Newsgroup sci.physics 215770

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Gary Cruse wrote:
>              Nah, time is just to keep everything
>              from happening at once.
Maybe everything IS happening all at once. Time may be a way to sort it
all out.

I heard that somebody once did an experiment similar to ye olde
double-slit experiment, except that the light source was set such
that it typically allowed only one photon to be present within
the apparatus at any time.  I'm told that with such a setup,
interference fringes are still produced (which is to be expected
under QM), but that if a detector is placed at one or both of the
double slits, only two bright spots are produced (also to be
expected under QM, from what I understand of it).
First off, does anybody know where I can find this experiment
written up?  A professor of mine isn't familiar with it and it'd
be nice to be able to show her something definitive.
Secondly, how does one go about detecting the passage of a photon
through one of the slits without affecting the properties of that
photon (that is, without absorbing it, or changing its wavelength
or phase (do photons have phase?))?  The only method of detecting
photons that I'm familiar with involves absorbing the photon
involved (a photocell).  I can postulate methods that, it seems
to me, would not absorb the photon but should sap some energy
from it (hence changing the wavelength) -- a SQUID, perhaps.
Thanks!
--
-----------------------------------------------------------------
                Everyone can be taught to sculpt;
         Michelangelo would have had to be taught not to.
Jason Cooper                              jcooper@acs.ucalgary.ca

John   wrote:
>Seems I've seen a lot of dust on unpainted metal fan blades and squirrel
>cage rotors. If the blades were grounded, then electrostatic attraction
>would be ruled out as the *only* cause for dust accumulation.
>
>In addition to dust in the air I'll bet there are cooking oil vapors
>etc. which could deposit on the blades and glue the dust in place.
>
>Even though the blades may have small area compared to other dust
>collecting devices, like CRT screens and knick-knacks, the motion of the
>blades sweep out larger areas over time. Some mean free path calcultions
>are in order.
>
>This sounds like a good science fair project or PHD thesis topic(%>)
By all means... and in the more excruciating detail the better.  :-)
I second the cooking oil mechanism.  The distinction between dust
held in place merely by static attraction and dust glued into a fiber
reinforced mess by oil residue can be observed at once by someone
cleaning the nasty thing.  I suppose this is why practical observation
is called "getting your hands dirty".
Still,  perhaps like CRT's there is a dual mechanism.  Static fields
may increase absorption cross-section,  condensed atmospheric
pollutants keep the dust glued in place.  But in the case of a fan,  I
would think airflow would be the major explanation of increased cross
section -- as you say a detailed analysis should look at a detailed
fluid dynamics simulation of flow ;).  Wait!   Here is the explanation
of why fan blade dust seem to be more oily than other dust in the same
environment:  The large airflow simultaneously gives them a larger
capture opportunity,  but only for material that can stick to them in
the wind.  So first we condense out the oil vapor,  then we stick
some dust fibers to it,  then more oil vapor,  until we have a real
yuckster.  If you place a ceiling fan near the kitchen,  where warm
air laden with cooking residue would find itself first,  this
completes the picture.
Further research requires funding.  Thanks.  That was an excellent
procrastination.  ;)
Ed

[Anthony Potts wrote]
>[LBsys wrote]:
>> Not quite. If the sprocket wheel pulled 10 feet of
>>  track (wrt the tank), the tank will have moved 10
>> feet. As the lower part of track stays motionless
>> wrt the ground, the upper part has to make up 
>> for that and thus moves with twice the speed - 
>> wrt the ground, not the tank (and not the
> Whilst what you say is correct, I will just briefly defend
> what I said as correct but not entirely clear.
> I was not making enough effort to distinguish what each
> piece of movement was relative to. I hope that my second
> post with an explanation will clear it up.
Well, it cleared up a certain confusion, for sure, but only deepened the
given misconception:
Im Artikel ,
Anthony Potts  schreibt:
>So, the driving wheel's circumference has a linear velocity twice that of
>the tank's velocity, and the force pulling the tank forward is therefore
>twice the force measured at the circumference of the driving wheel.
Sorry, not quite (ahem, if we can subsume an error of 100% as 'not quiet
;-)...
The upper part of the driving wheel has a a linear velocity twice that of
the tank's velocity WRT the GROUND. The lower part at the same time ZERO.
WRT the tank the circumference has a uniform linear velocity of 1 (i.e.
groundspeed) at all points. And that goes for any wheel, no matter if
there's a tread laying around it (which is easily visualized, when
shortening the tread to exactly circle the wheel just as it's outer rim).
If you just visualize the front wheel being in contact with the tread at
ground level (which it isn't only for the reason of climbing), than you
might visulaize the tread just as pieces of carpet, which some people pick
up behind the tank just to quickly lay it out in front of it again. Then
the sprocket wheel rolls along a surface like any other wheel.
Thus the force pulling the tank forward is just the same as if there was
no tread and the sprocket wheel would just dig the ground.
BTW: Take it with humour, sometimes intuition fails even the best of us
;-). What probably killed yours was the picture of the stationary
'puller', like Ed Green wrote. But in fact the 'puller' is the sprocket
wheel - which is moving along with the tank....
Cheerio
The most dangerous untruths are truths slightly deformed.
Lichtenberg, Sudelbuecher
__________________________________
Lorenz Borsche
Per the FCA: this eMail adress is not to 
be added to any commercial mailing list.
Uncalled for eMail maybe treated as public.

