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Questions tagged [physical-constants]

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Planck's Constant to the Kibble Balance

A tad confused on how the Kibble balance actually relates to the Planck Constant. There's most likely a very simple relationship that I'm missing (I feel this is a stupid question ..) If the Kibble ...
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1answer
60 views

On existence of other physical constants and the assumption in proof for special theory of relativity? [closed]

The only constant (that exist in the world) is speed of light in vacuum or c which is close to 3X10^8 m/s so 'all other constants should be variables (to some degree) w.r.t. this constant'. If this is ...
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Are all lengths multiples of the Planck length? [duplicate]

If the Planck length is the smallest unit of length that has any physical meaning then is it true to think that all the lengths are multiple of the Planck length?
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What is a free parameter?

Soft question here, but I was wondering just what exactly free parameters are? I have a murky understanding on the concept but I would much appreciate someone shedding some light on the matter. Is ...
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1answer
58 views

How I can get the numerical factor in the relation between string coupling and YM coupling?

I'm trying to understand some references about Wilson loops being used to test AdS/CFT. Some of them are Nadav Drukker, David J. Gross: An Exact Prediction of N=4 SUSYM Theory for String Theory ...
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Is it a coincidence that the maximum number of protons in a stable nucleus is on the order of $\alpha_s / \alpha_{EM}$?

The equivalent to the fine-structure constant for the strong interaction is about $\alpha_s \approx 0.5$ at the proton rest mass energy (and it runs pretty slowly with energy, so it's on this order ...
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1answer
47 views

Changes to Planck constant historically [closed]

I looked all over to get the various accepted value of the Planck constant since 1900. But there is never any record about the history of this mysterious number.
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${}$Can the gravitational constant $G$ be calculated theoretically? [duplicate]

In an answer to a previous post of the same question, it was stated that the gravitational constant can not be determined theoretically yet! From my perspective, the reason this is not yet possible ...
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1answer
47 views

Can a sequence associated with a physical constant lead to natural laws?

I wondered if we know the universal constants (e.g. gravitational constant, etc.) with high enough precision, and try to find a sequence of which it is part of, or a sequence that converges to it, ...
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50 views

Physical quantity of energy and it's dimension in the article

I am reading an article about Permutation glass by Mobolaji Williams and not sure about the unit (value/dimension) of energy on Fig.3. If $k_BT_c$ is measured in Joules, then I am not totally sure ...
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83 views

Can anyone explain the Planck area?

First of all, I am not an expert. I was reading about the holographic principle and came across the Planck area. It says that Planck area is the square of Planck length and there were some pictures ...
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1answer
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Is there a way to test whether dimensionless physical constants are rational?

This question is related to a question asked 3 years ago on SE (I was not the OP), but not quite the same. I would like to know if there is a way to test whether a dimensionless physical constant is ...
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If light speed determines the rate of time, could the speed of light be varying over time and we be unaware of it?

Is this a valid implication/alternative explanation of universal red-shift? I thought I'd ask this when I read this question about light clocks. I had speculated this way: If light speed determines ...
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105 views

Numerical coincidence with constants

Is there a reason why $$\dfrac{h}{ec}\sim 1.38\cdot 10^{-23} V/(m\cdot s^{-2})$$ ? Dimensionally, the Unruh effect temperature reads $$T_U=\dfrac{\hbar a}{2\pi k_B c}$$ from which the ...
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2answers
60 views

When using Planck's constant, how do I know when to use electron volts or joules?

Planck's constant may be written as 6.63 x 10^-34 Js or 4.14 x 10^-15 eVs right? But how do I choose which one I should use to figure out an answer to a question?
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Are there known expressions for the running of $\alpha_{em}(Q^2)$ and $\sin^2(\theta_{W})$?

I'm doing some integrations in Hadron physics, involving form factors and the like, and these terms show up often. I can get okay results if I use the values these couplings take at $Q^2=0$, but a ...
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1answer
120 views

Undoing problems caused by setting $c = 1$ { or “Undoing $c = 1$” }

In the mathematical derivation of equations for physics, and involving wave propagation in particular, the propagation speed at the start of the derivation is often set to one (c = 1). I am working ...
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Does $G$ effectively change in an expanding metric?

