This is one of several shortened indices into the Quantum Physics Sequence.

Hello!  You may have been directed to this page because you said something along the lines of "Quantum physics shows that reality doesn't exist apart from our observation of it," or "Science has disproved the idea of an objective reality," or even just "Quantum physics is one of the great mysteries of modern science; no one understands how it works."

There was a time, roughly the first half-century after quantum physics was invented, when this was more or less true.  Certainly, when quantum physics was just being discovered, scientists were very confused indeed!  But time passed, and science moved on.  If you're confused about a phenomenon, that's a fact about your own state of mind, not a fact about the phenomenon itself - there are mysterious questions, but not mysterious answers.  Science eventually figured out what was going on, and why things looked so strange at first.

The series of posts indexed below will show you - not just tell you - what's really going on down there.  To be honest, you're not going to be able to follow this if algebra scares you.  But there won't be any calculus, either.

Some optional preliminaries you might want to read:

  • Reductionism:  We build models of the universe that have many different levels of description.  But so far as anyone has been able to determine, the universe itself has only the single level of fundamental physics - reality doesn't explicitly compute protons, only quarks.
  • Joy in the Merely Real: If you can't take joy in things that turn out to be explicable, you're going to set yourself up for eternal disappointment.  Don't worry if quantum physics turns out to be normal.

And here's the main sequence:

