Tuesday, December 4, 2012

Shorter Take Away Lessons From Modern Physics

Time and space are relative.

The rate at which time passes is particular to the observer and depends upon things like one's speed and the strength of the gravitational field that one is in.  Likewise, the laws of physics are fundamentally "background independent" with the form of space-time being different for each observer although in all cases consistently translatable between observers.

A central aspect of the translation rules is the insight that nothing in the universe goes faster than the speed of light in a vacuum, causing weird distortions from one's intuition when something is moving very fast.

There are surprisingly few laws of physics (e.g., the second law of thermodynamics, the Standard Model weak force that governs things like nuclear radiation) that have a form that is not symmetric between going forward in time and going backward in time.  The Standard Model asymmetries, moreover, all adhere to a deep symmetry called CPT symmetry that states that processes and the reverse of those processes proceed in exactly the same way except that a quantity called "charge parity" (CP) is flipped at precisely defined rates where there is not perfect time reversal symmetry.

Everything is made of randomly interacting fundamental particles which are in turn made from the same mass-energy stuff.

Matter and energy are ultimately interchangeable version of the same mass-energy stuff which are related by E=mc^2.

At the deepest level, everything boils down to interactions between fundamental particles of which there are a couple of dozen kinds, and these particles behave and interact with each other in a manner that is profoundly random.  Some of the random ways that these particles behave in isolation is contrary to our common sense about what is physically possible that is based on real world scale phenomena immensely bigger than the size scale of these particles.

In particular, there are three fundamental forces in addition to gravity, there are twelve kinds of massive particles that are not force carriers (six quarks, three charged leptons and three neutrinos),  there are nine kind of massless force carriers (photons and gluons), and there are four kinds of massive force carriers (W+, W-, Z and Higgs bosons).

But, the universe if far less complex than it would seem because the vast majority of Standard Model fundamental particles and composite particles made up of Standard Model fundamental particles are extremely unstable, and/or operate only at very short ranges.  Protons, neutrons, electrons, neutrinos and photons are the only things in the Standard Model that have any meaningful kind of stability that operate outside atomic nuclei in real life, and only a modest percentage of theoretically imaginable combinations of protons and neutrons form stable isotypes of periodic table elements.

The laws of physics are extremely accurate and there is virtually no controversy within physics regarding the theoretical predictions of those laws, and scientific effort to refine this consensus continues.

There is an essentially perfect consensus of physicists on the theoretical predictions of the laws of physics as set forth in the theory of general relativity and the Standard Model in the conditions found in all experiments that have been conducted and replicated to date and the theory matches the experimental result to within reasonable levels of experimental imprecision (including so called "mysterious" dark energy) with one exception.    The precision of the match between theoretical expectations and experimentally measured results is exquisitite.

There are a number of theoretical constants in the Standard Model that are not known with much accuracy which are the subject of ongoing investigation.

Scientists still haven't figured out dark matter although there are some good theories out there to describe its effects.

The one exception to the consensus is that there is no consensus on the theoretical basis for the phenomena observed by astronomers that is described as "dark matter".  Dark matter effects most certainly exist and are observed in every galaxy and galactic cluster and in other respects as well.  And, the controversies over what dark matter effects have been observed are practical and technical experimental measurement and theoretical calculation method controversies, not deep scientific differences of principle over the laws of nature or epistomology or the like.

But, dark matter effects have not been described by applying general relativity to observed luminous matter, and the Standard Model does not seem to include any particle that would behave as dark matter appears to behave.  No single consensus dark matter theory describes all observed dark matter effects, has accurately predicted new dark matter effects, and has a mechanism that has been directly measured. 

The majority view is that there is some kind of dark matter than is not made up of protons and neutrons and is not simply a bunch of neutrinos, the lacks electromagnetic charge and is distributed more or less in filiments at a large scale and in halos around galaxies, made up of some unobserved type of particle with properties that are not precisely known that accounts for these effects without any modification of general relativity. 

A minority view is that some or all of dark matter is accounted for by a slight inaccuracy of general relativity in weak gravitational fields (probably due to quantum gravity effects) and that the remaining dark matter effects are caused by unobserved but basically ordinary types of matter that are already known to science but can't be seen because they don't generate light or heat.

Active astronomy observations and direct dark matter detection experiments are trying to bring more evidence to bear on the question so that we can determine which theory is right.

We don't perfectly understand the laws of nature, but the gaps in our knowledge are in places we can't examine experimentally.

General relativity and the Standard Model are theoretically inconsistent with each other, but this doesn't have much practical impact because it is usually clear that one or the other theory provides the dominant explanation and that the other has negligible relevance to a problem.  But, these theories may not be valid in extreme circumstances such as those involving very high energies and extreme masses, for example, around black holes and the Big Bang.  Many of  the unsolved problems of physics (other than dark matter) involve cosmology (i.e. the theoretical description of how the Big Bang gave rise to the universe.

There is a cottage industry in pursuing various strategies for resolving the known inperfections in the existing theories of physics and considering if there are alternatiive versions that could also be consistent with experimental evidence.  But, the vast weight of experimental evidence highly constrains what kinds of theories could be viable.  The predicted deviations from existing theories must happen in domains that we can't currently explore experimentally or relate to dark matter.

So far, these beyond the Standard Model theories are regularly being cut down by new experimental results disproving or constraining them, while new definitive beyond the Standard Model experimental results have cropped up in the last fifty years (apart from the discovery that neutrinos have mass).

All other legitimate controversies in modern physics involve matters where experimental results have not been replicated or are not yet available with sufficient certainty, where different theories are experimentally indistinguishable (at least to date), or where theoretical predictions are not possible to generate due to uncertainty about the theory or because the math is too hard.  In short, other legitimate controversies in modern physics do not undermine a near universal consensus on the nature of the laws of physics insofar as they apply to what can be measured experimentally.

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