Just because it’s falsifiable doesn’t mean it’s good science.

Flying carrot. 

Title says it all, really, but it’s such a common misunderstanding I want to expand on this for a bit.

A major reason we see so many wrong predictions in the foundations of physics – and see those make headlines – is that both scientists and science writers take falsifiability to be a sufficient criterion for good science.

Now, a scientific prediction must be falsifiable, all right. But falsifiability alone is not sufficient to make a prediction scientific. (And, no, Popper never said so.) Example: Tomorrow it will rain carrots. Totally falsifiable. Totally not scientific.

Why is it not scientific? Well, because it doesn’t live up to the current quality standard in olericulture, that is the study of vegetables. According to the standard model of root crops, carrots don’t grow on clouds.

What do we learn from this? (Besides that the study of vegetables is called “olericulture,” who knew.) We learn that to judge a prediction you must know why scientists think it’s a good prediction.

Why does it matter?

The other day I got an email from a science writer asking me to clarify a statement he had gotten from another physicist. That other physicist had explained a next larger particle collider, if built, would be able to falsify the predictions of certain dark matter models.

That is correct of course. A next larger collider would be able to falsify a huge amount of predictions. Indeed, if you count precisely, it would falsify infinitely many predictions. That’s more than even particle physicists can write papers about.

You may think that’s a truly remarkable achievement. But the question you should ask is: What reason did the physicist have to think that any of those predictions are good predictions? And when it comes to the discovery of dark matter with particle colliders, the answer currently is: There is no reason.

I cannot stress this often enough. There is not currently any reason to think a larger particle collider would produce fundamentally new particles or see any other new effects. There are loads of predictions, but none of those have good motivations. They are little better than carrot rain.

People not familiar with particle physics tend to be baffled by this, and I do not blame them. You would expect if scientists make predictions they have reasons to think it’ll actually happen. But that’s not the case in theory-development for physics beyond the standard model. To illustrate this, let me tell you how these predictions for new particles come into being.

The standard model of particle physics is an extremely precisely tested theory. You cannot just add particles to it as you want, because doing so quickly gets you into conflict with experiment. Neither, for that matter, can you just change something about the existing particles like, eg, postulating they are made up of smaller particles or such. Yes, particle physics is complicated.

There are however a few common techniques you can use to amend the standard model so that the deviations from it are not in the regime that we have measured yet. The most common way to do this is to make the new particles heavy (so that it takes a lot of energy to create them) or very weakly interacting (so that you produce them very rarely). The former is more common in particle physics, the latter more common in astrophysics.

There are of course a lot of other quality criteria that you need to fulfil. You need to formulate your theory in the currently used mathematical language, that is that of quantum field theories. You must demonstrate that your new theory is not in conflict with experiment already. You must make sure that your theory has no internal contradictions. Most importantly though, you must have a motivation for why your extension of the standard model is interesting.

You need this motivation because any such theory-extension is strictly speaking unnecessary. You do not need it to explain existing data. No, you do not need it to explain the observations normally attributed to dark matter either. Because to explain those you only need to assume an unspecified “fluid” and it doesn’t matter what that fluid is made of. To explain the existing data, all you need is the standard model of particle physics and the concordance model of cosmology.

The major motivation for new particles at higher energies, therefore, has for the past 20 years been an idea called “naturalness”. The standard model of particle physics is not “natural”. If you add more particles to it, you can make it “natural” again. Problem is that now the data say that the standard model is just not natural, period. So that motivation just evaporated. With that motivation gone, particle physicists don’t know what to do. Hence all the talk about confusion and crisis and so on.

Of course physicists who come up with new models will always claim that they have a good motivation, and it can be hard to follow their explanations. But it never hurts to ask. So please do ask. And don’t take “it’s falsifiable” as an answer.

There is more to be said about what it means for a theory to be “falsifiable” and how necessary that criterion really is, but that’s a different story and shall be told another time. Thanks for listening.

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[I explain all this business with naturalness and inventing new particles that never show up in my book. I know you are sick of me mentioning this, but the reason I keep pointing it out is that I spent a lot of time making the statements in my book as useful and accurate as possible. I cannot make this effort with all my blogposts. So really I think you are better off reading the book.]