Assume that we have a flat FRW metric expressed in conformal time $\eta$ so that the line element is $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where $a=1$ at the present time $\eta=0$ and the ...
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3answers
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Is the value for the speed of light in vacuum irrelevant? [duplicate]

Does it matter that the speed of light, $c$, is 299,792,458 meters per second in a vacuum? Would the universe be different if $c$ were, for example, 1 meter per second? Or should things be the same, ...
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Can we get the values of $G$, $h$ and $c$ to be numerically equal if we use a convenient system of measurement?

The speed of light in vacuum is approximated to be $3×10^8\ ms^{-1}.$ But, if we change the units, we can get a different number. For example, it won't be $3×10^8$ if we used $ft\,s^{-1}$ instead of $...
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112 views

Importance of $\alpha/\pi$

What is the physical meaning or importance of $\alpha / \pi$ ($\alpha$ being the fine-structure constant)? I have seen that it is the parameter in the expansion of the g-factor but I can not reach to ...
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1answer
33 views

Light speed in vacuum over space and time [duplicate]

Is the light speed the same, constant, in all the universe locations? I mean, is it he same in the surface of the Sun compared with speed in space inter galaxies? Also: is light speed now the same as,...
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2answers
121 views

How does one obtain $\hbar$ as $\frac{h}{2\pi}$?

I'm reading Dirac's Principles of Quantum Mechanics. He defines $\hbar$ to be the real number satisfying the following relation $$ uv - vu = i\hbar[u,v]$$ where $u$ and $v$ are dynamical variables, ...
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1answer
63 views

The expression of the fine-structure constant post-May 2019 [closed]

Those of us who are engineers were never fond of the common expression from physicists that $$ \alpha = \frac{e^2}{\hbar c} $$ implying that the units of the elementary charge are in "$\sqrt{hc}$". ...
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1answer
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Deriving Planck's Constant from Wien's Displacement Law

So I'm reading an introductory book on Quantum Theory (David Park, 3rd ed.) and I am having trouble with the following question: "According to Wien's displacement law, the wavelength $\lambda_m$ at ...
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1answer
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Why $µ_0$ and $ε_0$ are not considered to be $4π$ and $1/4π$? [closed]

I did some algebra... In Planck unit, if make $\mu_0 = 4\pi$ and $\epsilon_0 = \frac{1}{4\pi}$ you get: $$\mu_0 = 4\pi \cdot \frac{m_p l_p}{t_p^2 I_p^2} = 1.2566368452237765 \cdot 10^{-6} N \cdot A^...
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Update: Does the Planck scale imply that spacetime is discrete? [duplicate]

This question has already been asked Does the Planck scale imply that spacetime is discrete? however I'm wondering if there has been any change in the community since it was asked roughly 8 years ...
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6answers
176 views

Conversion of $R = 8317\ \mathrm{\frac{J}{kg\cdot mol\cdot K}} $ to $ 8317\ \mathrm{\frac{J}{kg\cdot mol\cdot {^\circ C}}}$

I'm trying to understand the unit conversions when one unit is obtained by shifting the value of another one by some constant. In particular, the unit conversions from Kelvin to Celsius scale: $$T(\...
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1answer
109 views

Is there a physical observable with the same units as $c/G$?

Dividing the speed of light $c$ by the gravitational constant $G$ yields the dimension mass*time/area or mass/(length * speed) Is there a physical quantity used in textbooks with this dimension? I ...
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Planck's constant calculated by photoelectric effect laboratory is off

I conducted an experiment today where I had to use a photocathode of unknown material (model: Daedalon Corporation Photoelectric Effect EP-05) and study the photoelectric (PE) effect to make a ...
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1answer
88 views

Regarding the Boltzmann entropy formula, is the Boltzmann constant really arbitrary?

In the top answer to this question (Is the Boltzmann constant really that important?) I read that the Boltzmann constant is just a dummy factor which converts energy to temperature. But that allows ...
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1answer
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Which reference should I use for molar masses of chemical elements?

This question is more related to the academical application of Physics than to Physics per se. I'm currently doing my thesis, and it involves a lot of molar masses in the calculations. I would like to ...
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What is a global limit in gauged supergravity?

