Quantum Physics - Schrodinger’s Cat: The great thought Experiment

Superposition of the Cat

The cat is in superposition, both dead and alive?
The cat is in superposition, both dead and alive?

Thought Experiment

Schrodinger’s cat is a thought experiment which was devised by you guessed it, Erwin Schrodinger. It was based around a paper called EPR, so called because it was written by Einstein, Podolsky and Rosen.

EPR, pointed out the strange nature of quantum superposition (the combination of all possible states of a system), and was developed to challenge the Copenhagen Interpretation, which states that the superposition or wave function collapses on the moment of observation.

Now to show the incompleteness of quantum mechanics Schrodinger developed a thought experiment, which applied quantum mechanics to a cat. The purpose of the experiment was to impose a superposition from a subatomic particle to a macro (large scale) system, by coupling the particle with the cat.

If we put a cat in a sealed box where the cat’s life depends upon the state of a particle, according to the Copenhagen interpretation it meant that the cat was in superposition, it is both dead and alive until the box is opened.

Schrodinger did this to show the strangeness of Quantum Mechanics and as a counter argument to the Copenhagen interpretation.

Erwin Schrödinger

The great Erwin Schrödinger
The great Erwin Schrödinger

Schrodinger proposal

This is what Schrodinger proposed:

“One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.

It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.”

See the video below for a detailed explanation of the thought experiment:

Schrödinger's cat Thought Experiment

The point of this thought of experiment was to try to find out when does a system stop being described in the quantum sense and start being described in the classical sense. It was also performed to highlight the apparent paradox that we cannot believe a cat is both dead and alive, yet the theory says it is!

Now this theory brought a lot of other interpretations and arguments I will discuss just a few:

The Copenhagen Interpretation

The Copenhagen interpretation, countered Schrodinger’s argument with the following, they stated that the collapse of the wavefunction is caused when you open the box, at the time they argued that the observation takes place then the particle hits the Geiger counter hence collapsing the wavefunction before the box is open.

However more work has been done recently on the Copenhagen interpretation and the results we get from it are extremely accurate, what needs to be understood is what constitutes a measurement, is the mere consciousness of the cat enough to collapse the wavefunction?

Objective Collapse Theories

Objective collapse theories means that the wavefunction collapses spontaneously, before the box is opened. It does this when a time, mass, temperature reach a certain threshold. Hence the cat would have settled into a definite state before it was observed.

This theory requires a slight change but has big implications as it requires the wavefunction to collapse by the passage of time. So this means that the state is determined by the one before is and the one before that one etc etc, all the way back to very beginning of time. It’s state was pre-determined from the beginning of the universe, wow and we thought we had free will!

Splitting of Worlds

Everett's Many-Worlds Interpretation

Everett's many-worlds interpretation is pretty much in the title. Everett suggested that the observation of a system is not a “special process”. That in fact both the dead and alive states exist but are decoherent. So when the box is opened two worlds are created one in which the cat is alive and one in which it is dead.

Since they are decoherent there can be no communication between the two. When an observer opens the box they become entangles with the cat and hence you observe a single state with no interference from the other. See video below for more info on this interpretation.

One of the main difficulties is distinguishing this interpretation from the Copenhagen interpretation as there is no communication between the separate worlds. However there was a thought experiment thought up by Max Tegmark, however I will discuss this in another hub.

THE book on Schrodingers Cat

In Search of Schrdinger's Cat. John Gribbin
In Search of Schrdinger's Cat. John Gribbin

This is the best book on the subject, it is extremely readable and extends on many of the topics touched on here. A book I highly recommend.



Even though it was a purely theoretical thought experiment applications of the principles have been found in quantum computing, namely the state of the cat matches that of the qubit where they are all in superposition i.e. all 1’s and 0’s. For more information check out some of the great books listed. Also for more quantum physics experiemts check out more in the series.

