ask a physicist

[fuindordm]

Okay, so, let's go with a hard one...

What do you think of pilot wave theory?

For those who haven't heard of it - "plot wave theory" is an old interpretation and formulation of quantum mechanics. Instead of having wave-particle duality, you have a real physical particle, and a real physical wave. The two are connected, so that the movement of the wave impacts the particle, and vice-versa. The ultimately important bit is that this interpretation tosses out quantum indeterminacy - the universe is deterministic in pilot wave theory.

Doesn't that just push the problems of QM down a level? Instead of matter particles having a wave nature, this postulates a new field in nature that can supply the waves, and then you have to think about what this field is made of and how it interacts with different kinds of matter particles...and in the end you don't do any better than standard QM because the predictions are still probabilistic and you still can't explain how the wave collapses to an eigenstate at the point of measurement.

It's kind of like dark matter: as long as you don't understand something, you might as well make the theory as simple as possible with one unknown rather than a hodgepodge of alternative explanations for the different types of observations that call for dark matter. QM is not simple, but the pilot wave seems like a big complication with no real payoff--the particles might have a deterministic veneer but the nature of reality described by the theory is no more deterministic than before.

But maybe I'm not remembering enough details, and the theory is better than I think. Did any of its proponents have a good candidate for the pilot wave?

 

[garnuk]

I just saw the movie Ant-man, and I have to ask. Does the idea of a "molecule" which "reduces the amount of space between atoms", sound like nonsense to a physicist as much as it did to me?

 

[Umbran]

Doesn't that just push the problems of QM down a level?

This is a Q&A thread, not a discussion thread. So, while I have thoughts on the matter, I'll leave it to the OP if he chooses to address it.

 

[Umbran]

I just saw the movie Ant-man, and I have to ask. Does the idea of a "molecule" which "reduces the amount of space between atoms", sound like nonsense to a physicist as much as it did to me?

It is a superhero movie. Of course the physics us hokey as anything.

I just saw the film last night - I recall them repeatedly referring to it as a "Pym particle" (what they call in the comics). I don't recall them calling it a molecule, but I may have missed it.

That said... I can imagine an as-yet undiscovered fundamental particle that alters the electromagnetic force could change bond distances (and thus shrink the space between atoms), or alters spacetime (say, through gravitation) having such an effect. The latter allows for funny gravity/quantum effects down when you reach the Planck scale for distances to get us to the Microverse! Woot! Micronauts!

 

[was]

This may be a bit silly, but is it theoretically possible to harden glass to the point that it could physically break through a brick wall? a.k.a. the Kool-Aid Man theory...

 

[freyar]

Sorry for a short delay, everyone --- had a couple of busy days.

Okay, so, let's go with a hard one...

What do you think of pilot wave theory?

For those who haven't heard of it - "plot wave theory" is an old interpretation and formulation of quantum mechanics. Instead of having wave-particle duality, you have a real physical particle, and a real physical wave. The two are connected, so that the movement of the wave impacts the particle, and vice-versa. The ultimately important bit is that this interpretation tosses out quantum indeterminacy - the universe is deterministic in pilot wave theory.

It should tell you something about my answer that I had to look things up to make sure I wasn't forgetting about a new variant or something. Short answer: not a big fan. I don't like the loss of locality vs other interpretations (more below), and it's also pretty ugly.

Longer answer with explanations: quantum physics is very weird compared to our intuition from daily life or even a very rigorous understanding of Newtonian mechanics. Many brilliant physicists of a century ago (including Einstein) had real problems coping with it. A famous example of that weirdness is the Schrodinger's cat thought experiment, in which a poor feline's fate is tied to a quantum event like a nuclear decay and ends up 50% dead/50% alive until someone looks at it. In a standard view of quantum physics, that cat is neither and both dead/alive until you look at it --- we say that standard quantum physics lacks "reality" in that . In classical physics or a pilot wave version of quantum mechanics, that cat is either dead or alive, but you just don't know which. The thing about the pilot wave version is that, just like normal quantum mechanics, if you perform the experiment 100 times, about 50 of the cats will come out dead and 50 alive. But don't do this experiment, because then you should be arrested for cruelty to animals (quite justly IMO).

