Thread on the (unfinished) history of interpretations of quantum physics, summarizing what I learned from the book What is Real? The Unfinished Quest for the Meaning of Quantum Physics by Adam Becker ( @FreelanceAstro)
Science doesn't happen in a vacuum, it is not immune to our imperfect human biases, political agendas, social dynamics, and economics. Its progress is dictated by wars, funding, charming personalities, peer pressures from the popular crowd, and cargo-culting.
We need interpretations for scientific theories so that we know where to look and how to progress forward in the research, what questions to ask. As Einstein said, "It is the theory which decides what we can observe"
An interpretation of quantum mechanics is not straightforward to interpret like Newtonian mechanics because the math and predictions of quantum mechanics are foreign to the world we exist in day to day.
Quantum physics uses infinite collections of numbers called wave functions to describe the world. An object (like a particle)'s wave function determines its behavior & the Schrödinger equation (1925) says that wave functions always change smoothly & deterministically.
Max Born discovered in 1926 that a particle's wave function at a given place gives the probability of measuring the particle in that place, and the wave function collapses when particle is measured, as the probability is suddenly 1 where it was measured and 0 everywhere else.
So the wave function behaves smoothly with no jumps except upon measurement when the wave function collapses and becomes 0 everywhere except for the place you measured it at.
So, that leads us to Big Scary Problem Number 1: What is measurement, and why is it treated differently by the laws of physics than everything else? Does it require a conscious observer? If a mouse looks at something, does that count as "measurement"?
As John Bell asked: Was the world wave function waiting to jump for thousands of millions of years until a single celled living creature appeared? Or did it have to wait a little longer for some more highly qualified measurer -- with a PhD?"
The "Copenhagen Interpretation", dripping in logical positivist influences, was, and arguably still is the dominant interpretation among physicists. The interpretation was touted by Niels Bohr and his popular posse including Werner Heisenberg, Wolfgang Pauli, Max Born and others
Bohr was very charismatic and took many of young physicists under his wing at his institute in Copenhagen. He was well respected, well connected, a slow and collaborative worker, peculiar to those around him. Bohr obscure in the way he spoke and wrote, leading to much confusion.
So what is the Copenhagen Interpretation? It's actually a collection of interpretations with the illusion of a unified front. But basically it goes something like: the quantum world can only be considered "real" in conjunction with some kind of measurer to observe it
It says that the measurement problem doesn't matter because it's senseless to talk about something that happens when we're not looking, we shouldn't have scientific theories for things we can't measure. Particles don't have definite positions until we measure them.
In the 1920s and 30s, Bohr, Heisenberg and others presented these arguments backed by Heisenberg's Uncertainty Principle and Bohr's notion of "complementarity". Heisenberg even claimed in 1927 at the Fifth Solvay Conference that quantum physics was a closed theory.
This interpretation is guided by the then-popular philosophy of science, logical positivism: that all statements beyond making measurable predictions are meaningless.
Einstein vehemently disagreed with positivism, because it claims that reality only exists in our minds. He disagreed with the Copenhagen Interpretation for this reason, and because he was certain that the current state of quantum physics was incomplete as it violated "locality".
Locality is the principle that an event in one location can't instantly influence an event that happens somewhere else. It is at the heart of Einstein's relativity. Copenhagen implied that upon measurement, the wave function at other locations instantly jumps.
Though people often mischaracterized Einstein's qualms as a desire for determinism at the heart of physics, the violation of locality was his big objection, famously detailed in the 1935 EPR paper 1935 as "spooky action at a distance", we'll call this Big Scary Problem Number 2.
Scientists like Schrödinger and Einstein were unconvinced by the Copenhagen Interpretation, calling for a more complete theory of quantum mechanics,
In 1932 the famous, mighty, never-wrong John Von Neumann published a (now known to be incorrect) proof that theories of quantum mechanics that assigned definite positions to particles in between measurements, so desired by Einstein were actually impossible.
In 1935 Grete Hermann published a paper pointing out a flaw in Von Neumann's proof, but was largely ignored. After all, what could a woman from outside of the regular physics community possibly see that the great Von Neumann couldn't?
By the late 1930s, the theoretical debates on quantum foundations were cut short by more pressing issues: Nazi Germany was on the rise and war brewed on the horizon. Many Jewish physicists living in Germany and surrounding areas fled to the US/UK.
In 1939 German physicist Otto Hahn had split the atom, leading to physicists' worries about atomic bombs playing a role in the war. Bohr concluded that building an atomic bomb "can never be done unless you turn the United States into one huge factory".
Instilled with fear about the Nazi's creating an atomic bomb first, Eugene Wigner, Edward Teller, Leo Szilard, and Albert Einstein wrote to FDR about the threat. It wasn't for two more years after the attack on Pearl Harbor that the Manhattan Project really got going in 1941.
