Physical Axioms and Attractive Forces

[Gregg] "Above the critical point the substance is a fluid. There is no physical distinction between vapor and liquid."

[tvf] "Well, there is one in my mind's eye. Assuming that "vapor" means "gas" and "fluid" means "liquid", the former transfers momentum by random collisions and supports only longitudinal waves; whereas the latter transfers momentum to other molecules already in contact by means of vibration and supports both longitudinal and transverse waves."

I'm going to step out on this limb and say that Gregg is partly right, and that Tom would be partly right if his assumption were right. But it is not. Just take a look at the dictionary definition.

=== (from Wikipedia) ===
A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. It is a subset of the phases of matter and includes liquids, gases, plasmas and, to some extent, plastic solids.
===

(There are probably other definitions that have features we should consider. We may need to discuss this in more detail.)

Gregg is wrong because vapors (gases) and liquids are always fluids without regard to the critical point. But I think he is right about the physical distinction part. I am assuming he means that after you compress a gas so much that the molecules are always touching (like they do in a liquid), then the distinction between a gas and a liquid goes away.

Tom's assumption that the term fluid refers only to liquids is wrong. All non-solids are technically fluids. But outside of the technical world, fluid is commonly used as a synonym for liquid.

No fluid can support the propagation of shear (transverse) wave energy within the bulk of the medium. Because the particles flow relative to each other for any non-zero applied force, there is no restoring / connecting force available to "pass the energy" to the next layer of particles.

Fluids can however support the propagation of surface waves. Surface waves are a hybrid phenomenon with particle motion that is both longitudinal and transverse (IOW, circular). They can only occur at the physical boundary between media that don't mix without agitation (like air and water, or oil and vinegar). And some type of restoring force must be present (like gravity, or perhaps surface tension). Surface wave amplitude drops rapidly to zero with distance from the media boundary.

LB

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Larry Burford</i>
<br />=== (from Wikipedia) ===
A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. It is a subset of the phases of matter and includes liquids, gases, plasmas and, to some extent, plastic solids.
===<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">All solids flow on a long-enough time scale. Glass is a good example, with a mesured flow rate that is significant on a time scale of centuries. So this definition includes everything, which makes it not very useful, especially for present purposes.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">No fluid can support the propagation of shear (transverse) wave energy within the bulk of the medium. Because the particles flow relative to each other for any non-zero applied force, there is no restoring / connecting force available to "pass the energy" to the next layer of particles.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">I can't see this. Things that flow very slowly but mainly imitate solids or liquids (such as glass or water) have no problem tramsnitting transverse waves (such as light) centrally through the medium.

In sound waves, a molecule is pushed, travels at the rms speed of tha medium, collides with another molecule relaying its momentum, then rebounds to more-or-less its original location. Vibration speed or direction of the original molecule is mostly irrelevant. In transverse waves, all molecules start in contact while vibrating. A pushed molecule relays its momentum forward at the rms vibration speed, but also transfers its random vibrational momentum in random directions that commonly include a sideways component. That sideways momentum continues only until so many molecules are displaced that their collective "weight" exceeds the sideways force, and the restoring forces push back.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Fluids can however support the propagation of surface waves.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">My initial impression is that we have little for value for understanding elysium to be learned from surface waves. We need to consider waves propagating through the deep ocean. Either sound waves (as from whales) or shock waves (from underwater explosions) might provide valuable information about how underwater waves behave. To me, this seems crucial to making sure the next deductive step about the nature of elysium is in the right direction. -|Tom|-

Tom:"Water is (very closely) incompressible."

The density of a fluid certainly changes with temperature. Water may not be the best example because it behaves funny over temperature.
Mercury may be a better one because it's an element.

Compressing a substance means lowering the average distance between molecules (adding kinetic energy to the system).
So if a fluid can change density, there must at least be some room between molecules. I would say that makes a fluid compressable, all be it not by much.

(When compressability is low, the resulting adiabatic process will also be hard to notice.)

___

Larry:"Fluids can however support the propagation of surface waves."

So can boundary layers within a fluid or gas when temperature distribution is not homogenous.

[tvf] "The word 'entrainment' puts us at a disadvantage in thinking about a pressure-dominated medium."

