The real meaning of probability density?

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Discussion Overview

The discussion revolves around the concepts of probability density and probability amplitude in quantum mechanics (QM). Participants explore the definitions, implications, and distinctions between these terms, focusing on their mathematical and physical interpretations.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the meaning of probability density, asking if it represents a probability per unit volume or something else, and how it differs from probability amplitude.
  • Another participant asserts that amplitude is a term used by physicists, while density is a precise mathematical term related to the absolute value of the magnitude squared in QM.
  • A participant defines a probability density function (pdf) as a non-negative function that integrates to one over all space, highlighting the challenge of dealing with infinitely many possibilities where individual probabilities are zero.
  • There is a suggestion that a probability amplitude is a complex-valued function whose norm integrates to one over all space, referencing the Schrödinger Wave Equation.
  • One participant seeks clarification on the domain of the pdf, questioning the term "everywhere" in the context of its non-negativity.
  • Another participant responds by stating that a pdf must be non-negative over its defined domain, emphasizing that it cannot take negative values anywhere within that domain.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of probability density and probability amplitude, indicating that multiple competing interpretations remain without consensus.

Contextual Notes

Participants highlight the complexities involved in defining probability density functions, particularly in relation to infinite possibilities and the conditions under which these functions are defined.

jeebs
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In QM people always talk about probability amplitudes and probability densities, but I've never really given these terms much thought and have always just ignored the density/amplitude part and focussed on the probability part.

So what is meant by a probability density - is it what it sounds like, ie. a probability per unit volume, or something different? And how is this distinct from a probability amplitude?
 
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jeebs said:
In QM people always talk about probability amplitudes and probability densities, but I've never really given these terms much thought and have always just ignored the density/amplitude part and focussed on the probability part.

So what is meant by a probability density - is it what it sounds like, ie. a probability per unit volume, or something different? And how is this distinct from a probability amplitude?

A probability density function is any function that is everywhere non-negative and integrates over all space to the number one. The basic problem is that when you have infinitely many possibilities and the probability of each possibility is zero. So you have a probability density function and to calculate a probability you integrate over a positive interval.

A probability amplitude is (I think, maybe not always) a complex-valued function that has the property that the integral of its norm over all space is one. The famous Shroedinger Wave Equation is like this.
 
PatrickPowers said:
A probability density function is any function that is everywhere non-negative...

everywhere... where?
 
jfy4 said:
everywhere... where?


The pdf is nonnegative over the domain on which it is defined. This is just a fancy way of saying that a pdf cannot have a negative value anywhere over its domain.
 

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