
Real-Life Schrödinger’s Cats Probe the Boundary of the Quantum World - digital55
https://www.quantamagazine.org/real-life-schrodingers-cats-probe-the-boundary-of-the-quantum-world-20180625/
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asafira
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)

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dsmithatx
I wonder, is there any science that making conscious decisions could be
related to quantum mechanics? The mechanics of both feel similar to me.

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imh
The connection between consciousness and quantum mechanics is in the same
realm as chemtrails and homeopathy.

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monktastic1
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._

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coldtea
From the article...

> _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._

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empath75
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.

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Koshkin
... except that the cat _was_ the measurement device...

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blotter_paper
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.

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AnimalMuppet
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.

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sebringj
Many Worlds helps explain how wave-particle duality could be plausible in some
sense. How does one explain non locality????

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ahelwer
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.

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danShumway
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.

[0]:
[https://web.archive.org/web/20120206011811im_/http://zs1.smb...](https://web.archive.org/web/20120206011811im_/http://zs1.smbc-
comics.com/comics/20100620.gif)

~~~
ahelwer
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.

~~~
danShumway
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."

[0]: [http://www.4dtoys.com/](http://www.4dtoys.com/)

~~~
gus_massa
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.

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5DFractalTetris
Ain't this me?

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monktastic1
> 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."

~~~
Koshkin
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.

~~~
monktastic1
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.

