Warp wrote:
>   But if you observe it going only through one of the slits, the interference
> pattern doesn't appear, IIRC.

Correct.

>   Wouldn't that be kind of evidence that when the interference pattern appears,
> it did to through both slits? 

Not really, no.

> When someone forces it to go through only one of
> the slits (by observing it) the interference pattern disappears.

That would only be true if the observation occurs before the particle goes 
through the slits. If it *really* goes through both slits, wouldn't it have 
to do so *before* it gets to the detectors on the screen side of the slits?

The whole "quantum eraser" thing is designed to prove you can measure which 
slit it went through after it has already hit the screen, and you still 
erase the interference pattern.  If the interference pattern was based on 
the particle turning into a wave and going through both slits, that wouldn't 
work.

>> You basically never see a wave or measure a 
>> wave. You always measure a particle, even as the whatever goes through the 
>> slits, even *after* the whatever goes through the slits.
> 
>   But does the interference pattern remain if the particle is measured?

No. The "interference" is caused not by the particle, but by the slits.

> If it does, then *that* would be indicative that the pattern is not
> appearing because the photon behaved like a wave.

Correct. But the pattern goes away even if you measure the path of some 
*other* particle *after* the particle that went thru the slits has already 
been measured. Hence, it's not a property of the particle/wave by itself.

>   Contradicted how? "We forced the photon to pass through only one of the
> slits and what do you know, the interference pattern disappeared." That
> would be confirming evidence, not contradicting one.

Contradicted by the fact that the pattern also disappears if you measure 
which slit some other particle went through *after* the original particle 
has already been measured. In other words, it also disappears if you decide 
whether or not to look which slit it went through *after* it has already 
been detected.

You have your choice of the particle looking at your experimental setup and 
telling itself backwards in time whether to go thru one slit or both, or 
just accepting that the particle never goes through both slits.

What you're doing is saying that the medical screening procedure *causes* 
cancer, because every time you do the blood test and it comes up positive, 
the person has a higher chance of being sick.

>   Explaining the reason why the interference pattern appears even though
> the photon does not behave like a wave would help.

http://vega.org.uk/video/subseries/8

http://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/0691024170

I'm trying to. You get an interference pattern because the probability for a 
particle to go somewhere is based on 2D numbers that when you multiply them 
wind up "interfering" with each other. But there's no wave there.

Watch this, which is Feynman answering exactly your question:

http://www.youtube.com/watch?v=_7OEzyEfzgg

>   I don't think you can talk about amplitudes after claiming so firmly that
> photons are not waves.

As I said, "amplitude" is a technical term that means a two-dimensional 
probability. It's not talking about the distance from the trough to the 
crest of a wave. You can replace it with any word you want, but you won't 
understand what quantum mechanics are talking about if you don't know what 
the word means.

-- 
Darren New, San Diego CA, USA (PST)
   Human nature dictates that toothpaste tubes spend
   much longer being almost empty than almost full.

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