HDTenEightyP/NetNews huggingface dataset:
From: pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt)
Subject: Re: Report on Philosophies of Physicists
Organization: Computer Science Department,  Stanford University.
Date: Mon, 14 Sep 1992 19:02:45 GMT

In article <TORKEL.92Sep14095330@isis.sics.se> torkel@sics.se (Torkel Franzen) writes:

>In article <1992Sep14.001412.27940@CSD-NewsHost.Stanford.EDU> pratt@Sunburn.
>Stanford.EDU (Vaughan R. Pratt) writes:
>
>   >It was
>   >Torkel Franzen who brought up your latter meaning [set theory as
>   >a foundation for all of mathematics].
>
>  Really? I'm more than a little puzzled by this. Allow me to quote
>from the article of yours to which I originally responded:
>...
>  I can't make any sense of this if you are not talking about set
>theory as a foundation for all of mathematics, but rather of the
>mathematical uses of set theory in topology etc.

Some 19th century mathematicians said they cared about set theory as a
foundation for mathematics.  I said essentially the same thing and you
challenged me to show that they meant "foundation for all mathematics"
rather than merely "foundation for their mathematics."  I concede that
I can't do this, you win your challenge.

Enough of this thread, it's gone stale.

-- 
======================================================| God found the positive
Vaughan Pratt   pratt@cs.Stanford.EDU   415-494-2545  | integers, zero was
======================================================| there when He arrived.
From: mcirvin@husc8.harvard.edu (Mcirvin)
Newsgroups: sci.physics
Subject: Re: Did electric/magnetic symmetry "break"?
Date: 7 Sep 92 15:49:12 GMT

terry@aslws01.asl.dl.nec.com (Terry Bollinger) writes:

>Anywho, I finally dug out the error of my ways, but have never been fully
>satisfied about the broader question -- as in, why the heck IS there a ratio
>of 68.5-to-1, 

Just because e^2/4pi (Heaviside-Lorentz units, hbar=c=1) is 1/137, instead
of 1/2.  If it were 1/2, the Dirac monopole charge would be the same 
size as e.  It's the *product* that's fixed. 

As for why e^2/4pi is so much smaller than 1/2, nobody knows! 
It's fortunate, though, because it makes it possible to do
lots of calculations in QED.

>and why IS space so biased towards particles with the lesser
>charge magnitudes? 

Well, if e were much bigger, the magnetic charge would be smaller than
e.  So I suppose this is really the same mystery. 

-- 
Matt McIrvin, Cambridge, Massachusetts, USA 
Xref: sparky sci.physics:14432 sci.astro:9593
From: bweiner@ruhets.rutgers.edu (Benjamin Weiner)
Newsgroups: sci.physics,sci.astro
Subject: Re: VLBI (was: My son would like to know about photons...)
Date: 10 Sep 92 22:12:22 GMT
Followup-To: sci.physics
Organization: Rutgers Univ., New Brunswick, N.J.

Dave Ring writes:
>I think I understand this [classical EM approx] explanation, except
>that I don't see why separate photons should be correlated since in
>cases of interest the measured radiation is thermal.

Despite the title of this post, I don't really know what goes on
with VLBI.  I do know, however, that the Hanbury-Brown and Twiss
interferometer is also called the "intensity interferometer"
precisely because it doesn't rely on phase information.

Maybe it's more productive to think of the light from e.g. a
point on a star as interfering with itself (as you vary the 
spacing in a H-B and T interferometer, as the world turns in VLBI)
rather than different points' emission interfering with each other.
Unfortunately most explanations of stellar interferometry seem
to gloss over the coherence question.
Newsgroups: sci.physics
From: jfreter@triton.unm.edu (The Myth)
Subject: Re: E=mc2?
Date: Fri, 04 Sep 92 23:57:53 GMT
Organization: University of New Mexico, Albuquerque

In article <1992Sep4.205540.26583@u.washington.edu> mcfarlan@corona.math.washington.edu (Thomas J. McFarlane) writes:
>In article <10585@sun13.scri.fsu.edu> jac@ds8.scri.fsu.edu (Jim Carr) writes:
>
>>Thus relativistic mechanics is not at the foundation of the physics 
>>of the bomb.  Indeed, from the perspective of 1943, quantum mechanics 
>>was hardly required since the models used to make sense of the data 
>>concerning uranium fission were macroscopic ones and not true quantum 
>>mechanical calculations.  
>>
>  It's a little hard for me to swallow that QM was hardly needed.
>Was this really the case? 
>

Well, if you want more accuracy, then yes QM is needed.  But, I've done
reactor calculations for the UNM core that were as non-relativistic and
non-QM as I could and still ended up getting within 5% of the accepted 
values for critical mass, reactor periods, and rod worths for this core.
The QM doesn't really appear in reactor mechanics for pratical use, they
only appear if you wish to try and manually calculate cross-sections and
decay constants (half-lives).