  • Quantum Explanations: Quantum mechanics doesn't deserve its fearsome reputation.  If you tell people something is supposed to be mysterious, they won't understand it.  It's human intuitions that are "strange" or "weird"; physics itself is perfectly normal.  Talking about historical erroneous concepts like "particles" or "waves" is just asking to confuse people; present the real, unified quantum physics straight out.
  • Configurations and Amplitude: A preliminary glimpse at the stuff reality is made of.  The classic split-photon experiment with half-silvered mirrors.  Alternative pathways the photon can take, can cancel each other out.  The mysterious measuring tool that tells us the relative squared moduli.
  • Joint Configurations: The laws of physics are inherently over mathematical entities, configurations, that involve multiple particles.  A basic, ontologically existent entity, according to our current understanding of quantum mechanics, does not look like a photon - it looks like a configuration of the universe with "A photon here, a photon there." Amplitude flows between these configurations can cancel or add; this gives us a way to detect which configurations are distinct.
  • Distinct Configurations: Since configurations are over the combined state of all the elements in a system, adding a sensor that detects whether a particle went one way or the other, becomes a new element of the system that can make configurations "distinct" instead of "identical".  This confused the living daylights out of early quantum experimenters, because it meant that things behaved differently when they tried to "measure" them.  But it's not only measuring instruments that do the trick - any sensitive physical element will do - and the distinctness of configurations is a physical fact, not a fact about our knowledge.  There is no need to suppose that the universe cares what we think.
  • Where Philosophy Meets Science: In retrospect, supposing that quantum physics had anything to do with consciousness was a big mistake.  Could philosophers have told the physicists so?  But we don't usually see philosophers sponsoring major advances in physics; why not?
  • Classical Configuration Spaces: How to visualize the state of a system of two 1-dimensional particles, as a single point in 2-dimensional space.  A preliminary step before moving into...
  • The Quantum Arena: Instead of a system state being associated with a single point in a classical configuration space, the instantaneous real state of a quantum system is a complex amplitude distribution over a quantum configuration space.  What creates the illusion of "individual particles", like an electron caught in a trap, is a plaid distribution - one that happens to factor into the product of two parts.  It is the whole distribution that evolves when a quantum system evolves. Individual configurations don't have physics; amplitude distributions have physics.
  • Feynman Paths: Instead of thinking that a photon takes a single straight path through space, we can regard it as taking all possible paths through space, and adding the amplitudes for every possible path.  Nearly all the paths cancel out - unless we do clever quantum things, so that some paths add instead of canceling out.  Then we can make light do funny tricks for us, like reflecting off a mirror in such a way that the angle of incidence doesn't equal the angle of reflection.  But ordinarily, nearly all the paths except an extremely narrow band, cancel out - this is one of the keys to recovering the hallucination of classical physics.
  • No Individual Particles: One of the chief ways to confuse yourself while thinking about quantum mechanics, is to think as if photons were little billiard balls bouncing around.  The appearance of little billiard balls is a special case of a deeper level on which there are only multiparticle configurations and amplitude flows.
  • Decoherence: A quantum system that factorizes can evolve into a system that doesn't factorize, destroying the illusion of independence.  But entangling a quantum system with its environment, can appear to destroy entanglements that are already present.  Entanglement with the environment can separate out the pieces of an amplitude distribution, preventing them from interacting with each other.
  • The So-Called Heisenberg Uncertainty Principle: Unlike classical physics, in quantum physics it is not possible to separate out a particle's "position" from its "momentum".  The evolution of the amplitude distribution over time, involves things like taking the second derivative in space and multiplying by i to get the first derivative in time.  The end result of this time evolution rule is that blobs of particle-presence appear to race around in physical space.  The notion of "an exact particular momentum" or "an exact particular position" is not something that can physically happen, it is a tool for analyzing amplitude distributions by taking them apart into a sum of simpler waves.  This uses the assumption and fact of linearity: the evolution of the whole wavefunction seems to always be the additive sum of the evolution of its pieces.  Using this tool, we can see that if you take apart the same distribution into a sum of positions and a sum of momenta, they cannot both be sharply concentrated at the same time.  When you "observe" a particle's position, that is, decohere its positional distribution by making it interact with a sensor, you take its wave packet apart into two pieces; then the two pieces evolve differently.  The Heisenberg Principle definitely does not say that knowing about the particle, or consciously seeing it, will make the universe behave differently.
  • Where Physics Meets Experience: Meet the Ebborians, who reproduce by fission.  The Ebborian brain is like a thick sheet of paper that splits down its thickness.  They frequently experience dividing into two minds, and can talk to their other selves.  It seems that their unified theory of physics is almost finished, and can answer every question, when one Ebborian asks:  When exactly does one Ebborian become two people?
  • Where Experience Confuses Physicists: It then turns out that the entire planet of Ebbore is splitting along a fourth-dimensional thickness, duplicating all the people within it. But why does the apparent chance of "ending up" in one of those worlds, equal the square of the fourth-dimensional thickness?  Many mysterious answers are proposed to this question, and one non-mysterious one.
