Beyond Schrodinger's Cat - a Television Appearance

[Qfwfq]

When we watch televison, what appears on the screen is a representation of an event, not the event itself. What is being transmitted is information. Naturally the waves by which it travels makes use of the existing energy and matter between the transmitter and the receiver. However it is not the original energy and matter that makes the complete journey. The pixels on every tv screen are composed of matter and energy that already exist at each destination, and change their form according to the information received. Such is the wavicle nature of tv and radio transmission.

This is exactly why the cat is being observed, even if nobody is watching any of the screens, or even if they're all switched off, even if the cameras are switched off but there's enough light inside the box to make the cat visible. You can't see a dead cat on one screen and a live one on the other, regardless of how you designed the camera and equipment.

 

To use a close analogy: when a tidal wave heads towards the coast, is it a single wall of water in motion? No. The water that finally breaks on the coast is not the same water that overturned a ship 30 miles away. So what is it that moves? You might call it an 'information wave', changing the form of the existing water as it travels, but it is not the water itself that is moving.

I'm saying there may be a difference between a random event directly observed and information about a random event sent via wave transmissions - transmissions that themselves depend on quantum uncertainty.

This isn't a close analogy at all, only a weak one. Apart from the fact that the wave wouldn't overturn a ship so far from the coast, it isn't the same water but it's the same energy that's propagating. Not so when you amplify and process an electronic signal or when you convert it between optical and electronic.

 

As far as I'm aware, no experiment has been performed in which a quantum event has been strategically placed in the proximity of a tv or radio transmitter.

What's so magical about a tv or radio transmitter? What's wrong with scintillators, streamers, photomultiplier tubes, bubble or cloud chambers and potographic emulsions?

 

A precise measurement with a photon detector does this in the twin slit experiment. Without the detector, the human eye cannot discern which slit the particle entered - so for all intents and purposes the event remains unobserved. What is seen, however, is the interference pattern. You might call this a wave transmission of the two possible paths the photon could have taken. As we also know, this transmission stops the moment a precise measurement is made.

Any circumstance that allows distinguishing which slit the thing went through disturbs the interference pattern.

 

But here is the most important point: the collapse of a wave function - like the wave function itself - does not appear to be a single event in space time, but actually seems to depend on the point of view of an observer (conscious or otherwise). If I'm not mistaken, it's analogous to the way Einstein regarded time - as existing differently, relative to each perspective. A wave function may have collapsed to one state from a given point of view, but can still exist in relation to another observer who just sees the interference pattern.

It doesn't matter which observer. There is the quite different matter of which observable it's possible to know the value of.

 

There have been a number of variations of the twin slit experiment. Interestingly, you don't need the particle necessarilly to be detected in order to reduce the wave function. If you place your particle detector at Slit A, there is a 50/50 chance that you will get a reading if the particle entered Slit A. If it entered Slit B, no measurement will occur. However, no interference will occur either, because the probability that the particle entered Slit B is now almost 100%. So here we have curious situation in which the mere presence of a particle detector eliminates the interference, regardless of whether it detected the particle or not. It's potential to detect a particle, even if it didn't, was sufficient to reduce the wave function.

Quite simply because the detector can't work without at the very least destroying the coherence between the two possiblities. This takes all the mystery away, gone is the magic and the poetry. Find a sure way of determining which slit it went through that doesn't change the amplitude and phase and we're in trouble, we get the paradox.

 

In the case of being able to tell whether a cat is dead or alive, a photon detector seems slightly superfluous. The human eye ought to be sufficient to collapse any wave function that occurs. But now of course we're on a different scale. What precise level of interaction or observation is required? I don't know. We have yet to witness a quantum interference pattern that represents two possible events in the macro world.

Show me a material that can be superconducting at 3000K and I might begin to believe there's some chance that a cat just might be in a superposition of two eigenstates.

