Does Schrodinger's Cat Qualify as an Observer in Quantum Mechanics?

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SUMMARY

The forum discussion centers on Schrödinger's Cat and its role as an observer in quantum mechanics. Participants agree that any macroscopic system, including the cat, can be considered an observer due to decoherence theory, which states that interactions with particles like photons collapse quantum superpositions into pure states. The conversation also explores superfluidity and Bose-Einstein condensation, particularly in liquid helium, highlighting its unique properties such as flowing without viscosity and quantized angular momentum. The discussion concludes with a call for mathematical examples to clarify concepts like superposition, mixed states, and pure states.

PREREQUISITES
  • Understanding of quantum mechanics principles, particularly decoherence theory.
  • Familiarity with Schrödinger's thought experiment and its implications.
  • Knowledge of Bose-Einstein condensation and superfluidity, especially in liquid helium.
  • Basic grasp of quantum states, including superposition, mixed states, and pure states.
NEXT STEPS
  • Research "Quantum Decoherence" to understand its impact on macroscopic systems.
  • Study "Bose-Einstein Condensation" and its applications in superfluidity.
  • Explore "Quantum Superposition" and its mathematical representations using kets and density matrices.
  • Investigate the "GRW Theory" (Ghirardi, Rimini, and Weber) for alternative interpretations of quantum measurement.
USEFUL FOR

Students and professionals in physics, particularly those interested in quantum mechanics, quantum computing, and theoretical physics. This discussion is also beneficial for anyone seeking to understand the implications of observer effects in quantum systems.

dgorman
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I'm sure you've all heard of Schrödinger's thought experiment regarding Quantum Mechanics.

The question is: does the cat constitute an observer?
 
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What if the cat is dead? Well, the cat won't die and come back to life if you open the box, so something else has to be going on here.
 
According to decoherence theory, even a stray photon "counts" as an observer.

- Warren
 
Yeah I know, I am observant. Wait, you weren't talking about me. Sorry. :biggrin:
So dead cats can observe, and the box can observe? Or is it something special about photons?
 
Any particle interacting with an object in a mixed state will collapse the superposition and puit the object into a pure state.

- Warren
 
Ah. Now i know exactly what you mean.
 
The idea is called "decoherence," and it's the reason we don't see quantum-mechanical effects [often] on the macroscopic level. All the billions of particles interacting with the system -- atoms of air, infrared photons, etc. -- all serve to keep macroscopic objects like cats out of quantum superpositions.

Plenty of good sites abound:

http://en.wikipedia.org/wiki/Quantum_decoherence

- Warren
 
It's rather odd though. That a single particle would determine the cats fate. That theory implies that if the cat breathed, it's fate would be determined. The cat would have to be in a complete vacuum to stay alive/dead. :biggrin: (alive/dead) sounds weird...)
 
He'd also have to be at absolute zero, which isn't physically possible. Really, a cat is a pretty poor laboratory for quantum-mechanical effects, no matter how you kill it.

- Warren
 
  • #10
no matter how you kill it
I went with curiosity and never looked back. :biggrin:
 
  • #11
BTW, there are some ways to observe macroscopic quantum mechanical effects, but they don't involve house pets -- are you familair with superfluids?

- Warren
 
  • #12
are you familair with superfluids?
Sure, I love orange juice!
But seriously, what are they?
 
  • #13
Well, I'll give you a bit to start with. If you take a bucket of liquid helium and cool it down to within a few degrees of absolute zero, it undergoes a phase transition called Bose-Einstein condensation. All the atoms hurry to enter the same quantum state, because that minimizes their total energy. So you wind up with a bucket full of atoms that have all agreed to be in the same state. What happens when you try to use it like a normal fluid?

Well, you'll notice it flows without viscosity. That's right, it flows without resistance through even the very smallest pores in your container, and through even the smallest pipettes. Why? Because all the atoms are already in their lowest energy state. Since they're all doing the same thing, though, the walls of a pipette can't smack them around too much -- you can't smack around one, you have to smack around every single last trillion of them.

Superfluids also will only permit certain values of angular momentum, e.g. 3 or 5 or 7 rotations per second. Even if you spin the bucket at 4 rotations per second from now to eternity, the helium atoms won't care. Angular momentum is quantized for their collective quantum state, and they'll only rotate at 3, 5, or 7, and never, ever at 4.

And the list goes on. You can basically consider a bucketful of liquid helium to be like one giant macroscopic quantum object.

- Warren
 
  • #14
That is very odd sounding. I never really took the time to think of a way for liquid to flow without viscosity. Makes you wonder what Schrödinger was thinking about the cat.
 
