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.

 

Closing Loopholes in Quantum Mechanics

Violations of Bell’s Inequalities and Loopholes in Quantum Mechanics

Cosmic Bell Experiment: Closing Loopholes in Quantum Mechanics: imageRecall that, in 1935, Einstein, Podolsky, and Rosen wrote their famous paper that became known as the EPR paradox.  In it, they pointed out the bizarre consequences of the mathematics of quantum mechanics.  If two particles were in an entangled state, then measurement on one of the particles would immediately affect the results of a measurement on the other particle, even if the two particles were arbitrarily far apart at the time of the measurements.  This non-locality was later called “spooky action a distance” by Einstein.

In the 1960’s, John Bell came up with a set of equations, inequalities, that quantified the disagreement between the predictions of quantum mechanics and that of a purely local theory (i.e. one that assumed the distant measurement could not affect the local measurement).  Since then, violations of these inequalities have been experimentally verified on numerous occasions.  Thus, the inescapable conclusion is that nature does make use of non-locality, some how.  However, this conclusion is based on the assumption that nothing else unusual or unexpected is happening during the experiment.

Scrutinizing Loopholes in Observed Violations of Bell’s Inequalities

Many different variations of the experiments have been done.  See, for example, my discussion at Quantum Weirdness: The unbridled ability of quantum physics to shock us.  Many more, different types of experiments have also been done.  In some of these experiments, the violation is more dramatic – not just a matter of the frequency of apparently correlated outcomes.  These experiments are go or no-go; they are designed to look for an event that would not happen under a purely local theory.  See Do We Really Understand Quantum Mechanics? or Do we really understand quantum mechanics? Strange correlations, paradoxes, and theorems for more in-depth discussions.

Given that the implication of these experiments is so profound, scientists have gone to great lengths to ensure that there is not some more benign, classical, local, or deterministic explanation that has been missed.  One possibility is that, since we do not detect every photon due to limitations in detector efficiency, we are detecting a special subset of events.  Another possibility is that the detector settings are not actually independent or random.  Typically, detector settings are chosen randomly; for example, by a quantum random number generator.  But if there were even some slight correlations between the choice of detector settings and some sort of local hidden variables in the system being tested, then the observed violations of Bell’s inequality could be explained without resorting to non-locality.

Closing the Settings-Independence Loophole

Physicists at the Kavli Institute for Cosmological Physics in Chicago, and at MIT, have come up with a brilliant (and FUN) way to avoid the settings-independence loophole and also potentially further quantify non-locality.  See their paper  Testing Bell’s Inequality with Cosmic Photons: Closing the  Settings-Independence Loophole.

Cosmic Bell Experiment Setup: to Close Loophole in Quantum Mechanics: image

Fig. 1 from http://arxiv.org/abs/1310.3288; Schematic of the proposed “Cosmic Bell” experiment. Cosmic sources are used to determine detector settings in a Bell-type experiment.

Their idea is to use distant quasars or the Cosmic Microwave Background (CMB) to determine detector settings.  They would chose two distant quasars on opposite regions of the sky, or two separate patches of the CMB with sufficient angular separation.  Photons from these sources would be coming from events whose past light cones do not overlap.  These photons would then be used to determine the detector settings.

This experiment will close the settings-independence loophole (assuming the results remain consistent with QM and non-locality!).  If something unexpected is seen, it will enable mapping non-local correlations as a function of the overlap between light cones of the two independent photon sources.

Of course, the experiment will not be without some challenges.  The authors refer to a potential “noise loophole”.  They have to ensure that the cosmic photon detectors are not triggered by more local sources of photons, such as light pollution, scattered star light, zodiacal light, etc.  They also need to account for the impact of the intergalactic medium and Earth’s atmosphere on the cosmic photons.  It will be interesting to see where this leads in the coming years!

Fun with Quantum Computing at University of Bristol

Physics is Fun, at University of Bristol

Run your own quantum computing experiments

Quantum Computing at University of Bristol imageHave fun with quantum physics and quantum computing!  Gain practical experience using the resources offered by University of Bristol: Qcloud.  Test out your quantum experiments in their online quantum processor simulator; includes reference material and users guide.  Then, you can (starting 20 September) register and run your experiment in their lab: “create and manipulate your own qubits and measure the quantum phenomena of superposition and entanglement.

