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Schrödinger's Cat

101 said:
This is why my retort made mention of the standard Judeo-Christian omniscient God. He's observing every moment of probabliity at every moment of space and time. Thus, His observations define reality long before you or I or Schrodinger has opened the box and believed ourselves to have just collapsed a probability wave. The cat's dead as soon as God notices it's dead, which isn't dependant on the box being open or closed, as He can 'see through it' as it were.

But if Gods act of observation has defined reality, then we live in a 'clockwork' universe and have no free will.

The box is incidental to the thought experiment. It's still the observer who defines reality by collapsing the wave.
 
Cavynaut said:
I might be wrong, but I think that the point is that you cannot possibly know whether the cat is alive or dead until you open the box and observe it. It's as though the observer somehow creates the reality that he/she observes. By seeing the state of the cat, a probability wave is collapsed and the universe at large is then informed of the state of the cat (through greater and greater probability waves).
Yup, the point of the thought experiment is an attempt to bridge the quantum world of the microscopic and the classical world of the macroscopic.

Physicists are happy with the idea that a microscopic particle can be described as a superposition of two, or more, states. They are also (fairly) happy with the idea that the act of observation can cause the "collapse" of the wave function into one of those states.

One nice analogue of this process is what you can do with polarised light. If you have enough polaroid sunglasses you might be able to try this at home. ;)

If you shine a light through the lens of polaroid sunglasses, you convert the light into a linearly polarised state. What you can now do is to put another pair of sunglasses into the path/beam and slowly turn them through 90°. You should see the light grow darker until, at 90°, nothing (or at least a very good approximation of nothing) gets through.)

Now comes the sneaky part. ;) Get a third pair of polaroid sunglasses, and stick a lens between the other two pairs. Turn it through 45° from the first (or halfway between the firts and second) and Hey Presto! you have light getting through again!

Why?

If you have read this far, you're probably wondering why I'm boring you with talk about sunglasses, and not superpositions of wavefunctions. ;) Well, this is exactly what we have done in this little experiment.

First of all, with the first lens we have generated linearly polarised light with a polarisation of (for the sake of simplicity) 0°. With the third (or intermediate) lens at 45°, we are effectively measuring the linear polarisation at 45°. It turns out that light with a polarisation of 0° can be thought of as a superpostion of light with a polarisation of 45° and light with a polarisation of 315°, hence making the measurement can collapse some of the light into the 45° polarisation. Now when we put the polaroid lens with a polarisation of 90° into the beam we can see that the 45° beam is actually a superposition of light with polarisations of 0° and 90°, and hence some of the light will make it through.

We can now see the very real effect of "collapsing" a wavefunction, and how something everyday can be thought of as a superposition of states. The problem with quantum mechanics is to determine the point at which measurement (and hence collapse) occurs. The point of the cat is that it is very much a macroscopic object, not a funny little thing like an electron or photon. Whilst we can easily think of photons being in a superpostion of two completely different states (like we did above) it is somewhat harder to think of a cat as being a superposition of dead and alive.

So the question is, "Do we need a conscious observer to collapse a wavefunction by making a measurement, or is there something subtler at play here?" Answers on a postcard, please. ;)
 
As Marion says Niles, putting a conscious cat in a box is not easy, so it has been anaesthetised of course! It's not conscious so can't observe its own demise.
 
Fortis said:
So the question is, "Do we need a conscious observer to collapse a wavefunction by making a measurement, or is there something subtler at play here?" Answers on a postcard, please. ;)

And that question is the answer to why I find quantum physics so interesting. I can't pretend to understand it all that well, and the maths involved is mind boggling (to me anyway), but it is certainly a fascinating subject.
 
Niles Calder said:
Why can't the cat collapse it's own wave function?

But the cat does collapse the wave function. It knows it's own state, it's the observer outside the box who doesn't.

