1 Quantum puzzle baffles physicists

My problem with this thought experiment is how can you claim, based on the outcome of this thought experiment, anything about the real world.

The article contains the following sentence: "One of the two friends (Alice) can toss a coin and - using her knowledge of quantum physics - prepare a quantum message to send to the other friend (Bob). Using his knowledge of quantum theory, Bob can detect Alice's message and guess the result of her coin toss"

For me this text is not clear, is not complete and does not give enough detail. What means: "her/his knowledge of quantum physics"? What is a "quantum message"? How does Bob "guess"?

What baffles me the most is the answer on the question: What have human observations to do with the physical state of any process. Specific what have human observations to do with the half-life of a radio active process? IMO nothing. IMO it is much better to replace the cat with a counter and count the number of gamma particles released. Ofcourse you can claim that before you look at the counter, the counter is in a set of states simultaneous (like both dead and alive) but that is of no physical significance.

Nicolaas Vroom

2 Quantum puzzle baffles physicists

3 Quantum puzzle baffles physicists

4 Quantum puzzle baffles physicists

In the book "In search of Schroedingers cat" at page 203, John Gribbin writes: Thew apparatus in the box is arranged so that the detector is switched on just long enough so that there is a fift-fifty chance that one of the atoms in the radioactive material will decay and that the detector will record a particle."

That means you have to perform this experiment first 1000 times in order to establish what the half-life is.

Next he writes: According to the strict Copenhagen interpretation, just as in the two-hole experiment there is an equal probability that the electron goes through either hole, and the two overlapping possibilities produce a superposition of states (1), so in this case the equal probabilities for radioactive decay and no radioactive decay should produce a superposition of states (2)".

The part near (2) is not clear to me. What is clear that when you look in the box, you have a 50% chance that the cat is alive and a 50% chance that the cat is dead. Because that is the way the experiment is set up. As such IMO to claim that the cat before you look is in a superposition state is of physical significance. The part near (1) is not clear to me. Interference between water waves is explained because the wave goes to both holes.

>	I'll leave others to answer your other questions.

Nicolaas Vroom

5 Quantum puzzle baffles physicists

This argument may be against counterfactual interpretations of quantum mechanics. The argument of Frauchiger and Renner depends upon whether a third observer makes a measurement to verify that Alice and Bob agree. If there are counterfactual definiteness in the quantum world this argument hold, in that the third observer can find that Alice and Bob do not agree, and this holds even if the third observer makes no measurement. Counterfactual definiteness means there would have been a type of outcome had a particular measurement been made. Quantum interpretations that permit counterfactual definiteness are the deBroglie-Bohm interpretation, the Stochastic interpretation of Nelson, similar to deBrogle-Bohm, and the Transactional Interpretation of Cramer. I am sure there may be others. Quantum interpretations have in recent years multiplied like bunnies.

If we reject counterfactual definiteness the result of Frauchiger and Renner is then not consistent or false in such a system or interpretation. The truth of Frauchiger and Renner conclusion depends upon counterfactual definiteness. We might then see this result as less about quantum mechanics as it is with interpretations of QM. Remember that QM is a completely deterministic physics in that a wave function is unitary evolved. However, this describes the evolution of amplitudes that define probabilities for outcomes in a measurements. These outcomes are completely stochastic, and their occurrence is with an actual detection or measurement. This is where we get into nettlesome issues of wave function collapse, many worlds and the rest.

Raamsdonk illustrates how spacetime might be built up from large N entanglements of states. This large N might run into the problem that Frauchiger and Renner illustrate. The occurrence of a classical spacetime that is resistant to quantization, except at the most extreme Planck energy, means there is some firmness to the idea that classical reality is separate from quantum physics. The reality of a quantum wave or state is slippery.

I am rather agnostic about the result of Frauchiger and Renner; I am not sure whether this has some impact on nature. We do have this conundrum with the dichotomy between quantum and classical physics, where Bohr saw this as a necessary feature of his Copenhagen Interpretation. However, we also have a sense that any classical object is ultimately built up from quantum particles, waves or fields. In some ways nature performs an einselection, to use Zurek's term, the quantum bases that pertain to the classical world. Some might argue it is not nature that does this, but rather minds or the action of a mental being coming aware of of quantum outcomes.

