What Constitutes Supporting Evidence?

Continuing the discussion from Theory of Everything?:

This is getting a little off topic so I’m creating a new thread for it.

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I think this is a matter of a difference of opinion as to what constitutes supporting evidence. I would go back to how I define it as anything observable that lends support to a claim. I think that’s a pretty legitimate definition. Are there any reasons that it wouldn’t be legitimate?

Please explain to me, then, how in your view the bouncing droplet experiments lend support to pilot wave theory, since that is what we are disagreeing about. I don’t see it.

Simply on the basis that it’s an observation about reality, and therefore PWT suggests an explanation of unobservable reality that is consistent and conforms with observable reality.

So, on the exact same basis, would you claim that vibrations in guitar strings are observable support for the existence of electromagnetic waves?

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I’m not clear on what you’re getting at here? How does observing a string vibrate give a visual correlation to a wave? If you want a visual correlation waves in water would be a much better correlation. I don’t see much, if any, correlation with strings vibrating? Or am I not understanding your point?

Okay, substitute water waves in place of string vibrations. Same question. Do water waves constitute observable support for the notion that there can be a kind of wave that propagates in a vacuum with no medium?

So now you’re adding to the equation. Waves that propagate in a vacuum with no medium is a somewhat different metaphysical claim than simply the existence of electromagnetic waves.

However, if I’m not mistaken, waves in water is where the idea came from that similarly there might be waves in unobservable reality. If so, that would be a case where observable reality inferred a possible explantation of an unobservable reality, wouldn’t it?

The whole idea of electromagnetic waves is that they propagate in a vacuum with no medium. If light had a medium, they would be “luminiferous ether waves” rather than electromagnetic waves.

This isn’t an inference; it’s a creative inspiration entirely independent of whether the thing you’re saying is inferred is actually true.

What I’m trying to get across here is that the bouncing drop experiment at best is a way of visualizing pilot wave theory, but not even a very good one, and it doesn’t provide any rational justification for the truth of PWT over standard QM, nor does it even succeed in showing that PWT is conceptually coherent or workable. (In that regard, guitar strings and water waves are better at “supporting” EM theory than the bouncing drop model is at “supporting” PWT.)

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This is a metaphysical inference. No one really knows that such is the case. There’s not very good evidence to support no medium. I would say there’s stronger evidence to support a medium than no medium.

Not sure what you are saying here. Can you clarify this a bit more please?

That’s how you see it. But to me if there’s a metaphysical claim that corresponds to observable reality, that’s arguably a good reason to prefer it over claims that don’t have any correspondence with observed reality.

Ever observe matter making decisions, or observed an instance of any matter’s existence being dependent on whether or not it had been measured?

You’re really denying that light waves have been propagated in a vacuum?

If not, please explain what exactly you’re denying here.

Okay. I’m going to resist being pulled along a tangent trying to convince you that there really is no medium for light propagation (other than to note that that is crazy talk, unless you are defining away what it is to be a medium until there’s no distinction between a medium and a vacuum - I suppose you could call that a metaphysical debate, if you wish).

As for the main topic of this thread, I have to confess I just don’t understand how you don’t understand the point I’m trying to make. The fact that we can bounce a droplet on the surface of some oil just doesn’t do anything to support PWT over standard QM, because both PWT and standard QM imply that we can bounce droplets (well, both defer to whatever classical physics says in this case) so the observation of bouncing droplets doesn’t discriminate between PWT and standard QM. In terms of what is observable they are entirely on level ground.

The best way I can construe what you are trying to say here is that the bouncing drop model shows that PWT is conceptually coherent, because if you can model something in reality then it obviously can’t be inconsistent or self-contradictory. But even that doesn’t really follow here, because the dynamics of the bouncing drop are entirely different than the dynamics of pilot-wave theory (that’s why I said it isn’t even a very good model).

So the best you can actually say is that the bouncing drop model provides a visualization for PWT, and if we can visualize something then it isn’t immediately or obviously incoherent - but that doesn’t get you very far. The incoherence could be hiding in the limitations of the visualization. But by now, in any case, it doesn’t depend at all on actually observing bouncing droplets, only on being able to imagine or conceptualize them. This isn’t observational support.

This is not to say that don’t think there are good reasons to accept some kind of pilot-wave theory - I think there are. But none of them are empirical, and definitely none of them have anything to do with bouncing droplets.

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OK. Maybe I should clarify by saying there are metaphysical aspects to the question. As far as I know, if there is such a medium, it’s not observable. So there’s really no way to know for sure whether or not it exists. Failed attempts to detect it cannot prove that it doesn’t exist. They can only suggest it. I’ll leave it there so as not to get off on a tangent.

But suffice it to say that just as in any case of abductive reasoning, all the evidence both for and against has to be weighted and then a judgment has to be made as to what best explains the evidence. But I don’t see that it’s possible to objectively established it one way or the other.

Here I think it would be good to clarify what you mean by “standard” QM? I think it’s important to remember that I’m concerned with the explanations, not the descriptions.

And a clarification on what you mean by “dynamics” would be helpful as well.

