Could someone explain the Schrodinger's Cat experiment to me?

[DarkStalker]

Could someone explain the "Schrodinger's Cat" experiment to me?

Hello,

I wanted to have an idea of what this experiment is. I just passed my 10th grade and so I don't quite grasp the meaning as it's been explained in Wikipedia. To quote from it:

"A cat, along with a flask containing a poison, is placed in a sealed box shielded against environmentally induced quantum decoherence. If an internal Geiger counter detects radiation, the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when we look in the box, we see the cat either alive or dead, not a mixture of alive and dead."

@ Bold: This is where I lose them.

My question regarding this are:
=What is this experiment used to prove/observe?
=How can something be both alive and dead at the same time?
=Is this, by any means, connected to the "Many-Worlds Interpretation"?

Can someone please cut it down for me to the point that someone with grossly limited knowledge on Physics can understand? Thanks in advance.

 

[tiny-tim]

Hello DarkStalker!

=What is this experiment used to prove/observe?
This thought-experiment doesn't prove anything …
it's just a description of a situation.

=How can something be both alive and dead at the same time?
It just is … that's the way quantum reality works.

=Is this, by any means, connected to the "Many-Worlds Interpretation"?
No.

Can someone please cut it down for me to the point that someone with grossly limited knowledge on Physics can understand?

There isn't really anything to understand … you either accept it or you don't:

in classical reality, you can say "the cat has a definite state, it is either alive or dead"

but in quantum reality, you can't say "the cat has a definite state".

 

[alexepascual]

I wonder when it was that you read this in Wikipedia. It appears that the article has been improved since you read it.
I suggest that you go back to it and see if you find a better explanation now.
Still, there is a flag on top of it that says: This article may be confusing or unclear to readers.
I'll see if I can give you a short explanation.
Quantum mechanics says that small particles such as an electron, atom, etc. can be in a "superposition of states". This is quite in contrast to what we observe in every day life.
Big objects are always at a definite place, move at a definite speed, etc.
Now, in the case of quantum systems (small systems with quantum behavior), when one interacts with another you get what is called "entanglement". (see the EPR experiment)
All the analysis of quantum systems involved in the beguinning the interaction between a quantum system and a classical (as opposed to quantum) measurement apparatus.
Of course the apparatus was some big (macroscopic) thing. Now, it seemed that this distinction between microscopic quantum systems and the macroscopic classical ones seemed arbitrary.
So some started to suggest that macroscopic objects (which by the way are composed of many small microscopic particles) should be considered to behave also according to quantum mechanics. This is when Schrodinger, who was not very happy with the superpositions) suggested his cat experiment as a way to show that something was wrong with the then current interpretation of quantum mechanics. After the cat interating with the quantum system, it should get entangled with it and end up in a state of both dead and alive at the same time, which obviously we never see. So this is a paradox. And that's what Schodringer intended it to be. But not everybody agreed with him in 1935. Some argued that the cat could be in a superposition and gave some explanation as to why we never see it. One of the explanations was in fact the many-worlds interpretation.
Today, most physicists think that as soon as the quantum superposition (of whatever triggers the poison vial to break) is amplified to something of a macroscopic size, the superposition is destroyed and you get a dead cat or a live cat (way before you open the door). The process that is said to explain how this happens is called "environmentally induced decoherence". But eventhough decoherence can be interpreted according to the old school ideas (the Copenhagen Interpretation) this phenomenon can also be interpreted according to the Many Worlds Interpretation.
If you really want to understand these things, I suggest that you continue reading. There is a lot to learn about this. It is not simple. Physicists have been arguing about it for all these years and will probably continue arguing until it is settled (if it ever gets settled).
Today there are other experiments that show the weird behavior of quantum systems and a few that may show weird behavior of macroscopic objects too.
In order to understand these experiments you'll have to learn a little math though. I suggest you study as much physics and math as you can.