Meanwhile, Werner Heisenberg, filled with German patriotism and loyalty was working for the nuclear program on the other side. The Nazi project, however was riddled with mistakes and scientific personnel decisions based on political alignment rather than scientific capabilities.
Confirming Bohr's prediction, the Manhattan Project basically did turn the US into a giant factory, spending $25 billion, employing 125k people, all over the US and Canada. In 1945, an atom bomb was dropped on Hiroshima.
The post war landscape changed many things, for one thing, the US government continued to pump incredible amounts of funding into physics projects for defense, because of this (certain specialities within) physics became a much more popular subject to study than it was pre-war.
Though physics was heavily funded and many began to study it, questions on theoretical foundations weren't taught or questioned. Big Science had arrived and it was only concerned with profitable or militarizable progressions in the field.
Meanwhile, Heisenberg was doing damage control on his reputation within the physics community, many of whom had family and friends killed by Nazis, many of whom were jews.
Heisenberg insisted he wasn't a real Nazi, and tried to smooth things over with Bohr and other physicists. In this vein, in 1955 Heisenberg invented what hadn't been there before, a single unified "Copenhagen Interpretation", coining the term himself
Though this unified Copenhagen Interpretation was a myth, it sat well with the post-war Big Science community, who weren't interested in opening the Interpretation of QM can of worms again.
In 1952, David Bohm came up with a pilot-wave interpretation of QM, similar to a theory Louis de Broglie brought up in 1927 but abandoned due to criticisms from Pauli and Kramers which he couldn't find adequate responses to.
The pilot-wave interpretation depicts a world of particles that existed and had definite positions whether anyone was looking or not. The particles each had "pilot waves" to determine their motion, which also were deterministic, though the mathematics still lined up.
The pilot-wave interpretation was one of those that Von Neumann's proof said couldn't exist. Though Bohm's ideas were dismissed by the physics community as "juvenile deviationism." Bohm wasn't able to defend his ideas as he was blacklisted from the US for his communist ideology
Bohm was arrested, blacklisted, and forced to leave the US to Brazil, and unable to travel to Europe or elsewhere to present and defend his ideas, and he found very little support for his ideas anywhere within the physics community.
Our next dissenting physicist was Hugh Everett, who, under the guidance of John Wheeler at Princeton took issue with the unexplained "wave function collapse" upon measurement, and was determined to take the consequences of physical laws seriously.
Everett came up with an interpretation of quantum physics with a universal wave function that never collapses. In his view, measurement isn't a special phenomenon, but is simply what happens when the observer becomes entangled with the wave-function it observes.
To be continued soon because I'm hungry and tired of typing :)
So Everett's interpretation of quantum mechanics is that there is a single universal wave function that always obeys the smooth, continuous Schrödinger equation, and that measurement only looks jumpy because we only see one piece of the puzzle, we ourselves are split into
different branches of the wave function. Just like we can say particles are in a superposition of states(with nonzero probabilities at multiple locations), so too we can say that we become part of that superposition, with different "probabilities" at different locations.
Thus, the smooth, deterministic universal wave function splits into different branches proportional to these probabilities, and there are an infinite amount of universes that continue to branch into more and more.
So where do these probabilities come from in the first place? This question is still up for grabs, but the way I think of it is that the probabilities represent proportions of the universes and that we only see them as probabilities from our limited single branch view.
So anyways Everett was like "here's my theory" and Wheeler liked it and all but he was very diplomatic and was friends with Bohr and other devout Copenhageners. So he checked with them, and they obviously didn't like it so he made Everett revise his thesis.
In the end Everett published his work under the title "Relative State Formulation of Quantum Mechanics" and drastically downplayed the "branch into multiple worlds" part, and focused more on the math of the universal wave function.
Everett's ideas are a lot to handle, especially because you and I have never experienced this branching, "I simply do not branch" said a skeptical Bryce DeWitt, but Everett replied "I can't resist asking: Do you feel the motion of the earth?"
Everett never wanted to be an academic, and after he published his thesis and received his PhD in 1956, he went to work for the Pentagon, not caring enough to endlessly defend his ideas to the academics. Bohr, Rosenfeld & others dismissed his ideas as stupid and hopelessly wrong
What about Big Scary Problem #2: Nonlocality? John Bell was an Irish physicist who wanted rigor & honesty from science. He knew something was wrong w/ Von Neumann's proof of the impossibility of hidden-variable theories when he saw a living counter example: Bohm's pilot-waves
A few years later while on sabbatical, he finally found the time to prove it, revealing an unjustified assumption at the proof's core. Hidden variable theories could exist if they had "contextuality" meaning that the outcome of a measurement depends on other things being measured
In other words, contextuality means that you can get different answers for A if you measure A with B than you do if you measure A with C. If you measure the energy of a particle along with momentum, you get a different answer for "energy" than if you measured energy + position
Just because the measurement alters the answer you get, doesn't mean that there is no quantum world before you measure it. So he proved Von Neumann's proof wrong, but was still disturbed, as was Einstein about the nonlocality that showed up in Bohm's interpretation.
