# Q: What would happen if an unstoppable force met with an unmovable, impenetrable object?

Mathematician: Sometimes, when we don’t use language carefully enough, we can get ourselves into philosophical trouble. For example, consider the following statement:

If a barber shaves all those men (and only those men) who do not shave themselves, does he shave himself?

If the barber shaves himself, then he is shaving a man who shaves himself, which is something that (by definition) he does not do. On the other hand, if the barber does not shave himself, then there is a man who doesn’t shave himself that the barber doesn’t shave, which again contradicts our definition of the barber.

So what is the answer? Well, the question has no answer, because the definition we use for our barber contains within it a logical contradiction. What’s more, it is impossible for such a barber to actually exist in the real world, since the razor burn associated with simultaneously shaving yourself and not shaving yourself is too much for any single person to withstand.

What would happen if an unstoppable force met with an unmovable, impenetrable object?

Well, let’s suppose that we define an “unstoppable” force to be one that can move absolutely any matter. Furthermore, let’s define an “unmovable” object to be one that cannot be moved by any force. In that case, this question is unanswerable, because like the barber paradox above, it relies on contradictory information. By definition our force can move anything, but then, also by definition, there is an object that the force cannot move. This is a bit like saying “suppose X is true, and not X is true. Then is X true?”. Here  X is the idea that the force can move anything, and not X is the idea that there is at least one object that cannot be moved by the force (which in this case is our unmovable object). Hence, this question has no answer because it relies on assumptions which contradict each other.

This entry was posted in -- By the Mathematician, Brain Teaser, Philosophical. Bookmark the permalink.

### 29 Responses to Q: What would happen if an unstoppable force met with an unmovable, impenetrable object?

1. Bill Stone says:

I think I like your explanation better than the one I came up with myself:

An immovable object and an irresistible force cannot both exist in the same universe due to their mutual exclusivity, hence they can never meet, making the question moot.

To the inevitable follow-up of “but what if the immovable object somehow crossed over into the irresistible force’s universe (or vice-versa)?” my response is “it would depend*”.

*With the visitor’s properties being such that its very existence contradicts the physical laws of the foreign universe then either its entry into that universe would be barred, or its properties would change such that it complied with the foreign universe’s laws, or the foreign universe would have its physical laws changed in order to permit entry to the visitor. Whichever, you still don’t end up with an immovable object and an irresistible force together in the same universe.

2. Oliwier Ptak says:

Can God create a stone he cannot lift?

I think that’s the same kind of question…

3. Arjun Pachory says:

They would pass through each other! While sounding odd and counter-intuitive, that’s the only solution where both their states are maintained.

4. GtoXic says:

Arjun Pachory: Not necercarilly. Let’s say that the immovable item cannot be morphed or have it’s state(s) changed in any way, shape or form, then what would happen?

5. Kevin Woolie says:

If a barber shaves all those men (and only those men) who do not shave themselves, does he shave himself?

Yes, but only once. Before he shaves himself, he is in the set of men who do not shave themselves. Afterwards, he is in the set of men who shave themselves, so he will not shave himself again.

6. Sean says:

Would not the properties of each object be transferred to the other so that in effect it would be as if they never interacted at all as object a which was unstoppable is now in the position previously occupied by object b and has now become immoveable while object b has adopted the properties of object a and become unstoppable however for the this to work i am assuming that objects a and b were both impenetrable otherwise i would have to assume the density of object a would be subject to destruction as the original question stated the second object or object b in my example is indeed impenetrable from the get go but hey i am assuming the properties of each object are indeed transferable

7. Erik says:

If the irresistible force’s energy cannot be deflected or in any way diminished, and the immovable object is indeed immovable and unchangeable, would not the energy simply pass through the object, retaining its strength, much like a Newton’s cradle? Kinetic energy is transferred through the objects, leaving them unchanged and unmoved. Now if factors like gravity and air resistance were removed…. Funny, those things work because of gravity, but gravity is what eventually stops them.. just a thought.

8. john says:

Can anyone find any flaw in this?
Within the confines of special reativity this apparent paradox may have a solution. This lies in the terms themselves. The term ‘immoveable’ implies a body at rest. If we consider an observer on the object then he is indeed at rest but may not be stationary relative to the universe and certainly not to the irresistable force. Similarly an observer located on the irresistable force may also consider himself to be at rest and the immoveable object to be moving, he may also accept that he may be moving through space as this irrestable force, either view is valid.
If we represent each by a snooker ball, lets call the red one the force and the white one the immoveable object. Following impact, momentum is conserved so the objects both adopt new trajectories within the same universe. The guy on the white ball still thinks he is immoveable albeit the universe has started to move differently. The collision shock may be interpreted as an effect of the whole universe suddenly changing direction while he stays still.
He could even say ‘wow! I am so immoveable that when that peice of the universe hit me then the whole universe shifted’
The other guy still thinks he is irresistable and the universe has also changed the way it moves relative to him, with the same interpretation of the collision.
That guy could say ‘wow! I am so irresistable that when I hit that chunk of the universe then the whole universe had to move’
The collision interpretation is consistent with relativity.
Therefore the paradox only exists for a third observer who starts off in the frame of reference of the universe because both terms are paradoxical from that relative viewpoint. This becomes most obvious when the third observer is stationary compared to the white ball. To be a valid argument the basic precepts of causality, and conservation need to be upheld and these examples do this.

