A quantum experiment by Feynman:

He did a double slit experiment a long time ago where detectors were placed by the double slits and found if the detectors were active, the wave function collapsed to make particles, a different pattern than a wave. There is a sci fi novella about that in the latest issue of Amazing sci fi mag. The gist of that story is using animals to try to make the wave function collapse and in the story only humans can do that. Does anyone know of a real experiment of that kind? You know, where observing the outcome of a quantum measurement changes the measurement, but does it happen if the observer is a parrot or duck or dog or some such. Has this test been done for real?

Here is a link to the actual feynman work:
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DoubleSlit/DoubleSlit.html

Originally posted by sonhouse
He did a double slit experiment a long time ago where detectors were placed by the double slits and found if the detectors were active, the wave function collapsed to make particles, a different pattern than a wave. There is a sci fi novella about that in the latest issue of Amazing sci fi mag. The gist of that story is using animals to try to make the wave ...[text shortened]... eynman work:
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DoubleSlit/DoubleSlit.html

I have always been very suspicious one of the main interpretations of quantum physics that basically says something does not “exist” unless it is “observed” (and therefore there is no objective reality!) because, if that was true, then who observed the first “observer”? What constitutes an “observer” is not something that is logically defined in physics and I see that fact as a fundamental flaw in that interpretation.

I see no reason why a parrot can be an observer in the slit experiment or, indeed, a much simpler life form. Why can’t a singled-celled amoeba be the “observer” in the sense it can respond either one way or in another way according to the result of the slit experiment? Or how about a robot or even a dumb computer? And if all these things can “observe” then why can’t a single molecule “observe”? -I see no reason why not thus I conclude that all what is required for something to “observe” in this context is to interact -that is all! -in which case, the words “observe” and “observer” are totally inappropriate words to use because, in this context, something devoid of intellect can be the “observer” (and therefore there can be an objective reality! -if I am correct). “interaction” and the “interacter” (I.e. the thing that interacts with something -this is a word I just made up for convenience!) would be more appropriate words.

Originally posted by sonhouse
He did a double slit experiment a long time ago where detectors were placed by the double slits and found if the detectors were active, the wave function collapsed to make particles, a different pattern than a wave. There is a sci fi novella about that in the latest issue of Amazing sci fi mag. The gist of that story is using animals to try to make the wave ...[text shortened]... eynman work:
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DoubleSlit/DoubleSlit.html

I thought much of quantum theory is based on the idea that the very act of measuring or observing the thing had a fundamental effect on the result.

Originally posted by Andrew Hamilton
I have always been very suspicious one of the main interpretations of quantum physics that basically says something does not “exist” unless it is “observed” (and therefore there is no objective reality!) because, if that was true, then who observed the first “observer”? What constitutes an “observer” is not something that is logically defined in physics and I see that fact as a fundamental flaw in that interpretation.

Isn't this idea just an extension of the branch breaking in the forest question? I presume you are happy with the idea of there being no sound if nobody is around to hear it.

Can't existence itself merely be an interpretation of something else in the same way that sound is an interpretation of moving air?

Originally posted by sonhouse
He did a double slit experiment a long time ago where detectors were placed by the double slits and found if the detectors were active, the wave function collapsed to make particles, a different pattern than a wave. There is a sci fi novella about that in the latest issue of Amazing sci fi mag. The gist of that story is using animals to try to make the wave ...[text shortened]... eynman work:
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DoubleSlit/DoubleSlit.html

This is a somewhat confusing issue - that often seems more difficult then it is.

As you correctly pointed out, an electron can be thourgh of as both a particle and a wave. The wavefunction of the electron is dependant on both time and space (Ie: the value of the wavefunction can change with time AND every different point in space).

The modulas squared of the wavefunction (if Y is the wavefuntion then by modulas squared I mean |Y|^2) gives the probability of finding the electron at any given value of time and position.

The problem of the observer is more simply then it first appears. All it means is that we have no way of predicting which slit the electron will go through and the only way to know for certain which slit it passes through is to observed it.

If a single electron is fired at the slits and the outcome is not observed then we have no way to knowing which slit it went through - effectivly all we can say is the probability that it went through either slit, which is determined by the wavefunction of the electron, which is determined by the position of the slits and the trajectory of the electron. If we observe the electron (using a detector of some sort as we cant actually see them) then we know for certain which slit it used.

This is what is meant by "the wavefunction collaspes when it is observed".

In short, the same process happens whether it is observed or not, us observing us does not change how it happens. But the idea of collapsing a wavefunction just means that if we happen to observe it we know for certain which slit it used.

I dont think I have explained this very well at all, feel free to ask for clarification.

PS: Another interesting quantum phenomenom is Quantum Uncertainty. Where if we observed one property of a quatum object it effects its other properties. Also, if we observe some quantities it prevents us from observing others. eg: if you observed the position of an electron exactly you can obtain no information at all about its momentum (thus have no idea how fast it is going).

When I drive my car, I usually don't bother to know what speed I have. But when I feel that a police is observing me, I always drive in a legal speed.

So, observation changes my speed.

Originally posted by FabianFnas
When I drive my car, I usually don't bother to know what speed I have. But when I feel that a police is observing me, I always drive in a legal speed.

So, observation changes my speed.

When you know what speed you are going, do you know where you are or do you suddenly get lost?

Originally posted by Wheely
Isn't this idea just an extension of the branch breaking in the forest question? I presume you are happy with the idea of there being no sound if nobody is around to hear it.

