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71 votes
Accepted

How was the first atomic clock calibrated?

More specifically, caesium atomic clocks realize the second (see this Q&A for the meaning of realization) or, said another way, they are a primary frequency standard. Generally, when a new primary ...
Massimo Ortolano's user avatar
42 votes

What's the problem with my Young slits experiment?

The angle between maxima in the double-slit pattern is $$ \theta \approx \frac\lambda d $$ for wavelength $\lambda$ and slit separation $d$. I wild-guess that the slits in your photograph are about 5 ...
rob's user avatar
  • 91.5k
35 votes

Given fluids expand non-linearly, how were physicists able to make a linear temperature scale?

There are physics answers to your question. To answer your worry at the level of the thermometer scale we see in our house thermometers, where the temperature is given equal intervals from -30C to +...
anna v's user avatar
  • 234k
23 votes

Is there any advantage in stacking multiple images vs a single long exposure?

Stacking is something that is done all the time in infrared astronomy. This is done because CCD technology doesn't work for wavelengths in the range of roughly 2 to 10 microns, and beyond, so they use ...
Sean E. Lake's user avatar
  • 22.7k
23 votes

Given fluids expand non-linearly, how were physicists able to make a linear temperature scale?

Simplistically, you can always define a linear temperature scale. For instance, you could call the freezing and boiling points of water (at some standard pressure) 0 and 100 and then construct a ...
Tony's user avatar
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22 votes
Accepted

How is time measured in particle experiments?

The Higgs is a challenging example because the tabulated quantity is the decay width $\Gamma$, from which a mean life $t≈\hbar/\Gamma$ is inferred. That is, nobody starts a clock when the Higgs is ...
rob's user avatar
  • 91.5k
20 votes

Given fluids expand non-linearly, how were physicists able to make a linear temperature scale?

There are already great answers, but I would like to address how one could operationally define temperature. As Tony already pointed out, we define a temperature scale that is linear. The scale ...
Superfast Jellyfish's user avatar
17 votes
Accepted

How to fix a bad KF flange?

I have fixed flanges like this by sanding. If your flange is aluminum, this won't be too hard; if it's stainless, it might be fairly tedious and painful! Start with a coarse-grit sandpaper, around ...
fiddlehead's user avatar
16 votes

Is there any advantage in stacking multiple images vs a single long exposure?

The voice of bitter experience, here, to tell you about a problem that a properly working observatory shouldn't have to worry about. But I did the time I was working on a "serious" astronomy project. ...
dmckee --- ex-moderator kitten's user avatar
15 votes
Accepted

Is there any advantage in stacking multiple images vs a single long exposure?

If your exposures are short enough (a fraction of a second), you can even combat turbulence in the atmosphere. The trick is to do very many short images then pick the ones where a (bright) point ...
ProfRob's user avatar
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15 votes
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Continued calibration of atomic clocks

My question is how is are the world's atomic clocks continually calibrated to compenstate for the vargaries of the Earth's rotation and orbit around the sun? They aren't. The atomic timescale is ...
BowlOfRed's user avatar
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14 votes

How can we 'see'/measure/detect particles during experiments?

In short: The Physicists analyse the final particles in the decay chain and derive from them the properties of the interesting particles. More detailed: The particle detectors consist of various sub-...
Gnorkx's user avatar
  • 183
14 votes

Given fluids expand non-linearly, how were physicists able to make a linear temperature scale?

The temperature scale we use nowadays is not based on thermal expansion of liquids or any other property of a substance which varies monotonically with temperature. Different types thermometers such ...
Vishnu's user avatar
  • 5,306
14 votes

One-way speed of light experiment, no clocks or mirrors (with simulation)

Unfortunately, there is simply no possible way to measure the speed of light independently of your synchronization convention. In this case, if you use the standard isotropic synchronization ...
Dale's user avatar
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14 votes
Accepted

Experimentally Measuring the Velocity of Water coming out of an Orifice

Water is incompressible. So you can put a bucket under your stream and let it run for some time. That way you measure the volume of water coming out per time [units m$^3/$s]. Divide by the cross-...
rfl's user avatar
  • 6,475
13 votes

How is time measured in particle experiments?

