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eimos
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Can anyone explain the casimir effect to me? I've only just got back to reading parallel worlds and it's mentioned quite alot, yet i have no idea what it is. Though i think it may have something to do with vacuum energy.
eimos said:Can anyone explain the casimir effect to me? I've only just got back to reading parallel worlds and it's mentioned quite alot, yet i have no idea what it is. Though i think it may have something to do with vacuum energy.
Bob S said:Here is an example of the Casmir effect ...
map19 said:the vacuum of space is most assuredly not "full of radiation"
I disagree.
Vanadium 50 said:You don't get to disagree. This is not a matter of opinion.
EM radiation is a very specific thing: far-field solutions to Maxwell's equations. Space is most assuredly not filled with this, and if you shield against it, you still have Casimir forces.
One of my neighbors is a PhD physicist who was a naval commander before retirement. He says there definitely is an attractive "force" between two ships when they are very close, and it is attributed to the relatively calmer wave structure between them.Vanadium 50 said:From "Interstellar Technologies"? Are you sure that's a reputable site?
Bob S said:One of my neighbors is a PhD physicist who was a naval commander before retirement. He says there definitely is an attractive "force" between two ships when they are very close, and it is attributed to the relatively calmer wave structure between them.
Vanadium 50 said:From "Interstellar Technologies"? Are you sure that's a reputable site?
Born2bwire said:Exactly, we could always enclose a volume of vacuum with reflectors on the exterior surface and absorbers on the interior, ensuring that radiation does not flow into the volume and any existing radiation inside will be dissipated...
Who cares? It's a thought experiment in regards to map19's statements. There is a background level radiation in the universe but it is not assumed to exist in the calculation of Casimir effects. The vacuum fluctuations that can be used as a means of calculating the Casimir effects are not directly observable. There are no photons and the expectation of the field values are zero. These are different from the background radiation.conway said:It's not so easy. Any "aborber" must absorb a full quantum of energy; otherwise it will just re-radiate it. If the total amount of energy at a certain frequency is less than one quantum, how do you "absorb" it?
Born2bwire said:Who cares? It's a thought experiment in regards to map19's statements...
map19 said:the vacuum of space is full of radiation
conway said:I'm not sure you can experimentally disprove the claim that the vacuum is full of classical e-m energy.
Vanadium 50 said:Note that your statement is different than map19's.
(My neighbor is the only person I know who can write out the proof of the Brachistochrone problem w/o referring to his book collection. He is very sharp.) This effect, whether due to true Casmir forces or not, is taught to graduates of the U.S. Naval Academy in Annapolis. This has nothing to do with the trustworthyness of Interstallar Technologies.Vanadium 50 said:And the fact your neighbor used to be in the Navy makes "Interstellar Technologies" reputable exactly why?
conway said:I actually knew that.
But more to the point, in re-reading the thread, I see that I was responding equally well to your point about space being "most assuredly not" full of radiation; Bob S merely gave a better, more operational version of the same claim.
Vanadium 50 said:Note that your statement is different than map19's.
Just out of curiosity, how many of the people here who are attempting to explain the Casimir effect have calculated it themselves, and how many have read about it in popularizations?
conway said:I don't know what you mean when you say "who cares?". You suggested a thought experiment and I offered the opinion that your thought experiment wouldn't work. Do you think I'm wrong or are you just saying you don't care if it works or not?
I would say that I care at least to the extent of being interested in whether or not I raised a valid rebuttal to your thought experiment.
Born2bwire said:There are theoretically perfect absorbing boundaries that have been derived for EM waves, like the perfectly matched layer...
Bob S said:From Bob S
One of my neighbors is a PhD physicist who was a naval commander before retirement. He says there definitely is an attractive "force" between two ships when they are very close...
conway said:Yes, Z_0 =377. You can make one by rubbing a pencil on a piece of paper. But this is a classically valid theoretical model based on the unbroken continuity of matter. I don't believe you can construct such a perfect boundary using real atoms.
map19 said:Thank you for that. This is analogous to the cutoff frequency in a waveguide. As I said before, wavelengths have to be at least a half wave inside the boundaries to exist. Other wavelengths are quickly damped. For a waveguide the cutoff is transverse distance a, the cutoff wavelength is 2a.
So waves outside the two shiips hulls would have long wavelengths and waves inside would only be allowed to be short. The longer waves would dissipate their surface energy against the outer sides of the ships, pushing them together.
Large scale Casimir.
Calculating Casimir force is not hard, it's a mathematical exercise. Following it up with an experiment that gets the same result, that's what a physicist would do.
My belief that EM from the quantum vacuum drives Casimir is still with me. External EM would no doubt be a boost to the process.
Born2bwire said:The impedance of free space is not an absorber. Defining a perfect absorbing boundary is more subtle than that, you have to have a lossy medium that is reflectionless at all angles of incidence but this has been defined. It doesn't matter whether or not we can actually make this, it's a thought experiment.
