Postdiction vs. Prediction

And it is precisely what they did, and called it a prediction…

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That’s nice, but it is clearly prediction to me and the >200 authors of the paper.

Like I said, maybe you should set them right. How would you propose to do that? Why argue with me if you’re not going to argue with them?

So you think that:

Is prediction?

The different Ms, at least one dating from 2013, make different predictions, do they not?

Why are you arguing with me and not with them? According to you, the whole series of papers is nothing but postdictions, correct?

Which different Ms are you talking about? The gas dynamics and stellar dynamics results? Those are different observations, not predictions.

But let me ask again:

Well you didn’t know if I did or didn’t argue with them. Even if I didn’t, it will be because prediction/postdiction distinction is not important for the crux of the paper, the observation of the image of a black hole.

Not the entire series of papers; I said Papers V and VI.

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I’d bet a lot of money that you didn’t and you won’t.

That’s the crux for laypeople, I agree.

Thanks for clarifying. And both, according to you, are entirely postdictions, correct?

Why did you switch from papers V and VI to paper IV, then?

Kindly correct the following sentences from V to make your position clear:

“Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87’s large scale jet are aligned, then the black hole spin vector is pointed away from Earth."

Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets.

Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models,”

" The compact source shows a bright ring with a central dark area without significant extended components. This bears a remarkable similarity to the long-predicted structure for optically thin emission from a hot plasma surrounding a black hole (Falcke et al. 2000)."

" Evidently the angular radius of the observed photon ring is approximately (Figure 1 and Paper IV), which is close to the prediction of the black hole model given in Equation (1)."

" In the GRMHD models the bulk of the 1.3 mm emission is produced within of the black hole, where the models can reach a statistically steady state. It is therefore possible to compute predictive radiative models for this compact component of the source without accurately representing the accretion flow at all radii."

" Our simulations already predict full polarization maps, albeit for our simple eDF model. Comparison of model polarization maps of the source with EHT2017 data are likely to sharply limit the space of allowed models (Mościbrodzka et al. 2017)."

“…their phenomenology, despite being observed on mass scales that differ by eight orders of magnitude, follows very closely the one predicted by general relativity.”

That would explain it to me.

@Mercer

Here’s my reply for your questions on from the other thread:

Why did you switch from papers V and VI to paper IV, then?

I honestly don’t remember, where did I go to paper IV?

As I said before,

All of your complains basically go back to that statement:

“Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87’s large scale jet are aligned, then the black hole spin vector is pointed away from Earth."

Yup, they have models for black hole shadows for all the spins a=[0,1), then fit for the image (the shadow) to get the spin. Then they get the value for the spin. The same for the spin vector direction.

Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets.

This is the same as before. Fitting for different spins, they found that a!=0.

Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models ,”

Yup, tested in the same way as before, through parameter search Edit: after the data is taken.

This is exactly analogous to the mass case:

Edit: But now what they are doing is that, first take the image of the black hole, then fit which spin fits the data.

Please don’t try to argue just by ctrl+f-ing words like “tests” and “predictions”, but also understand what they are doing to see if they are truly predictions.

Anyway, you haven’t answered my question,

@PdotdQ don’t get too frustrated. @mercer is just speaking from his experience as a molecular biologist. I get what you are saying.

@swamidass since you know something about me that @mercer doesn’t, certainly this whole exchange looks very comical from your point of view :rofl:

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Very much so.

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If the EHT collaboration had taken a picture of some other object, but applied the same analysis techniques to it (i.e. treating it as a black hole), would they been able to find a usable fit and find “M”?

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Perhaps not, but the measurement of M is just data fitting, a type of measurement, not a proper prediction. Of course this is a measurement deeply enmeshed in a model. So it is probably well phrase as “the observations along with the model predict a mass” noting we have not actually measured the mass in a direct way.

Testing between alternate models or a null hypothesis (no black hole) is close to how science works, without as much regard for chronological ordering.

Molecular biology is different than astrophysics in that we can actually do prospective experiments to test specific hypothesis, and many molecular biologists take this as the “gold standard” of good science. Trust me. I know. As a computational biologist I’ve had to explain how my work is legitimate and experimental too, many many many times. @mercer is just echoing the field on this, though computational biology and genomics is creating a large culture shift as my peers get tenure.

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If the answer to the question is no (i.e. you can’t coherently apply the EHT analysis to non-black hole objects), then at some level the people fitting data to the BH image to find M are verifying a prediction of GR, even if they are not the ones actually doing the predicting. Maybe it’s more accurate to say the prediction was Einstein’s, and the EHT people @PdotdQ mentioned are mostly working out the implications of assumptions in GR and seeing how they fit to the data.

That being said, I agree with @PdotdQ’s original comment, which is that there are a lot of scientists (physicists and astronomers) who don’t really make predictions, but instead make theory-laden measurements. In Kuhnian terms, they are scientists working within the widely accepted scientific paradigm. In fact, this describes the vast majority of the daily work of scientists. In fact I would describe my own experiment, measuring the EDM of an electron, as doing just that: we are using established techniques, refined and customized for this particular species of molecule and experimental setup, in order to measure a property of the electron based on theoretical assumptions that everyone agrees upon.

