Amateur Notes on "Quantum Mechanics as Classical Physics"

I am slow to mature. That is why I squandered myself in graduate school. I could have embraced the opportunity to think critically about the philosophy of physics, in which I was at least up to my knees. I instead glibly dismissed philosophy as secondary to prediction. Quantum Mechanics poses the greatest and most interesting philosophical problems and only now, when graduate school is vanishing on the horizon or, at any rate, eclipsed by towering pragmatics racing towards me (mortgages, careers, children), am I taken, with ever more frequency, by thoughts of the philosophy of physics.
Compensating this lack of remit to study is a comparative freedom of choice about how I study. Reading that would have been deemed frivolous by my graduate adviser I am now free to pursue for pleasure. Hence Charles Sebens' 2013 paper "Quantum Mechanics as Classical Physics," which develops a purely classical interpretation of Quantum Mechanics of a novel, Bohmian-flavored variety.

Interested readers should read the version on the Archive. I can't hope to reproduce anything by a quick and probably inelegant if not misleading summary here, but the basic idea is to create a sort of supererogatory interpretative framework for Quantum Mechanics by adding not a single Bohmian particle, but one for many universes in such a way that the dynamics are preserved and then cleverly realizing that the so-called "Pilot Wave," which corresponds to the Wave Function in more ordinary interpretations, can be completely removed, replaced instead by a regular Newtonian force between the Bohmian trace particles.

This results in a many-universe interpretation of Quantum Mechanics (with the same predictions as any other interpretation) but without a wave function. I'm interested in what I believe to be one aspect of this interpretation: it seems to be that worldlines never cross in this way of thinking, so that, if we jump up and up and up to slightly absurd questions like "Are there me's in other universes who have made different decisions than I have?" The answer is "no," in the following sense: because world lines never cross, there was never a time where two universes (and hence two versions of yourself) shared exactly the same state and then diverged. In other words, in each universe, while there may be many beings who resemble any individual in many respects, none of them share identical pasts. If you resent some decision in the past, as I resent not thinking about philosophy more in graduate school, and torture yourself by imagining some parallel person who made different decisions (with the help of some vague thoughts about the Interpretation of Quantum Mechanics) take heart: there is no such moment in the past where you could have chosen differently. You past is fixed and distinct from all those other versions of yourself, none of which were ever identical to you at any point.
At least that seems to be the case when you think about it this way.

Take it to the limit!

You've probably heard about absolute zero temperature but what other limits do physicist deal with?  It turns out that the universe if full of limits.  There are probably limits to the number of limits in the universe, but that is for another post.  Here are a few of the limits in physics.

1.  Absolute Small

Artist rendition of a quark.

Get used to the name Planck, because it will come up a bunch.  On the small side of things we have something called the Planck Length (which, only barbarians like myself would chuckle at) which is 1.6 x 10^-35 meters.  Planck length is where every known force in physics is meaningless except possibly quantum mechanics. Lets try a reference point:  A quark radius ( 3 quarks reside inside a proton) is "2000 times smaller than a proton radius, which is about 60,000 times smaller than the radius of a hydrogen atom, which is about forty times smaller than the radius of a DNA double-helix, which is about a million times smaller than a grain of sand." (ref.)  The Planck length is about 100000000000000000000 times smaller than a proton or 50000000000000000 times smaller than a quark. Put another way: if the Earth was the Planck Length, the entire known universe would be a quark.  And that takes us to our next limit:

2. Absolute Big

The universe is a tie-dye football.

I can't see anyway around the fact that if we live in a universe with edges, that represents a limiting length scale.  So the known universe is about 46 billion light years across.  It doesn't make sense use physics to describe things bigger than that.

3. Absolute Short

Firefly was so good that it
was only one season long

Akin to "light years" meaning the distance that light travels in 1 year, Planck time is the time it takes for light to travel the ever so short Planck Length.   This works out to be about 10^-43 seconds.  This length of time is so short that there I can't even construct a meaningful example.  Physicists talk about attosecond laser pulses which are 1 billion times faster than a nanosecond.  If 1 attosecond was the Planck Time then 100,000,000 universe lifetimes would be 1 second. Seasons of Firefly can be measured in Planck time.

4. Absolute Long

Again, in a universe that starts a clock, physics won't meaningfully describe anything outside of this
time.  Thus the oldest thing physics has anything to say about is the universe which is about 13.8 billion years old.  Graduate school is often measured in universe lifetimes.

Wife didn't want her pictures used so
you'll have to settle for this person.

