gwyn@Brl-Vld.ARPA (05/11/83)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> It is disappointing to see how many respondents fail to understand the r^ole of measurement unit standards and fundamental constants in physics. Perhaps this indicates something about the way the subject is taught or the effect of lack of a firm epistemological base for the current state of the science. In any case, herein follows a much abbreviated exposition of the nature of the speed of light. Fundamental physical laws are not obtained by "curve-fitting" empirical data, nor are they arbitrarily decreed by a supernatural agency. In the case of relativity theory (mostly the special theory, although insights from generalized field theory are useful for setting an overall framework for discussion), from a few simple principles (none of them a curve fit) one can deduce the existence of a special "fundamental" speed. Comparison with other knowledge such as Maxwell's equations leads one to conclude that light travels with this fundamental speed. The numerical value of "c", the fundamental speed, is not given by the theory. If you measure the actual speed of light in your favorite system of distance and time units, then you can assign that approximate value to c USING YOUR UNITS. According to this approach, it makes no sense to ask what things would be like were c different from what you would measure; it can't be! A more profound understanding of c's relationship to measurement unit standards can be obtained by considering Minkowski's four- dimensional "space-time" hyperspace, or its generalization to a differentiable four-dimensional manifold with metric signature (+ + + -) in general relativity and beyond. The idea is that a local coordinate transformation can reduce the metric tensor to diagonal form under some circumstances (absence of electromagnetic fields, etc.); that is, the generalization of Euclidean distance after this "free-fall" transformation to an "inertial frame" would be: ds^2 = (f dx)^2 + (e dy)^2 + (d dz)^2 - (c dt)^2 , where (x,y,z,t) are local Cartesian 3-space coordinates and the time coordinate measured in arbitrary units. People (not just theoretical physicists) are clever enough to use compatible units of distance for the three 3-space axis directions, so f = e = d by usual convention. In fact, theoreticians usually choose compatible units for time, so f = e = d = c by theoretical physics convention. In this case, "ds" units are normal taken to be compatible also, so f = e = d = c = 1 is the usual theoretical physics choice. In the case f = e = d = 1 which assigns ds units compatible with distance, the speed of light is exactly c. The theoreticians therefore have chosen units such that the speed of light is precisely 1. The confusion seems to arise because most people insist on using incompatible units for distance and time, in which case the numerical value of c will depend on their choice of unit standards. It may be hard to determine an accurate value for c using random distance/time units, since for example "meters" are related to the circumference of the Earth and "seconds" are related to the rotational period of the Earth -- neither of which has any apparent ties to fundamental laws of physics (and therefore, the constant "c"). The reasons distance and time are hard to consider as "the same type of thing" lie in the opposite sign for the time coordinate in the diagonalized metric. Since many physical field laws (including electromagnetic propagation) are tied strongly to the metric tensor, the "speed of light" c (for which read the "quotient of my space/time unit standards") is NOT a freely adjustable parameter in the laws of physics. Such ideas as "the speed of light may change with time and/or distance" are obviously inconsistent with the r^ole that "c" plays in interpreting the metric tensor. An additional note for cosmologists: Since "ds" is an invariant, the choice f = e = d = 1 is not really free either. Natural distance units would be tied to some physical phenomenon (obvious examples are: the "radius of the universe"; the "size of a nucleon"). The most natural generalization of general relativity, that is, the one making the fewest physical assumptions, leads directly to a cosmology with a natural distance unit. Anyone who is really curious about this can drop me a note and I'll send out a copy of my thesis.