In the fission process, the most energetic neutrons that are born only have
an energy of ~10 MeV.  The mass of the neutrons is 981 MeV.  A simple,
top of the head shows that the Newtonian momentum, and energy can still
be used and only give you an error of ~1% or so.

Jerry
From: bweiner@ruhets.rutgers.edu (Benjamin Weiner)
Newsgroups: sci.physics
Subject: Re: How old is the universe?
Date: 14 Sep 92 21:21:18 GMT
Organization: Rutgers Univ., New Brunswick, N.J.

DOCTORJ@SLACVM.SLAC.STANFORD.EDU (Jon J Thaler) writes:
>The answer to the second question is not known for a simpler reason -
>we don't
>know if there is enough matter in the universe to cause it ever to crunch
>back together.  This is the famous "dark matter" problem.

Well, just one part of the famous dark matter problem.  Anyway, I
don't think any of the cosmologists or astronomers who work on this
think \Omega can be greater than 1.  That is, minimal hypothesizing
gives not enough baryonic matter to "close the universe," there might
be other dark matter; many people want just enough, i.e. \Omega = 1,
so that we can have inflation, but that's already stretching.  And if
\Omega = 1, the universe expands asymptotically, i.e. the turnaround
time is at infinity.

The universe will expand forever, you heard it here first (not in the
Physical Review newspaper).
Newsgroups: sci.physics
From: pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt)
Subject: Re: Why Do Mirrors Reverse Left/Right and Not Up/Down?
Organization: Computer Science Department,  Stanford University.
Date: Fri, 11 Sep 1992 06:31:28 GMT

In article <Sep.10.19.00.46.1992.2310@ruhets.rutgers.edu> bweiner@ruhets.rutgers.edu (Benjamin Weiner) writes:
>I hate to add more to this topic, but recently while driving
>(I _do_ live in New Jersey) I found another demonstration that
>mirrors do not reverse left to right: when you're looking in the
>rear view mirror, and the car behind you is signaling, if the
>turn signal that's blinking is on your left, that means it will
>turn to your left

Excellent observation.  Moreover if the vehicle you're looking at is an
ambulance, you will see "AMBULANCE" spelled out in the correct
direction in your mirror.  Hence the word was written from left (side
of the vehicle) to right as usual.  Next time the topic of mirror
reversed writing on the front of vehicles comes up, feign disbelief and
bet whatever the market will bear that it's written left to right.
When they say "but I meant my right," reply that that's not the usual
criterion when naming the location of items on automobiles.

Incidentally my vote as to why mirrors *appear* to reverse left and
right goes to all those who pointed out that we normally rotate about a
vertical axis.  The FAQ should say something about this.

-- 
======================================================| God found the positive
Vaughan Pratt   pratt@cs.Stanford.EDU   415-494-2545  | integers, zero was
======================================================| there when He arrived.
Newsgroups: sci.physics
From: prim!dave@germany.eu.net (Dave Griffiths)
Subject: Re: Is Spacetime Absolute?
Organization: Primitive Software Ltd.
Date: Mon, 7 Sep 1992 00:53:41 GMT

In article <mcirvin.715719878@husc8> mcirvin@husc8.harvard.edu (Mcirvin) writes:
>prim!dave@germany.eu.net (Dave Griffiths) writes:
>
>>I understand that special relativity gets rid of the idea of absolute
>>3D space and absolute time, but is there any such thing as absolute spacetime?
>
>>I think what I am asking is, is there an objective spacetime fabric out there
>>through which all particles move? Or does a different universe exist from the
>>perspective of every particle?
>
>In special and general relativity there isn't a preferred coordinate
>frame with which to express positions in spacetime, but there is a
>single manifold through which everything moves.  The geometry of
>that single spacetime in general relativity depends on the distribution
>of energies, momenta, and forces, but statements about the curvature
>that results can be expressed in such a way that they are the same
>for everybody.  In that sense there is absolute spacetime in these
>theories.  What happens when gravity is quantized is less clear.
>

Just to muddy the waters, here is a quote from Einstein's Relativity:

"There is no such thing as an empty space, i.e. a space without field.
Space-time does not claim existence on its own, but only as a structural
quality of the [gravitional] field."

