Shut up and calculate!

[gusl]

Shut up and calculate!

Why do physicists care about interpretations of QM? Do differerent interpretations make different predictions? If so, shouldn't they then be called theories instead?

Comments

Is this a trick question?

[pbrane.livejournal.com]

Why do physicists care about interpretations of QM?

We don't.

:)

Re: Is this a trick question?

[gustavolacerda.livejournal.com]

oh, who does then?

Re: Is this a trick question?

[pbrane.livejournal.com]

Philosophers, mostly. And crackpots.

[spoonless.livejournal.com]

I care very much, although since I'm interested in both philosophy and physics, you could say that it's the philosopher in me who's interested.

I would have to disagree with pbrane here. Although many physicists do not care, there are plenty who do. But arguing about it tends to take place mostly outside of the formal paper-publishing arena so you could argue that it shouldn't be called "physics". If you ask me, the physicists who do care (such as myself) do so for good reason.

It's about having the simplest possible theory to describe the world. When you use an awkward theory with extra unjustified assumptions in there (such as a collapse postulate) it can mislead one into extending the theory in the wrong ways when the next layer of experimental results is uncovered.

I've been thinking of writing a paper proposing something I call the "Quantum Doomsday Device", it would be an objective experimental test for the many-worlds-interpretation. However, if it fails the world would be destroyed so it's unlikely to ever get funding :) There are other experimental tests for MWI which have been proposed, although they are currently at the level of thought experiments and we don't have anywhere near the technology to perform those experiments.

[spritedreams.livejournal.com]

it's the philosophers who care, not the physicists ...

[darius.livejournal.com]

Yeah, as I understand it David Deutsch discovered quantum computing because he cared about interpretation.

Shut Up and Prove!

[r6.livejournal.com]

Shut up and prove!

Why do mathematicians care about interpretations of mathematics? Do different interpretations make different theorems?

[spoonless.livejournal.com]

Although many physicists do not care, there are plenty who do. But arguing about it tends to take place mostly outside of the formal paper-publishing arena so you could argue that it shouldn't be called "physics".

And actually, I should point out that there have been plenty of physics papers published on interpretting quantum mechanics. It's just that the meat of the debate goes on behind the scenes. Different physicists think different ways so they have their own pet ways of interpretting it and their own preferences and ideas about how things should be done. Only the boldest publish on it though.

Re: Shut Up and Prove!

[gustavolacerda.livejournal.com]

ha. What's an interpretation of mathematics?

Under one meaning, I can see that different interpretations can lead to different understanding. Understanding is, in my view, very different from proof. In fact, one could say that I gave up being a mathematician because of its emphasis on proof rather than understanding.

Re: Shut Up and Prove!

[r6.livejournal.com]

Usually the debate is between Platonism (http://en.wikipedia.org/wiki/Philosophy_of_mathematics#Mathematical_realism.2C_or_Platonism) and Formalism (http://en.wikipedia.org/wiki/Philosophy_of_mathematics#Formalism).

Re: Shut Up and Prove!

[gustavolacerda.livejournal.com]

Well, mathematicians are said behave like Platonists. But in this case, the two interpretations are certainly empirically equivalent. In the case of QM, I'm not so sure.

I suspect that Platonist mathematicians don't mind incomputable things nearly as much as people like you or me.

[gustavolacerda.livejournal.com]

Let me get back to the original questions:

Do differerent interpretations make different predictions? If so, shouldn't they then be called theories instead?

[spoonless.livejournal.com]

It's hard to say what counts as a prediction. Some interpretations do not differ from others enough to make different predictions, but if we discover any small modification to them in the future they'd likely have radically different consequences so it's good to go with the one which is most likely to be true given what we know now. Others, such as many-worlds, do have predictions for experiments which might be able to be done, depending on what counts as a prediction and depending on the practical limitations of how much strong AI we can build. An irrefutable experiment that would test many-worlds is to build strong AI which uses a quantum computer for a brain. But we don't know yet whether we can build such a thing. In addition there is the "quantum suicide" experiment which any observer can perform for herself, and if MWI is true, she will know with certainty that it is. But if it is not true, she will be killed and won't find out anything. I'm extending this to a "Quantum doomsday device" which can do the same thing but for the entire world, so that we can all share in the objective certainty that many-worlds is true. However, it has the same consequence that if we're wrong about it the whole world would be destroyed. So you may or may not see that as a prediction since it doesn't make it "falsifiable".

