Hey everyone! This is similar to work I do in my PhD. Feel free to ask me any questions! (I believe Ralf, the first author of the paper cited in the article, will likely come to my group for a postdoc)
1) Putting aside feasibility with current technology, do you think Schrödinger's cat is as absurd as Schrödinger thought it was? Do you think that cats, or humans, could be placed in a state of quantum coherence? If not, why?
2) I expect some long-winded disclaimer before you answer this, but do you personally find the Everett interpretation of quantum mechanics or the Copenhagen interpretation to be more probable?
Not quite. The measurement problem only exists because of this recognition that I only experience one outcome. The key word is experience.
Contrary to the author's loose writing, Schrodinger's Cat (and environment) is in a superposition before it entangles with me. Decoherence (which is just a name for really complicated entanglement) does not change that; it only makes it hard to demonstrate the superposition.
And yet, after I entangle with it, I am no longer free to treat the system as a superposition. If you try to pin down when "I entangle with it," you will notice yourself trying to make precise when "I experience it." Clearly these problems are linked.
To quote Ed Witten:
> I’m not going to attempt to define consciousness, in a way that’s connected with the fact that I don’t believe it will become part of physics. And that has to do, I think, with the mysteries that bother a lot of people about quantum mechanics and its applications to the universe.
> Quantum mechanics kind of has an all-embracing property, that to completely make sense it has to be applied to everything in sight, including ultimately, the observer. But trying to apply quantum mechanics to ourselves makes us extremely uncomfortable. Especially because of our consciousness, which seems to clash with that idea. So we’re left with a disquiet concerning quantum mechanics, and its applications to the universe. And I do not believe that disquiet will go away. If anything, I suspect that it will acquire new dimensions.
>Schrödinger’s point was not, as often implied, the apparent absurdity of quantum mechanics if extrapolated up to the everyday scale.
And a few paragraphs down:
>Schrödinger wanted to show how Bohr’s notion that nothing is fixed until it is measured could lead to logical absurdity if we imagined blowing entanglement up to everyday size.
There’s a subtle difference between the two statements that is more clear if you read the entire article.
He’s saying that Schrödinger didn’t think that _quantum mechanics_ in its entirety was absurd at everyday scales, only that the indeterminacy prior to measurement led to absurdities.
Correct me if I'm wrong, but I think you may be placing special importance on the cat as a measuring device because it is conscious (at least until poisoned). If we assumed that consciousness has nothing to do with measurement, and that two particles interacting is enough for decoherence of one particle relative to another, the cat isn't a measuring device any more than the many constituent atoms in a bucky ball are. If a bucky ball can be in a state of coherence, why not a cat?
I know Schrödinger meant for his thought experiment to be obviously absurd, but I think he may have been underestimating the absurdity of the universe and thus accidentally proposing an entirely possible contraption that could not be feasibly built in his own time.
The radiation detector is a valid measuring device. The hammer that smashes the vial of poison is a valid output device. The cat is a valid observer. But even the cat and the hammer are redundant - the radiation detector is enough.
When learning quantum mechanics, you have to let go of the commonly-held belief (often unknown to the holder) that everything in our world can be explained in terms of concepts that are familiar to us and our human-sized environment. You cannot reason your way through QM with classical analogies. You find the wave-particle duality confusing; why? Only because you cannot imagine a human-sized object that is both a particle and wave. Let go of both of those concepts - forget about particles and waves as you know them - and accept that QM objects fall into new ontological categories which do not map well onto any classical concept. This is fine! As a child, you learned new ontological categories all the time. Everything was alien to you. You've learned new categories before, and you can learn them again.
If we cannot use the language of everyday experience to understand quantum mechanics, what can we use? The answer is math. We have found math to be an incredibly effective language for describing quantum mechanical phenomena. Superposition isn't really like a particle being "in two different places at the same time", it's a complex linear combination of state 0 and state 1. There are differences between those things, and the differences are fundamental to understanding quantum mechanics.
A corollary to the inability to meaningfully explain QM with classical analogies is that all pop science dealing with QM is absolute, irredeemable garbage and always will be. Stop reading it.
