God Vs. The Delayed Choice Quantum Eraser

This is the name of an experiment first proposed in 1982, and is the one that I have had in mind when talking about God and quantum mechanics. I realized that most of the comments I have received seem to be taking me to be talking only about the standard double-slit experiment; this is of course my own fault since I haven’t done a very good job of indicating what I had in mind. So, let me describe these results and then reformulate the argument.

We have to build up to this, so let’s start with the quantum eraser experiment. Here is how Brian Greene describes the experiment in his recent book The Fabric of the Universe.

A simple version of the quantum eraser experiment makes use of the double-slit set up, modified in the following way. A tagging device is placed in front of each slit; it marks any passing photon so that when the photon is examined later, you can tell through which slit it passed…when this double-slit-tagging experiment is run, the photons do not build up an interference pattern.

As he goes on to point out, this is what we would expect. Since we measure the photon’s path, we get the photons acting like particles. But then it gets weirder. As Green continues, the quantum eraser asks,

What if just before the photon hits the detection screen, you eliminate the possibility of determining through which slit it passed by erasing the mark imprinted by the tagging device?

The answer, as it turns out, is that the interference pattern shows up again. Which, is , uh, weird. But again it gets weirder with the delayed-choice quantum eraser. Greene describes it thus,

It begins with [the set-up of the quantum eraser], modified by inserting two so-called down-converters, one on each pathway. Down-converters are devices that take one photon as input and produce two photons as output, each with half the energy (“down converted”) of the signal. One of the photons (called the signal photon) is directed along the path that the original would have followed toward the detector screen. The other photon produced by the down-converter (called the idler photon) is sent in a different direction altogether. On each run of the experiment we can determine which oath a signal photon takes to the screen by observing which down-converter spits out the idler photon partner. And once again, the ability to gleen which-path information about the signal photons– even though it is totally indirect, since we are not interacting with any signal photons at all– has the effect of preventing an interference pattern from forming.

OK, so far so good. This is just a fancier version of what we have already talked about, with the exception that we are now no longer causally interacting with the signal photon. Everything we know about the signal photon we learn by observing the idler photon. But even so, we get the photons acting like particles. But we aren’t done yet. Again Greene

 Now for the weirder part. What if we manipulate the experiment so as to make it impossible to determine from which down-converter a given idler photon emerged? What if, that is, we erase the which-path information embodied by the idler photon? Well, something amazing happens: even though we’ve done nothing directly to the signal photons, by erasing which-path information carried by their idler partners we can recover an interference pattern from the signal photons[!!!!!!]

OK, so what this seems to show is that it is not anything that we do to the photon that determines which way it will behave. Rather what determines this is whether or not we are able to know which path the photon takes to the detector. Nothing changes here except our ability to know which path the photon took.

We can hammer home this point with one further modification of the experiment. Suppose that we set it up so that we could only get which-path information from some of the photons (and further that which ones we get this information about is random). Again Greene.

Does this erasure of some of the which-path information– even though we have done nothing directly to the signal photons– mean that the interference effects are recovered? Indeed it does– but only for those signal photons whose idler photons [had their which-path information erased]…If we hook up equipment so that the screen displays a red dot for the position of each photon whose idler photons [had their which-path information erased] and a green dot for all others, someone who was color-blind would see no interference pattern, but everyone else would see that the red dots we arranded with bright and dark bands– an interference pattern.

So, it is the knowledge of which-path information that determines which way the photons behave. Since God always has which-path information, whether he obtains it in such a way as to effect the physical world or not, He will never see the interference pattern. Or in other words, the wave like nature of reality will be hidden from Him.

Sheez! That took longer than I thought!!  

New Virtual Presentation!!

So, I just got a recording of my What is a Brain State? talk which I gave at the 2006 Towards a Science of Consciousness conference. I used that to record the narration to the powerpoint slides, and voila! A new virtual presentation. The conference uses a service called ‘conference recordings’ and it was easy to get the recording, but I think in the future I am going to try and record the narration as I am giving the talk…I think my mac laptop has a built in microphone…

This was by far the largest audience for a talk that I have had, and I was extremely nervous! So, I apologise in advance for all of the ‘um’s

Some Cool Links

(via David Pereplyotchik)

Below are links to some examples of talks that fall well within the cognitive science arena. I’ve found, however, that many of the non-cogsci talks are more interesting, because they introduce one, often in a vivid way, to a subject matter that is less familiar. (For instance, Wade Davis’s talk on anthropological fieldwork was, for me, genuinely exciting.)