Part of my confusion results from trying to imagine 4D spacetime. I imagine
first of all Euclidean space with the Universe as a big 3D cube, then
extend that to a 4D hypercube where it is still possible to assign
coordinates (x, y, z, t) to every event and where this 4D spacetime still
has an objective existence.

(Not to mention the problem of, "if spacetime is curved, in what dimension
is it curving?" :-)

Where am I going wrong?

Dave Griffiths
Xref: sparky sci.physics:14589 sci.math:11366
From: gjm11@cus.cam.ac.uk (G.J. McCaughan)
Newsgroups: sci.physics,sci.math
Subject: Re: Report on Philosophies of Physicists
Date: 13 Sep 92 22:18:34 GMT
Organization: U of Cambridge, England

Vaughan Pratt claims that set theory was always used foundationally,
citing the use of set theory in general topology as proof.

This sort of "foundational" use is Very Different from the idea of
set theory as a foundation for all of mathematics. Not just because
of the difference in scale; the point is that in one case set theory
is used because the concepts of set theory are what are needed, while
in the other the point is that everything can be done in terms of set
theory, albeit unnaturally [note: the point with general topology is
exactly that the most natural way to do things was in terms of sets],
and that this means that if set theory is consistent, so is everything
else. I do not believe that the early general topologists were using
set theory because they thought it was a good way to make sure that
what they did was consistent. They were using set theory because that
provided the most natural language for doing general topology in.

It's sort of like category theory, perhaps. C.T. is mostly used because
it provides a natural language in which to talk about some things that
occur a lot in mathematics. Very few people have any interest in trying
to found mathematics on category theory in the way they once wanted to
found mathematics on set theory.

-- 
Gareth McCaughan     Dept. of Pure Mathematics & Mathematical Statistics,
gjm11@cus.cam.ac.uk  Cambridge University, England.    [Research student]
Newsgroups: sci.physics
From: trivedi@yukawa.uchicago.edu (Anil Trivedi)
Subject: Re: Why Do Mirrors Reverse Left/Right and Not Up/Down?
Organization: Enrico Fermi Institute, University of Chicago
Date: Fri, 11 Sep 1992 11:37:58 GMT

qb7g6@fel.tno.nl (Maarten Landzaat) writes:
 > Quite a while ago, I asked the net the question mentioned in
 > the subject line. A little while later I detailed my question 
 > because so many people answered "They don't!". That question
 > was something like "Why do people THINK that left and right
 > are reversed, but not up and down?". 
...
 > 
 > ..but the question remains: why is there a "left/right" disagreement,
 > and not an "up/down" disagreement? 

As someone pointed out, the only thing mirrors reverse is Front/Back,
defined along the axis perpendicular to them. For example, if a
mirror is lying on the x-y plane then z-components of vectors, e.g.,
velocities, will be reversed in the "image world". (If you are moving
along the z-direction then your image will have exactly the opposite
velocity. This effect is real, not psychological, and can be verified 
experimentally, e.g., by measuring Doppler shifts.)

Now to your continuing puzzle as to why people *disagree* on Left/Right
and not on Up/Down.

As I said earlier, the crucial difference is that we (normally) define
Left/Right with respect to our bodies, but Up/Down with respect to Earth's 
center. For example, people mingling in a room will agree on Up/Down,
but each has own Left/Right. In particular, *someone facing you* has
an *exactly opposite* notion of Left/Right.

What a mirror does is to create a *someone facing you*, by reversing
Front/Back. (This is real; I cannot adopt a viewing angle in which
you and your image are not facing each other.) That done, the Up/Down
agreement due to its *locally* objective definition (with respect to
Earth's center) and Left/Right disagreement due to its subjective
definition are just as they would be with a real person facing you.

If you tell the (real) person to turn around, both of you will begin
to agree on everything (well, almost); unfortunately, you cannot tell
your mirror image to turn around. :-)

Well, here is an amusing and instructive variant: Unlike mirror images, 
a photographic negative is 2-dimensional, and does not have any 
3rd-dimensional (Front/Back) information in it; your brain generates
some convenient interpretation. This time you can actually turn the
negative around, view it from the other side, and watch the subjective
Left/Right change.

-----
From: mitsu@nic.cerf.net (Mitsuharu Hadeishi)
Newsgroups: sci.physics
Subject: Re: Report on Philosophies of Physicists
Date: 5 Sep 92 04:45:20 GMT
Organization: CERFnet

In article <5399@dove.nist.gov> gilligan@bldrdoc.gov (Jonathan M. Gilligan) writes:
>     Whereof we cannot speak, thereof we must remain silent.

