Seismic Forces Above Grade ARE "Real" - The Mea Culpa 1

[KootK]

1.0 INTRODUCTION

1) Like Jon Snow, I know nothing. You've been warned.

2) This is going to be a multi post thread setup, TLDR be damned. To that end, I ask that nobody respond until I've made it clear that I'm done with the initial setup.

For some time now on this forum, I have been recommending that people keep in mind that the above grade seismic forces shown in blue below are not real but, rather, "fictitious forces" representing dynamic, inertial effects. That, in keeping with d'Alembert's principle that many of us learned about in university. Tomfh described this succinctly in another, recent thread:

Oh you meant our force arrows on the building (which combine into base shear) don't just appear from thin air and start pushing on the building?

I've been suggesting that folks focus on viewing internal member stresses as arising from imposed displacements generated by the base excitation. While I still think that perspective has some utility, I've come to believe that the notion that inertial forces are something less than "real" is incorrect. Internal forces do arise from imposed displacements but those displacements are "imposed" by real forces I now feel.

Oops... my bad.

 

[KootK]

2.0 THE STATE OF AFFAIRS WITH RESPECT TO UNDERSTANING INERTIA

As far as I can tell, the world still has not come to an agreement on just what inertia is physically. We have operational definitions for inertia that allow us to quantify and predict its effects but we lack a physical understanding for what it truly is. If anyone would like to know more about this, the book shown below is an excellent read on the subject.

Two notes on inertia that I feel are salient:

1) Mach's principle suggests that inertia is the sum total of the gravitational effect of every scrap of matter in the universe on every other scrap of matter in the universe.

2) I believe that Einstein was once quoted as saying that he hadn't fully sorted out the mechanics of inertia but that he was confident that it would turn out to be an "inductive" version of gravity. Another "spooky action at a distance" thing.

For me to proceed, I'm basically going to have to propose my own, high level theory of what inertia "is". What I've come up with is, I believe, consistent with #1 and #2 above.

 

[KootK]

3.0 A LITTLE GENERAL RELATIVITY

John Wheeler was once quoted as saying: "spacetime tells matter how to move; matter tells spacetime how to curve”.

Drilling down deeper on that statement:

1) Energy also curves spacetime as Wheeler knew full well.

2) It's not matter that curves space time, per se, but the mass associated with matter.

3) Mass is just a label for the subcategory of energy that is associated with stationary matter. Mass is engergy.

4) Since mass is energy, this means that energy is the only thing that curves space time.

SUMMING IT UP

I believe that there is one -- and only one -- fundamental way to cause something to move in this universe of ours:

A) Pump some energy into the spacetime nearby the thing that you want to move such that spacetime curvature is induced.

B) Stand back and watch your thing slide around spacetime in a version of geodesic, "free fall" motion.

 

[KootK]

@phamENG: consider red flagging your post to be deleted such that the setup can be pure once again.

4.0 THE ANALOGY OF A VISCOUS EARTHQUAKE (STILL NOT DONE)

I'll be making reference to this analogy in section 5.0 such that it will make sense why I'm bothering with it now.

Imagine that we have a stick stuck up into a tank of water or some other viscous fluid through a magical slot that allowed us to impose a time varying displacement at the base.

I don't think that anyone would disagree that:

1) It's a dynamic situation with respect to the movement of the masses in the tank.

2) While the viscous forces on the masses would be "induced" by the base motion, the viscous forces themselves are perfectly real.

3) One way to describe the viscous forces would be to envision them as the energetic "cost" of having disrupted the static harmony of the surrounding fluid field.

 

[Tomfh]

3.0 A LITTLE GENERAL RELATIVITY

John Wheeler was once quoted as saying: "spacetime tells matter how to move; matter tells spacetime how to

On the topic of General relativity, this video is good at explaining how curved spacetime = gravity.

https://m.youtube.com/watch?v=wrwgIjBUYVc

It’s much better than that silly rubber sheet analogy.

 

[KootK]

5.0 WHAT I THINK INERTIA AND ABOVE GRADE EQ FORCES REALLY ARE (STILL NOT DONE -- NO COMMENTS YET PLEASE)

Firstly, I recognize that it is a bold thing to try and propose my own theory of inertia when, as best I can tell, the scientific community itself has not yet settled on a definitive mechanism for it. Whatever... I'm keeping this ridonkulously high level.

I think that it must work something like this:

1) Each mass has its own gravity well that locally modifies the curvature of the surrounding space-time.

2) When an earthquake hits, the base motion attempts to impose a corresponding motion on each of the above grade masses.

3) The motion imposed upon each mass causes its local gravity well to accelerate/plow through the surrounding space time.

4) The plowing of spacetime has an energetic cost that manifests itself as a force. Spacetime fights back a bit.

So, while I'm still not done with the setup, this is my explanation for why above grade seismic forces are "real" forces. In an imperfect way, I see this as the accelerative analog to the viscous earthquake that I proposed as a thought experiment in my previous post. In particular, I find these three aspects of the theory intellectually satisfying:

A) It is, effectively, induced gravitation as Einstein suspected that it would be.

B) Mach's principle is sort of acknowledged in that the surrounding space-time field through which the plowing would occur would be the result of everything in the universe exerting its gravity on everything else in the universe.

C) It means that equilibrium is satisfied on the seismic model of a building which is, of course, nice. No more "fictitious forces" or "equivalent forces". Real forces in real equilibrium all of the time.

The elephant in this particular room, I think, is the question of why spacetime offers opposition to acceleration but not velocity. For now, I'm simply going to sidestep that as it's not part of my mandate here. If Feynman didn't know the answer to that question, what chance do I really have anyhow? Additionally, given that I know next to nothing about what spacetime is, it's a bit ridiculous for me to harbor expectations for how it ought to behave. I'm already overextended with the seismic stuff.

Galileo discovered a very remarkable fact about motion, which was essential for understanding these laws. That is the principle of inertia—if something is moving, with nothing touching it and completely undisturbed, it will go on forever, coasting at a uniform speed in a straight line. (Why does it keep on coasting? We do not know, but that is the way it is.)

 

[Tomfh]

How Mach's principle and inertia works aside, we know the earthquake forces are real, because the storeys are accelerating back and forth.