What is the Meaning of Directly Observable?

Continuing the discussion from The Pilot Wave Interpretation of Quantum Mechanics:

I’d like clarity on what “direct” detection means to a physicist.

Let’s take black holes as an example. We have collected the data to image a black hole “directly”, but have to now run a massive algorithm to reconstruct it. What are we going to see? Not the black hole, but the gas swirling around it. It it will be a negative image. Is this a direct image? Perhaps more direct than in the past, but “direct” appears to be very subjective here.

Let’s take LIGO and gravity waves as another example. We are measuring vibrations in space between orthogonally situated mirror interferometers. Once again a lot of work goes in to processing, refining, and cleaning up this data to make a “detection”. How “direct” is this really?

Let’s take Bell’s inequality violations. These are discernible only in the statistical distributions of the outcomes of a large number of experiments. Any a single result tells us literally nothing. We need hundreds or thousands of results, aggregated together, to infer a distribution. The theory predicts the distribution. So is this really a direct observation?

There may be some rules here. I am not a physicist, but it seems that our notions of “direct” and “indirect” are very difficult to pin down in a reliable sort of way. Maybe you could help @dga471, @pevaquark, and @PdotdQ?

A direct detection means that what you are trying to observe only interacts with your detectors. Note that this is a definition from a theoretical astrophysicist, which is a double whammy: theoretical means I don’t actually take data; astrophysicist means I don’t do experiments (all our detections are observations not experiments).

Scientifically, there is never the claim that we have detected black holes directly - those are just buzzwords for the press. The claim is a detection of the photon ring that surrounds the black hole. I should know, I am part of the EHT.

The direct part just refers to the fact that the gravitational waves interact directly with your interferometers. The cleaning algorithm etc is not relevant in this definition.

Of course the definition of direct detections are somewhat arbitrary, however these seem to be standard at least in astronomy.

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I believe you, and that is what is contributing to my confusion here.

Okay, but where does the “detector” begin or end? Isn’t that somewhat subjective and a matter of perspective?

It is a matter of perspective but not subjective. Philosophically, direct detection in astronomy takes a very human-centrist perspective. Detectors are always human-made.

For example:

  1. Gravitational waves made some changes in the orbits of a couple of stars. Humans made telescopes to see these stars and notice the orbital changes. This is not a direct detection of gravitational waves, because these stars (which are not human-made) are not part of the detector.
  2. Gravitational waves made some laser wobble. This is a direct detection, as the laser and its detector are human-made.
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Edge case determination: Does Super-Kamiokande directly or indirectly detect neutrinos? How about IceCube?

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Every scientist would say that Super-Kamiokande and IceCube give direct detections.

In the case of Super-Kamiokande it is pretty clear: everything is human-made. But I agree that the case of IceCube is confusing. I would argue that in the case of IceCube humans turn ice (raw materials) into detectors by drilling into it and fixing sensors on them.

This is somewhat different than the case of using the decay in the orbits of stars to detect gravitational waves, as humans did nothing to the stars but observe them from afar.

By the way, I used to be on the other side of the fence, and I used to joke with my observer buddies that gravitational waves have been directly detected in the 70s. They managed to convince me that the mainstream (for astronomers) human-centrist definition of direct detection makes sense - especially since any other definitions seem to render the adjective “direct” in direct detection useless.


However you didn’t present a “human centric” definition. You presented a “detector centric” definition. Detector is so poorly defined as to be whatever we want it to be, right? We call neutrino flashes in Antarctic ice “direct” by just saying that the continental ice sheet is part of our detector. We use the sun’s gravity to “directly” test for gravitational lensing, so is the sun part of the detector too?

There is some philosophy of science here. Polayni talks about how the scalpel becomes an extension of the surgeons hands, so it is the surgeon. My pathologist colleagues have their eyes extended by microscopes. As a computational biologist, my perception is extended with machine learning algorithms. None of this is ordinary or direct perception. As soon as I exit my body, the line between detector and nature does not seem to be a clear boundary.

I’m just not sure I understand how to clearly adjudicate what is direct and indirect as you’ve laid out. Perhaps there are some disciplinary rules. Sure. I am okay with this, but how could it be much more than convention?

The human-centric definition is in the definition of the detector. I would agree that IceCube is a bit of an edge case, in which it depends on what one means by building a detector. Gravitational lensing from the Sun is a different kind of detection in which one is detecting a phenomena not an object. I think here the correct word is “observing”, in the sense that we observed gravitational lensing by the Sun.

It is a matter of convention. The convention is that if the object you are trying to detect only interacts with your (human-made) detector, it is considered a direct detection. Once the convention is chosen, it is not arbitrary what is considered a direct detection or not.

There might be edge cases that are not clear cut, perhaps there might even be a gradation of how direct a detection could be. Nevertheless, it is not true that almost nothing in physics now days is directly observable.

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All fine, as long as we remember that “directly observable” in this sense is a term of art, and does not correspond to what the person on the street would mean by it.

(Note: I considered adding a ground-based cosmic ray air shower detector as a third edge case. I’m pretty sure you would consider that indirect detection.)

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@PdotdQ has spoken about the convention in astronomy. In atomic and particle physics, it seems that many observations are indirect, in the sense that there’s a long chain of logical inferences you have to make before you actually read a number that is your data. As technology progresses, this chain becomes longer and longer. The boundary between “human” vs. “nature” is more blurred. (I think @jongarvey once reminded me that in Galileo’s time, looking through a telescope was considered indirect and untrustworthy because people could be deceived by optical illusions.)

Some things are simply not directly detectable by their very nature, yet people believe that they exist anyway. For example, the people figured out the existence of the Higgs boson was by discovering several decay paths of the Higgs into other particles, some of which in turn decayed into photons, electrons, or muons (particles which can be detected very “directly”), which led them to believe that there was a new boson with a certain mass. Was this “direct” detection? Clearly, concluding that the Higgs boson exists required a complex previously agreed upon theoretical framework (the Standard Model) from which one could begin to analyze and interpret the data. The amazing experimental successes of the SM have mostly been in this kind of detection. This is why I think “direct” vs. “indirect” detection are not very useful criteria in atomic or particle physics to judge the strength of evidence for an experimental claim or theoretical prediction.


That is my sense too. The original context was discussing the “direct” detection of a pilot wave. I was unclear that adjective had any meaning in that context.

Which led us down this rabbit trail.

I suppose the lesson is that different subfields have different meanings of “direct.” In the context of an individual field, it might be clear, but it is going to be hard to precisely and unambiguously define “direct” observation versus “indirect” observation in a global sense. I’m not sure, in the end, that really has much to do with the strength of the theory.

Yeah, going back to the original context, it makes more sense to think about what PW theory predicts differently compared to Copenhagen and test that experimentally, regardless whether it’s a direct or indirect test.

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Yeah, I can agree with this conclusion.


What is observable in biological bodyplans has never been observed. Only results , after mechanism, have been observed. YET evolutiondom uses these aftereffects to say they proved evolution. A complaint of mine on intellectual status of evolutionary thinking.
Common descent is case in point. They see common genetics/anatomy etc and go AHA thus common descent.
That is to say a line of reasoning ALONE is proof for evolution.
Creationists have to struggle to show that ANOTHER line of reasoning would not just be a option but nullififies the exclusive option of common descent which makes disappear its evidence.!!
Common descent PROOF is bot based on observation but on exclusive channel reasoning.
Evolution is not only not directly observable but not observable like these examples from physics.

That is false @Robert_Byers. There are several lines of reasoning. Catch up?

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I think i’m rioht here. i’m saying, carefully, that common descent conclusions are based on a conclusion that likeness can only come from a original stamp. A mere line of reasoning. This is the big point. Regardless of other examples for common descent.
Its based on observationism and comparisonism. Then allowing only one option for how that results could be.
its not real science. Raw data and a logical conclusion with exclusivity for options.

I once heard someone say that even our data is based on theories, and there is a lot of truth in that. The direct measurements at LIGO are based on the theory of how light interacts and how interferometry works. We could even get really specific and point to the quantum and classical theories involved in the rest of the equipment, from the microprocessors to light detectors. I’m definitely not saying that we should ignore data because it is derived from theory, but we should keep in mind that science is often theories all the way down.

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Are you talking about the Duhem-Quine thesis?

I think the defense against this is that while we cannot test the theory in isolation, in practice what is done is that the assumptions are already previously tested enough that we have confidence in them.

For example:
We cannot test that gravitational waves exists with LIGO without also testing whether our theory of light interaction is correct. However, the many previous experiments with lasers give us confidence that our theory of light interaction is correct.


Completely agree. Scientists are also careful to include controls that should detect known sources of noise or error.