Real-Life Schrödinger’s Cats Probe the Boundary of the Quantum World
quantamagazine.org
quantamagazine.org
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?
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.
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.
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.
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.
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.
[0]: https://web.archive.org/web/20120206011811im_/http://zs1.smb...
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.
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."
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.
In a way it is, though - when you measure, you get one or the other.
It might be real, but the problem is nobody knows what the physical mechanism for a collapse would be.
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.
Is there any reason to filter out nonlocal theories beyond vague notions of aesthetics?
Is that what you had in mind, or?
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.
Entanglement breaks locality, but not causality.
[0] and [1] are a public lecture, [2] is colloquium
[0]: https://www.youtube.com/watch?v=OBPpRqxY8Uw
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.
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.
But it's not! It is exactly what one observes in an experiment.
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."