The Double Slit Experiment - What does it say? What does it not say?

(RiderOnTheClouds) #1

I hear a lot of sensationalist information about the Double-slit experiment (like here), which I find fascinating, but since I’m no quantum physicist I’d like some clarification on some things.

1 - Is it applicable to the macro-world or only the quantum-world?
2 - Does it suggest that matter does not exist until observed?
3 - If 2 is so, does this suggest that mind is separate from the body, and indeed, more fundamental than the body?
4 - If 3 is so, is this evidence that the mind can survive death?


1: Yes.
2: No.
3: Not sure what QM has to do with that.
4: Sure, if you freeze the head quickly enough.

I don’t think a physician with the tag-line ‘biocentrism’ is going to be the best source for quantum theory.

(RiderOnTheClouds) #3

Thanks, I’ve seen Christians such as Inspiring Philosophy latch onto these sensationalist claims, I should have said that this was the main reason for my post

(Mitchell W McKain) #4

credentials: Masters of physics from the University of Utah specializing in theoretical physics.

The Double Slit Experiment - What does it say?
Light from a single coherent source goes through two holes (generally rectangular and long in the direction perpendicular to the distance between them). If light is a wave then you expect an interference pattern when the light passing through these two holes interfere with each other. If light consists of particles then you don’t expect interference. What happens is you see interference unless you use detectors at one or more slits to determine which hole the photons (light particles) go through. So it would seem that light only behaves like particles when you are looking. Most physicists believe, however, that this is about the interaction between the light and the detectors which alters the entanglement or group behavior of the photons.

It is difficult to make sense of this question since the interference patterns are macroscopic and visible to the naked eye. There was some question at some time whether quantum indeterminacy had any impact on the macroscopic world, but we now know that because of chaotic dynamics that it most certainly does have an impact. But that is a somewhat different issue than the double slit experiment.

No, it does not. Since the double slit experiment can be performed with any particle and not just light then it does imply that everything – both light and matter is simultaneously both wave and particle at the same time. This means that matter may not be what many think it is, though it does not mean that there is no distinction between matter and forms of energy like light. In particle physics we distinguish two kinds of particles: fermions and bosons. The fermions (like electrons, protons, and neutrons) are the matter particles because they follow a rule that prevents them from occupying the same space or state and thus they build structures like atoms and molecules. The bosons (like photons and gravitons) do not follow this rule and they are generally thought to be the force carriers – the means by which forces like electromagnetism and gravity act between different matter particles.

The double slit experiment and quantum physics in general has no bearing on the philosophical problem of mind and body – nor does it support any claim that the mind is more fundamental.

The double slit experiment and quantum physics in general has no bearing on the the question of whether the mind can survive death.

For reference… my position on these two latter philosophical/religious questions are as follows:

  • I believe that the mind is no less physical than the body but that it is a living organism in its own right with its own needs and inheritance - a memetic organism rather than a genetic organism. Thus there is an effective duality between mind and body rather than an absolute one as in Descartian dualism. Thus the mind can die just like the body can and usually this is at the same time but not necessarily.
  • I believe what survives after death is the spirit not the mind, and the spirit has both body and mind just like we do when we are alive. I believe the spiritual mind and body are created by the living physical mind and body – particularly by the choices they make. I do not believe in (the essentially pagan) ideas of the soul either as something being inserted into the body at conception or birth, or as something moving around from body to body. The use of this word in the Bible are from words which would be better translated as person, self, life, mind, spirit, heart, or will.

(Tim) #5

Did the Greek philosophical influence conflate what early Christians accepted as what the soul and spirit are? Was there a switch in definition?

The soul is not the image of God in Genesis 1, especially if your use of spirit is the original purpose of soul. The shell was in the image of God. After the Flood no one had knowledge of what God’s image was. One reason why men were not to carve just any image and worship it as God. It does not matter to me what term is used as what as long as all understand the differences in function.

My take in keeping with the Bible both OT and NT is: God created the shell out of dust in God’s image. God breathed, animated the body with a soul. We are not the shell, and we are no longer the image of God. How do we define the image of God, if we have no clue of what it looks like or actually is? God stressed that throughout Scripture that no living human could ever view nor have contact with God’s image.

I think humans decided to identify the image of God with the term Angel because the meaning was messenger which the function of the image of God actually was. It was direct communication with God. It is also closely related to the concept of sons of God. Not just a familiar nor genetic relationship, but a direct communication relationship.

The image is neither a soul or spirit in function, but if God is a spirit being, then the image would be of spirit as well. It is also the eternal part, because God claims the soul that sins, shall die. God struggled with human spirit, but the soul would forever be seperated if sin was the only thing the soul could do.

We still have a fleshly body which is not us, but will go back to dust.

We are only the soul and spirit in function. Currently we have no control over the spirit, because God has separated us from us to stop the struggle. Jesus was provided to restore us to our spirit.

So the Greeks introduced mind into the mix way before Christianity. They needed something to differentiate between physical and spiritual. They were not even sure which bodily organ to use.

