Schrodinger's cat gets a reality check

February 12, 2015 by Eric Cavalcanti, The Conversation
If kitty goes in, will she really be alive and dead? Credit: Robert Couse-Baker/Flickr, CC BY-SA

It's a century-old debate: what is the meaning of the wave function, the central object of quantum mechanics? Is Schrödinger's cat really dead and alive?

I was recently involved in an experiment conducted by Andrew White's Quantum Technology Lab at the University of Queensland that has now provided the most significant evidence on that question in years. And it doesn't look good for the cat.

To understand the importance of this result, we need to delve into its history. At the root of there is something of a reality crisis. Multiple interpretations of the theory exist, and they paint very different pictures of the world. One of the major contentions centres around what we should make of the quantum wave function.

In short, the wave function describes the of a physical system. But unlike in classical physics, where a complete specification of a state determines all of its properties (for example, a particle's position and velocity), the quantum state in general only gives probabilistic predictions.

In fact, the wave function seems to describe bizarre situations, like physical systems existing simultaneously in multiple states, such as different positions or velocities. It gives very precise probabilities for the possible outcomes of laboratory experiments, but it defies an intuitive interpretation.

Some of the founders of the theory, such as physicists Niels Bohr and Werner Heisenberg, suggested that until an observation of one or another property is made, questions like "where is this particle, really?" simply don't make sense.

Under this view, it's not that the particle is really here or there (and we just don't know until we look). Rather, for Bohr the very meaning of "position" depends on the existence of a measurement that detects it.

Physicist Erwin Schrödinger's famous thought experiment was designed to show how, if quantum mechanics is taken literally and to its ultimate implications, even macroscopic systems, like cats, would be in such "superpositions" of states – such as the cat being both dead and alive – which is an apparently absurd conclusion.

It's all in your head

An indeterminate reality was unacceptable for Albert Einstein, who famously said: "Do you really believe the moon exists only when you look at it?" Einstein believed instead that the wave function should be understood as representing our limited information about the actual state of physical systems.

A first blow to Einstein's view came in 1964, when John S Bell showed that any model that describes an objective reality underlying quantum mechanics must include some sort of non-local connection between distant systems, in an apparent violation of Einstein's own theory of relativity.

And contrary to Einstein's wish, in all objective interpretations known to date (such as the Many Worlds interpretation, objective collapse models, and de Broglie-Bohm theory), the wave function is a real physical object (with one very recent exception, where the wave function plays no explicit role, but the cat is literally dead and alive in ).

In 2007, however, Robert Spekkens from the Perimeter Institute published a seminal work showing that it was possible to reproduce many of the counter-intuitive aspects of with a model where the wave function plays the "epistemic" role Einstein longed for.

Other fragments of quantum theory were later shown to fit similar models, but the question was open whether or not this was possible for all of quantum theory. Could Einstein's dream be revived?

To understand what this kind of model is, imagine I hold two decks of cards: one contains only red cards, the other only aces, and I ask you to pick a card from one, without knowing which is which.

In an epistemic interpretation, the wave function would play the role of the deck you pick the card from. It gives you some information about the card – like if you pick from the aces deck, you're sure to pick an ace of some sort – but this information is not itself a property of the card. In fact, it is possible that you have picked an ace of hearts, which is compatible with both decks.

A wake-up call came in 2012, when Matthew Pusey, Jonathan Barrett and Terry Rudolph showed that in any objective model of quantum theory, the wave function must be a real property of individual systems, unlike the deck of cards. But, their theorem had an extra assumption that has called the implications of the theorem into question.

Reality check

However, a series of theorems published within the last year, starting with work from myself and colleagues, puts strong bounds on the viability of epistemic models, even without those extra assumptions.

These theorems consider the fact that some pairs of quantum states cannot be distinguished on a single experiment. This is analogous to not always being able to tell whether a randomly picked card came from the red deck or the ace deck. If you pick a non-ace card, you can be sure it came from the red deck. If you pick a black ace, you can be sure it came from the ace deck.

But if it's an ace of hearts or an ace of diamonds, it could have come from either. Counting the cards in the decks, we can determine how often this is supposed to happen.

In an epistemic interpretation, the fact we can't distinguish quantum states should be at least partially accounted for in this way. But the theorems show that this explanation simply cannot work. For some specially constructed quantum states, the "decks" corresponding to them cannot have anywhere near the right amount of cards in common, so to speak.

These predictions were partially confirmed by the experiment I was involved with, performed by Martin Ringbauer and the Brisbane team led by Alessandro Fedrizzi. They followed an improved version of our theorem due to Cyril Branciard, a co-author in the study.

The experiment involved preparing single photons (particles of light) in those specially designed states and subjecting them to a number of alternative measurements. The results give bounds on how well a model like the one outlined above can describe the statistics they observe.