5. Absolute Hot

Did you know that there is also a theoretical hottest temperature?  At 1.41x10³²  Kelvin, matter would have too much energy to be kept together by any known forces and thus would be necessarily driven to a lower energy state and a lower temperature.  This temperature is known as the (you guessed it)  Planck Temperature.

6. Absolute Cold

Drive all of the energy out of an atom and you get to the coldest temperature possible; Absolute zero (on the Kelvin scale).  These temperatures have been approached in laboratory settings.  Absolute zero is also the number of people who liked 1997's Batman and Robin.

I'll give you the cold shoulder. This
image works on both the cold and
loneliness levels.

7. Absolute Loneliness

Physicists are notoriously socially awkward but that is not the kind of loneliness about which I speak.  Observations of particle creation are always in pairs (or greater).  If a highly energetic photon creates an electron, it also created an anti-electron or positron. (I always secretly wished electrons were called "negatrons", but I digress).  If a positron annihilates with an electron, it usually creates two 511 keV photons emitted back to back. (Sometimes it creates three photons with Energy totally 1022 keV). 1022 keV is the energy equivalent of the mass of two electrons or 1 electron and 1 positron using the famous E=mc^2.

A single proton is actually a combination of two up-quarks and 1 down-quark.  Try to isolate a quark and you'll have to add so much energy that you create more particles.  In fact an isolated quark has never been observed.  Quarks are only ever found in either pairs or triplets (or rarely in greater numbers).  So it turns out that two is the loneliest number.  Eat it Three Dog Night! 

What is Dark Matter?

Pop quiz:  What makes up 27% of the universe and doesn't appear to interact with ordinary matter except by way of the gravitational force?  Well, unless you have zero intuition, the answer is obviously "dark matter". 

"Luke, you don't know the essence of Dark Matter."

But what is dark matter?  Why do people think that dark matter exists?   Why do people say that dark matter makes up 27% of the universe?  As an experimentalist dark matter gives me significant pause for a variety of reasons.  First of all, It hasn't been detected. Conveniently (or inconveniently) it doesn't interact via strong or weak nuclear forces nor electromagnetically, we have yet to have it light up a detector.  So why do scientists say it exists?  Two words: Galactic Rotation.

The visible mass of rotating galaxies don't rotate slowly enough.  Adding unseen "dark matter" can make the observed rotation speed match the calculated speed.  This effect was first noticed in 1930 by Fritz Zwicky.

Why is dark matter supposed to make up 27% of the universe?  Well, because you need a hell of a lot of it to correct galactic rotation apparently.  And if you were thinking that ordinary matter makes up the rest of the universe, think again.

Dark matter is eclipsed by "Dark Energy" which comes in at a whopping 68% of the universe.  That leaves about 5% of the universe to be made up of "ordinary" matter.  And we still don't understand all of that.  Bottom line: at minimum we understand less than 5% of the known universe. (Dark Energy is estimated from measurements of the acceleration of the expanding universe.  A topic for another time.)
Recently the elusiveness of dark matter has cause something of "rebel alliance" of astronomers who are championing a rather contrarian theory called Modified Newtonian Dynamics (MOND) that supposes that gravity changes from an inverse square law to one that makes galactic rotation curves fit observations.  A fuller article attempting to debunk dark matter can be found here.  The only thing that I find less satisfying than dark matter is probably the MOND scenario.  MOND seems to fly in the face of some of the near and dear suppositions of physics: that physics is obeyed here, there and everywhere. 

To my mind, neither theory is really satisfying.  The mystery is still intriguing.  If dark matter only acts gravitationally, would not "dark matter stars" form?  and if they are somehow prohibited from binding as nuclei do, would they not at least form some kind of massive oscillating structure?...  Would dark matter not also be present here in ordinary matter adding to the mass and confounding mass measurements?  I leave these questions for the reader.

Welcome to InterestingPhysics.com!

Hello and welcome to a website devoted to good physics content.  The goal of this project is engage people who are curious about natural phenomena.  And that should be just about everyone.  Because, honestly, if you aren't interested in some kind of natural phenomena then you're probably dead.  So a little bit about myself:  I am a PhD Physicist trained in the dark arts of experimental nuclear physics.  I have spent time within universities, national labs and industry.  I'll tend to write and share about nuclear related phenomena because that is what I know.  Other physicists with other kinds of training will hopefully contribute to the content as well.  They will probably stick to their areas of expertise as well, but it should be noted that physicists tend to think that they are experts at everything.  Anyway, now that an introduction is out of the way, On to Interesting Physics!