Imagine the experiment being performed in a room with only one observer. Reporters are waiting outside the room to hear the result and inform the rest of the world (it's a very popular cat after all). The observer looks in the box and sees the state of the cat, thereby collapsing the wave function. However, the reporters outside the room still don't know and for them the cat is still in a superposition of states.
 
err...I guess it's probably just me.... but didn't anyone think to use a see-through box? That way, you can fit the cat, the geiger couter, the poison gas, douglas adams, the zebra crossing and the half-glass in, and see whether the zebra crossing is half dead or not....

Or is there a flaw to the idea of defeating a thought experiment with a realistic solution?
 
Fayyaad said:
err...I guess it's probably just me.... but didn't anyone think to use a see-through box?
The whole point of this experiment is to stress the strange link between 'observation' and the 'collapse of the wave function' in quantum physics. In other words, something only seems to happen when it is 'observed'. The experiment focuses the mind on what exactly 'observing' means, and why does it 'collapse the wave function' ?

Using a glass box defeats the whole idea: it leaves proceedings under continuous observation, so there is no time for the mysterious superposed quantum states (dead cat/alive cat) to exist.

Whole books have been written on this one subject. I doubt that a MB thread will get very much further with it!
 
Thinking out of the box - sorry!

I like Schrodinger's analogy a great deal. His reductio ad absurdam comparrison between the (philosophically problematic) particle/wave duality of electrons and a cat suspended between the mutually exclusive states of life or death equally reminds me that we hold imprecise definitions of 'labels' like 'particle' and 'wave'. One must constantly bear in mind, however, that such scientific terms and axioms are not in themselves properties of reality, but are rather flexible explanations that form part of a (scientific) MODEL of reality which derrives its virtue not from its versimilitude per se, but rather from its ability to predict and explain in its own terms. That is to say science is good/helpful not because it is 'true' but because it works.

Schrodinger shows us either that the microscopic and macroscopic worlds appear to allow sharply differing behaviour and obey different laws, or (less radically) that our conception of particles, waves, and the behaviour acceptable to each is erroneous. This shouldn't be a shock - perhaps we need some new terms and axioms that don't suffer this problem - perhaps a more fundamental entity that underlies both. Does human observation affect the subject? Perhaps, but a more a more fruitful approach would seek to tidy up our scientific framework in order that seeming parardoxes do not arise. If the model isn't working (say its illogical or internally flawed) then it should be refined, rejected, or replaced as Newtonian physics was in the wake of relativity. Popper had it right with his inductive 'conjectures & refutations' whereby scientific 'truths' can only be said to be contingently true having survived numerous attempts at falsification.

Great Stuff,
Hope this makes some sense:)
 
So the question I want answered is...If the Pope shits in the woods, and there is no-one about to hear him, does he make a sound?

Sorry own little universe, I'll get my metaphorical coat!!:D :confused::cool:
 
Of course, if the cat did exist in some limbo of being both dead and alive until the box was opened and someone peeked, and if it was discovered that the cat was now dead (after collapsing the wavefunction) then it could truly be said that "curiosity killed the cat." ;)
 
Not here; our roads are silvery grey, so they paint the black bits too...
 
The roads are made from crushed granite and are all sparkly...
 
Schrödinger's cat comes closer
The physicist Erwin Schrödinger famously said that quantum theory would allow the existence of a cat that was simultaneously living and dead.

Now a team of physicists has published the recipe for making a large object - not cat-sized, but certainly bacterium-sized - in such a quantum quandary1. A tiny mirror, they propose, can be in two places at once.

Scientists are resigned to atom-sized entities being capable of such feats. But they generally assume that at larger scales a phenomenon called decoherence intervenes, stamping out quantum weirdness and fixing everyday objects to a single, definite location.

William Marshall of the University of Oxford and his coworkers outline a scheme for evading decoherence to achieve a quantum superposition of states in an object with around a hundred trillion atoms. This is about a billion times larger than anything demonstrated previously.

