How does Gravity exist?

[Steel]

8 minutes ago, Green Baron said:

Really ? In this case i meant a ball lightning above and stay out of the discussion of "Kugelblitz" because Kugelblitz in German means ball lightning.

It is a little confusing. Wikipedia

[Green Baron]

I ninja'd you with a Kugelblitz. Or was it a ball lightning ?

[Brotoro]

3 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? 

Now THAT is an interesting question, for which I’ve never received a satisfying answer (although I asked about it in the context of degenerate pressure in white dwarfs and neutron stars).

Edited September 13, 2017 by Brotoro

[kerbiloid]

Just electromagnetic repulsion between the atomic electron shells.

[Steel]

3 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? 

 

16 minutes ago, Brotoro said:

Now THAT is an interesting question, for which I’ve never received a satisfying answer (although I asked about it in the context of degenerate pressure in white dwarfs and neutron stars).

 

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.

Edited September 13, 2017 by Steel

[Human Person]

3 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? 

It is Electromagnetism! 

Well, two others beat me to it. See explanation above.

 

Edited September 13, 2017 by Physics Student

[JonathanPerregaux]

Curious fact: If you could exist in the exact center of the Earth, you would be weightless. Why? Because you would have an equal amount of mass all around you. The gravitational pull of that mass in all directions would cancel out any gravitational "pull" you would experience.

The Earth, in free-fall around the sun, is also weightless.

[Human Person]

The question I have is: Does Dark Matter actually exist or do we just have to figure out gravity a little better? Has anyone ever seen dark matter anyways?

[kerbiloid]

Another funny fact: if you could see planet cores instead of planets, you would see a Pluto-sized (R = 1200 km) yellow star instead of the Earth.
Because the temperature of the inner core is estimated to be 6000 K (exactly like on the Sun surface).

Though, if you prefer outer cores, you would see a Mercury-sized (R = 2500..3000 km) orange star instead of the Earth (T = 4400 K).

Edited September 13, 2017 by kerbiloid

[Human Person]

7 minutes ago, JonathanPerregaux said:

Curious fact: If you could exist in the exact center of the Earth, you would be weightless. Why? Because you would have an equal amount of mass all around you. The gravitational pull of that mass in all directions would cancel out any gravitational "pull" you would experience.

The Earth, in free-fall around the sun, is also weightless.

And: if earth was an empty sphere, you would be weightless in the entire space inside of it.

[Green Baron]

11 minutes ago, JonathanPerregaux said:

Curious fact: If you could exist in the exact center of the Earth, you would be weightless. Why? Because you would have an equal amount of mass all around you. The gravitational pull of that mass in all directions would cancel out any gravitational "pull" you would experience.

The Earth, in free-fall around the sun, is also weightless.

That is true for every point inside a hollow sphere. It's called the Shell theorem.

And, yes, any stable orbit means weightlessness or "microgravity" if there are still a few atmospheric molecules.

Ninja'd :-)

Edited September 13, 2017 by Green Baron

[Steel]

9 minutes ago, Physics Student said:

The question I have is: Does Dark Matter actually exist or do we just have to figure out gravity a little better? Has anyone ever seen dark matter anyways?

There is reasonably good evidence that it exists in some form [1], if we assume that GR is somewhere close to full description of gravity. Otherwise GR is wrong (or at least incomplete) and we have very little understanding of how gravity truely works.

Assuming one of the current theories of it is correct, no one has ever seen it and no one ever will. This is because, by it's definition, it does not interact with light and so cannot be seen or detected using electromagnetic radiation.

 

[1] https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence (I know it's wikipedia, but it's good enough for the purposes of this discussion)

Edited September 13, 2017 by Steel

[HebaruSan]

12 minutes ago, Physics Student said:

The question I have is: Does Dark Matter actually exist or do we just have to figure out gravity a little better? Has anyone ever seen dark matter anyways?

Here's a physicist responding to that question at a presentation at Fermilab (assuming the timestamp hyperlink thing works, otherwise jump to 50:11; the more detailed explanation starts around 8:50 or so):

 

Edited September 13, 2017 by HebaruSan

[Brotoro]

1 hour 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.

Of course, atoms repel each other because of the electromagnetic force between their electrons. But I think the reason mikegarrison says that it involves the Pauli Exclusion Principle is because you then have to answer the question "but what is keeping the electrons in place?". Or, skip atoms completely and go to the question of degenerate electron pressure in a white dwarf or degenerate neutron pressure in a neutron star. The force of gravity is acting inward to try to collapse these objects, but they are not collapsing...so some outward force must be acting to counterbalance gravity. This force cannot be the result of the electromagnetic force because it operates for any fermions, even uncharged fermions. That quite real outward force is NOT the result of any of the four fundemental 'forces', but instead results from the PEP.

Edited September 13, 2017 by Brotoro

[Steel]

31 minutes ago, Brotoro said:

Of course, atoms repel each other because of the electromagnetic force between their electrons. But I think the reason mikegarrison says that it involves the Pauli Exclusion Principle is because you then have to answer the question "but what is keeping the electrons in place?". Or, skip atoms completely and go to the question of degenerate electron pressure in a white dwarf or degenerate neutron pressure in a neutron star. The force of gravity is acting inward to try to collapse these objects, but they are not collapsing...so some outward force must be acting to counterbalance gravity. This force cannot be the result of the electromagnetic force because it operates for any fermions, even uncharged fermions. That quite real outward force is NOT the result of any of the four fundemental 'forces', but instead results from the PEP.

It only appears to be a force from a layman perspective, it is not actually a force.

In objects with densities as high as in white dwarves, the collection of matter that makes up the object basically ceases to behave as a group of distinct particles in continuum and becomes one large quantum system, which we call degenerate matter. In a quantum system particles may only have certain energies corresponding to certain quantum states. One property of fermions in a quantum system is that they cannot share these quantum states, thus to add another fermion into the same volume, it must occupy a different state with a higher energy.  This means that compressing degenerate matter requires energy, and this behavior manifests as degeneracy pressure that opposes gravity, but isn't a force.

 

This is one of those things that makes you suddenly realise just how weird quantum mechanics is, when a quantum system can resist gravity without an opposing force.

Edited September 13, 2017 by Steel

[Brotoro]

28 minutes ago, Steel said:

It only appears to be a force from a layman perspective, it is not actually a force.

In objects with densities as high as in white dwarves, the collection of matter that makes up the object basically ceases to behave as a group of distinct particles in continuum and becomes one large quantum system, which we call degenerate matter. In a quantum system particles may only have certain energies corresponding to certain quantum states. One property of fermions in a quantum system is that they cannot share these quantum states, thus to add another fermion into the same volume, it must occupy a different state with a higher energy.  This means that compressing degenerate matter requires energy, and this behavior manifests as degeneracy pressure that opposes gravity, but isn't a force.

 

This is one of those things that makes you suddenly realise just how weird quantum mechanics is, when a quantum system can resist gravity without an opposing force.

You see, it's the "That effect which is counterbalancing the force of gravity is not a 'force'..." song and dance that seems to be sidestepping the issue. Particles are trying to move inward under the force of gravity. Interactions with other particles are preventing that from happening. Just saying "It's not a force" doesn't change the fact that particles are interacting with measurable effects without it being caused by any of the four fundemental force interactions that can occur between particles.

Unsatisfying.

Edited September 13, 2017 by Brotoro

[Steel]

11 minutes ago, Brotoro said:

You see, it's the "That effect that is counterbalancing the force of gravity is not a 'force'..." song and dance that seems to be sidestepping the issue. Particles are trying to move inward under the force of gravity. Interactions with other particles are preventing that from happening. Just saying "it's not a force' doesn't change the fact that particles are interacting with measurable effects without it being caused by any of the four fundemental interactions that can occur between particles.

Unsatisfying.

It's not sidestepping the issue, I'm just trying to explain it to you without lying (a la popular science) about what is actually happening (or at least what we think is happening).

