Sunday, 19 February 2017

Confronting Quantum Woo. The Double Slit Experiment.

Let's continue with our dissection of the alleged quantum/conscious connection by moving further into the Collective Evolution article, an exploration that will lead us straight to Young's double-slit experiment, without question one of the most famous and crucial experiments in the history of physics. In addition to showing that the double slit experiment doesn't suggest that consciousness collapses the wave function of a particle, I'll attempt to go further and show that the double slit experiment suggests considerable evidence that "consciousness hypothesis" offered by quantum-woo proponents must be false. To do this I'll first suggest a hypothesis for consciousness inspired wave-collapse as I couldn't actually find one in Lanza's book. The article linked above, QUANTUM THEORY SHEDS LIGHT ON WHAT HAPPENS WHEN WE DIE: THE AFTERLIFE gives its evidence for Robert Lanza's "theory" that consciousness relates somehow to quantum physics.
"His theory implies that our consciousness does not die with us, but rather moves on, and this suggests that consciousness is not a product of the brain. It is something else entirely, and modern science is only beginning to understand what that might be. This theory is best illustrated by the quantum double slit experiment. It’s is a great example that documents how factors associated with consciousness and our physical material world are connected in some way; that the observer creates the reality."
To consider this relationship an element of science we first need a working hypothesis. The suggestion of a hypothesis offered by Lanza and others in regard to consciousness caused wavefunction collapse can be phrased very informally as:
"Consciousness can exist separately from matter. This consciousness can be shown to have a physical effect in the collapse of the deBroglie wavefunction of a travelling particle. Consciousness is otherwise physically imperceptible. This effect this best shown in Young's double slit experiment." 
This is my interpretation, as I mentioned above Lanza fails to offer an explicit hypothesis in his book and I've struggled to find one elsewhere. Let's call this the QC hypothesis, and return to it in a moment.

I'm sure many of you are familiar with Young's double slit experiment, but I'll give a brief introduction for those that aren't.The double slit experiment was revolutionary as it was our first outright hint that there is more to matter and energy than first suspected, namely that both light and matter on scales far smaller than that of our macroscopic world display both wave and particle characteristics (Actually, a more accurate description would be that both matter and light can be modelled as a wave and a particle. In reality, light is neither). The concept of particle and wave duality was staggering enough for light, but a series of experiments involving electron diffraction showed that matter possessed the same duality of nature. If you want more details about the double slit experiment, the Wikipedia page is actually a pretty decent resource. Rather than wax on about the finer points, I'll use a computer simulation to show the results of the double slit experiment and what this tells us about the nature of matter. Consider below, when reading the phrase collapse of the wavefunction, this refers to an electron switching from a wave-like behaviour to a particle-like behaviour. For example, the appearance of a single dot on a fluorescent screen is particle-like.

We start with a hypothetical apparatus set up as shown below.

Figure 1:
An electron gun fires electrons through two narrow slits onto a fluorescent screen. Where an electron hits the screen a white dot is left behind. We aren't going to concern ourselves too much with the widths of the slits(1nm) or the energy of the electrons(38V).

Figure 2
As the electrons stream through the slits one by one they appear on the screen seemingly randomly. In fact, this underlines how the probabilistic nature of quantum physics can still be reconciled with the deterministic nature of classical physics. We cannot predict with any certainty where the next particle will strike the screen, but we can predict with absolute certainty the overall distribution of a large number of electron hits.

Figure 3
The final distribution shows wide bands of electron hits punctuated with thin black bars showing virtually no elections strikes. These bars don't line up with the blockages in the equipment, and the distribution clearly doesn't resemble what we would get from firing hard projectiles such as bullets through scaled up slits where we would expect a graph of the hits to be two "humps" corresponding to the openings, with some lying between. Compare that to the graph yielded by our electron firing simulation in Figure 4 below.

This fringe pattern is destroyed if we run the experiment again, but this time with one of the slits, slit 1 in the case below, closed. Figure 5 below.

The fringe pattern is known as interference and it can be explained easily using an analogy to water waves. When two waves meet at the point of their maxim amplitude the overall amplitude is increased as in Figure 6 ( left. The waves are in phase.

