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Of course it's plausible!

Your world, your rules.

Since you have what is basically a human doing the shrinking & expanding of the metal, the obvious means is a kind of magic. As you already said: the mutant squeezes the metal so that the atoms press closer together, then reexpands the metal to its normal dimensions.

Consequences:

• eventually, if he squeezes the metal enough, he might end up with a neutronium bauble.


• if the number of atomic particles remains constant, then the weight will remain the same; it'll just be in a smaller space.


• durability should remain the same as well. Magic after all!


• the only open question is how Hephaistos keeps the metal squozen down into bauble form: we would need incredible pressure generating machines & technology to accomplish this feat, and would need to keep applying those forces in order to keep the metal compressed. He must also be making use of some kind of force field that maintains the bauble's diminutive size. Presumably, if he falls unconscious or is killed, his weapons will lose their magical integrity and will explosively expand back to their original volume.

Nice idea, but unfortunately not very realistic

Here's why:

• First off: Of course the atoms would be closer together but they would also repel each other. Which means as soon as you release the pressure, the atoms would push each other away and the weapon becomes bigger again, maybe not as big as originally but definitely quite close to that


• Heat is an important factor. If you pump your tire with a hand pump and you touch the shaft of the pump afterwards you will notice that it will be quite hot. If you compress a weapon like you described there will be a lot of heat that has to be transported away. Else your weapon would either melt or lose its properties (see below)


• Even if two pieces of metal or composed of the same element they can be different in the shape of the metal crystals. E.g. If you heat iron in a forge and cool it down slowly it will be softer and more flexible. If you cool it rapidly it will be very hard but break more easily. So the components in your weapon would probably loose their properties if you compress them.


• Let's say you could compress them regardless of the facts above: How would you do it? If you apply pressure in one direction the weapon will squeeze towards the other direction. So you'd need to apply the pressure from all direction and it has to be exactly the same amount of pressure. Else your weapon will deform and will be useless after inflation.


• And last but not least: How would you inflate it again without deformation? You'd need to reverse the process described above with perfect precision

While not realistic per se, as pointed out by the answer of CKA, it can work as an "arbitrary" superpower, as long as it is consistent. So, what would happen if this Hephaïstos actually had the power of squeezing metal atoms together by thousandfolds if not millionfolds? He would create degenerate matter, the stuff white dwarf stars are made of.

The compression would heat the metal to immense temperatures, so I will assume that the extra heat is temporarily stored by the power itself, and put back when it stops acting. Heat transfer depending on surface, it may have an immense heat capacity for its size, assuming the stable degenerate matter somehow keeps working like a metal in that regard.

If the power stops instantly, then those baubles are bombs, possibly equivalent to nuclear weapons.

If his power has an "inertia", and the bauble only slowly expands back to its original size once the effect stops, he can still use the expansion to break pretty much anything, but in the way of a hydraulic piston instead of a bomb. Inversely, the compression itself may be used to crush things with irresistible force, possibly up to nuclear fusion level. If you want to avoid this, the compression or expansion effect may be resisted externally due to how the power itself works - if expansion is limited, an object could be kept indefinitely in its compressed state if, for example, encased in something hard.

Baubles massing several tons are extraordinarily scary things. They will fall through the ground if you leave them there. So as Hephaïstos is strong enough to wear it, he is also strong enough to throw them, and there is nothing (barring other superpowers) that can survive that level of armour-piercing. If he needs less armour-piercing and more impact damage, simply compress it less, to have more contact surface for the same projectile mass and speed. If he can compress and throw shrapnel, armor-piercing dust would also be a terrifying weapon. And of course, a bauble knife or cutting wire will cut through anything. Conversely, compressed armor will stop any non-superpowered attack with sheer mass and density.

Also note that Hephaïstos is not only immensely strong, to be able to carry those, he also has an incredibly resistant skin (and clothes), or some sort of telekinetic powers. Otherwise, the baubles will go through him as easily as through any ordinary matter. (This type of required secondary superpower is similar to how Superman can grab a car by its bumper without ripping said bumper apart.)

And of course, let's not forget that he is wearing literally tons of equipment, so a car hitting him will stop the car, not send him flying.

Metals are to some degree compressible, and they may be compressed even further undergoing some phase transitions to denser modifications. Plutonium is a metal famous for having many modifications and a more dense modification created under high pressure is one ingredient of a nuclear bomb.

However, under given conditions (pressure and temperature) only one modification is stable and the other modifications are usually unstable or in rare cases meta-stable (like diamond, a meta-stable modification of carbon/graphite under normal conditions). A phase transition is always mass preserving and the chemical identity of the material is unchanged, but the stability of an object is usually not guaranteed and sharpness of weapons is surely not preserved.

