Skip to main content

Schrodinger's cat gets a reality check

Schrodinger's cat gets a reality check

19 hours ago by Eric Cavalcanti, The Conversation
Schrödinger's cat gets a reality check
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.
Source: The Conversation search and more info website
4.5 /5 (17 votes)
This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).
The Conversation

Popular posts from this blog

Hidden Wiki

Welcome to The Hidden WikiNew hidden wiki url 2015 http://zqktlwi4fecvo6ri.onion Add it to bookmarks and spread it!!!
Editor's picks Bored? Pick a random page from the article index and replace one of these slots with it.
The Matrix - Very nice to read. How to Exit the Matrix - Learn how to Protect yourself and your rights, online and off. Verifying PGP signatures - A short and simple how-to guide. In Praise Of Hawala - Anonymous informal value transfer system. Volunteer Here are five different things that you can help us out with.
Plunder other hidden service lists for links and place them here! File the SnapBBSIndex links wherever they go. Set external links to HTTPS where available, good certificate, and same content. Care to start recording onionland's history? Check out Onionland's Museum Perform Dead Services Duties. Introduction - Clearnet search engine for Tor Hidden Services (allows you to add new sites to its database). DuckDuckGo - A Hidden S…


Good News [May 08, 2015]: IDM developers got smarter, but the crackers are always a step ahead. Follow this article and send an email to if you are desperate. I can NOT post any crack here for legal reasons. Happy Downloading with IDM. ;) *********** first tip is to use latest crack for idm from idm universal web crack and make sure u are using all latest vers I am sure many of us are too much dependent on Internet Download Manager a.k.a. IDM. The main reason didn’t permanently switch to linux was IDM. I mainly use it for batch downloading and download streaming videos. Till yesterday, IDM was working fine with me (of course with fake serial numbers, keygen, crack, patch etc. which could be found with little effort). But few days ago, with the latest update version 6.18 build 7 (released on Nov 09, 2013) Internet Download Manager was literally had a breakthrough and crushed all the serial numbers, …

DoubleAgent Attack Turns Your Antivirus Into Malware And Hijacks Your PC

Short Bytes: Cybellum security researchers have uncovered a new attack mechanism that can be used to take control of your antivirus and turn it into a malware. Called DoubleAgent, this attack exploits an old and undocumented vulnerability in Windows operating system. This Zero Day code injection technique affects all major antivirus vendors and has the power to hijack permissions. The security researchers from Cybellum have found a new technique that can be used by the cybercriminals to hijack your computer by injecting malicious code. This new Zero-Day attack can be used to take full control over all the major antivirus software. Instead of hiding from the antivirus, this attack takes control of the antivirus itself. Called DoubleAgent, this attack makes use of a 15-year-old legitimate feature of Windows (read vulnerability)–that’s why it can’t be patched. It affects all versions of Microsoft Windows. Cybellum blog mentions that this flaw is still unpatched by most antivirus v…