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In my fictional story I am playing with the idea of a sort of density gloves that can change density while keeping volume the same. Obviously you can punch people with diamond hard gloves with these things, but what are other uses? How dangerous are they to the people on the punching end? Since increased density increases the weight, how heavy can they get until they're inconvenient?

The principal

The gloves change density(magic, tech, biological whatnots, whatever works for your answer its pretty open) and can do it really quickly. More dense makes the glove heavier, less dense they get lighter, but volume always stays the same. Going across the spectrum from densest to least dense takes two seconds at most.

Limits

The most dense it can get is about three times the density of the densest element, at 70 grams in 1 cubic cm. The least dense(is there a better word for that?) it can get is 10^-20 grams per cubic cm.

More details can be added if you need them

Sort of a secondary question, but if two people have these gloves, what does a fight between them look like?

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    $\begingroup$ Those sound great for picking after my dog! Low density for tactile sensation during the pickup phase (gotta be a good neighbor and get it all), then increased density to reduce ductility between the squishy bag and my fingers (because it's still warm). $\endgroup$
    – user535733
    Commented Oct 19, 2020 at 21:21
  • $\begingroup$ How's the mass preservation work in this case? If it does work, would it transfer mass from wearer's body into the gloves and back? $\endgroup$
    – Alexander
    Commented Oct 19, 2020 at 21:25
  • $\begingroup$ are the gloves elastic? i can imagine catapulting them, or just simply throw the dense glove to your opponent as range attack. $\endgroup$
    – Li Jun
    Commented Oct 19, 2020 at 23:31
  • $\begingroup$ How does the densification affect velocity? Does it just keep going in the same direction, same speed, only with more mass/momentum/energy, or does it have to play games to keep energy constant? $\endgroup$
    – Cort Ammon
    Commented Oct 20, 2020 at 5:36
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    $\begingroup$ You have a conceptual problem. Density does not mean hard. Or strong. Or stiff. It only means (for a fixed volume) heavy. $\endgroup$
    – WhatRoughBeast
    Commented Oct 20, 2020 at 22:56

4 Answers 4

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Ignoring the conservation of mass has physics-breaking implications.

The biggest implication of these gloves is that you have an unlimited, free source of perpetual energy. Turn the density all the way down, and use an electric motor to lift the glove to some height. Now turn the density all the way up, and let the glove fall and charge a battery on its way down. You will harvest more energy than it took to lift the glove in the first place, meaning you have a truly limitless, renewable, clean source of energy! The amount of energy earned will be proportional to the ratio of upper and lower density limits, so you could turn 1 Joule of energy into 1020 joules in just one cycle. Using these gloves to punch people would be an utter waste.

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    $\begingroup$ Also, it can be a solid vacuum balloon. You don't need to use a motor to lift it. $\endgroup$
    – notovny
    Commented Oct 19, 2020 at 23:16
  • $\begingroup$ Have the glove in a karate chop shape and it will do for an spectacular guilhotine. Light on the way up, heavy as Mjölnir on the way down. $\endgroup$
    – The Square-Cube Law
    Commented Oct 20, 2020 at 2:01
  • $\begingroup$ Physics-breaking indeed! Best use: put glove on flywheel. When at bottom, zero the density. When at top, make maximum. This flywheel form a perpetual motion machine that generates energy! Use this unlimited energy to conquer the universe. But do so quickly, because you are likely destroying the universe in the process. $\endgroup$
    – user79911
    Commented Oct 20, 2020 at 7:33
  • $\begingroup$ hang on - you can restore physics pretty easily by requiring the density change takes energy to happen and this is equal or greater than the energy gained - this does give interesting communication issues though $\endgroup$
    – jk.
    Commented Oct 20, 2020 at 12:35
  • $\begingroup$ @notovny Right, there are infinite ways to build a perpetual motion machine with these gloves, using a motor, or a balloon, or a flywheel, or basically any apparatus that allows you to lift the glove when it's light and drop it when it's heavy. $\endgroup$
    – Nuclear Hoagie
    Commented Oct 26, 2020 at 17:57
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They're as useful as any previous steel gauntlet

I can't think of any reason why in combat you wouldn't always use the highest density you can physically throw. Anything less simply opens you up to getting the snot beaten out of you by your opponent. Worse, these gloves will amplify the advantages between people of varying strengths. Without the gloves, the two of you are fighting with more-or-less the same disadvantage (the strength of the hand's muscles and bones). But the stronger person can throw a higher density glove, which increases the advantage to them.