I recall in the 90’s in the comic book Magazine Wizard, a Physics Professor was asked this question. What would happen if the UNstoppable Juggernaut ran into the Immovable Blob. The Prof, after explaining that two such objects could not exist in the universe, theorised that either:

Juggernaut would bounce off (be redirected) of the Blob. There by they both would retain their titles

Juggernaut would be stopped dead in his tracks and the Blob would go flying off. There by they both lose their titles.

this defines a paradox due to two mutual exclusive concepts, but if considered virtually, there will be infinite continuous transfer of energy between the two.

11. Landon says:

HaldBadger must be right, besides it was not stated that the force could not be redirected, oonly that it couldn’t be stopped. Though it does defeat the purpose of the question, which is to have no answer.

12. chris santiago says:

@bill stone i think that was Isaac Asimovs thought ….just sayin,,,

13. Stuey says:

Lets just say that there is an unstoppable force, which hits an unmovable object. And both are to big to redirect the other. Would the resulting exchange of energy simply just distroy the universe

14. brian tran "the asian" says:

15. The Cool Dude says:

The barber is actually a woman, or instead has no hair to be shaved.

16. l2fen says:

their characteristics are materialized,in other words they take a physical form (note:they are only one thing in our universe that can’t be stopped by any kind and they are only one thing that can’t be moved by any kind)

*if someone didn’t understand I can explain in details fell free to wright me on [email protected] or skype:niki.niki165 (please people whit no academy degree do not contact me)

17. Jim says:

Considering that in the world of physics it’s quite often that things are capable of doing or being two mutually exclusive things (particle/wave duality, superpositions, and so forth), then the answer to “If God can do anything, can He make a stone so heavy that He can’t lift it” is “yes and no”. Both are true. he can simultaneously be able to lift it AND be unable to lift it. When trying to understand God, you have to remember that God is farther above us than we are above microbes. Do you think a microbe understands much of anything that you can? So it should not be a surprise that what we call logic often fails when trying to understand God.

18. Carl Brohus says:

I actually maybe have an answer for it… The unmovable object meets the unstoppable one and it reverses it direction. Just my theory. Might be or might not be true…

19. ken tolbert says:

It would bounce. Change direction.

20. The closest i came is that it would bounce off .

21. James says:

Neither can really exist at all. An immovable object would have to push back on any force with equal and opposite strength to remain truly immovable. This cannot happen because there is a set amount of energy in the universe that cannot be exceeded. If an object were able to push back equally on any force regardless of its size it would in theory have to contain equal energy to the entire universe because if it is truly immovable the entire combine energy of the universe should not be able to move it. This would break the laws of reality and is therefore impossible (The only way such an “object” could exist is if it were the universe itself). An unstoppable force on the other hand (assuming that by unstoppable we mean nothing is able to affect its movement in any way), cannot exist due to the idea that every action has an equal and opposite reaction. If a force acts upon an object or another force, no matter how small would cause the force to lose energy, therefore breaking our definition of unstoppable.

As a side note my friend asked me a different paradox when I mentioned this one to him, he asked me “what’s an immovable force?” With that he has me stumped.

22. Michael Gunn says:

can you give me the mathematical equation for when an unstoppable force meets an immovable object, and here’s a little something else what happens?

23. Shuarau Nureni says:

Scientists Are Great!

24. Eric says:

an object has to have some physical properties right? whereas a force doesn’t necessarily(?)

e.g. let the immovable object be me, on a couch, with a beer on a Friday evening– and the unstoppable force my girlfriend’s nagging. She doesn’t need to stop but I don’t have to move.

25. Luciano says:

Many wonderful theories! I was curious what if the immovable object was a black hole and the unstoppable force was the matter being sucked into it?

P.s. I am not as smart as you guys so be nice!

26. Luciano says:

Another thought. It is very possible for both to exist at the same time, however the only constant is the immovable object. The unstoppable force can only be stopped or de-flected by the immovable object but only the immovable object.

27. Luciano says:

I seem to be obsessed with this question, i keep thinking about it. I really like johns theory of the the universe shifting or accommodating to both properties. The unstoppable force would have to be able bend space time and gravity to remain unstoppable the immovable object remains immovable and the unstoppable force remains unstoppable, but only cause the unstoppable force was able to avoid the the immovable object. Time has a big part to play as well nothing is infinite so neither can be unstoppable or unmoved forever which contradicts there meaning but the two properties can come into contact with each other within the time allowed, while both retaining there properties. Also it is possible for the same object to be unstoppable and immovable at the same time but only for a limited amount of time.

28. Norm says:

Infinity would happen. An uncountable amount in time and space.