  • #15
Quantum mechanics is pretty "weird," for sure, if by "weird" I mean "contrary to our everyday experience." It just takes more specialized apparatus than a cat to let you see it with your own eyes. :smile:

- Warren
 
  • #16
chroot said:
Well, I'll give you a bit to start with. If you take a bucket of liquid helium and cool it down to within a few degrees of absolute zero, it undergoes a phase transition called Bose-Einstein condensation. All the atoms hurry to enter the same quantum state, because that minimizes their total energy. So you wind up with a bucket full of atoms that have all agreed to be in the same state. What happens when you try to use it like a normal fluid?

I don't think that that counts as a BEC. http://www.aip.org/physnews/update/532-2.html announcement would seem to imply otherwise. I'm pretty sure BEC is a transition in a gaseous state, without becoming a liquid or a solid.
 
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  • #17
Sorry, you're wrong. Superfluid liquid helium is a BEC. Please do some more reading.

- Warren
 
  • #18
chroot said:
Well, I'll give you a bit to start with. If you take a bucket of liquid helium and cool it down to within a few degrees of absolute zero, it undergoes a phase transition called Bose-Einstein condensation. All the atoms hurry to enter the same quantum state, because that minimizes their total energy. So you wind up with a bucket full of atoms that have all agreed to be in the same state. What happens when you try to use it like a normal fluid?

Well, you'll notice it flows without viscosity. That's right, it flows without resistance through even the very smallest pores in your container, and through even the smallest pipettes. Why? Because all the atoms are already in their lowest energy state. Since they're all doing the same thing, though, the walls of a pipette can't smack them around too much -- you can't smack around one, you have to smack around every single last trillion of them.

Superfluids also will only permit certain values of angular momentum, e.g. 3 or 5 or 7 rotations per second. Even if you spin the bucket at 4 rotations per second from now to eternity, the helium atoms won't care. Angular momentum is quantized for their collective quantum state, and they'll only rotate at 3, 5, or 7, and never, ever at 4.

And the list goes on. You can basically consider a bucketful of liquid helium to be like one giant macroscopic quantum object.

- Warren

That also fascinates me.

But you say 3, 5 or 7. Does it extend to 1 and -1, or is it an approximation? Is zero a valid angular speed?

And is there a phenomenon of superposition of different speeds?
 
  • #19
swansont said:
I don't think that that counts as a BEC. http://www.aip.org/physnews/update/532-2.html announcement would seem to imply otherwise. I'm pretty sure BEC is a transition in a gaseous state, without becoming a liquid or a solid.

Instead of liquid, gas, solid, you should be thinking that helium-4 is indeed a boson (why is that? because it has an overall integer-valued spin!) which means that macroscopic quantities of the stuff follow Bose-Einstein statistics, which means that an indefinite number of them can collapse into the same state (unlike fermions which obey the Pauli exclusion principle) and so we can create macroscopic chunks of matter for which all the constituent particles are in the same state, which gives the Bose-Einstein condensate its unusual properties.

Marissa* said:
That also fascinates me.

But you say 3, 5 or 7. Does it extend to 1 and -1, or is it an approximation? Is zero a valid angular speed?

Yes, 1 and 0 are valid values for angular momentum. Usually we talk about the absolute value, but whenever direction matters we see negative spins like -1 along with positive ones.

And is there a phenomenon of superposition of different speeds?

Yes! It might surprise you to know that any quantum state can be written as a superposition of different speeds. But the same is true if the word speed is replaced by "Energy", "Position", or any other physical variable you can think of!
 
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  • #20
dgorman said:
does the cat constitute an observer?

Yes. Any macroscopic system is an "observer".
 
  • #21
chroot said:
Any particle interacting with an object in a mixed state will collapse the superposition and puit the object into a pure state.

- Warren

Can you give a mathematical example? I mean something with kets and density matrices. I'm having trouble differentiating "superposition", "mixed state" and "pure state". You can have a pure state that's in a superposition wrt some basis, no?
 
  • #22
dgorman said:
I'm sure you've all heard of Schrödinger's thought experiment regarding Quantum Mechanics.

The question is: does the cat constitute an observer?

Correct me of I'm wrong but the cat does not collapse its own state. The observer must be outside the system being observed. So, there is still a superposition of states in the system.
 
  • #23
Anybody familiar with the GRW Theory (Ghirardi, Rimini and Weber)?

http://philosophy.rutgers.edu/FACSTAFF/BIOS/PAPERS/LOEWER/loewer-schroedingers-cat.pdf
 
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  • #24
hello can anyone send me some technical notes about Schrödinger's cat
thanks
 
  • #25
I don't have any notes on it... but what would you like to know?
 

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