 

I think I won't give quite all the :cup: to Pyro, I'll have some of it myself. :)

[Pyrotex]

This is exactly why the cat is being observed, even if nobody is watching any of the screens, or even if they're all switched off, even if the cameras are switched off but there's enough light inside the box to make the cat visible....I think I won't give quite all the :eswirl: to Pyro, I'll have some of it myself. :warped:

That's okay, 'Quaff' -- I think I've had enough [burp!].

 

Schroedinger's Cat is one of those subjects in Physics that really bother me. I can see the logic of all sides here, but trying to pin the solution on "observation" or "intelligence" or "how many brain cells does it take to collapse a wave function?" all seem rather scatter-brained to me.

 

I am tempted to push all of them off the table and start over. :shrug:

[sebbysteiny]

As of this writing, the multiverse seems to be the only model of quantum theory that allows any reality to exist independently of observation/interaction.

 

 

 

Wow, I never thought the 'multiple universe' explanation, widely considered one of the worst scientific arguments ever, would ever come in handy for anything.

 

Really? According to a recent poll, 58% of leading quantum theorists now consider the multiverse view to be the most useful model - among them Steqhen Hawking. Of course this doesn't prove it is correct.

 

Followed by reasonably accurate explanation of the multiverse model.

 

The problem with the multiverse model is that 1) it can't be proven, and 2) it makes no unique predictions. Technically, 1 and 2 are both the same problem but it is such a big problem that it is worth mentioning it twice. This makes it a very bad scientific theory, and probably one of the worst that has ever for some strange reason been granted mainstream scientific 'acceptance' [in that it is explained in most scientific text books].

 

There are other ways of viewing the physics without resorting to an almost infinate number of universes whose existance we can never be aware of.

 

The only use of the multiverse theory is as an analogy to help explain the basics of quantum mechanics to people exposed to it for the first time.

 

I find it highly doubtful that the multiverse theory got such a large appeal. I strongly suspect that it was Quantum Mechanics that got the vote and not the little part that wonders into the idea that multiple universes exist that will never effect us and can never be detected.

 

We can agree that direct observation of an event means there is sufficient interaction between two systems for a wave function to collapse.

 

Well obviously. A nuclear bomb is sufficiently powerful to crack a nut.

 

If you want to know the very minimum that collapses a wave function, look at PGRDave's post 15.

 

However, you are not relying exclusively on EM waves. Your eyes receive photons that originate from the event itself. The same is not true of TV transmissions.

 

When we watch televison, what appears on the screen is a representation of an event, not the event itself. What is being transmitted is information. Naturally the waves by which it travels makes use of the existing energy and matter between the transmitter and the receiver. However it is not the original energy and matter that makes the complete journey. The pixels on every tv screen are composed of matter and energy that already exist at each destination, and change their form according to the information received. Such is the wavicle nature of tv and radio transmission.

 

I agree, but I think that is still enough to collapse the wave function. A direct measurement has been made in the box and the information contained from that measurement is being communicated outside the box.

 

I'm saying there may be a difference between a random event directly observed and information about a random event sent via wave transmissions

 

What is the difference? Technically, a direct observation requires EM waves to hit cones. This sparks a chemical reaction. The results of that chemical reaction are detected by nerves. This is converted into electric signals that go through the nerves and enter part of the brain for processing. Once processed, the image now is converted back to electricity and sent to another part of the brain and so on until, right at the end, it is converted into an "observation". How much more indirect can you get?

 

The real answer is, again, post 15.

 

There is no reason for either of the transmissions to be rejected, because there is no single observation or interaction taking place.

 

Other than the detection of life signals by an instrument that sends signals straight outside the box.

 

does not appear to be a single event in space time, but actually seems to depend on the point of view of an observer (conscious or otherwise). If I'm not mistaken, it's analogous to the way Einstein regarded time - as existing differently, relative to each perspective. A wave function may have collapsed to one state from a given point of view, but can still exist in relation to another observer who just sees the interference pattern.

 

Coming to the main point I want to make. The problem here with the analogy is that there cannot be an observer living in wavefunction that has not collapsed according to all observers.