You can browse the talks by clicking on the topic links at the bottom right of each video’s page. Or just start here

Enjoy.

David Pereplyotchik

58th Philosophers’ Carnival

Welcome to 58th edition of the Philosophers’ Carnival!

I am happy to be hosting the carnival again and glad to see that it seems to be doing well. I always liked the way that Avery did the 46th (international) Carnival and so I modeled this edition on his ‘psuedo-conference’ format. What follows is, indeed, a ‘narrow cross-section of philosophy from accross the web’.

Special Session on the Employability of Philosophers

  1. Presenter: Tom Brooks, The Brooks Blog
    The truth is out there: employers want philosophers
  2. Respondent: Rich Cochrane, Big Ideas
    The Value of a Philosophical Education

Symposium on Philosophy of Science

  1. Sharon Crasnow, Knowledge and Experience
    Is Science Based on Faith?
  2. Matt Brown, Weitermachen!
    Common Sense, Science, and “Evidence for Use”

Symposium on Race and Liberty 

  1. Richard Chapell, Philosophy, et cetera
    Implicit Interference
  2. Joseph Orosco, Engage: Conversations in Philosophy
    It’s Only Racism When I Say It Is

Invited Session

 Symposium on Philosophy of Consciousness

  1. Tanasije Gjorgoski, A brood comb
    The Myth of ‘Phenomenal/Conscious Experience’
  2. Richard Brown, Philosophy Sucks!
    Priming and Change Blindness
  3. Gabriel Gottlieb, Self and World
    Pre-reflective Consciousness: A Fichtean Intervention

Symposium on Metaphysics and Epistemology

  1. Marco, El Blog de Marcos
    Truthmaking and Explanation
  2. Kenny Pearce, blog.kennypearce.net
    What Does Bayesian Epistemology Have To Do With Probabilities?

Symposium on Philosophy of Religion

  1. Dave Maier, DuckRabbit
    D’Souza vs. Dawkins
  2. Enigman, Enigmania
    Is the Free-will Defence Defensible?
  3. Chris Hallquist, The Uncredible Hallq
    What’s the deal with philosophy of religion?

I hope you enjoyed! Be sure to check out future editions of the Philosophers’ Carnival.

    Submit your blog article to the next edition of philosophers’ carnival using our carnival submission form. Past posts and future hosts can be found on our blog carnival index page

On the Off Chance you Missed It

David Chalmers and one of his graduate students have launched MindPapers: A Bibliography in the Philosophy of Mind and the Science of Consciousness. This is a truly amazing resource as it includes all kinds of on-line papers! It is also searchable and has many other ‘capabilities’…I just hope it doesn’t one day take over the internet and steal my credit card info!!! 🙂

I think by far the best part is Part 7: Philosophy of Cognitive Science, section 3: Philosophy of Neuroscience, sub-section f: Philosophy of Neuroscience, Misc. ;^)

Metaethics and Cognitive Science

There has been some discussion over at Brains about what the ideal cognitive science program would look like which got me to thinking. I am writing my dissertation in metaethics and I have always thought that my project was a cogsci project. I don’t mean merely that my metaethical theory is informed by research in cognitive science. I mean that metaethical theorizing is itself part of cognitive science, or at least should be.

 Now I am not trying to critisize Eric for not including metaethics in his ideal program, though I don’t know if I share his idea that mathematics is essential for study in cognitive science and that philosophy can be ‘picked up’ later, but I guess I am more just curious about what people think. Is metaethics a part of cognitive science?

Does God Know About Quantuum Mechanics?

I was answering a comment from Richard C. which made me think of this.

It has been established via experiment that Einstein was wrong and that randomness is a fundamental feature of the quantuum mechanical description of reality. Scientists are even now using entaglement in the lab to ‘teleport’ information (in the form of transfering states fromone entangled atom to the other) inthe hopes of making this suprising fact about nature useful (relativity physics has never even come close to being so useful!). The question, then, is can an omniscient being know in advance the outcome of the random quantuum events? Either way you answer there is trouble.

