Friday, March 5, 2010

Dark matter, the modern aether

Today, my 16-year old son asked me what dark matter was. I was surprised that he would even have heard about dark matter, but it turns out that even junior science magazines talk about the search for dark matter these days. I must say that I'm not too happy about that. The junior science article, like many other, present dark matter practically as a fact.

The reason this makes me rather nervous is because of the rather obvious parallel with aether. Just like the luminiferous aether, dark matter is something that was postulated when no physical evidence justified it, in order to preserve existing theory.

Those of you who were already dabbling in physics during the 1850s1 may recall that luminiferous aether was hardly a ridiculous idea at the time. Aether was very simply the medium carrying light waves, much like air or water carry sound waves. It was initially postulated by Isaac Newton to explain things like refraction. According to Wikipedia, Augustin Fresnel proposed in 1818 a theory of light as a transverse wave in aether. To quote Wikipedia, from this point on, no one even seems to question its existence. In other words, the existence of aether was postulated in order to preserve the existing theory of waves. All existing waves required a medium, such as air or water, therefore it was natural to assume that light waves also needed a medium to carry them.

The key point to remember here is that the brightest minds of the time did not question aether at all. Some of them, like Newton or Fresnel invented it. Later, the vast majority of scientists were busy trying to refine the concept to make it work. Yet today, luminiferous aether is seen as the canonical example of an obsolete physics theory. Einstein's relativity made the very notion of aether not just useless, but actually wrong. Relativity simplified things by removing the need for a system of coordinates that would be special, but this simplification meant that aether could not exist, because otherwise aether itself would have defined a system of coordinates that was unique.

Back to dark matter. We find ourselves in a similar situation today. There's something about the universe that we very plainly, very visibly do not understand. The original problem, as identified by Fritz Zwicky, was that galaxies do not spin the way they should according to our best theory of gravitation, general relativity. They behave as if there was more matter in them than we can see.

The operational keyword here is as if. At the moment, we really have no idea whether it's the theory of gravitation that is flawed, or whether there really is 95% of the universe's mass that we can't detect. Talking about "dark matter" is choosing one option over the other. It's pretending that we know, when in reality we still lack a model that really explains all the evidence. In my humble opinion, the jury is still out on what this model will look like.

In short, I'm unhappy about references to dark matter made as if it was a settled topic, a known, validated scientific fact on a par with photons or Pluto. Maybe the problem is with the terminology. Talking about dark matter rather than, say, "gravitational anomaly in galaxies" (GAG) is a good way to preserve the illusion that we know what we are talking about. It makes it sound real. But just because we gave it a fancy name doesn't make it more real than aether or the tooth fairy.

Let's be humble and honestly face the simple fact that our model of mass and gravitation breaks down in face of quite a bit of physical evidence. We find ourselves in the situation of physicists in 1850 whose aether-based theories predicted phenomena like aether drag and aether wind, which experiments repeatedly didn't find. It's exciting, it's fun. It's a good thing for physics, because it means there is something new to be found.


Note 1: My editor tells me it's considered bad taste to live past 150 on this planet. My apologies to those of my readers I might have offended...

5 comments:

Mauro said...

Ever heard of Modified Newtonian Dynamics? It's an alternative to dark matter that fits the observed data quite well.

Mauro said...

... oops, I should have read your post more carefully. Apologies. Please ignore my other comment.

Dan Truong said...

Yup, Dark Matter is now common middle school "knowledge".

Jimmy Neutron Season3,Episode11...

"Evil Jimmy wreaks havoc around town and creates a cloned Earth with a dark matter chip! "

http://www.yidio.com/show/jimmy-neutron

If there was indeed dark matter and luminuferous aether, we'd have a lot of gray matter. It's clearly not the case, so one or both theories must be false. Wow! I'm now a physicist. :)


More seriously,

I think your gripe is misplaced: If anything your complaint should be that we should teach clearly that physics is a set of models. It can be assumed to be exact until we find it flawed, or reaches the limits of its precision. Most often teachers teach knowledge as hard indisputable fact.

There's nothing wring with saying there's dark matter. Is newtonian physics obsoleted by quantum physics or relativity, or other screwed up model?

I guess you call a model a theory until enough rocks have been thrown at it that it doesn't fall apart while applied for the scope it was dreamed up in the beginning. Eventually the armor gets chinks and a new way of looking at things is tossed on the table.

Christophe de Dinechin said...

Dan,


Thanks for the link to the Jimmy Neutron episode. It reinforces my point that dark matter is now considered a scientific "fact".

I think your gripe is misplaced [...] There's nothing wrong with saying there's dark matter.

My gripe is about the un-scientific process of inventing an entity with the sole purpose of fitting experimental data, of preserving the theory against evidence. It's very hard to disprove something that is defined as having exactly the properties you need.

If Newton had talked about the "apple harvesting fairy" to denote this invisible, undetectable entity that makes apples fall on the floor exactly as we observe, what knowledge would we have gained? Instead, his theory was about all objects. It was predictive. That's the reason it can still be used to compute the motion of most objects today.

What is predictive about dark matter? How is this a theory or a scientific fact, something that we can falsify? We don't look at a galaxy and say "based on some theory, I predict that there will be such and such amount of dark matter, therefore stars will move like this." Instead, we look at the galaxy and say "based on how it moves, it must have this and this amount of that invisible, undetectable stuff."

And it doesn't even work right without adding another bit of invisible stuff called dark energy!

I much prefer approaches like MOND or scale relativity. At least, these ideas try to address the issue from a theoretical angle, rather than merely fit theory to data.

Nathan said...

I completely share your assessment of dark matter. It is disheartening that there is so little dissent in the scientific community against a theory that has no observable evidence.

I would add to your argument that because dark matter and the aether are both unobservable, they are free to take on increasingly bizarre properties to accommodate whatever evidence mounts against them. With the aether, theory required it to be rigid enough to vibrate 400*10^12 times per second, yet to be of such a density that it produced not the slightest observable resistance to an object moving through it. With dark matter, theory requires it to have properties unlike any other observed form of matter, yet to not have any known mechanism for production, and to exist in increasingly bizarre patterns around galaxies. Furthermore there are galaxies that have rotational discrepancies (the same kind that led to the dark matter hypothesis in the first place) that are proven not to have any dark matter! http://arxiv.org/abs/0707.2459

Dragan Hajdukovic of Cern has an alternate theory of dark matter (different from MONDs) that I find very appealing: http://arxiv.org/abs/1106.0847