KFL@MIT-MC (05/12/83)
From: Keith F. Lynch <KFL @ MIT-MC> From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> ... the generalization of Euclidean distance after this "free-fall" transformation to an "inertial frame" would be: ds^2 = (f dx)^2 + (e dy)^2 + (d dz)^2 - (c dt)^2 , where (x,y,z,t) are local Cartesian 3-space coordinates and the time coordinate measured in arbitrary units. People (not just theoretical physicists) are clever enough to use compatible units of distance for the three 3-space axis directions, so f = e = d by usual convention. In fact, theoreticians usually choose compatible units for time, so f = e = d = c by theoretical physics convention. You are saying that asking "what happens if c is changed" makes as much sense as asking what happens if the ratio between vertical distance and horizontal distance was changed? In this case, "ds" units are normal taken to be compatible also, so f = e = d = c = 1 is the usual theoretical physics choice. In the case f = e = d = 1 which assigns ds units compatible with distance, the speed of light is exactly c. The theoreticians therefore have chosen units such that the speed of light is precisely 1. Ah yes. If you let c (the speed of light), h-bar (Plank's constant), and G (the gravitational constant) all equal 1, you have a perfectly consistent and well defined system of units in which there are natural units of time, distance, mass, etc. One problem with doing it this way (other than the practical problem that the value of G is not known to very many places) is that it tempts people to assign all units a dimensionality of unity, which greatly impairs one's ability to debug physics equations. The reasons distance and time are hard to consider as "the same type of thing" lie in the opposite sign for the time coordinate in the diagonalized metric. Of course you are assuming that general relativity is the last word. General relativity is built on special relativity, and special relativity begins by assuming some of the things it is often used to 'prove'! The only reason not to throw it out is that it manages to predict the results of many experiments with pretty good accuracy. There are almost certainly some very serious bugs in relativity, as was demonstrated in Bell's theorem, which shows that general (and special!) relitivity as it is generally understood is incompatible with quantum mechanics as it is generally understood. The most natural generalization of general relativity, that is, the one making the fewest physical assumptions, leads directly to a cosmology with a natural distance unit. Anyone who is really curious about this can drop me a note and I'll send out a copy of my thesis. Yes, I would like a copy please. ...Keith
gwyn@Brl-Vld.ARPA (05/12/83)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> There was indeed a circular argument in one of Einstein's early developments of special relativity, but the theory can be arrived at using alternative approaches. The general theory is not _________logically based on special relativity but rather includes the latter as a special case. The appeal to agreement with experiment was only sought by Einstein to sell the general theory once he had developed it; of course some physical arguments and probably the world's finest physics intuition went into formulating this theory. I was careful to avoid mentioning quantum theory because of its poor fit to relativity theory. The Bell inequalities do show that classical quantum theory does not mesh well with special relativity, but then that is obvious from general considerations. It is interesting that the recent emergence of gauge theories as fundamental physical principles is much closer to Einstein's work on a unified field theory than physics has been for fifty years. I have long been convinced that Einstein really did know what he was about!
RWK%SCRC-TENEX@MIT-MC@sri-unix.UUCP (08/19/83)
From: Robert W. Kerns <RWK%SCRC-TENEX@MIT-MC>
Date: 19 August 1983 02:06 EDT
From: Keith F. Lynch <KFL @ MIT-MC>
[Submitted to physics by dciem!ntt]
"According to next month's issue of Science 83, the participants in the
international Geneva Conference on Weights and Measures in October will
adopt a new definition of the meter; it will be the distance travelled
by light in 1/299792458 of a second (in a vacuum, I presume)."
So after that date any scientist who thinks he is measuring the speed
of light is actually doing nothing of the sort. He is measuring the
length of the meter!
Isn't it amazing how much of 'reality' is made by such means?
Funny; I had though it was already so defined. Does anybody
know how a second is defined?
-------rsl@SPA-Nimbus@sri-unix.UUCP (08/19/83)
From: Richard Lamson <rsl at SPA-Nimbus>
Date: 19 Aug 1983 0421-EDT
From: Robert W. Kerns <RWK@SCRC-TENEX>
Date: 19 August 1983 02:06 EDT
From: Keith F. Lynch <KFL @ MIT-MC>
[Submitted to physics by dciem!ntt]
"According to next month's issue of Science 83, the participants in the
international Geneva Conference on Weights and Measures in October will
adopt a new definition of the meter; it will be the distance travelled
by light in 1/299792458 of a second (in a vacuum, I presume)."
So after that date any scientist who thinks he is measuring the speed
of light is actually doing nothing of the sort. He is measuring the
length of the meter!
Isn't it amazing how much of 'reality' is made by such means?
Funny; I had though it was already so defined. Does anybody
know how a second is defined?