Does anyone know what a global limit (rigid limit), where the gauge coupling constant is zero, in gauged supergravity is?
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2answers
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How did Maxwell figure out the speed of light?

The Wiki article is about 2 graduate years of physics beyond my understanding. What is a good high-school rendition of his thought process: regarding his use of the "distributed capacitance and ...
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Are particle decay times universal constants?

Basically what the title says. For neutrinos, for example.
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What is the dimension of the weak gauge field couplin constant NOT in natural units?

What is the dimension of g and g', NOT in natural units, but in terms of mass, length, time, and permittivity?
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What parameters determine the probability of virtual photon emission/absorption?

Suppose an electron is producing an electric field by emission of virtual photons and interacting with other particles. What parameters determine the probability that it will emit at least one virtual ...
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1answer
48 views

Precise value of spin in Nms?

What is the exact value of the spin of a particle with a 'spin' of 'one'? In units of Nms (Newton-meter-second)? And does a boson really have a spin of exactly one, or has that been 'normalized'? ...
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3answers
213 views

Redefinition of everything on May 20th, 2019 [closed]

A couple of issues: So after May 20th, 2019, what exactly will be the defined value of $\hbar$? What will be the defined number of elementary charges in a Coulomb? Then $\mu_0$ and $\epsilon_0$ will ...
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Why is the resolution or measurement uncertainty of $G$ so bad?

I sorta know how the Cavendish apparatus works, and I know that "Gravity is an extremely weak force" for subatomic particles in comparison with the E&M or nuclear forces that act on these ...
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45 views

Schrödinger equation from Dirac–von Neumann axioms

Using Dirac–von Neumann axioms, it isn't too difficult to derive $$\frac{d}{dt}\left|\psi\right>=ikH\left|\psi\right>,$$ where $H$ is some hermitian operator. However, how would one show that $k=...
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103 views

Electrostatic in 2D: dimensional analysis

After reading this very interesting post about the electric field and the electric potential of a point charge in 2D and 1D, I've understood that, for the $2D-$case, the following formulas hold: $$ \...
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2answers
50 views

When was Coulomb's constant made/established?

Charles-Augustin de Coulomb lived from 14 June 1736 – 23 August 1806. Coulomb's constant is $$k_{\text e}=\alpha\frac{\hbar c}{e^2},$$ a form of Planck's constant is included, but Max Planck lived ...
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46 views

Why is it important to measure permittivity and permeability complex of materials?

I know that in general, $\epsilon(r,\omega)$ and $\mu(r,\omega)$ are complex tensors, so now how do you resolve maxwell equations? I don't understand why there are techniques to measure them if you ...
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1answer
83 views

Time-varying gravitational constant a 'well-posed' question?

To my knowledge, some famous physicists such as Dirac and Dyson (perhaps among others) have advocated for the possibility that Newton's gravitational constant $G_N$ might be "time-dependent". I have ...
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1answer
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How did Newton use his theory without the value of $G$? [duplicate]

I know that Newton extensively used his theory of gravity to calculate many things, but how did he do so without actually knowing the value of the gravitational constant $G$? Did he estimate it ...
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2answers
95 views

Are $2$ and $1/2$ universal constants? [closed]

For example, if the equation for energy were: $$E = mc^{2.713397972993}$$ clearly $2.713397972993$ would be a universal constant. And in the Einstein field equation: $$R_{\mu \nu} - \tfrac{1}{2}R \...
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1answer
220 views

Fine Tuning of the Universe

I'm an A level student looking into the fine tuning of various constants. Physicists explain the extensive effects that would happen if these constants were to be changed/different and hence, how this ...
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82 views

What is the role of $\hbar$ in quantum mechanics? [duplicate]

Planck's constant $\hbar$ appears in the Schrodinger equation: $$i\hbar \frac{d|\psi\rangle}{dt}\ = \hat{H}|\psi\rangle$$ which implies for stationary states, $$|\psi(x,t)\rangle=e^{-iE_n/\hbar}|\...
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1answer
157 views

What's going on with the Hubble constant?

I'm doing a small project on LIGO for a third-year project at University. I want to write something about how LIGO can be used to measure $H_0$, which would be useful because the current state of the ...