When two waves meet with maximum amplitude and minimum amplitude, the waves cancel. Unsurprisingly, this is destructive interference. Figure 7 ( left. The waves are said to be out of phase.

These patterns are achieved every time this experiment is conducted. Even though the precise build is random and probabilistic, the final distribution is deterministic and fully predictable. Remember this, it's crucial for later when we reassess the consciousness/wavefunction collapse hypothesis. 

These wavefunctions, known as deBroglie waves, are a mathematical interpretation of how a particle propagates through space, they are composed of all the possible positions of the particle at any time and the probability of finding a particle at that particular point. In what follows you'll see why it's necessary to describe the propagation of a particle through space as a wave function. In an attempt to understand these effects in the above experiment, we reopen both slits and turn down the current of the electron gun to allow one electron at a time to hit the screen. Remarkably the fringe pattern returns, albeit slowly, defying the idea that it is the wave function of one electron is interfering with its neighbour. In fact, it's clear that the electron interferes with itself.

Stop laughing at the back!

The consequence of this is we are forced to abandon the classic idea of a particle possessing a single defined trajectory through space. The passage above demonstrates that the particle can be considered passing through each slit, with the contribution of each slit in the wave pattern causing constructive and destructive interference. How does physics explain this? Well.... we can't. We can offer interpretations of this phenomena, such as the Copenhagen interpretation which states that quantum systems don't possess definite properties prior to measurement, only probabilities that reduce to certainties on measurement. There are other interpretations such as many worlds interpretation, but it's the Copenhagen interpretation I'm most comfortable with. There is good experimental evidence to support the idea that quantum qualities become definitive only upon measurement. Einstein argued against this and suggested quantum systems contain hidden variables in his EPR arguments, which were countered by Bell's inequalities and later answered by a modification of the same known as the CHSH inequality. As I don't want to digress too much I won't discuss those further here, but it's well worth a Google search if just to see how even opposition to an idea in science can sometimes strengthen and refine that idea.

So, we're left with an ambiguity, a hole in our carefully crafted quantum science. You may imagine this is where quantum woo merchants begin their machinations, as that's often the tactic of the pseudoscientist, to insert pseudoscience into a gap in conventional understanding. But, It's actually in our attempts to resolve this ambiguity and obtain what we refer to as which way information, that our quantum woo merchants operate.

Figure 8:
In an attempt to resolve the mystery of which slit the electron passes through, we introduce a new element to our experimental setup. We scatter photons off the electrons to see if the electron is in the vicinity of slit 1 or slit 2 and follow their path to the fluorescent screen.

Figure9&10: When we observe the results from this test on our fluorescent screen the interference pattern is gone, replaced by a distribution that is just the sum total of particles passing through the slit. The wavefunction now collapses prior to reaching the screen. It is as if our act of observation itself caused the wavefunction to collapse, and this is most certainly the quantum-woo merchant's interpretation of
this experiment. This brings us to our first misunderstanding supporters have of quantum physics and I can't express how fundamental this is to every conclusion they reach following this.

The Fundamental Error 1: It is not the presence of the conscious observer that collapses the wavefunction, it's the action they perform on the system that causes the wavefunction collapse. 

Let's consider this first in the case of the example given above, electrons fired at a fluorescent screen. When we bombard the electrons with photons the interaction changes the state of the system catastrophically as despite lacking mass, photons do carry momentum.

Mathematically we can show this quite easily if the wave is made of a superposition sum of all the possible states of the particle with constants that represent the probabilities of finding the particle in any of an infinite amount of positions, the probabilities of these possible locations must be 1 as the particle is certain to be somewhere. Therefore if we make the probability of one of these possible states 1 by locating the particle, the other probabilities must be zero! Thus, the wavefunction "collapses" from a superposition to a simple value. I've show this below crudely in a form that describes a "which slit" superposition.

