Decoherence and the Quantum to Classical Transition; or Why We Don’t See Cats that are Both Dead and Alive

Conflating Science with Pseudoscience

Galileo Cartoon for Decoherence and the Transition from Quantum to Classical article: imageThe spreading of misinformation and misconceptions about the quantum world can be lumped into two different categories.  The first category are people who mean well, who want to advance science and scientific understanding.  Maybe they write a book, give public lectures, or create news articles about recent events in quantum science, for examples.  However, they use misleading analogies, miss essential features, fail to properly address alternatives to a failing orthodoxy, or mischaracterize apparently paradoxical phenomenon.  As a result, they end up misleading or confusing the general public or their students.  Another failure mode within this category is the use of excessive hype.  Due to their own passions or the desire to spread the excitement of physics, they mislead about the implications of quantum physics in general.  They over-promise when describing the latest incremental step in theoretical or experimental physics; or they mislead about the nature of reality.

The second category is just plain fraudulent; people who deliberately make things up to deceive others for profit.  Prominent examples of this include books and talks like the ones by Deepak Chopra, and movies like What the Bleep Do We Know!?  Rest assured, there is no such thing as quantum healing.  You cannot change your quantum state through your thoughts.  Real harm is done by these quacks when, for example, someone forgoes proven medical treatments for pseudoscience.

My contention is that because we do not do enough to mitigate the negative impact of the first category, the fraudulent category is able to spread easily and quickly amidst fertile grounds.  The public is susceptible to charlatans peddling pseudoscience and quackery by throwing in sciency sounding phrases, and references to quantum physics that no one (including themselves) understands.  Moreover, their claims have no relationship to reality.

There will always be a certain number of people eager to believe whatever pseudoscience or pseudo-religion these hucksters want to sell.  But, if we want to influence the fraction of the public that is interested in separating fact from fantasy, we need to be clearer and more precise in our own presentations of physics.  Moreover, if we want to retain our credibility with the general public as we seek to dispel the drivel these hucksters distribute, we need to make sure we are precise about what QM is and what it is not, what we understand about it and what we do not.

Misconceptions about the Quantum to Classical Transition

Schrodingers_cat_experiment: image

Experimental setup for the Schrodinger’s cat thought experiment. Image from Wikipedia.

One example that contributes to the confusion is the parable of Schrödinger’s cat.  A cat, a flask of poison, and a radioactive source are placed in a sealed box (this is a hypothetical thought experiment, of course – no cats were harmed…).  If an internal monitor detects a single atom decaying, the flask is shattered, releasing the poison that kills the cat.  Naïve application of the Copenhagen interpretation of quantum mechanics leads to the conclusion that the cat is simultaneously dead and alive.  Up until it is measured by a conscious observer, the atom is in a superposition of having decayed and not decayed.  And this superposition allegedly extends to the radiation detector, the vial of poison, the hammer to break the vial, the cat, the box, and to you as you wait to open the box.

People trot out Schrödinger’s cat whenever they want to tout how strange QM is.  “See how weird and paradoxical QM is, how bizarre and unintuitive it’s predictions, how strange the universe is?  Anything is possible with quantum mechanics, even if you don’t understand it or I can’t explain it.”   No, quantum mechanics is not an “anything goes” theory.  A cat cannot be simultaneously dead and alive, regardless of whether or not we observe it.

References to the role of the observer or of consciousness in determining outcomes contributes to this mess.  Even in interpretations of QM that refer to a special role for an observer or a consciousness (interpretations that I believe miss the target of reality), the observer cannot control or manipulate outcomes by choice or thought.  He/she is merely triggering an outcome to become reality; the particular outcome that nature chooses is still random.  You cannot decide to pick out a different wave function for yourself.  Additionally, interpretations of QM that do not have any need for a special role for a conscious observer (and are thus, in my opinion, better approximations of reality) are readily available.  See, for example, the Transactional Interpretation.

Isolating the Environment in Classical Physics

In “Decoherence, einselection, and the quantum origins of the classical, Wojciech Zurek had this to say:

“The idea that the “openness” of quantum systems might have anything to do with the transition from quantum to classical was ignored for a very long time, probably because in classical physics problems of fundamental importance were always settled in isolated systems.”