 

[Naty1]

In classical physics, the large scale world we typically observe around us, things are in one state or another, say a coin is EITHER heads or tails when flipped.

The essential concept in quantum theory is that unlike the classical large world, the sub atomic world not only has a state such as (x) say heads OR tails say (y) but also superpositions like a(x) + b(y)...that is combinations!...so things than be "weird" combinations...but only until we observe them...

Check out wikipedia, Quantum Superpostion for more...
http://en.wikipedia.org/wiki/Quantum_superposition

And by the way, this "weirdness" has never been experimentally observed...we only observe classical results...it was theoretically uncovered via mathematics...but the classical results we can and do observe match predictions extremely well...what it MEANS is still the subject of discussion/debate/interpretation after perhaps 80 or more years...

 

[qmpash]

There are much better explanations here but if you want a simplistic one, here it is. The "cat" is used to explain the duality that often appears in qm; for example the duality of photons (light.) Do one experiment, such as the Young dual slit experiment and you prove the wave nature of light. Shine a light on certain metals and you produce a current; something that only a particle can do.
So, it doesn't make any difference whether the cat is dead or alive. It depends totally upon the observer and what he does that determines whether the cat is dead or alive. Similarly; it makes no difference whether a photon is a particle or a wave; it depends upon the experiment you are doing at the time.
I hope this helps.

 

[Bob_for_short]

It is better to consider a box with a cat at t=0, without the Schroedinger mortal stuff. The box may be in two states - with and without a cat. (Cats like boxes, especially when they are comfortable in them.) Initially the box is with cat: |1>. But with time the cat quits the box so at t > 0 the box is in a superposition state: a|1> + b|0>. The reason for quitting the box may be internal (hunger, for example) and/or external. Anyway, when t → ∞, the box state becomes |0> (an empty box). See an experiment here:

 

[map19]

At the time Schrodinger invented the experiment it was often said in physics that an intelligence was required to 'collapse the probabilities" in QM. This view was taken to the extent that one prominent physicist stated that 'the moon is not there if nobody is looking'.
I'm not sure where the dividing line is between quantum superposition and the macro world is, but a cat is definitely above it.
Schrodinger's cat is an attempt to show the stupidity of this view.
You can soon reduce the experiment to nonsense by suggesting, for example that the expt. is filmed and the film is stored and only looked at a week later. Does the probability collapse then ? or when it's filmed ?
Supposing nobody looks at the expt. is the poor cat suspended forever ?
Maybe a flea on the cat knows if the cat is alive or dead, does this flea collapse the probabilities ?
Etc Etc.
The cat is either dead or alive in reality and this doesn't depend on who looks at it.

 

[muppet]

In classical physics, the large scale world we typically observe around us, things are in one state or another, say a coin is EITHER heads or tails when flipped.

The essential concept in quantum theory is that unlike the classical large world, the sub atomic world not only has a state such as (x) say heads OR tails say (y) but also superpositions like a(x) + b(y)...that is combinations!...so things than be "weird" combinations...but only until we observe them...

Check out wikipedia, Quantum Superpostion for more...
http://en.wikipedia.org/wiki/Quantum_superposition

And by the way, this "weirdness" has never been experimentally observed...we only observe classical results...it was theoretically uncovered via mathematics...but the classical results we can and do observe match predictions extremely well...what it MEANS is still the subject of discussion/debate/interpretation after perhaps 80 or more years...

Some might take issue with the statement that the superposition has never been experimentally observed. What is uncontroversial is that the results of measurement are always single-valued and well-defined to within the limits of experimental accuracy. But there's extensive evidence that these superpositions exist prior to measurement. (Insofar as such a statement can be true independent of what interpretation you ascribe to QM).

