Tuesday, March 28, 2017

Galileo and Dark Matter

I was drafted recently to give a talk about Galileo, a subject on which I am no expert. So I relied heavily on Rocky Kolb's book, Blind Watchers of the Sky (I wrote my very first physics paper with Rocky back in 1981, back when we were both, uh, about 12 years old), and on Michael Flynn's brilliant, if somewhat irreverent (coming from me, the latter is higher praise than the former), The Great Ptolemaic Smackdown.

When doing scientific research, you quickly come to realize that progress in science consists of thousands of blind alleys (for you), punctuated by the occasional breakthrough (for someone else). In the textbooks, all of the blind alleys get airbrushed away (who has time to study incorrect theories?), leading to the erroneous impression that science has been been one long march to the Truth. But it never happens that way.

For example, one of Galileo's main arguments for the Copernican model was his theory of the tides. He noticed that as the Earth moves around the Sun, the surface is moving faster at midnight than at noon, since in the former case the motion around the sun is in the same direction as the rotation of the Earth, and vice-versa at midnight.

So Galileo claimed that the water would bunch up at noon and spread out at midnight, leading to a low tide at midnight and high tide at noon (or maybe the other way around?)  A couple of problems here:  there are two high tides and two low tides a day, not one, and they occur at different times of the day, not just noon and midnight.  Galileo's theory was completely wrong, which is why you've never heard of it (except for you, Michael...)  But (and this is the weird part), I drew a diagram exactly like this one for my cosmology class a few weeks ago. Why?

The earth is now riddled with underground dark matter detectors.  As the faint breeze of dark matter passes through the earth, one or two of the particles might strike a detector. But there's another signal we can look for -- the solar system itself is moving through the dark matter halo of our galaxy. During one part of the year, the earth moves in the same direction as the motion of the solar system through the galaxy, and six months later in the opposite direction. So there ought to be a yearly modulation of the signal in any detector. Furthermore, the rotation of the earth ought to produce a daily modulation (just as in the diagram above). So maybe Galileo was on to something after all.

As both Flynn and Kolb point out, Galileo had a very "modern" personality (i.e., a modern academic personality). He wanted priority for all of his discoveries, tried to avoid teaching duties, and thought that his academic rivals were idiots (and wasn't shy about saying so). That sort of person might thrive at a present-day U.S. university, protected by tenure and modern political institutions, but these were poor survival skills for Renaissance Italy. Flynn suggests that Galileo might have had Asperger's. I think it's obvious that he was a time-travelling physicist from the 21st century who got in over his head.


Kathy said...

I love the blind alleys of science and the history of science. Also logical theories which were widely believed and then disproven, like Phlogiston (didn't the chemist who first isolated oxygen think of it as dephlogisticated air?).

Or the luminiferous ether notion. The quality of the Morley-Michelson experimental set ups, by them and others, was impressive; all struggling to measure something that wasn't there.

There's much to learn from negative results, too.

TheOFloinn said...

Detecting the aether using Michelson-Morley is like detecting the rotation of the earth by comparing east wind to south wind. It doesn't work if the air rotates with the earth -- or the aether moves with the planet. As Duhem pointed out: you can never test just one hypothesis and Not-Q doesn't point to any particular Not-P.

One reason for not detecting dark matter may be that it isn't there:

Kathy said...

I love the idea that dark matter isn't there, because it would then revolutionize something else (or reveal were living in a buggy simulation?) Kind of what happened with Mercury's orbit and the nonexistent Planet Vulcan.

But I can think of other reasons why dark matter hasn't been detected:

1) It's not what our instruments are designed to detect (and therefore not what our theories or hypotheses says it might be)

2) It's been detected but not recognized as such (highly unlikely)

3) It can't be detected because it really doesn't interact with anything, even itself, except gravitationally (also highly unlikely, and how would you go about proving it?)

4) It's not something we can detect with existing technology. Think of trying to detect radio waves back in 200 BCE.

But the more time passes without any kind of positive detection, the more I tend to think there may be nothing to detect.

Robert Scherrer said...

The dark matter detectors continue to push down the limits on the rate at which dark matter interacts with ordinary matter. But there is a "nightmare scenario" here -- if dark matter really exists, but interacts only gravitationally. We live in interesting times.

Kathy said...

Assuming dark matter only interacts gravitationally, even with itself, could we even know it's matter? Might it be an attractive force distinct from gravity?

I keep recalling Haldane: the universe it's not stranger than we imagine, it's stranger than we can imagine.

Robert Scherrer said...

There are certainly theories in which the dark matter is actually just a modification to gravity. The problem is that these theories tend to be even more contrived than the idea of dark matter itself.

robk said...

if we change DM in Aether that we have this same idea which was use to detect Aether over 100 years ago :) earth move with the sun through the Aether so that must be change in aether flow in year ;-)