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Does the ghostly Casimir effect really cause ships to attract each other?
Some stories are just too good to be true. And according to physicist Fabrizio Pinto, that's exactly the case for an analogy routinely used by physicists to illustrate the mysterious Casimir effect.
Read the story here.
Posted by Nicola Jones on May 04, 2006
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Actually, he's wrong, the old tales are right. Anyone with a pool can easily demonstrate that items left floating in the pool long enough will end up stuck together which is against the odds of thermodynamics.
Posted by: MrX_TLO | May 5, 2006 04:23 PM
I have no clue about the Casimir effect, but there clearly is one minor flaw in the article. It says the flat sea picture shows a small rowing boat pulling the two ships moored too close together apart. Said small rowing boat is BETWEEN the two ships and not in contact with either. It might be fishing, or drifting, or lugging stuff between the two ships. What it is not doing unless it has a line tied around on off-stage pully, is pulling them apart. It'd have to be outside the two ships on one side or the other to do that. As we (almost) all discover in our youth, pushing on strings does not work.
Posted by: vtcodger | May 5, 2006 04:57 PM
Historical myths are just the ones that are safe to talk about. The overwhelming majority of myths are in the textbooks as accepted theories. Most frequently they start as speculations by founders of the field. Over time, they become standard lore, and it takes overwhelming evidence -- way more than exists for the myth -- for any alternative to be considered viable.
An excellent example is the origin of comets, for which there never was any evidence of any kind. Every detail being discovered today either fails to support or actively contradicts it. Still, NASA press releases still trot out the "remnants of solar system formation" line.
Posted by: Nathan Myers | May 5, 2006 07:43 PM
you need to consider the Bernoulli effect. Bernoulli is known for explaining the principles of lift with airfoils. The case also applies here. When there is less pressure between two bodies in water than the surrounding area, the two bodies will eventually come together.
Posted by: peter | May 5, 2006 11:02 PM
Moored ships in tidal waters, even in a flat sea, are subject to "venturi effect". Water flowing between 2 objects accelerates in the narrowed space, causing a lowered pressure, which would, in this case, tend to draw two ships together.
This effect can become greatly magnified as the speed of the water between them increases, as when ships are underway on parallel courses, as they are when fueling, or transfering goods underway.
Posted by: Remember Mr. Venturi? | May 6, 2006 01:13 AM
I think your writer meant "quash" rather than "squash". Unfortunately, a rather common error nowadays but your editor should have caught it.
Posted by: E.B.Mullen | May 6, 2006 03:00 AM
Two ships moored nearby each other will, due to waves or currents or the lack thereof, either remain the same distance apart, increase their distance aprart, or move closer together.
Nobody much cares about the first two as they pose no danger to the ships. Notice will only be taken in the third instance.
And when notice is taken, explanations will be sought.
But is there really anything that needs explaining?
.
Posted by: Ick of the East | May 7, 2006 07:00 AM
MrX_TLO, your conclusion about the items in a pool is correct, but I don't see how it can be attributed to the Casimir effect or how it is against thermodynamics.
Assume that objects are floating completely randomly in your pool. It doesn't happen randomly, but assume that it does. Also assume that there is no force interacting between the objects. They will randomly strike each other on occasion. When they strike, water will rise between the two objects by capillary action. In order to separate, they must now overcome the surface tension between them. In other words, even though there was no force initially (Casimir or otherwise) once they've struck, there is now a force holding them together. If we remove the assumption of randomness, and allow the objects to move with the currents in the pool or with the wind, etc. the chance of collision increases. Hence, items in your pool are stuck together by surface tension and not by Casimir.
Posted by: Chad Welch | May 7, 2006 05:23 PM
Another possibility is that one ship may lie in the wind shadow of another. If the windward ship steals the wind out of the sails of the leeward then all else being equal they could move together. The windward ship could also steal wave energy from the leeward, effectively pushing their hulls together.
But as for the situation with floating objects on calm water in a closed pool, here the average reflected wave energy coming from every direction can be assumed to have become equal but there is presumably a similar mutual shadowing effect on pairs of objects causing them to migrate together over time. If so, then the average wave energy in the gaps would be smaller and it seems to me this would be broadly analogous to Casimir. (Note: It is also analogous to the didactic 'particle shielding' theory of inverse-square gravity discussed by (among others) Feynman.
Posted by: Martin Shough | May 9, 2006 02:00 PM
The ships are free floating, not moored.I was told that the effect was also reported in the world litterature: Herman Melville's "Moby Dick" and Philip Roth's "Rites of passage". It is not a myth, the original paper Am.J.Phys. 64. 539-541 (1996) gives the quantitative theory to calculate the attractive force, given Ships rolling amplitude, weight, metacentric height, "Q" oscillator quality factor and wave period. An example for two 700 ton clipper ships gives 2000 Newton, quite reasonable. The theory gives also another effect: Repulsion. An atom is attracted to a conducting plate but a ship in a wave field is repelled from a steep cliff. This is due to a difference in boundary condictions between Electromagnetic waves and Seawaves. This repulsion was already known to the Cape Horn sailors of the Cape Horn Society, Hoorn Holland. Caussé's error: Caussé put his ships in a "Flat Calm" without any waves. That won't work. However, already a small swell suffices if its period matches the natural period of the ships and we have resonance magnification. A long light swell can easily have been overlooked by the mariners on board. The second possibility is that Caussé should have put his ships In "Calm with Big Swell" which after all to me seems less likely.
S.L.Boersma Delft The Netherlands
Posted by: S.L.Boersma | May 11, 2006 02:25 PM
I have a few critics to the article Am.J.Phys.64, 539-541 (1996). First, the author (S.L Boersma) presents things as if Caussé had reported on this force in the situation of a big swell but no wind. I have read that this might not the case. (I didn't have access to Caussé's book.) It is true that re-reading carefully the article, he avoids stating that (which is not how you first perceive the text). Second, on the physical side: Boersma says that the waves between the ships cancel each other (destructive interferences), hence that the radiation pressure is essentially zero on the inner side of the ships (if I understood correctly). But waves travelling against each other will never cancel for all times: they will produce a stationary wave. I guess the radiation pressure of such a wave is essentially zero, so that might be ok. However, the fact that the two ships roll simultaneously is irrelevant for that.
Posted by: Armando Sano | July 19, 2006 02:49 AM