  • On Being Decoherent: When a sensor measures a particle whose amplitude distribution stretches over space - perhaps seeing if the particle is to the left or right of some dividing line - then the standard laws of quantum mechanics call for the sensor+particle system to evolve into a state of (particle left, sensor measures LEFT) + (particle right, sensor measures RIGHT).  But when we humans look at the sensor, it only seems to say "LEFT" or "RIGHT", never a mixture like "LIGFT".  This, of course, is because we ourselves are made of particles, and subject to the standard quantum laws that imply decoherence.  Under standard quantum laws, the final state is (particle left, sensor measures LEFT, human sees "LEFT") + (particle right, sensor measures RIGHT, human sees "RIGHT").
  • The Conscious Sorites Paradox: Decoherence is implicit in quantum physics, not an extra law on top of it.  Asking exactly when "one world" splits into "two worlds" may be like asking when, if you keep removing grains of sand from a pile, it stops being a "heap".  Even if you're inside the world, there may not be a definite answer.  This puzzle does not arise only in quantum physics; the Ebborians could face it in a classical universe, or we could build sentient flat computers that split down their thickness.
  • Decoherent Essences: Decoherence is implicit within physics, not an extra law on top of it. You can choose representations that make decoherence harder to see, just like you can choose representations that make apples harder to see, but exactly the same physical process still goes on; the apple doesn't disappear and neither does decoherence.  If you could make decoherence magically go away by choosing the right representation, we wouldn't need to shield quantum computers from the environment.
  • The Born Probabilities:  The last serious mysterious question left in quantum physics:  When a quantum world splits in two, why do we seem to have a greater probability of ending up in the larger blob, exactly proportional to the integral of the squared modulus? It's an open problem, but non-mysterious answers have been proposed. Try not to go funny in the head about it.
  • Decoherence as Projection: Since quantum evolution is linear and unitary, decoherence can be seen as projecting a wavefunction onto orthogonal subspaces.  This can be neatly illustrated using polarized photons and the angle of the polarized sheet that will absorb or transmit them.
  • Entangled Photons: Using our newly acquired understanding of photon polarizations, we see how to construct a quantum state of two photons in which, when you measure one of them, the person in the same world as you, will always find that the opposite photon has opposite quantum state.  This is not because any influence is transmitted; it is just decoherence that takes place in a very symmetrical way, as can readily be observed in our calculations.
  • Bell's Theorem: No EPR "Reality": (Note:  This post was designed to be read as a stand-alone, if desired.)  Originally, the discoverers of quantum physics thought they had discovered an incomplete description of reality - that there was some deeper physical process they were missing, and this was why they couldn't predict exactly the results of quantum experiments.  The math of Bell's Theorem is surprisingly simple, and we walk through it. Bell's Theorem rules out being able to locally predict a single, unique outcome of measurements - ruling out a way that Einstein, Podolsky, and Rosen once defined "reality".  This shows how deep implicit philosophical assumptions can go.  If worlds can split, so that there is no single unique outcome, then Bell's Theorem is no problem.  Bell's Theorem does, however, rule out the idea that quantum physics describes our partial knowledge of a deeper physical state that could locally produce single outcomes - any such description will be inconsistent.
  • Spooky Action at a Distance: The No-Communication Theorem: As Einstein argued long ago, the quantum physics of his era - that is, the single-global-world interpretation of quantum physics, in which experiments have single unique random results - violates Special Relativity; it imposes a preferred space of simultaneity and requires a mysterious influence to be transmitted faster than light; which mysterious influence can never be used to transmit any useful information.  Getting rid of the single global world dispels this mystery and puts everything back to normal again.
  • Quantum Non-Realism: "Shut up and calculate" is the best approach you can take when none of your theories are very good.  But that is not the same as claiming that "Shut up!" actually is a theory of physics.  Saying "I don't know what these equations mean, but they seem to work" is a very different matter from saying:  "These equations definitely don't mean anything, they just work!"
  • Collapse Postulates: Early physicists simply didn't think of the possibility of more than one world - it just didn't occur to them, even though it's the straightforward result of applying the quantum laws at all levels.  So they accidentally invented a completely and strictly unnecessary part of quantum theory to ensure there was only one world - a law of physics that says that parts of the wavefunction mysteriously and spontaneously disappear when decoherence prevents us from seeing them any more.  If such a law really existed, it would be the only non-linear, non-unitary, non-differentiable, non-local, non-CPT-symmetric, acausal, faster-than-light phenomenon in all of physics.
  • Many Worlds, One Best Guess: Summarizes the arguments that nail down macroscopic decoherence, aka the "many-worlds interpretation".  Concludes that many-worlds wins outright given the current state of evidence.  The argument should have been over fifty years ago.  New physical evidence could reopen it, but we have no particular reason to expect this.
  • Living in Many Worlds: The many worlds of quantum mechanics are not some strange, alien universe into which you have been thrust.  They are where you have always lived.  Egan's Law:  "It all adds up to normality."  Then why care about quantum physics at all?  Because there's still the question of what adds up to normality, and the answer to this question turns out to be, "Quantum physics."  If you're thinking of building any strange philosophies around many-worlds, you probably shouldn't - that's not what it's for.

