Tag Info

Hot answers tagged

24

quantum dots. nanoscale semiconductor materials that can confine photons in 3 dimensions and release them a measurable time after. based on material used the decay time is known empirically. frequency is also known. the latter is sufficient to calculate the energy of one photon. the former is then sufficient to calculate the rate of photon re emission from ...


20

which experiment gave scientists the reason to believe nuclear fission/fussion existed Fusion was first. Francis William Aston built a mass spectrometer in 1919 and measured the masses of various isotopes, realizing that the mass of helium-4 was less than 4 times that of hydrogen-1. From this information, Arthur Eddington proposed hydrogen fusion ...


19

There are a variety of methods used to measure distance, each one building on the one before and forming a cosmic distance ladder. The first, which is actually only usable inside the solar system, is basic Radar and LIDAR. LIDAR is really only used to measure distance to the moon. This is done by flashing a bright laser through a big telescope (such as ...


13

Binary objects provide the invaluable opportunity to observe objects interacting. The only way to observe most solo objects sitting in space is by the light they emit all by themselves. (All stars emit a low background of neutrinos resulting from their nuclear fusion, but only the Sun is close enough to observe that. A supernova emits a burst of neutrinos ...


13

In the double slit experiment, if you decrease the amplitude of the output light gradually, you will see a transition from continuous bright and dark fringe on the screen to a single dots at a time. If you can measure the dots very accurately, you always see there is one and only one dots there. It is the proof of the existence of the smallest unit of each ...


12

The practical answer (which I also wrote in a comment on the linked question) is that you turn the intensity of the light source down until the expectation value for the number of photons on the optical path is low enough to suit you. If $\bar{n} = 0.1$ then very few of the events that are recorded on the screen will come from events where more than one ...


10

our lab has an ultra-high vacuum stm system (10-11 torr), and all parts that go in the vacuum system has to be extremely clean. Here is what we do: first i want to point out that the material you use for UHV is very important too. The commonly accepted material is 316 stainless steel and oxygen free pure copper. For other specialized material, you should ...


10

A pendulum of (long) length L will tick with a period of $2\pi\sqrt{L\over g}$, and air resistance can be made negligible for a mm-sized oscillation of a heavy object on a several-meter long rigid arm. You need to determine the location of the center of mass accurately to know the effective value of $L$, but this can be done arbitrarily accurately by ...


9

The Aethrioscope (see Wiki page with this name) was invented in 1818 by Sir John Leslie and the basic idea for a pyrometer (see Wiki pahe with this name) was conceived in the late 1700s by Josiah Wedgewood. These were calibrated by comparing observed colour with that of hot metals / clays (as appropriate) of known temperature. The idea was to heat a small ...


7

Try a styrofoam cooler: They're about $5 at Walmart. Your homemade dewars should fit easily inside and should be reasonably sealed from atmosphere. We used to use this in my Senior Lab in undergrad to freeze samples overnight, there was always plenty left in the morning, so I imagine that the combination should get you near that 3 day mark.


7

As an "infrared survey technician" part of my job was keeping our infrared camera topped off with liquid nitrogen. We had a large dewar (about the size of a barrel) at the shop that was filled bi-weekly. For daily excursions we used a smaller dewar (about the size of a pony keg). Both of these dewars were specifically designed to contain cryogenic liquids, ...


7

Alternatively I would look around the lab for an infrared thermometer. There exist in the market close focus ones that go down to 6mm in close focus option ( so as not to advertise, google space accurate infrared thermometers microscopes where I found the number in a one of the first hits). I would choose a large ant, or attract more by a spot of honey ...


6

This is not really an answer to the question in the title, but a description of why the proposed short baseline neutrino speed measurement is exceedingly difficult. It related to the question in the sense that it explains the limits of the precision with which $\delta t$ can be extracted in a neutrino experiment, without even touching on the kind of ...


6

Triangulation. The Earth is not stationary, it moves in a 150 million km (1 AU) radius orbit around the Sun. If you measure the apparent position of a star at different points in that orbit a near enough object will appear to be displaced by a measurable amount, this displacement is called parallax, which is typically measured across a 1 AU baseline. A ...


6

Similar situations can arrise in experimental work on large scale machines, and there is a body of knowledge that gets passed around. The "can it hold pressure" test suggested by other answer works best if you can apply a pretty good over-pressure and have either a sensitive pressure gauge in the system or can afford to wait over night to see how tight ...


