As of May 31, 2023, we have updated our Code of Conduct.

# Tag Info

Accepted

### How do you make more precise instruments while only using less precise instruments?

I work with an old toolmaker who also worked as a metrologist who goes on about this all day. It seems to boil down to exploiting symmetries since the only way you can really check something is ...

### Can photons be detected without being absorbed?

It is indeed possible, as demonstrated by the group of Serge Haroche in 1999 using so-called quantum non-demolition Ramsey interferometry. The idea was to observe the presence or absence of a photon ...
Accepted

### How do laser "tape measures" work?

EDIT updated (improved) description of phase detection circuit There are two principles used in these systems. The first is the time-of-flight principle. As you noted, if you wanted to get down to 3 ...

### How do I experimentally measure the surface area of a rock?

I would ignore answers that say the surface area is ill-defined. In any realistic situation you have a lower limit for how fine a resolution is meaningful. This is like a pedant who says that hydrogen ...
Accepted

### Why is a leading digit not counted as a significant figure if it is a 1?

Significant figures are a shorthand to express how precisely you know a number. For example, if a number has two significant figures, then you know its value to roughly $1\%$. I say roughly, because ...

### How do I experimentally measure the surface area of a rock?

The problem is that as you increase the measurement precision, so will increase the result you get. The result of a meaningful experiment should converge with the increase of the precision, this does ...

### How do I experimentally measure the surface area of a rock?

The way I would do it is to first dip the rock in thinned fingernail polish. Let that dry, and then dip the rock into hot liquid wax. Let the wax cool. Peel the wax off the rock and measure the ...

### How do you make more precise instruments while only using less precise instruments?

The more you measure things and add or multiply those measurements, the greater your errors will become. Not necessarily. If the errors in a series of measurements are independent and there is no ...

### How do I experimentally measure the surface area of a rock?

I would prefer an accuracy to at least one hundredth of the stone size. Weigh the stone. Dip the stone in thin paint; let excess drip off. Weigh the stone. Repeat steps 1-3 with a square, 1 cm2 ...
Accepted

### How is temperature defined, and measured?

Definitions First and foremost, temperature is a parameter defining a statistical distribution, much as the statistical parameters of mean and standard deviation define the normal probability ...

### Can photons be detected without being absorbed?

Yes, according to a paper by the Rempe group [Science 342, 1349 (2013)], photons can be detected after reflection by an optical resonator that contains a prepared atom in a superposition of two states....
Accepted

### Why can interaction with a macroscopic apparatus, such as a Stern-Gerlach machine, sometimes not cause a measurement?

It's a very good question, since indeed if the original Stern-Gerlach machine had a well-defined momentum, then you are right that there could be no coherence upon rejoining the beams! The rule of ...

### How long is a second?

A second is a second long by definition, but if you measure any time in seconds, the number of seconds you infer will be subject to an error of at least $\mathcal O(10^{-15})$ because of the ...

### How do I experimentally measure the surface area of a rock?

Fully wrap stone really tight in aluminium foil. (Of course it will crinkle; press the crinkles tightly down.) Soot the whole thing with a candle, just enough so it's completely black. Carefully ...
Accepted

### What is the current limit for a photon's rest mass?

The limit in the 2020 Particle Data Group Summary is $m_\gamma < 10^{-18}\mathrm{eV} \approx 10^{-27}m_\text{proton}$, based on a 2007 analysis of the magnetohydrodynamics of the solar wind. The ...
Accepted

### How is the speed of nucleons in the nucleus measured?

If you shoot an electron or a proton at a nucleus at moderate energies (a few hundred $\mathrm{MeV}$ to a few $\mathrm{GeV}$) it will usually either bounce off the whole nucleus or break up the ...

### How do you make more precise instruments while only using less precise instruments?

One thing I haven't seen mentioned is amplification. Amplification: Imagine you have a lever that is 10 cm on one side of the pivot and 1 m on the other. Then any change in position on the short side ...

### What does it mean that dimensionless physical constants cannot be calculated but only measured?

It may be helpful to point out that most dimensionless physical quantities (like $\alpha$ or $e$, under a suitable choice of natural units) are not unambiguously “calculable” or “non-calculable” ...

### What's exactly the new definition of kilogram, second and meter?

The SI system is now defined entirely by physical constants. There are no more “prototype” artifacts. How it works is thus: https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-concise-EN....
Accepted

### If the LHC-calculated mass of the Higgs is wrong, how long will it take to determine this with confidence?

Most of the reproduction of results in particle physics comes from two sources: Competing experiments running nearly simultaneously. In this case both ATLAS and CMS got comparable results. Now, they ...
Accepted

### What is the quantum mechanical definition of a measurement?

Until we have an accepted solution of the Measurement Problem there is no definitive definition of quantum measurement, since we don't know exactly what happens at measurement. In the meanwhile, ...

### How do you make more precise instruments while only using less precise instruments?

That's a really nice one! I'm not an expert on experiments and measurements but this is how I see it: The ultimate calibration tool is always nature. We pick special phenomena which rely on certain ...
Accepted

### Why don't we use absolute error while calculating the product of two uncertain quantities?

It basically comes from calculus (or more generally just the mathematics of change). If you have a quantity that is a product $z=x\cdot y$, then the change in this value based on the change of $x$ and ...
Accepted

### What does it mean that dimensionless physical constants cannot be calculated but only measured?

In "natural units", we set $\hbar=c=\epsilon_0=1$. In these units, the equation you wrote down becomes \begin{equation} \alpha = \frac{e^2}{4\pi} \end{equation} In this notation, it is ...

### How do laser "tape measures" work?

The timing circuit doesn't have to run that fast. It just needs a time-to-digital converter which has a high enough resolution (0.1ns is nearly trivial with off the shelf CMOS technology) and then it ...

### How do I experimentally measure the surface area of a rock?

The task isn't well-defined. Do you include cracks? If yes, you'll see finer and finer cracks adding to the surface, and in the end you'll be at atomic level and find it hard to even define what's ...

### How can you measure the zero resistance in a superconductor?

You can take a superconducting magnet and set it up to run in persistent mode. Here, you have a current running around and around in a loop of superconductor with no external source. If the current ...

### How can you measure the zero resistance in a superconductor?

You take a real resistor with small resistance, measure the current, then you put your superconductor in series with it. If it's really superconducting, the current stays the same.