Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Warp wrote:
>>> Darren New <dne### [at] sanrrcom> wrote:
>>>> No. The "interference" is caused not by the particle, but by the slits.
>>>   How does the photon know there was another slit nearby, and behave
>>> differently if that was so?
> 
>> Nobody knows.
> 
>   But it seems that everybody knows that it does not happen by the photon
> going through both slits?

Correct.  Or, rather, nobody has ever measured anything that would imply the 
photon goes through both slits as a wave. Nobody has ever measured the 
actual wave-like nature of photons, other than that the probability you 
measure one of those particles at a particular place shares much of the same 
math as waves. (The probability that *two* photons do something does *not* 
share the same math as waves.)

The photon, in some very rare cases, might go through both slits, but not at 
the same time as a wave. Instead, it might actually go in a circle and go 
through both. But it's vanishingly improbable, and maybe even impossible if 
the probabilities actually add up to zero.

Essentially, every single time you find a photon, you find an entire photon. 
There's never a case where you get a half a photon. Hence the "quantum" 
nature of it all. You don't get half a photon just like you never ever find 
an electron with half a charge, and just like you don't find a coin that 
comes up half heads half tails. The only "interference" you get is patterns 
when you measure where lots of photons go.

>   (After all, it would make sense: If there's no other slit to go through,
> then it can't go through two slits, and thus its behavior is different than
> if there was another slit and it went through both.)

But it only goes through one slit if you measure which slit it went thru. 
This is true even *if* you make the measurement *after* the slits. You never 
see the photon going through both slits, like you would if it were actually 
a wave.

>   Or is this again that kind of almost-supernatural phenomena like the
> photon somehow knowing that one year from now it will be measured which
> slit it went through and thus it will already know to not to produce the
> interference pattern?

Again, it's not a question of one photon making an interference pattern. 
It's a bunch of photons making an interference pattern. Any one photon shows 
up all at once and makes a single discrete mark wherever it hits. It's just 
more likely to hit in one area than another.

It's sort of like saying that at a sufficiently small scale, there aren't 
waves in water either. If there's a "high" wave, you're more likely to see a 
molecule of H2O high above the average surface level than if there's a 
trough. You can calculate the probability of H2O molecules being in a 
particular place, but you're never going to see an H2O molecule interfering 
with itself. (At least, not in the "classic" size of water waves.) You can 
ask "how does the beach get more wet near the waterline than higher up if 
water isn't waves?" It's because you're more likely to find an H2O molecule 
close to the waterline than higher up the beach. But that doesn't mean H2O 
is made out of waves.

But in short, yes, it's the same weirdness, which is how people know it's 
not waves per se. Being a wave would imply it's actually the photon itself 
that's controlling where it goes. But we already know you can control where 
the photon goes retroactively, which would obviously make no sense if it was 
deciding based on being a wave. If it's interfering *because* it is going 
through both slits, you wouldn't be able to change whether it went thru both 
slits or not after it already hit the detector. There's something deeply 
unintuitive going on that nobody understands.

BTW, it's also pretty easy to prove it's not something in the photon that we 
can't measure controlling how it works. That's what Bell's Inequality is about.

-- 
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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