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Results tagged with physical-constants
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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.
7
votes
Can Planck's constant be derived from Maxwell's equations?
If you're talking about deriving the value of Planck's constant, then no, that is not possible. The value is simply a consequence of our chosen unit system.
If you're talking about deriving the fact …
- 77.3k
18
votes
Accepted
Do we take gravity = 9.8 m/s² for all heights when solving problems? Why or why not?
No, the value $9.8\frac{\mathrm{m}}{\mathrm{s}^2}$ is an approximation that is only valid at or near the Earth's surface. You can go a few miles up or down and it'll still be good enough, but once you …
- 77.3k
50
votes
Accepted
Is speed of light and sound rational or irrational in nature?
Something I posted on reddit answers this question quite well, I think:
"Rational" and "irrational" are properties of numbers. Quantities with units aren't numbers, so they're neither rational nor …
- 77.3k
2
votes
Derivation question of WKB method
It does not mean each term of order of $\hbar$ have to equal zero since $\hbar$ is constant, does it?
It's true that $\hbar$ is a known constant in the real world. But the theoretical model you'r …
- 77.3k
10
votes
Accepted
Planck time, distance, mass? Why do we take those values?
Excellent question! As far as I know, there is nothing like a rigorous justification that quantum gravity effects should kick in at the Planck mass/length/time. But there is an intuitive justification …
- 77.3k
6
votes
What is the significance of Planck force?
There is no particular physical significance; it's just a unit. Of course, in any system where such a large force is exerted, our current theories should not be accurate, and a quantum theory of gravi …
- 77.3k
3
votes
What is the opposite of the Planck length?
Planck's constant is not at one end of a spectrum, so it has no "opposite" in this sense. In particular, it's not a minimum length. There is an argument for a minimum measurable length that is on the …
- 77.3k
25
votes
Accepted
How can Planck units be consistent with conflicting dimensions of mass?
The short answer is that it can, if $M = 1 = M^{-1}$. In this way of looking at it, all quantities in Planck units are pure numbers.
The longer answer is that there are two different ways of thinki …
- 77.3k
52
votes
Accepted
Does the speed of light in vacuum define the universal speed limit?
It's the second one: the reason the speed $299792458\ \mathrm{m/s} = c$ is special is because it's the universal speed limit. Light always travels at the speed $c$, whatever that limit may be.
The re …
- 77.3k