	I could speak about it, and go on and on, but I don't think
I have time to do justice to the subject matter, and if I said just
a little about it I believe it could well do more harm than good.
However, anyone interested in such things I heartily recommend
that they take a look at these other source texts (Wittgenstein's
in particular is quite good if you prefer a Western-style discursive
presentation rather than the rather intutive presentations used
by many Eastern philosophers.  Unless you're willing to suspend
disbelief, i.e., assume that the sometimes seemingly nonsensical
things that Eastern philosophers often say actually have some basis,
then I would recommend the Western readings, particularly Wittgenstein,
which, although also difficult to understand, are more gradual and
less "obscure".  That is, you can at least understand each sentence,
even if the paragraph might be rather strange-sounding; Eastern
philosophers are fond of making even single sentences almost impenetrably
difficult to fathom.)

Mitsu

Xref: sparky sci.physics:14654 sci.math:11443
From: gjm11@cus.cam.ac.uk (G.J. McCaughan)
Newsgroups: sci.physics,sci.math
Subject: Re: Probability Question
Date: 14 Sep 92 23:25:27 GMT
Organization: U of Cambridge, England

In article <1992Sep14.232348.27497@infodev.cam.ac.uk>, gjm11@cus.cam.ac.uk (me) writes:

> In article <1992Sep14.212528.29098@uwm.edu>, gunter@csd4.csd.uwm.edu (David O Gunter) writes:
> 
> >                         |"How arrives it joy lies slain, and why unbloomed the
> > david gunter            | the greatest hope ever sown . . ." 
> > gunter@csd4.csd.uwm.edu |           -Thomas Hardy
> > gunter@mcs.anl.gov      |               "Hap"                            
> 
> Are there meant to be two "the"s there? I'm sure Hardy didn't write them both
> (though I've never read that particular poem).

Erm, of course it mightn't be a poem, especially since it doesn't sound very
metrical. I've never read *anything* by Hardy called "Hap".

-- 
Gareth McCaughan     Dept. of Pure Mathematics & Mathematical Statistics,
gjm11@cus.cam.ac.uk  Cambridge University, England.    [Research student]
Newsgroups: sci.physics
From: mcirvin@husc8.harvard.edu (Mcirvin)
Newsgroups: sci.physics,sci.math
Subject: Re: Elevator mayhem
Date: 15 Sep 92 22:46:37 GMT
Article-I.D.: husc8.mcirvin.716597197
 <mcirvin.716585471@husc8> <1992Sep15.203557.21099@ichips.intel.com>

bhoughto@sedona.intel.com (Blair P. Houghton) writes:

>Not quite.  Say at 10' off the ground, the elevator is moving
>at -50mph and ejects you at 50mph wrt itself.  Then, instead
>of being 10ft from the ground moving 50mph, you're 10ft from
>the ground moving 0mph.  All you have to do now is keep from
>landing on the cat.

I assumed that the original poster meant that the elevator passenger's
legs were the ejection device.  Under those conditions this would be
rather hard to do.

>It's not perfect, though.  The trouble is, how fast does the
>elevator accelerate you from 0mph wrt itself to 50mph wrt itself?
>If it does it as fast as the ground will, then you might as well
>just press all the buttons and laugh, 'cuz yer hamburger.  If,
>however, the ejector is more gradual than a simple impact, then
>you're probably going to be okay.

This is correct.
-- 
Matt McIrvin, Cambridge, Massachusetts, USA 
Newsgroups: sci.physics,sci.optics
From: peri@cco.caltech.edu (Michal Leah Peri)
Subject: Re: Help on Fiber-optics
Organization: California Institute of Technology, Pasadena
Date: Sun, 13 Sep 1992 06:04:31 GMT

gapdrr@gsusgi2.gsu.edu (Daniel R T Rothman) writes:

>How, you ask, does this work when you wrap a fiber optic cable around a 
>corner? ...
>Now you ask, what happens if you KEEP crimping the
>cable tighter? It breaks. The thing is glass you know. I beleive that 
>fiber optic cable is designed to maintain total internal reflection at
>ANY curvature that can be achieved before material failure.

Take a fiber optic cable and wrap it around a rod or something, but not
so tightly that it breaks, and measure the transmission. You will find 
that the transmission *decreases* as the fiber is wrapped to smaller
and smaller radii.  This is because light is leaking out (not all is
totally internally reflected).  The light that does come out the 
end is spread over a larger angle (than for an un-wound fiber).  This
effect is called focal ratio degradation, because the outgoing light 
has a higher focal ratio than the incomming beam.