This also could use some clarification. Are you saying that there can be incoherence in observation? I assume we’re talking about observation in the context of what would be considered accurate observation.

My take on this is if an idea corresponds with any observable reality, then it’s a lot more probable than an idea that has no correspondence with any observable reality whatsoever.

If we abandon the yardstick of reality in passing judgment on ideas, I think we risk loosing touch with reality altogether and absurd ideas like “something from absolutely nothing” start to take hold.

So regardless of the fact that it’s not an actual “quantum” observation, the fact is that there is no such thing as a “quantum” observation. So the next best thing to it is an observation that “corresponds” to how we know the quantum realm “behaves.”

And I think the observation in question fits quite well with that behavior and therefore is observational evidence that supports PWT. I don’t see why that’s so hard to see.

I said I wasn’t going to do this but here I am… last comment in this tangent, seriously this time. :smiley: Failed attempts to detect the luminiferous aether certainly do tell us something, namely, that if such a thing exists, its properties are such as to really strain the meaning of the word “medium”. In some ways it would be more reasonable to identify the aether with empty space, the vacuum, than to maintain the idea of the aether as some kind of material that fills the vacuum. So there’s a question of semantics as well.
(Perhaps one could argue that this identification strains the meaning of “empty space”, and that it’s better to keep the idea of a medium filling space while rejecting that it is material in any way… okay, so it’s both a metaphysical and semantic question.)

Moving on.

By standard QM I mean something like the Copenhagen interpretation.

By that I mean the behavior of the entities described by the theory and the interactions between them. Here’s a couple of very important differences:

  • in the bouncing drop experiment there’s a complicated interplay between the droplet and the oil surface. The droplet partially determines the motion of the surface, and the surface partially determines the motion of the droplet.
  • in PWT on the other hand, the wavefunction evolves entirely independently of the particles whose motion it governs.
  • in the bouncing drop experiment, the drop and the oil surface exist together in 3d space.
  • in PWT, the wavefunction is defined in a many- dimensional abstract configuration space, separated from the particles it governs in 3d space.

No, but there’s no point in clarifying here until you grasp my more basic point:

There’s absolutely no reason for this to be the case. The “yardstick of reality” is that our theory has to be consistent with the evidence, not that unobservable reality must somehow resemble observable reality! Really, we should expect unobservable reality to not resemble observable reality in some important respects.

The other yardstick of reality is that our theory has to be self-consistent and coherent. Observable reality gives us examples of such things, but just because unobservable reality must “resemble” observable reality in that way, doesn’t mean it must resemble it in any other way. This is where I think you are getting confused.

Do you understand why I say that the observational evidence does not discriminate between PWT and standard QM? This has to do with Bayesian reasoning, really.


Water waves through a double slit will interfere constructively or destructively, and provide a model for QM as well. As it turns out, that model is severely limited in its usefullness. If you are interested, this accessible book has a strong experimental focus on the two slit experiment, and the progressive development of it’s ever more sophisticated variations, you may find helpful.
https://www.amazon.com/Through-Two-Doors-Once-Experiment/dp/1101986093Book - Through Two Doors at Once

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Another book that explores quantum foundations for a lay audience: What Is Real?, by Adam Becker. Highly recommend it.

Also would highly recommend Travis Norsen’s Foundations of Quantum Mechanics for a more detailed, but still very accessible, intro to interpretive issues in QM. Undergraduate level.

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My point is, since we don’t know what kind of “substance” it might be, any so called “failed” attempts could possibly be a matter of it being humanly undetectable, or looking for the wrong kind of “substance”. Regardless, what is really meant by a “true vacuum” of space?

To say it’s “nothing” makes no sense. Whatever it is it’s still necessary for matter to exist. So whether or not we know or can know what it is, it still has to be “something”. And if it’s “something”, then it seems to make sense that there would exists absolute location within that “something”.

At least that is the only way I can make sense of it. But I’m sure we could go around it circles on this for eternity. Just seems to be one of those issues that no one can be sure about. :slight_smile:

The one that says the so called “wavefunction” doesn’t know what to be until it gets measured. Right?

Could it be that there’s a blurring of an explanation and a description here? Theories are explanations and mechanisms are descriptions, right?

What “evidence” exactly are you referring to here?

I think we all agree that spacetime coordinates would exist in a vacuum of space. What I haven’t seen is evidence that a set of coordinates defined by a specific inertial frame has primacy over others.

The way I have always thought of it is that the wavefunction is a set of probable outcomes that becomes one outcome as soon as there has to be an outcome. Until an electron interacts with something it doesn’t have to be at any one specific point in space. In my experience, “measured” injects too much anthropocentrism into the science.

Interesting. So wouldn’t it follow that absolute coordinates existing in space would entail absolute space? How can you have absolute coordinates if what they are located in isn’t absolute? Or maybe I’m not following what your point is?

Not exactly… again, if you want to get a good handle on what some of the different interpretations are saying, I really highly recommend this book.

There’s no clear dividing line there. Descriptions of phenomena are highly theory-laden, especially in fundamental physics.

I’m referring to all empirical evidence here. All the data of what we observe in the natural world around us.