“Reality is that which, when  you stop believing in it,  

doesn’t go away.” That’s how Phillip Dick  summed it up. Sounds simple enough but it’s  

ridiculously difficult to make sense of in  quantum physics. It’s somewhat of a problem  

because we think that quantum physics describes  everything. Really? Well, that is the question,  

right. Because in quantum physics it can happen  that different observers arrive at different  

conclusions about what is real. And if reality is  a matter of opinion, then it isn’t really real.  

A group of physicists now say they have found the  origin of reality. And that is really interesting.

The trouble with quantum mechanics is  that it’s unclear just what it describes.  

The central element of quantum mechanics is the  wave-function. From the wave-function we calculate  

probabilities for measurement results. So far,  so clear. But what does that mean? Some say,  

the wave-function describes what particles  do. Others say, no it just describes what  

we observe about the particles. If you believe  the latter, then quantum mechanics is really a  

subjective theory. It doesn’t tell you what’s  real. It just tells you what you observe.

Physicists believe in the subjectivity of quantum  mechanics to different degrees. Some find it  

appealing because it seems to suggest a link  to consciousness. An extreme case is probably  

Chris Fuchs who defends an interpretation called  QBism that is basically solipsistic. He says,  

you can’t know that anything is real other than  you, yourself. It’s the only interpretation that  

comes with built-in customer support: if you  disagree, you are not real. Most physicists,  

to be fair, just ignore the problem. Shut  up and calculate is still very popular.

But it isn’t that easy. Because in the past years  we have seen a number of papers, theorems even,  

that say that in quantum mechanics there  can be instances in which different  

observers come to different, inconsistent  conclusions about what “really” happened.

This is extremely confusing because  it’s a self-contradictory statement.  

Quantum mechanics is supposed to describe  reality and yet it seems that if we use  

quantum mechanics there is no reality that will  fit the bill. So what are we even talking about?

If this hurts your brain, it should.  The underlying conundrum is that we  

don’t understand what a measurement  is in quantum mechanics. You see,  

a detector is also made of particles and it should  also behave by the rules of quantum mechanics.  

Yet if we do it this way, we arrive  at the issue that reality isn’t real.

The authors of the new paper now say  they’ve figured it out. They work with  

an approach called quantum Darwinism. It’s  named after Charles Darwin because the idea  

is that a quantum system reproduces  its information in the environment,  

and the measurement result is what is best at  reproducing, so that we can then read it out.  

The idea is 20 years old or so and goes back to  Wojciech Zurek, but so far it’s been rather vague.

I’ve never been a fan of this idea  but let me postpone my misgivings  

for a moment. Don’t worry, I am German,  I am extremely capable of postponing joy.

One has to give credit to the authors that  they have really made a lot out of this. In  

the new paper they say they have found a “precise  characterization of the onset of classicality”,  

that is, they have been able to quantify  when quantum effects go away. And they  

say they have shown that this happens gradually.

So if observers make only a few  measurements of a quantum object,  

or the measurements are not particularly good,  they might not agree. But the more they measure,  

the more they will agree. And this  is why we all share the same reality.

The neat thing about the paper is that  it acknowledges that yes, those worries  

that observers can disagree on reality are  correct, but this is only for small systems  

or imprecise measurements. The more precise the  measurements, the more observers will agree.

This is another win for Quantum  Darwinism, after they just showed a  

few months ago that they had experimentally  confirmed some of the key predictions.

Ok, now to my misgivings. This approach depends  on how you define the “environment”. This means  

you must know what is the system you are  trying to measure and what is the thing  

that you are measuring with. In quantum  mechanics, there is no such distinction,  

you have to put this in by hand. Which means  you have basically postulated the problem away.

This is why I give this paper a 3 out of 10 on the  bullshit meter. I think it’s mathematically fine,  

but they didn’t really understand the  problem they are trying to solve. But  

it teaches us an interesting lesson.  If reality is what survives repeated  

copying into the environment, then  Twitter is the most real thing ever.

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