Anyway, the pilot wave theory is a way to explain the probabilistic nature of quantum mechanics as a measure of our ignorance rather than as a fundamental thing. It seems to have the benefit that it allows a derivation of the Born Rule (named for Max Born) that tells us how to assign probabilities based on a wavefunction. However, it has a couple of big flaws. One is that it does require quite a bit of extra machinery to it that seems unnecessary and is pretty ugly. What's probably worse in most physicists' minds is that it is written in terms of mathematics that violates locality (there looks like there is instantaneous communication in the math), and it's pretty tricky to avoid that instantaneous communication. I'd also say that it looks like it would be difficult to describe a quantum field theory with pilot waves (though apparently it has been done).

Of course, this raises the question of the "right" way to think about quantum mechanics. There are quite a few ideas. The standard presentation of quantum mechanics is the "Copenhagen Interpretation" since Niels Bohr, one of the earliest quantum physicists and promoter of this interpretation (and his collaborators) worked in Copenhagen. This interpretation says that the cat turns alive or dead when someone "measures" it. That leads to a lot of questions about what counts as a measurement and requires breaking the world into a "quantum part" and a "classical part." This is of course silly, since all physics is supposed to be quantum. But this is still a popular view, since it was how the mathematics were developed, and most physicists learn to "shut up and calculate" (yes, that's a quote often attributed to Feynman but probably is due to David Mermin instead).

I personally prefer something like the "many worlds approach," in which the whole universe is quantum and branches into many possibilities any time an interaction happens between two different systems. Apparently the Born rule can be derived in this approach, as well, though that's a recent result that may not have been scrutinized a lot yet. I'm also intrigued by the "consistent histories" approach, which basically says that interactions with the (quantum) environment is important for the evolution of a quantum system (like Schrodinger's cat). But I can't explain that one in too much detail because the explanations I've seen are philosophically very heavy and honestly hard for me to decipher. I do have a sense that it's somewhat related to the "many worlds" approach.

Anyway, as fuindordm and Umbran have indicated, there's a lot to say on this subject, and I think a thread just devoted to this topic might be really interesting if someone cared enough to start one.

Incidentally, as of the last informal poll taken, a plurality of physicists prefers the Copenhagen Interpretation.

 

[fuindordm]

Incidentally, as of the last informal poll taken, a plurality of physicists prefers the Copenhagen Interpretation.

Interestingly, though, Bohr himself did not favor this interpretation. He considered the wavefunction to describe an inseparable relationship between the system being measured and the experimental apparatus. He never took steps to try to formalize that idea with mathematics, but he expressed this view several times in letters as a way of interpreting the wave function without insisting that it has to "collapse" non-locally; in effect, when you make a measurement the relationship between the observer and the observed changes.

 

[Umbran]

However, it has a couple of big flaws. One is that it does require quite a bit of extra machinery to it that seems unnecessary and is pretty ugly. What's probably worse in most physicists' minds is that it is written in terms of mathematics that violates locality (there looks like there is instantaneous communication in the math), and it's pretty tricky to avoid that instantaneous communication. I'd also say that it looks like it would be difficult to describe a quantum field theory with pilot waves (though apparently it has been done).

As for locality - a number of folks have issues with standard quantum entanglement for similar reasons.

And, for the reader, I'd like to comment on "ugly". I am guessing freyar is using it in the way I would - and to a physicist the word has two related meanings.

"Ugly": the math is inelegant. There has been a striking tendency for math that turns out to be a good model to have a kind of simplicity of expression that is aesthetically pleasing to a mathematician.

"Ugly": The math is difficult to actually do anything with. The model may be simply expressed, but as soon as you actually try to calculate things, it blows up in complexity to the point that you snap your pencil in frustration. Doing the orbital mechanics of the solar system in spherical coordinates centered on the Sun isn't child's play, but doing the same in geocentric rectangular coordinates is pretty darned ugly.

Pilot wave QM is certainly ugly in the second sense. Using it to calculate even fairly simple things is a pain in the chalk.

