# Does the cat collapse itself?

#### Brainguy

Hi, I'm an 11 year old kid so I won't get any mathematics too complicated, but I don't want an answer simplified too much.
So in schrodingers cat experiment, the cat and the Geiger counter used to detect the decaying atom do not decohere the system because they are a part of that closed system. But then i say, if we want to know what's going on inside that box, couldn't we just replace the cat with a person, and the poison with something less lethal? Would the person inside the box just have one version of himself sent to a parallel universe, with the other version stepping out of the box with a definite answer as to what happened inside? But that would have to mean that a conscious person had both if his superpositions collapse, ergo having two states of himself exist in the group of universes we supposedly live in. So is this experiment possible? Would we get the disappointing, seemingly classical result I suggested?
Ty!

Related Quantum Physics News on Phys.org

#### DaveC426913

Gold Member
Hi, I'm an 11 year old kid so I won't get any mathematics too complicated, but I don't want an answer simplified too much.
So in schrodingers cat experiment, the cat and the Geiger counter used to detect the decaying atom do not decohere the system because they are a part of that closed system. But then i say, if we want to know what's going on inside that box, couldn't we just replace the cat with a person, and the poison with something less lethal? Would the person inside the box just have one version of himself sent to a parallel universe, with the other version stepping out of the box with a definite answer as to what happened inside? But that would have to mean that a conscious person had both if his superpositions collapse, ergo having two states of himself exist in the group of universes we supposedly live in. So is this experiment possible? Would we get the disappointing, seemingly classical result I suggested?
Ty!
Cat or person makes no difference. Person in box will collapse the state of everything in the box.

However there are still two states existing from an observer outside the box: one where the person in the box did observe the hammer break the poison vial, and one where he observed the hammer to not break the poison vial.

Once the box is opened, the observation of the external observer will collapse those two states in one.

#### Brainguy

That's the fastest answer I've ever gotten :) I hadn't even left the site yet. So the person in the box collapses it only for himself, but for the person on he outside, it is still a wave function? How is that possible? It wouldn't mean much to call it alternate realities for the two people, especially when talking about quantum weirdness, but how does that happen? Ty!

#### phinds

Gold Member
Dave what do you think about the argument that goes as follows. The external observer collapses two states inside the box, but then his lab assistant who went to take a pee comes back and has two possible states to observe; first his boss saw the cat dead and second his boss saw the cat alive. Taking this to its logical extreme you get that it's not possible to collapse anything in the first place because it requires the whole universe to be in on it.

I find this an interesting point of view but don't know enough to judge it on any merits.

#### Drakkith

Staff Emeritus
2018 Award
That's the fastest answer I've ever gotten :) I hadn't even left the site yet. So the person in the box collapses it only for himself, but for the person on he outside, it is still a wave function? How is that possible? It wouldn't mean much to call it alternate realities for the two people, especially when talking about quantum weirdness, but how does that happen? Ty!
Hold on, this is a complicated theory to explain completely and there are many misconceptions with the cat in the box. First of all, the cat in a box is only a thought problem. It serves to represent the fact that until you measure something, you don't know what the outcome is. It could be any of the possibilities!

If you pull out a ruler and go to measure something, do you know what the exact size will be? Let's say you take a guess and come up with 6 inches for the length of a stick. Until you measure it, you don't know whether it is more than 6, less than 6, or so close to 6 that you can say it is exactly 6. Like the cat in the box, you don't know what it is until you measure it! (In this case, our "observation" will be the measuring of the stick) As far as you know it could be any of them! While this isn't the usual explanation, I don't know how in depth your knowledge of science is, so I'm trying not to make it too complicated.

Many people take this a step further and say that ALL of the possibilities exist until they are observed. I think that is an issue for people well beyond you and I at the moment, so I wouldn't think too much on it right now.

#### DaveC426913

Gold Member
Taking this to its logical extreme you get that it's not possible to collapse anything in the first place because it requires the whole universe to be in on it.
Remember that this can not really happen. Particles are interacting with each other continually throughout the universe. It is only because of the hidden nature of the contents of the box that the wave functions can get superposed.

#### phinds

Gold Member
Remember that this can not really happen. Particles are interacting with each other continually throughout the universe. It is only because of the hidden nature of the contents of the box that the wave functions can get superposed.
Clearly I'm going to have to do more reading on this. The whole thing about wave function collapse is just something I can't seem to get my head around.

Saying " this can not really happen" seems to imply that there is nothing strange with the cat. It's always either alive or dead (and if dead, there's some point at which it became dead) we just can't know which one until we open the box. My belief, from what reading I HAVE done, it that it's more complicated than that, otherwise there would never have been an issue in the first place. I think the cat thing has gotten about as much play in laymen's physics conversation as anything except maybe black holes.

#### Christopher G

As far as I know, Schrodinger originally used that argument to illustrate the disconnect between quantum processes (wavefunction collapse in particular) and what we experience.

It would be ridiculous for the cat to be both dead and alive. So the wavefunction must have collapsed at some time, but when? This is still a bit of an unresolved issue.

