Isn't the classic answer the double slit experiment?

Even if there are so few photons in the experiment, if you wait long enough, you can see the standard interfence bands formed by the photons going through both slits. So one photon goes through both slits at the same time.

But the moment an intelligent observer determines which slit the photon actually went through, the fringes disappear and you just get two lines, one for each slit.

re the answer by bb2193. Thank you. But this only repeats what we are always told. All we know is that if we try to "see" the electron going through the slit it destroys the interference effect. Perhaps it just stops the electron going through. No reason to postulate "collapse of the wave function": "superimposition": "an inteligent observer"  - (at the evening class in quantum mechanics I attended at Edinburgh University, the lecturer reckoned observation by a chicken would be sufficient).

What, if any, is the demonstration that the "Observation by an intelligence"  is crucial ?

Still hoping - the questioner.

I believe that Schrödinger's thought experiment with the cat is widely misinterpreted. The issue is with the level of intelligence that observes the probability collapse.

In our species-chauvinist fashion, we assert that the quantum state is resolved when a human observation is made.

However dumb the cat is (and my cat is definitely smarter than me - it is dictating this now), it can presumably distinguish between being alive or dead. So the quantum state should be resolved by the cat, not the person. Schrödinger was illustrating that the Copenhagen Interpretation must be false, because it relies on a subjective definition of the quality of the required observation.

Personally, I feel any self-respecting cat would promptly pop into the wardrobe via the fourth dimension for a nap, leaving the observer appropriately Zen-baffled on opening the box. (What is the sound of no cat napping?)

I have a small demonstration that I use on people who make dubious assertions about probability.

First, I toss a coin, palm it, and pretend I am covering it up in the usual fashion. Most people readily agree there is a 50% chance it is heads, although a fair number agree the chance is 50% that it is heads or tails (I would argue that is 100% certain normally, although 0% in the present situation).

I then take a peek, and tell them it is heads. And then they agree the probability of heads is now 100%, and I suggest we must play poker sometime.

Eventually they agree that the 50% chance of the toss has become a 50% chance I am lying to them. This is wrong because I always tell them the truth when I tell them I am lying. That's usually a good time to reveal that there is no coin anyway.

Further to the discussion about the role of the experimenter in the collapse of the probability wave function, this puts Schrodinger's cat firmly among Heisenberg's pigeons.

http://en.wikipedia.org/wiki/Free_will_theorem

I would very much like someone who has access to the necessary equipment to perform the following suggested experiment or let me know if this has already been done and what the results were.

I should apologise in advance for my lack of proper scientific terminology as I have had no formal physics training other than the thought experiments I have posed to myself and reading a few books on physics and astronomy and articles online.

Firstly, I understand that when light is shone through two slits it causes a pattern of light and dark on the screen behind thus suggesting that light is a wave.  

Secondly,  when the rate at which the light is emitted is slowed down so it can only be a photon(?) at a time there is still an interference pattern.  Suggesting the photon goes through both slits at the same time and somehow interferes with itself….giving birth to quantum physics and a whole lot of outlandish theories (I have an even more “outlandish” theory myself)

Finally, it gets even stranger, when any kind of device is put in place to measure or count which slit each light particle is going through, the pattern changes to diffuse. At last being what you would expect to see from individual particles. 

The strange part is, of course is how can just the act of observing cause a diffuse pattern. If the measuring device is switched off it goes back to bands of light and dark. 

I hope I have this reasonably correct so far ?

SO…..what would happen if the device that measured which slit photons went through was powered by some kind of voltaic cell that was placed where the light bands are. 

That is to say only when the light was making the banded pattern would the power be sufficient to switch on the device measuring which slit the light particles went through….which will then make the pattern diffuse….which will switch off the machine…….which will then make the pattern banded……which will switch on the machine…..ad infinitum. 

This seems to cause the same paradox as Schroedinger’s Cat but a lot easier to test and more animal friendly…..a bit of Hawkins style humour there…haha. 

If this is the first time someone has suggested this and it does prove useful in any way I would very much like to make my parents proud and have this referred to as the “McTavish Twist”.   The first part being my surname and “twist” because I love twists in both science and cocktails !!!  Both are refreshing and alleviate the boredom..

One suggested result is that the measuring device switches on and off at the speed of its slowest component. If there was nothing slowing down its rate of switching then it would be very interesting to measure the number of times it changes state per second ?  Although I imagine this may take some clever machinery indeed as the resulting figure should be the smallest possible measure of time.  A “universal tick” ?  (maybe we can call it a “McTavish Tick” or am I getting greedy !!).  I imagine this should link mathematically to the maximum speed of light in some way.

Another possible outcome is that the machine will not switch on even though the light level appears sufficient to power it.  This could happen because it is only really at half power…..being half in the diffuse pattern (that is to say half in another state/universe).  It may be interesting to note where the “tipping point” is ?  Does it really work out to be exactly one half or does it lean more towards one state ?  Again this ratio may link mathematically to another well known number.

I imagine the actual results of the experiment will lead as usual to more questions.  I would very much like confirmation of the results so my own thought experiments move on to new questions too. 

Thank you sincerely for your attention.

Ian McTavish