The shortest lifetime that was directly measured is that of the neutral pion, or $\pi^0$, the lightest meson at a mass of $m=135\,\textrm{MeV}$. It decays to (predominantly) two photons and it has a ...
tobi_s's user avatar
  • 1,321
13 votes

Continued calibration of atomic clocks

Modern metrology supports the issues you address by having multiple time systems, each with their own peculiarities. Any activity that needs a clock to measure time chooses a time system which meets ...
Cort Ammon's user avatar
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12 votes
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How can we 'see'/measure/detect particles during experiments?

This answer is to be read in parallel with the one by Gnorkx. This is one of the most recent particle detectors, CMS: CMS detector in a cavern 100 m underground at CERN's Large Hadron Collider. ...
anna v's user avatar
  • 234k
12 votes
Accepted

What are the "x" marks in a bubble chamber image?

They’re “stereo fiducials”: marks used to calibrate from the distances on the (somewhat deformable film image) to accurate 3D positions during the scanning process. On the chambers I'm familiar with, ...
Bob Jacobsen's user avatar
  • 14.5k
11 votes
Accepted

Laser beam alignment: best practices

Safe Alignment Since you're using visible wavelengths, you can align everything looking at it through webcams. They're so cheap and they see very well for many applications out to 11oonm. Put them ...
Selene Routley's user avatar
11 votes
Accepted

Why do scientists need to measure extremely small intervals of time?

It all depends on the phenomena one is studying. If you need to make an analysis about the general pick up of a car engine, i.e. how much time it take to go from 0 to 100 Km/h, one does not need to ...
Ishika_96_sparkle's user avatar
10 votes

Is there any advantage in stacking multiple images vs a single long exposure?

I haven't done this for astronomy, but have used an astronomy CCD down a microscope for electroluminescence and have also used cooled imaging CCDs for spectroscopy. Although I have often set the ...
Chris H's user avatar
  • 784
10 votes

How are photons told apart by a PET scanner?

Scatter is indeed a big problem. We try to deduce whether detected photons are scattered by measuring their energy. With a modern PET detector, energy resolutions on the order of 12% FWHM (full width ...
Floris's user avatar
  • 119k
9 votes
Accepted

How do non-mechanical solid-state optical switches work?

It looks like there are at least two ways to go about this: Bistable MEMS MEMS (Microelectromechanical systems) are very small structures, with features from $1-100\mu m$ in size, generally made with ...
Mark Omo's user avatar
  • 283
8 votes
Accepted

Why squared mass difference between neutrino 3 and 2 is a absolute value?

The experiments we've done so far are sensitive to the difference of the squares of the masses $\Delta m_{ij}^2 = m_i^2 - m_j^2$, not the square of the differences $\left( m_i - m_j \right)^2$. As ...
dmckee --- ex-moderator kitten's user avatar
8 votes
Accepted

Identifying the Higgs boson at LHC

In particle physics (as with most sciences) we are rarely ever concerned with analysing single events. What we look at are distributions of the same measurement(s) made many times. From performing ...
dukwon's user avatar
  • 2,064
8 votes

How to fix a bad KF flange?

I can think of three possible fixes: 1) The easiest "work-around" involves using a teflon gasket. Teflon is known to cold flow and conform to the shape that it is clamped between. With sufficient ...
David White's user avatar
  • 12.2k
7 votes
Accepted

Measuring air density - where is my huge error coming from?

Your method is the problem, Imagine for an instant trying to measure the density of helium by the same method, your balance would measure a negative weight (the balloon rise), and hence a negative ...
Jeannette's user avatar
  • 473
7 votes

Is there any advantage in stacking multiple images vs a single long exposure?

I don't know about astronomy, but one reason it can be useful in normal photography is to combat camera shake by auto-aligning the images before blending them. This can be useful if you want to take a ...
N. Virgo's user avatar
  • 34.3k

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