Is Reading a Signal on a PMT "Direct" or "Indirect" Measurement?

That would seem to get to the heart of the topic. We would have to ask what empirical measurement really is. In the lab, we measure light as a change in voltage in a photomultiplier tube (PMT). The change in voltage itself is measured through other means. So is anything verified? Is all empirical measurement subjective?

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Well, a distinction between direct and indirect empirical verification might be in order. What’s directly accessible empirically could be referred to as direct empirical verification. And what’s only indirectly accessible empirically through detection could be referred to as indirect empirical verification. Both would be objective if there are no subjective theories involved. The former would be more empirically informative, and the latter would be more limited in empirical information.

Do you think detecting a photon on a PMT is direct or indirect empirical verification?

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There are tons of theories involved in the detection of a photon by a PMT. First, we have the theory that a photon will produce a change in voltage in the PMT. We then have various theories for amplifying that change and displaying it for the scientist. Outside of very basic observations, the vast majority of empirical measurements nowadays is based on theory.

So how does that affect your position on direct and indirect evidence?

How exactly are the theories involved in measurements? I thought theories were explanations of phenomenon? Is it the equations on which a theory is based that are the being used for obtaining the measurements?

Take the PMT as an example. A photon causing a change in voltage in the PMT is based on theories. The equations are based on theory. Measuring the voltage in the PMT is based on theory. It is theories all the way down.


OK. But are we talking about causes, or measurements?

Both. If I want to measure fluorescence in a sample I am ultimately depending on a measuring device that depends on certain theories about light and electricity being true. I am not directly observing photons, but measuring the theoretical effect they have on my equipment.

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Well, that’s not really answering my question. I thought we were talking about measuring something. That’s different than the explanation of what causes something. So if you want to talk about causes fine. But to make progress in the conversation will require being mindful of distinctions. Otherwise it just makes things confusing.

So I think what we’re trying to discuss is whether or not measurements in science today involve subjective aspects. So I’m just wondering how the explanation of the cause of what’s being measured is all that relevant to what we’re talking about?

I am saying that measurements and explanation for causes are one in the same in many cases. If you put a chunk of metal on an analytical electronic balance you aren’t directly measuring mass. Instead, you are probably measuring the resistance across a circuit. You are using the theory that the mass is causing the change in resistance, and that is your measurement.

Not sure I follow you. Please correct me if I’m wrong, but I would assume first of all that what’s involved in making the balance, among other mechanical considerations, is using different equations from relevant theories to figure out how to put the circuits together in the balance in such a way as to give the correct measurement of whatever is being measured.

But the balance giving the correct measurement doesn’t depend on the theories explanation. It depends on working out the theories equations correctly and putting the circuits together correctly to get the correct results when measuring an object. And assuming the equations have been confirmed to work would make them an objective aspect of the production process.

Sure, knowing about the theories is needed by those working out the equations so they know which equations to use, but that’s the only aspect I can see where the theories would play a role. As long as the equations used are confirmed there’s nothing I can see that would be subjective about the measurement. The explanation of why the balance gives the correct measurement may be subjective, but not the actual measurement. Does that make sense?

Also, I notice the conversation seems to have shifted a little from direct/indirect back again to objective/subjective. They’re both relevant, but just wanted to point that out to avoid confusion.

Sure, but there is a parallel. The indirect/direct dichotomy is as illusory and useless as the objective/subjective distinction.

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Just to put a plug in for philosophy: What you describe is called the “Theory Ladenness of Measurement” by philosophers of science. It challenges, perhaps irrefutably, the idea that scientific quantities can be defined solely operationally.

The topic is part of the more general issue of scientific realism: That is, it it reasonable to believe that the unobservables entities or structures postulated by scientific theories are real? (After all, theories and what they postulate keep changing, or so it seems).


So, then, it seems that even materialism is not truly concrete. We are directed toward the hard sciences to look for the facts, the truth. But, in the end, are things not as knowable as assumed? (Or have I gone too far with this conclusion?)


I am not seeing any inconsistency between theory ladeness of measurement, and the idea that quantities can be defined operationally.

I enjoy some philosophy, especially epistemology, but sometimes it can seem a bit far from what happens in practice. I don’t have much expertise in philosophy but I am an experimental physical chemist who teaches Quantitative and Instrumental Analysis classes. My comments below come from that experience.

I usually define a measurement as a signal that has some (mathematical) dependence on a change in some physical property of the sample. Practically, we don’t need to know why the dependence is there. We can (and students typically do) treat the translation from physical property to signal as a “black box”. We typically try to distinguish between three contributions to the signal:

  • the signal due to the analyte - this should have some functional form (linear is nice but not necessary)
  • signal due to random error - this generally follows some distribution (often Gaussian) and provides some uncertainty to the measurement
  • signal due to systematic error - this could be operator (mis-calibration), instrumental (background noise), or perhaps due to some other variable

So is the signal on a PMT direct or indirect measurement? I’m not sure the question makes a whole lot of sense really. I think it could be like asking is something is hot or cold. A PMT is a fairly direct measurement of photons in that it’s possible to “see” individual photon hits, but it’s also indirect in that the signal is still just a current flow caused by a cascade chain reaction of electron impacts that started with a single photon. I don’t need theory to record the current and look at its dependence on how much light hits the detector, but it’s often helpful when trying to interpret odd results.

Another example from my area is Beer’s Law (the undergrad’s favorite): A=\varepsilon b c . In this case we have a mathematical relationship between absorbance (A) measured and concentration (c) of analyte. Students don’t care why Beer’s Law works and are happy to use it as a “black box” tool. However, if you know the theory of how it’s derived you can be aware of problematic points (i.e. that it becomes nonlinear at high concentrations). I have no real way to determine how direct or indirect this measurement of concentration is.


I agree @jordan.

Concepts like independent corroboration might be valuable here.

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Thanks for those very pertinent links. From what I can gather from this particular link, it would seem like there are measurements with elements of theory involved that have been crossed checked in some way with other types of measurements and confirmed, which seem to me would fall into the category of objective. Whereas when testing to confirm an entire theory there will be subjective aspects to the measurements until they are confirmed by cross checking with other types of measurements. Does that sound like more or less the gist of what’s being said?

I’m not sure the reason for the wording of the title? Maybe @T_aquaticus (sorry I think this was actually @dga471 who titled this) can clarify what he meant by that? It may be I said something to that effect somewhere? Or maybe a mixing up of distinctions? I’m more concerned with direct and indirect in terms of empirical access to a particular entity, e.g., object, event, etc. I’m still kind of unsure about how to approach that particular distinction. But that’s another discussion.

With measurement I’m more concerned with the objectivity of the measurements. I think we’re trying to sort out if theory-laden measurements would be considered objective vs subjective. I’m leaning more towards objective when confirmed by alternative principle of operation measurements, and subjective when not. How do you see it?