An easy way to understand this physically is by analogy. Imagine me asking you to determine the location of tennis balls I'm firing into a darkened room. The only instrument I'm going to give you to do this is a tennis racquet. There's no way of you doing this without fundamentally changing the state of the ball. You may be able to give me the position of the ball at impact, but you would find it impossible to give me information about the ball after impact. The act of measurement has destroyed the information you had, there is no way you can violently examine that system without changing it fundamentally.

We call any measurable quantity in quantum physics, an observable. Obviously, this is a name that doesn't help the layman distinguish between an "observation" and a measurement". The choice of name seems to suggest that observation is a crucial part of quantum physics, rather than the true meaning of the name: a quantity which can be observed. Other examples of observables are energy, momentum and spin, the above principle applies to these qualities too, any attempt to know one destroys the wavefunction. We often find proponants of quantum woo run versions of the above experiment in which the electrons are fed through spin selectors, therefore collapsing the wavefunction for in the same way as a bombardment of photons does for which way information. We can define this by considering quantum systems to be in extremely delicate balance, with slight perturbations causing collapse.

This is what we find anytime a quantum system interacts with a classically defined object. It's nothing to do with an observer as the following thought experiment should show.

Thought Experiment 1

Of course, the big question is can measurements occur without an observer? The key to considering this idea is to remember that the CHSH inequality has shown there are no hidden variables. Before a measurement is made a quantum system has no determinable observables. In light of this consider this thought experiment.
An isolated nucleus of Uranium 238 exists in the far reaches of space, it has zero momentum. It emits an alpha particle via the process of alpha decay and becomes a thorium 234 nucleus. The conservation of momentum tells us that the total momentum of these two daughter particles must be zero, therefore the momentum in an undefined direction of the alpha particle must be matched by an equal momentum in the negative direction of the thorium 234 nucleus.  Neither direction can be known, until the thorium 234 particle interacts with a particle of dust with a defined location. Suddenly both daughter particles have a definitive directions and thus momentum vectors, they must as there is a physical effect on the space dust.
This interaction can be considered a crude form of measurement, the wave function for both particles has collapsed, no observer necessary.

Finally, let's return to our QC hypothesis, to see how what we've covered strongly implies it is inconsistent with reality.

"Consciousness can exist separately from matter. This consciousness can be shown to have a physical effect in the collapse of the deBroglie wavefunction of a travelling particle. Consciousness is otherwise physically imperceptible. This effect this best shown in Young's double slit experiment." 

Let's  question for a second what we would expect to find if the double-slit experiment was repeated in a world in which the QC hypothesis is true. Surely, an unavoidable consequence of the fact that we can't detect or protect our experiment from incursions and interactions from disembodied consciousness is that we should expect that there would be occasions in which the wavefunction collapses for no discernable reason. It would be as if we'd attempted to gather which way information with the later addition of a photon source and a conscious observer, despite us doing no such thing. If the QC hypothesis were true we would expect to see random wavefunction collapses. This has never been shown to happen. I think that strongly implies that the above hypothesis is incorrect in some way. Either consciousness does not exist separate from matter, or consciousness is not responsible for wavefunction collapse.

 Or Both.

Finally, just for fun.

Thought experiment 2

Plinkett and Nadine are conducting the double slit experiments with the electron gun set to a slow voltage with the particles released at 2 minutes intervals. They do not attempt to collect which way information. As Plinkett observes particle 2 hit the screen at 2:00, Nadine attempts to escape. Plinkett corners her in the living room 3 minutes later. Too exhausted by the struggle to return to his basement lab, he decides to watch the experiment progress on the VHS relay his friends at Lightning Fast VCR set up for him and that he began recording at time 0:00 when the first particle hit the screen. Plinkett tells Nadine when they turn on the VCR they will see particle 3's impression on the screen. Nadine agrees but adds that if they rewind the tape they will see the point appear at 4:00. Plinkett argues that they will not as their was no conscious observer present at this time. He believes the particle will not be present on the tape before 5:00 despite being automatically fired at 4:00. His finger hovers over rewind. 
Who is correct? 
Correct answers get a pizza roll. 

Computer package used: Open University S207, The Physical World: Electron Diffraction. 

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