Questions tagged [absolute-units]
Absolute units, or natural units, are a system of units where certain dimensionful constants are set to 1. This often simplifies various formulae.
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How exactly are natural units used in everyday life? [closed]
I’m currently trying to understand natural units, and I have read that natural units are used frequently in everyday life. For example: “I live 100s away".
But I do not understand how this is an ...
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Books or other sources for natural units, Planck units, dimensional analysis etc. for someone with only high-school physics knowledge
I'd like to know more about natural units, Planck units, dimensional analysis, etc., and things like how units are "created" by man or by the universe, universal constants and where they ...
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How to convert quantities between SI units and a natural unit system?
Let's say I'm working in a natural unit system defined by a set of physical constants set to dimensionless numbers. How can I convert quantities between that natural unit system and a more ...
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What would be the Earth acceleration in the units where $\hbar=c=1$? [closed]
In my calculations, I have to use the units in which the Planck constant and light velocity must be taken as unity.
Now, what would be the value of Earth's gravitation force $\implies g = G\cdot\frac{...
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Is the exact definition of the Planck units important?
Wikipedia says there are other, possibly better ways to define the Planck Units based on whether we want to factor in or out a $2$ or a $\pi$ or a $4\pi$ as the case may be. Most of them represent ...
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What is the significance of Planck units?
There are many questions asked about different Planck units.
This question is just a generalization of all of those questions.
Planck units are considered to be natural units. The thing I don't ...
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In natural units, where $\hbar = c = 1$, what is $G$?
This seems like a simple question, but I cannot wrap my head around it. If $\hbar = c = 1$ then length is time, and mass is inverse length or inverse time. Hence $G$ should have dimensions of length ...
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What is the decay width and why is it given in energy units?
I'm reading Thomson, Modern Particle Physics, and in chapter 16 author says that the decay width of the Z boson is $\Gamma_Z =2.452 \pm 0.0023 \,\mathrm{GeV}$. He also says the total width of the ...
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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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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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Is there any difference in dimensionless quantities, $G=1$ and the fine sturcture constant, from prediction prospective?
In the Planck unit, $G=1$ as dimensionless. The fine structure constant is also dimensionless. I had an impression that predicting the speed of light is not meaningful, since it has unit/dimension and ...
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How to rigorously put back dimensions in equations involving natural units?
I was watching the first lecture of Special Relativity by Leonerd Susskind (link:Youtube) whereby setting the speed of light to 1, i.e. $c = 1 \dfrac{[l]}{[s]}$, where $[l] = 3 \cdot 10^8 [m]$, we get ...
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What is the Planck scale magnetic field strength?
Using the constants $\mu_0$ (or $\varepsilon_0$), $c$, $\hbar$, $e$ and $G$, it is possible to define two quantities with units of magnetic field :
\begin{align}
B_1 &= \sqrt{\frac{\mu_0 c^7}{\...
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Is the Planck force a truly "Planck unit"?
The Planck force appears to be defined as the ratio of the Planck energy to the Planck distance, $ F_P = E_P/l_P $ that can be rewritten as $$ F_P = \frac{ E_P }{ l_P} = \frac{ c^4 }{ G }. $$
Isn't it ...
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Is there an absolute zero for internal energy?
Of course we can arbitrarily define a reference point to call zero. However, I was under the impression that internal energy and enthalpy were relative and had no absolute zero, and only changes or ...
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What's the link between Planck Mass squared and $1/8πG$?
Good morning,
In an equation of an article, we said that 1/8πG = Mass of Planck^2. But 1/8πG = 596175243.8, is much larger than the Planck Mass^2 = 1.383*10-16 kg. Is there a conversion to do? If not, ...
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Understanding the units of cosmic string number density
I am reading this old paper: https://arxiv.org/pdf/1309.6637.pdf and trying to work out the units in equation 63. It gives the number density of cosmic strings in the radiation era as
$$
\frac{n(\ell,...
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What does the qualifier NOMINAL exactly refer to?
This question may not specifically belong to the physics domain, and rather perhaps to some engineering one, yet I couldn't find a better place to ask it either, trying my chance here...
