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Results tagged with physical-constants
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user 199630
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.
3
votes
Newtonian gravity equation / Big $G$; not duplicate from search results
The period of the Hulse-Taylor binary, a system 21,000 light-years away, is decreasing at just the rate predicted based on gravitational waves carrying away energy, using the standard value of $G$ tha …
- 52.2k
3
votes
Accepted
The expression of the fine-structure constant post-May 2019
My prediction is that the 2019 metrological redefinitions will have absolutely no impact on how theoretical physicists use natural units. They will continue to think of $\hbar$ and $c$ as $1$, and of …
- 52.2k
2
votes
Accepted
Dimensionless quantity using fundamental constants
It has the value $4.1\times10^{88}\text{ s}^{-2}$ so it isn’t dimensionless: it has the dimensions of inverse time squared. You can rewrite it more simply in terms of the Planck time
$$t_P=\sqrt{\fra …
- 52.2k
2
votes
Is there a way to test whether dimensionless physical constants are rational?
This is a meaningless question. Dimensionless physical constants such as the fine-structure constant or the muon-electron mass ratio can be measured only to some limited precision. To know whether a n …
- 52.2k
5
votes
Are particle decay times universal constants?
Physicists think that the half lives of various kinds of unstable particles, measured in the rest frame of the particle, are the same everywhere in the universe. In that sense you could call them univ …
- 52.2k
1
vote
Accepted
How to deal with constants on computer simulations?
You can map linearly from pixels to real-world distance units. For example, if you are doing a solar system simulation out to Pluto, you might want it to cover a square 15 trillion meters on a side si …
- 52.2k
3
votes
Why $µ_0$ and $ε_0$ are not considered to be $4π$ and $1/4π$?
They do indeed have these values in Planck units. This is because in Planck units $1/4\pi\epsilon_0$ and $c$ are set to $1$, and
$$c=\frac{1}{\sqrt{\mu_0\epsilon_0}}.$$
Planck units are not in every …
- 52.2k
3
votes
Is $\hbar, c, e$ truly independent?
$\hbar$ was initially introduced during $E=\hbar\nu,$ thus connected to Lorentz/Maxwell $c$ and electron orbits which carries $e$.
The historical context in which $\hbar$ was introduced has nothi …
- 52.2k
9
votes
Why was the Planck constant $h$ fixed to be exactly $6.62607015\times10^{−34}\text{Js}$ and ...
It is an updated experimental value that matches measurements by Kibble balances and by counting atoms in silicon spheres to determine Avogadro’s number. The 2010 measurement was presumably consistent …
- 52.2k
5
votes
Accepted
What is the significance of Planck units?
All Planck units are constructed from $c$, $\hbar$, and $G$. The speed of light $c$ is the defining constant of Special Relativity. Planck’s constant is the defining constant of quantum mechanics. New …
- 52.2k
4
votes
Why does the fine-structure constant $α$ have the value it does?
One theory is that we live in a multiverse where physical constants such as $\alpha$ are different in different universes. This theory is speculative but based on plausible physics such as cosmic infl …
- 52.2k
5
votes
Accepted
Can we set cosmological constant equal to one?
Yes, you can do this if you want, because $\Lambda$, $\hbar$, and $c$ have independent dimensions, just like $G$, $\hbar$, and $c$ do. If you do, $G$ will be the dimensionless number $10^{-60}$, so yo …
- 52.2k
15
votes
The fine structure constant
Mathematical constants such as $\pi$ and $e$ have values that are determined by their definitions. For example, you can define $\pi$ as the ratio of the circumference of a circle to its diameter, and …
- 52.2k
4
votes
Accepted
Can anyone explain the Planck area?
The Planck length is usually defined as
$$l_P=\sqrt\frac{\hbar G}{c^3}\approx 1.6\times 10^{-35}\,\text{m},$$
where $\hbar$ is the reduced Planck constant, $G$ is Newton’s gravitational constant, an …
- 52.2k
2
votes
Accepted
Derivation of $G=\frac{\hbar c}{m_\rm{P}^2}$
It’s simply because the Planck mass is defined as
$$m_\text{P}=\sqrt{\frac{\hbar c}{G}}\approx 21.8\text{ micrograms}.$$
This particular combination of the reduced Planck constant, the speed of light, …
- 52.2k