If you say that God cannot know the outcome of the events then there is an obvious limitation of God’s knowledge. With respect to quantuum mechanics He can do no better than us! He knows the outcome of the events in the form of probabilities, but just like us He is unable to say in any given case what the outcome will be. But the Quantuum Mechanics is surely the greatest discovery in the history of the universe! For, if this is true then we have discovered God’s knowedge of the universe…but this sounds crazy! So it seems to me that there is strong pressure to say that God does indeed know the outcome, in advance, of all quantuum events.

But then there is a seperate problem. Forget for the moment the issue of whether His foreknowledge is compatible with the outcome being truely random and consider the double slit experiment (I assume you know what that is, if not let me know and I’ll give a description). One of the strangest things that we have found out about it over the last thirty years or so is that if there is a way for us to know the path that the photon actually takes, and so determine which slit it actually travels through, then the interference pattern no longer manifests. What we get is ‘nothing but us particles down hir sir’. In Green’s book The Fabric of the Cosmos he details experiemnts he calls ‘quantuum erasures’ where they showed that what matters is whether someone could know the path taken by the photon. Tis is obviously extremely strange and anti-common sense, but it is a robust experimental finding. But now consider God. If He knows the path that the photon takes then it will not act like a wave. It will act like a particle. So from God’s point of view particle physics has to be correct. Since He is always holding the door of the refrigerator open, metaphorically speaking, the light inside will always be on. But this really reduces to the first option in claiming that God can’t have any direct knowledge of quantuum physics.

In fact one might think that if God did in fact exist then we couldn’t have discovered quamntuum mechanics in the first place.  

Evidence for something String-ish

There has been a lot of debate lately about string theory and testability (see for instance Hanging on by a Thread) . It seems to be one of the only theories in the modern time that is taken seriously by physicists in spite of the fact that it cannot be empirically tested, and in fact it is hard to see how it could be so tested. This gets a lot of people upset as it looks like string theory can’t be falsified and so shouldn’t count as a scientific theory.

Ed Witten is famous for, among other things, arguing that string theory does make one prediction. It predicts gravity. It is concievable, he argues, that some alien society discovered string theory before they discovered gravity. In that world gravity is a prediction of string theory and not of quantuum mechanics. So it merely an historical accident that in our actual world we discovered gravity first and string theory second. So there is a sense in which string theory has already made one bery important ‘prediction’ albeit one that is already established.

Is this all the evidence for string theory that we can muster? I have been thinking lately that perhaps we can get some evidence for something that is at least more string-ish than particle physics if we think about special relativity….So perhaps the most famous result of the special theory of relativity is the equivelance of mass and energy captured by E=MC^2. There have been two philosophical interpretations of this equivelance (see the Stanford Encyclopedia entry linked to above). There are those who take it to show that the properties picked out by ‘energy’ and ‘mass’ turn out to be the same property, and those who take it to show that there is no ontological distinction between fields and matter, or in other words that there is just one kind of stuff out there. It seems to me that either way one interprets the equivelance of mass and energy it provides some reason for thinking that a theory like string thoery will turn out to be correct (as opposed to a theory like particle physics).

So, say that you take it to show that there is only one kind of fundamental stuff out there that we can describe as either energy or mass. This is evidence for something string-ish in that string theory posits only on fundamental entity, viz. the string. Particle physics, on the other hand, posits a zoo of particles that are all made from different stuff. Electrons are made from one kind of stuff quarks from another kind of stuff. Thus particle physics as standardly construed seems fundamentally at odds with the ‘one stuff’ interpretation of E=MC^2.

On the other hand, let’s say that you take the equivelance to show that energy and mass are the same property, though that property may be had by several different kinds of stuff. This is evidence for something string-ish in that string theory posits that the one property is actually string vibration and so can explain what the property is as well as how it will seem to us that one converts into the other (i.e. the pattern of vibration changes).  What is the candidate property that particle physics offers? It’s only answer is ‘a property, we know not what’. Thus the two fundamental properties of physics are rendered completely mysterious.

Now, I don’t think this second argument is as decisive as the first, and I also think that the property interpretation is more likely to be true, so I tentatively conclude that special relativity does give us some reason to prefer a string-ish theory over particle physics…but I will have to think about it some more…