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# Unstoppable Force/Immovable Object Solution?

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The famous paradox: what happens if an unstoppable force meets an immovable object? It has many variations, unstoppable bullet v.s. bulletproof vest, etc. However, I think there may be three viable options.

1. This is a true paradox, and thus unsolvable.

2. Perhaps the unstoppable force was diverted from its course. The immovable object would not have moved, and the force would not have ceased.

3. The unstoppable force passes through the immovable object, not colliding with any of its atoms as it does so.

Please let me know if my reasoning is faulty or if there are other solutions to this paradox.

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Well, from a scientific perspective both those things are misnomers.

"Unstoppable Force" - I would argue forces are fundamentally unstoppable. You can counteract a force, but you can "stop" it. I.e. when I hold up a book, I'm counteracting the force of gravity that is trying to pull it down towards the earth's mass, but I'm not stopping gravity, it's still acting on the book.

"Unmovable Object" - This depends on the frame of reference. An object is never moving in it's own frame of reference, but is always moving in some other frame of reference, or else, like, our universe would be some wasteland where of absolute zero temperature, if there were no particles moving. I.e. the ground beneath my feet seems like it's not moving to me right now, but it's actually rotating around and circling from the frame of reference of the sun or an observer in space. On the most fundamental level, the electrons in any object are always moving, the atomic bonds are always vibrating.

So I think the fundamental answer to the question is that it's not a true paradox, it's using words to make things seem like a paradox, but since the words don't correspond to anything real or truly meaningful, its in truth a semantic illusion.

(But if you want to play with it, you could try arguments based on the 'frame of reference' principle )

Edited by Yoruichi-san
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When I imagined this, I pictured it to be the collision of a force with infinite energy that is not affected by friction or entropy (never stopping or slowing down) and a solid object at 0K in a fixed, unchanging point in space. That's my view, anyway. I never really thought about the semantics before.

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I'm not sure what a "force with infinite energy" actually means, but if a strong force was applied to anything at 0K, it would no longer be at 0K. A force with enough energy would destroy the object, if not by mechanical means, then by breaking the bonds b/w or even within the atoms, sending its pieces or particles flying throughout space. Since a force must have direction, there will be a net movement in the center of mass of all the debris. And don't try "indestructible", that's not atoms work, unless this is some fictional universe where things are not made of atoms and the laws of physics are different, in which case, I have no idea .

As for being affected by friction or entropy, take an object sliding down on inclined plane. The force of gravity is what is pulling it down. The force of gravity remains the same, it's a fundamental force. Friction will slow the object down, it will counteract the force of gravity, but it will not affect the force of gravity itself.

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Maybe the problem becomes simpler if we reduce it to "a force so powerful it can pass through any object" hitting "an object that cannot be penetrated, shifted, or broken."

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No, both those things are, by the laws of physics, impossible. The more powerful a force is, the more likely it will interact with the fundamental forces, particles, and bonds of molecules, atoms, quarks, etc., there's no way it'd just "pass through" the object. On the other side of the coin, all molecules, nuclei, etc. have threshold energies at which the bonds will break, particles will excite, decay, fuse, etc.

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The paradox isn't necessarily realistic, it's hypothetical. In the Grandfather Paradox, time travel is a factor, but that doesn't exist either (in the way that it is worded).

Edit: typo

Edited by Kikacat123
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Precisely. It has no correspondence to real concepts, it's a semantic bastardization of scientific terminology, like the "evolution -> progress -> good" or "entropy -> chaos -> evil" arguments.. Trying to explain with real physics (as 2 and 3 of your OP does) is kinda like trying to explain what goes on in Harry Potter with physics...;P

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What?! Harry Potter is real! Anyway, in my OP I was just trying to answer a hypothetical with a hypothetical. It's all speculative, so I guess, like you said, the real paradox comes if you try to interpret the semantics.

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• 3 months later...

The meaning of this paradox is more philosophical than physical. It is about two people going head to head with identical aptitude, where either won't give an inch and the situation is bound to continue forever. There will never be a victor or a loser, how they can conclude battle is by disengaging from it, not by trying to win it.

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As for being affected by friction or entropy, take an object sliding down on inclined plane.  The force of gravity is what is pulling it down.  The force of gravity remains the same, it's a fundamental force.  Friction will slow the object down, it will counteract the force of gravity, but it will not affect the force of gravity itself.

As gravity is a force that increases with mass, its force is affected. When an object slows down, its mass inherently increases -- though it may be unnoticeable due to the small scale we usually find such objects so affected. So small that most of our present day instruments will record no increase.

The more powerful a force is, the more likely it will interact with the fundamental forces, particles, and bonds of molecules, atoms, quarks, etc., there's no way it'd just "pass through" the object.  On the other side of the coin, all molecules, nuclei, etc. have threshold energies at which the bonds will break, particles will excite, decay, fuse, etc.

Unless the probability is 1, as "more likely" implies that the probability is not, there is a contradiction in the statement. Even with a probability of 1, the possibility can still exist. Take, for example, the probability of picking a random number that contains the digit 1 from the set of natural numbers. This probability is 1, yet it is still possible to pick the number 5 from the set, and the numeral 5 is not composed of a numeral 1.

The paradoxical nature of the problem does lie in semantics, and a lack of clear definitions. What do 'unstoppable' and 'unmoveable' mean? As mass and energy are akin as shown in the equation E=mc2 , one can say that unmoveable is when E=0, and unstoppable is when E = ±∞. Both these two "possibilities" do not actually exist with our understanding of the physical nature of the universe, yet mathematically they can each exist, but not at the same time. By definition, if one exists the other cannot. If the question, then is referring to these "possibilities", then it is not a paradox, but a fallacy.

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