It's not the first proposal for achieving quantum effects in a big system. But unlike others, it is feasible with current technology. For example, mirrors like those Marshall and colleagues invoke can be made just ten thousandths of a millimetre square - about the size of a red blood cell, weighing around five billionths of a gram.

Cat's cradle

The plan goes like this. The mini-mirror, pasted on the end of a tiny arm, is hooked up to a conventional quantum object: a single photon of light in a quantum superposition. The photon is made to bounce back and forth between the small mirror and a much larger one, making the small mirror oscillate on its springy arm.

Under normal circumstances, this would be like trying to use the flapping of a fly's wing to push a yacht's sail during a storm. Vibrations of the mirror caused by heat would swamp any influence of the lone photon.

The researchers propose to calm this stormy background by cooling the apparatus to less than two thousandths of a degree above absolute zero. The mirrors would also be in a very high vacuum so as not to be disturbed by colliding gas molecules.

Beam time

In the hypothetical experiment, the light beam passes through a beam splitter, a kind of semi-mirror that lets some photons through and reflects others. Any photon can end up on one of two possible paths. Or it is possible to arrange things so that a photon effectively follows both paths at once, in a quantum superposition.

The mirror would have to be in a very high vacuum as not to be disturbed by colliding gas molecules

This enables the photon to interfere with itself, just as two light beams interfere when they cross paths, creating light and dark bands where their waves add or cancel out.

The photon can transfer its superposition to the small mirror, so that it is in two positions at once. When this happens, the photon's self-interference disappears. The researchers calculate that the system will cycle back and forth between a superposition of photon states (in which case one can detect an interference pattern) and a superposition of mirror positions (for which there is no photon interference pattern).

References
Marshall, W., Simon, C., Penrose, R. & Bouwmeester, D. Towards quantum superpositions of a mirror. Physical Review Letters, 91, 130401, doi:10.1103/PhysRevLett.91.130401 (2003). |Article|
 
Do we have a physicist on the board who can explain the above? I would love to understand it but fear I've reached my limit with this!
 
Upon noticing that there were new posts on this thread, my initial thought was Oh, no. Schroedinger's Cat has been ressurected . Then I started to think about what the implications of such an outcome would be in the model. Then my head hurt, so I stopped.
 
Slightly off thread, as usual these days. I'm reminded of Pavlov's cat, as related by Eddie Izzard:

Pavlov's cat results:
Day 1: Rang bell, cat f*cked off, oh dear.
Day 2: Rang bell, cat answered door
Day 3: Rang bell, cat said he'd eaten earlier cheeky bugger
Day4: Went to ring bell, but cat had stolen batteries
Final day Day 5: Went to ring bell with new batteries, but cat put paw on bell so it only made a thunk noise. Then cat rang his own bell, I ate food.
 
101 said:
Oh, no. Schroedinger's Cat has been ressurected

is that possible with a cat who may or may not be alive or dead? :confused: :D
(and what about the pint glass, mr adams, gieger counter, poison gas and the zebra crossing)?
 
Just to confuse matters further... does the expectation of the observer affect the result? A cat-hating person who had had an accident on a zebra crossing whilst reading H2G2 might want the cat dead.

Jane.
 
If the cat has been in the box since April then it'll probably be well and truly dead by now.

You could tell this by not lifting the lid, but just by having a sniff because I'm sure it would be a sticking festering pile of goo so many months on.
 
Tyger Lily said:
If the cat has been in the box since April then it'll probably be well and truly dead by now.

You could tell this by not lifting the lid, but just by having a sniff because I'm sure it would be a sticking festering pile of goo so many months on.

Mustn't... post... obvious... inuendo... temptation... too strong... logoff, logoff, logoff
 
I'm sure someone's said this already but...

you can not be sure what the outcome of an event may be and by observing the reaction you change the outcome. Or to put it another way (god I wish I#d thought of the teenage girls :( :

A person goes for a walk around town and their friend desides they wants to get a hold of him.

