Planck's Unit and universal constants

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Discussion Overview

The discussion revolves around the nature of Planck's units and the concept of universal constants in physics. Participants explore the significance of constants such as the gravitational constant, Planck's constant, and the speed of light, questioning their fundamental nature and how they relate to human-defined measurement systems.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the need for Planck's units, suggesting that traditional measurements are arbitrary and wonders what can be considered truly fundamental in the universe.
  • Another participant argues that constants like the speed of light and Planck's constant can be considered fundamental because they maintain consistent relationships in measurements, assuming those constants do not change.
  • It is noted that while the numerical values of constants depend on the measurement system, their constancy across different contexts is what makes them significant.
  • A challenge is raised regarding the definition of units, specifically the claim about the diameter of an electron, which is contested as electrons are considered point-particles.
  • A later reply emphasizes that fundamental quantities must be measured rather than stipulated, reinforcing the idea that measurement is essential to understanding their values.

Areas of Agreement / Disagreement

Participants express differing views on the nature of fundamental constants and their definitions. While some agree on the constancy of these values, there is no consensus on what constitutes a fundamental measurement or the implications of these constants.

Contextual Notes

There are unresolved questions regarding the definitions of units and the implications of measuring fundamental constants, as well as the assumptions underlying the discussion of what is considered fundamental.

shounakbhatta
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Hello,

I am reading Planck's Unit. What I understand is that what is the need for Planck's unit. Kilogram, meter and second are measurement, rather arbitrary measurement, which humans have done in the past. The question is what do we know that is constant through the universe? We know (1) The gravitational constant (2) Planck's constant and (3) The speed of light. So if we could measure, say mass M, in terms of gravitational constant and Planck's constant and speed of light, then we can get something which is fundamental.

My question is what is fundamental? G,c and h bar, how can they be called universal constants if they are again measured by us, human beings?

Thanks.
 
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They are fundamental in the sense that a perfect length prototype for a light-second (=distance light travels in a second) and a perfect clock will always keep synchronized - light will need one second to travel along that length. In the same way, an atom will always the same fraction of the Planck mass, and the acceleration between two objects with the same mass will always stay the same - assuming those 3 constants do not change.

In the Standard Model of particle physics, they are not considered as fundamental constants - it is assumed that they are constant (and this has been measured with incredible precision, using the idea described above), and then they can be set to 1.
Fundamental parameters are always dimensionless - they are real (or complex) numbers, and independent of the measurement system.
 
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The numerical value of "G", "h", and "c", etc. depend upon choice of measuring system (which is what I presume you mean by "measured by us, human beings") but the fact that they are constant, not changing from place to place or time to time, does not.

Some texts do, as mfb suggests, choose units of measurement to make those constants equal to 1. If I remember correctly, the unit of length is the diameter of an electron, the unit of time is the time it takes light to cross the diameter of an electron, etc.
 
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HallsofIvy said:
If I remember correctly, the unit of length is the diameter of an electron, the unit of time is the time it takes light to cross the diameter of an electron, etc.
?
There is no (known) diameter of an electron, it is probably a point-particle.

The second is defined in terms of the frequency of radiation emitted in a specific atomic transition, together with the fixed numerical value for the speed of light this fixes the length as multiple of that wavelength as well.
 
Thank you for the details.
 
Think about it. If something is fundamental than measuring it is the only way we can know its value. It cannot be stipulated, it must be measured.
 

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