# Physics experiment: what does it demonstrate

I saw a show on quantum mechanics on the science channel (which sucked, btw) where they showed a demonstration of some principle which I didn’t understand. It invovled a static cylinder and a rotating cylinder attached to a crank handle. They were arranged concentrically with the rotating one inside the other, IIRC. The space between them was filled with glycerin and a line of ink was injected into it. The crank was turned three times and the ink appeared to dissolve. However, when the crank was turned three times in the opposite direction, the line of ink rematerialized. What does this demonstrate and how does it work?

Thanks for your help,
Rob Dennett

Maybe the cylinders were either electrically charged or magnetic, and it was an experiment in magnetic polarization?

My guess is that they were trying to show that time can be bidirectional on the macro scale, in certain situations. See Feynman diagrams for examples of how time can be interpreted in various ways on the particle level.

I suspect it was to show that even apparently disordered things (the ink after cranking) there may be information if one knew how to extract (uncrank) it. This may relate to the argument about whether quantum events are truly random or whether there is some underlying rule or law.
Without seeing the prog it is hard to tell what that (admittedly pretty - Ive seen it done live) demonstration has to do with quantum physics

I can’t say for sure what they were trying to demonstrate about quantum physics…if you wanted to know what it demonstrates in fluid mechanics, I’d have a better idea.

As for “how does it work,” I can tackle that for you.

The cylinders, as you describe, are made to rotate relative to one another (doesn’t matter which one is doing the moving, it’s all about their relative motion). For this example, lets assume the outer cylinder reamins stationary, and the inner cyliner is rotating. The glycerin filling the gap wants to stay attached to the surfaces of both cylinders, and it does. The fluid touching the outer cylinder stays with that surface, and remains still. The glycerin in contact with the inner cylinder says in contact with that wall, and rotates along with it. But the fluid in the middle has to help make the transition, and so moves at various speeds through its thickness. As you move through the glycerin from the outer, stationary wall, the fluid will be rotating faster and faster until it’s going as fast as the inner wall when you get there. This phenomenon, and the stress it creates in the fluid, is called “shear”.

Shear is intimately tied to viscosity. In fact, viscosity is a fluid’s inherent resistance to shear. If the fluid in the gap is air, the resistance to turning the inner drum is almost nothing. If water, it’s somewhat higher. And if it’s glycerin, it will be quite noticeable, because glycerin is a very viscous liquid.

But that high viscosity makes glycerin just the stuff to use for this experiment. Because of the glycerin’s high viscosity, the dimensionless quantity called “Reynolds number” that would apply to this experiment would be very, very low. All other things being equal, the Reynolds number for the same experiment using water would be 1200 times higher. Why is this important? Because the Reynolds number determines how the fluid behaves as is moves past itself. If it’s really low, there will be laminar flow, where the the different “layers” of fluid move over each other smoothly, resulting in the velocity distribution I described above. If the Reynolds number gets fairly high, the fluid motion will be turbulent, and things will get mixed up. Irreversibly mixed up…you can’t undo turbulence.

So turning the cylinder slowly with glycerin in the gap makes for a very low Reynolds number, maybe something like 50. This is called creeping flow, and is definitely laminar. As the cylinder turns, the ink line gets smeared out in this shear region, and it gets smeared out uniformly and predicably. It actually stays a continuous sheet of ink wrapped around the inner cylinder, but eventually becomes so thin as to be invisible. Because it stretches slowly and smoothly, reversing the cylinder can reverse the shear, and it all goes back into place.

Now if everything else stays the same, except the glycerin is replaced with water, the Reynolds number in the experiment may be something like 60000. That may likely induce turbulent flow as the water tries to slide past itself, but trips in the process. The ink line will get swirled and mixed into oblivion, and no amount of reversing can undo it.

Another key factor that makes this demonstration work is that glycerin has a very low mass diffusivity coefficient compared to something like water. Put a line of ink in motionless glycerin, and it will stay in tact for a long time. Put a line of ink in very still water, and it will diffuse to become evenly distributed in no time. With glycerin, you have more than a few millieconds to run the experiment, and you can go nice and slow to avoid turbulent mixing.

That was an outstanding explanation, aerodave.

Just to second his point, the demo has nothing to do with quantum mechanics. It’s a demonstration of laminar flow.

I’ve actually performed this demo in class back when I was teaching physics. (Someone before my time had put together the cylinder apparatus, and it still had glycerine in it.)

It’s actually a pretty common demo. A simple Google search using the keywords “glycerin laminar flow demo” came up with this link for the second result.

Fantastic explanation.

I just came in to add a link to a video .

“Chaos” by James Gliek explains the rationale for this experiment.

The experiment demonstrates that the motion of the ink in the viscous liquid is still, within limits, reversible.

One of the niftiest uses of this I saw was in an article in Scientific American several years ago for Photon Echoes. This is a weird optical behavior in which a collection of atioms (or molecules, or other material that can come in bulk and has light-absorbing and emitting energy levels) is excited by a pulse of light, allowed to evolve through time, then shocked with another pulse. After a time equal to the times between the first two pulses, most if not all of those atoms will collectively emit another big pulse of light.

the explanation is complex, and involves the Bloch vector of the overall system (you have to use a specific timing for that intermediate pulse), but the action of the Bloch vector is very much like that system of cylinders with viscous liquid and ink in it. The initial pulse is like the initial ink spot. Letting the system evolve is like rotating the cylinder and smearing the ink. The “pi pulse” of light is the act of reversing the direction you turn the cylinder, and the ink spot(s) lining up again is like the resulting “echo”.

Hrm. Could the pulse of light be structured to contain information? A message? How long can the process be extended?

Would it be possible to pulse a message into the medium…wait a loooooong time…and then apply the pi pulse…so the original message would appear later (after an equal interval of time) but be unrecoverable until then? Might this have implications for time-lock safes and the like? Or some kind of cipher application?

Sailboat

Big topic. I just looked for a cite on the internet, and couldn’t find a brief answer. I couldn’t even find the Scientific American article on them. You can google around on your own. Find the Sci Am article if you can. There’s a brief, but (especially if coherent optical effects are new to you) unstaisfying treatment in Amnon Yariv’s Quantum Electronics.

aerodave’s explanation is nice and, I think, accurate.

This brings to mind an oddity about small life forms. If a creature lives in a small environment, where Reynolds numbers are small because the characteristic dimension is small, they can’t locomote with a single reversing degree of freedom in their anatomy. Larger creatures can, like scallops or clams. They open slowly and water moves between their shell halves, then they close quickly and the water jets out and shoots them hingeward. But Reynolds numbers have to be large for behaviors to be unidirectional in time. This puts a lower limit of 2 degrees of freedom on the mobile anatomy of tiny creatures.