# Schrodingers cat question

1. Feb 22, 2014

### rede96

I have no background in quantum physics (or any physics really) its just a topic I find really interesting.

I watched a video on schrodinger's cat and am struggling to understand the cat being in a state of superposition. In the thought experiment I saw there was an radioactive atom with a half life of one hour in a box with a geiger counter connected to some poison. And of course the cat. It was explained that statically there was a 50/50 chance the atom would decay but of course according to the Copenhagen interoperation it was until you opened the box that you would find out if the cat was alive or dead and until the point the cat existed in both states.

What I was wondering is why couldn't they just put a timer on the Geiger counter, which started the moment the experiment was started. Once the lid was lifted, say 90 minutes after the start of the experiment, and if the cat was found dead, then they could have just checked the time counter to see when the atom decayed. If it was say after 60 minutes then doesn't that sort of suggest that cat didn't exist in both states before the box was opened?

Or am I missing something?

2. Feb 22, 2014

### ChrisVer

Schrodinger's cat experiment tells you that you cannot determine the system's state until you somehow interact/observe it.
On the other hand you cannot know whether the cat is dead or alive in that experiment's set-up. As you said,in your suggestion, you had again to open the box and see the cat is dead. Before that though, you only knew the chances were 50-50. What if the cat was still alive let us say?

Also, I am kind of skeptic on the timer thing- it can play the role of an observer.

3. Feb 22, 2014

### phinds

I think Schrodinger really came up with the cat thing to show how silly the Copenhagen interpretation can be.

Personally, I'm really confident that the moon is there whether I'm looking at it or not and I think the cat is always either alive or dead.

4. Feb 22, 2014

### rede96

The point was in my suggestion that yes I did have to open the box to find out if the cat was dead but if I checked the timer and found the atom decayed before I opened the box I would know the cat was dead before I opened the box. So it wasn't opening the box that determined the outcome. Which was in contradiction to what was being said.

Of course if the cat was alive I'd just repeat the experiment, but at some point I would find the cat dead.

5. Feb 22, 2014

### phinds

I think you are missing the point here that the Copenhagen interpretation does not require a PERSON to make the observation. If you have a mechanism that operates a timer based on the decay, THAT is an observation and according to Copenhagen the cat was both alive and dead up until the mechanism observed the decay and turned off the timer. That is, your solution has not avoided the problem with the Copenhagen interpretation.

6. Feb 22, 2014

### ChrisVer

That was the main thing...when I posted about the "experiment's set-up" I meant exactly the set-up of Schodinger's experiment. The observation is done by the timer...

7. Feb 22, 2014

### Catflap

It was a joke.

But it illustrates a basic concept. Any interaction between particles represents an 'observation'. The world proceeds between interactions in a state of uncertainty, which is resolved when the result of the interaction 'happens'.

It was probably the worst joke in the history of science. There's nothing less funny than a joke that has to be explained over and over and over and over....and even then, few people ever 'get' it.

8. Feb 22, 2014

### ChrisVer

Of course a few people get it. The problem with that experiment is that it considers a classical object (a cat) and people are quiet unfamiliar of thinking an $\frac{1}{\sqrt{2}} [ |alive> \pm |dead> ]$ cat....
However in QM objects this illustration seems plausible, and once you understand the QM illustration you can somehow imagine the cat's thing...

9. Feb 22, 2014

### phinds

But your OP said that you had solved the problem of the superposition of the cat. MY point was that you did not. Your scenario still has the cat in superposition, it just changes the timing of when the superposition ends from when the person opens the box to back when the timer stopped.

OOPS ... I've confused your posts with the OP's post. My apologies.

10. Feb 22, 2014

### Maui

Quantum mechanics isn't a complete description of reality. You can't really hope to explain 'cats' or derive the conflict at the Maidan in Kiev with the formalism.

11. Feb 22, 2014

### rede96

I understand that the outcome can't be predetermined but I am struggling to get my head around how the cat can exist in two states.

Maybe its just the thought experiment that is confusing me, as I could have done the same thing with a glass box and watched the whole thing unfold. During the process I wouldn't have seen two cats, one alive and one dead. I would have just seen the Geiger counter trigger, the poison release and the cat die.

The other thing that confuses me is if I watched the whole thing unfold, does that mean I predetermined the outcome?

All very confusing!