Now it's a large town and noone knows where the person is so the friend sends out several other people to find them.

They wander the town with torches looking for them, themselfs walking around in the same town.

One of the people, entirly by acieent, stumbles onto the missing person's path and not sure if they've seen them shines a torch in the persons face. Startled by the light in their face the person (who was the origional subject of the search) runs in the other direction.


Or to put it another way...by finding the person they where looking for the searchers changed where he was.

I'm sure it all makes sence now :D
 
Hi guys. I've been 'advised' to read a book called "In Search Of Schrodinger's Cat" by John Gribbin as part of an introduction into quantum physics for AS-Level Physics. I was wondering if anyone has read this book and if it is worth buying, since I am the stereotypical student with no money.:confused: :(
 
Have just read it - I know nothing about quantum physics, but I just cannot agree with the assertion that 'the cat is both dead and alive'.

Surely it is either dead or alive?

Heather. :confused:
 
No, it hasn't picked one yet, so it's both. That's the fundamental weirdness of quantum physics. It works better with photons than with cats...
 
An unobserved object in an isolated system cannot accurately be said to exhibit any characteristic whatever. If the object has no effect on another system, no intelligible statement can be made about it.

In other words, an observer must make a measurement or an effect must be felt somewhere for one state or another to become manifest.

If no one looks in the box--i.e., if the cat has no contact with any outside reality--the cat exists in neither state and both states at once. Our phenomenological reality expresses itself this way all the time, though it seems to go against common sense.

To say the cat "is" alive or "is" dead without measuring its state directly requires that we suppose, guess, hypothesize or simply opine. The measurement makes the fact. Without observation no fact exists, and no statement has any real meaning outside of poetry.
 
Dunamis said:
Hi guys. I've been 'advised' to read a book called "In Search Of Schrodinger's Cat" by John Gribbin as part of an introduction into quantum physics for AS-Level Physics. I was wondering if anyone has read this book and if it is worth buying, since I am the stereotypical student with no money.:confused: :(


ive read it and yes if you can follow it its quite good... however it may be advanced past A level and lead to trouble!... I would get it from the libury not bye it as its not something to refer to.
 
This thread was dead but I've brought it back to life.

Experiment makes Schrodinger's cat choose—things can be real, or certain, but not both

Experimenting within quantum theory is an extremely complex process, where common intuitions are regularly inverted within shifting reality. Over the years several quantum features and methods of their study have been identified. Now scientists have investigated a new set of assumptions and proposed a novel experiment, to test the consequences of making quantum theory more intuitive.

"While quantum theory is the science behind almost all of our technology, its disconnect with our everyday intuitions is still worrisome and actively researched," says lead author Associate Professor Daniel Terno.

"How do you find your way in a reality which is shifting, where the opposites are allowed to coexist? Moreover, how do you conduct experiments in it? These are the questions that must be answered when dealing with the floating world of quantum mechanics."

Throughout the development of quantum theory, a set of reasonable ideas has led to strange paradoxes, such as the famous Schrodinger's cat, which is neither dead nor alive.
Another of the most famous (and useful) results of quantum mechanics is that every object can behave as a particle or as a wave, given the right conditions. Associate Professor Terno and colleagues proposed a new experiment in 2011, which was realised by dozens of research groups worldwide. This proposal made complicated experiments much simpler, such as an experiment formerly requiring 40 meters of optical cable now being performed on just a single chip. ...

More information: "Is wave-particle objectivity compatible with determinism and locality?" Radu Ionicioiu, Thomas Jennewein, Robert B. Mann, and Daniel R. Terno, Nature Communications, 2014 DOI: 10.1038/ncomm5997

Provided by Macquarie University

"Experiment makes Schrodinger's cat choose—things can be real, or certain, but not both." September 26th, 2014. http://phys.org/news/2014-09-schrodinge ... -real.html
 
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