit ;-)

Edited September 12, 2017 by Green Baron

[kerbiloid]

49 minutes ago, Green Baron said:

Will you ask me now what is "above" ? :-)

No, why, that's all clear.
"Above" means "located farther from the local potential field center in the same radial direction".
Or sometimes "located closer to the axis of rotation" if speak about a centrifuge.

But in physics there are at least two masses (gravitational and inertial), and their equivalence is just postulated and observed, but not theoretically proven.
And there are different theories how this characteristics appears at all — from "local space-time curvature" to "resultant gravity of all bodies in the Universe pulling this one" and even different quantum abracadabra.
Also, relativistic and quantum theory have very different views on the gravity and the mass.

So, "gravity is property of mass" sounds absolutely similar to "dense bodies attract to each other because the nature doesn't tolerate emptiness", that's what I mean.

Edited September 12, 2017 by kerbiloid

[YNM]

7 hours ago, kerbiloid said:

What is mass?

Mass is "gravity".

Mass is also "lazy".

 

If you could take it that way !

[wumpus]

1 hour ago, kerbiloid said:

But in physics there are at least two masses (gravitational and inertial), and their equivalence is just postulated and observed, but not theoretically proven.

The unity of gravitational and inertial  mass is the main point of general relativity, and it has been proven remarkably well.  I doubt any update of general relativity would be able to separate the two forces.

[kerbiloid]

18 minutes ago, wumpus said:

The unity of gravitational and inertial  mass is the main point of general relativity, and it has been proven remarkably well.  I doubt any update of general relativity would be able to separate the two forces.

The Equivalence principle is just empirically postulated due to the observable equality of both masses in known experiments.
But it doesn't with necessity follow from any theoretical construction.
It just defines a scope of applicability of the relativistic theory in its current state, just as speed light limit.

Physics deals in some cases with inertial mass, in some cases - with gravitational mass, but it just presumes that they are equivalent.
Because experiments show that they are equal (inside achievable range of accuracy ), but nothing proves they are the same.
"=" doesn't mean " ".

Edited September 12, 2017 by kerbiloid

[Green Baron]

So ?

[wumpus]

5 minutes ago, kerbiloid said:

The Equivalence principle is just empirically postulated due to the observable equality of both masses in known experiments.
But it doesn't with necessity follow from any theoretical construction.
It just defines a scope of applicability of the relativistic theory in its current state, just as speed light limit.

Physics deals in some cases with inertial mass, in some cases - with gravitational mass, but it just presumes that they are equivalent.
Because experiments show that they are equal (inside achievable range of accuracy ), but nothing proves they are the same.
"=" doesn't mean " ".

Depends how you look at general relativity.  Einstein''s point wasn't so much that they were equal experimentally, but there was no possible experiment to show that they could be different (i.e. "are you in an accelerating elevator or a gravitational field?").  If you want to claim they are different, you still have to overthrow/replace a lot of general relativity.  The whole basis of the thing is that not only that special relativity was true, but that inertial mass is gravitational mass (or gravitation fields act as accelerating fields of reference and all physical laws still hold).

Einstein assumed they were the same ( " ") and worked out how that would affect physics.  He called those effects "general relativity".  If they aren't the same, somebody is going to have the enormous task of creating a substitute with nearly similar results that could replace general relativity.

[kerbiloid]

1 minute ago, wumpus said:

If you want to claim they are different

Nobody really knows, are they different or the same.
Just until some experiment or theory can show that the are different, Occam makes to treat them as one.

2 minutes ago, wumpus said:

you still have to overthrow/replace a lot of general relativity.

Absolutely not, like you don't have to overthrow Newton's mechanics, knowing that it is applicable not always.
Relativistic theory has its own scope of applicability, accurately designated with its postulates.
It is by definition not applicable outside the conditions listed in these postulates, because if/when they will appear, the relativistic theory just will become a part of more general theory.

7 minutes ago, wumpus said:

Einstein assumed they were the same ( " ")

He just took this assumption (as well as other postulates) delimiting the working field.
His starting points are Lorentz transformation and topological theories of Henri Poincaré and Hermann Minkowski, which operate with 4d space-time with an absolute limit of speed and (when they deal with mass) make no difference between the masses.
At that moment this was enough, because experiments and theories even now still give no clear answer are the masses equivalent.

[NSEP]

Because the programmer wanted to add more effect to this empty mass of gas, he implemented gravity and now he got cool and awesome stars, galaxies and planets and stuff.

Maybe this?

(its hypothetical though, anyways bye)

[Green Baron]

Yeah, the graviton is speculated to be the gravity carrying particle. Nothing special there.

@kerbiloid, pls. give me an example where inertial mass is unequal to gravitational mass. You say no theory supports the equivalence, but General relativity is based on it. Is that no support ?

So what exactly is in your eyes the problem with "Gravity is a property of mass" ? and i personally add provokingly: ", be it inertial or gravitational" because the two are only different methods of measuring the same thing, right or not ?

Gravity or gravitation is the weakest of the four fundamental forces. An electromagnet lifts a car, atoms keep a courtly distance from each other and electrons for example have a deep antipathy to shake hands. It is only that when very much mass is concentrated that gravity overrules the other forces. Not of importance for every day life :-)

Edited September 12, 2017 by Green Baron

[kerbiloid]

1 hour ago, Green Baron said:

give me an example where inertial mass is unequal to gravitational mass.

