From henry@spsystems.net Mon Jul 27 10:00:44 1998 Path: news.wwa.com!gail.ripco.com!newsroute.bconnex.ca!feed.nntp.acc.ca!newsfeed.wli.net!su-news-hub1.bbnplanet.com!news.bbnplanet.com!nntp3.crl.com!nnrp1.crl.com!not-for-mail From: henry@spsystems.net (Henry Spencer) Newsgroups: sci.space.science Subject: Re: Why does light travel at the speed it does? Date: Mon, 27 Jul 1998 15:00:44 GMT Organization: SP Systems, Toronto, Canada Lines: 42 Approved: sci-space-tech@isu.isunet.edu Message-ID: References: <6p5is4$s89$1@heliodor.xara.net> NNTP-Posting-Host: crl3.crl.com Xref: news.wwa.com sci.space.science:20523 In article <6p5is4$s89$1@heliodor.xara.net>, David Potter wrote: >Why DOES light travel at constant speed of c? >I mean, what makes it travel this fast? As others have already mentioned, c is determined by the permeability and permittivity of space. But this obviously just moves the question one step further, to why those properties have the values they do. Oversimplifying... Empty space is not really empty. The Uncertainty Principle is usually written in terms of position and momentum, but it can also be written in terms of energy and time. What this means, roughly, is that over very short periods of time, the exact amount of energy present in a system becomes uncertain. In particular, it is possible to "borrow" energy from nowhere, provided you return it quickly. Notably, you can borrow the energy needed to create a particle, provided you destroy it again almost immediately. This process is constantly happening even in empty space; the "vacuum" in fact is a sort of soup of virtual particles, leading very temporary existences. This has measurable consequences for physics and is not just a hypothetical abstraction. (One particularly striking measurable consequence is the Casimir Effect. Put two metal plates very very close together in vacuum. In the space between them, one particular type of virtual particle -- photons with wavelengths longer than the plate spacing -- cannot exist; the soup is a bit thinner between the plates. And the soup outside presses inward, pushing the two plates together. This pressure is real, it has been measured.) The electromagnetic properties of empty space are determined by the density of the virtual-particle soup. In particular, light travels at a finite and fixed speed because it interacts with the soup. And what determines the density of the soup? Well, um, er, it's the fundamental constants of nature that do that... and precisely why those constants are the way they are is an unsolved problem. (There are way too many "fundamental" constants in orthodox quantum theory, and there are strong hints of underlying relationships, but last I heard, the theorists hadn't gotten very far on dealing with the question.) -- Being the last man on the Moon is a | Henry Spencer henry@spsystems.net very dubious honor. -- Gene Cernan | (aka henry@zoo.toronto.edu) From henry@spsystems.net Mon Jul 27 10:09:30 1998 Path: news.wwa.com!gail.ripco.com!newsroute.bconnex.ca!nntp.abs.net!howland.erols.net!nntp3.crl.com!nnrp1.crl.com!not-for-mail From: henry@spsystems.net (Henry Spencer) Newsgroups: sci.space.science Subject: Re: Why does light travel at the speed it does? Date: Mon, 27 Jul 1998 15:09:30 GMT Organization: SP Systems, Toronto, Canada Lines: 31 Approved: sci-space-tech@isu.isunet.edu Message-ID: References: <6p5is4$s89$1@heliodor.xara.net> <1226.508T1939T13164235@lr.net> NNTP-Posting-Host: crl3.crl.com Xref: news.wwa.com sci.space.science:20526 In article <1226.508T1939T13164235@lr.net>, Ash R. J. Wyllie wrote: >>Now it turns out to seem that the speed c is fundamental, and that the >>other parameters are probably derivative. Oh well. > >But are they? There is the Casimir(sp?) effect of two metal plates held near >each other. The speed of light parallel and between the plates is >theoretically (and measured I think) faster than the speed of light in a >vacuum. Uh, no, not correct. The Casimir effect is the inward pressure, caused by the space between the plates being emptier than a normal vacuum. Since it's the interactions with the virtual particles of a normal vacuum that determine the speed of light, there has been *speculation* that the speed of light might be higher between the plates of a Casimir-effect apparatus. As I understand it, the theory says "yes, but only in the direction perpendicular to the surfaces, and only by a really really tiny amount". The predicted effect is far too small to be measurable. (It is still enough to be rather unsettling to physicists, and much effort has been spent trying to find flaws in the prediction, but that's another story.) >Quantum mechanics explains this as the suppression of some virtual >photons, but I suspect the permittivity or permeability are being affected by >the plates. One implies the other: finite permittivity and permeability are the result of the presence of virtual particles even in empty space, and suppressing some of the particles certainly could change the properties. -- Being the last man on the Moon is a | Henry Spencer henry@spsystems.net very dubious honor. -- Gene Cernan | (aka henry@zoo.toronto.edu)