On 9/16/2011 10:33, Warp wrote:
>    The diffraction pattern isn't there even with elemental particles if you
> measure which slit the particles go through.

It's a little more complicated than that, really.

>    How do you set up a situation where it's not possible to tell which slit
> the football went through? I don't think it's physically possible.

The same way you do it with electrons or photons or whatever. You don't look.

>    You could perhaps try having the experiment in an absolute vacuum (something
> which is already very hard), and in an environment with no electromagnetic
> radiation of any kind, that could hit the ball and tell its trajectory
> (maybe it would be theoretically possible, but I'm not sure it is in
> practice). Also anything else that could hit the ball and hence possibly
> tell its trajectory (eg. neutrinos and cosmic rays) would have to be
> completely absent.

Not really. You get the diffraction/interference pattern even with photons 
going through air, so there's no reason to expect a football going through 
air is going to be more affected.

>    In fact, thinking about it. would the ball and the walls themselves emit
> photons that could tell the trajectory? Are they black body radiators?
> I suppose that if that's the case, the ball and the walls would have to be
> cooled to absolute zero to stop them from emitting any particles.

It's not a question of emitting particles or not. It's a question of whether 
you *measure* whether they're emitted.

>    If yes, what explains the deviation in the trajectory of the ball?

That's just how the quantum world works. The same explanation you give for 
photons or electrons applies to footballs, as far as I know (or as far as 
anyone knows, last I heard).

> One of the fundamental interactions (gravity, electromagnetism, strong
> interaction, weak interaction)?

Well, for the football, it would be quantum electrodynamics, which is the 
interaction of photons with electrons.

> How can they deviate the ball so much?

Remember that the diffraction pattern is going to be on the same scale as 
the wavelength of the football. The wavelength of the football is 
exceedingly tiny, quite possibly smaller than a proton, for example. So it 
doesn't take a large deviation.

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

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