Science questions not worth a thread I: I'm a moron!

Venus also does not have plate tectonics, but Mercury does as announced on this Monday!

There are certainly theories why this is for Mars (too small, would require water), but I guess most of these just went into the recycle bin

It might be linked to the magnetic field question: It has been argued that the core of Venus lacks convection, because no subducting plates bring cool material into the interior of Venus. But it might also be the other way around: because Venus has more or less thermalized, there is neither plate tectonics nor a magnetic field.

I have read more about an inductive response to the Jupiter's magnetic field. But there are also scientists arguing for a dynamo, so I do not think this has been settled, yet.

It is in orbital resonance with the other major moons, though. I would expect the moons exert little force on each other by now,

Certainly reasonable, but since it is two out of four, you can make up plenty of reasonable theories that fit the data, so I am not convinced of any explanation.

 

Theorizing, but if a planet's core has cooled to the point where there's no longer a liquid, or semi-liquid outer core or mantle, then there isn't a bed for the continents to 'float' on, and so it would be minimally geologically active. And would also probably not have a magnetic field, because it wouldn't have a metallic inner core rotating at a different rate than a metallic outer core.

Early in its history Earth collided with a small planet. The resulting impact created the Moon, and left Earth with a higher metals content than it would have had otherwise. Speculating that that could have contributed to Earth's core remaining hot.

 

Oh man, last post was 2016.

Archimedes Principle states that the buoyant force is equal to the weight of the displaced fluid. Moreover the buoyant force opposes gravity, meaning up. I was wondering why the buoyant force is always up and I came across this while googling.

http://physics.bu.edu/~duffy/sc527_notes01/buoyant.html

But this confuses me more because wouldn't the buoyant force also depend on the height of the object? Taller objects should experience a greater force and an infinitely flat object should experience no force. This seems like an important tidbit which I don't remember being taught in school. Or am I just thinking about this wrong?

 

Taller objects experience a greater pressure difference between top and bottom, but if the volume of your submerged object does not change, there will be less area to push on. The greater pressure but smaller area give the same overall buoyant force. Really flat objects with the same volume will have a tiny pressure difference acting on a huge area, giving the same buoyant force.

 

Newton's 3rd Law repealed?

https://gizmodo.com/224739/dread-silent--monster-weapon-spins-out-120000-rounds-per-minute

120,000 rounds per minute no recoil

How is this possible?

 

It isn't. There is an interesting discussion on it over at https://www.physicsforums.com/threads/dread-weapon-system-power-requirements.66613/

 

Thank you, CKS

 

Does quantum degeneracy pressure fit somewhere into the four fundamental forces? A thing that prevents matter from collapsing upon itself seems like it would be a force yet it doesn't seem to correspond to gravitational, electromagnetic, strong, or weak forces.

 

I don't think so

my impression is basically that the particles are packed so densily as possible

anyway I think it sounds more like a pseudoforce like centrifugality

(pretty sad I can't give a better answer on something I have studied)

 

Degeneracy pressure comes from Fermi's exclusion principle, not quite sure I can pin this down to a fundamental force right now. But most likely EM. As strong and weak only apply to the inside of the core, and gravity has negligible effect on the scale of individual atoms.

 

I suspect you are essentially right. After much searching I found this which basically says the wave function for fermions doesn't allow it. So it's not really a force, it's just math...which isn't a terribly satisfying answer to me. But to really understand it I'd need a deeper study of QM and the Schrodinger equation.

If this were true I don't know how that would explain neutron degeneracy pressure since neutrons are electrically neutral.

 

Fermi exclusion principle is indeed the root cause. That, however, has nothing to do with EM. At the core it comes from symmetry.

The way to think of it is that particles occupy the lowest energy states possible. For degenerate matter, all the lowest energy levels are already filled. As you squeeze it some more, some of these levels become more energetic. This is essentially saying that they have to vibrate faster. So when you try to compress a neutron star, you are forcing the neutrons vibrate faster. This requires energy. This energy depends on the change in volume, so you have dE/dV which has the same dimensions as dF/dA, so we call it a pressure.

In other words, degeneracy pressure comes about because you need to give things more internal kinetic energy to reduce their volume, for very deep underlying reasons due to symmetries in quantum field theories. You are not doing work against any of the fundamental interactions (gravity/EM/Strong/Weak), so it is not due to any of them. It isn't really like the centripetal force, which just arises because you are working in a non-inertial reference frame.

 

What makes fermions fermions then? If every interaction boils down to the 4 forces there has to be some interaction between them that let's them not fall into the same state.

 

Antisymmetry on particle exchange.
If you have two fermions x1 and x2, the following is true by definition
Ψ(x1, x2) = - Ψ(x2, x1)
Particles that do not follow this rule, are not fermions.

From this it immediately follows that two identical fermions cannot occupy the same state, because if x1 = x2, then Ψ(x1, x2) = Ψ(x2, x1). This equation and the above equation can only be true at the same time if Ψ = 0. Maybe you can think of it as some kind of interference: The wavefunction of one particle cancels out the wavefunction of the other particle in such a way that they can never be at the same place.

You can describe it as an interaction and the statement that every interaction boils down to the 4 forces is wrong. It is not called a force, because it is very different to the 3 forces in quantum field theory (excluding gravity, which is an issue of its own). The forces are bosonic fields, so it doesn't make sense to group it together with this interaction of fermionic fields.

 

So graphite and diamonds are both pure carbon. Both can melt although diamonds need to be kept at a much higher pressure otherwise it tends to turn into graphite as it is heated. Other than the temperature and pressure combinations needed to melt them, do melted diamonds and melted graphite have any different properties? Or is it all just liquid carbon?

 

Once melted it would be Carbon in liquid form. What makes the two different is that the atoms are arranged different in the solid phase.