 

[Fredrik]

The point of this thought experiment is to show that QM implies not only that microscopic systems can be in superpositions of different states, but also that the same thing holds for large and complicated systems. To some people, this suggests that there are "many worlds". To others, it only means that a wavefunction can't be a mathematical representation of the objective properties of a physical system. The alternative is that it's a mathematical representation of the statistical properties of an ensemble of identically prepared systems. Another way of saying that last thing is that we shouldn't think of QM as describing reality, but instead as just a set of rules that tells us how to calculate probabilties of possible results of experiments.

 

[bjacoby]

Hello,

I wanted to have an idea of what this experiment is. I just passed my 10th grade and so I don't quite grasp the meaning as it's been explained in Wikipedia. To quote from it:

The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when we look in the box, we see the cat either alive or dead, not a mixture of alive and dead."[/I]

@ Bold: This is where I lose them.

Passing the 10th grade has nothing to do with not understanding this. Like the proverbial kid of old, yes indeed, the emperor has no clothes on!

All of the word-salad and hand-waving does not change the fact that none of this makes any logical sense. Understand that Quantum Mechanics is what I call the "science of ignorance". Nobody knows what a "probability wave" is or what kind of medium supports and propagates these mythical "probability waves". The fact that it's probability says that the details are basically unknown. But men in their adult ignorance and arrogance have to try to take that one step further and proclaim that the unknown is "unknowable"!

However in spite of the illogical illucidity of these theories, you you should be made aware that any quest for reason, truth, or ultimate structures of reality will not further any career in science nor aid in the passage of any course work in science. For now such a quest must simply remain it's own reward.

 

[muppet]

Passing the 10th grade has nothing to do with not understanding this. Like the proverbial kid of old, yes indeed, the emperor has no clothes on!

All of the word-salad and hand-waving does not change the fact that none of this makes any logical sense. Understand that Quantum Mechanics is what I call the "science of ignorance". Nobody knows what a "probability wave" is or what kind of medium supports and propagates these mythical "probability waves". The fact that it's probability says that the details are basically unknown. But men in their adult ignorance and arrogance have to try to take that one step further and proclaim that the unknown is "unknowable"!

However in spite of the illogical illucidity of these theories, you you should be made aware that any quest for reason, truth, or ultimate structures of reality will not further any career in science nor aid in the passage of any course work in science. For now such a quest must simply remain it's own reward.

I'd disagree with a great deal of this. I would agree that the process of the collapse of the wavefunction isn't well-defined or understood. But nobody talks about probability waves. Rather, everyone talks about probability amplitudes. Too much is made of "wave-particle duality" in popular science books because it's a strange concept that's easy to describe (if not to explain properly ), and it's a phenomena that really only occurs when you expand states in a particular mathamatical representation to find out where something will be. Also, in relativistic quantum mechanics the picture changes considerably, and the existence of particles themselves is down to an operator-valued field which satisfies a wave equation. So the idea of a "medium of probability waves" is wrong. Furthermore, foundational work is being done, in some extremely well-regarded universities. See Imperial or Bristol in the UK, for example. A lot of it is motivated by the experimental importance of decoherence in quantum computing. You might think it's sad that people have needed an experimental motivation in order to take the question seriously, and I'd agree with that, but to portray all physicists as distinterested in truth, beauty or reason is unfathomable. Hell, chemistry is essentially predicted by demanding U(1) gauge invariance. (In the suitably qualified sense that all electromagnetic interactions- including the responses of electrons to nuclear potentials, and those of other electrons, are explained within the framework of QED; to account for the existence of nuclei, you just have to enlarge your symmetry group to the non-abelian gauge group of the standard model- and then you're talking about >90% of physics.). How much more elegant a theory do you want?

 

[Halcyon-on]

If it happens to you to kill a cat and be processed about this crime, you could always appeal to the court saying that it was a quantum mechanics aleatoric event, that it was a indeterministic event beyond your control. if the court has a good knowledge of QM they will find you not guilty, since it was not your free-will.

 

[alxm]

For an Emperor with No Clothes, Quantum Mechanics certainly has produced an incredible amount of real results and practical uses.