Comments 35 comments

Helen 7 years ago

A really good explination of a complex topic, really well writen and is absolutely fascinating! Has this experiment ever been performed?

Nolimits nana 7 years ago

All right, my mind's been blown. Does this hub really exist if I don't read it?

Ain't quantum physics cool!

Frieda Babbley profile image

Frieda Babbley 7 years ago from Saint Louis, MO

Love this. My son is really into physics and enjoyed his class so much. He thinks about things like this all the time. Think I'll pass this onto him. Thanks.

And congrats on the HubNugget nomination: http://hubpages.com/hub/Nuggets-of-the-Green-Corn-...

dipless profile image

dipless 7 years ago from Manchester Author

Thanks Friea, Is an amazing topic and I just want to show people how crazy the quantum world is, If he has any questions pass them onto me? Also this is becoming an entire series of articles check out my cental hub for more links http://hubpages.com/education/Quantum-Physics

Thanks again

Frieda Babbley profile image

Frieda Babbley 7 years ago from Saint Louis, MO

I've checked that out and let him know about it. I'm posting on our private readers and writers club network that I put together for the kids for the summer. He's anxious to start reading them. I'll be sure to let him know that we can ask you questions about it. Thanks much for that!

dipless profile image

dipless 7 years ago from Manchester Author

That's no worries at all

RKHenry profile image

RKHenry 7 years ago from Neighborhood museum in Somewhere, USA

I agree with Helen. Excellent explanation of a difficult topic. Love the videos you added. By the way, this hub got my vote. Congrats on being nominated.

ripplemaker profile image

ripplemaker 7 years ago from Cebu, Philippines

Dipless, quantum stuff makes my head spin.  LOL But this is really very interesting stuff.  Now I know who to go and ask for help if I need someone to explain to me in the most simplest way possible that my brain could possibly absorb. 

Congratulations for being a Hubnugget Wannabe!!  Calling all Quantum lovers out there to vote for this hub.  This link will take you to the Green Corn Moon hub of Patty where exciting things are happening with the Hubnuggets!  http://hubpages.com/hub/Nuggets-of-the-Green-Corn-...

Patty Inglish, MS profile image

Patty Inglish, MS 7 years ago from North America

Physics and cats together - what could be MORE fun?

KCC Big Country profile image

KCC Big Country 7 years ago from Central Texas

Wasn't it Quantum physics guru Fred Wolf that said in "What the Bleep Do We Know?" that "quantum phyics isn't weirder than we know, it's weirder than we CAN know" or something along those lines. I love studying up on it. Thanks for the hub!

wandererh profile image

wandererh 7 years ago from Singapore

Ever heard of retro causation? I am thinking that this might actually be a form of the Schrodinger's Cat experiment in action as I really can't wrap my mind around the possibility that the past can be changed. Would love to see a hub on that. :)

urgr8 7 years ago

This is gr8 hub really well explained to a laymen thks

dipless profile image

dipless 7 years ago from Manchester Author

@RKHenry Glad you enjoyed it thanks for the support

dipless profile image

dipless 7 years ago from Manchester Author

@ ripplemaker, thanks again for the support, was really chuffed to be nominated for a hubnugget, is just a topic which is close to my heart :) And yes if you ever want anything explained or written about physics just let me know and i will do my best :)

dipless profile image

dipless 7 years ago from Manchester Author

@Patty Inglish, MS I totally agree two of my favourite things combined into 1 topic my idea of heaven ;)

dipless profile image

dipless 7 years ago from Manchester Author

@KCC He did indeed, what a great scientist, he is so so good at explaining these concepts in simple ways a real inspiration

dipless profile image

dipless 7 years ago from Manchester Author

@wandereah retro causation, now there is agood topic I will have to do a hub on that...I am not sure that it directly applies to this experiment however it is an angle i never thought of it from i will ponder it for a while... It is believed to have some link with entanglement and the concept of non-locaility you can take a look at http://hubpages.com/education/Quantum-Physics---Be... for more info.

dipless profile image

dipless 7 years ago from Manchester Author

@urgr8, thanks i am glad you enjoyed it

Pearldiver profile image

Pearldiver 7 years ago from Tomorrow - In Words & NZ Time.

Nicely put together Dipless ... This has helped me understand the paralels of Bumble Town politics and options/outcomes available for the mayor's cat. Good Luck in the HubNuggets.


dipless profile image

dipless 7 years ago from Manchester Author

Lol glad that I helpped you understand the weird and wonderfulness of Bumble town, and yes great way to dispose of the cat ;) Thanks for the support :)

RedElf profile image

RedElf 7 years ago from Canada

Interesting information, well presented. Congratulations on your nomination

glassvisage profile image

glassvisage 7 years ago from Northern California

I never would have thought that I would ever read something about quantum physics... or seen an experiment involving cats such as this! Only on HubPages... :)

dipless profile image

dipless 7 years ago from Manchester Author

@RedElf, thanks for your commentglad you found it interesting

@glassvisage makes me pleased to have introduced this strange world to someone who has never read about it. hope you found it fascinating

B.T. Evilpants profile image

B.T. Evilpants 7 years ago from Hell, MI

I read about this experiment many years ago, and it holds my attention today, just as it did then! Excellent hub!

ripplemaker profile image

ripplemaker 7 years ago from Cebu, Philippines

Dipless, I just read the Hubpages newsletter and saw your hub on first place. Congratulations!!! :-) Yipppeeee! Let's celebrate!!!

dipless profile image

dipless 7 years ago from Manchester Author

@B.T. glad you enjoyed it is a really interesting subject :D

@ ripplemaker, yeah a celebration is in order... yippppeeeee!!!!! Thanks again :)

metaphysician profile image

metaphysician 7 years ago

Quantum physics is truly an interesting subject. These theoretical experiments really spins the brain hard. Quantum as a science explains well for what spiritual is trying to teach particular from the eastern myths.

The quantum world and the mind is closely related perhaps the ultimate reality is in the mind.

Keith J. Schiller 7 years ago

I opened Schrodinger's box, and found the basics of dimensional definitions. They have already been discovered, but not defined. Opening the box came with a powerful sensation. The dimensions are based on mathematical heirarchy. From singularity to multiplicity, (Arithmatic) to direction, (Geometry) to volume combined with direction, and numbers, (Calculus)

I know that this is correct but I am not a mathmatician,

If interested write me at k.j.schiller@comcast.net

Have fun!

Lou 5 years ago

stuped this comments

lobobrandon profile image

lobobrandon 5 years ago

Awesome hub!! This should be ranked #1 instead of the other crazy articles. Thanks to your forum post I found you and these hubs :)

I'm surely following you. I'll be writing a few hubs as well after my exams on anything that you miss and surely I'll link back to you. Looking forward to it :)

dipless profile image

dipless 5 years ago from Manchester Author

@lobobrandon thank you for your kind words :) Glad you enjoyed it.

I'll be building up the series and expanding it over the coming month to try and encourage people to look at quantum physics and the exciting reality of the universe in which we live.

Look forward to reading some of your Hubs :D

lobobrandon profile image

lobobrandon 5 years ago

You'll have to wait till the end of Jan as first I'll be doing a 60 hubs in 20 days challenge from Jan 10th or so and after that will do these :D (quantum physics and other science related hubs)

dipless profile image

dipless 5 years ago from Manchester Author

No worries well, good luck with the 60 hubs in 20 days,I should really challenge myself to something like that!

lobobrandon profile image

lobobrandon 5 years ago

Thank you soo much. But, I'll stick to high quality and well researched hubs as I want traffic more than quantity :D

dipless profile image

dipless 5 years ago from Manchester Author

You are very welcome. Good Luck.

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