6 Quantum puzzle baffles physicists

We did this experiment in 1968. Apollo 8. Three cats called Frank, Jim and Bill (Borman, Lovell and Anders).

There were three points in that mission when they were cats in the box with a state unknown. Entering lunar orbit, leaving lunar orbit and during maximum heating at re-entry (after hitting the upper atmosphere at escape velocity, 45,000 km/hr). That mission had a 1/3 chance of failing with the loss of three cool cats.

I don't see strong evidence for the superposition of anything more than the thoughts and memories of observer's facing overt uncertainty in the outcome of their own actions, while utterly cutoff from interaction with the outside world.

I believe the observable observer (like a cat), is more than capable of collapsing its own wave function (in a box or anywhere), in the absence of their being independently observed by another observable observer.

Perhaps the question is more tractable if we ask about Schrodinger's gall stone. An observable that is NOT an observer might serve this debate better.

mj horn

7 Quantum puzzle baffles physicists

8 Quantum puzzle baffles physicists

The article contains the sentence: "It consists of agents who are using quantum theory to reason about other agents who are also using quantum theory." To understand that sentence you must know what the definition of "quantum theory" is. I don't think the article does.

>	In my opinion the conclusion, however, is not convincing. It's nicely explained here: https://www.scottaaronson.com/blog/?p=3975 ]

This document is not very convincing.
It contains the sentence: "Hardy’s paradox involves the two-qubit entangled state." That may be true and all what is following may be true, but what I miss is a description of a real experiment.
The same feeling I get with a sentence like: "The schroedinger cat experiment starts with an entangled cat" or a cat in an entangled state (or something equivalent)

The article also contains this sentence: "Thus, suppose Wigner entangles his friend with a qubit, like so:" How do you do that?

>	On 10/13/2018 5:56 PM, Nicolaas Vroom wrote:

>>

The first sentence of this article reads: "New twist on thought experiment yields conflicting results" The subject of the article is a new version of the Erwin Schroedinger's cat thought experiment in which the cat is replaced by two human beings (Alice and Bob) and with two observers. (one for Alice and one for Bob)

>	So can we agree then that the original Schroedinger's cat is no problem? If you accept the existence of superpositions you can have superpositions involving a cat. Or a human (inluding the human mind).

It is not a matter for me to accept superpositions, it is a matter for "you" or anyone to explain what it means.
To explain what it means: that a cat is both alive and dead. See also my reply to Phillip Helbig, where I discuss the issue if the coin is both head and tail.

>>	For me this text is not clear, is not complete and does not give enough detail. What means: "her/his knowledge of quantum physics"? What is a "quantum message"? How does Bob "guess"?

>

The sentence seems to indicate that Alice prepares a quantum message to send to Bob. If she actually does send it then of course Bob can detect it. But the sentence is not clear, perhaps it is meant that Bob can already detect it while Alice has only prepared it, without sending it. (And if that is really meant, then we do not believe the article of course.)

I agree

>>	What baffles me the most is the answer on the question: What have human observations to do with the physical state of any process.

>	Humans *are* the physical state of a process! (The functioning of their body.)

Strange.

>>	Specific what have human observations to do with the half-life of a radio active process? IMO nothing. IMO it is much better to replace the cat with a counter and count the number of gamma particles released.

>	But then the counter would be in a superposition of having counted no particles, and having counted one, having counted two, etc.

Such a claim IMO is of no physical significance.

>	But back to your quantum message.

SNIP

>	There is no puzzle if you accept superpositions.

Why do I have to accept superpositions? It does not make sense accept something when what you want accept is not clear.

>	(And if you don't, you can't even describe a simple atom.)

Please explain.

Nicolaas Vroom

9 Quantum puzzle baffles physicists

Consider the most? simple experiment.
1. We put a coin in the box. I shake the box.
2. Before we open the box is the state of the coin now "both head and tail"?
3. Is the coin in a superposition state? (IMO both no)
4. You open the box and you look inside.
5. Does that mean that there is a collapse of the wave function?
6. You close the box.
7. Is the coin (again) in a superposition state? (IMO no)
8. I open the box and I look inside.
9. Does that mean that there is (again) a collapse of the wave function?