To be truly dualistic one would have to have control of soul and spirit. Because the soul is as you describe an extension of the physical body. Even though it seems separate it was not what the Greeks were shooting for. Since the goal was to get to the spirit, and they probably did not read the Hebrew’s version of creation in Genesis they only identified the soul. Thus we are stuck with a split physical definition and property and not the contrast with the physical and spiritual. Are we spiritual with just a soul?

Is changing the word soul in Genesis to spirit correct though? If you view the function as a seperate physical entity, how can it mean spirit? The flesh was animated, but the input of the soul can only be physical or both, but can never be strictly spirit. If the soul is just animation of the physical it does not follow that it made dust something that it could never be. The dust was changed in form when “turned into the image” but was still physical, and the animation is what makes you you. At what point are you you generations after God created multiple humans both male and female?


Honestly, the world’s greatest physicists disagree vehemently on the answers to these questions; I don’t think you will get a definitive reply here. Copenhagen interpretation, many worlds, hidden variables - they all have their variants and defenders. It’s a real rabbit hole.

Having just been through a few QM popularized reads, I found myself wondering how the classical definitions of the attributes of God may play into this. If God is omniscient and omnipresent, that would infer He observes all of nature, the sparrows of the firmament and past event horizons, down to the last plank length. That need have nothing to do with determinism, but what would be the implications for collapsing the wave function, and ultimately the entire entangled universe, to the reality we in turn observe? That the wave function collapses with observation is the Copenhagen interpretation, and God observes. I would be interested in any thoughts.

(David Heddle) #7

I’ll nitpick on that. It is sometimes taught that way, but it is a mistake, because it is a misguided (IMO) attempt to give a classical explanation to a quantum phenomenon, wavefunction collapse. But all it really does (again, IMO as a garden-varierty at-best average physicist) is show you that the classical explanation gives an order of magnitude estimate. QM of course postulates that it is the wavefunction collapse–regardless of how impactful the observation, that is the explanation. Ingenuous experiments get around this problem (via entanglement) and show that it does not require pinging the photon (or electron) to destroy the interference pattern.

Here is a really cool video about one such experiment, the Quantum Eraser. The slits are monitored with the same detectors and yet you do or do not get inference based on a remote measurement of the entangled particle.

By the way, the woman who does these videos has the best set of accessible QM videos (and most entertaining) I have ever seen. Her youtube channel is Looking Glass Universe. I highly, highly recommend.

EDIT: typo

(Mitchell W McKain) #8

That is an example of what I said not a counter-example. Measuring an entangled particle alters the wave function which governs the behavior of the entangled group of particles. It doesn’t change in the slightest the fact that consciousness has absolutely NOTHING to do with the result. If nobody watches the experiment, it will not alter the measurements in any way at all.

P.S. “Wave collapse” is no longer the accepted term, we now use the word “decoherence.”

I watched the quantum eraser video. And here is my answer to here final question about what does this mean: Decoherence is a non-local phenomenon. Changing the term from “wave collapse” to “decoherence” obviously doesn’t change the non-local nature of the thing.

(David Heddle) #9

Your response makes no sense. You should really develop a thicker skin and not take any disagreement as an attack.

What I replied has nothing to do with consciousness. I am only addressing your comment that the loss of the interference pattern is often explained as a result of the interaction with the detectors. (In fact, I agree with you, that that is a common explanation.) However this explanation is (IMO) misleading and wrong. In spite of the fact that it is a common quasi-explanation, the destruction of the interference pattern is not a result of the detector disturbing the photon as it isolates it to one slit or the other. QM teaches that observing the photon (human or machine, doesn’t matter) collapses the wavefunction. It makes no claim about the type of observation or a threshold of the disturbance. If the knowledge of which slit the photon went through can be obtained by entanglement, the pattern is destroyed, even though you didn’t bang on the photon with some sufficiently low wavelength probe to isolate it.

(Mitchell W McKain) #10

I disagreed with your quibble, did that upset you? I did not say “banging on the photon” or even “interaction with the photon”. I said interaction with the light altering the “entanglement or group behavior of the photons.” To be sure the physics terminology is to say it is an interaction with and alteration of the wave function, but I think my common English description captures that quite well.


Listen to @heddle! He’s a physicist and probably has the double split experiment tattooed on his bicep. Either that or ‘Double Split Experiment’ was the name of the progressive jazz garage band he played in.

(David Heddle) #12

Mitchell is also a physicist. Again, my comment is narrowly directed at the common classical explanation of the destruction of the interference pattern. I do not like it!

(Mitchell W McKain) #13

No tatoos or garage bands I am afraid … (nothing so cool at that in my case)… The problem was more to do with the hazzards of nitpicking. If one disagrees with the minor issue it one tends to reinforce the central point one was making as well.

The whole thing might have worked better as a clarification. To say that the destruction of the interference pattern wasn’t simply the result of the detectors pushing the photons off course but an alteration of the whole wave function. In that case the quantum eraser experiment would have been right on target to show this because it alters the wave function remotely.

(system) closed #14

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