The pin is only immovable if you paddle it with a ping pong bat, otherwise it isn’t
Same with the ball, it’s unstoppable with a paddle but when it hits a brick wall it aint
Why?
An immovable object cannot be moved by anything, including the bowling ball. By stipulating that the bowling ball and the pin never meet, you make the question of the pin’s movability undecidable. Same goes for the bowling ball. Put another way, by using the ping pong paddles, you have merely shown that the pin is immovable by ping pong paddles, not that it is absolutely immovable.
In your bowling ball/pin scenario, the bowling ball would be traveling around the pin at exactly the speed of light (which isn’t possible, though). If the orbit of the ball were to decay, and eventually crash into the pin, it would be cataclysmic. To the pin, the ball is careening toward it at the speed of light. To the ball, the pin is also coming at it at the speed of light.
So, a UO and IO are identical things. One is not greater than the other. They don’t have to necessarily meet, but logic tells us there’s no difference between the two. And if they do meet…
Boom.
Kinda boring, but there it is.
I would point out the OP specified “unstoppable object”, not unstoppable force. Both the objects in the OP are the same type of thing, objects whose current inertial state cannot be changed. Both could be particles with no interaction cross section with any force in the universe.
Jonathan
Are you describing a massless particle? If so, wouldn’t that define it as not matter? An object would have to consist of matter.
So by your definition of immovable, no object can be called immovable until we’ve tested everything against it? That would seem to be saying there are no immovable objects .period.
I would define an immovable object as one for which any force that tries to move it, fails. A force (unstoppable or not) for which it is impossible for it to even try to move the object would not affect its immovability status.
Yes, that arguments may work in this universe. But people have made the statement that it’s logically impossible for an immovable object and unstoppable force to coexist. That means we can consider any logically consistent universe.
True, and in our universe, matter must interact with other matter through certain force (although you can have massless particles such as photons, which are real objects). In this thread we are speculating on something that, to the best of our knowledge, cannot exist.
The question from the OP was:
You could argue that the word hit implies in interaction, but I would say the Earth is “hit” by numerous neutrinos every day the majority of the pass through without ever interacting. If you speculate a universe in which particles can exist that have no interaction with other particles, then you can have both an immovable object (such a particle at rest), and an unstoppable object (such an article in motion). Such particles would have to come into existence at inertial equilibrium and remain there for the life of the universe.
Now, if we change the wording to: Suppose an irresistible force comes interacts with an unmovable object. What happens? That seems more of a logical contradiction, but even so it may not be possible to imagine a universe where both could exist. It seems to me, that you could have an irresistible force and an unmovable object in the same universe, but only if you had exactly one of each (and by one force I mean one vector of force only). In that case what would happen, is what happened in Alan Dean Foster’s The End of the Matter. The object would appear to move, but in actuality, the entire universe would shift around it. If you had two objects or two forces, it would rip the universe apart.
Jonathan.
An unstoppable force is, by definition, one that cannot be stopped by anything in existence. If the unstoppable force exists, the immovable object does not, and vice-versa. Asking what happens when they meet is like asking what happens when you divide an object into three equal halves.
Please note my OP states OBJECT not force
ETA: I’m starting to wish I’d never asked
“Force” is in the traditional formulation of the problem, but it makes no logical difference. (And this is essentially a logic puzzle, not a thought experiment in physics.)
nobody here a fan of general kinematics? i’m understanding an unstoppable force to mean cannot be stopped - i.e. always moving at its constant velocity. an unmovable force, i take that to mean that from every other frame of reference, no matter what is done to it, will not move - a physical impossibility, but not one too difficult to imagine.
now, what happens when you have something traveling with a constant velocity strike another and retain the same velocity while imparting none of it to the object struck? it harks back to those many hours doing intro phys hw about bliliard balls doesn’t it? a perfectly elastic collision.
now if you want to get beyond the simplicity established by perfectly elastic collisions, i suppose that you could define unmovable as something that will get from point A to point B no matter what. Then you rethink the unmovable object as something beyond just an unmovable billiard ball and try to imagine its real world equivalent - a black hole will do nicely. It is massive enough to not be affected by the gravity of others around it making it effectively free of outside forces and thus stationary. now, when a spaceship goes into a black hole and assuming the hole hasn’t done anything funny like ripped space-time, the ship will essentially travel forever since it is still moving ahead at constant velocity, and the black hole has stretched space time to “make it longer”. So, bottom line… the unstoppable object will try to hit the immovable object, but it never will, which is more or less what everyone else is saying thus far.
The second statement does not follow from your definition, as shown by my bowling pin and ball counter-example. My bowling ball cannot be stopped by the pin, because it is forever orbiting. And yet nothing can move the pin.
Not at all: If there exist objects with which one of our hypothetical absolutes do not interact, then we need not consider those objects at all. Can the bowling pin be moved by the ping-pong paddle? No, no matter how hard we paddle the pin, it doesn’t budge. Can the bowling pin be moved by the bowling ball? No, because it can never interact with the ball. Since there is no object in the universe which can move it, it can’t be moved at all. Likewise, the bowling ball can’t be stopped by the pin, since they never interact.
Prepositions are important.
Just in case anyone is wondering why I asked.
I remember many years ago reading a Captain Marvel comic book.
One of the questions posed our hero was that in my OP, and the answer was:
The UO would hit the IO and they would change places, the UO would become the IO and vice versa.
Daft innit?
Fair enough. Although, I was trying to keep the basic fundamentals of particle physics and GR in check. Anyone can simply creating particles that don’t exist or make any sense to get around the issue.
In QM, everything supposedly has a particle associated with it. Those that don’t have mass, is not matter. And therefore, cannot be applied to creating objects out of them, hence subverting what the OP was getting at.
Anyhoo, it’s all fantasy anyway. In this made-up universe, why couldn’t the UO just deflect off the IO at the opposite angle, like light off a mirror? In fact, that’s a pretty good analogy, if I do say so myself. 
Okay. Say you own a circus with a side show, and among the side show attractions are a Strong Man and a Heavy Lady. These attractions are at the opposite ends of the midway, they do not interact and have in fact never even met. The Strong Man’s sign advertises that he is so strong that he can lift any person in the whole world. The Heavy Lady’s sign advertises that she is so heavy that no person in the whole world can lift her. Do you see a logical problem with the signs?
I was attempting to refute the assertion that it is logically impossible for both an immovable object and an unstoppable object to exist in the same universe. If we posit that GR applies, then nothing is unmovable simple because nothing can be stationary in all frames of reference.
Not to nitpick, but it depends on how you define object. Is a photon an object? What about an electron? To me an object is something that exists and is defined. Mass, charge, energy; all attributes of objects, but not required to make the object exist.
That may be a valid solution, depending on the framing of the question. Does “unstoppable” mean unchanging speed(scalar, direction does not matter) or velocity (vector, direction does matter)
This comes back to the difference between unstoppable object and an irresistable force. Your example works on the second, but not the first. You could, logically, have both The Heavy Lady (“No one can push her aside!!”) an The Slippery Ell (No one can hold him in place!!"). All the eel has to do is go between her legs and no contradiction in logic.
Jonathan
Whether there is a problem with the signs depends on the answer to “is it possible for the Strong Man to attempt to life the Heavy Lady?” If your statement “they do not interact and have in fact never even met” is extended with “and never will meet”, then it is impossible for them to interact. And there is no contradiction. No one has or will ever succeed in lifting her, and he has not and never will fail to lift anybody.