Bob S said:From Bob S
One of my neighbors is a PhD physicist who was a naval commander before retirement. He says there definitely is an attractive "force" between two ships when they are very close, and it is attributed to the relatively calmer wave structure between them.
Originally Posted by Vanadium 50
From "Interstellar Technologies"? Are you sure that's a reputable site?
(My neighbor is the only person I know who can write out the proof of the Brachistochrone problem w/o referring to his book collection. He is very sharp.) This effect, whether due to true Casmir forces or not, is taught to graduates of the U.S. Naval Academy in Annapolis. This has nothing to do with the trustworthyness of Interstallar Technologies.
Here is another version from Chemistry Daily
http://www.chemistrydaily.com/chemistry/Casimir_effect:
An effect analogous to the Casimir effect was observed by 18th century French sailors. Where two ships are rocking from side to side in conditions with a strong swell but light wind, and the ships come closer together than roughly 40 m, destructive interference eliminates the swell between the ships. The calm sea between the ships has a lower energy density than the swell to either side of the ships, creating a pressure that can push the ships closer together. If they get too close together, the ships' rigging can become entangled. As a countermeasure, a handbook from the early 1800s recommends that each ship should send out a boat rowed by 10 to 20 sailors to physically pull the ships apart.
Here is another from http://scienceweek.com/2004/sa041119-6.htm (see ref 5.)
3) Although the Casimir effect is deeply rooted in the quantum theory of electrodynamics, there are analogous effects in classical physics. A striking example was discussed in 1836, in P. C. Caussee's L'Album du Marin (The Album of the Mariner)(5). Caussee reported a mysteriously strong attractive force that can arise between two ships floating side by side -- a force that can lead to disastrous consequences. A physical explanation for this force was offered only recently by Boersma (1996), who suggested that it originates in the radiation pressure of water waves acting differently on the opposite sides of the ships. His argument goes as follows: the spectrum of possible wave modes around the two ships forms a continuum (any arbitrary wave-vector is allowed); but between the vessels their opposing sides impose boundary conditions on the wave modes, restricting the allowed values of the component of the wave-vector that is normal to the ships' surfaces. This discreteness created in the spectrum of wave modes results in a local redistribution of modes in the region between the ships, with the consequence that there is a smaller radiation pressure between the ships than outside them.
References (abridged):
1. Casimir, H. B. G. Proc. Kon. Ned. Akad. 51, 793-795 (1948).
2. Bordag, M., Mohideen, U. & Mostepanenko, V. M. Phys. Rep. 353, 1-205 (2001).
3. Kenneth, O., Klich, I., Mann, A. & Rezen, M. Phys. Rev. Lett. 89, 033001 (2002).
4. Boyer, T. H. Phys. Rev. A 9, 2078-2084 (1974).
5. Caussee, P. C. L'Album du Marin (Mantes, Charpentier, 1836).
map19 said:perhaps you could email instead of posting. mphilo@xtra.co.nz
I would also be interested if anybody knows the correlation between calc numbers and experimental numbers. I expect only about an 80% correlation if em has not been accounted for.
nrqed. your post about the ships is what I said, in different words.
I see an analogy between the ships, frequencies in a waveguide, and the Casimir effect.
Of course the analogy is not exact, when are they ever ? we're talking water, rf, and quantum stuff. But it is similar.
Born2bwire. I think that external em would add to the Casimir force. Why would it not ?
If it's there and given the cutoff inside the plates, there is going to be an imbalance with many more frequencies external than internal, resulting in more momentum transfer outside than inside.
Bob S said:As reported in the professional physics literature in 1995, an entirely classical, that is, non-quantum, treatment of the Casimir force is all is needed to explain the mysterious attractive force
Halcyon-on said:Bob S: can you give me the exact reference of the paper you pointed out.
I'm very sorry if what I'm going to say cannot be used in fiction movies or starships stories, but it turns out that that the Casimir effect is one of the biggest and most misleading myths of quantum mechanics. The fact is that the "Casimir’s original goal was to compute the van der Waal’s force between polarizable molecules at separations so large that relativistic (retardation) effects are essential. [...] These results were derived using the
standard apparatus of perturbation theory (to fourth or-
der in e) without any reference to the vacuum."
ref{ Title:"The Casimir Effect and the Quantum Vacuum"
arXiv:hep-th/0503158v1
Journal:Phys.Rev. D72 (2005) 021301
Abs: In discussions of the cosmological constant, the Casimir effect is often invoked as decisive evidence that the zero point energies of quantum fields are "real''. On the contrary, Casimir effects can be formulated and Casimir forces can be computed without reference to zero point energies. They are relativistic, quantum forces between charges and currents. The Casimir force (per unit area) between parallel plates vanishes as \alpha, the fine structure constant, goes to zero, and the standard result, which appears to be independent of \alpha, corresponds to the \alpha\to\infty limit.}
Born2bwire said:That is Jaffe's paper is it not? It is a good read.