It's quite logical if you ask me. Following some basic postulates (some of which are 'self evident', some of which are empirical but well-verified) it's all built up quite logically - through math.

It appears weird and nonsensical because it cannot be rationalized and explained in terms that 'make sense' in terms of everyday human experience. I don't see why Nature would or should be forced to obey that constraint. Sure, it'd be nice if Newtonian physics held true at every scale. But we never had any reason to believe that.

Of course quantum theory is not the "final theory of everything". Nobody said it was. Unlike Newtonian physics, it's been free of that unwarranted assumption from the start. The search for the Next Big Thing has been ongoing from the start.

 

[DaveC426913]

if the court has a good knowledge of QM they will find you not guilty, since it was not your free-will.

Alas, a clever judge will further conclude that the cause-effect chain continues to sentencing, and will find you guilty since he has no free will either.

 

[Halcyon-on]

Actually the "Schrodinger's Cat" experiment prove that logical inconsistencies of QM. It works incredibly well, as we know from QED. For this reason physicist should start to think more seriously to alternative (deterministic) theories from which QM emerges at effective level.

 

[ZapperZ]

Actually the "Schrodinger's Cat" experiment prove that logical inconsistencies of QM. It works incredibly well, as we know from QED. For this reason physicist should start to think more seriously to alternative (deterministic) theories from which QM emerges at effective level.

What logical inconsistencies?

Are you confusing your personal tastes with "logic"?

Zz.

 

[zonde]

The issue with Schrodinger's Cat can seem quite trivial but in order to see controversy between macroscopic realism and QM one has to remember that probability of finding cat alive or dead can be influenced by procedure of opening the box.

 

[alxm]

Actually the "Schrodinger's Cat" experiment prove that logical inconsistencies of QM.

"Schrödinger's cat" shows logical inconsistencies to the same extent that Zeno's Paradox does. It's a problem posed in a certain way to give an apparent contradiction.

As with Zeno, the reasons for this contradiction are to be found in the question itself. Zeno fools the listener by shrinking distance and length at the same rate. "Schrödinger's Cat" assumes that the 'observing' system can be treated classically while the 'observed' system is quantum.

The fact that there's yet to be a fully satisfactory answer to the problem of Schrödinger's Cat doesn't mean that QM is logically inconsistent. All it does is tell us that we've yet to fully understand the quantum-to-classical passage and decoherence. That's a subject of active research.

 

[Halcyon-on]

This is way the Schrodinger cat experiment are usually addressed as a paradox.

Paradox : A paradox is a statement or group of statements that leads to a contradiction or a situation which defies intuition; or, it can be an apparent contradiction that actually expresses a non-dual truth (cf. Koan, Catuskoti). Typically, either the statements in question do not really imply the contradiction, the puzzling result is not really a contradiction, or the premises themselves are not all really true or cannot all be true together. The word paradox is often used interchangeably with contradiction. Often, mistakenly, it is used to describe situations that are ironic. [http://en.wikipedia.org/wiki/Paradox]

Paradox: (logic) A self-contradictory statement, which can only be true if it is false, and vice versa. [http://en.wiktionary.org/wiki/paradox]

Refereeing in particular to the latter definition you see that a paradox has a inconsistency with respect, for instance, to the Aristotel truth table. If the cat is alive than is not dead or vice versa. In logic, when one find such a paradox is forced to conclude that there is one or more hypothesis false. The hypothesis in the Schrodinger cat experiment are the QM postulates.

 

[Bob_for_short]

Actually the "Schrodinger's Cat" experiment prove that logical inconsistencies of QM. ... For this reason physicist should start to think more seriously to alternative (deterministic) theories from which QM emerges at effective level.

1) It is the isolated radioactive atom that is in superposition, not the cat.

2) Wave function describes a set of decay events, not one.

 

[Halcyon-on]

1) It is the isolated radioactive atom that is in superposition, not the cat.
2) Wave function describes a set of decay events, not one.