It's so many ticks of a cesium clock, I think. It's funny, I had read
the same story about defining the length of the meter and never figured
out that it meant that it means there is no independent measure of the
speed of light. Anybody have any idea how one would operationally
measure the length of the meter given the new definition?abc@brl-bmd@sri-unix.UUCP (08/21/83)
From: Brint Cooper (CTAB) <abc@brl-bmd> Actually, a more intriguing question than how to measure the length of a meter independently is how to "measure" the speed of light (or of anything else). Philosophically, speed is a somewhat more abstract entity than distance. Consider the endless debate over whether instantaneous frequency (rate of change of phase angle of a sinusoidal waveform) actually exists! Still intrigued, Brint
stanwyck@ihuxr.UUCP (08/22/83)
This speed of light issue of is interest to me, as some of the creation-science (THEIR words) people are suggesting that a physicist in Australia has determined that the speed of light is slowing fairly rapidly, thus suggesting that radio-active decay is related to the speed of light (which used to be faster) so argon and related dating systems are invalid because they assume present light speeds..... etc. Does anyone know anything about who this Aussie scientist is? or where he/she is published? or if decay is related to speed-of-light? Or any other factual matters related to such? ( I don't need flames for or against the creation-science people, just trying to verify or refute a statement so made.) advTHANXance. don stanwyck : ..!ihnp4!ihuxr!stanwyck : 312-979-6667 : btl @ naperville, il
rh@mit-eddie.UUCP (Randy Haskins) (08/22/83)
No, it is still possible to measure the speed of light. It's just that the measurments now have to be done in feet. They will then be converted to meters. Remeber, the speed of light is independent of the measuring system. :-* *= fill in the missing character. -- Randwulf (Randy Haskins); Path= genrad!mit-eddie!rh or... rh@mit-ee (via mit-mc)
kwmc@hou5d.UUCP (K. W. M. Cochran) (08/22/83)
My understanding is that (Einsteinian ?) physics relates the speed of
light to the size of the universe, although what the relationship is
I can't remember. Does anyone know? Thus if the universe is expanding,
the value of c may be changing. Yes ? No ?
Ken Cochran hou5d!kwmcntt@dciem.UUCP (Mark Brader) (08/24/83)
(Earlier net articles, one from me, pointed out that the meter is now going
to be defined in terms of the speed of light, that the speed of light will
*by definition* be 299792458 m/s, and that therefore anyone who thinks they
are measuring the speed of light will actually be measuring the meter.)
No, it is still possible to measure the speed of light. It's just
that the measurments now have to be done in feet. They will then
be converted to meters. Remeber, the speed of light is independent
of the measuring system.
Randwulf
Well, in THIS country at least, the foot is defined as 0.3048 m exactly. This
This makes the speed of light in a vacuum in Canada exactly 299792458/1609.344
(or about 186282.397051221-) miles per second.
I'm not sure of the current definition but there used to be 3 kinds of foot
in the USA; one was 0.3048 m, another was 1200/3937 m, the third in between.
Mark Brader, NTT Systems Inc., Toronto, Canadagwyn@brl-vld@sri-unix.UUCP (08/24/83)
From: Doug Gwyn (VLD/VMB) <gwyn@brl-vld> I wonder what the operational significance of "the speed of light is slowing" would be. After all, the fundamental velocity (which happens to be the speed of light in vacuuo) can be used to relate standards of distance and time. Surely the claim is not being made that the second changes as a function of time??
gwyn@brl-vld@sri-unix.UUCP (08/24/83)
From: Doug Gwyn (VLD/VMB) <gwyn@brl-vld> There is no relationship between the speed of light and the size of the universe. Perhaps you're thinking of the conventional explanation of the Hubble effect (red shift vs. distance). Seems to me we went over this topic a few months ago. The speed of light in the units I normally work in is precisely 1. This is not a dirty trick but is permissible since the velocity in question is more fundamental than any time or distance standards, insofar as relativity goes.
KFL@MIT-MC@sri-unix.UUCP (08/25/83)
From: Keith F. Lynch <KFL @ MIT-MC>
[Submitted to physics by dciem!ntt]
"According to next month's issue of Science 83, the participants in the
international Geneva Conference on Weights and Measures in October will
adopt a new definition of the meter; it will be the distance travelled
by light in 1/299792458 of a second (in a vacuum, I presume)."
So after that date any scientist who thinks he is measuring the speed
of light is actually doing nothing of the sort. He is measuring the
length of the meter!
Isn't it amazing how much of 'reality' is made by such means?speaker@umcp-cs.UUCP (08/31/83)
Perhaps I'm dim, but I don't really see the problem with
measuring the speed of light.
First fix an arbitrary length to be your unit of measure and call
it the meter. Then measure the distance traveled by a beam of
light in C ticks of the cesium clock. You will always have the
standard meter and the cesium atom to refer to later... in the
same way that you use light beams to gauge the value of c, the
speed of light.