 

Quantum Weirdness: The unbridled ability of quantum physics to shock us

Double slit experiments and the root of quantum weirdness

Quantum weirdenss: quantum superposition meets life cartoon

You are probably familiar with the legendary double slit experiment.  It is a simple, straight-forward experiment that introduces the wave-particle duality and the weirdness that is at the foundation of quantum mechanics.  Many people read or hear about the generic double slit experiment and assume that it is the end of the story.  Worse, many intrinsically curious people erroneously assume that this phenomenon is understood.  Or, they believe that they have a benign explanation for a particular experimental result.  Hence, they don’t dig deeper into the challenge and excitement that can be found in the quantum world.  Digging through the layers of experimental results helps ensure we are questioning the implications of our hypothesis, and testing whether it remains consistent and valid.

In the typical double slit experiment, a beam of light is directed at a pair of slits in a wall.  The resultant interference pattern is then observed on a screen or some other sort of detector.  You can see a five-minute introduction to the double slit experiment by Dr. Quantum: “Double Slit Experiment”.  Photons are typically used, although according to de Broglie and confirmed by Davisson and Germer, electrons or any other particle will work.  Using photons greatly simplifies the technical difficulties and cost of the experiment.

Quantum Weirdness: double slit interference pattern imageThe experimental apparatus consists of a coherent light source, a wall with two narrow slits (the width and separation of the slits are comparable to the wavelength of the light), and a detection screen behind the wall.  A narrow beam of light strikes the pair of slits.  The pattern on the detection screen is an interference pattern.  So what, light is a wave.  Or, at least it acts that way sometimes.  We can easily calculate the details of the interference pattern using the dimensions of the experimental setup and the frequency of the light.  The two different paths, (source – slit A – screen; or source – slit B – screen), have different path lengths.  Hence, the two interfering waves have different phases and can add constructively or destructively.

Doubling the dosage of quantum weirdness

The rub comes in when you dial down the intensity of the beam of light so that only one photon at a time is passing through the slits.  One photon can’t pass through both slits, right?  It can’t interfere with itself, right?  Well, the interference pattern is still there.  Quantum mechanics tells us that the photon is in a superposition of states.  See Feynman’s QED: The Strange Theory of Light and Matter (Princeton Science Library) or Cox and Forshaw’s The Quantum Universe: (And Why Anything That Can Happen, Does)for excellent conceptual descriptions of how to visualize what the particle is up to.

Quantum Weirdness: Wave-Particle Identity Crisis image

I can contemplate how a single “particle” can act like a wave sometimes, a particle other times, or some combination thereof.  I can do that without feeling like I am losing my grip on reality.  But, here is where things start to get really weird.  If you modify the above experiment so that you can tell which slit the photon traveled through, the interference pattern goes away.  It is as if each photon realizes they are being watched and stops their shenanigans.  But how do they “know” they are being watched?  How do they know that it is time for wave function collapse”?  In wave function collapse, the photon discards the superposition of states and selects a specific value or path (a specific eigenstate).

The quantum measurement problem

This is the crux of the quantum measurement problem:   Why are there two different processes describing the evolution of a particle’s wave function?  These two processes are (1) the continuous evolution described by Schrödinger’s equation, and (2) the spontaneous collapse into a specific eigenstate.  What triggers the collapse?  Why can’t we observe a superposition or the collapse process? How can a wave function that is spread out over arbitrary distances collapse seemingly instantaneously?  Some scientists have argued that wave function collapse is triggered by an interaction between the observer and the photon, or between the measurement apparatus and the photon.  For example, the momentum that is imparted to the photon by the act of measurement may break the superposition. The problem with this argument, however, is that clever experimentalists have already devised and carried out experiments using interaction-free measurements.

Now for some truly bizarre quantum weirdness

If you have followed the discussion so far, it is still not safe to unfasten your seatbelt and start walking about the cabin.  This is the point where I start to feel my grip on reality slipping away.  Particles are doing something we completely do not understand, in apparent response to some trigger we completely do not understand.  Consider delayed choice experiments and quantum eraser experiments.  In delayed choice experiments, the decision to determine which path the photon used is made well after the photon has passed the slits*.  Yet, the results seem to indicate that the photon has retroactively collapsed its wave function and chosen a single path.  If you mark through which slit each photon went, the interference pattern is destroyed.  This happens even if you mark the path without disturbing the photons movement.  In quantum eraser experiments, photons are tagged based on which path they took. By itself, that tagging destroys the interference pattern.  However, when this path information is discarded (erased), the interference pattern is restored.

