What happens if a very heavy impact hits a person's Holtzmann shield?

Question

The basic purpose of a personal shield is that it's bulletproof, transferring so little impact to the wearer that unlike modern body armour, the wearer isn't even knocked back or winded.

But physically, a bullet is not carrying that much total energy or momentum, with the danger ordinarily being that it will deliver that energy very abruptly to a very small area. That allows a shield to plausibly work like a bubble that transfers the impact over the wearer's whole body and/or slow down the impact to make it less abrupt, and so diffuse the energy to the point of harmlessness. After all, the momentum of a 60g bullet spread uniformly over a 80kg+ human isn't worth mentioning.

But are there any instances in the books or other media of a shield impact that is heavy and/or energetic enough that this diffusion wouldn't be effective? The mental image that inspired this question was firing a ballista bolt at an Imperial soldier - the energy/momentum from that seems to be at least enough to knock someone off their feet even perfectly spread out.

Answer 1

Here is a list of relevant passages from Dune, the first novel of the series by Frank Herbert (key phrases in bold):

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Chapter 4, Paul and Gurney in a weapons training session:

"The shield turns the fast blow, admits the slow kindjal!"

Paul fell back, parrying. He felt the field crackling as shield edges touched and repelled each other, sensed the electric tingling of the contact along his skin.

[Paul] crossed to the dummy, slapped the switch on its chest with his rapier tip and felt the defensive field forcing his blade away.

Chapter 25, Duncan Idaho blocks Sardaukar from reaching Paul, Jessica, and Kynes:

With his mother beside him, Paul leaped for the door, seeing Idaho blocking the passage, his blood-pitted eyes there visible through a shield blur, claw hands beyond him, arcs of steel chopping futilely at the shield. There was the orange fire-mouth of a stunner repelled by the shield. Idaho's blades were through it all, flick-flicking, red dripping from them.

Chapter 33, Paul duels with Jamis

A shield would repel too fast a blow, admit only the slowly deceptive counter.

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Here's what I don't see in these passages:

• A heavy projectile impacting a shield worn by a person.

Here's what I do see in these passages:

• Weapons wielded by human arms/hands impacting a shield worn by a person.
• Lightweight, non-lethal projectiles ("stunner") impacting a shield worn by a person.

In other words, Herbert portrayed shields as an advanced form of personal armor. He didn't include any scenes where larger or more energetic projectiles were pitted against personal shields in order to show the results. He did write that gun emplacements, buildings, and IIRC even spaceships had shields, but included no scenes with attacks on them. The shield around the Emperor's camp at the end of the book was shorted out by the storm rather than defeated by weapons.

Even the descriptions we do have of shields vs. light weapons (above) are not 100% consistent with each other. Some descriptions paint a picture of weapons being stopped in their motion or their motion reflected back toward their source. Others paint a picture of weapons being deflected just enough to miss the shield's wearer. But the first book lacks the kind of description of shield effects that would provide an answer to your question.

Answer 2

Dune's Appendix Terminology of the Imperium is quite clear: defensive shield is utilizing the Holtzman Effect which, in great simplification:

nullifies gravity within certain limits prescribed by relative mass and energy consumption.

Which means that technically there's no energy transfer between object. In fact - if Robert L. Forward is correct - the only variables that matter in that situation is mass of each colliding objects, and if the incoming object has less mass then it will be repelled, the more violently the bigger the mass difference.

It is because, theoretically of course, gravity nullification does not mean what one would think it means, to borrow from Inigo Montoya.

To oversimplify it (to the point of ridiculousness): it will literally counteract the momentum of the incoming object. In less ridiculous term it is more elegant formulation of the concept of "negative gravity" (and again - it doesn't mean opposite of gravity, that is rising instead of falling), based on the idea that gravity is a wave and thus can be cancelled out.