From time to ...
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What is the logic behind Planck units?
I was curious to know the logic behind “Planck Units”, I read this question but did not understand it. Do you have a better (simpler) explanation for setting $c = G = \hbar = 1$?
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Why do universal constants have the values they do?
This is meant to be a generic question of the type that we get repeatedly on this site, in different versions:
The origin of the value of speed of light
The gravitational constant G theoretically?
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Why do we treat the action as dimensionless in QFT?
When determining whether the couplings in a QFT Lagrangian are relevant/irrelevant/marginal, we set $\hbar = c = 1$ and use the fact that the action is dimensionless to find the dimensions of the ...
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Physically, how are constants like $c$ or $\hbar$ allowed to be unitless in natural units?
This seems like a rather elementary question but it has been causing me some troubles.
Suppose I want to construct a unit system were $c = 1$ and $\hbar = 1$.
The constraint $c = 3 \times 10^ 8 m/s= 1$...
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When working with natural units, how do my other variables change? [closed]
I'm trying to plot energy splitting
as a function of $a$ (where $g=1$). When I set $\hbar=1$ such that $a=[t^{-1}]$, how does the value of this variable change to keep the equation consistent?
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Confusion on Absolute Temperature Scales used in solving Carnot Cycle problems
I have a Carnot Cycle problem where the temperatures of the hot and cold reservoirs are given in Celsius and I'm asked to solve for the cycle efficiency. Of course, My first step is to convert Celsius ...
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Meaning of the Planck Temperature
I don't understand what makes the Planck Temperature the "absolute hot". To my understanding Temperature is just a measure of the kinetic energy of the particles, so is the Planck ...
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Why is the Schwarzschild radius associated with the tiniest micro black hole formed by a Planck mass twice the Plank length?
If one calculates the Schwarzschild radius, $r_s$, of a Plank mass $m_p=2,18*10^{-8} (m)$ one gets:
$$r_s=2{\frac{G{m_p}}{c^2}}=1,48*10^{-27}*2,18*10^{-8}=3,22*10^{-35}(m)$$
Now the Planck length $...
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Isn’t natural units prone to mistakes?
Suppose I am deriving a length contraction formula using natural units. If I arrive at $L = L_0 \sqrt{1 - v^2}$, I know that I should divide $v^2$ by $c^2$ to get the correct answer in SI units. But ...
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How do I determine what physical constants can be combined to derive a set of natural units?
I am looking into natural units (units of measurement based only on universal physical constants). Different systems of natural units use different physical constants as their defining constants. If I ...
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Factors of $c$ when giving masses in natural units?
I am starting a course on particle physics, and have been introduced to natural units. I am slightly confused, because we are using 'natural units', and yet masses are stated as, for instance, $139.6\ ...
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What physics laws justify Planck's units? [duplicate]
It is usually said that Planck units have no scientific ground, yet they are useful indeed because many laws collapse, make no more sense at, say, Plancks length or time.
Can you mention a couple of ...
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Numbers in Dimension Analysis
When I learned dimensional analysis for the first time, I know that the dimension, for example, of the velocity can be written like this $$[V]=LT^{-1},$$ but in QFT the action for example is ...
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When we set $c=1$ and $\hbar=1$, why is energy still measured in $eV$?
When we set $c=1$ and $\hbar=1$, we often see in particle physics that mass and energy are expressed in terms of $eV$.
This doesn't make sense to me. If we are choosing a new unit system where for ...
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Why are there just 3 main units ($L$,$T$,$M$) in physics?
Most physics books define physical units in terms of length, time and mass. Some books add temperature. And yes, the SI unit system has 7 base units, but some are clearly redundant.
Why are exactly ...
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Solution to Klein-Gordon equation in terms of $\vec{p}$ and $\vec{k}$
A general solution to the Klein-Gordon equation can be written as:
$$\phi = \int d^3k \frac{1}{(2 \pi)^3 \sqrt{2\omega_k}} \left(a(\vec{k})e^{-i(\omega_kx_o-\vec{k}\cdot \vec{x})}+a^{\dagger}(\vec{k})...