As for quantum ontologies... you can pretty much go hog wild.

#### romsofia

That's the fastest answer I've ever gotten :) I hadn't even left the site yet. So the person in the box collapses it only for himself, but for the person on he outside, it is still a wave function? How is that possible? It wouldn't mean much to call it alternate realities for the two people, especially when talking about quantum weirdness, but how does that happen? Ty!
The experiment is really to show how to know when a measurement has taken place. Some physicists say, you have a combination of the cat being alive, and dead. However, once the observer (person checking the box) looks inside and the cat is dead, then the observer pretty much killed the cat because there was a chance the cat was still alive, and thus, the observer made a measurement

Some physicists choose to believe that the measurement takes place when the Geiger counter is triggered by an atom decaying.

As I said before, the experiment is asking "how do we know when a measurement has taken place?"

EDIT: Never learned about this, but I looked in the back of "Introduction to Quantum mechanics second edition by David J. Griffiths", and he use no math to explain it, so I would check it out if you could!

Last edited:

#### Christopher G

In reality you have a combination of the cat being alive, and dead.
I don't agree with this, even though the general prescription for QM would imply this is what happens.

The original argument (if memory serves) was to point out this scenario as ridiculous, and illustrate that quantum mechanics seems to break down at the macro level (at some point the wavefunction collapses).

Last edited:

#### romsofia

I don't agree with this.

The original argument (if memory serves) was to point out this scenario as ridiculous, and illustrate that quantum mechanics seems to break down at the macro level.
Ah yes, it says most physicists call it ridiculous; however, there is no proof (that I know of) to say the cat cannot be the linear combination of the two! I had to offer both sides of the argument (the one that says the cat is in both states until the observer opens the box, and the one that when the Geiger counter slams the hammer down due to the atom decay).

I should probably edit my post and take out the "In reality" part haha

#### riezer

I think superpositions can only occur to observables like position, spin, momentum. Since being dead and alive are not observables. Then it's not possible to have superpositions of dead and alive.

But in Many Worlds, it seems the rule changes such that everything can be in superposition... you just separate them into alternative histories.

#### Christopher G

I think superpositions can only occur to observables like position, spin, momentum
That's all a cat is made of. A collection of particles with the mentioned properties.
Living is the particles in a certain state, so is dead.

So (to play devils advocate) QM does say that the cat will be alive/dead at the same time.
But few people actually think that's what happenes.

#### riezer

Hi, I'm an 11 year old kid so I won't get any mathematics too complicated, but I don't want an answer simplified too much.
So in schrodingers cat experiment, the cat and the Geiger counter used to detect the decaying atom do not decohere the system because they are a part of that closed system. But then i say, if we want to know what's going on inside that box, couldn't we just replace the cat with a person, and the poison with something less lethal? Would the person inside the box just have one version of himself sent to a parallel universe, with the other version stepping out of the box with a definite answer as to what happened inside? But that would have to mean that a conscious person had both if his superpositions collapse, ergo having two states of himself exist in the group of universes we supposedly live in. So is this experiment possible? Would we get the disappointing, seemingly classical result I suggested?
Ty!
Always remember that in Copenhagen, the collapse is only in the equations. This is because no collapse agent can be found. So you have to look at the world thru the lens of equations. And only observables can be in superposition. I think Copenhagen is like a dumb down mode in which one has induce in onself some kind of Schizophrenia in viewing the world as mere equations on paper. Physicists love its mindset because it would prevent them from thinking deeper.

For those who want to think deeper. They are offered Many Worlds which split worlds and this sound a bit silly.

For Bohmians, collapse doesn't occur too.. instead something is piloting the wave.

We need to entertain others. I wonder if the above 3 are the only "main" possibilities. Can't there be something else better?

#### DaveC426913

Gold Member
Some strange views here.

Physicists love its mindset because it would prevent them from thinking deeper.
Physicists are the deepest thinkers of all, since they more than any other profession, deal with the very foundation upon which reality itself and all other sciences rest. You can't get any deeper.

The reason they deal with equations is that any other form of thinking uses analogies and models which compare themselves to things we already know, and thus, since they're comparisons, must be flawed.

We need to entertain others. I wonder if the above 3 are the only "main" possibilities. Can't there be something else better?
Are we doing science here or writing a movie script?

#### Brainguy

The fact that the cat is a macroscopic object can be ignored, because it's only meant to describe how it works at the subatomic level. It IS in a superposition because if you measure a photons horizontal/vertical polarization, and get 100% V, and then measure it's diagonal/antidiagonal polarization and get 100% D, and then measure it on H/V again, you have a 50% chance of getting H and a 50% chance of getting V! it proves that measurements DO affect systems by collapsing their wave function.

#### Christopher G

The fact that the cat is a macroscopic object can be ignored, because it's only meant to describe how it works at the subatomic level.
It's not meant to describe how it works at the subatomic level. You cannot ignore the fact that the cat is a macroscopic object. If you have read my previous posts you will see that Shrodinger came up with that thought experiment to show how ridiculous it would be for a cat to be both live and dead.

