Understanding this quote by Feynman

Question

This might be more of a question on semantics and interpretation and if this doesn't meet community guidelines, feel free to let me know and I'll delete it.

It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong.

Feynman said that once.
I do agree with this to a certain extent. But consider this - the wave theory was unable to explain things like the Photoelectric Effect and Compton Scattering, whereas the particle theory (by which I mean the existence of photons) couldn't explain interference and diffraction.
Well, aren't both the theories wrong then?
Ideally, there should be one theory that explains all of this. Then why do we take both of these to be true?

I'd simply leave this as a glib remark but what Feynman is echoing is some form of Popperian view of science which nobody (not even Feynman) strictly believes in. :) There are various critiques (somewhat along the lines of what you touch on) of the Popperian view by Kuhn, see, e.g., his The Structure of the Scientific Revolution. — , Mar 07 '22 at 17:30
Until we understand physically what a "wave-particle" actually looks like and how it interacts with its environment you will have two or more theories trying to describe the same phenomena in a complementary fashion. Apart of pilot wave deterministic theory there is no other known theory currently that seems to do that. — Markoul11, Mar 07 '22 at 17:35
Because we don't want to confuse our brains with the facts. Clearly, your intuition is correct. Both theories are wrong. Unfortunately, (or fortunately!), they are both incredibly accurate predictors of actual behavior. The message that many take from this is that there must be some underlying deeper reality or truth that the properties and predictive algorithms of both theories are consequences of, like biology is a consequence of Physics, or electronics is a consequence of Quantum Mechanics. But noone knows for sure what that deeper theory is as yet. — Charles Bretana, Mar 07 '22 at 17:52
My answer here is relevant https://physics.stackexchange.com/questions/687930/how-exactly-do-physicists-reject-and-accept-theories-models-in-physics/687943#687943 and this https://physics.stackexchange.com/questions/71150/what-evidence-refutes-a-pre-eternally-existent-cosmic-egg/71156#71156 — anna v, Mar 07 '22 at 18:29
@DvijD.C. As best I recall, Kuhn's response to Popper is wholly unrelated to the OP's concerns. (In any case, Popper's actual views have been widely misconstrued ever since they were first published; he was never a naïve falsificationist.) — J.G., Mar 07 '22 at 18:40
@J.G. I thought Kuhn's response relates to OP's concern in that Kuhn rejected the idea that there is any definitive experiment that can wholly rule out a framework -- at least not in a clean way in which a simplistic Popperian view might suggest. I might be wrong, I read Kuhn a long time ago at this point — , Mar 07 '22 at 18:54
Note that he doesn't say "if it doesn't agree with ALL experiments, it's wrong". A theory may have a limited domain of applicability, such as the examples you gave. So this brief statement captures a truth, but its domain of applicability has to be carefully established. — tobi_s, Mar 08 '22 at 04:14
My take (be it this Feynman quote or similar statements from Asimov, Neil dG-T, Sagan, et. al., is simpler: wackos' and religious fundies' "theories" are completely disconnected from reality. — Carl Witthoft, Mar 08 '22 at 14:09
Another relevant quote might be All models are wrong, but some are useful — johnDanger, Mar 08 '22 at 21:58
There are no perfect hammers. But there are people who use them on screws. There's wrong and then there's wrong. — candied_orange, Mar 08 '22 at 22:02
I think it helps to put this into historical context: many of the ancient Greek 'scientists' believed that you could deduce the nature of reality through logic which led to logical (but wrong) ideas such as that we see by shooting particles (rays?) out of our eyes. Science has created so much knowledge because it, fundamentally, starts with the assumption that what we observe is the true nature of reality and the logic we use to explain it is only correct if it aligns with observation. — JimmyJames, Mar 09 '22 at 15:50
Ahh, they are indeed both wrong when you assess by set criteria; but when you counteract the wrongness of one with the "not-wrong"ness of the other, then you end up with some tangled mess of wrongness that's just about "not-wrong" enough that it just might be right. — Andrew Corrigan, Mar 10 '22 at 14:17
You're right, both theories are wrong. Light is not a wave and not a "particle" (in the classical, billiard ball sense), but something else entirely. We do have a theory that explains all our observations of light, and it does not say "light is both a classical particle and a wave". — ZachMcDargh, Mar 10 '22 at 18:12

score 21 · Accepted Answer · answered Mar 07 '22 at 16:56

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[...] the wave theory was unable to explain things like the Photoelectric Effect and Compton Scattering [...]

