Sunday, September 6, 2009

Theory Vs. Experiment. II

Most of the science we know was discovered based on the mismatch between what we thought we knew and experimental/observational results. There are a host of historical discoveries that were precipitated by the discovery of such discrepancies. But as you can see in these examples, the time between the discovery of a problem and its resolution can be years, even decades.

Here's a list of discoveries which started with discrepancies relevant to 'missing mass' which existed at one time and have been resolved.

Discrepancies found in the orbit of the planet Uranus (discovered 1821):
Hypotheses: breakdown of Newtonian gravity
Undiscovered planets
Resolution: Planet Neptune discovered, 1846 (See Wikipedia: The Discovery of Neptune)
Time to resolution: 25 years

Discrepancies are found in the proper motions of the relatively nearby stars Sirius and Procyon (1844)
Hypotheses: Something massive but very faint, that did not emit enough light to be seen in the glare of the primary star, was orbiting these stars.
Resolution: Faint companion stars are found orbiting Sirius (1862) and Procyon (1896). These stars would turn out to be white dwarf stars. (see Wikipedia: White Dwarf)
Time to resolution: 18 and 52 years

Discrepancies found in the orbit of the planet Mercury (discovered 1859)
Hypotheses: Undiscovered planet between Sun and orbit of Mercury. Proposed name is Vulcan but repeated searches do not find it.
Resolution: Postulation of the General Theory of relativity, 1915 (See Wikipedia: Perihelion Precession of Mercury)
Time to resolution: 56 years

Beta-decay of atomic nuclei is found to violate conservation of energy and angular momentum (discovered 1911)
Hypotheses:
  • Beta-decay violates these conservation laws
  • there is an extra particle, electrically neutral, spin 1/2, very small mass, emitted in beta-decay that is not detected by current technologies (neutrino hypothesis, 1930)
Resolution: Neutrino detected, 1956 (See Wikipedia: Neutrino)
Time to resolution: 45 years

Atoms with the same nuclear charge are found to have different atomic masses (discovered 1913) (See Wikipedia: Isotopes). The mass of atomic nucleus for many elements is about twice the number of protons.
Hypothesis: tightly bound states of electrons and protons make up the difference in mass
Resolution: Discovery of neutron, 1932 (See Wikipedia: Neutron)
Time to resolution: 19 years

Shortage of neutrinos emitted from the Sun (discovered 1968).
Hypotheses:
Resolution: Neutrino oscillations, 2003 (See Wikipedia: Neutrino Oscillations)
Time to resolution: 35 years

What many people forget is that in the years between discovery of the problem and the resolution, there was often much contention between scientists. In a number of cases, there were experiments performed which reinforced some hypotheses.

In the case of the neutrino, theories of its interaction were developed which allowed theorists to treat it as a real particle and make numerical predictions. This capability also played a role in the eventual discovery as it enabled researchers to better estimate what level of technology would be needed for a direct detection.

Here's the big discrepancy in astronomy that has yet to be resolved. The is the focus of current controversy

Discrepancies: Rotation curves of galaxies doesn't match the visible matter distribution (discovered 1933). Clusters of galaxies have galaxies moving too fast to be gravitationally bound.

Hypotheses:
For information on the current state of searches for various particles beyond the Standard Model, check out the reviews at Particle Data Group, 2009 Reviews, Tables, and Plots

Frankly, I think the undiscovered subatomic particle option is most likely. It has the advantage of being the simplest solution that does not violate constraints from other observations. One could make the point that there seems to be an interesting hierarchy in the family of particles related to what interactions different classes of particles 'feel' (marked with an 'X').


Forces:gravityweakE&Mcolor (strong)
Quarks

X


X


X


X
Electrons, muon, tau

X


X


X


-
Neutrinos

X


X


-


-

It appeals to a sense of symmetry (a surprisingly successful concept in particle physics) that there should be one more line
Forces:gravityweakE&Mcolor (strong)
'Dark Matter'

X


-


-


-
In addition, the history has strongly favored the discovery of new particles just when we think we've found them all.

Consider the example from 1936, after the identification of electrons, protons, and neutrons, all the particles needed to build atoms. Carl Anderson discovered the muon in cosmic rays (see Wikipedia: Muon). It was such a surprise that one physicist commented “Who ordered that?”

Science involves finding solutions to difficult problems and sometimes it takes many years. I suspect there were cranks and crackpots exploiting the gaps in our understanding in the case of the older discrepancies, just as creationists and EU advocates try to exploit the more modern problems that are at the frontiers of our current knowledge.

In spite of the claims of pseudo-scientists, real scientists did the work and eventually solved the problems. They also improved on the measurements, sometimes revealing new discrepancies. Today, experiments are running that measure neutrino oscillations by measuring neutrinos that pass through the Earth emitted by reactors around the world (so there is a calibrated source). For more examples of science that started out as astronomical observations, see "The Cosmos in Your Pocket: Expanded and Revised".

Comments illustrating more examples from physics and astronomy are welcome.

5 comments:

Anonymous said...

The Dark Matter item is a lot more complicated than described here.

As already noted, the initial discovery, by Zwicky in 1933, concerned a rich galaxy cluster, not spiral galaxies. And the apparent discrepancy - in the estimated mass of the cluster - concerned the total estimated mass of stars in the cluster's galaxies. Today we know that much of this discrepancy is due to incorrect values of input parameters, much is due to the mass of the inter-galactic medium (a hot, low density plasma whose estimated mass exceeds that of all cluster galaxies combined, many times over), etc. However, there's still a big discrepancy, and a great deal of evidence to suggest that it's not 'baryonic' (i.e. not composed of H, He, etc, whether ionised or not).

And that's just a few words on CDM (cold dark matter) in rich galaxy clusters; there's an equally complicated story concerning rotation curves of spiral galaxies, a somewhat more straight-forward story concerning cosmology, and so on.

The remarkable bottom line is that a single hypothesis (or theory) - CDM - can apparently account for such a diverse range of observations, of so many different kinds of objects. Of the competing hypotheses, only some relativistic version of MOND comes close in terms of its explanatory and predictive power.

Nereid

Jon Voisey said...

What about whether or not those "spiral nebulae" where just nebulae or "island universes"?

Original observations date back nearly as long as there were telescopes and resolved with Hubble.

W.T."Tom" Bridgman said...

Hi Jon!

My theme was 'discrepancies' in mass and energy where the cause/source has been found - in many cases just how they expected - Neptune, neutrinos, etc. Primarily to build an historical analogy with the current Dark Matter issue.

Jon Voisey said...

All right then, what about the discovery of "forbidden" states for electrons from the observed spectral lines, originally dubbed "Nebulium". I think that falls more along the lines you were looking for since it deals with energy states.

W.T."Tom" Bridgman said...

I've got lots of atomic examples in the section "A Tale of Two or Three Element" in this paper
The Cosmos in Your Pocket: Revised and Expanded.

I've got some similar historical examples I'm preparing for Part III of this thread, primarily as a response to some EU advocates on a e-mail thread I'm in. ;^)