This begs the following questions: What happens to the discarded part of the wave function when wave function collapse occurs?  Some people argue that the wave function is not real, it just encodes our knowledge of the situation.  But when the tagging is erased, how does the quantum system “know” what superposition to re-enter?

Quantum Weirdness: delayed choice quantum eraser experiment imageDelayed choice quantum eraser” experiments combine all of the absurdities of the above experiments.  This experiment is arranged to identify which one of the paths the photon uses.  And, this information can be erased after the fact.  See the adjacent figure from Wikipedia.  Photons are emitted one at a time from the lower-left and then subjected to a 50% beam splitter.  After the beam splitter (green block), photons travel along two possible paths, the red or blue lines. Reminds me of a Tokyo subway map.  In the top diagram, the trajectory of each photon is known. If a photon emerges at the top of the apparatus, it appears to have come by the blue path.  If it emerges at the side of the apparatus, it appears to have come by the red path.  As shown in the bottom diagram, a second beam splitter is introduced at the top right.  This can direct either beam towards either path, erasing the which-path information.  So, the decision whether or not to remove the path information is made after-the-fact.  If you remove the path information, the interference pattern is restored.  The photon appears to recover its superposition properties.

Mind-blowing quantum paradoxes

Indeed, quantum physics is rich with paradoxes and non-intuitive behaviors.  While contemplating a certain experiment, it is important to ensure you have a complete picture of what the theorists and experimentalists are trying to tell us.  By merely considering one particular experiment, it is possible to convince yourself that you understand what is happening.  But, other experiments may contradict or invalidate your conceptual line of reasoning.  It seems to me that there is some deep, underlying concept or unifying principle that we are missing.  Some key piece of the puzzle that will show us that superposition and entanglement are fundamental, and apparently not constrained by space and time.  There must be some (comprehensible) reason for why matter behaves like this.

* Many experiments use alternative path-separation devices, such as mirrors or beam splitters.  Additionally, different techniques have been used to “tag” specific paths or to detect the photons.  The math and the concepts are the same in these various setups.  Different arrangements help to clarify the results and resolve concerns over subtle technical issues.
 
Einstein quote image"The supreme task of the physicist is to arrive at those universal
elementary laws from which the cosmos can be built up by pure
deduction. There is no logical path to these laws; only intuition,
resting on sympathetic understanding of experience, can reach
them."
				- Albert Einstein

 

Contrary to Popular Belief, Einstein Was Not Mistaken About Quantum Mechanics

Misconceptions and assumptions concerning quantum mechanics

I get somewhat frustrated every time I read another blog post, book review, or journal article that claims Einstein was wrong about quantum mechanics (QM).  It must make for good headlines and is almost cliché.  First, these articles often give the misleading Einstein and quantum mechanics imageimpression that Einstein was the only physicist who had concerns with quantum mechanics during its development and exposition.  That simply is not true.  Many physicists (Schrödinger, de Broglie, Podolsky, Rosen, and several other major figures) had concerns.  Additionally, the relatively small fraction of physicists that are active today in the foundations and interpretations of quantum mechanics continue to debate the meaning, the implications, and the completeness of the theory with great vigor.  There is not yet a general consensus among experts as to the answers to some of the most fundamental questions about the implications of quantum theory in its present form.

For decades, there has been a common misconception among many physicists that the conceptual problems with QM were already resolved or that any remaining questions were purely philosophical.  Contributing to this state of affairs, many textbooks focused solely on the computational aspects.  If interpretations or foundations were discussed at all, the focal point was on the Copenhagen interpretation.  There was little or no discussion of other viable formulations, and the solutions to conceptual problems that these formulations offered.  The prevailing interpretation of QM does not give a clear answer to the question “what, if anything, is objective reality”. Some alternatives, such as de Broglie-Bohm mechanics, do.  According to de Broglie-Bohm mechanics, particles are objective point-like objects with deterministic trajectories. These trajectories are guided by wave functions, which also objectively exist.

Alternatives to conventional quantum mechanics

I am not at all claiming that de Broglie-Bohm mechanics in its current form is the final word.  And I am not claiming that we need to immediately replace our existing paradigm with it, without further consideration or modification.  However, de Broglie-Bohm mechanics has not been properly vetted by generations of physicists.  I think failure to fully consider and evaluate such approaches may be blinding us to the way ahead.  The prevailing, fractured conceptual understanding of QM may be holding us back from making the next theoretical and technical leap in our quest to understand the universe.