This represents the first large class of quantum models to be ruled out since Bell's theorem started being tested in the 1980s.

If further experiments confirm the implications of the theorems, viable epistemic models of quantum mechanics will be essentially ruled out. If we want an objective reality, à la Einstein, the must be real, dead and alive cats and all.

But there are alternatives. One could be to reconsider assumptions of the framework used to derive the theorems, perhaps by introducing backwards-in-time causality or parallel universes. However, no approaches of this form have yet managed to produce an epistemic interpretation.

Or else, we can deny that a purely objective description is possible at all. However it may be, the weirdness of is here to stay.

Explore further: Researchers describe the wavefunction of Schroedinger's cat

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3.7 / 5 (3) Feb 12, 2015
Classical mechanics and quantum mechanics, as well as general relativity and superstring theory are all mathematical MODELS of reality. The models are all deterministic mathematical constructs, and we know *exactly* how quantities behave and operations occur in the models.
It is by drawing an ANALOGY between any given model and the observable universe that we test the models, and to the extent that the analogy holds up, we accept the model as being a useful tool in predicting the behavior of the observable universe.
A 'real world' entity such as a photon is exactly that: a real world entity, and NOT a 'wave' or a 'particle' or any other mathematical object which was used to construct one of our models.
To the extent that QM fails to correspond by analogy to the actual observable state of the cat, it must be considered an inadequacy of the model.
Never conflate your models with reality. A cat is NOT a wave function, nor is a wave function a cat.
not rated yet Feb 12, 2015
Models aren't reality, certainly, but models can limit what reality can be. For example, if 25 + 25 = 50 and I have two quarters, I don't have 75¢ in reality. You would have to throw out mathematical logic to be able to claim that 25 + 25 doesn't equal 50 (although some Governments try this). And if you throw out mathematical logic, you have nothing left with which to make a claim.
In this case, what is claimed is that the mathematical models tend to rule against the epistemic interpretation.
3.1 / 5 (7) Feb 12, 2015
'Objective Realism' can not be maintain as experiments continue to refute that view point. A 'scientific positivist' point of view is supported by epistemic considerations.....

In QM, the act of observation seems to create the properties that the quantum system did not have prior to observation, because the conditions for observation to be possible, necessitates that the underlying reality be forced to conform to our a-priori intuitions, ...our 'forms of thought', ...or conceptual structure,.... dependent upon the way the mind synthesis experience at the macroscopic scale.....

The result is that QM does not tell us about 'Independent Reality', but rather of our Experience of reality, since we add or force the wave-function to collapse into conceptual values.
3.7 / 5 (3) Feb 12, 2015
....i.e. the concept of 'position' with respect to say an electron is simply not defined until it is observed, ...until it interacts with the macroscopic apparatus designed to conform the underlying reality (the "electron") to the concept of 'position'.
5 / 5 (1) Feb 12, 2015
There's a good reason people doubt explanations that deny objective reality. ... The 25+25 quarter analogy above only works because we implicitly trust that magical quarters can't appear out of thin air. It's the same reason people trust that cats can't be alive and dead in the same universe at the same time. Parallel universes (or, in other words, one universe that is itself in a state of tremendous superposition) makes a lot more sense to me.
3.7 / 5 (3) Feb 12, 2015
IOW..... What is called an 'electron' is not a particle nor is it a wave,... in itself (independent objective reality) it is unknowable underlying reality. It is only a 'particle' to the extent that experimental detection apparatus projects the underlying reality to the concept of 'position'.
5 / 5 (1) Feb 12, 2015
"In QM, the act of observation seems to create the properties that the quantum system did not have prior to observation"

I heard a Copenhagen guy on Youtube attempting to argue that reality being effected by our "choices", (in terms of thinks like quantum measurement) undermines the Everett interpretation. Its a bad argument, because in the Everett interpretation, each path unfolds deterministically: There are no choices, only epistemic uncertainty in our brains about what we will do, whether we are in the universe where we will "choose" to do A or B. So experiments like Delayed Choice Quantum Eraser, if a photon or set of photons creates the interference pattern earlier, than they must all go right at the splitter later down their path, because it has been pre*determined* they are in the universe where that will happen. I like this view, because you get rid of the dualism of conscious "actors" and "observers" as different than other physical systems.
5 / 5 (1) Feb 12, 2015
Has anyone considered that for any two observers separated by any amount in space and time, the reality must be different simply due to the fact that the information of the rest of reality reaching them has to be different, as dictated by the speed of light.

Therefore two people observing the same event from different sides of a table will necessarily occupy two different realities and no objective consensus can be reached between them.

One may as well see the cat dead and the other alive, because effectively they both occupy slightly different universes, and the state of the matter may not need to be the same in both.
3 / 5 (2) Feb 12, 2015
I like this view, because you get rid of the dualism of conscious "actors" and "observers" as different than other physical systems.

