# Questions tagged [physical-constants]

This tag is for questions relating to physical constants which are any of a set of fundamental invariant quantities observed in nature and appearing in the basic theoretical equations of physics. Accurate evaluation of these constants is essential in order to check the correctness of the theories and to allow useful applications to be made on the basis of those theories.

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### Can all physical quantities and units be derived from the seven base SI units? [closed]

I understood that from cesium that we derived the unit for second and procedurally with other universal constants we derived the other SI units. Can all other “physical” units be derived from the ...
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### Why is finding a mathematical basis for the fine-structure constant meaningful?

I was reading QED by Richard Feynman and at the end he mentions that: There is a most profound and beautiful question associated with the observed coupling constant, $e$ – the amplitude for a real ...
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### Why did we choose Coulomb's constant $k$ as $9\cdot10^9 \rm Nm^2/C^2$ while define unit of charge instead of any other number?

I understand that while defining charge, Coulomb had to choose any arbitrary value of $k$ to describe unit of charge. But, why did we chose $9\cdot10^9 \rm Nm^2/C^2$ as the value of $k$, but not any ...
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### Definition of the gravitational constant in 1+1 gravity

In this paper, the author formulates a $(1+1)$-dimensional theory of gravity by taking the trace of the Einstein equations $$\left(1 - \frac{D}{2}\right)R = 8\pi G_D T,\tag{2}$$ (where $G_D$ is the ...
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### Is the Planck mass the "lower limit" for gravity?

The Planck units are often treated as being the "lower limits" to things: the Planck length for length, the Planck time for time, etc. But the Planck mass, which is about $2.2\times10^{-5}$ ...
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### Planck Length Calculation

On this question, from long ago, the Planck length is calculated as the length at which the Reduced Compton Wavelength is equal to the Schwarzschild Radius. However, in the calculation, the scalar &...
1 vote
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### Mass of the Prototype $\rm kg$

With the new SI kg based upon Planck's constant $h$, is there any plan to eventually measure the former IPK using a Watt Balance, and determine how far its mass deviates from the new definition of 1 ...
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### Gravitational Constant with ENM Units?

To give some context, there's a conspiracy 'theory' that I saw called Electric Universe that says that gravity is not a fundamental force and instead is a "incoherent dielectric acceleration"...
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### How is Planck's constant relevant in quantum mechanics?

Planck’s constant relates the frequency of a photon to its energy but how does that relate to other quantum particles? For example in the Schrödinger equation it is used according to my book “to ...
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### Two values of Rydberg Constant

In my textbook I came across two values of Rydberg Constant: \begin{align} R&= 109\,677\ \mathrm{cm}^{-1} \\ R_h&= 2.18 × 10^{-18}\ \mathrm J \end{align} when we are calculating energy of the ...
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### Do we really know the universal gravitational constant?

We've all heard $$F_g=\frac{gm_1m_2}{r^2}.$$ However, since I took physics, we've discovered "dark energy," which if I have any concept of the current thinking is caused by space being ...
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### What is the value of the ideal gas constant in J/(mol*°C)? [duplicate]

I've seen in some sites that it is the same as in J/(mol*K), is that true? $8.314 \frac{J}{mol×K}=8.314 \frac{J}{mol×°C}$
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### Why is the uncertainty of the Gravitational Constant $G$ many orders of magnitude larger than that of other important fundamental constants?

Why is the numerical range of uncertainty of the Gravitational Constant $G$ many orders of magnitude larger than that of other important fundamental constants? What are the challenges in precise ...
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### Ideal Gas law and Universal gravitational constant

In a hypothetical universe with a different Universal gravitational constant $G$, will the nature of ideal gas change by any means? i.e, will $PV = nRT$ be no longer applicable in that case?
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### How were luminous intensity changes for each EM frequency measured in the blackbody experiments of the late 19th c., which led to up to Planck’s Law?

If in today’s parlance it would be either ‘spectral density’ or ‘spectral radiance’ that was in fact measured then as opposed to ‘luminous intensity’ please feel free to clarify. Kindly note that my ...
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### Dimensions of constants

We know that pure numbers are dimensionless then how come universal constants like the gravitational constant have a dimension cause they are also equal to some numerical value and if the numerical ...
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### Why Normalise by $h$ in the Partition Function for Classical Harmonic Oscillator?

I was wondering if anyone could explain the reasoning behind the $h$ normalization constant when calculating the partition function for a classical harmonic oscillator. I know that the partition ...
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### Relation between Coulomb's law and Fine-structure constant

There should be some relationship between them due to the nature of FSC, but I could not find anything about it.
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### If the fine-structure constant was very large could positronium have negative mass?

