Ben Franklin and the speed of light
Dennis Kehoe
I'm putting together a few momentum problems for a general high school
class and I came across one that stated that Ben Franklin objected to
the theory of light being a particle by stating that it would have the
momentum of a 10 kg ball moving at 100 m/s. The problem asks for the
mass that he ascribed to these proposed particles. The problem is an
easy one, but I have other questions. As an historical point of
discussion, would anyone know:
1. What value was given to the speed of light at this time. Was it a
reasonable estimate? Empirical?
2. On what basis could he make such a statement? i.e. he must have had
a value for the mass in mind. How did he arrive at such a value of the
mass or was it simply stated in so-and-so's theory that he was objecting
to?
Thanks for any thoughts on this matter.
Dennis Kehoe
Powers Catholic High School - Flint, MI
dke...@tir.com
Harry H Conover
: Friends,
: I'm putting together a few momentum problems for a general high school
: class and I came across one that stated that Ben Franklin objected to
: the theory of light being a particle by stating that it would have the
: momentum of a 10 kg ball moving at 100 m/s.
[Moderator's note: Quoted text trimmed. -P.H.]
This is a remarkably good question that I have never heard addressed
before. I'm a physicist, and I can't recall a historic moment being
cited when first the speed of light came into question.
Here's an opportunity to educate for someone! ;->
Harry C.
Michael Mandelberg
> These days, I believe radar ranging measures the scale of the solar
> system most accurately; the speed of light is DEFINED to be 299792458
> m/s, the meter and second being defined in terms of radiation from
> atomic transitions (something which can be measured in the laboratory).
This gives the misleading impression that the speed of light is a matter
of definition, which it certainly is not. It is the units which are
defined in terms of physically measured quantities, not the other way
around. As there are only two units involved here, length and time,
there are only two measurements necessary, not the three mentioned
above.
I. Michael Mandelberg
Department of Physics and Astronomy
The Johns Hopkins University
A.J. Tolland
> This is a remarkably good question that I have never heard addressed
> before. I'm a physicist, and I can't recall a historic moment being
> cited when first the speed of light came into question.
Well, Galileo tried to measure the speed of light with an
experiment. He stood on a hill with a lantern, and had a friend stand on
a distant hill with another lantern. He raised his lantern. When the
friend saw Galileo's lantern go up, he raised his lantern. Galileo
counted the time between when he raised his lantern and when he saw his
friend raise his lantern. I'm not sure, but I think this is mentioned in
one of Sagan's books.
The experiment was inconclusive, of course, but it shows that
Galileo was thinking about light's speed.
--A.J. Tolland
Toby Bartels
>These days, I believe radar ranging measures the scale of the solar
>system most accurately; the speed of light is DEFINED to be 299792458
>m/s, the meter and second being defined in terms of radiation from
>atomic transitions (something which can be measured in the laboratory).
Well, you can't define all three of these things independently
and expect actual light to travel at the speed you specify.
Until 1983, the metre and second were defined by atoms (krypton & cesium),
and the speed of light was measured to be right about 299792458 m/s.
Nowadays, the second is defined in terms of cesium,
and the metre is 1/299792458 of the distance light travels in a second,
making the old measure in terms of krypton only approximate.
-- Toby
to...@ugcs.caltech.edu
Michael Sanders
>Friends,
>I'm putting together a few momentum problems for a general high school
>class and I came across one that stated that Ben Franklin objected to
>the theory of light being a particle by stating that it would have the
>mass that he ascribed to these proposed particles. The problem is an
>easy one, but I have other questions. As an historical point of
>discussion, would anyone know:
>1. What value was given to the speed of light at this time. Was it a
>reasonable estimate? Empirical?
>
The Danish astronomer Olaus Roemer, in the 1670s, noticed that the periods of
the moons of Jupiter were slightly shorter when Earth and Jupiter were
approaching each other, and longer when they were receding. He correctly
interpreted this as an effect of the finite speed of light, deducing a value
low by only about 25%, about 230,000 km/sec.
I do not know what other determinations there might have been between Roemer
and Franklin, who was born in 1706.
--
(T.) Michael Sanders internet: san...@umich.edu
Physics Department URL: http://www-personal.umich.edu/~sanders
University of Michigan phone: 734/936-0799
Ann Arbor, MI 48109-1120 FAX: 734/764-6843
Matthew J McIrvin
>Friends,
>I'm putting together a few momentum problems for a general high school
>class and I came across one that stated that Ben Franklin objected to
>the theory of light being a particle by stating that it would have the
>momentum of a 10 kg ball moving at 100 m/s. The problem asks for the
>mass that he ascribed to these proposed particles. The problem is an
>easy one, but I have other questions. As an historical point of
>discussion, would anyone know:
>1. What value was given to the speed of light at this time. Was it a
>reasonable estimate? Empirical?
>
>2. On what basis could he make such a statement? i.e. he must have had
>a value for the mass in mind. How did he arrive at such a value of the
>mass or was it simply stated in so-and-so's theory that he was objecting
>to?
I know that Franklin was Embassador to France and that on 7 April 1795
the National Convention of France decreed the new "Republican
Measures" to be legal measures in France, but did Franklin make such a
statement using metric units? That would mean that Franklin
enthusiastically embraced the metric system which he may well have but
in reading many things that Franklin wrote I don't recall him
discussing things in terms of km, meters, etc. but in yards, miles,
and even leagues. What might be the reference?
C. Cagle
Martin Ouwehand
san...@lhe.physics.lsa.umich.edu (Michael Sanders) writes:
] I do not know what other determinations [of the speed of light]
] there might have been between Roemer and Franklin, who was born in 1706.
I think it is possible to derive a value for the speed of light from the
phenomenon of aberration which was finally explained by Bradley in the 1720's.
--
| ~~~~~~~~ Martin Ouwehand ~ Swiss Federal Institute of Technology ~ Lausanne
__|_________ Email/PGP: http://slwww.epfl.ch/SIC/SL/info/Martin.html __________
On atterrit et on paye les consommations ! [Pypo l'intello - Tramber]