Negligence

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If you are not blessed with the negligent gene, Physics is not for you...shift ho!...to Pure Math, or better, Eastern Philosophy.

When we were in College, we didn't hear of Sears or Resnick-Halliday. So, we were taught Mechanics by an innocent teacher from our Math Dept. And he prescribed Loney for us. That was enough repulsive for us, students of Physics. The trouble with our Math Teachers of Mechanics was that they heard that a projectile moves in a parabola. That's it! Since they knew all about parabolas, not to talk of ellipses (closed orbits) and hyperbolas (open orbits), they would play with all exotic and intricate properties of Conic Sections and 'invent' problems in Mechanics to suit them. It is like you have a screwdriver and are looking for screws...

But such problems have no immediate relevance to the physical world we live in. Let us say we do a typical projectile problem at the Class XII level and get a Horizontal Range of 50 meters. And then take a cricket ball and throw it with the same parameters supplied in the problem sheet. And you would find the Range would be anything between 20 meters and 30 meters. Because you have conveniently done: "Let us neglect air friction". So your text book Mechanics is a lot useless for Sports Physics. And it would be worse for the Artillery Unit of your Army. You not only can't neglect air friction, but also the spin of the earth...you have to take into account the Centrifugal and Corialis forces if the cannon shell has to hit the enemy and not result in friendly fire. And if you are employed by NASA, it is even worse if you neglect air friction...ha!

Not all students of Physics turn up as Physics Teachers and spend the rest of their lives ignoring air friction, spin of the earth, googlies and such inconvenient stuff. They may be employed in the so-called Applied Physics. So, each and every moment they should know what to ignore and what not.

When Einstein brought GR out of his hat, he didn't write his Field Equation and sleep off...he took the enormous trouble of making verifiable astronomical predictions and gave numbers. That was because in his youth he was employed as a Clerk in a Patent Office and day in and day out he had to examine every one of thousands of applications and judge if they make sense or no...which requires a Physicist's intuition of what to neglect and what not.

There was an eminent scientist by name Clifford Truesdell (Google for him if you wish). He developed a branch of Mechanics (and later Thermodynamics with a KGPian) called Rational Mechanics. Somewhere along the line he apparently made a comment like: Newtonian Mechanics is nothing but the study of a class of second order differential equations. That rather was a sweeping statement, since it implied that if you know those differential equations, you know Mechanics. Feynman made a remark like: Folks who think that they know the Physics since they know the Math used there, contribute very little to Physics and even less to Math.

You may think that these coarse things like viscosity of air and spin of the earth are a nuisance at the macroscopic level while QM which deals with atoms and molecules is neat and you don't have to bother what to neglect and what not. The situation is even worse at the Quantum level. It is a mess.

Let us take a single Ammonia molecule (not easy). And heat it from Absolute Zero (with a Bunsen burner???)

We know that the Ammonia molecule (NH3) has the shape of an umbrella (don't ask how we know it) with the three Hydrogen atoms forming an equilateral triangle joined with spokes (bonds) to the heavier Nitrogen atom at the top of the pyramid.

At first the molecule is quiet...as quiet as can be. As you heat it slowly you find that the thing starts rotating. And you can apply the rigid rotor Schrodinger Equation, solve it and get some reasonable answers. Here you are 'assuming' that it doesn't run (translate), its atoms don't vibrate, the electrons in each atom are in their ground states, the protons and neutrons in each nucleus are quiet, the quarks in them don't misbehave and so on...

Then you find that as the temperature rises, the umbrella does a flip-flop-flip-flop-flip-flop as if it is caught in a whirlwind. Even neglecting all the motions mentioned above, you get your Ammonia Maser and your Nobels.

Heat it further...and the atoms vibrate and electrons start jumping and if you cleverly separate the rotations and vibrations as Born and Oppenheimer did, you have a damn good explanation for the very complicated band spectra that result. Heat it further and the bonds break and the molecule has dissociated into its atoms....band spectra are replaced by line spectra and you have to do a lot of 'negligence' if you wish to explain them by QM. Heat it further and all the electrons will evaporate and you are in the regime of Chandra's Astrophysics. And further and further till the proton and neutrons in the nuclei start evaporating and you can 'neglect' everything except hard core Nuclear and then Astro Particle Physics...maybe you are in the interior of stars and supernovae. Heat it further and everything breaks up and you are back to our First Three Minutes... and then the LHC maybe...

Where did you start and where did you end!...At each stage neglecting and neglecting and neglecting...

Physics is verily the Science of Negligence...

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