KFL@MX.LCS.MIT.EDU (11/07/86)
From: "Keith F. Lynch" <KFL@MX.LCS.MIT.EDU>
From: "James J. Lippard" <Lippard@HIS-PHOENIX-MULTICS.ARPA>
... When asked to explain how we can see stars millions of light years
away if the universe is very young, [a creationist] ... indicated that
the speed of light is decreasing.
In ... 1934 ... Frank R. Edmondson ... said that by 1941, conditions
would have changed enough that it would be possible to determine [whether
the speed of light is decreasing] ...
Can anybody tell me what was finally determined?
For the past ten years it has been possible to measure the speed of
light to 9 place precision. During that period no change has been
measured. One might argue that it only recently stopped slowing down
I suppose.
A more common creationist answer to the question is that the light
was created in transit, complete with information about nonexistant
supernova explosions, fictitious recessional velocities, spectral
spectral lines, etc. Another common answer is that we are mistaken
about the scale of the universe and everything we see is within 6000
light years or so. A less common answer is that scientists have good
evidence for creationism (including a small universe) and are
deliberately deceiving the rest of us to promote irreligion or secular
humanism or free love or whatever.
Since 1982 the length of the meter (and hence of the inch, mile, etc)
has been defined in terms of the length of the second and the speed of
light. This means that the speed of light is now constant by
definition. This also means that since 1982, nobody has measured the
speed of light. Anyone who thought he was doing so was actually
measuring the length of the meter!
...Keithzdenek@heathcliff.columbia.edu (Zdenek Radouch) (11/08/86)
In article <241@sri-arpa.ARPA> KFL@MX.LCS.MIT.EDU writes: [In the reply to the question whether the speed of light is decreasing] > > Since 1982 the length of the meter (and hence of the inch, mile, etc) >has been defined in terms of the length of the second and the speed of >light. This means that the speed of light is now constant by >definition. Let me remind you that the constant speed of light is not an implication of the new definition of meter. The speed of light is assumed to be constant. The definition is based on that assumption. It doesn't make any sense to use this definition when questioning constant speed of light. If we find that speed of light really is changing, we are going to get rid of the definition. You are right in one thing; given this definition, it's going to be difficult. Consider this: I am going to define the length of meter in terms of the length of the second and the speed of my car. Then I can say that the speed of my car is constant... zdenek ------------------------------------------------------------------------- Men are four: He who knows and knows that he knows, he is wise - follow him; He who knows and knows not that he knows, he is asleep - wake him; He who knows not and knows that he knows not, he is simple - teach him; He who knows not and knows not that he knows not, he is a fool - shun him! zdenek@CS.COLUMBIA.EDU or ...!seismo!columbia!cs!zdenek Zdenek Radouch, 457 Computer Science, Columbia University, 500 West 120th St., New York, NY 10027
pmk@prometheus.UUCP (Paul M Koloc) (11/08/86)
In article <241@sri-arpa.ARPA> KFL@MX.LCS.MIT.EDU quotes: > From: "James J. Lippard" <Lippard@HIS-PHOENIX-MULTICS.ARPA> > > ... When asked to explain how we can see stars millions of light years > away if the universe is very young, [a creationist] ... indicated that > the speed of light is decreasing. To my understanding the speed of light is a weak inverse function of gravitational field (energy density) and should have INCREASED in the period after the big bang. By this very late time the rate of change in volume of the universe (average gravity field density and corresponding increase in the rate of the speed of light) should have slowed to a relatively imperceptible amount. As an engineering physicist, this problem hasn't come up, but I like to see a comment or two from the relativists or astronomers. +---------------------------------------------------------+--------+ | Paul M. Koloc, President: (301) 445-1075 | FUSION | | Prometheus II, Ltd.; College Park, MD 20740-0222 | this | | {umcp-cs | seismo}!prometheus!pmk; pmk@prometheus.UUCP | decade | +---------------------------------------------------------+--------+
bill@utastro.UUCP (William H. Jefferys) (11/09/86)
The constancy of the speed of light does not have a physically
invariant meaning. Whether you claim that the speed of light is
constant, or that it is changing, the statement has meaning only
with reference to the _physical method_ that was used to make
_measurements_ of the speed of light. Different measurement
methods can be expected to give different results.
For example, you could choose the Krypton-86 standard as your
standard of length, and the atomic second as your standard of time.
This was done until very recently.
Or you could choose the length of the platinum-iridium
bar stored at the International Bureau of Weights and Measures
in Paris as your standard of length, and you could use the old
definition of the second in terms of the Earth's orbital
period as your standard of time.
The speed of light might be constant if you used one of these
methods and variable if you used the other; or both methods could
give variable results (varying at the same or different rates), or
both could give constant results. What would happen in each case
is a subject of experimental determination.
Once a method of measurement has been specified, one finds that
the measured speed of light will vary as some monomial in certain
_dimensionless_ physical parameters. For example, if we used the
first method to measure the speed of light we would find that
c ~ 1/(alpha^2 * beta)
where alpha is the fine structure constant and beta is the ratio
of electron to proton mass. The speed of light, measured in this
way, would be found to vary if and only if either alpha or beta
(or both) varied.
On the other hand, the speed of light determined using the second
method varies as
c ~ 1/(alpha * alphaG * beta)
where alphaG is the dimensionless gravitational coupling
constant G*M^2/ch, and where M in turn is the proton mass.
