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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On 9/16/2011 11:23, Warp wrote:
> Alain<aze### [at] qwertyorg>  wrote:
>> For a footbal, it would be in the 10e-20 to 10e-30 m range. So, your
>> interference pattern would be smaller than a proton.
>
>    That starts being awfully close to the planck length.

I guess if 15 orders of magnitude is "awfully close"... :-)

I do wonder how big something would have to be for the wavelength to be 
close to the planck length.  It would be cool if that was, like, close to 
the size of the observable universe.

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

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Darren New <dne### [at] sanrrcom> wrote:
> It's not a question of emitting particles or not. It's a question of whether 
> you *measure* whether they're emitted.

  Those particles are going to hit something. It's very unlikely that they
will go to the end of the universe without hitting anything.

  Now, how does the football (or atom, or particle) "know" that what those
emitted particles hit is not "measurement"? Does it make a conscious
decision like "yeah, these particles are not being measured by any conscious
mind, nor any device which would record the difference, so it's safe for
me to do the interference dance"? How does the traveling object know the
difference between those emitted particles hitting a rock or hitting a
measurement device that can tell the difference?

  Or is it the *interaction* between the traveling object and the particles
that hit it (or which it emits), particles that *could* be used to measure
which slit the object went through, that makes the difference?

-- 
                                                          - Warp

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Am 14.09.2011 20:46, schrieb Alain:
> Le 2011/09/14 04:44, Invisible a écrit :
>>>> It ALWAYS make a _sound_.
>>>
>>> How do you know?
>>
>> If Schroeder's cat dies and no one observes it, is it still dead?
>
> Shroeder's cat is always both dead and alive untill someone does observe
> it. Then, the cat is ether alive or dead.

http://cheezburger.com/View/5209969152

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> Alain<aze### [at] qwertyorg>  wrote:
>> For a footbal, it would be in the 10e-20 to 10e-30 m range. So, your
>> interference pattern would be smaller than a proton.
>
>    That starts being awfully close to the planck length.
>

We ended up with much smaller values:
A person, around 10e-50.
A medium car, around 10e-75.
The Earth, in the 10e-200 range.

I don't remember the formula and don't think that I still have it, after 
over 30 years ;)

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Le 2011/09/16 16:31, Darren New a écrit :
> On 9/16/2011 11:23, Warp wrote:
>> Alain<aze### [at] qwertyorg> wrote:
>>> For a footbal, it would be in the 10e-20 to 10e-30 m range. So, your
>>> interference pattern would be smaller than a proton.
>>
>> That starts being awfully close to the planck length.
>
> I guess if 15 orders of magnitude is "awfully close"... :-)
>
> I do wonder how big something would have to be for the wavelength to be
> close to the planck length. It would be cool if that was, like, close to
> the size of the observable universe.
>

Not even close, the Earth fit the bill, if I remember corectly.
The Sun would be a few order of magnitude smaller.

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Alain <aze### [at] qwertyorg> wrote:
> Le 2011/09/16 14:23, Warp a écrit :
> > Alain<aze### [at] qwertyorg>  wrote:
> >> For a footbal, it would be in the 10e-20 to 10e-30 m range. So, your
> >> interference pattern would be smaller than a proton.
> >
> >    That starts being awfully close to the planck length.
> >

> We ended up with much smaller values:
> A person, around 10e-50.
> A medium car, around 10e-75.
> The Earth, in the 10e-200 range.

> I don't remember the formula and don't think that I still have it, after 
> over 30 years ;)

  So if the interference pattern of an object the size of a person is
15 orders of magnitude smaller than the planck length, does that mean
that there is no interference pattern, and hence no interference?

-- 
                                                          - Warp

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On 9/16/2011 14:56, Warp wrote:
>    So if the interference pattern of an object the size of a person is
> 15 orders of magnitude smaller than the planck length, does that mean
> that there is no interference pattern, and hence no interference?

Only if a tree in the forest makes a noise if nobody is there to hear it. ;-)

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

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On 9/16/2011 14:11, Warp wrote:
>    Now, how does the football (or atom, or particle) "know" that what those
> emitted particles hit is not "measurement"?

Well, that is indeed the fundamental problem exposed by Schrodinger's cat.

> measurement device that can tell the difference?

It doesn't matter if it hits a measuring device. It only matters if you look 
at the measurement.

>    Or is it the *interaction* between the traveling object and the particles
> that hit it (or which it emits), particles that *could* be used to measure
> which slit the object went through, that makes the difference?

No, because you can take the measurement as to which way it went, *then* 
erase that measurement after the particle has already hit the sensor, and 
you will or won't get interference depending on whether you don't or do 
erase the measurement after the fact.

https://secure.wikimedia.org/wikipedia/en/wiki/Quantum_erasure

https://secure.wikimedia.org/wikipedia/en/wiki/Delayed_choice_quantum_eraser

It's not a question of whether it goes thru one slit or two, or whether you 
interfere with it or not at that time. It's (weirdly enough) a question of 
whether you know what the answer was supposed to be.

Remember, tho, that each particle lands in exactly one place. One particle 
doesn't give an interference pattern like one wave would. Each particle in 
the two-slit experiment is just more or less likely to hit different places 
on the screen, but each hits at only one place.

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

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Darren New <dne### [at] sanrrcom> wrote:
> It doesn't matter if it hits a measuring device. It only matters if you look 
> at the measurement.

  And how does the particle know that the measurement was looked at?

> >    Or is it the *interaction* between the traveling object and the particles
> > that hit it (or which it emits), particles that *could* be used to measure
> > which slit the object went through, that makes the difference?

> No, because you can take the measurement as to which way it went, *then* 
> erase that measurement after the particle has already hit the sensor, and 
> you will or won't get interference depending on whether you don't or do 
> erase the measurement after the fact.

  But that has nothing to do with whether someone "looked" at the measurement
or not. It has to do with whether the two possible paths of the emitted
particle were kept separate or whether they were merged before the particle
hit the measurement device. If I understand correctly, the interference
pattern would disappear if the emitted particles are kept separate even if
nobody "looks" at the result. It has nothing to do with an observer, only
with how the original particles and the emitted particles interact.

  While at macroscopic levels it's hard to understand how particles affect
each other from a significant distance (and even time) this way, it kind of
makes sense, if you imagine that the particles are somehow "bound" together
even though they are in their own separate paths: This way what happens to
one particle affects what happens to the other. If the path of the
"measurement" particle is merged, it affects the original particle (even
through time, via some quantum oddity), and if it's not merged, it also
likewise affects it.

  However, what does not make sense even in this context is that the particle
somehow "knows" whether someone "looked" or not.

-- 
                                                          - Warp

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