So what does happen when an Unstoppable Force meets an Immovable Object?

Or is this one of those “If a tree falls and no-one is there to hear it” questions? Can any force be described as Unstoppable? Is light unstoppable? And how can Immovable be defined? I’m pretty sure that even the Earth could be “dunted” off its orbit if hit with a big enough asteroid or comet. Any scientific breakdown of this question, or is it just a physics riddle?

Both cannot exist at the same time.

It’s a meaningless question, since both actors in the proposition are impossible.

Or, if you wish a suitable answer to the question: your knob drops off.

Light is not unstoppable. You can verify this for yourself by observing a light source behind an opaque barrier. For example, if you put a flashlight against a dictionary, do you see any light going through the dictionary?

Yes, rather like the concept of “A Pizza Too Big to Eat” or “More Beer than We Can Drink.”

This, too, could cause your knob to drop off.

Every time this comes up I reply with “the unstoppable force goes through the immovable object and continues on its path.” What’s wrong with that? Is it too glib?

According to All-Star Superman #3, “They surrender.”

It’s not a physics riddle, but rather a popular cliché with no basis in reality.

My understanding was that light just reflects off things and continues infinatley… So it bounces off the dictionary, into the eyes of a passer-by, allowing him to “see” the dictionary… as was my understanding.

All pysical objects (even a mirror) absorb some of the light that strikes them. The light that is not reflected is absorbed and changed into heat energy, (not always right away) this light is effectively stopped. As to the OP Peggy Lee said it best “Somethings gotta give, somethings gotta give, somethings gotta give!” OK bad joke. But now I have that stupid song stuck in my head!!

In practice there are objects that are immovable by the forces you have at hand. In that case you insert dynamite and reduce them to manageable size.

Since force is a vector, any reflection will constitute “stopping” the light force.

To follow up on mstay, think of a neutrino ploughing through the Pyramid of Giza. Happens all the time.

Gee, seems to me we’ve had this one before.

1.) as has been pointed out, both items, by definition, cannot exist. Ambrose Bierce, in his wonderful Devil’s Dictionary, says the word to describe such situations is incompossible, a word I would dearly love to use more often. I thank you for giving me the opportunity to use it again. Bierce points out that the word makes for an elegant and delayed-action insult. “Sir, you and I are incompossible!”

2.) There is a myth intended to deal with just this situation. I suspect it’s a pretty late one, because it shows every sign of being a late sophistry rather than a primitive myth. There was a hare said to be so swift it could not be caught, and a hunting dog – I believe it was Laelaps – who could catch anything he chased. Inevitably, Laelaps took off after the uncatchable hare one day, and the result would destroy the foundations of logic, so Zeus took care of it by turning both participants into stone, so the issue could never be resolved. Me, I think there were just a couple of rock formations somewhere resembling (to the willing eye) a hare and a hound, and some mythically-minded tour guide came up with this little gem of Greek logic to explain it. It’s fully as serious as the stories they tell about the rock formations ayt Polar Caves in New Hampshire, or Bryce Canyon in Utah. Or just about any limestone cave with stalctites and stalagmites.

Excellent. We have ourselves a testable hypothesis.

Put the flashlight against the dictionary. Make a good light seal with some silly putty or somesuch.

Turn the flashlight on for about three minutes, then turn it off. If our hypothesis is correct, that will “accumulate” a bunch of the infinitely-bouncing light in the sealed chamber, no?

Then pull them apart, being careful to look away from the bright flash of all that accumulated light so you don’t go blind.

I’m only being a little tongue-in-cheek here – this actually might make a good experiment for talking to kids about how light behaves.

An unstoppable force is one which cannot be stopped by any object in the universe. And an immovable object is one which will stop any force in the universe. So you can see where, by definition, you can’t have both in the same universe.

Light cannot be stopped. You can cause light to cease to exist (as, for example, by sticking a dictionary in front of a flashlight), but you cannot stop it. Never will you have a stopped photon.

Neutrinos, by contrast, can be stopped, but it’s really, really difficult. On the other hand, it’s also not nearly as easy to cause a neutrino to cease to exist as it is for a photon.

Here’s my thought experiment involving light in a confined space.

Build a sphere out of a two-mirror-like material - one which reflects light from one side and passes light through the other side. Build the sphere so the reflecting side is on the inside.

Now put it inside a well-lit room. The sphere is now being illuminated and the light is passing through its surface and entering the sphere. And after the light enters the sphere, it crosses through the inside of the sphere and hits the opposite interiro surface, where it is reflected back. So it travels across the inside of the sphere again and hits the opposite side, where it is again reflected.

Now turn off the lights in the room so it’s completely dark. Is it still light inside the sphere? If it is, would you get a flash of light in the dark room if you shattered the sphere with a hammer and let the light “escape”? If it’s dark inside the sphere when the room lights go out, where did the light inside the sphere go and how did it pass through the reflective surface?


An unmovable object doesn’t stop a force by definition. It’s just unmovable.
An unstoppable force just can’t be stopped. Nothing prevents it’s deflection.
What’s the best deflector in theory? An unmovable object. Ricochet!

I’m going to respectfully disagree, at least in part. That is, as a physics riddle, I agree the proposition posits two mutually incompatible things and, as such, isn’t a particularly interesting paradox. As a social cliche, however, it captures something meaningful. It often happens that a (nearly) irrestible force (the formulation I prefer) meets a (nearly) immovable object, e.g., two people who are each used to getting their respectives ways dispute whether to take a particular action. The cliche describes pithily and usefully what’s going on. Now, what happens, of course, is that one or the other eventually wins, thus showing which was in fact less irresistible or immovable, as the case may be.