1)the cat life is entangled with the decay of the atom once that you put it in the box.

2) the decay probability of the atom is given by the wave function, but the decay of a single atom is an event, a single bang!

the killer of the cat is the quantum dice.

 

[ZapperZ]

This is way the Schrodinger cat experiment are usually addressed as a paradox.

Paradox : A paradox is a statement or group of statements that leads to a contradiction or a situation which defies intuition; or, it can be an apparent contradiction that actually expresses a non-dual truth (cf. Koan, Catuskoti). Typically, either the statements in question do not really imply the contradiction, the puzzling result is not really a contradiction, or the premises themselves are not all really true or cannot all be true together. The word paradox is often used interchangeably with contradiction. Often, mistakenly, it is used to describe situations that are ironic. [http://en.wikipedia.org/wiki/Paradox]

Paradox: (logic) A self-contradictory statement, which can only be true if it is false, and vice versa. [http://en.wiktionary.org/wiki/paradox]

Refereeing in particular to the latter definition you see that a paradox has a inconsistency with respect, for instance, to the Aristotel truth table. If the cat is alive than is not dead or vice versa. In logic, when one find such a paradox is forced to conclude that there is one or more hypothesis false. The hypothesis in the Schrodinger cat experiment are the QM postulates.

There are NO paradox here. If you make an observation, you only have one value. Done! So where is the paradox?

When you make an observation of a non-commuting observable, only THEN can you detect the effect of such superposition. It is why we have bonding-antibonding bonds in chemistry! But you still haven't made any observation that would be considered as a paradox in this case.

None of the issues being brought up here is new. There have been many previous threads on this very issues that have been addressed.

Zz.

 

[Halcyon-on]

There are NO paradox here. If you make an observation, you only have one value. Done! So where is the paradox?

I was only reading the wikipedia introduction to the Schrödinger's cat: "Schrödinger's cat is a thought experiment, often described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935." Then I followed the link of paradox and, as suspected, I have found that it means a logical contradiction. This reference to paradox is also common in many QM books.

Frankly speaking I feel 100% alive (or I do not feel at all at 100%) even if I would be in a Schrodinger box with the cat. Doesn't matter what people outside the box think.

I'm not saying that QM doesn't work or that it is wrong. I perfectly know that QM in its formulation is a very powerful tool for physicists (see QED). I want just to point out that it give rise to a long series of paradoxes and inconsistencies with other fundamental principles of classical mechanics (energy conservation, particle number conservation, least action principle, ecc...). Deny this to me means to don't have a clear vision of QM. The most important physicists of our era, from Einstein to Schrodinger passing by Feynman ("nobody has never fully understood QM"), shown to have these paradoxes well clear in their mind.

 

[Bob_for_short]

Many think that Classical Mechanics (CM) is deterministic and QM is not. QM needs many experiments to get full information about the wave function. In fact, CM also uses many measurements and what is deterministic in it is the average particle position. Any CM observation includes multi-photon exchange with a body and the average is obtained by summing up the spread readings. In QM there is also the notion of the inclusive cross section and it is the closest QM result to CM one.

 

[kote]

I was only reading the wikipedia introduction to the Schrödinger's cat: "Schrödinger's cat is a thought experiment, often described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935." Then I followed the link of paradox and, as suspected, I have found that it means a logical contradiction. This reference to paradox is also common in many QM books.

From Wikipedia:

The logician Willard V. O. Quine distinguishes falsidical paradoxes, which are seemingly valid, logical demonstrations of absurdities, from veridical paradoxes, such as the birthday paradox, which are seeming absurdities that are nevertheless true.​

Typically paradoxes are of the veridical type. It is something that is true but appears not to be possible. In the case of Schrodinger's cat, this thought experiment was developed to show the logical inconsistency of Bohr's explanation of QM. It is supposed to be falsidical. Schrodinger's experiment is a version of a previous thought experiment developed by Einstein in a letter to Schrodinger, which made this explicit. Whether it is falsidical or veridical is a topic of debate.