The venerable John S. Bell had this to say about de Broglie’s wave theory (see Speakable and Unspeakable in Quantum Mechanics):

“Is it not clear from the smallness of the scintillation on the screen that we have to do with a particle? And is it not clear, from the diffraction and interference patterns, that the motion of the particle is directed by a wave? De Broglie showed in detail how the motion of a particle, passing through just one of two holes in screen, could be influenced by waves propagating through both holes. And so influenced that the particle does not go where the waves cancel out, but is attracted to where they cooperate. This idea seems to me so natural and simple, to resolve the wave-particle dilemma in such a clear and ordinary way, that it is a great mystery to me that it was so generally ignored”

 And this about Bohmian mechanics:

 “In 1952 I saw the impossible done. It was in papers by David Bohm. Bohm showed explicitly how parameters could indeed be introduced, into nonrelativistic wave mechanics, with the help of which the indeterministic description could be transformed into a deterministic one. More importantly, in my opinion, the subjectivity of the orthodox version, the necessary reference to the “observer,” could be eliminated. … But why then had Born not told me of this “pilot wave”? If only to point out what was wrong with it? … Why is the pilot wave picture ignored in text books? Should it not be taught, not as the only way, but as an antidote to the prevailing complacency? To show us that vagueness, subjectivity, and indeterminism, are not forced on us by experimental facts, but by deliberate theoretical choice?”

EPR and quantum entanglement

The famous “EPR paper”, so-named due to its authorship: A. Einstein, B. Podolsky, and N. Rosen, “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”, laid out some of Einstein’s main concerns.  These included lack of an objective physical reality in which deterministic properties of observables exist regardless of measurement.  And nonlocality, in which a measurement process carried out on one of a pair of entangled particles can seemingly affect the other particle’s properties, instantaneously and without regard to distance.Real-world quantum entanglement cartoon  Einstein continued to voice his objection to this fundamental property of quantum mechanics: “because it cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance.”  (Max Born, ed., The Born-Einstein Letters: Friendship, Politics and Physics in Uncertain Times (Macmillan, 1971), p. 178).

After Einstein’s death, the phenomenal John Bell figured out how to quantify the “spooky” part of the intrinsically probabilistic behavior of a pair of entangled particles.  See his papers in Speakable and Unspeakble in Quantum Mechanics.  Years later, experimentalists such as Freedman, Clauser, and Aspect, confirmed that Nature really does make use of this spooky action at a distance, or nonlocality.  But to what end?

Although nonlocality has subsequently been confirmed experimentally, it is ludicrous to criticize Einstein for his concerns about a theory that included it.  It would be a sad day for science if such a huge paradigm shift swept over the community without raising a few hairs.  Additionally, physicists still do not understand how the nonlocality is achieved, nor its implications.

The quantum measurement problem

A related issue is wave function collapse and the “measurement” problem. The measurement problem manifests itself in the fact that there are two rules for how a quantum state evolves in time. The Schrödinger equation tells us how the wave function (or more generally, the state vector) evolves in time when a quantum system is not being “observed” or “measured”.  With the Schrödinger equation, you can calculate the probabilities for possible outcomes to different measurements, and how those probabilities change over time.  This evolution of the state vector while no one is looking is continuous.  However, instantaneous collapse of the state vector into a particular eigenstate occurs upon measurement.  Why the discontinuity in the descriptions of the two processes?  What constitutes a measurement?  What are the dynamics for wave function collapse?  Does this mean that wave functions (or state vectors) are approximations to some more complete description of quantum systems?

The Dalek Interpretation of Quantum Mechanics cartoonThe collapse postulate is ad hoc, based on the fact that we never observe superpositions of quantum states.  The core of the measurement problem is the inability of QM to explain the abrupt transition from linear evolution of the wave function, to non-unitary wave function collapse.  Steven Weinberg summarizes it thusly: “during measurement the state vector of the microscopic system collapses in a probabilistic way to one of a number of classical states, in a way that is unexplained and cannot be described by the time-dependent Schrödinger equation.”