There is a similar problem with Special Relativity and Classic Newtonian Mechanics. Let's pick an usual object like a pool ball. To understand it completely you must apply Special Relativity, but for everyday calculation you can approximate its behavior with Classic Mechanic and the approximation is good to play pool, throw it at a target and whatever you usually need. Moreover, you can apply all the theory of a Rigid Body and analyze how it rotates. The problem is that there are no rigid bodies in Special Relativity, because any change must affect all the ball instantly. So a rigid body is sometimes a very good approximation, but there are no true rigid bodies.
In a quantum object in some experiments they can be approximated very well as a particle, and in other experiments they can be approximated very well as a wave. But this are only two good approximations that make the calculations much easier but are not the exact behavior of the object.
I think that at the beginning of discovery of quantum mechanics nobody know what was happening and only know that some object were weird and sometimes they ware well approximated as particles and sometimes they ware well approximated as waves. Later they discovered the "true" nature of these object that sometimes can be approximated as a wave or as a particle, but it is much more difficult to explain to someone that doesn't want to learn the math.
Just to somewhat gently push back on this a little bit, math is probably not enough if you want someone to really get how quantum mechanics work at a fundamental level.
So quantum superposition isn't just two things at the same time, it's a complex equation that combines the two states. Great. Except that non-math majors don't think of linear equations as a singular 'thing' that describes a state. That in itself is a huge paradigm shift for people like me. I'm not used to having an axis in an equation not refer to a continuous range that I can move through.
There's a comic[0] on this springs to mind, that being able to plug equations into something is not the same as understanding it. Most tutorials or technical articles I read about quantum mechanics skip concepts and assume that showing the math is enough. In doing so, they skip all of the hard, useful parts of education and focus only on the language and terminology we can use to talk about something.
I spent a lot of time in physics, statistics, and calculus where I understood the math and not the concepts. I graphed things in 4 dimensions before I encountered Flatland, but I didn't understand the 4th dimension before Flatland. I worked with complex numbers all the time in calculus, but I didn't start to understand complex numbers until years later when I started watching Numberphile.
So I'm at least a little bit skeptical of claims that that quantum mechanics are different. We've had to build new paradigms to understand a lot of stuff in the past; concepts like infinity don't map to tangible analogies, but we can still build tutorials and scenarios that demonstrate interesting behaviors and properties.
Right now the only people I can find trying to do that for quantum are the pop-science writers, and like you said they're mostly all crap. It's kind of frustrating.
I think quantum mechanics really is different. Consider a fundamental aspect of the quantum world, familiar to anybody reading Scott Aaronson's blog: the 2-norm.
People are familiar with how probability works in the classical world. For a given event with several different possible outcomes, like flipping a coin, the probability of those outcomes sums to 1 (here, 1/2 + 1/2 = 1).
In the quantum world, probability doesn't work like that. Rather, the sum of the absolute value of the probability squared equals one. A quantum coin flip has probability (actually called amplitude) 1/sqrt(2) for heads and 1/sqrt(2) for tails. |1/sqrt(2)|^2 + |1/sqrt(2)|^2 = 1.
This has huge implications. It means probabilities can be negative, since |-1/sqrt(2)|^2 + |-1/sqrt(2)|^2 = 1 (actually it goes further - the probabilities can be complex numbers!). It means negative probabilities can interfere and cancel out positive probabilities. And a whole bunch of other wild stuff. All of quantum mechanics (or so Scott claims) follows from this.
I submit that a world where something as basic as the probabilities of all outcomes summing to 1 not holding is a world so alien that attempts to explain it in terms of our own experience will only end in confusion.
Sure, but we don't need to explain everything in terms of our world to still try to explain concepts. We can use contradictions, or games[0], or experiments, or any number of mechanisms.
Infinity doesn't really map to anything else that I understand. Some of the better tutorials I've gotten on infinity are literally just saying, "hey, look at this scenario. Do you think it works the same way as with really big numbers? Well it DOESN'T! Now watch me shove even more guests into your hotel! :)"
I'm not pushing back against the idea that quantum mechanics is a completely new paradigm, I'm pushing back against the idea that math, by itself, is a good enough mechanism for teaching new paradigms.
The math/pop-science dichotomy actually reminds me a bit of my early physics lessons on electricity. I'd either get a really awful analogy (electricity is just like water flowing through a pipe), or I'd get the formula to add up the resistors and calculate voltage and then get told, "well, this is all you're going to need to know for the test."
Neither approach was concerned with understanding how electricity worked; one just said "think of it as something else", and the other said "don't think about it."