(Did you really read at least one of my posts?)
Until such example gets discovered, gravitational and inertial masses are presumed to be equivalent, and the Equivalence principle is presumed to be satisfying the scientific picture of the world.
Once such example gets discovered, this will mean that the experimental results get out of scope of the Equivalence principle, and the presumption gets unsatifying in common case.
This presumption (aka Equivalence principle) is purely empirical and doesn't follow with necessity from any theory, it just defines the scope of applicability of the physical theory currently being used as general.
More details you also can find here.

1 hour ago, Green Baron said:

You say no theory supports the equivalence

Me? Where could I say this?
Don't you know that the General relativity is based on the Equivalence principle?

1 hour ago, Green Baron said:

"Gravity is a property of mass"

Gravity is function of gravitational mass (as a value implemented to describe the ability of a physical body to make gravity).
It is not a property of mass. Even "mass" is very uncertain value which even doesn't have any conservation law.

"Energy" and "momentum" are used in the theoretical mechanics to describe mechanical systems, they have corresponding conservation laws. While inertial "mass" is just a derivative abstraction from them.
(See Hamiltonian mechanics for more details).

Gravitational "mass" is just a derivative abstract coefficient from empirical measurements of gravitaty facts.

As currently there arre no facts proving that their difference gets out from the measurement accuracy, these two abstract values are presumed equivalent (in most theories).
But "equal" doesn't mean "the same". Are they "the same" or just "equal", is not known and nothing tells that must be "the same".

1 hour ago, Green Baron said:

Gravity or gravitation is the weakest of the four fundamental forces.

It is not the weakest, it's manifestations are observable on greater scales of space-time (compared to the human-size scale).
This may be or may not be a result of the gravity own features, or a result of different dimensional scales or something else, this is unknown.

1 hour ago, Green Baron said:

Gravity or gravitation is the weakest of the four fundamental forces. An electromagnet lifts a car, atoms keep a courtly distance from each other and electrons for example have a deep antipathy to shake hands. It is only that when very much mass is concentrated that gravity overrules the other forces. Not of importance for every day life :-)

Everyday lack of gravity is especially notable in zero-G.

[Green Baron]

I don't believe in wikipedia :-)

Ok, it is a matter of definitions. For you General Relativity is presumed to describe gravity and presumed to be a valid and you like to add something like "only".

I find that there is nothing better than multiple empirical verifications of theoretical assumptions. I don't question cognition and am content with mass as an si unit like meter, second or kelvin, which all are in the end as relative as mass. I might be wrong.

37 minutes ago, kerbiloid said:

Everyday lack of gravity is especially notable in zero-G.

If you are consequent than zero gravity does not exits. That'll mean no space and no time.

There is no lack of gravity, but a state where different gravitational forces are in an equilibrium.

Just sayin' to avoid confusion ;-)

Or am i wrong ?

Edited September 12, 2017 by Green Baron

[Steel]

21 minutes ago, kerbiloid said:

It is not the weakest, it's manifestations are observable on greater scales of space-time (compared to the human-size scale).

It categorically is the weakest. It just happens to be one of the two fundamental forces that (at least appear to) have infinite range.

Edited September 12, 2017 by Steel

[kerbiloid]

22 minutes ago, Green Baron said:

I don't believe in wikipedia :-)

No problem. Here you go.

22 minutes ago, Green Baron said:

I find that there is nothing better than multiple empirical verifications of theoretical assumptions.

You can make thousand correct experiments and understand nothing without a theory.
For example. (Let's say) all experiments of Archimedes and Newton were multiple, empirical, absolutely correct and demonstrable.
None of them shows any signs of... how they call this thing?... "relativity"!
Ergo, there is no relativity.

But now the experiments show that relativity exists.
Did relativity appear in XIX century?
Or the old theoretcial assumptions (proven with multiple empirical verifications) just had limited scope of applicability which was reached in XIX century?

So, we now post factum postulate that the classical (Newtonian) mechanics scope applicability is limited with low speed and low gravity where Lorentz transormation is insignificant.

The same with relativity postulates. They are called "postulates" because they are axiomatic, i.e. just postulated, not logically figured out.

22 minutes ago, Green Baron said:

There is no lack of gravity, but a state where different gravitational forces are in an equilibrium.

There is only one gravitational force there, at it aims at the center of the Earth (more or less).
It is compensated by inertia which represents the conservation laws.

20 minutes ago, Steel said:

It categorically is the weakest.

It moves galaxies and keeps them together.
That's the human's rulers are short, not the gravity is weak.

20 minutes ago, Steel said:

It just happens to be one of the two fundamental forces that (at least appear to) have infinite range.

All three other forces unite into one ((electomagnetic + weak) + strong) and can be treated as three manifestations of one fundamental force on different scales.
So, unlikely it is correct to say that weak and strong have finite range. They are measurable manifestations of the triple force on short range.
And one of main physical problems currently is "how to unite gravity and electro-weak-strong force in one."

_{(It's late evening here, I'll be back tomorrow).}

Edited September 12, 2017 by kerbiloid