Is this experiment trully an experiment with a fifty-fifty chance? NO. The coin can be fake.
To be sure you have to perform this experiment first 1000 times. That's why this cannot be a thought experiment.

>	I read today (bonus points if you can guess where) that a better experiment just puts the cat (in eternal?) sleep.

In an ad which advertises sleeping pills

> >	Next he writes: According to the strict Copenhagen interpretation, so in this case the equal probabilities for radioactive decay and no radioactive decay should produce a superposition of states (2)".

>	Note that this is just standard quantum mechanics and has nothing in particular to do with the article which started this thread.

The significance is in the usage of the word "superposition" and in the clearity of the text.

> >

The part near (2) is not clear to me. What is clear that when you look in the box, you have a 50% chance that the cat is alive and a 50% chance that the cat is dead. Because that is the way the experiment is set up. As such IMO to claim that the cat before you look is in a superposition state is of physical significance.

>	Did you mean "of NO physical significance"?

Ofcourse.

>	The whole point of this thought experiment is to get rid of "IMO".

If you want to get rid of "IMO" than you have to perform real experiments. which are clear and which can be repeated as much as possible. IMO when you study the Copenhagen Interpretation, in the beginning this was the opinion of a small group of people. Some agreed and some disagreed. The purpose of this tread is to show that there are at least some which have different opions, today.

>	It is well known that our intuition often fails when quantum theory is involved. Saying "it just ain't so" won't cut it. The thought experiment is an obvious extension of the Copenhagen interpretation. What it means is another question.

I prefer to call it a chalenge. The problem is: the Copenhagen is an interpretation of what?
Both should be a description of a certain experiment. See below.

> >	The part near (1) is not clear to me. Interference between water waves is explained because the wave goes to both holes.

>

This is the Central Mystery of quantum mechanics: repeat the double-slit experiment with a source (of light, electrons, bucky balls---it doesn't matter) so weak that only one particle at a time is registered on the screent. The same interference pattern builds up over time. Block one of the slits, do the experiment again, and the interference pattern goes away.

The problem is, I do not see any mystery in this experiment. There must be a physical explanation.
For water waves this is simple. They go through both slits. If their is a statistical difference, between a single electron going through one slit or to two slits (holes) than the next step is to perform the same experiment with different distances between the two holes (rims). I bet there is a difference.
That's why this can not be a thought experiment. My understanding is, that what we call one-electron is much more than a simple (point) particle.
It can be much more, like a particle surrounded by a cloud of something. Part of this cloud can go through the other hole, which influences its behaviour.

Anyway to call the one-electron in a superposition state is too simple.

Nicolaas Vroom.

10 Quantum puzzle baffles physicists

That is why I asked! You are of course free *not* to accept the most widely accepted theory of modern physics. If you accept it, we can explain things to you by using this theory. If you have another theory of your own, or if you want stick to 19th-century classical physics, then it is you who has to do the explaining..

>	it is a matter for "you" or anyone to explain what it means.

You are of course right (and there is quite some literature about it, I would say). Likewise, for the alternative mentioned, one would have to explain what a 'classical particle' or a 'classical field' actually means! (And deal with the intrinsic problems and paradoxes..)

Anyhow, in short: quantum mechanics looks a bit as if there are a large number of simultaneous realities, each with a complex number describing its "amplitude". That's all.

Some people *claim* that those amplitudes describe 'probabilities' but that is not what the theory claims. It would imply that one of the realities is the 'true' reality, but there is no hint of that in the equations of the theory.

Some people *claim* that the simultaneous realities can suddenly all cease to exist, except one (the 'collapse') but this is not at all what the theory describes. So those people are *not* giving an 'interpretation' of QM (as they euphemistically may tell you) but they are actually *rejecting* the theory (the theory contains no collapse, but only unitary time evolution of all amplitudes).

Some people *claim* that the different realities are all equally real and that with each event they 'split up' in branches to create even more realities. That is also not what the theory says. (It is claimed in some versions of the 'many worlds' view).

If we just ignore all those claims, then QM is just the description of parallel realities each with its own amplitude, which summed together can be seen as a state in the so-called Hilbert space.

That's the short answer..

...

>>>	For me this text is not clear, is not complete and does not give enough detail. What means: "her/his knowledge of quantum physics"? What is a "quantum message"? How does Bob "guess"?