Conclusions

So, objective reality is not understood, nonlocality is not understood, wave function collapse is not understood.  We could go on.  My impression, based on trends in the literature, is that more and more of the community of physicists is recognizing the holes that remain in our conceptual understanding of the quantum world.  As more and more theoretical and experimental physicists struggle with these issues, perhaps we will get closer to a breakthrough.

Additional material

Here is a YouTube video with a quick introduction to entanglement: Quantum Entanglement – The Weirdness Of Quantum Mechanics.  And a ScienceDaily article on quantum entanglement, including links to additional background information on quantum mechanics.

Comrade on the quest

To my delight, just as I finished writing and editing this post, I found the following article on the electronic preprint archive, arXiv.org.  Submitted today by Pablo Echenique-Robba, who apparently shares many of my views on the current state of QM:

Title: Shut up and let me think! Or why you should work on the foundations of quantum mechanics as much as you please.

 Abstract: If you have a restless intellect, it is very likely that you have played at some point with the idea of investigating the meaning and conceptual foundations of quantum mechanics. It is also probable (albeit not certain) that your intentions have been stopped on their tracks by an encounter with some version of the “Shut up and calculate!” command. You may have heard that everything is already understood. That understanding is not your job. Or, if it is, it is either impossible or very difficult. Maybe somebody explained to you that physics is concerned with “hows” and not with “whys”; that whys are the business of “philosophy” — you know, that dirty word. That what you call “understanding” is just being Newtonian; which of course you cannot ask quantum mechanics to be. Perhaps they also complemented these useful advices with some norms: The important thing a theory must do is predict; a theory must only talk about measurable quantities. It may also be the case that you almost asked “OK, and why is that?”, but you finally bit your tongue. If you persisted in your intentions and the debate got a little heated up, it is even possible that it was suggested that you suffered of some type of moral or epistemic weakness that tend to disappear as you grow up. Maybe you received some job advice such as “Don’t work in that if you ever want to own a house”.   I have certainly met all these objections in my short career, and I think that they are all just wrong. In this somewhat personal document, I try to defend that making sense of quantum mechanics is an exciting, challenging, important and open scientific endeavor. I do this by compulsively quoting Feynman (and others), and I provide some arguments that you might want to use the next time you confront the mentioned “opinions”. By analogy with the anti-rationalistic Copenhagen command, all the arguments are subsumed in a standard answer to it: “Shut up and let me think!”

 

The Folly of Physics: Interpretations of Quantum Physics, Part 1

The issue

With this post, I begin to layout some concerns that I have with descriptions and interpretations of quantum physics.  We still do not have a conceptual understanding of what the heck is going on in quantum mechanical processes.  Albert Einstein took issue with several aspects of quantum theory: the inherent randomness, the nonlocality, and the lack of realism, for example.  We may need to accept these aspects of nature, but is it asking too much to be able to understand how/why/what the universe is doing in these situations?

Quantum physics is a fickle mistress

Quantum Mechanics (QM) is perhaps one of the most successful hypotheses in the history of physics. That is, if you evaluate success based on agreement with experiment and ability to make predictions that are later confirmed by experiment.  And, quite frankly, that is (quite appropriately) how science judges hypotheses and theories.  Thousands of experiments have been performed, verifying the accuracy and relevance of QM.  These experiments include emission or absorption spectra predictions and measurements, magnetic moment predictions and measurements, and multiple variations of the double slit experiment, to name a few.  Physicists, chemists, and engineers have subjected matter to all kinds of bizarre tests that have validated the theory’s un-intuitive predictions.  Additionally, QM is not just a theoretical curiosity.  The range of technologies based on it is staggering.  Without QM, we would not have PCs, iPads, smart phones, laptops, modern TVs, modern medical imaging equipment, the microchips that control everything from our cars to our refrigerators, and so on.

Unexplained behavior in quantum physics experiments. imageYet, after all this, we still do not understand HOW quantum physics works.  Even though the theory is a century old, we are far from a proper conceptual understanding of what it actually means and HOW the universe pulls off this behavior.  How does a particle manage to take every possible path?  How does a wave function seem to collapse, essentially instantaneously, across arbitrary distances?  How do entangled particles influence each other, seemingly without regard to time and space?  Why do certain quantities have to be quantized, rather than continuous?  These difficulties are related to the fact that a complex-valued state vector is used to describe a physical system.  So another way to ask these questions is, “why are complex quantities and a state vector required to quantitatively describe behavior at the quantum level?”