New to LessWrong?

New Comment
26 comments, sorted by Click to highlight new comments since: Today at 10:00 AM
But there won't be any calculus, either.

Hmm... I certainly had to look up calculus to follow you and your second derivatives.

With that kind of introduction, I thought you were going to address the Seed article:

on realism.

Brian Miller: That'd be under Bell's Theorem.

RI: But I didn't actually use the second derivative, I just mentioned that it was being taken, so it doesn't count! Right?

Brian Flanagan: I address all these points in various posts.

The foregoing is an example of what William James called the philosophy of "nothing but." Its practitioners can be counted upon to reduce any phenomenon, however mysterious, to: "O, that is nothing but (blah blah, quack quack)."

Closer to home, Gell-Mann has recently opined that Bohr & Heisenberg "brainwashed" a generation of physicists into thinking QM was complete.

"The overcoming of naive realism has been relatively simple. In his introduction to his volume, An Inquiry Into Meaning and Truth, Russell has characterized this process in a marvellously pregnant fashion:

'We all start from 'naive realism,' i.e., the doctrine that things are what they seem. We think that grass is green, that stones are hard, and that snow is cold. But physics assures us that the greenness of grass, the hardness of stones, and the coldness of snow, are not the greenness, hardness, and coldness that we know in our own experience, but something very different. The observer, when he seems to himself to be observing a stone, is really, if physics is to be believed, observing the effects of the stone upon himself. Thus science seems to be at war with itself: when it means to be most objective, it finds itself plunged into subjectivity against its will.'

Apart from their masterful formulation these lines say something which had never previously occurred to me." (Einstein)

"If you ask a physicist what is his idea of" yellow light, he will tell you that it is transversal electromagnetic waves of wavelength in the neighborhood of 590 millimicrons. If you ask him: But where does yellow come in? he will say: In my picture not at all, but these kinds of vibrations, when they hit the retina of a healthy eye, give the person whose eye it is the sensation of yellow." (Schrodinger)

"It seems clear that the present quantum mechanics is not in its final form [...] I think it very likely, or at any rate quite possible, that in the long run Einstein will turn out to be correct." (Dirac)

"Anyone dissatisfied with these ideas may feel free to assume that there are additional parameters not yet introduced into the theory which determine the individual event." (Born)

To be specific about Flanagan's points:

The foregoing is an example of what William James called the philosophy of "nothing but." Its practitioners can be counted upon to reduce any phenomenon, however mysterious, to: "O, that is nothing but (blah blah, quack quack)."

Discussed in several places on Overcoming Bias. In this sequence, mainly Quantum non-realism. But also Fake Reductionism and Angry Atoms.

Closer to home, Gell-Mann has recently opined that Bohr & Heisenberg "brainwashed" a generation of physicists into thinking QM was complete.

Quantum Non-Realism, If Many-Worlds Had Come First

"But where does yellow come in? he will say: In my picture not at all, but these kinds of vibrations, when they hit the retina of a healthy eye, give the person whose eye it is the sensation of yellow." (Schrodinger)

Where Physics Meets Experience

"It seems clear that the present quantum mechanics is not in its final form [...] I think it very likely, or at any rate quite possible, that in the long run Einstein will turn out to be correct." (Dirac)

Einstein was correct. Spooky Action at a Distance: The No-Communication Theorem

"Anyone dissatisfied with these ideas may feel free to assume that there are additional parameters not yet introduced into the theory which determine the individual event." (Born)

Bell's Theorem: No EPR "Reality".

I realize this is a long series, but in the name of all cute kittens, don't tell me what it doesn't address until you actually read it! For the love of Belldandy, Montressor! This is a medium-sized book we're dealing with, designed specifically to reveal quantum mechanics as non-mysterious! Does it really not occur to you that these points might be, oh, addressed?

Hmm, I wonder what Steven Hawkings has to say about this theory.

JT http://www.FireMe.To/udi

Thanks, Eliezer, and fair enough, but in the context of "Hello! You may have been directed to this page because you said something along the lines of Science has disproved the idea of an objective reality," ...

I'm not sure how the Seed article on Zeilinger's work fits in here:

"But whereas Bell's work could not distinguish between realism and locality, Leggett's did. The two could be tested."

