If the vacuum is a real mechanical medium—stiff enough to carry light—then a sharper question follows naturally:

Why don’t we feel it at all?

After all, we do feel air when it moves past us.
We do feel water when we swim.
So why does the vacuum seem perfectly transparent to matter, yet actively shape the behavior of light?

The answer lies in a fundamental mechanical distinction: the difference between motion and deformation.

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Sensing Requires Strain, Not Presence

You do not feel a medium simply because it exists around you.
You feel a medium only when your motion produces differential strain within it.

A simple analogy makes this clear.

An object that is neutrally buoyant in a fluid does not feel the fluid at rest. Even when it moves at constant velocity, it experiences no force—because the fluid moves with it. Only when the object accelerates or when relative motion produces shear does resistance appear.

Presence alone is not detectable.
Only relative deformation is.

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Matter Moves, Light Deforms

This distinction explains everything.

• Matter (defects of the medium) translates with the surrounding stress–flow configuration.
• Light is not a translating defect—it is a propagating shear deformation of the medium itself.

Matter does not continually shear the medium during uniform motion.
Light exists only as shear.

That single difference is decisive.

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Why Stiffness Matters Only to Waves

In an elastic medium, stiffness governs how fast disturbances propagate, not whether objects can exist inside the medium.

Transverse waves obey:$$c = \sqrt{\frac{S}{\rho}}$$​​

where:

• $S$ is shear stiffness
• $\rho$ is density

Light “feels” stiffness because light is a shear wave.

Matter, by contrast, does not probe stiffness unless it attempts to deform the medium. During uniform motion, it does not.

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Why the Medium Is Invisible to Our Senses

Every sensory mechanism you possess—touch, pressure, sound—works by detecting gradients:

• Pressure differences
• Shear forces
• Compression and rarefaction

The vacuum medium around you is:

• Nearly perfectly uniform
• Extremely stiff
• Extremely low-loss

Uniformity produces no gradients.
No gradients produce no sensation.

You are not blind to the medium.
There is simply nothing for your senses to register.

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Why Light Reveals the Medium Instead

Light cannot exist without interacting with the medium.

Every photon corresponds to:

• Local shear displacement
• Stress transport
• Elastic response

That is why:

• Light bends near mass
• Light slows in regions of reduced stiffness
• Light carries energy and momentum through space

Light is a diagnostic of the medium’s properties.
Matter, during free motion, is not.

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A Useful Rule of Thumb

If something moves with the medium, it feels nothing.
If something moves as a deformation of the medium, it feels everything.

Matter belongs to the first category.
Light belongs to the second.

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What This Means Going Forward

This distinction explains why:

• The vacuum can be mechanically real yet experientially empty
• Motion does not reveal a background substance
• Optical phenomena reveal structure that mechanics alone does not

It also sets up a deeper question—one that troubled physics for more than a century:

If the medium exists, why doesn’t motion through it reveal an absolute state of rest?

That question is not an objection to a mechanical vacuum.
It turns out to be one of its strongest predictions.

We’ll take that up next.