Why is it so hard to visualize quantum physics?

We can visualize general relativity (GR).  It is understood as the interplay between matter and spacetime.  Apart from some warping and dilation, GR makes intuitive sense.  There is a speed limit and strict enforcement of locality.  With some mathematics, we can readily convince ourselves that causality is safe.  Electromagnetic and nuclear interactions are described mathematically and conceptually as due to the exchange of particles called bosons.  These particles (photons for electromagnetism, gluons for the strong nuclear force, W and Z bosons for the weak nuclear force) account for the transfer of momentum and energy between fermions (i.e. quarks, electrons, protons, etc.).  They also account for the transfer of conserved quantum numbers.  In none of these fundamental forces do we have “spooky action at a distance”, or nonlocality.  We do have virtual particles, which is another story and takes some time to get used to.  But at least, even then, we have a picture in our heads of what is going on.

In quantum physics, the state vector is not a physical description of the system.  And the evolution of the state vector seems to occur in two distinct phases.  First, a continuous evolution of the state vector occurs as the system evolves in time and space (described, for example, by the Schröedinger equation).  Then, there is an abrupt and discontinuous collapse of the state vector, into a particular eigenstate; the dynamics of which are not understood.  A common misconception is that this state vector collapse is caused by the interaction with the measurement device.  But clever, interaction-free measurement processes have been devised.  The collapse of the wave function has been verified in situations where interactions play no role in the measurement.  At least no known interactions.

Why should we care about conceptualizing quantum physics?

Given that QM works so well, and (so far) no experiments have contradicted it, why should we care how it is interpreted?  QM does not provide a physical description of a process.  The old adage is to just “shut up and calculate” (David Mermin).  However, this lack of a conceptual understanding may be what is holding physicists back from uniting the two pillars of modern physics, QM and GR.  It may be the key to understanding many of the most fundamental and provocative questions physicists are struggling with:

  • How do we unite the two theoretical paradigms of modern physics, QM and GR?
  • What happened at (and before) the origin of the universe?
  • What will be the ultimate fate of the universe?
  • What is driving the apparent acceleration of the universe’s expansion (i.e. what is dark energy)?
  • What happens inside a black hole?
  • What is time?

It may also be necessary for the next great leap in technology, such as quantum computing.  Besides all that, I just want to know.  Is that asking too much?

Competing interpretations of quantum physics

At least to some extent, I think QM has been a victim of its own success.  Since it has worked so well, there is little less motivation to fix it.  Additionally, it is very difficult to distinguish between the predictions of some of these different interpretations.  So it will be difficult to experimentally validate the correct interpretation, at least for some time.

Many different interpretations have been offered up over the years.  I will dig into some of these in later posts.  To name a few (Frank Laloë, “Do We Really Understand Quantum Mechanics?”):

  • Statistical interpretation
  • Relational interpretation
  • Logical, or algebraic approach
  • Veiled reality
  • Additional (hidden) variables
  • Modified Schröedinger dynamics
  • Transactional interpretation
  • History interpretation
  • Everett interpretation
  • Modal interpretation

One of my favorites is the de Broglie-Bohm pilot wave interpretation.  In this model, a particle’s motion is determined by a wave.  Hence, you can reproduce both particle and wave-like behaviors and the predictions of generic QM.  However, there are some issues with de Broglie-Bohm theory.  These include things like relativistic invariance and the dynamics for how the particle and wave influence each other.  “We’ll talk about that later”.

Unfortunately, alternative explanations have not been given full and proper consideration over the past 86 years (since the 1927 Solvay Conference and the birth of the Copenhagen interpretation).  Some alternatives have been appropriately disproven.  However, others have just been over-ridden or ignored.  A common theme is that someone publishes a paper “showing” that some interpretation is not workable.  Later, someone else shows how that paper was in error.  People remember the first paper and continue to assume that a certain idea is untenable.  Various interpretations are confused with each other, or assumptions are confused with conclusions.  Scientists are erroneously lead to believe that a particular approach is not valid.

These myths are perpetuated in text books and lectures.  One example is de Broglie’s hidden variables theory, which was relegated to the scrap heap after the 1927 Solvay Conference.  It was resurrected after David Bohm developed his theory in the 1950s, and the similarities between it and de Broglie’s earlier work were noticed.  Another example: experiments confirming the violation of Bell’s inequality and hence confirming the concerns of the famous EPR paper (Einstein, Podolsky, Rosen) are often cited as confirming hidden variables theories are unworkable.  They actually show that hidden variables theories cannot sidestep the apparent nonlocality, not that they are altogether un-viable.