Why would such a dualism implicit in the Copenhagen interpretation bother one to such an extent as to propose what amounts to metaphysics? It is not a surprise that reality must change its cloths (to concepts) to become knowledge for us. An epistemological interpretation seems more reasonable than Everett imo.
5 / 5 (1) Feb 12, 2015
I wish you would explain what you think in my view "amounts to metaphysics". In fact, the very of idea of an "us", different from the rest of the universe, so that reality must transform itself to be known by us, smells to me of old religious concepts like that of a soul that is different from the physical world. We are just another part of the physical world.
3.7 / 5 (3) Feb 12, 2015
Nope, no religious or spiritual non-sense implied here. Our physical brains are subject to physical laws also, yes of course, but this does not imply that the mind synthesizes experience in accord with qm laws,.... in fact why would it having evolved at the macroscopic scale?

To propose multiple instances of events that by definition are not all observable is metaphysics.

"it carries too great a load of metaphysical baggage" - John Wheeler
3 / 5 (2) Feb 12, 2015
@Noumenon, But can it affect results? I remember a few years ago when that late night radio show had an episode where they were trying to find out if human thought could affect things and the attempt if I remember correctly was a computer generating random numbers and the test was to see if people could make it generate a certain number repeatedly around 11 pm that evening. The results were charted and the result seemed to indicate pretty strongly that when a whole lot of people listening on the radio thought at the computer it did generate the same number repeatedly. Since that point in time I haven't discounted anything when it comes to qm measurements.
5 / 5 (1) Feb 13, 2015
@24volts That experiment would be more convincing if they used something subject to quantum effects, such as a radioactive source. A computer cannot usually change it's state without crashing. Computer memory is analogous to a railroad track. The processor cannot jump tracks easily. Instead you end up with a train wreck, or a blue screen in windows

Note in other psychic experiments the computer is considered as a paragon of stability. So the experiment setups themselves are often fraught with contradictions
3 / 5 (2) Feb 13, 2015
" Albert Einstein, who famously said: "Do you really believe the moon exists only when you look at it?" "

That's exactly what happens on my PS3 games.
5 / 5 (1) Feb 13, 2015
Impressive views from everyone.
High fives.

Noumenon is in his element here.
Let's challenge him a bit.

Macroscopic scale?
As in, the Now is one of scale? For a synthesis of experience? From the mind?

I have stated explicitly that consciousness must have a purely physical basis. - Noumenon

Dec 9 2014 Hawking warns AI

You can not take away from Science what science never had - objectivity.

Do not expect Realists to change.
Philosophy can end the search for objectivity here. Ontology too.

That is not going to happen. That is the bread and butter of too many people.
You can not get more physical than that.

not rated yet Feb 13, 2015
@24volts That experiment would be more convincing if they used something subject to quantum effects, such as a radioactive source. A computer cannot usually change it's state without crashing. Computer memory is analogous to a railroad track. The processor cannot jump tracks easily. Instead you end up with a train wreck, or a blue screen in windows

Note in other psychic experiments the computer is considered as a paragon of stability. So the experiment setups themselves are often fraught with contradictions

I also remember such experiments planned for soccer matches and like due to the ,sometimes, high emotional focus of both spectators and viewers. Don't know if it ever happened.
3 / 5 (2) Feb 13, 2015
@Noumenon, But can it affect results? I remember a few years ago when that late night radio show had an episode where they were trying to find out if human thought could affect things....

I don't think the mind 'reaches out' and effects physical systems in an active way like that. I don't believe in anything psychic......
3 / 5 (2) Feb 13, 2015
@Russell / 24 volts

Rather it is passive, in the sense that the mind add's conceptual form for our knowledge.

People forget that the brain is just a bio-mechanism itself that 'operates' on experience. It must do so in a particular way, since it is a finite mechanism itself. It must have hard-wired 'forms of thought' as conditions for experience to be possible. We presuppose a-priori, certain concepts as a necessary condition for intuitive knowledge to be possible, given the nature of mind.

At the QM scale some of these concepts fail to order experience in accord with intuition, so we say QM is non-intuitive; space, time, absolute temporal simultaneity [GR], causality, counter-factuality, locality, separability, particle / wave..

"Useful as it is under everyday circumstances to say that the world exists "out there" independent of us, that view can no longer be upheld. There is a strange sense in which this is a "participatory universe". - John Wheeler
3.7 / 5 (3) Feb 13, 2015
There IS an objective reality existing independently of us,... it is just that it is unknowable 'as it is in itself',... Because it lacks conceptual form. It has not been conceptualized for knowledge. Once the mind synthesizes experience (brings together elements of sensibility into the mould of conceptual forms of thought), .. it becomes 'phenomenal reality' or 'empirical reality', which must have a component that is mind dependent.