So long as the gloves remain pliable, the only disadvantage to using them in highest-possible-density mode all the time is the loss of tactile sensation. You could use them to pound in lengths of rebar (although a hammer would be easier).

To make a long story short, you're basically asking, "my world has access to steel gauntlets — why would you use them?"

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  • $\begingroup$ Heavy steel gauntlets will slow you down. With these magic gloves you can lower the density, throw the punch, and only after it is in motion raise the density to max. Of course this might shatter your hands. $\endgroup$
    – Daron
    Commented Oct 20, 2020 at 10:56
  • $\begingroup$ @Daron That's true, but nothing's free (gloves that have no consequence are like godlike beings - boring) and without the added weight, they have no value other than to create an entirely rigid surface between the wearer's fist and the target. So they won't substantially add to any fight. Indeed, removing the limitation of weight renders them even more useless. $\endgroup$
    – JBH
    Commented Oct 20, 2020 at 13:31
  • $\begingroup$ They do have added weight, You raise the weight after throwing the punch. That way you can throw a 1000 ton punch without lifting 1000 tons. $\endgroup$
    – Daron
    Commented Oct 20, 2020 at 19:29
  • $\begingroup$ @Daron After starting to throw a punch the change to, e.g., 1,000 tons would cause the glove to fall the ground in a ballistic manner - but the energy put into the glove was long before the weight changed, so the only energy available for horizontal motion is for the lighter weight. F(x,y) = mA(x,y), gravity would not only drag the glove the ground almost instantly, it would rip the user's arm off in the process. $\endgroup$
    – JBH
    Commented Oct 21, 2020 at 1:34
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You can stop bullets with them

Newton's approximation for Impact Depth works as a good estimator of how far an impactor (like a bullet) will penetrate into a stationary target:

$$\frac{d_{target}}{d_{bullet}} \approx \frac{\rho_{target}}{\rho_{bullet}}$$

That is, the penetration depth ($d_{target}$) is proportional to the ratio of the densities of the impact medium and the bullet.

A 5.56mm NATO round is capable of penetrating 12mm of steel (density $8\ \mathrm{g\ cm^{-1}}$) at 100m. Its penetrating power in a material 8.75 times denser is therefore at least 1.3mm, but probably not substantially more. A completely reasonable 5mm thickness of material, therefore, should be able to absorb and dissipate the impact of modern military ammunition. You can literally catch bullets in your hands.

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The first thing to think of is the energy consumption. When you increase density without changing volume, you are (conceptually) compressing a solid. Compression on a solid or liquid is quite difficult-it changes the bonds and the interactions between the molecules of the solid. For reference, anthracite coal has a density of approx. 1.506 g/cm^3, while diamond has a density of about 3.5 g/cm^3. Often this will lead to crystallization or more ordered "stacking" of the molecules, but requires a massive amount of energy, which creates heat. In today's most common method of "densifying" a material, commercial lab-grown diamond presses must heat carbon to at least 1400 degrees C to cause it to reorder itself.

With a bit of handwavium you might be able to explain how the gloves can effectively melt and reform in their high density state, using fusion power and magnets to hold it all together. In reality though, wearing 1400 degree gloves would be slighly uncomfortable and, if you're a knight fighting a dragon with your wonder gloves, speed up the barbecuing process for your antagonist. Not to mention that the gloves must also have some source of extra material, possibly in a pocket dimension, to keep a relatively stable volume.

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