How does Gravity exist?

[HebaruSan]

Here's a physicist calling degeneracy pressure a quantum mechanical "force":

 

[PakledHostage]

2 hours ago, Steel said:

It doesn't. The point i'm trying to make is that in quantum systems, the particles don't behave as individual particles, the system acts as a whole.

So they're like the Borg?!

[qeveren]

14 hours ago, mikegarrison said:

Gravity pulls us toward the center of the Earth. What (obviously stronger) force keeps us from falling there? What creates the resistance that keeps one solid object from moving through another? The standard answer seems to be "the Pauli Exclusion Principle", but if everything comes down to four forces, which one is involved in that? Is that electromagnetism? 

As I (vaguely) understand it, degeneracy pressure doesn't arise directly from any of the fundamental interactions, but rather from something called the exchange interaction.

10 hours ago, Steel said:

The force that keeps us from falling into the centre of the Earth is just the mutual electromagnetic repulsion of the electrons in the atoms, not the PEP. The PEP only kicks in for super dense objects like white dwarves.

The PEP itself is not a force and nor does it act like one, it's just the slightly hand-wavy popular-science analogies that make us think it is a force. The PEP is actually a fundamental behavior of fermions in quantum systems. This thread explains quite well.

Didn't Dyson show that the PEP was responsible for the 'impenetrability of matter' back in the 60s?

[sevenperforce]

On 9/12/2017 at 1:32 PM, Green Baron said:

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 ?

The graviton does not "carry gravity" in the same way that photons are the gauge bosons for electromagnetic interaction. The graviton is a quanta of a gravity wave, and must exist per quantum mechanics, but doesn't do much.

In all measured instances, inertial mass is equal to gravitational mass. This is odd, because there is no physical reason why this should be so. One can model an object that is very heavy to lift in a gravity well but is very easy to move around in free-fall. Why don't we see this? It's a big mystery in modern physics.

Gravitational mass and inertial mass are not two different methods of measuring the same thing; they are two different things that somehow always turn out to be exactly proportional to each other, and so we use a single unit for both.

[Green Baron]

1 hour ago, sevenperforce said:

Gravitational mass and inertial mass are not two different methods of measuring the same thing; they are two different things that somehow always turn out to be exactly proportional to each other, and so we use a single unit for both.

I am not convinced. Wouldn't that be questioning the equivalence principle and GR ? (Edit: not that i see something irrevocably in it but it serves quite well, at least in the macroworld)

Because it is impossible to distinguish between an inertial and gravitational reference frame.

So were is the difference between them ?

Ok, i found this: https://www.physicsforums.com/threads/difference-between-gravitational-and-inertial-mass.689095/

 

Edited September 14, 2017 by Green Baron

[sevenperforce]

24 minutes ago, Green Baron said:

I am not convinced. Wouldn't that be questioning the equivalence principle and GR ? (Edit: not that i see something irrevocably in it but it serves quite well, at least in the macroworld)

Because it is impossible to distinguish between an inertial and gravitational reference frame.

So were is the difference between them ?

The difference between gravitational mass and inertial mass is not the same as the difference between gravitational reference frames and inertial reference frames, despite the similarity in name. An "inertial reference frame" is a frame in which there is no acceleration; a "non-inertial reference frame" is a frame in which there is acceleration. Non-inertial reference frames result from external acceleration, either due to some momentum-exchange impulse (like a rocket engine) or due to gravitational attraction. The point of general relativity's equivalence principle is (primarily) that you cannot determine whether a given non-inertial reference frame is the result of rocket impulse or gravitational attraction.

It would be possible to discover a substance with high gravitational mass and low inertial mass (or the reverse) without violating the equivalence principle.

[sevenperforce]

11 hours ago, qeveren said:

As I (vaguely) understand it, degeneracy pressure doesn't arise directly from any of the fundamental interactions, but rather from something called the exchange interaction.

Didn't Dyson show that the PEP was responsible for the 'impenetrability of matter' back in the 60s?

As someone else here with a physics degree, I'm going to weigh in on this side. Solid materials are solid because of the Pauli Exclusion Principle.

Electrostatic repulsion (the Coulomb Force) is responsible for pressure in gases. In a gas, molecules do not get close enough to trigger degeneracy-pressure exclusion; the outward pressure of a volume of gas (given by PV=nRT for anyone who remembers basic thermodynamics) is the aggregated force of all those molecules getting too close and exchanging momentum via Coulomb forces. You can decrease this pressure by decreasing the temperature of the gas; lower the temperature and you lower the average molecular velocity, decreasing the available momentum in interactions and thus decreasing the amount of pressure.

