When do you reject a theory?

Recently I’ve been wondering: when have you amassed enough evidence to reject a theory?  At some point astronomers decided to cast aside the geocentric cosmology in favor of the heliocentric cosmology.  Later, the Big Bang theory replaced static cosmologies.  How did they know they had enough evidence to reject an observationally supported theory?

Last week I found an interesting paper on radio galaxies.  Radio galaxies are normal galaxies with supermassive black holes in their center that shoot out a highly collimated jet of matter moving at 99% the speed of light.  This jet is mostly seen in the radio part of the electro-magnetic spectrum, hence the name “radio galaxy.”  When radio galaxies shoot these jets almost directly at us, things appear quite different; the radiation varies rapidly and is overall much, much brighter than when the jets aren’t directed toward us.  When a radio galaxy’s jets are directed toward us, we call it a blazar.

Radio galaxies can be further divided into two classes: FRI and FRII.  FRII jets (pictured below) are much more powerful and more collimated.  At the end of their jet, the material is hitting the extra-galactic medium (gas outside of galaxies) and heating up, producing a bright hot-spot.  On the other hand, FRI jets (pictured above) are simply less powerful and collimated.  You might think that FRI jets and FRII jets are just two ends of a spectrum, but that’s not the case.  Most jets clearly fall in one group or the other, with a clear dividing line between them.

As you might expect, there are also two types of blazars: BL Lacs and Radio Quasars.  A lot of work has been done to see which type of radio galaxies produce these blazars.  The emerging picture is that BL Lacs are FRI jets pointing at us, and Radio Quasars are FRII jets pointing at us.  Furthermore, there are some good reasons that come from physics that this should be the case.

Unfortunately, recent observations cited in the paper I read suggest that around a third of all BL Lacs are actually FRII galaxies pointing at us, not the expected FRI galaxies pointing at us!

Shall we just throw this whole theory of radio galaxies/blazars out the window now?  Not yet is my best answer.  The theory still explains a lot of concrete observations in terms of straightforward physics.  Furthermore, we don’t have any better theory to replace it with yet.  We might find some evidence, theoretical or observational, that will allow the current theory to remain intact.  What’s more likely, in my opinion, is that we will find more theoretical/observational evidence that suggests the original theory was on the right track but just too simple.

This sort of situation arises all the time in science: a dominant theory explains a wide variety of phenomena in terms of a few physical principles, yet problems remain.  Many times these problems are solved, but occasionally these problems persist, leading to a modification or collapse of the dominant theory.

Some scientists latch on to these problems, despite the supporting evidence, and hold them up as examples for why the scientific community is biased towards the dominant theory.  Critics of the Big Bang, evolution, and human-induced climate change, for example, reject the dominant theory but propose no better theory of their own.

So when do we reject a theory?  First, we have to understand the dominant theory well conceptually (saying that you can’t understand it doesn’t cut it).   We must also understand the entire body of supporting evidence.  Secondly there must be excellent data–which has withstood the test of time and several analyses–that, if true, completely undermines the theory.  Finally, a new theory must be in place that fully explains both the data that undermined the old theory and the body of supporting evidence of the old theory.

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