Im Artikel , meron@cars3.uchicago.edu
schreibt:
>Here I agree.  And this could be formalized into a law. 
>You could have an arrangement where a politician's 
>campaign promises are being collected in a formal
>document which he has to sign upon being elected 
>and which then is considered a binding contract.  
>Then, if he breaks a promise, it is a breach of contract
>with the usual penalties.  Hey, I like it.
Ohoh, then pass it on to your congressman :-)
>I'm rather sure.  I may be mistaken, of course, but I don't know about 
>any actual law that was broken in this case.
[...and later...]
>>
>>I faintly remember that the US stretched the laws quite a bit to
>>make it possible to move troops into SV.... 
>
>That's true, it did.  Or, rather, the US presidents did.
>By never declaring a war they got around the law
>stating that only Congress can authorize war.
Hm, hm. "Mitgegangen, mitgefangen, mitgehangen" (Gone with, caught with,
hung with [the other crooks]). McNamara must have known, that this IS a
war and his president was more than stretching the law. Thus staying in
the government as secretary of defense he is guilty just as well.
>The laws regarding Presidential pardons should indeed
>be changed.  I don't know what are the rules in Germany.
>In Israel the president has the power of pardon but he can
>only pardon people from a list that's submitted to him by 
>the Parliament.  He cannot add people to this list.
Sort of the same in Germany. But the difference to the US (just like in
Israel): The president is not the government (we have a Chancelor, Israel
has the Prime Minister etc.) Our President (with the right to pardon) is
more like the Queen, mostly with representational (and humanistic) duties.
>This was indeed a unique and unprecedented occurence in the US 
>history, having a president who was not elected by the people.  As I 
>said, no law can predicted any eventuality.
Easy: The president has the right to pardon .... . This right is not valid
for any member of his or a former government. The right to pardon a member
of [former] government[s] stays with the Congress with 2/3 of all votes.
>>Thus we deserve the best woman and men governing
>>us, not the most ignorant, self esteemed, TV-popular ones. And to make
>>that happen, we should create a system narrowing down on that, not on
some
>>idealistic misconceptions....
>
>I can't say that I disagree.  But, it all gets back to the point of 
>"who'll select these people?"  If not the public then who?  Some 
>paternalistic body of people who "know what's good for us"?  Can't 
>think about any such body that I would trust.  
How is the head of MIT or Harvard or UofChicago being chosen? By the
students???? Or are those who choose the head elected by the students?
And lets again ponder upon the idea of immunizing people against
dictatorship, fascism and the like. There is a book (I think an american
one) how a teacher made an experiment with his class [The Bridge???]. He
put up a certain group ideology, garnished it with a certain outfit,
songs, group rules etc. He of course was the leader and the pupils
followed willingly. No outsiders were allowed, etc.pp. Then he showed them
how near that all was to fascism... It's a fascinating story of how easy
people can be mislead. I'm convinced, that non of the participating pupils
can ever be mislead by another "Pied Piper of Hamelin". How about putting
such an experiment into the regular curriculae?
BTW: I wonder when someone will come up with the idea that this is not
physics nor science at all and ball us out =;-)....
Cheerio
The most dangerous untruths are truths slightly deformed.
Lichtenberg, Sudelbuecher
__________________________________
Lorenz Borsche
Per the FCA: this eMail adress is not to 
be added to any commercial mailing list