This is true. As a result, we know that classical electrodynamics is only an approximation, which fails when quantum effects become important. In many, many areas of practical interest, classical electromagnetism is phenomenally accurate, so as long as we stick to those areas it is very useful.

[...] whereas the particle theory(by which I mean the existence of photons) couldn't explain interference and diffraction.

This is not accurate. Photons are not classical particles, so they do not obey our naive intuition about how particles behave. They are part of quantum electrodynamics, and in that framework they both interfere and diffract.

To the best of our knowledge, every experiment we've ever performed is consistent with quantum electrodynamics, so if you're looking for a theory of electromagnetism which explains all of our current observations, that would be the place to look. However, QED is phenomenally complicated compared to classical electrodynamics, so if possible we use the latter (perhaps adding some quantum "corrections" by hand if needed).

Well, aren't both the theories wrong then?

This is certainly possible. We know classical elecromagnetism is "wrong" insofar as it is known to be inconsistent with experiment. The same is not true of QED, but it's entirely possible that there are experiments we have not yet devised which will expose some fundamental flaw in QED as well. If and when such experiments are performed, we will have to re-evaluate our understanding of electromagnetism once again - an exciting prospect!

answered Mar 07 '22 at 16:56

J. Murray

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"To the best of our knowledge, every experiment we've ever performed is consistent with quantum electrodynamics". General relativity, and in fact gravity in general, is not consistent with QED – thegreatemu Mar 08 '22 at 22:42
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@thegreatemu To which experimental results are you referring? – J. Murray Mar 08 '22 at 23:06
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@thegreatemu at the risk of getting into even more naive philosophy, doesn't that mean that QED says nothing about gravity, rather than it saying something wrong about gravity? – llama Mar 09 '22 at 00:54
@J.Murray QED explains the interactions between particles. You can extend that to macroscopic objects via statistical mechanics. Strictly QED has no gravity interaction, but you can somewhat naturally extend it by including a graviton. But even that would not be consistent with truly GR effects like the precession of Mercury or time dilation in a gravity well. For that you need a Grand Unified Theory of quantum gravity, which we do not yet know. – thegreatemu Mar 09 '22 at 18:00
Just want to point out that I'm not disparaging this great answer, just nitpicking a single statement =) – thegreatemu Mar 09 '22 at 18:01
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@thegreatemu No worries, I don't take your comment as disparaging at all. Are you familiar with QFT in curved spacetime? There's no problem modeling the effects you refer to as long as the curved background spacetime is treated classically. More to the point, the theory of GR may be ultimately incompatible with QED, but that's different from an experimental result - indeed, the lack of such results is part of why a quantum theory of gravity is so hard to formulate. – J. Murray Mar 09 '22 at 18:28
(Related)Since the wave theory is a classical approximation of the particle theory, is it not necessary that all classical phenomena are explained by the wave theory for it to be valid?Consider refraction- frequency remains the same but the wavelength changes.One could explain this using the conservation of energy using the particle theory as the energy of the photon is proportional to it's frequency but I didn't find a convincing explanation using the wave theory(energy only depends on amplitude!).Does this mean the photon formulation is more accurate ,or QED is the 'ultimate theory:? – Ambica Govind Mar 14 '23 at 13:45
@AmbicaGovind I would suggest abandoning the terms "wave theory" and "particle theory" when it comes to classical and quantum electrodynamics. They are both theories of fields, but the latter is a quantum mechanical theory which features the existence of photons. In any case, I don't really understand the question - classically, the relationship between frequency and wavelength (the so-called dispersion relation) depends on the properties of the medium, so if you change the medium you change that relationship. The frequency must remain the same at the interface [...] – J. Murray Mar 14 '23 at 14:06
[...] otherwise the field would be discontinuous there, and so it is the wavelength which must change. – J. Murray Mar 14 '23 at 14:07

score 15 · Answer 2 · answered Mar 07 '22 at 16:47

There is no single-sentence statement which can capture the whole of makes a theory good or bad. In its context, this statement is making the point that in the end what science is about is grappling with the natural world as it is, rather than a more abstract task such as creating beautiful structures of ideas.