In a solid, however, there's no relationship between temperature and "pressure" because you're not dealing with Coulomb alone. Granted, there is some electrostatic repulsion involved when my fingers press down on the keys of my keyboard, but forces can be overcome. The primary reason my fingers don't simply phase into the keys is that quantum mechanical effects (not quantum mechanical forces) prevent the electrons in my finger from occupying the same space as the electrons in the keyboard. A force occurs when two particles interact via an exchanged boson (e.g., the strong force with quarks exchanging gluons or the electromagnetic force with electrons exchanging photons); this quantum effect results from the fact that the two electrons CANNOT interact because of the combined wavefunction of the system.

Edited September 14, 2017 by sevenperforce

[Green Baron]

Ok, i know now that i suck at physics, the evidence is overwhelming

 

[kerbiloid]

16 hours ago, PakledHostage said:

So they're like the Borg?!

System is Cthulhu, particles are its tentacles.
Or a hand with fingers. You can't put fingers in any possible combination. Sometime one finger blocks any place for another one. But this doesn't mean that you can't place any finger into any position keeping others straight.

11 hours ago, qeveren said:

As I (vaguely) understand it, degeneracy pressure doesn't arise directly from any of the fundamental interactions

It starts at 3000 km under the Earth surface, where the pressure pushes the outer electrons into underlying electron shells if there is empty space.
Look at the table and find, say, Nickel.
You can see in nearby rows that second to last electron shell can hold 18 electrons.
But due to quantum effects, the last and the second to last shells do not get filled sequentially. So, nickel has 16 electrons and 2 vacations on the penultimate shell and 2 electrons on the outer shell (and due to this, normally this is a chemically active metal).
Under pressures inside the Earth depth, outer electron shells of nearby atoms electromagnetically press each other, and 2 electrons from the outer shell get pushed into the vacations of the previous shell.
Nickel instead of "normal" 16, 2 gets 18, (0), so its outer existing shell is completed, and it becomes chemically inert.

The greater gets the pressure, the worse are the things. But unlikely you need something except electromagnetism to press electrons inside atoms.

11 hours ago, qeveren said:

PEP was responsible for the 'impenetrability of matte

PEP is the legislative power. Four interactions are the executive power.
So, PEP doesn't effect something directly, while four interactions just perform orders.

Edited September 14, 2017 by kerbiloid

[sevenperforce]

On 9/13/2017 at 7:34 AM, p1t1o said:

I went to a lecture once at the Royal Academy by some eminent physiscist or other, and if I remember it correctly, it talked about (under certain theories or hypotheses) how gravity is so "weak" because it is capable of "leaking" into other universes or "branes". Its strength is effectively diluted between multiple layers.

Hence, dark matter is us detecting the gravity of masses in other universes.

And further, this could be harnessed to communicate between universes in a real way.

That is one hypothesis, yes.

On 9/13/2017 at 11:56 AM, p1t1o said:

Just popped into my head - a sufficient quantity of EM radiation in a volume has zero mass but *does* exert gravity (see: Kugelblitz, a hypothetical black hole formed purely from EM radiation).

How does that change things?

Electromagnetic radiation has zero rest mass. It does not have zero mass. A single photon of 480-nm sky-blue cyan light has a mass-energy of 4.138e-19 J, corresponding to a mass of 4.6 millionths of a billionth of a trillionth of a microgram.

Plot twist: the overwhelming majority of all mass is, in fact, relativistic mass, rather than rest mass. If I recall correctly, something like 90% of the mass of an oxygen nucleus is the relativistic mass of the gluons bouncing between quarks, rather than the rest mass of the quarks themselves.

Rest mass does exist, thanks to the Higgs field.

Edited September 14, 2017 by sevenperforce

[p1t1o]

51 minutes ago, sevenperforce said:

That is one hypothesis, yes.

Electromagnetic radiation has zero rest mass. It does not have zero mass. A single photon of 480-nm sky-blue cyan light has a mass-energy of 4.138e-19 J, corresponding to a mass of 4.6 millionths of a billionth of a trillionth of a microgram.

Plot twist: the overwhelming majority of all mass is, in fact, relativistic mass, rather than rest mass. If I recall correctly, something like 90% of the mass of an oxygen nucleus is the relativistic mass of the gluons bouncing between quarks, rather than the rest mass of the quarks themselves.

Rest mass does exist, thanks to the Higgs field.

Ah, I didnt realise that photons had relativistic mass. I thought they had momentum, but zero mass.

Relativistic mass is one of my favorite concepts.