It IS in a superposition because if you measure a photons horizontal/vertical polarization, and get 100% V, and then measure it's diagonal/antidiagonal polarization and get 100% D, and then measure it on H/V again, you have a 50% chance of getting H and a 50% chance of getting V!
Like I said, particles do not behave like macroscopic objects.

#### Delta Kilo

I think superpositions can only occur to observables like position, spin, momentum.
No, it is the other way around, Wavefunction can be in superposition, but when it comes to actual measurement only one outcome is observed. Also superposition is a relative thing, what looks like a single pure state in one basis look like a superposition in another.

#### Brainguy

Just want to bump this thread and maybe get another reply ty!

#### DaveC426913

Gold Member
Just want to bump this thread and maybe get another reply ty!
Uh. Three hours is a little slim to be needing a bump - even here at PF.

Was there more to say? I didn't see any new questions.

#### gamma5772

It is experimental fact, as has been pointed out, that for microscopic objects, the Schrodinger cat experiment indicates that the cat (photon, electron, what have you) actually is in a superposition.

Let us for now assume that quantum mechanics does not include wavefunction collapse. We can deal with that later.

The argument posed is this:
P1. It violates experience and intuition to have a macroscopic object in a quantum superposition. ("I've never seen a cat in a superposition before!").
P2. Quantum mechanics predicts that the Schrodinger cat experiment has as an outcome a macroscopic object in a quantum superposition.
P3. If a theory predicts an outcome that violates experience and intuition, then it is wrong.
C. Therefore, quantum mechanics (w/o collapse) is wrong.

There are several attempted resolutions to this problem.

First, we can accept the conclusion and try to define precisely when wavefunction collapse occurs -- something the orthodox interpretation leaves conveniently vague -- (which runs into problems such as why are we drawing the line between macroscopic and microscopic here? One such theory is the GRW interpretation.)

Second, we can reject P1, saying that we do see macroscopic objects in superpositions, all the time. This is the Many Worlds Interpretation (MWI). I personally find this the most satisfying and elegant interpretation of quantum mechanics. How would the cat experiment work? The cat would be in a quantum superposition of alive and dead (let's assume it is perfectly isolated from the outside world). When the scientist observes the cat, he would be in a superposition of observing an alive cat and observing a dead cat. When the scientist is recounting his experiment, he is in a superposition of publishing a paper saying he observed an alive cat, and of publishing a paper saying he observed a dead cat. In this way, we see that the radioactive decay in the cat's chamber caused a "split" in the world -- there are two entangled branches of the universal wavefunction, one in which the cat is dead and one in which the cat is alive. It would require an astronomical coincidence for these two branches to ever interfere again -- in jargon they have decohered. We might as well call them separate "worlds."

It is perhaps unsatisfying to consider yourself in a superposition -- ("I've never felt like I've been in a superposition before!") -- but how would you tell? It wouldn't feel any different.

Finally, with the MWI, it is possible to recover the appearance of wavefunction collapse and the correct probabilities, although there are some subtleties in what we mean by probability, etc.

(Note: I plead guilty to at least one count of false dichotomy. There are alternative interpretations of QM which neither have wavefunction collapse, nor have many worlds. One such theory is Bohmian mechanics.)

#### Christopher G

It is experimental fact, as has been pointed out, that for microscopic objects, the Schrodinger cat experiment indicates that the cat (photon, electron, what have you) actually is in a superposition.
The Schrodinger cat experiment does not indicate that the cat is in superposition. Is is certainly not experimental fact that the cat is in superposition.

People seem to have a very difficult time understanding what the point of the experiment is. (Don't take this as offencive!) It simply serves to illustrate (as I have said too many times in this thread) the difference between the quantum world and the macroscopic world; When does a measurement take place? If you don't look at a far away planet is it's weather/clouds in a massive superposition of all possibilities until a human comes by to look at it? Unlikely (what's so special about a human?); so when does it collapse?

Decoherence (in the way that you mean it with 'many worlds') is an attempt to allow objects to remain in superposition 'post measurement' while still seemingly collapsing. This may be true, and the whole universe may be in superposition, but it would still have to behave effectively as if it wasn't. This is because we don't see objects in superposition. So if they are, it must be suppressed immensely (we don't ever see two cats).

More importantly, Discoherence has nothing to say about WHEN a measurement takes place. (when do the worlds split?)

Last edited:

#### agentredlum

Using Quantum Mechanics is it possible to calculate a probability that Schrodingers cat is playing 'ragtime blues' when you open the box?

#### Christopher G

Using Quantum Mechanics is it possible to calculate a probability that Schrodingers cat is playing 'ragtime blues' when you open the box?
Yes, but he would only be able to do so for a few attoseconds as he would be in violation of the conservation of energy (to get a guitar), so he would have to strum quickly.

Last edited:

#### agentredlum

Yes, but he would only be able to do so for a few attoseconds at he would be in violation of the conservation of energy (to get a guitar), so he would have to strum quickly.
LOL

Using Quantum Mechanics is it possible to calculate the probability that conservation of energy does not hold so actually as you are calculating probabilities, the cat grows an extra head.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving