HDTenEightyP/NetNews huggingface dataset:
From: jac@ds8.scri.fsu.edu (Jim Carr)
Newsgroups: sci.physics
Subject: Re: simple question
Date: 8 Apr 92 17:58:13 GMT
Reply-To: jac@ds8.scri.fsu.edu (Jim Carr)
Organization: SCRI, Florida State University

In article <9204080901.aa00587@Paris.ics.uci.edu> kibler@turing.ICS.UCI.EDU (Dennis Kibler) writes:
>How is radioactive dating done?
>
> I understand that by knowing the current % (say Z) of X in Y,
>and the half-life of X, that we can project backwards
>and figure out the % (say Z') of X in Y at any previous time.

Yep.  The formula is 
                       t = tau_p ln{(P+D)/P} 
                                              where tau_p is the mean 
lifetime (not the half-life) of the parent nucleus.  This assumes no 
daughters present at time zero, but it also works if the daughter 
population can be estimated independently.

>So to figure out the age of Y  we need to know the original
>percentage of X in Y.  But how is that known?

In the case of metallic ores, the problem is one of geology: is this 
metal expected to be found with its daughters.  In the case of alpha 
decay, it can be assumed that the helium gas was not trapped in the 
rock when it was formed.  In the case of C-14, the problem is one of 
biology and cosmic-ray backgrounds.  Every case has different problems; 
I think you may find them discussed in a nuclear chemistry text. 

> Let me put it in a more concrete form. Suppose we use carbon
>dating to measure the age of a fossilized plant. I don't
>understand why we aren't measuring the age of the carbon,
>created I presume at the some time before the age of the earth,
>rather than the age of the plant.

C-14 has tau ~ 8000 years so it does not originate from the big bang or 
supernovae but rather from cosmic-ray reactions in the atmosphere.  One 
starts by assuming this production rate is constant, but much work has 
led to calibration curves that effectively tell you about the time 
variation of those reactions.  More on that later.  The key point is 
that living organisms exchange carbon with their environment and thus 
maintain a fixed ratio of C-14 to C-14 that is reflected in a particular 
specific radioactivity (that is, radioactive decays per gram of carbon 
per second).  This interchange ceases at death, and from that point on 
the drop in specific radioactivity tells you the age. 

Calibration has been done by correlations with tree ring records that now 
go back a long ways (about 7 or 8000 years I think) and Egyptian mummys 
that are 4 to 5000 years old.  The deviation is not much from linear 
until you get back to > 5000 years.  More information on the variations 
and other terrestial measures of the time-dependence of cosmic ray fluxes 
requires someone with more expertise than I. 

--
J. A. Carr                                    |  "The New Frontier of which I  
jac@gw.scri.fsu.edu                           |  speak is not a set of promises
Florida State University  B-186               |  -- it is a set of challenges."
Supercomputer Computations Research Institute |   John F. Kennedy (15 July 60)
Organization: University of Illinois at Chicago
Date: Monday, 6 Apr 1992 10:18:06 CDT
From: <U53644@uicvm.uic.edu>
Newsgroups: sci.physics
Subject: Re: My objections to the collapse postulate

Please pardon me if this is a silly question (I'm a Math/Elec.Eng. grad, not
physics), but can one tell that a wave function has collapsed at a certain
distant point before light from that point would have arrived? If not, then
does the simultaneous collapse of that wave function everywhere transmit
information at faster than light speeds? If not, what's the paradox?
From: jim@cs.UAlberta.CA (Jim Easton)
Newsgroups: sci.physics
Subject: Re: van de Graaff generators
Date: 14 Apr 92 23:59:40 GMT
Organization: University of Alberta, Edmonton, Canada

jeroen@dxcern.cern.ch (Jeroen Belleman) writes:

>So? Can anyone suggest a way to measure the potential of the sphere?

In the Nuclear Physics lab where I used to work we had a Van De Graff
which could go up to 7 Mev.  There were two methods of measuring
the terminal voltage.  One was with the analysing magnet.  ie. There
was a magnet that bent the beam at right angles and by measuring the
magnetic field (we could do that very accurately using NMR) we knew 
what the terminal voltage was.  In fact there was a slit the beam had to
go through and by comparing how much of the beam hit one side compared
to the other generated a control signal which controlled a corona
discharge on the inside of the tank containing the terminal.  Thus
the analysing magnet actually controlled the terminal voltage.

Off hand I would say this is not too practical for you unless you want
to generate a beam.  The second method involved what we called a
"generating voltmeter".  This was plate mounted inside the tank 
facing the terminal.  There was a grounded chopper plate rotating
in front of it so an AC signal was generated on the plate the amplitude
of which is proportional to the terminal voltage.  Calibrating this
voltameter was easy (for us) - we calibrated it from the analysing
magnet.