What you're saying about this effect being caused by interactions of particles with other particles is what is wrong with your statement. In a quantum system, you do not have distinct particle interactions. The system behaves as a whole, with the outcome of events determined by the state of each particle within it. It's a subtle difference, and one that is near impossible to get across to someone who hasn't studied the maths behind quantum mechanics which determines how the whole thing behaves. 

The long and the short of it is that degeneracy pressure is not caused by a "force" due to interactions between the particles, its is caused by the intrinsic behavior of the degenerate system as a whole. Maybe think of a white dwarf or neutron star not as a collection of electrons or neutrons, instead try to see it a one huge quantum blob. This "blob" has intrinsic behaviors that are totally different to that of a collection of normal matter and largely unrelated to the classical behavior of the stuff its made up of.

Edited September 13, 2017 by Steel

[Brotoro]

14 minutes ago, Steel said:

It's not sidestepping the issue, I'm just trying to explain it to you without lying (a la popular science) about what is actually happening (or at least what we think is happening).

What you're saying about this effect being caused by interactions of particles with other particles is what is wrong with your statement. In a quantum system, you do not have distinct particle interactions. The system behaves as a whole, with the outcome of events determined by the state of each particle within it. It's a subtle difference, and one that is near impossible to get across to someone who hasn't studied the maths behind quantum mechanics which determines how the whole thing behaves. 

The long and the short of it is that degeneracy pressure is not caused by a "force" due to interactions between the particles, its is caused by the intrinsic behavior of the degenerate system as a whole. Maybe think of a white dwarf or neutron star not as a collection of electrons or neutrons, instead try to see it a one huge quantum blob. This "blob" has intrinsic behaviors that are totally different to that of a collection of normal matter and largely unrelated to the classical behavior of the stuff its made up of.

I don't see how you can say there us no interaction going on between fermions. If just two fermions try to inhabit the same location with all of their other quantum states being the same, it will not happen because the rules of quantum mechanics prohibit it. One electron could sit there just fine. The other electron could sit there just fine. But try to put both there at the same time and it will not happen. How does this not involve an interaction between these two electrons?

Edited September 13, 2017 by Brotoro

[Steel]

8 minutes ago, Brotoro said:

I don't see how you can say there us no interaction going on between particles. If just two fermions try to inhabit the same location with all of their other quantum states being the same, it will not happen because the rules of quantum mechanics prohibit it. One electron could sit there just fine. The other electron couls sit there just fine. But try to put bith there at the same time and it will not happen. How does this not involve an interaction between these two electrons?

Believe me, I wish this were simpler!

Essentially, the second electron cannot occupy that filled state because the wavefunction of the system (basically a bit of maths that determines the probabilities of each possible outcome of the system, given its current conditions) literally does not allow another electron to exist there.  You may have heard that quantum systems behave probabilistically, well the PEP basically means that there is zero chance (that is exactly zero, not negligible, not incomprehensibly small, literally zero) that the second electron will be found in that same state, it can only ever be found in another state. The particles are not interacting to tell each other anything, its just that the system as a whole has a zero chance (a statistical impossibility if you will) that you will see the second electron in that state.

Edited September 13, 2017 by Steel

[Brotoro]

2 minutes ago, Steel said:

Believe me, I wish this were simpler!

Essentially, the second electron cannot occupy that filled state because the wavefunction of the system (basically a bit of maths that determines the probabilities of each possible outcome of the system, given its current conditions) literally does not allow another electron to exist there.  You may have heard that quantum systems behave probabilistically, well the PEP basically means that there is zero chance (that is exactly zero, not negligible, not incomprehensible small, zero) that the second electron can go to that state, it can only ever be found in some other state. The particles are not interacting to tell each other anything, its just that it is literally a statistical impossibility that you will see the second electron in that state.

If the fermions are not interacting... how does the second fermion know the first one is already in that primo parking location?

Edited September 13, 2017 by Brotoro

[Steel]

Just now, Brotoro said:

If the fermions are not interacting... how does the second fermion know the first one is alreay in that primo parking location?