Whether you believe that it is logically inconsistent to say that the cat ever actually is both alive and dead at the same time depends on the interpretation of QM that you believe. Neither Bohr nor Bohm would have ever said that a cat can be in a superposition of alive and dead. That a superposition could be "real" is a feature of some more recent interpretations, and was not something anyone initially considered to be possible.

 

[Count Iblis]

You can also think of this as follows. As long as the superposition of the contents of the box has not decohered (i.e. the superposition becomes entangled with the environment), there exists an observable corresponding to measuring the initial state (the observable being an operator that acts only on the degrees of freedom located inside the box).

It then doesn't matter if the cat is in a superposition of dead and alive, the cat could well be unambiguously dead. Also, it is not necessary to use a "quantum event" to trigger any change, as everything is quantum mechanical anyway.

Now, this is also true in classical mechanics. Time evolution does not erase information about the initial state. As long as there are no interactions between he contents of the box and the outside world, you can (in principle) observe the initial state.

 

[Count Iblis]

Whether you believe that it is logically inconsistent to say that the cat ever actually is both alive and dead at the same time depends on the interpretation of QM that you believe.

Note that superposition of dead and alive is not the same as somehow being dead and alive at the same time. Compare the two slit experiment where particles do not go through one slit or the other slit but they appear at the screen via an intermediary state that is the superposition of the two "classical" possibilities. But the particle, of course, never splits in two.

 

[Halcyon-on]

Great! I agree with you Kote.

I want also add that QM is a recipe that solve all the problems encountered at the scales investigated so far. And it actually works! But conceptually there are still so many open questions that motivate the study of new physics, from strings to twistors, from KK to SUSY. Hopefully, when there will be an improved explanation of QM (if any), all these paradoxes will be solved and finally the Schrodinger's cat will be free to die only when it will be its own turn.

 

[Bob_for_short]

...But conceptually there are still so many open questions that motivate the study of new physics, from strings to twistors, from KK to SUSY. Hopefully, when there will be an improved explanation of QM (if any), all these paradoxes will be solved ...

It is so because too few understand from where the CM determinism appears and how it is embedded in the theory. I repeat, experimentally any observation includes multi-photon exchange and only the average is deterministic. This average corresponds to the center of inertia of the observable body and the internal degrees of freedom are taken into account in making the average. In QED it is the same.

 

[Dmitry67]

Numerous experiments (on some smaller objects) have shown that in fact, smaller cats CAN be in a superposition. Even more, the ability to be in a superposition does not 'fade' when you increase the size of a system. For example, it was demonstrated that systems of billions of electrons can be in a superposition in a superconductive ring.

So if anyone claims the absurdity of a cat being in a superposition then he/she needs to draw a line which separates big cats from smaller ones, a line which separates microscopic world from a macroscopic one. And nobody could do this it remained in a status of a paradox until Quantum Decoherence was discovered.

 

[Halcyon-on]

It is so because too few understand from where the CM determinism appears and how it is embedded in the theory. I repeat, experimentally any observation includes multi-photon exchange and only the average is deterministic. This average corresponds to the center of inertia of the observable body and the internal degrees of freedom are taken into account in making the average. In QED it is the same.

Ok, maybe I use the word determinism in another way. QM can be regarded as a statistical theory but which is not deterministic. With this I mean that in a proper statistical system you usually describe the macroscopic variables (temperature, pressure,...) but in principle you can even determine the position and the velocity of every particle of the classical statistical system and predict exactly with a simulation the evolution microscopically. In QM you can describe only statistically observables but you cannot but the uncertain principle forbids to know the exact value of an observable in a give measurement. In the Schrodinger cat case you can say that after a lifetime of the atom the cat will be dead at 50%, but you cannot say the exact moment when the cat die. In a statistical system, in principle, you can know the mechanism that kill the cat with all the accuracy that you want and predict with arbitrary precision when it will bang.