[gustavolacerda.livejournal.com]

An irrefutable experiment that would test many-worlds is to build strong AI which uses a quantum computer for a brain.

why? it seems to me that quantum computers would be no different for all practical purposes.

[gustavolacerda.livejournal.com]

An irrefutable experiment that would test many-worlds is to build strong AI which uses a quantum computer for a brain.

why? it seems to me that quantum computers would be no different for all practical purposes.

[spoonless.livejournal.com]

And incidentally, I do plan on performing the quantum suicide experiment on myself before I die, as soon as I get old enough that not too many people will miss me when I'm gone (since it would involve my death in most of the quantum multiverse). I'm literally "dying to find out" which interpretation is correct. :)

Also, one more way in which MWI might be testable is through the string theory Landscape. Leonard Susskind, and others, has suggested that MWI might be connected with the inflationary multiverse (which is something I'd been personally speculating on ever since I heard about the latter) which means that it might be tied into certain anthropic predictions of various quantities like the cosmological constant. But this connection is far from being explored yet, and nobody knows how to think about it.

[spoonless.livejournal.com]

It would prove that observers can split along with everything else. The problem with the people who don't believe in many-worlds is that they refuse to accept that human consciousness is no different than any other type of matter. So strong AI would help them get over their superstitions and accept MWI.

[spoonless.livejournal.com]

One more comment: while many physicists will no doubt disagree with me, I don't think I would call the "shut up and calculate" version of quantum mechanics a physical theory at all since it doesn't provide a model or description of the external world. MWI is a fully fledged theory whereas shut-up-and-calculate is, in some sense, only a phenomenology (a bunch of equations that gets the right answer for particular experiments which have been done so far but makes no attempt at explaining how or why it comes out right).

[gustavolacerda.livejournal.com]

isn't regular death already equivalent to a quantum suicide?

[spoonless.livejournal.com]

I seriously doubt it. People have tried to make hand-wavy arguments to that effect, but personally I find them completely silly. The quantum suicide experiment precisely sets up two possibilities very carefully and allows them to decohere from each other into two separate branches. After repeating this a number of times, you can then get a probability 2^-n arbitrarily small but still there for living.

Ordinary death doesn't involve such a clean split between different possibilities. It involves someone dying through some classical means, which usually takes a while and involves them being aware of it as it's happening. In order to argue for something like Quantum Immortality you would have to show that given any death there is some branch of the multiverse where the person remained alive... since this is a completely arbitrary assertion not backed up by any real physics, I think the burden of proof is on someone who claims such a thing. My impression is that the QI argument arose from people who don't understand MWI, people who think it means "anything you can imagine happens... and since I can imagine remaining alive, therefore it must happen". That's not at all how MWI works.

That said, I won't rule out the possibility that there are some instances where a person could die in one branch and remain alive in another branch and it might work like the quantum suicide experiment, but the claim that that's always how it works is far fetched and silly in my opinion.

[spinemasher.livejournal.com]

I would state it as this, every good physicist does not care about different interpretations up until they realize they need to care about different interpretations. This is because every good physicist knows that just how you think about something determines what things you look for. One of the greatest challenges in science is learning to ask the right questions.

For example, we do not know whether quantum mechanics is just classical mechanics in a limiting version or if they are instead two distinctly unique versions stemming from some deeper underlying theory as yet not described (despite my word choice in language this is not to be confused with so-called hidden variable theories; I am instead talking about the fundamental categorizations and attributes of dynamical systems). The latest theoretical investigations are more and more suggesting that classical mechanics and quantum mechanics coexist and thus are not just related by a limit. This implies we ought to be looking for something we never looked for previously.

Now my example above is not about the current most popular different interpretations however; it could easily lead to an interpretation not yet described, which has far reaching consequences. But the point being that we would have arrived at these far reaching consequences of interpretation not by asking which interpretation was right but by instead asking what is the theory that best described nature.

So sure, shut up and calculate, but make sure you are calculating the right things and make sure you stop to ensure the things you calculate actually describe nature from all angles, not just one.