12. Feb 22, 2014

### phinds

You are missing the point of the replies. The cat does NOT exist in two states, that just a Copenhagen interpretation of the formal math of QM and Schrodinger's POINT was to illustrate that fact.

Also, according to even Copenhagen, much less common sense, if you watched there would be no "paradox" or superposition because the system would be under constant observation.

13. Feb 22, 2014

### WannabeNewton

This. The formalism in and of itself doesn't explain physically the transition that happens between unitary time evolution of the state of the system under the Schrodinger equation and the projection onto an element of a preferred basis upon measurement.

14. Feb 22, 2014

### phinds

Can you say that in English?

15. Feb 22, 2014

### Maui

He is saying what I said in the quote in a technical way - i.e. you need additional assumptions to get outcomes and.... errr....yes- atoms, molecules, cats, etc.

16. Feb 22, 2014

### ChrisVer

What you need to understand it, is kind of classical. For example think about a box with gas inside. You cannot determine the state (eg speed) of a molecule in the gas, except for if you measure it. You can speak about possible outcomes (distributions).
The main difference between that classical view (statistical mechanics) and the quantum mechanics, is that the statistics in classical physics come out of undetermined parameters (eg initial state) and our incapability of watching out the motion of billions of bodies.... The same thing happens with the dice, you can drop the dice and it will bring an outcome with 1:6 possibility (depends on the dice structure). In that case you would be able to determine every outcome if you knew all needed parameters (eg initial position, velocity, geometry of the dice etc). However statistically, you could say that dropping a dice you have 1/6 possibility of it coming out 1, 1/6 for coming out 2 etc (so it's in a superposition)
In QM , the possibilities, and so the superposition, are intrinsic and cannot be determined by any unknown parameters (Bell's experiment proved that). And according to Cop. School, which I think is closest even to the modern view, you cannot determine the outcome of an observation (dropping of dice) except for if you observe it. So observing the whole procedure let us say through a glass box, you would have immediately made the superposition to collapse in one state (cat: dead, cat :alive - no need for being 50-50 alive/dead)

Last edited: Feb 22, 2014
17. Feb 22, 2014

### bahamagreen

It is kind of a joke; the additional assumptions must include that the cat has been properly prepared so ensure that it is living the ninth of its nine lives when placed in the box.

18. Feb 22, 2014

### ChrisVer

Isn't the cat itself an observer? :tongue: in its reference frame, the collapse happens before our own... so does the reference frames play a role?

19. Feb 22, 2014

### Staff: Mentor

That's true - but the devil is in the detail of exactly in what way it showed it was silly.

According to Copenhagen there is a commonsense classical world out there and QM is a theory about observations that occur in that world.

In Schrodinger's cat the observation is at the particle detector, everything is commonsense classical from that point on - the cat is never alive and dead - it is alive or dead - period.

So what was its purpose? It bought to light the REAL issue with QM. Since its a theory about observations in an assumed classical world, and since that classical world is in fact composed of quantum objects how does such a theory explain it? In other words we need a fully quantum theory of measurement without this arbitrary classical cut. What Schrodinger's Cat showed is if you try that problems arise.

However since then a lot of work has been done, particularly in the area of decoherence, on developing such a fully quantum theory of measurement and how a classical world arises. Significant progress has been made, but some issues remain and research is ongoing.

The moon is there when you are not looking because it is being observed all the time by its environment. A few stray photons from the CMBR is enough to decohere a dust particle and give it a definite position. The moon its much larger and the number of photons it interacts with is so large its there regardless - no question.

Thanks
Bill

20. Feb 22, 2014

### Staff: Mentor

You guys are all missing the point of the thought experiment.

Schrodinger's Cat is utterly trivial in Copenhagen.

Copenhagen assumed the existence of a commonsense classical world observations appear in. The observation occurred at the particle detector - everything is classical from that point on. The cat is alive or dead - period.

Its real purpose was to show we need a fully quantum theory of measurement.

Maybe Weinberg can explain it better (see the section on Contra Quantum Mechanics):
http://scitation.aip.org/content/aip/magazine/physicstoday/article/58/11/10.1063/1.2155755 [Broken]
'Bohr’s version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wavefunction (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from?'

Scrodinger's Cat was simply a demonstration of the above. Since then a lot of work has been done fixing that issue.

There also seems to be possibly some confusion about exactly what Copenhagen says. Check out:
http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

Thanks
Bill

Last edited by a moderator: May 6, 2017