I would suggest that you could still use the same setup.  For
calibration you could run your VDG at fairly low voltage which is
directly measureable (say like about 30 KV) and note how much
AC signal one gets at the generating voltmeter  - it won't be much 
but it should be enough to get a reasonable calibration.

The column was built in layers of conducting and insulating plates.
There was a high value high voltage resistor between each layer the
purpose of which was to keep the field gradient more or less uniform.
(We were accelerating particles)  The current down this resistor chain
would be proportional to the terminal voltage.  Although we never used
it for that you could.

Lets assume that you are getting about 500 Kv and that you could draw
1 UA without loading it too much.  If I've done my calculations right
a string of 50 10000 Mohm resistors would be just about right.  Just 
remember when dealing with resistors of this high value a finger print
on the resistor will change its value.  You also have to be careful
what you are using to mount them on.

You could also use such a resistor chain to help calibrate a generating
voltmeter at lower voltages.

	Jim
From: DURRETT@SLACVM.SLAC.STANFORD.EDU
Newsgroups: sci.physics
Subject: Re: SLC (was Re: terminal particle energy ...)
Date: 17 Apr 92 19:49:52 GMT
 <16APR199209312039@sscvx1.ssc.gov> <8286@sun13.scri.fsu.edu>
Distribution: world,local
Organization: Stanford Linear Accelerator Center

Actually, a large part of the design of the looped portion of the SLC
was concern for the combination of aesthetics and environment. If the
collider had been built in a plane, the north loop would have been
above ground. This was considered a problem for a lot of reasons. First,
cooling would have been a problem(it always has been along the linac,
which, because of complaints from the locals is painted gov't green
rather than the more logical white. SLAC has always been cheap in their
approach to such things. They long intended to cool the linac housing
by spraying water on the building which holds the klystrons, only to
have a five-year drought get in the way). Second, the exposed hillside
which would have been torn up for this purpose, faces the city of Menlo
Park, whose residents are only too happy to take the money of poor
graduate students and transplanted post-docs, but don't want to have to
actually see any evidence of what goes on up the hill. Thirdly, there
is, oddly enough, some wildlife on the site, and some of it(some foxes
and deer) lives in this portion of the site. Had Menlo Park objected
insisted on an EIS, SLAC probably would have lost. But Jim is right,
Burt and others apparently discounted too much reports that non-linear
effects would cause major problems with the beams once they were
steered away from the linac. Hopefully, physicists will learn a lesson
from all this, but I'm not holding my breath. People like Burt Richter
will always be around to say, "No, it's OK. We'll do it really cheaply."
And as long as a few people continue to have success with this technique
the people who don't have to do the science, but do provide the funds,
will think under-funding works.

Roo-Dog Rip
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
Roo-Dog Rip is:                  <>SLAC, Stanford, the DOE and the
Derrell Durrett                  <>University of Colorado neither claim
D_DURRETT@COLOPHYS.BITNET        <>responsibility nor accept liability
D_DURRETT%KAOS@VAXF.COLORADO.EDU <>for my opinions. Gee- what a surprise.
From: shaw@s119.es.llnl.gov (Henry Shaw 510 423-4645)
Newsgroups: sci.physics
Subject: Re: simple question
Date: 10 Apr 92 00:09:54 GMT
Organization: Earth_Sciences_Dept.,_LLNL

In article <1992Apr09.141649.37267@watson.ibm.com>, platt@watson.ibm.com (Daniel E. Platt)
   writes:
|> In article <1992Apr8.193655.27457@cs.yale.edu>, rescorla@rtnmr.chem.yale.edu (Eric Rescorla) writes:
|> |> In article <1992Apr08.173630.1785@watson.ibm.com> platt@watson.ibm.com (Daniel E. Platt) writes:
|> |> >In article <1992Apr8.161532.24698@cs.yale.edu>, rescorla@rtnmr.chem.yale.edu (Eric Rescorla) writes:
|> |> >The concentrations have not been ~ constant over time.
|> |> >
|> |> >See some of Colin Renfrew's work...  You'd find it interesting...
|> |> It was my impression that they had been constant to within say 50%,
|> |> which was all I meant to suggest.
|> 
|> 50%?  Wow!  That's essentially 1 half-life... or around 8,000 years...
|> 

The pre-bomb variations in atmospheric C-14 concentration over the last ca. 8000 years were 
much less than 50%.  The maximum deviation from pre-bomb "modern" values 
(say, the value for the year 1950) occurs toward the end of the dendrochronological record 
(i.e. at 7000-8000 y.b.p.).  At that time, the atmospheric C-14/C-12 ratio was about 10% higher 
than the 1950's pre-bomb value.  This corresponds to an error of about -750 years -- that is, 
an "uncorrected" C-14 age obtained by assuming a constant atmospheric C-14 concentration is 
about 750 years too young.

A good review of atmospheric C-14 variations is Damon, PE, Lerman, JC, and Long, A, 1978, 
Temporal Fluctuations of Atmospheric C-14: Causal Factors and Implications.  Annual Rev. of 
Earth and Planetary Sci., v6, pp457-94.

-- 
Henry Shaw, Earth Sciences Dept., LLNL |    All opinions are my own.
Internet: shaw4@llnl.gov               |  
CIS: 76347,2523                        |
Fidonet: Henry Shaw at 1:161/55        |
Newsgroups: sci.physics
From: richard@locus.com (Richard M. Mathews)
Subject: Re: idealizations??
Organization: Locus Computing Corporation, Los Angeles, California
Date: Fri, 17 Apr 1992 02:25:15 GMT

platt@watson.ibm.com (Daniel E. Platt) writes:

>There's a problem with this in the relativistic limit -- something
>infinitely rigid would transmit the signal instantaneously.  An infinitely
>rigid ball only makes sense in a limit where the stiffness makes a
>transmission speed much greater than any of the other dynamics in the
>problem.

But if you are going to impose this much realism, then you must impose
the realism that we live in an atomic world.  There is no such thing as
a perfectly spherical ball (except for something like a single Helium
atom), and there is no such thing as perfectly flat surface.  The contact
"point" will involve at least 3 atoms from each object, so it will have
finite area, so the pressure won't be infinite, so you won't need infinite
rigidity.

The problem as presented is not realistic, so there is no right answer.

Richard M. Mathews			D efend
Internet:   richard@locus.com		 E stonian-Latvian-Lithuanian
UUCP:	    ...!uunet!lcc!richard	  I ndependence
MIL/BITNET: richard%lcc@UUNET.UU.NET
From: rcain@netcom.com (Robert Cain)
Newsgroups: sci.physics
Subject: Re: Time
Date: 15 Apr 92 05:03:39 GMT
Organization: Netcom - Online Communication Services  (408 241-9760 guest)

Or how about, "Time is Gods way of keeping everything from happening all
at once."

-- 
Bob Cain    rcain@netcom.com   408-358-2007

"There are some strings.  They're just not attached."
                                                    Victoria Roberts
From: jeroen@dxcern.cern.ch (Jeroen Belleman)
Newsgroups: sci.physics
Subject: van de Graaff generators
Date: 10 Apr 92 08:50:39 GMT
Organization: CERN European Laboratory for Particle Physics


A little more than a week ago, I related of the problems I had to get
a van de Graaff generator running. Things are much better now, and I
would like to compare its performance with others.

As suggested by Ron V. Parsons <parsons@sequent.com>, I replaced the
bottom PVC roller, with an aluminium one. That made the charging current
settle to about 2.5uA after a few seconds. I noticed however, that the
charging current was much higher initially, and that much charge was
left on the belt when it came down again. So I changed the top roller
for an aluminium one too. That did it. The current is now up to 9uA
and the thing starts spitting discharges all around about a second
after I switch it on. It works fine for both polarities.

The high voltage electrode is still the same aluminium foil covered
plastic ball, and it leaks corona discharges all over. In spite of
that, I now get the nice fat sparks I wanted. With a proper polished
metal ball, performance is likely to be far more impressive still.
I'm trying to find one...

I have a few more questions too:

Would some generator owners take the trouble to measure the charging
current of their van de Graaff? I.e. connect an Ampere meter between
ground and the high voltage electrode and then switch on. Not the other
way around: You might damage your meter.

Can anyone suggest a method to measure the output voltage? I do not
think suitable probes exist. The best I can get is a 40kV 1000MOhm
probe, which is clearly insufficient. I have considered using some
sort of Kelvin electrometre arrangement, but, one of the electrodes
being a ball, calibration is a problem. Yet I think that's the way to go.

So far, I get fat sparks from a polished 4cm diameter metal pipe over
a little more than 10cm. The more diffuse sparks from the pipe's end
easily exceed 50cm. You can make you own voltage estimates from that.

Jeroen Belleman
jeroen@dxcern.cern.ch
jeroen@cernvax.cern.ch
From: jim@cs.UAlberta.CA (Jim Easton)
Newsgroups: sci.physics
Subject: Re: Question about dimmer switch and lamp.
Date: 7 Apr 92 23:35:24 GMT
Organization: University of Alberta, Edmonton, Canada

rrr@hpdmd48.boi.hp.com (Rudi Rynders) writes:

> Can someone versed in physics please explain this phenomenon?
> Given an incandescent lamp with a dimmer switch. The lamp is on.
> One can then slowly turn the dimmer knob and reduce the light
> intensity until the filament is just barely glowing.
> If , however the lamp is off, and one slowly turns the dimmer
> switch up, no matter how slow, the light always comes on rather
> suddenly and much brighter than in the reverse process .
> Why is that so ?
> Thanks in advance!

> Rudi (Not a physicist-just an ME) Rynders
> (rrr@hpdmd48.boi.hp.com)

The resistance of the filament is very much dependent on its
temperature.  When the filament is cool the resistance is low
so that at a given setting of the dimmer most of the energy is
disapated in the dimmer and not the filament.  There is insufficient
energy being dissipated in the filament to raise its temperature
significantly.

In order to get it to glow one has to raise the current more and as
soon as it begins to glow, (temperature up) the resistance rises and
it absorbs a larger fraction of the energy so it would heat up faster.
This would happen quite suddenly and it would come on fairly brightly.

However if the filament is hot to start with (viz. approach the
same dimmer setting from the hot side) its resistance is high and
now for the same setting of the dimmer a greater fraction of the
energy is dissipated in the filament - enough to keep it hot.  

	Jim
Newsgroups: sci.physics
From: bud@ms.uky.edu (William K Glunt)
Subject: Re: idealizations??
Date: Wed, 15 Apr 1992 18:20:13 GMT
Organization: University Of Kentucky, Dept. of Math Sciences
X-Bytes: 

In article <1992Apr15.173142.6990@cis.uab.edu> menon@ftp.phy.uab.edu (Govind Menon) writes:
>The case of the hard sphere and plane is an interesting scenario, but by no 
>means realistic.  If however, you do wish to conduct a thought experiment, 
>constrained to idealizations, then permit me if you will to make yet another
>reasonable assumption.  
>
>There is nothing preventing a perfectly spherical and a perfectly planar 
>material from being infinitely rigid.  Subject to this condition the bodies

 How fast would such a sphere transmit shock waves in it's interior?



-- 
Dr. William K Glunt     | Are you ABNORMAL?
************************| Then you are probably better than most people!
U of Kentucky math dept | Are alien space monsters bringing a STARTLING NEW
home phone 606 258 8864 | WORLD?  from _The book of the SubGenius_
From: dbriggs@zia.aoc.nrao.edu (Daniel Briggs)
Newsgroups: sci.physics
Subject: RFD for IMAGE PROCESSING newsgroup
Date: 13 Apr 92 18:44:36 GMT
Article-I.D.: zia.1992Apr13.184436.4085
Organization: National Radio Astronomy Observatory, Socorro NM

[Third and last time that I'm going to try and post this.  Sorry about the
multiple posts, but something in the crosspost list is buggering things!]

I just thought I'd point out that there is a discussion about an possible
image processing group over on news.groups.  I think it's a *great* idea,
and about bloody time, too.  But I seem to be surrounded by computer
vision / computer graphics people.  Could we get some scientific image
processing people over there too?  If we don't act now, we're likely to end
up with comp.graphics.image.processing, which would be a gross restriction
on an otherwise tremendously useful group.  Actually, I'd like to see a
general imaging group, not restricted to conventional image processing
techniques, but expanded to included techniques of indirect observation,
calibration and processing, as well as general imaging theory.  Say
sci.imaging.  Do we have enough support for such a group?

-- 
| Daniel Briggs  (dbriggs@nrao.edu)                      |        USPA B-14993
| New Mexico Tech / National Radio Astronomy Observatory |            DoD #387
| P.O. Box O / Socorro, NM 87801   (505) 835-7391        |
Support the League for Programming Freedom  (info from league@prep.ai.mit.edu)
From: jarlo@ifi.uio.no (Jarl Magne Michael stensen)
Newsgroups: sci.physics
Subject: Time travel
Date: 8 Apr 92 10:58:25 GMT
Organization: University Of Oslo, UiO. Student At Dept. Of Physics
Originator: jarlo@sognsvann.ifi.uio.no


I just read an article in New Scientist (28 March 1992 Issue) written by
Marcus Chown entitled "Time travel without paradoxes." The article discusses some
aspects of time travel in relativity and quantum mechanics. The quantum-mechanical
"model" of time travel has something to do with parallell universes and explains the
paradox of going-back-in-time-to-kill-your-own-grandfather as not being a paradox at all.
The argument in the article goes something like this: 
Since quantum mechanics assigns probabilities for every possible (?) path in some 
time-space manifold, there exists a parallell universe (actually several), in which
it is possible to kill your own grandfather without it having any impact on the universe
you originally came from (whichever that is, since originally all of them should have
equal probabillity.) Now, if this is correct (read: a probable explanation) then
there actually IS something "paradoxal" here. (Or I just get it all wrong, which is the
most probable answer...:-) )
My problem is that if these parallell universes develop equally, then they are also equal at 
the point in space-time when you, in your ORIGINAL universe, enter the time-machine and
start travelling back to kill your own grandfather. In other words; you get a paradox
in any of these parallell universes, because they are equal at all times. If this should
work, as I understand it, there would have to exist a parallell universe where the
killed-grandfather-scene was "available" with a complete new future all ready for use! 
And that is not all: If this is correct, then there would have to be some mechanism to
generate a new universe for EVERY possible action and path for EVERY possible 
configuration of particles and energy. How is the configuration of the "universe" wave
functions that keep these universes apart, and how is probabillity divided amongst them?

Please clearify this brain-twisting problem.

If anyone has any refs on good books on quantum-mechanics and time,  
I would like to hear about them.

M&M
From: gilligan@bldrdoc.gov (Jonathan M. Gilligan)
Newsgroups: sci.physics
Subject: Re: FAQ
Date: 16 Apr 92 00:56:44 GMT
Organization: National Institute of Standards and Technology

I'm new to reading this newsgroup, but it seems to me that there are
enough questions about second quantization (what are virtual
particles, what is a photon, etc) that it might merit a place in an
FAQ list.

Also, something on how and where (FNL, CERN, SSC) the top and Higgs
might be found. Scott gave a nice brief summary a few weeks ago on how
you'd see the top, with reference to the Higgs Hunter's Guide and
Ferbel's book for further reading. This might be a model entry.

---Jon

Newsgroups: sci.physics
From: brahm@cco.caltech.edu (David E. Brahm)
Subject: Re: mass definition in particle physics
Summary: Pole in propagator
Organization: California Institute of Technology, Pasadena
Date: Wed, 8 Apr 1992 05:35:02 GMT

s5169180@deimos.ucc.umass.edu (S5169180) asks:
> What is the mass ?

Why, the mass is the pole in the propagator!  In a Feynman diagram, a
particle passing from one interaction to the next is represented
diagrammatically by a line, and algebraically by 1/(p^2-m^2).  If it is
kinematically allowed (by energy-momentum conservation), this factor is
(usually) dominated by p^2=m^2, and we call the particle "on-shell", a
"resonance", or an "external particle."  [Recall here p^2 = (energy)^2 -
(momentum)^2.]  The location of that pole can by shifted from the "bare
mass" (the thing in the Lagrangian) by loop effects (renormalization).

--
Staccato signals of constant information,  |  -- David Brahm
A loose affiliation of millionaires and    |  (brahm@coil.caltech.edu)
  billionaires and Baby ...                |---------------------------
These are the days of miracle and wonder,  | Disclaimer:  I only speak
And don't cry, Baby, don't cry, don't cry. |   for the sensible folks.
From: ss8c+@andrew.cmu.edu (Simon Szykman)
Newsgroups: sci.physics
Subject: Re: Xeroxing mirrors
Date: 13 Apr 92 17:40:37 GMT
Organization: Masters student, Mechanical Engineering, Carnegie Mellon, Pittsburgh, PA



Jon J Thaler says:
>I would suppose that a photocopier uses the diffuse reflection from
>the document to be copied.  Paper does not have much specular
>reflection (and that which it does have is not from the ink - try reading
>a glossy magazine when the reflected light shines in your eyes).
> 
>Your mirror, having *ONLY* specular reflection, is as invisible to the
>copier as the glass plate you put the paper on.
 

Actually, I think the problem is one of where the lens focuses.  Think
of it this way:
Say the light source of the photocopier is x inches below the glass.  
You could remove the mirror and put the same kind of light source x
inches above the glass (some copiers have a "flash", others a strip of
white or green light which scans across the glass) and achieve roughly
the same conditions.  The point is, that would result in a black copy
too.  Why?  I think it's because the lens focuses on the glass, and not
a distance x inches above the glass.  That's why you get a black copy
if you open the copier lid and make a copy without placing anything on
the glass.  Even if there are strong overhead lights above the copier,
that light doesn't cause the copy to come out white.

                            - Simon