"If quantum mechanics described the data, then the lights' polarizations didn't exist before being measured. Realism in quantum mechanics would be untenable."

"The data defied the predictions of Leggett's model by three orders of magnitude. Though they could never observe it, the polarizations truly did not exist before being measured."

"Leggett agrees with Zeilinger that realism is wrong in quantum mechanics ..."

"Late last year Brukner and Kofler showed that it does not matter how many particles are around, or how large an object is, quantum mechanics always holds true." (Macrorealism)

"It could very well be that the distinction we make between information and reality is wrong. This is not saying that everything is just information. But it is saying that we need a new concept that encompasses or includes both." (Zeilinger)

Just looking for an orientation from a layperson-with-some-physics-background perspective trying to resolve an apparent difference (if not contradiction).

Brilliant post! I kinda think a lot of your general points about science (in the intro) are also highly applicable to a personal interest of mine: Consciousness. I might follow your lead and start to 'curate' a collection of relevant papers books etc that demystify consciousness without belittling it. Once again, great work!

Is it going to be an explanation of Cramer's Transactional Interpretation?

Brian M: the basic rule is that if a physicist says something which sounds like mysticism, solipsism, or irrationalism, you ignore it. They are occupational hazards for the philosophizing physicist; you are hearing the effects of a "workplace injury" and nothing more.

To respond to the SEED article at slightly greater length... We can start by trying to get a grip on what they mean by "realism". Zeilinger himself says "to give up realism about the moon, that's ridiculous". So the so-called rejection of realism doesn't involve anything like the abandonment of belief in reality (whatever that could mean), just an abandonment of belief in the reality of some things. Calling that a rejection of realism may be rhetorical excess; it is as if I believed there was a cake in the cupboard, discovered there wasn't, and as a result proclaimed that realism about the cake had been falsified.

However, Zeilinger says, "on the quantum level we do have to give up realism". So what does that mean? We believe in things made of particles (like the moon), but not the particles themselves? We believe that big things, like the moon, have properties, but that small ones, like particles, do not? In the end, it seems we are to abandon the belief that small things have properties before we look. No, wait, we are to abandon the belief that small things have the properties we see them to have before we looked. Well, what if they had some other property before we looked, and then the act of looking (measuring, more precisely) perturbed them into a new state with new properties? That would seem to be entirely consistent with what they describe, but what does that have to do with the 'falsification of realism'?

Do I sound exasperated? Pardon me. It is just that there is so, so much nonsense propagated by physicists in the name of physics, and then further passed on by credulous people who are in no position to make an independent judgement about what they've been hearing. The situation is something like this: We have quantum mechanics, which works experimentally. Traditionally, the quantum states (wavefunctions) are not regarded as the fundamental reality of things, they're just a quasi-statistical description which happens to work. So, on the one hand, we have a variety of attempts to explain what the fundamental reality might actually be, and on the other hand, we have - complacency, basically. A frame of mind which is content to use QM as it is, apply it, extend it, but not to dig deeper. Returning to the attempts at a deeper explanation, we have, as SEED mentions, Bohm's theory, which is a nonlocal theory. So long as quantum mechanics continues to work experimentally, Bohm's theory will never be falsified, because it makes exactly the same predictions as quantum theory. On the other hand, Leggett apparently produced a nonlocal theory which does make slightly different predictions. Zeilinger's group did the experiments, quantum mechanics was right, Leggett was wrong - and this is trumpeted as a falsification of realism on the quantum level, for absolutely no good reason that I can see. It is, I suppose, a falsification of the particular postulates that Leggett was trying to uphold, but calling this a falsification of realism is like saying that not finding the cake in the cupboard was a falsification of realism.

Well, that cleared it up for me.

"...don't tell me what it doesn't address until you actually read it!"

Sorry, but I've heard it all before and remain unimpressed.

"Decoherence," e.g., is often cited as though it's established fact -- and never mind that it might more aptly be called "incoherence."