 

An observer, by definition has to be on a macro scopic size, and is certainly not on the quantum scale. Since *it is actually impossible to create a quantum state in a macroscopic scale* it is impossible to have an observer in a quantum supersition relative to another observer.

 

This is why the whole "alternitive word's" theory is not necessary. An electron does not feel or observe. As far as the electron is concerned, it's wave function contains some certain states and other uncertain states that are quantum superpositions of certain states. That is all the electron is. Measuring an uncertain state will change the electron by making a state that was otherwise certain uncertain. We don't need to say that an alternative universe is created when the electron is measured, merely that measurement actually changes the properties of a particle. And of course, by measurement, what we really mean is an interaction capable of distinguishing one state from another.

 

The only annoying thing is that we must accept that the actions of particles really is completely random and unpredictable leaving to the conclusion that so irratated Einstein to the point that he wasted about 10 years of his productive scientific career trying to disprove: that god plays dice.

 

Schroedinger's Cat is one of those subjects in Physics that really bother me. I can see the logic of all sides here, but trying to pin the solution on "observation" or "intelligence" or "how many brain cells does it take to collapse a wave function?" all seem rather scatter-brained to me.

 

Correct me if I'm wrong, but Schrodinger's cat is not scientifically valid *even as a thought experiment*. It's more celebrity science than real science.

[Simon]

You're not the only one to employ a pick-and-mix approach to quantum theory. By that I mean, you accept the observations of quantum mechanics but limit their implications to the sub-atomic level.

 

Also, you've been saying that even the minutest interaction/observation is always sufficient to reduce the wave function for every potential observer. From this, it follows that only when there is absolutely no interaction with anything can a given particle's wave function be preserved.

 

If this was really the case, there could never have been evidentiary basis for quantum theory at all. It would only be a mathematical construct. Twin-slit interference could never be witnessed. The observation screen itself would collapse the wave function.

 

Put simply: given that there is in fact discernible evidence of indeterminacy or superposition in anything, then by definition this proves that not every observation will eliminate it - for if it did there could be no evidence.

 

The valid question then becomes: what precise degree, quality or property of interaction is required to reduce the undetermined state of something to a determined state? This question lies behind my described experiment.

 

I would also say this. Given that there is evidence of indeterminacy, expressed in terms of the probabilty values of more than one outcome, it must be evidence of something that exists. If it was evidence of a non-existent reality - the 1930s Copenhagen view - again there would be no evidence.

 

So the next question is: what description of reality is consistent with evidence of an uncollapsed wave function?

 

I submit that the multiverse does qualify as such a description. Unlike David Deutch, I don't say that it has to be the correct one. However, I'm not aware of another model that attempts to describe any reality behind quantum theory.

 

In support of it, you could say it is compatible with Einstein's model of a fixed space-time in which events really do occur. The multiverse simply extends to a five dimensional space-time instead of four.

 

You may think the supposition of other universes in different states is a bizarre violation of Occam's razor. And yet anyone who already accepts Einstein's model of a four dimensional space-time in a sense already subsribes to this supposition. If you beleive in one universe that exists now, existed before and will exist in the future - then you are already describing non-identical universes in different states.

 

You could maintain that only the present universe was real. You could claim that it was scientifically unsound to take seriously the existence of past and future states of reality - because we're not directly aware of them. You could claim that the idea of different states of the universe existing in time should be considered, at best, a useful model for students studying the present moment - but they should not violate Occam's Razor by supposing that these countless hypothetical past and future states really exist. You could claim that there were no grounds to accept the existence of any reality except the present moment, but that there were plenty of competing theories to explain our memories and the phyisical evidence around us. You could claim that to make sense of the present did not require imagining a vast multitude of other moments, the existence of which was unproven and did not ultimaley tell us anything new about now.

 

:)

 

Simon

[Farsight]

And yet anyone who already accepts Einstein's model of a four dimensional space-time..

 

Sorry to butt in and nitpick, but that was Minkowski. Einstein was unhappy about it at the time.