(Of course, the reason that this measurement is interesting is that there are people who have made predictions about the value of the electron EDM, but those are mostly particle theorists, none of whom actually do the experimental work, which is based on “regular” atomic physics. Secondly, just as I said for the EHT case, one could say that the fact that we are able to successfully use basic atomic physics to take and fit data in a coherent manner to our daily experimental results is a testament to the explanatory power of atomic physics and quantum mechanics.)

Chronological Ordering and Evidential Status

Overall, I’m not sure that chronological ordering of the evidence is always relevant for describing the epistemic status of a theory. This has been an area of scholarship in contemporary philosophy of science. For example, Stephen Brush in Prediction and Theory Evaluation: The Case of Light Bending argues that the successful prediction of a new phenomenon is not as convincing as an explanation of past facts, “at least until competing theories have had a chance (and failed) to explain it”. In particular he uses the example of the historical verification of GR. After Einstein devised GR, he found it to be able to explain the advance of Mercury’s perihelion, a previously known fact. GR was able to precisely predict what Newtonian mechanics had failed to do. On the other hand, GR also predicted the phenomenon of light-bending, which Eddington famously confirmed via his observations in 1919. Now if one believes that chronological ordering is important, one could imagine that Eddington’s discovery should have been viewed as a stronger piece of evidence for GR than the post-diction of explaining Mercury’s perihelion. But according to Brush, the historical documents and scientific correspondence of the time show us that this was not necessarily the case:

It later became clear to the experts that the Mercury orbit was stronger evidence for general relativity than light bending. In part this was because the observational data were more accurate it was very difficult to make good eclipse measurements, even with modern technology (36, 37, 71) - and in part because the Mercury orbit calculation depended on a “deeper” part of the theory itself (36, 72) . The fact that light bending was a forecast whereas the Mercury orbit was not seems to count for little or nothing in these judgments…

But the most significant argument (though it was not often explicitly stated) is that, rather than light bending providing better evidence because it was predicted before the observation, it actually provides less secure evidence for that very reason. This is the case at least in the years immediately following the announcement of the edipse result, because scientists recognized that any given empirical result might be explained by more than one theory. Because the Mercury orbit discrepancy had been known for several decades, theorists had already had ample opportunity to explain it from Newtonian celestial mechanics and had failed to do so except by making implausible ad hoc assumptions (33). This made Einstein’s success all the more impressive and made it seem quite unlikely that anyone else would subsequently come up with a better alternative explanation. Light bending on the other hand, had not previously been discussed theoretically (with rare exceptions), but now that the phenomenon was known to exist one might expect that another equally or more satisfactory explanation would be found (74).

Brush goes on to explain that only once other competing theories had failed to explain light bending (about 10 years after the experiment), did light bending become just as important a piece of evidence for GR as Mercury’s orbit. Thus, in Brush’s view, what’s important isn’t the chronological order of the evidence, but how well the theory can explain all the evidence we have, regardless of their chronology.

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@swamidass is correct, getting “M” is just a matter of data-fitting. Note however, that non-GR black holes or even non-black hole objects can give a good fit (though we won’t know if this fit actually returns the mass of the object).

Exactly; Einstein made the prediction, not the many legitimate scientists who work on the EHT to fit the observation with models that include, among them, Einstein’s prediction.

It’s like this: If I have a bag with between 1-3 marbles in them, then I ask: “How many marbles are in the bag?”

Newton says: 1
Einstein says: 2
Crackpot says: 3

These are predictions.

What the EHT is doing is the following: to answer “How many marbles are in the bag?”,

Make the model that each marble is 1 gram each and weigh them. They can then fit for the number of marbles in the bag based on this model.

Suppose they get N=2. This number of marbles that the EHT came up with from this procedure is not a prediction. Nowhere in this procedure did the EHT team did any prediction. They can then say: Einstein’s prediction was right. But they themself did not predictions.

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7 posts were split to a new topic: What Constitutes a Scientific Explanation?

And as a geneticist, neuroscientist, cell biologist, and biochemist.

Neither of you has shown anyone using the term “postdiction” in any professional context. Heck, my autocorrect doesn’t even like it and splits it.

Sorry, but that’s just silly.

You’re now moving the goalposts from unsupportable pedantry about post- vs. prediction to trying to pretend that I am denying any utility of purely descriptive science.

I was a mouse geneticist in the 1980s-90s, a Golden Age of description:
https://www.nature.com/articles/349709a0

There’s not a speck of hypothesis testing in there.

Why would we expect to see it used in scientific papers? We are explaining that postdiction is a type of prediction.

Agreed. The same person never has to do the predicting and testing.

That’s not even close to @PdotdQ’s usage:

How can colleagues only do postdictions and never make predictions if the latter is merely a subset of the former?