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.

The whole system has a possible set of outcomes based on its current set up, external forces e.t.c, and each outcome has a certain probability of occurring. When you leave the system alone and come back to observe it, it will be in one of these outcomes. There are no interactions between members of the system in the same way as you get in classical systems. The PEP essentially means that in these systems, it will never be possible (i.e the probability of it occurring is exactly zero) for there to be a system outcome where two fermions are in the same quantum state.

[Brotoro]

4 minutes 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.

The whole system has a possible set of outcomes based on its current set up, external forces e.t.c, and each outcome has a certain probability of occurring. When you leave the system alone and come back to observe it, it will be in one of these outcomes. There are no interactions between members of the system in the same way as you get in classical systems. The PEP essentially means that in these systems, it will never be possible (i.e the probability of it occurring is exactly zero) for there to be a system outcome where two fermions are in the same quantum state.

I continue to be unsatisfied. But thank you for taking the time to discuss it. I enjoyed that interaction. Have a Like.

[Steel]

5 minutes ago, Brotoro said:

I continue to be unsatisfied. But thank you for taking the time to discuss it. I enjoyed that interaction. Have a Like.

I don't blame you. After all, Einstein wasn't particularity satisfied with quantum mechanics! It's unintuitive, complicated and difficult to explain; even with a physics degree I've barely scratched the surface.

No problem, I enjoy talking about his stuff with people. Also, by trying to answer your questions, you've helped me clear up some things in my own head that I didn't completely get before, so thanks! 

Edited September 13, 2017 by Steel

[wumpus]

3 hours ago, Physics Student said:

The question I have is: Does Dark Matter actually exist or do we just have to figure out gravity a little better? Has anyone ever seen dark matter anyways?

Since as Steel mentioned, the stuff simply doesn't interact with the electromagnetic force it is next to impossible to get it to show up in the LHC.  It might be interesting to take the assumptions of "how much dark matter is in the universe" and try to take an inventory of what the LHC produced.  Is there enough "missing mass" to match the expected amount of dark matter?  I'd assume that the LHC has *zero* missing mass as that would be the discovery of the century (either as a conservation violation or the observing the creation of dark matter: take your pick).  This leads to the question of why this thing doesn't make dark matter when it appears to make everything else.

As mentioned in this thread, gravity is unbelievably weak.  If the LHC creates dark matter, we aren't going to detect the gravity it produces.  Try to remember what it took to detect gravity waves: colliding black holes.  We need to detect subatomic particles in LHC (probably with relativistic mass, but still impossibly weak forces).  Maybe somebody can detect it via nuclear strong or weak forces, but they don't interact with the electromagnetic and gravity is just too weak (and nobody has any idea if dark matter interacts with the nuclear strong or weak anyway).

It is hard to tell, but  I think "dark energy" is a bigger finagle factor to existing equations than "dark matter".  Comparing them directly would be apples and oranges, but it seems the "dark energy" is the greater mystery, but doesn't get nearly the ink that "dark matter" gets.  Count me as one who thinks the whole dark matter/dark energy looks more like the "luminiferous ether" than "Pluto", but I'm only qualified to talk about the history of said science, not the actual  nitty gritty of the equations (and not much at that.  But considering how well  my history of science professor botched chemistry...)

[Human Person]

2 minutes ago, wumpus said:

It is hard to tell, but  I think "dark energy" is a bigger finagle factor to existing equations than "dark matter".  Comparing them directly would be apples and oranges, but it seems the "dark energy" is the greater mystery, but doesn't get nearly the ink that "dark matter" gets.  Count me as one who thinks the whole dark matter/dark energy looks more like the "luminiferous ether" than "Pluto", but I'm only qualified to talk about the history of said science, not the actual  nitty gritty of the equations (and not much at that.  But considering how well  my history of science professor botched chemistry...)

Do you see the pattern? Attracting and repelling forces are everywhere, from the smallest particles to the scale of the universe itself. The weaker the force, the longer its range.