The problem is that our best way to describe QM is using math, and the experiments can only verify that the predictions made with math agree with the results.
There are many interpretations of what the math mean, like many words or wave function collapse. But the interpretations are equivalent, in any experiment they use the same math to get the same results.
So you may like one interpretation or the other, but the only sure part is the math.
Games, analogies, think experiments are nice and are useful to understand how to apply the math and get an intuitive idea of how the math work and how some approximations may simplify the calculations.
I still sometimes move my finger in circles to think and do calculations about spin, in spite the spin of a particle is not exactly equal to the spin of a ball. It's a nice analogy and (for me) it's useful, but the real particles don't behave like balls or moving finger.
Another important part is to understand how to translate an actual lab setup to math and how to relate the result of the calculation and the experiments.
I totally agree, the poster understands the calculation rules, but calls the wave function or amplitudes the probabilities, where practically everyone calls their norm squared the probability (or probability density)
It was for pedagogical purposes. I mentioned they were actually called amplitudes. For the intended audience, the difference doesn't matter. Scott Aaronson uses a similar tactic on p110 of Quantum Computing Since Democritus.
As you mentioned Numberphile, what do you think of the YouTube channel PBS Space Time? It helped me finally grok quite a few bits of physics that never made sense to me before.
Everett was a physicist. So was Einstein, Schrodinger, Bohm and DeBroglie. Not all physicists have been sold on the Copenhagen interpretation of QM, where classical concepts don’t apply. That’s just one interpretation.
Everything also seems to behave like a split is happening. They are equally valid explanations, and neither has any supporting evidence that isn't shared by the other. We should probably assume the simpler explanation is more likely, but it's not even clear which explanation is simpler. :/
This dispute isn't about the probabilities or the math, it's about what the probabilities/math are modeling. Is it a non-classical wave-function woo that collapses, including backwards in time? Is it evidence for multiple realities? Is there a pilot wave or other hidden variables? Or is it mostly a matter of ignorance of the exact state of the quantum system, requiring us to use probabilities?
Consider particle decay: it's a fundamentally random event - there is not enough internal structure to suppose the existence of some internal timer, nor is there enough worlds to fill the eternity during which the particle can "choose" to decay at any moment.
For pop books I've only read Brian Greene. I don't think they were helpful for learning quantum mechanics; the reader just stumbles through bewildering scenario after scenario and has forgotten them all within a week.
QED is great, I think it really gets across one way of thinking about these things.
Agree with @ahelwer that Brian Greene isn't terribly useful. Have not read Zee & Smolin's popular books... but for what it's worth, Zee is a serious guy & writes great textbooks, Smolin is not.
This statement is a bit confused. The wave-particle duality is just a confusing and pretentious way of saying that both the model of using particles and the model of using waves are incomplete. They are incomplete because the correct (or at least more precise) model to describe nature, is to use quantum fields. This is a model that describe reality exactly at a vast range of scales. This is a model that supercede our old imprecise particle or wave models. The "many worlds" interpretation does nothing more than "interpret" this new confusing model in a way that is more palatable to our intuition.
Absolutely confused, I agree. I cannot wrap my head around any of this. But to go further into the fog as it were, is there a fuzzy interpretation of non locality, meaning how can one work out "intuitively" how spooky action at a distance works out or is there a story of sorts to explain how a structure could encapsulate that?
I never understood why non-locality is a problem to begin with. I mean, are we assuming that the universe is implemented on an infinite grid of computers, or what? (...which, before you get any ideas, I don't think would work with relativity.)
Is there any reason to filter out nonlocal theories beyond vague notions of aesthetics?
Einstein's theory of relativity is in large part a simple statement that the Universe has a speed limit (the speed of light), and all the complex and amazing facts that entails.
If they Universe doesn't have a speed limit after all, how is that everywhere we look there appears to be a speed limit, except in certain corners? How does the speed limit not hold in non-local action, and what prevents non-local action from making every thing else avoid the speed limit?
If I understand correctly (a big if), then non-locality can always be made to work as time travel. Because locality is the same as saying causation is limited to speed of light, and causality outside a light cone is always time travel from someone’s frame of reference (I’ve never seen a space-time globe demonstration of that last part, but it gets said often enough).
