# Tagged Questions

The result of sampling the property of a system

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### Has the speed of gravity been measured experimentally? How? [duplicate]

In Newtonian physics, changes in gravity propagate instantly. In general and special relativity, gravity propagates at the speed of light, $c$. From reading answers to questions about gravity on this ...
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### Does infinite actually occurs?

I tend to deny any claim that $X$ is infinite, where $X$ is anything from our observable universe. For instance: The number of grains of sand on Earth is infinite? No, it is a big number but not ...
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### Do we regularly measure and update our physical constants (By using the LHC)?

This question is motivated by sheer curiosity. I certainly do not expect that the free parameters we use in the standard model have changed in value since we started measuring them with a "modern" ...
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### Why is a silicon ball with an exact number of atoms a bad measure of mass?

I have some friends that are talking about why a silicon ball with an exact number of atoms is a good/bad measurement of mass (1kg) and things are getting pretty exotic. Is there a layman's ...
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### Are measurements of time and distance independent or always coupled by the speed of light

In texts about special relativity the light clock is usually described as a tool to measure time $t$. It counts how often light travels along a given distance $d$, i.e. $t = d/c$, where $c$ is the ...
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### How can the orbit of Jupiter's moons be used to calculate the speed of light? [duplicate]

How can the orbit of Jupiter's moons be used to calculate the speed of light? It seems this was one of the first methods and goes back to 1656.
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### How does one describe a state with a density matrix after measuring position?

My question is about position measurement in non relativistic quantum mechanics. I've been taught that when you measure the value of an observable for some state of a system described by $|\psi\rangle$...
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### How might we measure the size of a proton? (Without high energy collisions) [closed]

What methods could we use to measure the diameter of sub-atomic particles. Please think of something inventive, like an indirect technique. Or name some of the lesser known techniques that have been ...
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### What are the longest half-lives we can detect experimentally? What stops us going further? Are we trying to?

Xenon 136, apparently, has a half-life of 2.11×1021 years. This strikes me as a humongously long time to run an experiment, clocking in at about 11 orders of magnitude longer than the age of the ...