 

You could maintain that only the present universe was real. You could claim that it was scientifically unsound to take seriously the existence of past and future states of reality - because we're not directly aware of them. You could claim that the idea of different states of the universe existing in time should be considered, at best, a useful model for students studying the present moment - but they should not violate Occam's Razor by supposing that these countless hypothetical past and future states really exist. You could claim that there were no grounds to accept the existence of any reality except the present moment, but that there were plenty of competing theories to explain our memories and the phyisical evidence around us. You could claim that to make sense of the present did not require imagining a vast multitude of other moments, the existence of which was unproven and did not ultimately tell us anything new about now.

 

Music to my ears.

 

I cringed at David Deutsche's QC multiverse lecture which starts off with a billiard-ball photon. It's like the guy has never heard of long-wave radio or the affect of beam-splitter position. Look mummy, this little here "electromagnetic shout" is in two places at once, wow, must be a multiverse.

[CraigD]

I agree with the several previous posters in this thread who claim that observing something via a television camera has the same effect on quantum superpositions that observing it with ones eyes does, so the experiment Simon proposes would result in either monitor B remaining blank, or both cameras showing the same image.

 

According to best present theory and experimental evidence, decoherence – the collapse of quantum wave functions – occurs whenever any information about a particle ensemble spreads outside that system. The role of vaguely defined ideas like “consciousness”, while the subject of much serious consideration in the early and middle decades of the study of quantum mechanics, appear to be disappearing from current thinking.

 

One reason are recent experiments in quantum computing, which are revealing both that quantum superpositions are possible, and how difficult it is to keep a particle ensemble from interacting with outside particles and losing coherence. Although no human (or even cat ;) ) intelligence is aware of this interaction, it nonetheless causes quantum computers to “break down” and cease to exhibit their desired “many states at once” behavior.

Correct me if I'm wrong, but Schrodinger's cat is not scientifically valid *even as a thought experiment*. It's more celebrity science than real science.

In light of these increased understandings, I’d judge sebby’s pronouncement correct. Modern theory predicts that a particle ensemble as complicated as a warm cat in a box interacts with itself far too much to maintain a state of quantum superposition, supporting in s sense a common objection raised by generations of science students to the Schrodinger's cat thought experiment – “wouldn’t the cat make some kind of measurement?”

 

On another level, it should be noted that, in all but some recent and very fringy variations of it, the many-worlds interpretation by definition predicts no experimentally observable phenomena that disagree with the Copenhagen interpretation. So, even if the Schrodinger’s cat experiment were not invalidated by current theory, neither it nor any other observable phenomena can be used to demonstrate one or the other of the Copenhagen or many-worlds interpretation correct. For this reason, these ideas are considered interpretation of quantum mechanics, not true theories, and are intended to organize thinking and promote discussion of quantum physics, not yield experimental predictions.

[Sajuuk]

I'd like to extend the analogy of Schrodinger's cat with quantum computing. A few years back, quantum computers were thought to be impossible because of decoherence. Indeed, the coherent state of a few entangled particles would simply not last long enough to allow us to make more than a handful operations on the system. Yet, some people discovered that decoherence can be seen as (non-unitary) "errors" in a quantum algorithm and now there is a lot of research in quantum error correction (or ways to fighting decoherence). Even thought they are hard to implement in experiments, there are algorithms that allow us to correct the errors in quantum system. I saw some several-qubit-systems that could remain entangled for a few seconds (the decoherence time is usually a few milliseconds) in NMR quantum computing. This is still far from thousands of qubits or even a cat, but this proves that decoherence is not a fundamental limit and that if no other phenomenas are involved, it could be theoretically possible for a macroscopic object to remain in an entangled state for a while. Research is still very active in quantum error correction (let's remember that the first experimental result was achieved only in 1998).

 

But this brings another problem : if decoherence can be "corrected" even in large scale objects, what is it like to be in an entangled state (from the cat's point of view)? Some assume that the cat will live simultaneously in two "parallel worlds" (Everett's idea). This could justify why there are two fundamental interactions in quantum mechanics : unitary operations (which says how particles interact in closed systems) and observations (how large systems interact with quantum systems). But at which scale do they differentiate? I saw a few lectures by 2003 nobel laureate Anthony Leggett this summer and he thinks that there could be new physics to be discovered between our macro-world and the quantum world, which would give more fundamental limits on quantum computing and explain how the transition from our scale to the microscale really happens (since we know that decoherence is not the true difference).

 

This is how I understand the problem of Schrodinger's cat and I think that most quantum information theorists are seeing it in a similar way( yet nobody truly have a solution).

[Simon]

Thanks for your input Craig.

 

Observing something via a television camera has the same effect on quantum superpositions that observing it with ones eyes does, so the experiment Simon proposes would result in either monitor B remaining blank, or both cameras showing the same image.

 

In fact I described only a single camera that would transmit to one or the other monitor depending on whether the crystal decayed. But your point remains the same - and you may be right. At this stage, all I can do is refer to the question: "what precise degree, quality or property of interaction is required to reduce the undetermined state of something to a determined state?" Until that is answered, I must remain agnostic.

 

According to best present theory and experimental evidence, decoherence – the collapse of quantum wave functions – occurs whenever any information about a particle ensemble spreads outside that system.

 

This seems to favour the Relational interpretation of quantum theory. It suggests that a given system can be small, large, simple or complex. The only crucial factor here is that no information about itself is shared outside. As long as this is the case, the wave function will be preserved from the perspective of another system. In the Relational model, interaction within a given system will collapse the wave for all potential observers inside, but leave the wave uncollapsed for all potential observers outside.

 

Self-evidently, my version of the thought experiment is premised on this relational interpretation.

 

Modern theory predicts that a particle ensemble as complicated as a warm cat in a box interacts with itself far too much to maintain a state of quantum superposition.

 

This contradicts the relational model. Is there really evidence to do this? It suggests that any degree of interaction within a closed system denies superposition, regardless of whether information is shared outside. If so, then quantum phenomena seems to be strictly limited to the micro world. Are we in a position to say that?

 

In addition, my version of Shrodinger's experiment requires that some forms of wavicle information can be shared outside the system without reducing decoherence to one probable state.

 

This is precisely what appears to be happening with twin slit interference - which also seems to fit the relational model. The particles interact with the observation screen and some with each other. I might also suggest that (consciousness aside) each particle 'knows' that it entered one of the slits in exactly the same way that a particle detector 'knows'. The only difference now is in the degree of interaction with another system - i.e the human observer. It could be said that when a particle detector is used, the systems have now interacted sufficiently to collapse the wave. In this scenario, the particle, the detector and the observer all 'know' which slit was entered. When no detector is used, interference is witnessed. Should we conclude (like the Copenhagen gang did) that the particle itself has no point of view, or even existence? I would say no. In so far as interference is evidence of an uncollapsed wave function, does this mean no information was shared outside the particle's system? Again, I would say no - based on simple logic. If no information was shared, then no interference could be witnessed.

 

Summary:

 

1) Observations in quantum mechanics suggest that certain forms of information can be shared outside a system while preserving the decoherence of that system.

 

2) Interactions within a given system may be capable of eliminating the wave function locally but do not automatically collapse the wave function to observers outside.

 

Conclusion:

 

These are two big 'ifs' required for my experiment. A third 'if' is creating a sufficiently closed system inside a sealed room. A fourth 'if' is whether television and radio signals would manifest themselves to the monitors as interference - thus sharing information as a wave function and not creating coherence between the two systems.

 

Simon

[CraigD]

At this stage, all I can do is refer to the question: "what precise degree, quality or property of interaction is required to reduce the undetermined state of something to a determined state?"