In investigating further, John Bell proved theoretically that either quantum mechanics as we have formulated it is wrong or that nature is nonlocal, which would be a *really* big deal. He published his findings in 1964.
What Einstein had feared with a world where locality doesn't hold is that science would be meaningless. If things can be affected instantaneously by something that could potentially be anywhere in the world, how could you really prove causality of anything?
In 1969, the Clauser-Horne-Shimony-Holt (CHSH) paper proposed an actual experiment to test Bell's inequality. The experiment should prove either that the predictions of quantum physics were wrong or that nature was nonlocal. In 1972, John Clauser and Stuart Freedman performed it.
Clauser and Freedman's 1972 experiment, followed up by a tighter experiment in 1974 by Alain Aspect proved once and for all that nature was non-local😵
However, this wasn't actually as bad for science as everyone thought it would be because later on it was proved that the non-local interactions of quantum physics could never be used to communicate a signal faster than light.
In the backdrop of all of this, a shift in philosophy of science was occurring, positivism was on the way out. In 1951 Willard Van Orman Quine published a paper called "Two Dogmas of Empiricism". Quine pointed out that there is no way to verify a single statement by itself.
Any individual statement relies on an enormous assumed base of knowledge about the world. If I look out my window and say "it's raining outside" it assumes a range of things.
"It's raining outside" assumes that my eyes convey an accurate picture of what is happening, assumes the window doesn't obscure or alter what's happening, and that the water falling in my vision is rain and not tears from a sad flying goat to name a few.
It's impossible to verify a single statement, you can only ever verify the whole of your knowledge about the world. This shook up positivism bc unobservable/unverifiable statements must have meaning -- bc there's no such thing as a perfectly "observable/verifiable statement"
Many other philosophers like Kuhn, Smart, Putnam, Popper, Maxwell, Hanson, and Feyerabend continued to point out flaws in positivist thinking and came up with an opposition: scientific realism: the idea that there is a real world out there whether anyone observes it or not.
As Grover Maxwell pointed out, what is and is not "observable" depends on what technology we have and can always change, like with the invention of microscopes. How can we expect our scientific theories to depend on this fluid, changing unstable foundation?
Though philosophers had moved on from positivism, it's knock off version within physics was still alive and well. Philosophy itself became unfashionable for physicists to talk about, dismissed by physicists concerned with getting grants for pragmatic applications of physics
Many opponents of Copenhagen within physics struggled, like Clauser who proved that nature was nonlocal struggled to get permanent academic jobs. Others were denied promotions and funding, deemed unserious. Young physicists were discouraged from pursuing research in these fields.
It wasn't until the 1980s, when large groups of physicists started openly questioning the Copenhagen Interpretation, nearly 60 years after it was born.
The conception of quantum computing in the 1980s by Richard Feynman and David Deutsch suddenly turned the theoretical questions of quantum foundations into pragmatic ones about how to build a quantum computer.
Old theories like Bohm's and Everett's were revived and new theories like the spontaneous-collapse theory, which alters the mathematical equations ever so subtly to solve the measurement problem.
In spontaneous-collapse theory, there is a real wave function that is governed by an equation similar to Schrodinger's, except sometimes, very rarely, randomly the wave function collapses (independent from any idea of "measurement").
Under this theory, individual subatomic particles, can live in their uncollapsed wave function in their superpositions, but, since objects in our macro-world are composed of so many particles, it's near-certain that at least one of them will collapse,
indicating why we never see macro objects like dogs in the same superposition as we see subatomic particles. Before he died in 1990, he stated the approaches he thought most promising to understanding quantum physics were spontaneous collapse theory or pilot-waves
Bell also said he entertained the many-worlds interpretation, though it seems extravagant. Many-worlds subsequently gained a lot of popularity among cosmologists, and quantum computing theorists like David Deutsch.
Though, even today, many physicists still subscribe to at least some version of the Copenhagen Interpretation. Partly because there is no single coherent Copenhagen Interpretation, it's easy to avoid pitfalls by switching to different versions of it.
Positivist-like ideas, despite being known to be flawed by philosophers, still prevail as many don't look beyond the field, beyond the funding, and beyond what is popular. I think this book makes an excellent case for keeping an open mind, because science isnt immune to our flaws
Science, the way we have it now, is probably flawed in many ways, waiting for some honest and rigorous people to see its flaws and communicate them widely. The best thing scientists can do is educate themselves about interpretations available, and hold them loosely in their minds
Theories that are susceptible to political and social influence, bad epistemic practices, and popularity should not be held too tightly for there is probably something we're missing. All we can do as scientists is be utterly honest and curious about what is really going on.
So I don't know how many tweets this was and if you made it this far you should probably check out the book🙂 It's really interesting and fun to read, and much better at explaining these topics than I was :) /end
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