[escepticos] $$ Distancia a la Luna, ondas gravitacionales

[arteaga@...>]

	Creo que esto podria interesar a los que estamos 
discutiendo sobre gravitacion. Es un articulo en ingles,
asi que ofrezco una breve descripcion.

	La distancia a la Luna se puede medir con un error 
de una pulgada (2.5 cm, mas o menos). El truco es medir 
cuanto tiempo tarda un laser en reflejarse en los reflectores 
dejados por los americanos y rusos. Este experimento lleva 
haciendose preiodicamente desde hace an~os, y en principio 
podria encontrar errores en la teoria de la gravitacion de 
Einstein. Pero no dicen nada mas... voy a escribir a este 
tipo preguntandole si tiene una pagina.

	( En otro orden de cosas, se ha aprobado otro 
experimento para detectar ondas gravitacionales; tomese una 
esfera de cobre de 100 toneladas cuidadosamente pulida, 
enfriese a 0.001 grados kelvin, cuelguese de un "hilo", 
usese un interferometro laser para medir desplazamientos 
ridiculamente pequen~os, y observese si esas vibraciones son 
consistentes con los efectos de una onda gravitacional. La 
intencion es que la esfera entre en resonancia con la onda 
gravitacional y se mueva algo. Esta antena podria empezar a 
funcionar en 2004 y sera 100 veces mas sensitiva que los 
otros experimentos funcionando hoy en dia, que no han 
conseguido detectar nada todavia )

	Santi



 From ayee en nova.astro.utoronto.ca Mon Dec 22 01:43:22 EST 1997
 Newsgroups: sci.space.news
 Subject: How far is the moon? Would you believe 15 billion inches? (Forwarded)
 Date: Sat, 20 Dec 1997 19:53:02 -0500

The University of Michigan
News and Information Services
412 Maynard
Ann Arbor, Michigan 48109-1399

Contact: Sally Pobojewski
Phone: (734) 647-1844
E-mail: pobo en umich.edu

News Release: December 16, 1997 (30)

How far is the moon? Would you believe 15 billion inches?

ANN ARBOR -- Sky watchers contemplating the moon this month may
wonder how far away it is. According to University of Michigan
astronomer Richard Teske, the moon is 15 billion inches from Earth --
give or take an inch.

Mix together the moon, four reflecting mirror arrays on its surface
and five Earth-based telescopes firing laser beams and you have a
recipe for one of the most accurate scientific experiments ever
undertaken. "Determining the moon's distance to within one inch is
an extraordinary accomplishment," Teske said. "These exquisite
measurements -- a kind of scientific slam-dunk -- are being used to
test Albert Einstein's theory of gravity."

The moon measurement project is an international effort. Mirrors
were placed on the moon's Earth-facing hemisphere by three teams of
Apollo astronauts and by one of the unmanned Soviet Lunakhod
missions. Telescopes used for beaming laser light at the moon are
located in Texas, Hawaii, France, Germany and Australia.

In the experiment, laser radiation launched toward the moon from
one of the telescopes on Earth bounces off one of the lunar mirrors
and returns to the same telescope about 2.6 seconds later.
Scientists determine the moon's distance by measuring as precisely
as possible the time it takes laser pulses to make the round trip.
Called laser ranging, the technique is equivalent to using radar,
Teske explained.

At stake is the dependability of Einstein's theory of gravity as
expressed through his complicated equations of relativity. The
theory is used to predict the moon's distance from Earth from
moment to moment as it orbits our planet once each month. The
laser ranging experiment checks out those predictions with actual,
accurate observations.

"Right now all is well with Einstein. Relativity's predictions seem
to be correct, reinforcing physicists' beliefs that the theory is
the best description we have for how nature operates," Teske said.
"But there are other theories of gravity that also are being
examined by the experiment, and it is still too early to rule them
out altogether. Since the laser ranging experiment will go on for a
few more years, we might eventually find that one of those theories
is preferable to Einstein's. If so, science will have taken a great
stride in our understanding of how the universe works."

Keeping track of the moon as it orbits around Earth is tricky,
Teske added. In addition to our planet's gravitational pull, the
moon is strongly tugged by the sun and weakly attracted by all the
planets and even some small asteroids. As a result, it meanders a
lot during its month-long orbit. A further complication is that the
moon doesn't simply circle around Earth's center.

"Both Earth and the moon circle one another around a common
point that is located 1,025 miles beneath Earth's surface," Teske
said. "As our planet rotates on its axis that point travels through
solid rock with the speed of a jet airplane, but with no discernible
effect upon us or the Earth."

Gravity experimenters also take into account the fact that the
moon's orbit is affected by random variations in the rate of the
Earth's rotation, which are caused mainly by the interaction of the
Earth's surface with its atmosphere. To compensate for these
variations, scientists must occasionally insert an extra second
into the stream of time.

"Both cycles will finally fall into step in the dim, far future
when the Earth will keep the same face toward the moon just as the
moon now keeps the same face toward us," Teske said. "Perhaps by
then nobody will much care whether Albert Einstein was right."


---
Andrew Yee
ayee en nova.astro.utoronto.ca