 

Although it’s a bit peculiar to study science at a science fiction writer’s website, I recommend Greg Egan’s “decoherence” webpage for a thoughtful approach to this question. Though neither Egan’s fiction nor nonfiction are “easy reading”, they’re much more understandable to a amateur like me than most academic physics. My only criticism of his website is that it’s often difficult to determine where the real physics ends and his fictional extrapolations begin.

[sebbysteiny]

At this stage, all I can do is refer to the question: "what precise degree, quality or property of interaction is required to reduce the undetermined state of something to a determined state?" Until that is answered, I must remain agnostic.

 

Although CraigD describes himself as 'an amateur' and I would probably describe myself as a 'professional' in that I studied the thing for almost 4 years at University, it's been almost 2 years since those studies so CraigD may easily get it right where I get it wrong.

 

Nevertheless I'll say what I remember and allow others to correct me if I'm wrong.

 

the precise degree, quality or property of interaction is required to reduce the undetermined state of something to a determined state is A MEASUREMENT. Anything that is capable of making the measurement of the state collpases the wavefunction, or rather alters the wave function.

 

Thanks to Hiesenburg's uncertainty principal, if you make a measurement of one property of a particle / wavefuntion, another property which was, prior to the measurement, certain now, after the measurement, becomes uncertain. In this way, you can change the velocity of a particle simply by measuing it's position (which would make the velocity change from certain to uncertain). So I think alter the wavefunction rather than collapse it is the correct expression.

 

But back to the problem, any measurment of the state of the particle is sufficient, a switch that is meant to check which state is inside the box is sufficient even if the output of that switch is connected to two TVs.

 

Lets go back to the two slit experiment. The point here is that THE POSITION OF THE ELECTRON HAS NOT BEEN MEASURED. Yes sure, it is limited to going through one or another slit. But there is no possible way of extracting the information as to which slit the electron went through even with perfect experimentation. The result is that the electron position state is a quantum supersition of both states so it interferes with itself creating an interference pattern.

 

But as soon as any effort is made to determine exactly which slit the electron went through ie THE POSITION, the position state is now fixed (making the velocity uncertain) and the electron can now not interfere with itself.

 

So anything capable of, with perfect experimentation, distinguishing between a reality in which the particle is in one state and a reality in which the particle is in another state is sufficient to collapse the wafefunction. One cannot just take two such measurements and assume that simply because you have not looked properly, the wavefunction has not been irrepairably altered.

 

The only crucial factor here is that no information about itself is shared outside. As long as this is the case, the wave function will be preserved from the perspective of another system. In the Relational model, interaction within a given system will collapse the wave for all potential observers inside, but leave the wave uncollapsed for all potential observers outside.

 

Without going into the ins and outs of quantum wavefunctions on a macro scale, which I seem to remember is impossible, the measurement made in your experiment is sufficient to "share information about itself outside" the system anyway.

 

But despite my strong criticisms, I do want to compliment you on your thoughts and ideas which, despite the gaps, show a great deal of understanding that one would normally expect from a University student.

[Simon]

Thanks Sebbysteiny.

 

Incidentally, when I was at University I didn't study quantum physics - so my interest in the subject is not backed up by specialised knowledge, nor a mastery of the equations that underly it.

 

The precise degree, quality or property of interaction required to reduce the undetermined state of something to a determined state is: A MEASUREMENT. Anything that is capable of making the measurement of the state collpases the wavefunction, or rather alters the wave function.

 

Can you or Craig address the following:

 

Rightly or wrongly, all three of us have clearly stated that sentience or consciousness are incidental as to whether a wave function is reduced or altered. We agree that a photon detector does measure which path the photon took - result, no wave function.

 

Does the observation screen - where interference is definitely witnessed - fail to make a measurement? If it made no measurement at all, then nothing could be witnessed. Unless I'm mistaken, the observation screen doesn't fail to measure which path the photon could have taken. Instead it measures both paths. What it fails to do is reduce to a single measurement.

 

As far as I can tell, this must be explained in other terms. I might phrase the question like this: "what precise degree, quality or property of measurement is missing whenever evidence of superposition appears.?"

