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

[Colibri]

@PBear42

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.

Of course I meant “no basis in reality” to mean physical reality (which you agree is impossible). The OP was asking about a scientific basis for the phrase, not a figurative meaining.

I actually agree with what you say about the rest of it; in fact, I thought to post something of the kind myself. The phrase is a bit of hyperbole used to describe an apparent impasse. It gives tension to a confrontation by asking you to guess which one of the two will fail to live up to its description.

[Q.E.D]

The pressure of the unstoppable force acting on the immoveable object generates infinite heat and the immoveable object vaporizes. Well, it’s just immovable; no one said anything about its thermal properties.

[Chronos]

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

I would be much more comfortable with a material that is completely frictionless, or one that’s completely unbreakable, or one that makes me completely irresistable to women. All of those are, at least in principle, consistent with the laws of physics. But a material which passes light in only one direction… That’s somewhat more difficult.

[Excalibre]

@PBear42

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.

Sure, but it’s ridiculous to try to discuss it as though it were a meaningful physics question. I’m amazed that people would even bring it up as a topic of conversation.

[Schnitte]

Cal Meacham:

According to Wikipedia, the dog’s name was indeed Laelaps, and the beast it was destined to catch was the Teumassian fox, sent by some deity as a punsihment to ravage the country.

I tried to find out if it’s an ancient myth or a rather recent one; I didn’t find it in a book with a collection of re-told Greek myths, but after some searching on the web I found a variety of different versions of the story - different in the names of the hunter employing the dog and of the god who had sent the fox to Earth as well as in the way the hunter had gotten possession of the dog. Soem versions also seem to include a javelin that could never miss its target.

According to one German site on the web, Ovid’s Metamorphoses contain a narration of this legend (seventh book, verses 763-793). Wikibooks offers the original Latin text of that passage, and while my Latin has become too rusty to understand the meaning of the text upon reading it, the name Laelaps appears in it (in the form Laelapa), and the first few lines seem to tell the story of a beast sent to a country and killing many sheep.

[Mangetout]

@Chronos

I would be much more comfortable with a material that is completely frictionless, or one that’s completely unbreakable, or one that makes me completely irresistable to women. All of those are, at least in principle, consistent with the laws of physics. But a material which passes light in only one direction… That’s somewhat more difficult.

Agreed; I think it would actually violate the second law of thermodynamics, because such a device would decrease local entropy without doing any work.

Maxwell's demon

[Puzzler]

@Chronos

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.

Actually, yes, it can.
Or, at the very least, to slow it down. Real slow.

[CalMeacham]

But a material which passes light in only one direction… That’s somewhat more difficult.

There certainly are devices which transmit light only in one direction. Such “Optical Diodes” (or Optical Isolators) are used, for instance, in Laser Fusion systems, where you don’t want reflected pulses going backwards through the gain medium, draining off gain and probably damaging the medium. They consist of a Faraday Rotator (which rotates the plane of polarization in different directions for beams propagating in different directions, inj distinction to your items with optical rotary power, which rotate it the same way) in conjunction with two or more polarizers.

Devices using Faraday Rotators are the only things I know of which can be used in such isolators, since just about everything else treats light passing in either direction the same way.
As for photons not being created or destroyerd, the answers above confuse me. Photons are being created and absorbed all the time. The energy isn’t destroyed, of course, but the photon loses its identity as the energy it contained goes into exciting electrons to higher energy levels in atoms, or to exciting multiple phonons in the structure of the absorber, or some such. It’s not like the case of electrons, where you can view an electron-positron annhilation as an electron reversing its flow through time. Photons don’t annhilate each other, and have no anti-particle.

[flight]

@Little Nemo

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?

I use to rack my brain over this one in elementary school. If you could create such perfect reflectors, then yes, it could work, but in reality there is no such thing. Reflection is never 100% efficient. Thus, every time the light bounces off part of your reflector it loses some photons. Light moves fast enough that all of the energy effectively immediately dissipates.

[CalMeacham]

We’ve answered that one about the light bouncing around the sphere before on this board. Even if you make the sphere as perfectly reflecting as you can, the number of bounces it’ll make is going to be finite – about rho/(1-rho) bounces, where rho is the reflected fraction of the light. So even if your sphere is 0.99999 refldecting, you only get about 100,000 bounces before it’s all absorbed. Let’s make your box or sphere a foot across, because it’s a convenient size, and because light travels about a foot in a nanosecond. With 99.999% reflectivity (which is a lot better than we can achieve for all angles), you’ll get light bouncing around for only 10^5/10^9 = 0.1 microsecond. Better swing that hammer fast if you want to let the light out.

[astro]

Gravy.

[sleestak]

I actually posed this question to a woman who was, at the time, one of the best impact physicist in the world*. A very smart lady. She did impact physics for a living, worked on designs for weapons and such at one of the U.S. national labratories.

Her answer was ‘Infinite impact’.

When I asked her the question she answered immediately. Regretably at the time I was not able to follow up as we were at a party and the conversation turned to other things. I am not sure if that was her quick pat answer or if she was serious. My thought is that she was serious knowing that the question is silly.

Slee

*This lady was a friend of my parents, they worked at the same national labratory. My Dad, a 4.0 PHD in math and one very smart guy, is not one for compliments. My Dad totally caught me off guard one night after a party at this ladies house when I was about 14. He took me aside and said “Remember her. It is not often that you meet someone who is the absolute best at what they do. She is the *best * in the world at what she does. You might, in your life, meet one or two other people who are the absolute best at what they do.” This was the only time he ever said anything like that to me about anyone. So, coming from him, this was extremely high praise.