For centuries, progress in our understanding of how the world works has been made by isolating the system under study from its environment.  In many experiments, the environment is a disturbance that perturbs the system under investigation and contaminates the results of the experiment.  The environment can cause unwanted vibrations, friction, heating, cooling, electrical transients, false detections, etc.  An isolated system is an idealization where other sources of disturbance have been eliminated as much as possible in order to discover the true underlying nature of the system or physical properties under investigation.

Portrait_of_Galileo_Galilei for quantum decoherence and transition from quantum to classical articleGalileo Galilei is considered by many to be the founding father of the scientific method.  By isolating, reducing, or accounting for the secondary effects of the environment (in actual experiments and in thought experiments) he discovered several principles of motion and matter.  These principles, such as the fact that material objects fall at the same rate regardless of mass and what they are made of, had been missed or misunderstood by Galileo’s predecessors.  A famous example is the experiment where Galileo dropped two metal balls of different size, and hence different mass, from the top of a building (supposedly the leaning tower of Pisa). Luckily, the effects of air resistance were negligible for both balls, and they hit the ground at roughly the same time.  He would not have been able to do the experiment with a feather and a steel ball, for example, because air resistance has a much more dramatic effect on the light feather than on the steel ball.  Interesting bit of physics why that is the case, but I’ll avoid the temptation to take that detour for now.

During an Apollo 15 moon walk, Commander David Scott performed Galileo’s famous experiment in a live demonstration for the television cameras (see the embedded video below). He used a hammer (1.32 kg) and a feather (0.03 kg; appropriately an eagle feather).  He held both out in front of himself and dropped them at the same time.  Since there is no atmosphere on the moon (effectively, a vacuum) there was no air resistance and both objects fell at the same rate.  Both objects were observed to undergo the same acceleration and strike the lunar surface simultaneously.

Superposition and Interference: the Nature of Quantum Physics

The situation is quite different in quantum mechanics.  First of all, the correlations between two systems can be of fundamental importance and can lead to properties and behaviors that are not present in classical systems.  The distinctly non-classical phenomena of superposition, interference, and quantum entanglement, are just such features.  Additionally, it is impossible to completely isolate a quantum system from its environment.

According to quantum mechanics, any linear combination of possible states also corresponds to a possible state.  This is known as the superposition principle.  Probability distributions are not the sum of the squares of wave function amplitudes.  Rather, they are the square of the sums of the wave function amplitudes.  What this means is that there is interference between possible outcomes.  There is a possibility for outcome A and B, in addition to A or B, even though our preconceived notions, based on our classical experiences of everyday life, tell us that A and B should be mutually exclusive outcomes.  Superposition and the interference between possible states leads to observable consequences, such as in the double-slit experiment, k-mesons, neutrino oscillations, quantum computers, and SQUIDS.

We do not see superpositions of macroscopic, everyday objects or events.  We do not see dead and alive cats.  Sometimes, our common sense intuitions can mislead us.  But this is not one of those times.  The quantum world is more fundamental than the classical world.  The classical world emerges from the quantum world.  So what happens that makes these quantum behaviors disappear?  Why does the world appear classical to us, in spite of its underlying quantum nature?

Coherence, and Then Naturally, Decoherence

Two waves are said to be coherent if they have a constant relative phase.  This leads to a stable pattern of interference between the waves.  The interference can be constructive (the waves build upon each other producing a wave with a greater amplitude) or destructive (the waves subtract from each other producing a wave with a smaller amplitude, or even vanishing amplitude).  Whether the interference is constructive or destructive depends on the relative phase of the two waves.  One of the game-changing realizations during the early days of quantum mechanics is that a single particle can interfere with itself.  Interference with another particle leads to entanglement, and the fun and fascinating excitement of non-locality.

Decoherence is the Key to the Classical World

The key to a quantum to classical transition is decoherence.  Maximillian Schlosshauer, in “Decoherence, the measurement problem, and the interpretations of quantum mechanics, states that

“Proponents of decoherence called it an “historical accident” that the implications for quantum mechanics and for the associated foundational problems were overlooked for so long.”

Decoherence provides a dynamical explanation for this transition without an ad hoc addition to the mathematics or processes of quantum mechanics.  It is an inevitable consequence of the immersion of a quantum system in its environment.  Coherence, or the ordering of the phase angles between particles or systems in a quantum superposition, is disrupted by the environment.  Different wave functions in the quantum superposition can no longer interfere with each other.  Superposition and entanglement do not disappear, however.  They essentially leak into the environment and become impossible to detect.

