I mean the Wikipedia article on it basically says as much: https://en.wikipedia.org/wiki/Bloodstain_pattern_analysis

> The validity of bloodstain pattern analysis has been questioned since the 1990s, and more recent studies cast significant doubt on its accuracy. A comprehensive 2009 National Academy of Sciences report concluded that "the uncertainties associated with bloodstain pattern analysis are enormous" and that purported bloodstain pattern experts' opinions are "more subjective than scientific." The report highlighted several incidents of blood spatter analysts overstating their qualifications and questioned the reliability of their methods. In 2021, the largest-to-date study on the accuracy of BPA was published, with results "show[ing] that [BPA conclusions] were often erroneous and often contradicted other analysts."

That's not the same as saying it could _never_ be done properly, but the analysis being done right now is not that, even though it's presented in courts as expert opinion with high certainty.

I think if you read this article you'll see (at least part of) the answer. Not only does a given action have to cause a definable result, the medium upon which the result is deposited has to provide enough detail to read the result back off it, and the people reading the result have to be able to be consistent and accurate in their analysis of that result.

With bite marks, the skin has to hold the bite mark (which it doesn't) and experts have to be able to map it back to real human teeth (which they can't). With blood spatter, there are analogous problems.

Which sounds miles from 'have no basis in evidence'. So that might have been a little hyperbolic.

I'm not sure why you're trying to play devil's advocate on that. The things they are claiming about the analysis techniques literally have no basis in evidence. Saying "blood spatters follow laws of physics" has nothing to do with what the experts are doing now

...and I countered with "the physics surely have validity", which the plentiful science done to date has corroborated.

Not playing devil's advocate; just doing some simple searches and calling out hyperbole.

These days folks have replaced science with advocacy. You know, that thing lawyers do to convince a jury. To the point folks think that you can arrive at a conclusion with advocacy. Instead of looking at the data.

I'd ramble on about how the world is going to pot, because folks thing a sarcastic remark or clever-sounding turn of phrase is 'just as good' as facts. But oh well.

You confused the GP statement about “blood spatter analysis” with a statement about “blood spatter”.

If someone critiques astrology you don’t reply that “planets follow physics”.

"Blood spatter analysis" is not a physics based discipline. The other comment responding to you with the analogy about astrology is good explanation of what is wrong with your reasoning

not so, if i square a number, and get 4, There is no basis in evidence for the claim that the number I squared was 2. it could equally likely have been -2. Science is capable of making no stronger a claim than that.

Blood splatter is still studied. What I think most forensic experts would tell you is they are really trying to compile as much evidence as possible that allows the largest reconstruction of what may have happened. It is not like the movies, or it shouldn’t be, where you find a single finger print and that determines who the killer is.

I think the key challenge to blood spatter analysis is not that there's no way it could be valid, it's that so far, it's based primarily on unsupported assumptions and statements. Those assumptions may well be correct, but the hard physics behind it has not been done yet, it's been just folks saying 'well I'm very clever and this is what I think'.

An internet search shows there is research done, particularly the 'hard physics' behind it..

Is there? Simulating forward sounds doable. Some fluid sprayed from here, with this mass and so on it lands as so. Backwards from a spatter pattern on an object sounds a lot harder, not least because there will be multiple possible answers and heaps of unknown variables.

I'm sure blood spatter has some place in the courtroom. The issue seems to be that there's no standard as to what kind of conclusions you can reach from it. So it's ripe for abuse. Pay some expert enough to say anything you want is true based on the blood spatter.

Everything follows the rule of physics. The existence of some way to do something is a very low bar to clear. There may be some way to determine if there's water underground, but that doesn't mean a dowsing rod can.

The actual current practice of blood spatter analysis is to the physics of blood splatters roughly what astrology is to the physics of the movement of stars and planets.

Could there be an astronomy equivalent for blood spatter analysis? In principle yes, but it doesn't exist yet.

Could the current astrology sometimes shed real light? In some cases yes, just like even an astrologer can tell you something true about where Mercury is likely to be on the sky tomorrow. But the vast majority of the claims of current experts are going to make claims a lot stronger than that.

And just like most astrologers, they will make those claims based on what they want to be true, or what they think you want to be true, not on what the evidence tells them.

Usually solving a PDE “backwards” is not possible. And that is exactly what those people are claiming to do.

IE: from a specific (discrete) distribution of temperatures in, say a length of wire, you cannot infer the distribution some time before.