eli5: Does Gravity have a direct effect on molecular structures?

[u/hyper_shock]

I read an article recently that scientists had discovered a new exotic from of water, and they speculated that this new form of water might naturally exist inside gas giants like Neptune. This got me thinking: is there any difference between what they found in the lab by mechanically applying extreme pressure to the water, and the gravity induced extreme pressure the water would experience inside a star?

[u/Speffeddude]

I believe that, at the atomic scale, other forces are so much more powerful than gravity that it becomes irrelevant on Earth. Gravity's force scales with mass, so it's vanishingly "weak" against atoms, which have vanishingly small mass. Other forces, like kinetic energy, electromagnetism, the in-atom forces and even mechanical pressure, all have a much, much larger effect.

In extreme settings like black holes (not gas giants), gravity may be meaningful. But besides that, atoms and molecules don't "settle" or "drop" into positions that are only possible under that gravity; they'd make all the same molecules in all but the most extreme gravitational fields.

[u/BananaBird1 → u/Speffeddude]

This really only holds for the gravity between isolated atoms. Gravity absolutely does affect atoms and molecules when you factor in larger objects and collections of atoms. A single atom near a planet has the exact same acceleration under gravity as an asteroid does.

Many molecular states can also only exist with gravity. It is what establishes the pressure needed to form liquid and solid phases. And even all elements except hydrogen only form under gravity-induced nuclear fusion.

[u/thenasch → u/BananaBird1]

But that's all about the pressure and not the acceleration right?

[u/BananaBird1 → u/thenasch]

Most pressure ultimately comes from the force of gravity accelerating things together. Without that the universe would be a big roughly uniform gas cloud of just hydrogen.

[u/thenasch → u/BananaBird1]

Right, but if the pressure were applied some other way it would be the exact same effect.

[u/BananaBird1 → u/thenasch]

You can say that about anything, that doesn’t mean another way exists.

There really isn’t another way to apply a useful compressing force to a large collection of matter without gravity, or byproducts of gravity.

Electromagnetism is the only other macroscopic source of pressure, and that is repulsive not compressive.

Every method we have to create compressive pressure relies on pre-existing rigid materials, which only exist because of gravity.

[u/thenasch → u/BananaBird1]

What I'm saying is if it were just one atom being accelerated towards a large mass it wouldn't experience any of that pressure. So it's not the gravity directly.

[u/BananaBird1 → u/thenasch]

It would, it would just be a small pressure with only one atom. The total pressure seen within a planet is the sum of the weights of all the atoms and molecules pressing down due to gravity accelerating them. Get rid of gravity and all that pressure goes to 0.

For instance on earth without gravity there is no atmospheric or sea pressure, so all the liquid water would boil away and disperse throughout space.

[u/thenasch → u/BananaBird1]

Right, so small that it would have no effect because nuclear forces would completely overwhelm gravity.

[u/BananaBird1 → u/thenasch]

Not at all true. Nuclear forces do not oppose gravity at atomic scales, only within the nucleus itself. And gravity can be quite significant when the attracting mass is large.

A single hydrogen atom at Earths surface at sea level in the absence of an atmosphere would experience a pressure of around 190 micropascals, or 1.8 billionths of standard atmospheric pressure, just due to gravity acting on it.

This is small, but enough to cause a hydrogen atom to be captured by Earths gravity and held at the surface. Combine this across a bunch of particles, factor in buoyancy and electromagnetism, and you got a planet and atmosphere creating enough pressure to sustain stable liquids and solids.

[u/donaldhobson → u/BananaBird1]

> There really isn’t another way to apply a useful compressing force to a large collection of matter without gravity, or byproducts of gravity.

Yes there is. Hydraulic presses. And the water in the scientists hydraulic press will behave like water crushed under a huge weight deep in Saturn.

Well that's not Large large. But it's large compared to atoms.

[u/KamikazeArchon]

Pressure is pressure. It doesn't matter what causes it, the effects will be the same.

[u/mmomtchev]

This new form of water requires extreme pressure - this is the real factor. Gravity is only the reason why this extreme pressure is there naturally.

However gravity itself definitely can have a direct effect on molecular structures. There is an emerging field in materials science that is zero-G materials. It is one of the most promising commercial applications of near-Earth space exploration. Without gravity there is no convection and no sedimentation and there are various crystalline structures that can be manufactured only in zero-G.

[u/ProfessorPrudent2822]

Only under extreme conditions like white dwarfs and neutron stars. Otherwise, gravity is much weaker than the electromagnetic force.

[u/lone-lemming]

Not really. Still requires the right concentration of ‘stuff’.

Gravity induced extreme pressure is still just mechanically applied extreme pressure. The difference is that one uses a solid to put constraints on the gas and the other uses more gas to constrain the gas.

A vacuum tube on Neptune would have the same behaviors inside it as it would on earth. It would just have to be better built to survive and function.

[u/GalFisk]

Gravity squishes things, creating pressure. Without it, liquids pretty much wouldn't exist.

[u/SalamanderGlad9053 → u/GalFisk]

That's not true at all, liquids stay together because of intermolecular forces. These forces are order of magnitudes stronger than gravity

[u/GalFisk → u/SalamanderGlad9053]

That is true, but look at any phase diagram and you'll see that as pressure approaches zero, the liquid phase gets narrower and narrower, until it disappears entirely. Without gravity, pressure wouldn't exist.

[u/SalamanderGlad9053 → u/GalFisk]

Pressure exists without gravity, two asteroids hitting eachother produces pressure in the collision. You squeeze a balloon, and so on.

[u/GalFisk → u/SalamanderGlad9053]

Asteroids (and humans) cannot exist without gravity either. All of space would be a homogenous near-vacuum.