De Broglie-Bohm mechanics at work?

Take a look at this amazing video from the Science Channel’s Through the Wormhole, on Wave/Particle dynamics with silicon droplets.  It shows how the results of the double slit experiment can be reproduced by a silicon droplet (the “particle”) riding on an actual, physical “wave”.  There are a lot of details that go into this experiment, including how the apparatus works and how it is filmed.  So it is definitely not a proof of de Broglie-Bohm mechanics.  However, it is intriguing, and offers an irresistible visualization that begs further investigation.

I think a significant factor in our failure to develop a consistent and deep conceptual understanding of quantum physics is rooted in the dogmatic presentation of the prevailing interpretation.  For decades, up-and-coming physicists have been indoctrinated in the “Copenhagen interpretation”.  Presented with the implicit assumption that interpretation questions are settled, many students don’t dig deeper.   The development of a proper, conceptual understanding has been further hobbled by misconceptions that are perpetuated through textbooks and instructors.  Students either assume the issue is resolved and look elsewhere for research opportunities, or they are discouraged by their advisors and forced to conform to availability of funding and job opportunities.

I recall being confused and frustrated, as an undergrad physics student, when the explanations in the textbook or provided by the professor, just did not make sense and did not seem to be consistent with what the mathematics implied.  For example, the meaning and implication of the uncertainty principle are often explained as being due to the unavoidable transfer of momentum to the observed particle during a measurement.  However, experiments have been done that show this is not the case.  Moreover, it is an intrinsic property of the mathematics, in which momentum and position are Fourier transformations of each other, like time and frequency in everyday applications of Fourier theory to acoustic or electromagnetic signals.

Conclusion

In the weeks and months ahead, I will expand on the specific points brought up in this article. It is entirely possible that Nature really is unknowable.  The Universe probably does not feel compelled to satisfy my desire for a visual, comprehensible model, unless it already intended to do that anyway.

Welcome to “The Fun is Real!” (Fun with Physics, that is)

Welcome to “The Fun Is Real“, a new blog that will explore wonders and mysteries of physics.  In particular, I am interested in the questions that are not yet understood.  These questions may be due to new experimental evidence that out-paces the theorists, like dark matter, dark energy, neutrino anomalies, etc.  Or it may be areas where the theory works, but we don’t have a conceptual understanding of how/why the universe does what it does.  One example of this is quantum physics and  quantum non-locality.

The Fun Is Real: wave-particle duality imageThe predictions of quantum mechanics have been confirmed, time and time again, by experimentalists, with greater precision than any other theory in the history of physics.  In the history of science, for that matter!  Additionally, the engineering breakthroughs that have created our information society, and the current trajectory of our technology, are dependent upon quantum mechanics.  Yet, we do not understand how the universe pulls off some of the tricks inherent in quantum physics.  We don’t understand why certain things are quantized.  And entangled particles seem to be able to affect each other over arbitrary distances, without regard to time.  I will expand more on these issues in future blogs.  I also invite your inputs and ideas on the discussions.

In addition to quantum non-locality, examples of other areas that you will see discussed here in the coming months include: (1) Given that a charged particle undergoing acceleration gives off electromagnetic radiation (i.e. emits photons), and a gravitational field is equivalent to acceleration, then why don’t charged particles emit photons simply due to being in a gravitational field? Or do they? (2) Would time exist if there were no matter? (3) Why does the universe insist upon the use of “imaginary”, or complex, numbers to communicate it’s behavior? (4) Where does inertia come from and why does gravitational mass appear to be the same as inertial mass?

I don’t accept anthropomorphic explanations.  That is, I don’t accept as adequate an argument that states “we would not be here if it were not so”.  That does not contribute to our understanding of the how/why of the universe.  I also don’t accept “the theory has to be that way to be consistent with the evidence”.  I want to understand.  I want to know why.  I want to know how.  I want to know how a particle can impact measurements done on it’s entangled partner, in apparent violation of locality and the speed of light; not just how to do the calculations.

This is a new website.  I am trying to make it interesting and accessible.  Let me know if you see problems or if you have ideas to make it better.  Remember, the physics may be theoretical, but “The Fun Is Real“.