Or in thinking about a medium that likely has other properties that are unknown and possibly counterintuitive, given our limited experience. Now that you mention it, this seems obvious. I've been using (and thinking in terms of) entrainment because you used it and because it conveys (some of) the essence of what is (probably) going on near gravitating masses. But if used uncritically it might lock us into thought patterns that lead in the wrong direction.

Do you have another term in mind that we should consider using?

[tvf] " ... the smaller anomaly created by Earth is more significant locally because it changes faster with distance than the Sun's anomaly."

Hmmm. The elysium "entrained" by Earth creates a local "bulge" (gradient) in the much larger bulge (gradient) of elysium "entrained" by Sol.

The LB bulge moves around within the Earth bulge. The Earth bulge moves around within the Sol bulge. The Sol bulge moves around within the Milky Way bulge. The Milky Way bulge ...

All the while (assuming a pressure model and dynamic "entrainment"), individual elysons are streaming past and through LB and Earth and Sol at high velocity. They don't get any closer together (a density gradient) as they move by, but push harder on each other (a pressure gradient).

The term entrainment still works, but it does suggest that parts of the medium attach to matter moving within it. Until we know for sure that this is what actually happens another more general term might be helpful.

I guess this is why I've been struggling with things like static vs dynamic entrainment. In the absense of another term to express the idea, I fall back on adding a modifier of some sort to a term that is in the ball park.

LB

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by tvanflandern</i>
<br /><blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by jrich</i>
<br />Clearly no substance is incompressible, even steel or diamond or <i>neutronium</i>. Changes in pressure always result in changes in density.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Water is (very closely) incompressible. My understanding is that the density of water is not significantly greater at seven miles depth than it is at the surface.

Can anybody quantify that? -|Tom|-<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Water is certainly compressible. It's Bulk modulus (from wikipedia) is 2.2 GPa. To increase the density of water by 1%, 22 MPa (0.01 x 2200 MPa) of pressure must be applied to it. Since the pressure of a column of water increases due to Earth's gravity (hydrostatic pressure) by about 10 kPa/m the pressure at the bottom of a 11 km column of pure water is about 110 MPa. So the density increase would be 110 Mpa/22 Mpa or about 5%. That's not a lot compared to gases, but it's significantly larger than zero. Of course in the non-pure water real oceans, salinity and temperature affect sea water density much more than pressure so its certainly possible that the density of sea water at the surface and at the bottom of the Mariana Trench is almost the same.

JR

[tvf] "All solids flow on a long-enough time scale. Glass is a good example, with a mesured flow rate that is significant on a time scale of centuries. So this definition includes everything, which makes it not very useful, especially for present purposes."

Technically true, but you're really stretching it thin. I propose that the Wiki definition above be amended to say " ... a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress <u>so long as the force is applied for less than &lt;a minute?&gt;</u> ... ".

Some of the referenced plastic solids might not flow within a minute (and be erroneously excluded), but glass definitely will not (and thus be correctly excluded.

[tvf] "I can't see this. Things that flow very slowly but mainly imitate solids or liquids (such as glass or water) have no problem tramsnitting transverse waves (such as light) centrally through the medium."

Light also propagates through air. But doesn't it do so because air is filled with elysium?

[tvf] "My initial impression is that we have little for value for understanding elysium to be learned from surface waves.

Agree strongly.

[tvf] "We need to consider waves propagating through the deep ocean. Either sound waves (as from whales) or shock waves (from underwater explosions) might provide valuable information about how underwater waves behave. "

Or earthquake waves? See final comment below.

[tvf] " ... To me, this seems crucial to making sure the next deductive step about the nature of elysium is in the right direction.

Agree strongly.

===

Subsurface earthquakes produce both longitudinal waves (p waves) and transverse waves (s waves). When either type reaches the surface they become surface waves (Rayleigh waves). As you say, surface waves offer little help in understanding wave phenomena withn bulk media.

They each have their characteristic propagation speeds, and this is used by seismologists to both locate the quake and to tell us things about the media through which the waves have passed. Transverse waves from a deep quake can travel around the planet several times, but they are stopped cold by an ocean or the liquid inner core of Earth. Longitudinal waves pass throuh oceans and the inner core without problem. Much of what we know about the inner core (especially its suspected liquid state) comes from the behavior of transverse waves when they reach it. They bounce, they absorb, they become "surface" waves at the interface, but they do not propagate.