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A question regarding dimensional analysis and a "Planck matrix"?
Let:
the speed of light in a vacuum, $c$,
the gravitational constant, $G$,
the reduced Planck constant, $\hat{h}$,
the Boltzmann constant, $k_B$
the electric constant, $\epsilon_0$
with dimensions $...
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How to convert $\rm cm^{-1}$ to $\rm eV/Å^2$? [closed]
As in the title, how to convert $\rm cm^{-1}$ to $\rm eV/Å^2$? Å stands for angstrom.
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What happens as you approach/cross the Planck temperature?
According to IFLScience, above the Planck Temperature (absolute hot) conventional physics breaks down.
My question is what happens as you approach this temperature, and, if it is possible, what ...
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Does electromagnetism have no free parameters?
In SI units, Maxwell's equations (in vacuo) seem to have two free "parameters" or "constants". The vacuum permittivity, however, can be eliminated by properly redefining the ...
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Do the Planck voltage and the Planck current have a natural physical interpretation in classical general relativity?
Most Planck units are a product of powers of all three of $\hbar$, $c$, and $G$, so we will not be able to fully understand their physical significance until we have a full theory of quantum gravity. ...
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What is the significance of Planck charge?
It seems for me that Planck units are somehow connected to limits where our current knowledge breaks down because of (quantum) gravitational effects. Please correct me if I'm wrong.
For example ...
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What happens to the $2\pi$ factor in natural units?
In natural units when we define $c=\hbar=1$ and we have that energy and mass have the same units because of $E=mc^2$. The same happens for time and space due to $x=ct$. Now, when we want to relate ...
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Units of a scalar field
Consider the Lagrangian density
$$\mathscr{L} = \frac{1}{2} \partial_\mu a \partial^\mu a + \frac{m^2}{2} a^2.$$
I understand why $[a]=m$, i.e. $a$ has mass dimension one. What and why are the units ...
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Is temperature of 1 Kelvin equivalent to 1 eV in natural unit?
We know that the Boltzmann's constant, $k_B$=8.617 $\times$ $10^{-5}$ eV/K. Now in the natural unit, $k_B=1$.
So can I say, in the natural unit, 1 K temperature is equivalent to 1 eV in energy? 300 K ...
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Using $ct$ axis instead of $t$ axis in special relativity
I've recently started studying the concept of space-time diagrams in special relativity, and I came across the concept of representing the time axis using $ct$, with units being that of length. Now I'...
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Subatomic natural units
In High Energy Physics it seems to be common use to measure everything in terms of eV powers, by assuming $\hbar = c = 1$ (dimensionless). Often times this system of units is referred as Planck units, ...
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What does it mean for $2\pi = 1$ in a "private system of units"?
I saw the following image of an excerpt from Robert Mill's Tutorial on Infinities in QED, floating about the internet:
The book is available here, however I don't think I have access to it, unless ...
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Why don't we take the universal gravitational constant $G$ to be equal to 1 in $F= \frac{Gm_{1}m_{2}}{r^2}$?
In the derivation of Newton's Second Law, we get to an equation $F=kma$. Since this equation is essentially defining force, Newton could have taken the value of $k$ to be anything. For the sake of ...
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Units in the geodesic equation / Schwarzschild metric
Most textbooks define the geodesic equation for a particle with unit mass, such that it looks like:
$$ \ddot{x}^{\mu} + \Gamma^{\mu}_{\alpha \beta} \dot{x}^\alpha\dot{x}^\beta = 0$$
Where "dot&...
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Einstein's Field Equations differ by a factor of $\frac{1}{c^4}$. Why is that?
I am trying to familiarize myself with General Theory of Relativity. I am by no means an expert in the field, and I am doing this as my own hobby. At any rate, I have come across Einstein's Field ...
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How can geometrized units have more than one constant equal to 1?
I can understand how you could manipulate units to make a certain constant equal to $1$, like $c$ or $G$, et cetera. But how can you make it so two constants (in this case $c$ and $G$) are equal to $1$...
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