"There is no way to remove the observer us from our perception of the world, which is created through our sensory processing and through the way we think and reason. Our perception and the observations upon which our theories are based are shaped by a kind of lens, the interpretive structure of our human brains." - Stephen Hawking

"The doctrine that the world is made up of objects whose existence is independent of human consciousness turns out to be in conflict with quantum mechanics and with facts established by experiment" - B. d'Espagnat
4 / 5 (1) Feb 13, 2015
Just a thought on location and position:

You can locate Schrodinger's cat relative to the box it is in and the box relative to the building it is in; the street it is on; the town; the country; the planet; the solar system; the galaxy and the universe.....
But where do you locate the Universe??? Where is that positioned???
Seems to me that you cannot locate the Universe and give it a true position so therefore everything inside the Universe also cannot be located with a true position which begs the question:
Where exactly is the cat????

5 / 5 (1) Feb 13, 2015
There IS an objective reality existing independently of us - Noumenon

You assert Teflon for 'us'. If there is an objective reality, then 'we' are not stuck to this.
What did we do?

Of course I see the need to conjuncture objective reality. A perform for indeterminism and all the rest QM entails. No other Abhandlung meets with more success than this.

Necessity and sufficiency demands are met.
And you turn the last objection raise against QM - that of imcompleteness - to your advantage:
Of course QM is incomplete! you exclaim.
Whatever QM leaves untreated (all that is physical by the way) IS the 'objective reality' 'existing' 'independent' of us.

You win.
For now.
(Even though we both know 'now' for us does not exist)

not rated yet Feb 13, 2015
Typos noted above:
not rated yet Feb 13, 2015
'where' is argued to this day to be observer dependent.
You can overwrite the word 'observer' with the word 'measurement' if you like.

Both words harbor or are burdened with further meaning labeled 'frames and references'
not rated yet Feb 13, 2015
Further typos: see brackets[...];

Whatever QM leaves untreated (all that is [not] physical by the way) IS the 'objective reality' 'existing' 'independent' of us.

not rated yet Feb 13, 2015
Last typo correction:

Interesting to note one's own typos in retrospect - I had to drink from a glass that was not my glass...of water.
not rated yet Feb 14, 2015
"But there are alternatives....However, no approaches of this form have yet managed to produce an epistemic interpretation."

Maybe the Local Time is a proper alternative.
not rated yet Feb 14, 2015
I guess Schrodinger's cat and Bell's inequality can be compared to card tricks. They're only fun as long as people can't see the slight of hand.
not rated yet Feb 14, 2015
oops! darn 'e'
not rated yet Mar 24, 2015
How big is a photon? It is a particle of light. Light has a frequency of energy peaks and troughs. To have a frequency there has to be more than one cycle of such. How many make a particle of light?
not rated yet Mar 30, 2015
I don't think we should be casual about asserting that a light bulb can be on and off at the same time as the phrase "on/off" is nonsensical and absurd. Now if we oppose that it doesn't just trouble physics, it troubles everything because we are saying that first order logic is either wrong or incomplete (not in the Godel sense). This would so upset the apple cart by saying that things reasoned by basic logic can't be taken as truth that I don't know what we should do. Alice (as in wonderland) might be real.

So I hang with the crowd that says qualities, quantities, and characteristics don't exist until measured. But I would not term this as measurement "creates" anything. Rather measurement [italics] requires [end] that Scrodiner provide an answer, a requirement that cannot be ignored or declined.

not rated yet Mar 30, 2015
If we would live at the water surface and observe it with its own ripples like the waterstriders, then the observable reality would appear very similar at the small distance scales. At these distances the water surface gets dominated with Brownian noise in another dimensions of underwater. This noise propagates much faster, than the surface ripples and it blurs the observations done with these ripples. Therefore the observations at quantum scale cannot be done in deterministic way in context of single observation - but with averaging of multiple observations they can be still extrapolated and made deterministic (quantum tomographic or stroboscopic measurements).

For example, when we are observing a tiny atoms in atomic microscope, then these atoms looks like pin-point dots, but this picture is a product of many averaged observations, during which the state of atoms may change temporarily. Single pass observation would always picture these atoms heavily blurred.
not rated yet Mar 30, 2015
You may imagine yourself as a object floating inside of dense elastic fluid, which vibrates and dissolves your boundaries temporarily. Therefore your shape is not so monolithic and persistent at small distance scales, as you're repeatedly dissolving and condensing from vacuum in high frequency. What we can see macroscopically is time-averaged result of these fluctuations.

For example the angled shape of water molecules is not an actual shape of water molecules - but time averaged result of many subsequent changes of these molecules. It has still meaning to believe, that this averaged result is real, as it determines the shape of snow-flakes, for example. It just doesn't exist momentarily.

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