Positronium is an atom with one electron and one positron. It's mass is 1.022MeV which is almost twice the electron mass: The ground-state (1S orbital) binding energy of -6.8eV reduces the total mass ...
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### Relationship between CFT coupling constants and gravity parameters in the AdS/CFT correspondence

The AdS/CFT correspondence relates a string theory in AdS to quantum field theory. Various versions of this correspondence exist, and I want to know the map between parameters in the field theory and ...
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### Why didn't Michelson and Morley assume or conclude that aether is homogenous and permeate all of space?

In the Michelson-Morley experiment, why didn't they assume or conclude that ether is homogeneous and permeate all of space instead of coming from an unspecified source or direction dubbed the ether ...
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### Einstein equivalence principle and running of couplings

It is known that a spacetime variation of the dimensionless gauge coupling constants of the standard models would lead to a violation of the Einstein equivalence principle (EEP). This point is ...
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### Is it possible to distinguish the type of space according to the speed of light in vacuum? [duplicate]

Why is the speed of light 300 000 km/s? The speed of light is slower in glass, water and diamond. Is the speed of light in water, glass and diamond also absolute? Can we calculate the material such as ...
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### Is the Planck length frame dependent? [duplicate]

In accordance with special relativity, if we take into account length contraction then ideally the Planck length which is the smallest possible length possible should be frame dependent. Well we know ...
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### Planck Units of the Universe

Is it a coincidence, or is it chosen this way, that the size of the width of the universe is approximately 10^61 planck lengths, the mass is 10^61 planck mass, and the age is 10^61 planck time?
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### I am confused about the apparent fact that the numerical values of the Planck units are unchanged by exchanging $\hbar$ for $h$ [closed]

Can you confirm that each of Max Planck's original equations for Planck units included $h/2π$. If yes, does this explain how $ħ$ can be used In Planck's formulas without changing the numerical results ...
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### Why are all Planck base units square roots of expressions in $\hbar$, $G$ and $c$?

Section 2.3.1 "Base units" in "The International System of Units" defines length, mass and time as the dimensions of "base units". The corresponding Planck units are ...
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### What natural processes are most sensitive to exact value of fine-structure constant?

My intuition (game theory PhD here, no serious background in physics) suggests that of all naturally observable processes protein folding should have the strongest dependence on exact value of $\alpha$...
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### What is the evidence against a variable gravitational constant? [duplicate]

I understand that our main supporting evidence for dark matter is the anomalous speed of objects orbiting around the edges of distant galaxies. Is there a reason why dark matter solves this problem ...
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### Why does the assumption that speed of light $c$ is constant exist? [closed]

This is I'm sure a very basic and juvenile question. Why do we take the assumption that $c$ is constant in all situations to be a law? It does work out that it fits the standard model well but it has ...
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### What is the name of the physical quantity which is the gyromagnetic ratio divided by $2 \pi$?

I am reading Abramowitz and Stegun's Handbook of Mathematical Functions and I come across Table $2.3$, Adjusted Values of Constants. In there we see: Gyromagnetic ratio of proton: $\gamma$, with value ...
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### How do we know that the speed of light is constant everywhere, not just here? [duplicate]

It might well be that universal constants, say the speed of light, are only constant as far as we can tell in our chunk of the universe - in the same way that the Earth looks flat in the area you live....
1 vote
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### Does expanding universe length units also expand? [duplicate]

Information I have read an explanation about the expanding universe which represents our universe as a metal plate that tends to expand simultaneously in all directions while being heated. One can ...
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### Are the physical properties of Tin-coated metals the same as of the substrate?

I am trying to perform a finite element analysis to plot static structural loading on a bare copper and compare the results with the tin plated copper. I do have the information about the electrical ...
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### How to add Newton's constant to the metric function?

My main question is, is it possible to manually add Newton's constant $G$ to the metric function of a black hole? Is there such a possibility for Black Brane? How to add? Should it be added to the ...
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### How can i find the value of Planck's constant using Black body radiation?

I'm working for a physics project for Highschool, and i'm currently thinking about using black body radiation to measure Planck's constant. I know there is an option using the photoelectric effect but ...
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$$\frac{Bohr_s}{c} \approx \frac{1}{137} \approx \frac{1}{137.035999084} = \alpha$$ Where $Bohr_s$ is the speed of the electron in the Bohr model of the hydrogen atom and $\alpha$ is the fine ...
What is the dimensions of Newtons gravitational constant in arbitrary dimensions in terms of teh curvature radius? I am looking at entanglement entropy which goes as $S \sim \frac{ l_4^2}{G_4}$. This ...