Both C and the meter are arbitrary, but always constant when used
wrt the value of c.
- Speaker
--
Mundane-Name: John T. Nelson
Full-Name: Speaker-To-Animals
UUCP: {seismo,allegra,brl-bmd}!umcp-cs!speaker
CSNet: speaker@umcp-cs
ARPA: speaker.umcp-cs@UDel-RelayKFL@MIT-MC@sri-unix.UUCP (09/07/83)
From: Keith F. Lynch <KFL @ MIT-MC>
Date: 29 Aug 83 15:25:58-PDT (Mon)
From: ihnp4!ihuxm!gjphw @ Ucb-Vax
My question: does Pauli's large number hypothesis allow for a
change in the speed of light in a vacuum with a change in the radius of
the physical universe?
Since the speed of light is now 299,792,458 meters per second BY
DEFINITION, the trivial answer is NO. What MAY change is the length of
the meter or the duration of a second. What does it mean for all the
meters to get shorter or for seconds to get shorter? This would mean
that all chairs, tables, molecules, terminals, people, and everything
else would get bigger (not that anyone would notice the difference) OR
that all clocks, atoms, modems, people, and everything else would
become slower (not that anyone would notice the difference). We could
'explain' an 'increase in the speed of light' in any of those three
ways with equal accuracy.
...Keithcmsj@ihuxm.UUCP (11/04/83)
In an effort to shed some facts on the continuing debate over whether the speed of light has changed through time, I offer the following information (gleaned from my 1st year college Physics text - Resnick and Halliday). Date Experimenter Method Speed Uncertainty (* 10**8 m/sec) (* 10**5 m/sec) 1600(?) Galileo Lanterns "If not instantaneous, it is extraordinarily rapid" 1675 Roemer Astronomical 2 unknown 1729 Bradley Astronomical 3.04 unknown 1849 Fizeau Toothed wheel 3.133 unknown 1862 Foucault Rotating mirror 2.98 5 1876 Cornu Toothed wheel 2.9999 2 1880 Michelson Rotating mirror 2.9991 .5 1883 Newcomb " " 2.9986 .3 1883 Michelson " " 2.99853 .6 1906 Rosa & Dorsey E&M Theory 2.99781 .1 1923 Mercier Standing waves 2.99782 .15 1926 Michelson Rotating mirror 2.99796 .04 1928 Karolus et al Kerr Cell 2.99778 .10 1932 Michelson et al Rotating mirror 2.99774 .11 1940 Huettel Kerr Cell 2.99768 .10 1941 Anderson Kerr Cell 2.99776 .14 1950 Essen Microwaves 2.997925 .03 1950 Bergstrand Geodimeter 2.997927 .0025 1950 Houston Vibrating Crystal 2.99775 .09 1950 Bol et al Microwaves 2.997893 .004 1951 Aslakson Shoran radar 2.998942 .019 1952 Rank et al Molecular Spectra 2.99776 .07 1952 Froome Microwave Interferometer 2.997926 .007 1954 Florman Radio Interferometer 2.997951 .031 1954 Rank et al Molecular Spectra 2.997898 .030 1956 Edge Geodimeter 2.997929 .002 ---------------------------------------------------------------- I assume full responsibility for any typographical errors introduced during my transcription from the aforementioned text. I would appreciate hearing from anyone who finds any errors in the table. And now, one editorial remark, if I may: Is it the speed of light that changes with time or is it our capability of measuring the speed of light that changes with time. I personally suspect that the latter is true. Chris Jachcinski *!ihnp4!ihuxm!cmsj
gwyn%brl-vld@sri-unix.UUCP (11/17/83)
From: Doug Gwyn (VLD/VMB) <gwyn@brl-vld> I took the couple dozen independent determinations of the speed of light posted to this list not long ago, and computed their weighted average: 2.9979291 _+ 0.0000014 Note that this is more accurate than the most accurate determination listed. Upon checking this "best estimate" for c against the individual experiments, I find that there is more scatter than one would expect on the assumptions: (1) The "true" speed of light is the same in each experiment. (2) Each measurement is independent of the others. (3) The standard deviations properly represent random error. However, there is no evidence of a monotone change in c with time.
UNIX%Ames-VMSB@sri-unix.UUCP (04/18/84)
EM waves - speed of light Matter < speed of light Gravity - speed of light (elementary General Relativity) Quantum Spin Correlation - no limit (true for other QM correlations too) ------