In the case of diffraction and photo-electric effect, both the Newtonian particle model is wrong, and the classical field theory model is wrong. In this sense it is not a case of "both/and" but "neither". The quantum field theory model combines particle-like and classical-wave-like aspects, but now the words 'particle' and 'wave' are serving as technical terms in physics discussions, whose ultimate meaning is cashed out in terms of a more careful and complete statement of what quantum field theory asserts.

(A related issue is that one should not immediately abandon an established theory on the first hint of some tension with laboratory data, since it could just be that the experimental apparatus went wrong or its precision was overestimated.)

score 7 · Answer 3 · answered Mar 07 '22 at 18:37

Particle-wave duality doesn't simply say the earlier it's-a-particle and it's-a-wave theories are "both right". It combines two ideas:

Measuring an observable ensures the physical state is an eigenstate of an associated operator, even if it wasn't before.
For some observables, the resulting states can be interpreted as particle-like. For some others, the resulting states can be interpreted as wave-like.

As we currently understand it, everything in nature that can in some experiments look like particles can also in some other experiments look like waves, and vice versa. Neither of the earlier theories can be reconciled with this. And such theories were in any case domain-specific - electricity (light) was believed to be particles (waves) - which our current understanding is not.

score 2 · Answer 4 · answered Mar 08 '22 at 18:08

It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong.

The term "theory" here means not just math equations, but implicitly some range of acceptable conditions. Today, nobody will use Newton's laws to describe movement near speed of light.

That doesn't mean Newton's laws are "wrong", it just means that the theory doesn't work very well in some circumstances.

score 1 · Answer 5 · answered Mar 09 '22 at 05:29

Yes, they're both wrong. Taking the 'semantic' approach, he is asserting a hierarchical relationship: first and foremost is 'experiment', second is explanation.

Some assumptions to make this a bit more substantive: I assume he is referring to the data or observations from the experiment, and that he is ignoring the possibility in mistaken theory/description of the experiment itself. (An experiment, minimally, makes assumptions about what is and is not measured, which, if wrong, leads to a mistaken sense of what the data is in the first place. We'll assume those assumptions are theory, too.)

Perhaps especially relevant in your example. Because we create theories, it may be tempting to maintain them, whether because of their apparent cohesion or elegance in illuminating complex phenomena. But the fact that they appear right 99% of the time, or are easier to understand/explain phenomena, are subjective appeals that are irrelevant in the face of the full set of observations. The advice is to not be fooled by the appeal of a theory that does not 'agree with experiment'; there must be something wrong with the theory/theories--whether or not you cannot conceive of how or why.

A note about a theory being only right or wrong: There is another semantic element at play here, which is to treat a theory as a singular, categorical explanation that is right or wrong. However, theories implicate many entities and often make multiple claims about relationships between them.

score 0 · Answer 6 · answered Mar 09 '22 at 18:32

There is a difference between "in a specific limit we need another theory" and "the theory disagrees with the experiment".

Newtonian mechanics works pretty well for all experiments and more which it was created to explain.
We still use wave description of light (and it works pretty well in certain limits)

What he wanted probably to address was the tendency to spare beautiful theories which are not triggered by experimental observation from culling and instead saving them by increasingly complex extensions is not a good trend in physics. Maybe he was a little ahead of the curve here... (one of my favorite books on the topic is "trouble with physics" by Lee Smolin).

Understanding this quote by Feynman

6 Answers6