The best your mutants can have are weapons made of very dense meta-stable metal causing heavy violence to the victims and breaking weapons made of low density metals. Live transformations from small to big or vice versa won't work. Depending on the tech level, those very heavy weapons are probably copy-protected (but still can be conquered or stolen by non-mutants).

*Your World, Your Rules*

Having a basic understanding of the physics in our world can help you formulate a reasonable alternative for your world.

Here are a couple of articles that explain atoms and nuclei,

• https://en.wikipedia.org/wiki/Atomic_nucleus


• http://academic.brooklyn.cuny.edu/physics/sobel/Nucphys/atomprop.html

Roughly paraphrasing the CUNY article,

An atom is about 10e-10 meters in size. Although atoms of different
elements vary in sizes, use a rough estimate of 10e-10 as the size of
any atom.

An atom is composed of a nucleus surrounded by an electron cloud. The
nucleus is about 10e-15 m in size (about 10e-5 or 1/100,000 of the
atom). A good comparison of the nucleus to the atom is a pea in the
middle of a racetrack.

The nucleus is composed of protons (positive) and neutrons (zero).
There is a nuclear force which is (mostly) attractive and acts between
protons and neutrons, and is stronger than the repulsive electric
force.

Your blacksmith could generate some 'field' or alter metals to,

1. manipulate (amplify) the nuclear force of metals


2. moderate (reduce or weaken) repulsive force between nuclei of metals


3. add another particle to the nuclei of metals (to do the above)

Option 3 makes the most sense to me, depends upon how your desired storytelling

Don't forget about inertia. You must push as hard to move a bauble as if it were its normal size & every movement continues in its direction & rotation until you put in the same energy as you did to get it started. (Consider handling a piano via an attached bracelet charm.) This also means that the force & pressure to push it and get the friction to twist it is increased.

On the other hand, you really don't need weapons or metals, you can just compress rocks or for that matter marshmallows, because when you throw them the target is going to have to donate the same amount of energy at the same pressures as you.

PS Asimov addressed some of these issues in the introduction of, text of, and essays about his novelization of the movie Fantastic Voyage, wherein humans & their submarine are shrunk to do surgery.

The size of atoms, the size of molecules, and the distance of metal atoms in the crystal grid are determined by the size of the electron hull, which in turn depends on the interaction of the electrons with each other and with the nucleus.

Electrons around nuclei behave a bit like three-dimensional standing waves, comparable to a resonating surface, but in space. The wave's shape is determined by the electric charge of the electron and proton, the mass of the electron, its energy and the vacuum permittivity.
The latter in turn is connected to the light speed; higher speed of light would decrease the permittivity which in turn would increase forces between charges, if I understand correctly. Let's assume that higher forces would result in smaller atoms and denser crystal grids. All your hero needs to do is to locally increase the speed of light. I would think that that is equivalent to locally speeding up time. Easy ;-).

I'll be looking at the energy side of this with my admittedly very cursory understanding of degenerate matter, because it's pretty interesting. If you want to compress metal down to small, but still macroscopic sizes it's probably alright to disregard anything to do with the nucleus and just look at the Fermi energy of the electron gas within the metal. The energy per electron is $E = hbar^2 (3pi^2*Electron Density)^{2/3}/2me $ in a Fermi gas. According to this table http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/fermi.html the value for iron would be $17*10^{28}$ free electrons per cubic meter. Which gives us about $11 eV$ of energy per free electron. If you want to cut the iron's density by a factor of say, ten, we have ten times higher electron density and an increase in energy of $10^{2/3}$, ca. $4.64$. So each free electron has an energy of about $51eV$ now. As said before, there's $17*10^{28}$ free electrons in a cubic meter of iron. Each of those has 40 additional electronvolts of energy now, which comes up to a total of $1.09 * 10^{12} J$ of energy, or $260 t$ of TNT equivalent. Yep, a block of metal is practically a small nuke. Formidable indeed if your mutant can also spontaneously decompress metal.

To answer 1): The density in a White Dwarf is a hundred thousand times larger than iron, but the nuclei still stay intact in the degenerate electron gas. Iron will stay iron even for a very high compression.
2): Yes, I see nothing that would indicate otherwise. It's just very dense.
3): Realistically it'd be a white hot nugget of extremely high temperature, see math above. Having it be stable requires handwaving. It makes intuitive sense that extreme density would also result in an increase in durability though, there's simply more mass to move out of the way if someone wants to punch a hole into it. Another cool property you could give these metals is superconductivity. Highly compressed hydrogen is theorized to turn into a superconducting metal for example, so it's not a farfetched idea to apply that here too. There's all kinds of cool stuff coming with that, like levitating in magnetic fields.