[CraigD]

Incidentally, when I was at University I didn't study quantum physics - so my interest in the subject is not backed up by specialised knowledge, nor a mastery of the equations that underly it.

My education is in Math. I had 8 academic credits of “Modern Physics”, which was about 75% dedicated to quantum mechanics. I’ve studied it informally for decades since my graduation. So my knowledge of quantum physics doesn’t qualify me as a specialist, either.

Does the observation screen - where interference is definitely witnessed - fail to make a measurement?

The important measurement in determining if an interference pattern appears or not is “through which slit did each photon pass?”. The observation screen fails to measure this.

Unless I'm mistaken, the observation screen doesn't fail to measure which path the photon could have taken. Instead it measures both paths.

The screen (or, more precisely, each photosensitive molecule on the screen) measure’s only “was a photon with a frequency sufficient to trigger my detecting it here?” It doesn’t “ask” or “know” how the photon got there.

[sebbysteiny]

My education is in Math. I had 8 academic credits of “Modern Physics”, which was about 75% dedicated to quantum mechanics. I’ve studied it informally for decades since my graduation. So my knowledge of quantum physics doesn’t qualify me as a specialist, either.

 

I had a sneaking feeling that CraigD was underselling his qualifications when describing himself simply as 'an amateur'.

 

Rightly or wrongly, all three of us have clearly stated that sentience or consciousness are incidental as to whether a wave function is reduced or altered.

 

Agreed.

 

We agree that a photon detector does measure which path the photon took - result, no wave function.

 

We agree in the substance of what you say that is relevant to this debate, but your description here is unfortunately not correct.

 

All particles have wavefunctions: photons; electrons; hydrogen atoms; and even tennis balls.

 

To be scientifically accurate, what you really mean, I think, is that a photon detector measures the position variable of that wavefunction so that it's position stops being a superposition of the two possible position states.

 

In other words, a particle is a wavefunction. Each wavefunction contains a number of certain properties, and for every certain property, it contains an uncertain property. So if you measure it's postion, it's momentum changes. Measure the momentum again and you now will be uncertain of the position. It's like a slippery soap bar where no matter how much you try and measure the thing, you cannot 'hold onto' it's properties (or technically states) firmly because every time you measure one thing, you lose information about another.

 

When you measure it on the screen, what you are doing is measuring the position state of the particle at the screen. As you have not measured it's momentum or angle of projection, there is no way to know exactly what path the particle / wavefunction travelled prior to arriving at the screen. At the moment the wavefunction passed through the slits, it's position state was uncertain as it could have gone though one or another slit and no amount of measuring at the screen will change that.

 

However, if at the screen, you were to try and measure the velocity of that particle as well as the position to work out which slit it went though, you would find it impossible to make an accurate enough measurement of both position and velocity states to make the necessary determination.

 

So in yet more words, there is not enough information at the screen (or anywhere once it has passed the slits) to tell which slit the electron went though even with perfect experimentation.

 

Does the observation screen - where interference is definitely witnessed - fail to make a measurement? If it made no measurement at all, then nothing could be witnessed. Unless I'm mistaken, the observation screen doesn't fail to measure which path the photon could have taken. Instead it measures both paths. What it fails to do is reduce to a single measurement.

 

I think the mistake here is that you have misunderstood exactly what 'the diffraction pattern' means.

 

A single electron is sent through the two slits. One does not get 'a diffraction pattern', one instead simply detects the electron somewhere on the screen.

 

However, after thousands of similar electrons passing through the slits, one can determine the probability density of an electron hitting a particular point on the screen after passing through the slits.

 

This probability density is in the form of a diffraction pattern which proves that the electron ACTUALLY INTERFERRES WITH ITSELF EVERY TIME.

 

Then, when an attempt is made to measure the exact slit in which the electron passed, the probability density loses it's defraction pattern.

 

So the only measurement made is what part of a screen any particular electron hits after it has passed through the two slits and when this measurment is made thousands of times, one can 'see' the diffraction pattern.