I typically love the many educational and entertaining short videos by Minute Physics. However, the video below about Schrödinger’s cat is misleading.  Well before the cat could enter into a superposition, coherence in the chain of the events leading up to his death (or not) has been lost to the environment.  The existence of a multiverse is not a logical consequence of the Schrödinger’s cat experiment.

Perhaps the muddled correspondence principle of the Copenhagen Interpretation could have been avoided, as well as myths and misconceptions about the role of consciousness and observers, if decoherence had been accounted for from the beginning.

The Measurement Problem

Decoherence occurs because the large number of particles in a macroscopic system are interacting with a large number of microscopic systems (collisions with air molecules, photons from the CMB, a light source, or thermal photons, etc.).  Even a small coupling to the environment is sufficient to cause extremely rapid decoherence.  Only quantum states that are robust in spite of decoherence have predictable consequences.  These are the classical outcomes.  The environment, in effect, measures the state of the object and destroys quantum coherence.

So does decoherence solve the measurement problem?  Not really, at least not completely. It can tell us why some things appear classical when observed.  But, it does not explain what exactly a measurement is and how quantum probabilities are chosen.  Decoherence by itself cannot be used to derive the Born rule.   Additionally, it does not explain the uniqueness of the result of a given measurement.  Decoherence never selects a unique outcome.

The Universe and You

International_Space_Station_after_undocking: image

The International Space Station (ISS). Image from Wikipedia.

With care, mechanical, acoustic, and even electromagnetic isolation is possible.  But, isolating a system gravitationally, i.e. from gravitons, is another challenge.  In orbit around the Earth, like the space shuttle or the International Space Station, you are still in a gravitational field with a flux of gravitons that is not that much different than here on the surface of the Earth.  The apparent weightlessness is due to being in a continuous state of free fall (an example of microgravity).  Various theories have been developed that use the pervasiveness of gravitons to explain certain aspects of our quantum universe.

So, yes, the atoms and subatomic particles in your body are entangled with the universe.  That does not mean that you can do anything about it, or use it to your advantage in any way.  There is no superposition, no coherent relationship between you (1) as a millionaire dating a super model and (2) not a millionaire and not dating a super model.  Sorry about that.

 

About Warren Huelsnitz

Former submariner, now high energy physicist. Neutrino physics and astrophysics, quantum physics and other mysteries of the universe. Engaged in life-long learning and pursuit of knowledge. Blogging about quantum mechanics and other issues at www.thefunisreal.com.
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4 comments
huelsnitz
huelsnitz moderator

@HenningD Yes, I think you are correct that the root causes extend back to him.  Ludicrous and unnecessary assumptions were given credibility, setting back progress.  And it has been impossible to shake that legacy.

I was not aware of your blog until checking out your link.  Very interesting material.  Thanks for commenting and providing the link.

HenningD
HenningD

@huelsnitz @HenningD It's kind of ironic that what started out as a pragmatic interpretation of QM essentially turned into a dogma and spawned all this pseudo science, isn't it?

Very much enjoy your blog, will work my way through your archive :-)


huelsnitz
huelsnitz moderator

@HenningD This paper discusses some of the back-and-forth during the development of quantum theory and doctrine: https://www.academia.edu/260503/_Mysticism_in_quantum_mechanics_the_forgotten_controversy_

The 'mysticism' movement in the social culture at the time is interesting, and implications for how that may have influenced physicists and popularization.

Many of the woo-woo masters of today repeat quotes from giants in the history of quantum mechanics to support their pseudoscience.  Even if a physicist had meant something as a metaphor, or later in life realized their foolishness, the quacksters don't hesitate to bend things to their desired meaning.

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  1. […] superposition interacts with the environment, it appears to collapse into a definite state (see “Decoherence and the Quantum to Classical Transition; or Why We Don’t See Cats that are Both Dead a…”).  Only tiny particle-sized entities can be maintained in quantum superpositions for any […]

  2. […] the reference to cats is strictly metaphorical.  Just as with the case of Schrödinger’s cat (Decoherence and the Quantum to Classical Transition; or Why We Don’t See Cats that are Both Dead a…), decoherence prevents macroscopic objects from displaying these quantum mechanical […]

  3. […] Conflating Science with Pseudoscience The spreading of misinformation and misconceptions about the quantum world can be lumped into two different categories.  […]