							--  Michal


Newsgroups: sci.physics,sci.math
From: torkel@sics.se (Torkel Franzen)
Subject: Re: Report on Philosophies of Physicists
Organization: Swedish Institute of Computer Science, Kista
	<1992Sep14.190245.7926@CSD-NewsHost.Stanford.EDU>
	<TORKEL.92Sep14233501@bast.sics.se>
	<1992Sep15.032800.14769@CSD-NewsHost.Stanford.EDU>
Date: Tue, 15 Sep 1992 05:21:19 GMT

In article <1992Sep15.032800.14769@CSD-NewsHost.Stanford.EDU> pratt@Sunburn.
Stanford.EDU (Vaughan R. Pratt) writes:

   >Well, I don't see how one could read my "mathematical community finding
   >that Cantor's theory provided an attractive and workable foundation for
   >mathematics" as their finding this for *all* mathematics unless they
   >actually *did* all mathematics.  So just from context this part *has*
   >to mean "their mathematics".  And that's all I intended in that
   >particular sentence.

  But doesn't "the mathematical community" do all mathematics? But I
see that I must interpret your statement as meaning that individuals
within the mathematical community found that Cantor's theory...etc.
All is now clear! People who regard something as a "foundation for all
mathematics" must do all mathematics. I will reread your article in
the light of this, but I fear it will take some time. For example, the
very next sentences after the one quoted:

  > There is no a priori reason why Cantor's set theory need be the only
  >workable foundation.  Nor is there any reason why alternative equally
  >workable foundations will all create the exact same class of
  >mathematical entities and theorems.

  Which mathematics is referred to here, and who is doing that mathematics?
Since you are the person talking, I guess you're referring to the
mathematics that you do. Since I don't know just what mathematics that is,
my reading flounders.
Newsgroups: sci.physics
From: ritley@uimrl7.mrl.uiuc.edu ()
Subject: AT WHAT HEIGHT IS GRAVITY PERCEPTIBLE?
Reply-To: ritley@uiucmrl.bitnet ()
Organization: Materials Research Lab
Date: Mon, 14 Sep 1992 19:20:53 GMT


At the risk of breaking with the tradition of using simple assumptions 
in stating physics problems (spherical cow, etc.), let me ask....

Given most humans' average athletic ability, musculature, metabolism,
etc. etc. --- at what altitude will a deviation in gravity from sea level
become perceptible (to someone)?  Any comments?
From: sichase@csa2.lbl.gov (SCOTT I CHASE)
Newsgroups: sci.physics
Subject: Re: Report on Philosophies of Physicists
Date: 6 Sep 92 18:42:44 GMT
Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA
Distribution: na
Reply-To: sichase@csa2.lbl.gov
NNTP-Posting-Host: 128.3.254.197
News-Software: VAX/VMS VNEWS 1.3-4   

In article <2738@news.cerf.net>, mitsu@nic.cerf.net (Mitsuharu Hadeishi) writes...

>>converse doesn't hold). "If you can't explain it without a lot of
>>effort, it must be bullshit", in effect.
> 
>	I've noticed this too among many people in the sciences.
>I believe it is a kind of "reverse envy"; many humanities people have

Nonsense.  Physicists have learned, at least within the domain of physics,
that this principle is a very good measure of understanding.  People who
*really* understand something can explain it simply and directly, without
undue reliance on the mathematics or on heavy-handed language.  Conversely,
it is all too easy to throw up a smokescreen of math and jargon when 
trying to deflect a question that you can't really answer.

When someone is trying to explain something to me, and ten minutes have gone
by without getting to the point, I know that the speaker doesn't understand
the issue as well as they think they do.

Also, experience shows that simplicity is a good tool for judging the likely
correctness of an hypothesis.  Complicated explanations are not likely 
to reflect underlying principles.

I don't think that envy has anything to do with it.  At most, it is a case
of physicists trying to export the principles which guide good physics research 
into domains in which they may not apply.  Understanding literature and art, 
which reflect human reality and social complexity, are necessarily more 
complicated than physics is ever likely to be, in the sense that there are 
no simple precise mathematical principles on which to construct the entire
edifice.

-Scott
--------------------
Scott I. Chase			"The question seems to be of such a character
SICHASE@CSA2.LBL.GOV		that if I should come to life after my death
				and some mathematician were to tell me that it
				had been definitely settled, I think I would
				immediately drop dead again."      - Vandiver