> Locality is the same as saying causation is limited to speed of light
Nope. When two qbits are entangled, operations on one affect the other instantaneously. This breaks locality. However, it does not break causality - we cannot get any classical information from one qbit's frame to the other. See the No-communication theorem: https://en.wikipedia.org/wiki/No-communication_theorem
You can have non-locality without time travel, or breaking causality. If I entangle two qbits, give you one, and we both go to opposite ends of the universe, then when I measure my qbit I'll know yours instantly collapsed to the same value. However, all we've done is created a shared random number generator. I can't communicate anything to you.
One might be tempted to think - well, why do we believe entanglement is a non-local phenomenon? Surely the qbits just decided at the time of entanglement how they would collapse, then remembered that? No. That is called hidden variable theory, which has been disproved.
It seems that it is more sensible to say that entanglement is locality. The notion of two points being close to each other appears to be a course-grained approximation of high amounts of entanglement in the vacuum. If you divide the space along a boundary and break the entanglement between regions on both sides, the effect is to increase the distance between them. So perhaps it is better to think of gravity, locality and dynamical spacetime, as the "hydrodynamics of entanglement"
[0] and [1] are a public lecture, [2] is colloquium
Of course - non-locality in its various forms is a fundamental feature of the quantum world. Nobody argues that the slits (in the two-slit experiment) are separated, or that a particle, when it has a definite momentum, can be found anywhere in the Universe...
I think understanding holography is a good way to develop some intuition for this. The basic idea is that our notions of spacetime aren't fundamental, but instead emerge from other structures where locality may not be evident.
The past 20 years of research into quantum gravity have been dominated by the development of toy models where our formulations of physics in 3+1 dimensions are interchangeable with theories in 2+1 dimensions, where a simple calculation in one theory may be translated into a complicated calculation in the other.
If the way we experience local spacetime is an emergent phenomenon of low-energy approximations of structures that don't involve locality, then it becomes possible to imagine that at high energies, locality might no longer be present.
I have experienced the same type of frustration and sympathize. The laws governing nature are simple and elegant, but they work at a level quite remote from the parameter regimes we experience macroscopically, so they indeed seem unintuitive.
There are many good resources online that can help (I encourage you to search for them), but the truth is that you need to spend many hours reading and solving problem sets to build something you can call intuition. I promise you, it is worth it, but there are many other things that are worth it too.
You missed my point. What one observes in experiment can not be explained by a wave theory or a particle theory alone, or in shorter (but uninformative) phrasing you can say that there is "wave particle duality". But what is the theory that actually works? It is not the theory of wave particle duality (which just stands for "we had to use two contradictory theories" before we can find the more precise theory). The more precise theory is that of quantum fields: there are no particles and no waves, just quantum fields that in certain parameter regimes are well approximated by particles or waves.
But this duality has never been offered by anyone as an "explanation" of quantum behavior. Because it is the quantum behavior - as directly observed in experiment; and it is quantum mechanics (rather than QFT as its relativistic form) that provides a clear explanation of this apparent duality.
> If you simply stick a cat in a box and link its fate to the outcome of some quantum event, you’re not likely to put it in a superposition of alive and dead, because decoherence will almost instantly force it into one state or the other.
This is a non-sequitur. Decoherence will not force it into one state, but merely prevent us (in practice, but crucially not in principle) from detecting interference between the two states. It is still in a superposition.
When am I required to stop treating it as a superposition? When it entangles with me. From the MWI perspective, there are then two copies of me, and each must say that there is no superposition. In other words, from "my" perspective, the superposition ends precisely with me.
Which part of "me"? My toe? My nose? No, clearly it has something to do with my consciousness. A materialist might say that consciousness is a strictly physical process, and when the relevant components entangle with the system, that is when the superposition ends (from my perspective). An idealist might say that consciousness is the very fabric of reality, and so it's obvious that "becoming conscious of an outcome" and that outcome becoming real are one and the same event.
Nonetheless, once this happens, nobody in "my world" can demonstrate interference, even in principle. In this precise sense, you do occupy a unique role in what you call "your world."
Even though it is possible to treat macroscopic objects as quantum systems, the science of quantum mechanics finds it necessary to continue to treat them classically; it is an interaction of a quantum particle with a classical object that constitutes "measurement." The cat is merely such a measurement device.
But crucially, it is two branches, each evolving classically. It is true that this system will obey the same statistics as a classically indeterminate one, but claiming that it's now a single classical system is disingenuous. It's a great place to sweep the measurement problem under the rug.