 

[Bob_for_short]

Ok, maybe I use the word determinism in another way. QM can be regarded as a statistical theory but which is not deterministic. With this I mean that in a proper statistical system you usually describe the macroscopic variables (temperature, pressure,...) but in principle you can even determine the position and the velocity of every particle of the classical statistical system and predict exactly with a simulation the evolution microscopically. In QM you can describe only statistically observables but you cannot but the uncertain principle forbids to know the exact value of an observable in a give measurement. In the Schrodinger cat case you can say that after a lifetime of the atom the cat will be dead at 50%, but you cannot say the exact moment when the cat die. In a statistical system, in principle, you can know the mechanism that kill the cat with all the accuracy that you want and predict with arbitrary precision when it will bang.

A classical statistical system looks as deterministic but it is not. I mean, any measurement is made with some precision, OK? So, even though you know the particle positions at t=0, they have some uncertainties. With time this makes predictions impossible.

Let us consider only one particle in an open space and let its initial coordinate is known exactly but the velocity has a small uncertainty delta_v. With time the particle position is determined with the precision delta_v*t which is growing with time. Now, if you consider this particle enclosed in a box, the uncertainty of its position in a closed space transforms in any possible position within the box. So you see, the determinism of CM and Classical Statistical mechanics should not be exagerated.

 

[Halcyon-on]

A classical statistical system looks as deterministic but it is not. I mean, any measurement is made with some precision, OK? So, even though you know the particle positions at t=0, they have some uncertainties. With time this makes predictions impossible.

Let us consider only one particle in an open space and let its initial coordinate is known exactly but the velocity has a small uncertainty delta_v. With time the particle position is determined with the precision delta_v*t which is growing with time. Now, if you consider this particle enclosed in a box, the uncertainty of its position in a closed space transforms in any possible position within the box. So you see, the determinism of CM and Classical Statistical mechanics should not be exagerated.

Ok. It is impossible to know exactly the initial condition even in a classical system, in a statistic system this leads to fact that only a statistical description makes practically sense. But the indetermination in QM is something different. The more you measure the velocity with accuracy the more you loose accuracy in the spatial measurements. So, once you fix exactly the coordinate the velocity of the particle is completely unknown, no matter how precise are your instruments. The indeterminism in QM is not a problem of experimental resolution, it is not a problem of your will to do good measurements, but it is a full impossibility to determine the value of an observable.

 

[Bob_for_short]

Ok. It is impossible to know exactly the initial condition even in a classical system, in a statistic system this leads to fact that only a statistical description makes practically sense. But the indetermination in QM is something different. The more you measure the velocity with accuracy the more you loose accuracy in the spatial measurements. So, once you fix exactly the coordinate the velocity of the particle is completely unknown, no matter how precise are your instruments. The indeterminism in QM is not a problem of experimental resolution, it is not a problem of your will to do good measurements, but it is a full impossibility to determine the value of an observable.

In QM there are "observables" that are always the same dispite spreading other "observables". For example, take a monochromatic light in a double-slit experiment. The photon frequencies are the same but positions change from one observation to another. The frequency is the energy, the position spread corresponds to the wave function. The wave function "measurement" needs many experiments. The energy, if it is an eigenvalue, does not need many. The momentum is reciprocal to the position, it is an argument of the wavefunction in the momentum space. In this respect making many measurements to find out the entire wave function does not differ from classical mechanical measurements that need many points for better accuracy (for determinism).

 

[Count Iblis]

I mean, any measurement is made with some precision, OK? So, even though you know the particle positions at t=0, they have some uncertainties. With time this makes predictions impossible.

This is not necessarily true. You cannot rule out discovering the exact laws of physics and the correct intitial coinditions. You then cannot rule out that a nontrivial mathematical theorem about the exact future state of the universe could be proven.