On the whole, it resembles a steaming pile of what "everybody knows."

I had a feeling it would come down to "it depends what you mean by realism" even though (1) realism as "preexisting properties" seems to have been disproved on a quantum level and (2) macrorealism apparently also fails.

So I'm supposed to ignore Leggett and Zeilinger? I did go read Quantum Non-Realism and came away non-enlightened, largely because of the use of the word "consciousness" which seems to be as fuzzy for physicists as quantum reality is for philosophers.

I don't think Zeilinger was "philosophizing" -- they were trying to hire an actual philospher.

Brian Miller, I referred you to Bell's Theorem, not "Quantum Non-Realism".

Just wanted to say thanks for all the time you spent putting this together. Really good stuff.

I must just say:

Stop talking down on Bohr&Heisenberg when you are doing exactly the same thing. MWI is a joke in the minds of serious physicists. Only David Deutsch (who got the most post-modern crazy ideas in history) claims it's truth, and you jump on the wave.

As Hilary Putnam put it "Because talking about probability in MWI is pointless, the whole hypothesis is incoherent" The bullshit desperate polls people pull up "LOOK HOW MANY BELIEVES IN IT" is a logical fallacy, appeal to authority.

Murray Gell-Man is often presented as a advocate of MWI, but he says he believes in ONE real universe. Hmm? Same with Stephen Hawking. As for Stephen Weinberg, he doesn't even pay attention to interpretations, search for his e-mail exchanges with Shelden Goldstein on the Bohm interpretation.

Bohm pilot wave theory was proposed already in 1926 by de-Broglie but was overlooked because of non locality which has been demonstrated against every loophole. So it really came first, and if it wasn't for Bohr, Heisenberg and Pauli, the quantum mechanics would be bohmian mechanics and solved long time ago. It saves realism, deals with, errrh, predicted non locality, before it was even demonstrated to be a fact of nature. It saves the single universe, energy conservation, the philosophy of SCIENCE and the scientific method and empericism.

Sorry hands down Bohm

MWI can't even be called science, not even pseudo science, pure ficiton.

Re: Bohm saving the (single) universe:

Most of Bohm's adherents do not seem to understand (or even be aware of) Everett's criticism, section VI [1], that the hidden- variable particles are not observable since the wavefunction alone is sufficient to account for all observations and hence a model of reality. The hidden variable particles can be discarded, along with the guiding quantum-potential, yielding a theory isomorphic to many-worlds, without affecting any experimental results.

Well, shows how little Everett understood bohm, the particles are not the hidden variables, the particles are our universe. The wavefunction alone is not enough to describe anything, if thts all that exist, nothing exists and we wouldnt have this conversation...but we exist so we do...

MWI also doesnt work with borns probability rule.

This appears to be an impressive series of articles. Kudos on writing it.

The impression that I get is that the measurement problem is still common to all QM interpretations. Not so much when 'exactly' does decoherence occur, but when, approximately, does decoherence occur. It occurs whenever there is a measurement, and possibly (rarely) at other times, although there is no experimental evidence for the latter.

Your impression is mistaken when it comes to Many Worlds; decoherence is a continuously occurring process which has nothing to do with any 'observers', but only with the way probability mass stretches itself out into different regions of configuration space according to the Schrödinger equation. It'll make more sense once you've read some of these posts, I promise.

(comment edited): Consider an experiment performed which illustrates the watchdog effect. A radioactive molecule has a half-life of an hour. The molecule is repeatedly measured every second, with a resulting delay in the decay of the molecule, consistent with the hypothesis that the half-life is reset upon each measurement.

This experiment seems to show that upon measurement, something happens, whether it be called collapse of the wave function or XYZ. And, if there is no measurement, that 'something' does not happen.

If you think that all worlds are just as real as our world, then under the MWI interpretation you can say that the multiverse is intact. However, the series of measurements has nudged our world to a part of the multiverse where the molecule decays later than it (probably) would have.