Can't existence itself merely be an interpretation of something else in the same way that sound is an interpretation of moving air?

…I presume you are happy with the idea of there being no sound if nobody is around to hear it…

Err, no! I was saying the exact opposite! Read what I said again and you should be able to deduce that I actually would believe that sound can exist even if nobody is around to hear it. That’s because I actually disagree with the interpretation of quantum mechanics that says things can only exist when they are observed.

Originally posted by MattP
This is a somewhat confusing issue - that often seems more difficult then it is.

As you correctly pointed out, an electron can be thourgh of as both a particle and a wave. The wavefunction of the electron is dependant on both time and space (Ie: the value of the wavefunction can change with time AND every different point in space).

The modulas squared o ...[text shortened]... can obtain no information at all about its momentum (thus have no idea how fast it is going).

The "waveform collapses when observed" thing comes form the fact that even if only ONE electron is sent though the double slits, it produces a wave pattern.

How does a single electron produce a wave patten? That's why they try to observe which slit it passes through, but once they've done that, there is no longer a wave pattern.

Originally posted by forkedknight
The "waveform collapses when observed" thing comes form the fact that even if only ONE electron is sent though the double slits, it produces a wave pattern.

How does a single electron produce a wave patten? That's why they try to observe which slit it passes through, but once they've done that, there is no longer a wave pattern.

The wavefunction gives the probability of where the particle is (via the modulas squared). Imaging there is a screen behind the slits and wherever and electron hits the screen a dot appears (say 1mm in diameter so it is easily seen with the eye), the dots are permenant so do not fade.

If electrons are fired at the slits one and a time (ie: with only one electron "in flight" at any given time, so a single electron is fired then the next electron is only fired after the first one has hit the screen) then over time a pattern will built up on the screen.

You are correct that indervidual electrons behave as waves, it is not an effect which needs multiple electrons interfering with each other. Each indervidual electron has a probability that it will hit the screen at any given point, this probability is found by considering the modulas squared of the wavefunction at that given point.

If lots of electron as fired then the resulting spot pattern is a way of visualising the modulas squared of the wavefunction, as the density of spots is proportional to the wavefunction magnitude.

This is why electrons are said to behave both as a wave and a particle. It is an example of wave-particle duality - where something has the properties of both a wave and a particle. Another common example is light; light behaves as a wave, but it is also made up of quanta called photons, and the quantised nature of light has been varified by many experiments.

Coming back to the topic of this thread. The electron has a wavefunction, but upon measuring the properties of the electron its values are determined, and so the wavefunction collapses to give particle-like behavour. In otherwords, if you dont know where the electron is you can say the probability of where it could be, but if you watch where it is you know for certain so there are no other possibilities.

This phenomenom is better understood by considering more quantum properties as well as position. There are lots of interdependant quantum numbers,( like total angular momentum and the 3 orthoganal angular momentum components). In many cases if you observe on property, it limits the values that other properties can take, as you know the values of one property and you know how the others behave on it. In this way, once you know a property by observing it the wavefunction is collapsed, as there are now less possible states the particle is in. However, there are also properties which you cannot know at the same time, (like position and momentum, time and energy etc...) due to quantum uncertainty.

There is a rival theory to the genrally accepted QM theory. It involves "quantum fields" which quide particles. So an electron would be a particle which floats in a quantum field, the flow lines of the field would be related to the wavefunction of the electron in normal QM, so the behavour of the electron is the same as in normal QM. This is perhaps as way to "think round" the problem of wave-particle duality, - particles are still particles but they are quided by waves.

I do not know the details of this theory tho, so it could have some massive flaws in it. The small amount I have read about it suggests it gives the same behavour as standard QM, but again, I do not know enougth about it to say for sure how it compares.

Originally posted by Andrew Hamilton
[b]…I presume you are happy with the idea of there being no sound if nobody is around to hear it…

Err, no! I was saying the exact opposite! Read what I said again and you should be able to deduce that I actually would believe that sound can exist even if nobody is around to hear it. That’s because I actually disagree with the interpretation of quantum mechanics that says things can only exist when they are observed.[/b]

I am surprised you believe that. Of course that may come down to purely how we define sound.

For me, in that forest, when the branch breaks, there becomes a potential for sound as the air is disturbed in a particular way. If no cilia attached via nerves to a brain wiggle about in response to that air movement and no brain turns it into a "thing" in your head, then there´s no sound.

Maybe you define sound as the movement of the air itself but for me that´s just a wave.

Originally posted by Wheely
When you know what speed you are going, do you know where you are or do you suddenly get lost?

I often ends up in Heissenberg...

Sound is the disturbance of the air, and it exists independent of anyone being around to hear it.

Originally posted by googlefudge
Sound is the disturbance of the air, and it exists independent of anyone being around to hear it.

I disagree. Sound is a construct of your head. Air being disturbed is just that, air being disturbed. If you were deaf air could be disturbed as much as you like but you´d hear no sound.

Originally posted by Wheely
I disagree. Sound is a construct of your head. Air being disturbed is just that, air being disturbed. If you were deaf air could be disturbed as much as you like but you´d hear no sound.

This question has always intrigued me. And I always find myself changing which side I'm on I do have a question though: Why doesn't the same theory apply to the other senses as well. If a star goes nova, we don't see the results until the light travels millions of light years to get to us, right? But none of us were around to see it, when it occurred (and I assume we can determine our "date" for the actual explosion) so what happened? No one was there, so light wasn't "disturbed until millions of years later, when there were optic nerves to witness it?