What is Equal Apriori Probability?

A fundamental postulate of statistical mechanics is that a macroscopic system in equilibrium is equally likely to be in any of its accessible microscopic states satisfying macroscopic conditions of system. It is called the postulate of equal apriori probability.

According to this postulate, the probability of finding the phase point for given system in any region of phase space is identical with that for any other region of equal extension or volume. This may also be stated as for a system in equilibrium, all accessible microstates corresponding to a given microstate are equally probable.

The basis for the postulate of Equal Apriori Probability is ergodic hypothesis. The constancy of volume element in phase space with time indicate validity of postulate of Equal Apriori Probability.

Explanation of words in "The Special Theory of Relativity"

Special: The word "Special" in name arises because we employ only unaccelerated reference frames, not all reference frames  that one can think of. In other words, we "specialize" to the way things appear when observed from uniformly moving reference frames.

Relativity: The word "Relativity"  comes from a phrase coined by Henry Poincare, an eminent French Physicist and Mathematician.

In 1904, Poincare was invited to address the international congress of Arts and Science, held in St. Louis to commemorate the 100th Anniversary of Losiana purchase. "Poincare" spoke of "Principle of Relativity".

In his words.....

Assume that you are in plane on its way from Chicago to Pheonix, another plane making the return flight, over wheat fields of Kansas. A farmer, looking up, notes that you are flying south-west at 500 miles/hr relative to his wheat fields. The pilot of return flight notes that the distance between the two planes is decreasing at about 1000 miles/hr. So far as the pilot is concerned, you are travelling at about 1000 miles/hr relative to his plane.

The essence is this:

Statements about uniform motion relative to a specified reference frame-wheat fields or another air plane are meaningful.

       A quantitative statement about uniform motion with out specification of a reference frame is not meaningful. Why? because our principle 1 says we cannot discern uniform motion with out recourse to some reference frame.

Take first the colloquial form of that principle: If we are in unaccelerated vehicle, its motion has no effect on the way things happen inside it. So by just doing experiments inside the vehicle, we have no way to assign a velocity to the vehicle. Only if we look out of window and there by use wheat fields of Kansas as an outside reference frame- we can decide on velocity (relative to that outside reference frame).    

Theory: It appears because principle 1 &2 are generalizations from observation and experiment.

 

Principles of Special Theory of Light

PRINCIPLE-1

Colloquial Statement: if we are in unaccelerated vehicle, its motion has no effect on the way things happen inside it.

Formal Statement: The laws of physics are same in all unaccelerated reference frames.

PRINCIPLE-2

The motion of light is not affected by motion of source of light.

what does MWe mean? what does the 'e' stand for?

M- Mega ; W-Watt; e-Electrical

Unit used to denote power level in Power Plants.

It is used to denote Electrical Power Generation by Power. It is denoted so as to differentiate it from thermal power generated. Not all thermal power can be converted into electrical but it depends on efficiency of plant. 

What causes the "white smoke" trailing high flying jet planes ?

Modern turbofan (jet) engines actually mix with fuel only a small portion of the air they suck in. The vast majority (90% or so) of air is "bypass air" that is compressed and then used for additional thrust. This air expands as it leaves the engine.


All engines that burn petroleum-based fuels (jets, piston engines, etc.) produce steam and CO2 as their exhaust. The CO2 is invisible on exiting the engine and stays that way, but in cold air, the steam condenses into liquid water or ice, forming a white mist that leaves a trail behind the aircraft. This trail is called a condensation trail, or contrail.


Contrails may or may not be visible, depending on atmospheric conditions. They are common above 26,000 feet, but uncommon below. Again, depending on atmospheric conditions, they may be completely absent, or they may disappear after a few minutes, or they may grow into ordinary cirrus clouds.