>>

The sentence seems to indicate that Alice prepares a quantum message to send to Bob. If she actually does send it then of course Bob can detect it. But the sentence is not clear, perhaps it is meant that Bob can already detect it while Alice has only prepared it, without sending it. (And if that is really meant, then we do not believe the article of course.)

>

I agree

Thanks. So now the problem is probably solved!

-- Jos

11 Quantum puzzle baffles physicists

What side is up when you put it in? Or is it in a superposition?

>	2. Before we open the box is the state of the coin now "both head and tail"? 3. Is the coin in a superposition state? (IMO both no)

Assuming that it was *not* in a superposition to begin with (since you forgot to specify that) it will then depends on how exactly you move the box what the outcome will be. There must be some 'dividing line' between scenarios where you make it flip, and scenarios where you don't. If your series of movements is close enough to that dividing line, then the result will be a superposition with significant amplitudes for both outcomes. For most movements, however, the amplitude for one of the outcomes is negligible, so practically speaking it will not be in a superposition of heads and tails. (But most likely it's still in a superposition of different locations in the box!)

>	4. You open the box and you look inside. 5. Does that mean that there is a collapse of the wave function?

No, objective collapse of the wavefunction was a wrong idea of the past.

>	6. You close the box. 7. Is the coin (again) in a superposition state? (IMO no)

As explained, there is almost certainly a superposition involved for things that are not exactly fixed (position, orientation, internal position of crystal defects in the coin, etc.)

>	8. I open the box and I look inside. 9. Does that mean that there is (again) a collapse of the wave function?

Same as before! forget it! Superpositions never will go away by themselves. That's what 'unitary time evolution' means.

Now it is clear that *in your opinion* all this cannot be true, but I'm only telling you how modern quantum mechanics describes the world. Which means that I do not claim that is is necessarily true, only that it is the best theory we have.

-- Jos

12 Quantum puzzle baffles physicists

>	Or is it in a superposition?

I can not answer this question because I do not understand the question. (IMO no one can)

> >	2. Before we open the box is the state of the coin now "both head and tail"? 3. Is the coin in a superposition state? (IMO both no)

>	Assuming that it was *not* in a superposition to begin with (since you forgot to specify that) it will then depends on how exactly you move the box what the outcome will be.

SNIP

>	so practically speaking it will not be in a superposition of heads and tails. (But most likely it's still in a superposition of different locations in the box!)

Sorry to say I do not understand anything. The only thing that you can claim is how can I answer: IMO both no when I do not know what superposition means. That is correct.

> >	4. You open the box and you look inside. 5. Does that mean that there is a collapse of the wave function?

>	No, objective collapse of the wavefunction was a wrong idea of the past.

> >	6. You close the box. 7. Is the coin (again) in a superposition state? (IMO no)

>	As explained, there is almost certainly a superposition involved for things that are not exactly fixed (position, orientation, internal position of crystal defects in the coin, etc.)

(*) The only thing that I know that before I look inside the box that 'I' do not know what the actual state is of the cat. That does not mean that the cat then is in a superposition state. In fact you can also set up the experiment such that the cat has a chance of 90% being dead or 10% being dead. As a matter of fact looking by a human being inside the box has nothing to do with what is inside the box. The only change is inside the human brain. (new knowledge).

> >	8. I open the box and I look inside. 9. Does that mean that there is (again) a collapse of the wave function?

>	Same as before! forget it! Superpositions never will go away by themselves. That's what 'unitary time evolution' means.

I don't understand.

>	Now it is clear that *in your opinion* all this cannot be true, but

See above after (*) IMO that is true for everyone.

Nicolaas Vroom.

13 Quantum puzzle baffles physicists

How can I accept a theory when I do not understand it. Understanding implies that it is clear. For example: I do not understand that before I look (or some one else) in the box that the cat is both alive and dead (simultaneous). What I understand is that when I look inside the box that there is 50 - 50% chance that the cat is either alive or dead. This is in agreement in the way the experiment is set up.

The only thing that can happen is that the cat dies. The question is when. Before that event the cat is alive. There after the cat is dead.