If only alphaG varied, then the first method would give a constant
speed of light and the second a variable one; similarly, the second
method could give a constant result and the first variable; and so
on, depending on how the _dimensionless_ parameters happen to vary.
Of course, as was pointed out by several others, under the current
definition of physical standards, the speed of light is an absolute
constant, by definition.
The point is that only changes in _dimensionless_ parameters
have an invariant physical meaning. So it is ultimately meaningless
to talk about "variations in the speed of light". Any physical
measurement that measures changes in the speed of light can in
reality only reflect changes in the underlying dimensionless
parameters.
As far as I have been able to determine, all _practical_ schemes for
measuring "changes" in the speed of light would have to reflect
underlying changes in alpha, alphaG, or beta.
To my knowledge, no changes in any of these fundamental parameters
have been firmly established experimentally. van Flandern claimed
to have detected a change in the ratio of the gravitational and
atomic seconds, but his results have not been confirmed and have
been strongly questioned. The limit on the cosmological variation
of alpha set by the Okolo natural nuclear reactor is very stringent,
as is the limit on beta (actually beta*gp, where gp is the form
factor of the proton) from observations of quasars.
This is discussed at great length in Barrow and Tipler's recent
book, "The Anthropic Cosmological Principle", especially section
4.5.
As far as Barry Setterfield is concerned (he is the Creationist
whose theory about "variations" in the speed of light brought
this whole subject up), the variations from current values of
the parameters that his theory would require are many, many
orders of magnitude, so large that life could not have existed
on Earth at the "creation" 6000 years ago. For example, making
reasonable assumptions I calculate that Adam and Eve would have been
squashed into a "primordial ooze" only a micron thick, because
the molecular forces holding their bodies together would have
been unable to withstand the (relatively much stronger) gravity
of Earth implied by his theory. So much for "Creation Science".
--
Glend. I can call spirits from the vasty deep.
Hot. Why, so can I, or so can any man; But will they come when you
do call for them? -- Henry IV Pt. I, III, i, 53
Bill Jefferys 8-%
Astronomy Dept, University of Texas, Austin TX 78712 (USnail)
{allegra,ihnp4}!{ut-sally,noao}!utastro!bill (UUCP)
bill@astro.UTEXAS.EDU. (Internet)bill@utastro.UUCP (William H. Jefferys) (11/10/86)
In my earlier article, the line reading > > c ~ 1/(alpha * alphaG * beta) > Should have read c ~ alpha * alphaG * beta The rest of the article is not affected by this typo. -- Glend. I can call spirits from the vasty deep. Hot. Why, so can I, or so can any man; But will they come when you do call for them? -- Henry IV Pt. I, III, i, 53 Bill Jefferys 8-% Astronomy Dept, University of Texas, Austin TX 78712 (USnail) {allegra,ihnp4}!{ut-sally,noao}!utastro!bill (UUCP) bill@astro.UTEXAS.EDU. (Internet)
hoey@nrl-aic.ARPA (11/18/86)
From: Dan Hoey <hoey@nrl-aic.ARPA>
Date: Thu, 6 Nov 86 23:57:34 EST
From: "Keith F. Lynch" <KFL@MX.LCS.MIT.EDU>
...
Since 1982 the length of the meter (and hence of the inch, mile, etc)
has been defined in terms of the length of the second and the speed of
light. This means that the speed of light is now constant by
definition. This also means that since 1982, nobody has measured the
speed of light. Anyone who thought he was doing so was actually
measuring the length of the meter!
...Keith
As I recall, the definition is chosen so as to make the speed of light
an integral number of meters per second. Do you know the number?
Also, I have heard that the integral number of m/s applies only at some
experimentally convenient ultralow pressure, and that the speed in
vacuum is still subject to errors in measurement of refractive index.
Can you corroborate or debunk this?
I mostly want the value to make an accurate not-just-a-good-idea-it's-
the-law (or my-limits) bumper sticker or button for a change. The
common approximation of 186000 mi/s is galling, since so many people
take it for scripture.
Speaking of scripture, my grandfather was into biblical numerology (for
lack of a better term). He took a phrase about the light of heaven
being a thousand times that of the earth, and wrote a paper about the
length of heaven's light year and the number of square LY in heaven and
the like. Of course he used 186000 mi/s and carried his results out to
dozens of figures. What stupefied me was his use of a year composed of
24*365 + 12/4 hours. This from a man who was alive in 1900!
And what cretin decided to metrify our speed limits into km/hr, anyway?
Dan Hoey
HOEY@NRL-AIC.ARPAKFL@MX.LCS.MIT.EDU (11/24/86)
From: "Keith F. Lynch" <KFL@MX.LCS.MIT.EDU>
From: columbia!heathcliff.columbia.edu!zdenek@Seismo.arpa (Zdenek Radouch)
Let me remind you that the constant speed of light is not an implication
of the new definition of meter. The speed of light is assumed to be
constant. The definition is based on that assumption.
But given the current definition, how could one falsify the
assumption that the speed of light is constant?
It doesn't make any sense to use this definition when questioning constant
speed of light.
Right. One can't question it.
If we find that speed of light really is changing, we are going to get
rid of the definition.
Given the definition, we can't find that it is changing.
Consider this:
I am going to define the length of meter in terms of the length of the
second and the speed of my car. Then I can say that the speed of my car
is constant...
Right. Your gas pedal then changes the length of the meter instead
of the speed of the car. Other than being horribly unintuitive and not
very useful in terms of physics and engineering, what's wrong with that?
...Keith