6

Gold foil is quite easy to hold you just hang it from a paperclip. The only difficulty is if there is a lot of static electricity in the air which makes it stick to things. (This is the main reason for the cold damp Cambridge's supremacy in early particle physics) Photographic film at the time wasn't sensitive and so in Marsden and Geiger's experiments ...


5

Firing the ball straight upwards is the obvious experiment. The vertical trajectory has an analytic solution even including air resistance, so you could probably estimate the muzzle velocity and the air resistance. If this is too boring, hammer nails into your block of wood so the ball hits the nails and is impaled on them. The experiment can be made more ...


5

Has anyone ever constructed an ultra-high vacuum system with half-assed, or no cleaning of parts? Haven't we all done that at some point? How'd it turn out? Badly! Water and hydrogen are easy to bake off the internal surfaces, but get any hydrocarbons, skin grease, silicone, etc on it and you'll be baking for days.


5

A little known method is the convergent point method (or moving cluster method). Stars in open clusters move parrallel through space and due to the perspective effect they will appear to move towards a common point on the sky. This point depends on the distance and the distance can thus be calculated if the point in the sky can be determined. I have never ...


5

I wish there was an easy answer, but this is actually somewhat complicated, and to some extent is more art than science. There are several simple models that are used to predict molecular geometry, one of the most common is the VESPR model. Based on this model, one can begin to perform calculations of energy associated with different vibrational modes of ...


5

Have a look at http://arxiv.org/abs/1009.1569. In this article Thomas Juffmann discusses some of the practical issues in doing these experiments. In principle these experiments aren't hard, but in practice there are lots of technical difficulties. For example the large molecules need to be all moving at the same velocity(i.e. the beam needs to be very cold) ...


5

This is one of my favorite questions in astronomy. It's extremely clever really. The Sun emits a certain colour of light that can be analysed. This colour is obviously completely white other than certain frequencies. For example, an object emitting a turquoise light would be emitting every colour frequency other than parts of red perhaps (there are other ...


5

What has been measured at CERN some days ago? Once you have an event (as @user1504 describes in his answer), i.e. a proton proton interaction, and have the four vectors of all the interaction products, i.e. jets, photons, leptons, you can generate invariant masses of the interaction products. The Higgs particle has been predicted theoretically and is ...


5

You do exactly the same thing: you "rotate" the state and then measure along whatever axis your measurement apparatus happens to measure. The only difference here is that the "rotation" does not necessarily correspond to a rotation in space like it does for a true spin. What follows is a detailed description of how we do rotations of a generic 2 level ...


5

'Every piece of knowledge in science has a beginning lying in someone's experiment' A view that has long stopped being correct. In fact, I'd say the idea of nuclear energy was first established when Einstein formulated mass-energy equivalence: $E=m_0c^2$, which shows that there are enormous amounts of energy hidden in ordinary matter, if you can find a ...


4

I just realized what the problem is. It actually doesn't have anything to do with the detector. When working in vacuum systems you have to worry about the dielectric breakdown of the air as the pressure is reduced. It turns out that the breakdown voltage hits a minimum around $\sim 1$ Torr depending on the species of the gas (see the curves below). This ...


4

Just spit-balling as I have never tried this. Use the best dewar(s) you can find. Silvered glass walls and a vacuum gap. Assuming you can take as much LN2 as you want, use the biggest dewar(s) you can find. Not only is there more there, but the surface area to volume ratio works in your favor. Keep the dewar(s) in a cool, enclosed place. You'd like them to ...


4

They just had to measure the capacitance of a capacitor! If you can build a precise dimensioned capacitor you can work out it's capacitance with an equation including the vacuum permittivity ($ε_0$). It's easy to build the dimensions of the metal plates in a capacitor to high accuracy, and you can measure current accurately so by measuring the capacitance ...


4

Liquid nitrogen is used because nitrogen is extremely abundant on earth. Nitrogen makes up approximately 78% of the atmosphere by volume. Hence, liquid nitrogen is rather easy to make (and consequently cheap). I've heard for instance that Fermilab buys liquid nitrogen for cheaper than what you pay for water. Liquid helium is useful for things that must go ...


4

I would expect stimuli-responsive polymers to have what you're looking for, and the keyword "stimuli-responsive" may be a useful search term. The stimuli-responsive polymers undergo conformational changes in response to changes in their environment. The primary environmental control variables people use are temperature, pH, and ionic strength. But people ...



Only top voted, non community-wiki answers of a minimum length are eligible