# 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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### 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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### 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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### 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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### 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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### 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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### 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 ...