***

So, if a compressed spring has a slightly higher mass compared to a uncompressed spring due to stored energy - I have heard this is literal and can be detected with a sufficiently sensitive experimental setup - is that relativistic mass?

***

So. Where does "rest mass" and "relativistic mass" fit into the inertial/gravitational mass discussion?

Are there now 4 categories? Rest-inertial, Rest-gravitational, Relativistic-inertial and Relativistic-gravitational?

Rest-I and Rest-G are obviously the masses we were already talking about, is Rel-mass associated with that distinction at all?

***

1 hour ago, sevenperforce said:

Plot twist: the overwhelming majority of all mass is, in fact, relativistic mass, rather than rest mass. If I recall correctly, something like 90% of the mass of an oxygen nucleus is the relativistic mass of the gluons bouncing between quarks, rather than the rest mass of the quarks themselves.

Wait, hang on.

So when someone quotes the "rest mass" of a proton, or nucleus or whatevs, that is not technically accurate? Does Rel-mass in this context behave identically to Rest-mass? I was under the impression that Relativistic and Rest mass had distinct properties.

[sevenperforce]

1 hour ago, p1t1o said:

So, if a compressed spring has a slightly higher mass compared to a uncompressed spring due to stored energy - I have heard this is literal and can be detected with a sufficiently sensitive experimental setup - is that relativistic mass?

Yes. Springs resist compression because, although the Pauli Exclusion Principle keeps them solid, a crystalline molecular structure makes some molecular bonds longer (and thus weaker) than others. Those molecular bonds are held in tension by electromagnetic force, which stores energy by creating photons to exchange between particles.

The application of external force allows those molecular crystals to twist, adding energy to the molecular bonds. This manifests on a subatomic scale as an increase in the number of photons being exchanged; since photons have relativistic mass, the spring becomes slightly more massive. When external force is removed, the molecular bonds expand again, the extra photons disappear, and the spring becomes less massive.

One can follow the mass-energy very neatly. Photons from the sun have relativistic mass. They strike chloroplasts, which use the photon mass-energy to break the molecular bonds in carbon dioxide and water, storing that same mass-energy in the chemical bonds of ATP. Some of the mass-energy is lost to heat and zips away as the increased relativistic mass-energy of surrounding gas molecules. I consume the ATP, and my body breaks it down and gives my muscle fibers the ability to contract under tension, performing work on a spring. Mass-energy moves out of my muscle fibers and into the spring's molecular substructure, as explained above. Suppose, then, that I place a ball bearing on top of the spring, then release it. The mass-energy of the gauge bosons (photons) in the molecular bonds disappears and moves into the ball bearing, which rockets away, its relativistic mass increased due to its newfound velocity.

1 hour ago, p1t1o said:

So. Where does "rest mass" and "relativistic mass" fit into the inertial/gravitational mass discussion?

Are there now 4 categories? Rest-inertial, Rest-gravitational, Relativistic-inertial and Relativistic-gravitational?

Rest-I and Rest-G are obviously the masses we were already talking about, is Rel-mass associated with that distinction at all?

See below!

1 hour ago, p1t1o said:

***

Wait, hang on.

So when someone quotes the "rest mass" of a proton, or nucleus or whatevs, that is not technically accurate? Does Rel-mass in this context behave identically to Rest-mass? I was under the impression that Relativistic and Rest mass had distinct properties.

In typical use, there are two kinds of relativistic mass: bound relativistic mass, and unbound relativistic mass. Bound relativistic mass exists in subatomic particle exchange; the gluons bouncing between quarks have rest mass, but their relativistic mass due to extraordinarily high velocities (just under c) is what gives the nucleus, and all visible matter, most of its mass. Unbound relativistic mass exists in particles which are free and are traveling relative to an observer.

For the purposes of calculation, bound relativistic mass can be treated as rest mass because it is at rest with respect to an observer. Bound relativistic mass contributes to the mass-energy tensor, producing gravitational fields. Unbound relativistic mass increases an object's inertial mass and follows the curvature of spacetime, but it does not contribute to the mass-energy tensor.

[Guest]

Photons have Mass? I didn't know they were Catholic.

Edited September 16, 2017 by Guest

[Ultimate Steve]

Maybe the field that makes up matter is moving through some weird compression anomaly?

I just spit a random word salad out of my mouth and it almost made sense.

[sevenperforce]

1 hour ago, Ultimate Steve said:

Maybe the field that makes up matter is moving through some weird compression anomaly?

I just spit a random word salad out of my mouth and it almost made sense.

That's...sort of...the Higgs field?