 

[freyar]

As for locality - a number of folks have issues with standard quantum entanglement for similar reasons.

Well, there is a difference in "nonlocality" in standard quantum mechanics (either Copenhagen or many-worlds or whatever) vs pilot waves. In standard quantum mechanics, the wavefunction covers the whole universe, so there's nonlocality in that sense. But there are absolutely no nonlocal interactions (in the Hamiltonian, for the technically-minded). Nothing happens faster than the speed of light. The reason weird things can happen is due to the fact that quantum properties of an object just don't have any operational meaning or reality unless there's an interaction between that object and something else. From a common-sense point of view, that's much more profoundly disturbing than nonlocality (after all, we'd all love a FTL space ship!), but it causes fewer problems with physics.

The nonlocality in pilot waves is nastier in the sense that there is an instantaneous interaction between objects at a distance from each other explicitly in the equations describing the theory (ie, in the so-called guiding equation which supplements the Schrodinger equation, if I understand it correctly). So, even though in the end the results have to be the same as standard quantum mechanics, you have to work hard to make sure that the nonlocalities don't show up. And it would be easy for something to break, leaving you with time travel paradoxes and such.

And, yes, the math for pilot waves looks ugly in both senses you (Umbran) describe.

 

[freyar]

I just saw the movie Ant-man, and I have to ask. Does the idea of a "molecule" which "reduces the amount of space between atoms", sound like nonsense to a physicist as much as it did to me?

It is a superhero movie. Of course the physics us hokey as anything.

I just saw the film last night - I recall them repeatedly referring to it as a "Pym particle" (what they call in the comics). I don't recall them calling it a molecule, but I may have missed it.

That said... I can imagine an as-yet undiscovered fundamental particle that alters the electromagnetic force could change bond distances (and thus shrink the space between atoms), or alters spacetime (say, through gravitation) having such an effect. The latter allows for funny gravity/quantum effects down when you reach the Planck scale for distances to get us to the Microverse! Woot! Micronauts!

Haven't seen the movie, but that does sound pretty hokey! But, just for fun, let's look at Umbran's suggestion that there's some kind of undiscovered particle that can change electromagnetism (and shrink atoms/molecules). Actually, if it were a particle, you'd probably have to stick one inside every atom, which wouldn't be convenient. Fortunately, every fundamental particle really comes from a field (like electromagnetic fields go with photon particles), and fields fill space. So we need a field that controls the strength of electromagnetism. This is not so far fetched, actually --- remember that the value of the Higgs field controls the masses of many fundamental particles in the Standard Model of particle physics.

It turns out we have to look beyond the Standard Model for a field that can control the strength of electromagnetism (really, we're talking about changing the value of the electric charge). But then it's not too hard to find something appropriate. Most extra-dimensional models (whether in string theory or not) as well as "supergravity" models include fields called moduli, which can control the strengths of different forces, etc. So you just have to find a way to adjust the value of the appropriate moduli fields for the person you want to shrink (but not anything else, I guess). That, of course, is the hard part; like the Higgs particle, corresponding moduli particles have to be heavy, which means it takes a huge amount of energy to change the value of the field, so you'd probably actually vaporize the person you're trying to shrink. Ooops!

The fact that moduli control the strength of fundamental forces is a very appealing part of extra-dimensional physics/string theory/etc. In normal particle physics, the strength of a force is just a number, and you just have to go measure it. There's no way to figure out why it takes a particular value. On the other hand, if the interaction strength is controlled by a field, you can in principle ask about what kind of physics determines what value that field should take. Of course this is a hard problem, but it means that at least theoretically you can try to predict from first principles the strengths of fundamental forces. It also opens up the possibility that maybe the field value and therefore the strength of the force was different in the very early universe. There have actually been a number of studies that looked at galaxies very far away to determine if the electromagnetic force strength changed even a tiny amount over the age of the universe. I think they're finally settling on the answer "no" (though it's not an easy measurement and I believe there have been claims in the past of "yes"), but, from a particle physics point of view, you'd expect the field value to be very well settled at its present day value long before galaxies could form.