>	If you accept it, we can explain things to you by using this theory. If you have another theory of your own, or if you want stick to 19th-century classical physics, then it is you who has to do the explaining..

> >	it is a matter for "you" or anyone to explain what it means.

>

You are of course right (and there is quite some literature about it, I would say). Likewise, for the alternative mentioned, one would have to explain what a 'classical particle' or a 'classical field' actually means! (And deal with the intrinsic problems and paradoxes..) Anyhow, in short: quantum mechanics looks a bit as if there are a large number of simultaneous realities, each with a complex number describing its "amplitude". That's all.

What have simultaneous realities and complex numbers to do with the Schroedinger's cat paradox?

>	Some people *claim* that those amplitudes describe 'probabilities' etc Some people *claim* that the simultaneous realities can suddenly etc Some people *claim* that the different realities are all equally etc

What does all of this has to do with the state of a cat in a box?

Nicolaas Vroom

14 Quantum puzzle baffles physicists

You study the theory (in the framework of studying all of Physics, I would presume). Then after you understand what the theory is saying you make your decision. That is also how most of the other phycisists have done it.

>	Understanding implies that it is clear. For example: I do not understand that before I look (or some one else) in the box that the cat is both alive and dead (simultaneous).

Then you do not understand that quantum mechanics describes the 'state' of the world as a sum of different realities. Before you understand the meaning of that concept (which is the concept of a Hilbert space) you can indeed not decide whether you accept the theory, that's true.

>	What I understand is that when I look inside the box that there is 50 - 50% chance that the cat is either alive or dead.

15 Quantum puzzle baffles physicists

[Moderator's note: Quoted text snipped. -P.H.]

>>>	2. Before we open the box is the state of the coin now "both head and tail"? 3. Is the coin in a superposition state? (IMO both no)

>>	Assuming that it was *not* in a superposition to begin with (since you forgot to specify that) it will then depends on how exactly you move the box what the outcome will be.

>

SNIP

>>	so practically speaking it will not be in a superposition of heads and tails. (But most likely it's still in a superposition of different locations in the box!)

[Moderator's note: Quoted text snipped. -P.H.]

>>	As explained, there is almost certainly a superposition involved for things that are not exactly fixed (position, orientation, internal position of crystal defects in the coin, etc.)

>	(*) The only thing that I know that before I look inside the box that 'I' do not know what the actual state is of the cat.

How do you know that? You are also a superposition of different states if we use quantum mechanics to describe you. In some parts of the superpositions you may think one thing and in other parts something else.

-- Jos

16 Quantum puzzle baffles physicists

17 Quantum puzzle baffles physicists

> >	What I understand is that when I look inside the box that there is 50 - 50% chance that the cat is either alive or dead.

To repeat: "What I understand is that before and when I look inside the box that there is 50 - 50% chance that the coin is either head or tail.

>	No that is wrong. That is only what a small minority of physicists think.

Then I agree with this minority.

>	They believe that at that point the state 'collapses' to one 'true' reality and there is a 'chance' for each of the previously present realities to become the chosen one.

That sentence is not clear to me. I do not understand it. How do you know that this minority also believes that?

>	If such a collapse does not happen, then there is no meaning to the concept of chance.

Before I open any box (generally speaking) I do not know what is inside. When I threw dice I do not know what the outcome is before I look. When I play cards I do not know which cards are in my hand before I look. When I want to know the weather I have to look outside. Does that mean that something collapses when I do that? And does something collapses when some else does this? Generally speaking nothing chances when you perform these observations except in the brains of the people present.

>	(And few physicists believe that such a collapse actually happens. Only those adhering to the Ghirardi-Rimini-Weber theory or the Penrose interpretation: https://en.wikipedia.org/wiki/Objective-collapse_theory>> ).

SNIP

> >	What have simultaneous realities and complex numbers to do with the Schroedinger's cat paradox?

>	O, that's easy: in the Schroedinger's cat paradox there are two of them (two simultaneous realities) and both of them have a complex amplitude (so that's two complex numbers).

I do not understand what "two simultaneous realities" mean. IMO there is only one "reality"

>	Of course we then are only using the most simple case. If we also distinguish between different positions in the box where the cat can be present, then there are more complex number, but that should be clear now.

Sorry, it is not clear to me.

Nicolaas Vroom.