# 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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### Planck constant in geometrized units? [closed]

How do I calculate the value of the Planck constant in geometrized units? I cannot find its value anywhere.
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### How do I calculate ¨Newtonian constant of gravitation over $\hbar c$¨ to get to the value in NIST? [closed]

I know this is a silly question, the definition of the value is the formula for the value itself but I have tried putting the constants in and I am not getting the same answer. What am I doing wrong?
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### Why can we set $c$ and $\hbar$ to 1 when it changes the result? [duplicate]

So in my QFT course, my professor said that you can set $c$ and $\hbar$ to 1. And he gave us an example: $$E = mc^{2}$$ And then set $c = 1$: $$E = m$$ This seems completely ludicrous to me to do. ...
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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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### Bremermann's limit vs Planck frequency

Bremermann's limit, as maximum possible computation power or CPU total computing frequency, is known to be on the order $10^{50}~\text{Hz}/\text{kg}$. Why max computation frequency for unit mass can ...
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### Dimensions of momentum?

I am learning realitivity in college and in our class our lecturer explained four-momentum. When I was reading a book in QFT. it writes the momentum as $p^{\mu} = (E,p^i)$. Why is one of the ...
116 views

### Doubt about the constant $\kappa$ of Einstein's field equations

I'm trying to understand each of the terms in this equation intuitively but I'm having a little trouble. I know that we can represent the equation in the following way: $G_{\mu\nu}= \kappa T_{\mu\nu}$ ...
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### Why did my professor write down the Einstein field equations like this?

Ok, so I was taking an online course where the professor wrote down the Einstein field equations like this $$R_{\mu \nu }-\frac{1}{2}g_{\mu \nu }R = 8\pi G\: T_{\mu \nu }.$$ But I saw it most commonly ...
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### Confusion regarding the appearance of $\hbar$ in the eigenfunctions of momentum operator

The two images below are from different books. One has the $\hbar$ in the root below which seems right to me as that gets the dimensions correct but other does not have a $\hbar$. I am confused as to ...
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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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### Is the Bohr radius deprecated?

The Bohr radius ($a_0$ or $r_{\text{Bohr}}$) is a physical constant, equal to the most probable distance between the nucleus and the electron in a hydrogen atom in its ground state. It is named after ...
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### Derivation of $G=\frac{\hbar c}{m_\rm{P}^2}$

I read that the gravitational constant can be expressed in terms of Planck length. $$G=\frac{\hbar c}{m_\rm{P}^2}$$ What is the derivation of this relationship?
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### Is my friend right about omitting $c^2$ in world famous tiny equation?

I know $E = mc^2$ says that inertial mass of a system is equal to the total energy content of a system in its rest frame. My friend told me the $c^2$ can be omitted from this equation because that's ...
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### 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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### Would setting the ideal gas constant to $1$ yield an attractive natural temperature scale? [closed]

In this recent question, there was a comment 'The "zero point" of Kelvin is natural, but the scale is not'. This led me to wonder whether setting $R = 1$ in the ideal gas law would be an attractive ...
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### How to find the corrsponding expression after working with natural units $\hbar=c=1$?

If one does long calculations in natural units how does one find the right expression in let's say SI units in the end? I know that natural units make the calculations easier and also help to show ...
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### Grasping systems of units

Perhaps it's that I've grown up with the SI system of units, but there's something about other unit systems that doesn't sit well with me. For example, Coulomb's law in Gaussian units can be written ...
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### A question about natural/geometrized units

I had a question about the following document- Natural units I understand the conversion factors. But if you look at the tables, they take an SI unit, say 1 kg, convert it into geometrized units, say ...
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### What's the matter with Planck mass $M_P$ in Einstein-Hilbert action?

The Einstein-Hilbert (EH) action is often written as $$S_{EH}=\frac{c^4}{16\pi G}\int d^4x \sqrt{-g}R\tag{1}$$ and often as $$S_{EH}=\int d^4x \sqrt{-g}M_P^2 R\tag{2}.$$ Comparing (1) and (2), one ...
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### Are ‘fundamental measures’ a thing?

The question I want to ask is: What measures are needed to describe the physical world and what are the fundamental ones of those, in the proper sense of the word fundamental? But that might be too ...
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### Why is the mass dimension of the covariant derivative 1?

I'm reviewing an exam, and I can't figure this one out. I know the covariant derivative, but I'm not seeing how it necessarily has a mass dimension.
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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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### Planck units and special relativity

The fundamental relation of special relativity linking time to position is \begin{align} dx=cdt \end{align} I am performing a change of variable to rewrite it in terms of energies as a Force F ...
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### What is the Planck Pressure?

What exactly is the Planck Pressure? According to wikipedia the Planck pressure "is not found except for shortly after the big bang or in a black hole". But wouldn't the pressure inside a black hole ...