The result of sampling the property of a system

learn more… | top users | synonyms

5
votes
2answers
2k views

Does photon have size measurement because of its particle nature

Does photon have size measurement because of its particle nature like electron's 3.86*10^-13m etc..
1
vote
4answers
145 views

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 ...
1
vote
1answer
174 views

Thought Experiment: Force on magnets in a Stern Gerlach Experiment

Background: In the SG experiment, an inhomogenous magnetic field affects a force on particles passing between two magnets. "Measurement" takes place when a screen is placed on one end, blocking one ...
-1
votes
0answers
21 views

Multiple contemporary measures of a photon's state

From quantum mechanics, when you polarize a photon using computational base and you try to measure its state using a diagonal base, you'll get with the 50% of probability $|0\rangle_{{} + {}}$ or ...
3
votes
2answers
78 views

How to (safely) measure the surface area of a human body? [on hold]

Here's a question just for curiosity. You measure mass on scales. You could measure volume by submerging in a bath with a snorkel system. But how could you measure a living human's surface area ...
0
votes
1answer
17 views

How could the BOE (barrel of oil equivalent) be measured?

The barrel of oil equivalent was given as an example of gigajoule scale energy. "The barrel of oil equivalent (BOE) is a unit of energy based on the approximate energy released by burning one barrel ...
0
votes
1answer
28 views

Calculate how many photons hitting a sample that are absorbed

In the determination of the quantum yield of a photoisomerization, $\Phi,$ the following is needed $$ \Phi = \frac{\mbox{Number of "reactions"}}{\mbox{Number of absorbed photons}} $$ The photon flux ...
3
votes
4answers
443 views

What truly is mass, and is there a direct way to measure it?

We know a mass of an object of one kilogram as an object that weighs W = mg = 9.8 N and we reference it to that, (when it should as a fundamental parameter describe weight not the opposite). But if we ...
2
votes
0answers
24 views

Venturi “Turn Down” [closed]

I'm sorry if this isn't the right forum for this question, but physics is the only stackexchange which had many results for Venturi. I'm trying to measure the flow of air through a 16mm hose/pipe ...
1
vote
2answers
116 views

How stellar aberration is measured?

A simple calculation shows that stellar aberration due orbital motion of earth is roughly 20 arcseconds. My questions are: Practically how this small value is measured? Does this value is in the ...
6
votes
1answer
54 views

Is there a tool to measure the chemical potential of a system?

Let's suppose I put you inside a room filled with a gas. You can measure its pressure directly with a barometer. You can measure its temperature directly with a thermometer. Can you measure its ...
1
vote
0answers
38 views

Is there any well-defined, measurable atomic size?

Is there any well-defined, measurable size at the atomic scale? I was under the impression that the van der Waals radius was a good way of describing the size of an atom, but it turns out to be a ...
0
votes
0answers
54 views

How to measure size of small particles of $mm$ level?

Is there any way to measure dimension of small particles (mm sized) such as rice grains accurately? I know there are techniques such as laser or x-ray diffraction, but the equipment needed are too ...
2
votes
0answers
26 views

How precisely do we know the van der Waals radius of hydrogen?

I've found several sources that say the van der Waals radius of hydrogen is $120 pm$. How precisely do we know this number? What order of magnitude is the error on this measurement?
0
votes
3answers
56 views

Measuring force of a punch

I'm trying to build a device that can mesure the force of a punch. ​ My initial plan was to build a platform with 4 springs (one at each corner) and an accelerometer in the middle. But, if the ...
3
votes
0answers
48 views

Simultaneous measurement of non-commuting observables without uncertainty

A pair of non-commuting Observables $\hat{X}$ and $\hat{P}$ does not have a common set of eigenfunctions, i.e., it can not be measured simultaneously. Let us for the sake of simplicity assume that ...
1
vote
2answers
580 views

Is it possible to use induction ampermeter to measure power consumption of electric water heater and dryer?

I hope this practical question is not OT and not too trivial for this forum. I am renting an apartment in a duplex with a shared water heater and dryer. Turns out, both water heater and the dryer are ...
3
votes
0answers
21 views

How are melting points of metals measured?

How are melting points of metals measured? What apparatuses are used and how do they "conceive" heat units?
1
vote
1answer
59 views

Why do scientists measure fundamental constants so precisely?

According to the last CODATA report the charge of electron is measured very precisely (10 decimal places). And it's not only about the charge of electron. Why do scientists measure fundamental ...
12
votes
7answers
652 views

Can a ultracentrifuge be used to test general relativity?

With today's ultracentrifuge technology, they can spin so fast that the sample can be subjected to accelerations of up to 2 millions Gs. That is equivalent to two solar masses. Has someone tried to ...
3
votes
1answer
64 views

Ideal shape of a water clock

The ancients undoubtedly discovered the ideal shape of a water clock by trial and error. In examining some ancient water clocks I notice the shape is different depending on the size. For example, a ...
2
votes
3answers
120 views

How is complex permittivity measured?

Complex relative permittivity is defined as $$\epsilon_r = \frac{\epsilon(\omega)}{\epsilon_0}=\epsilon_r^{\prime}(\omega) + i\epsilon_r^{\prime\prime}(\omega) = \epsilon_r^{\prime}(\omega) + ...
0
votes
0answers
33 views

In which book is it written that the only measurable physical quantity is the length?

Task. In any book I've seen thoughts and images that very clearly illustrate the sequence of actions and implicit assumptions when measuring something. The result has been that in any measurement key ...
3
votes
2answers
98 views

How mass is determined in dynamics?

Mass is one of the most core and complicated concepts in dynamics. I have tried many books but I still don't have a good idea of how the mass of any object is determined relative to another. In The ...
0
votes
1answer
48 views

Combining errors. Gamma spectrometry, Poisson distribution

I have run an experiment 3 times and measured the results by gamma spectrometry. For example I get values like this (1 $\sigma$): $100 (10)$ $90 (8)$ $110 (12)$ The above 1 sigma error is based ...
0
votes
0answers
50 views

The use of real numbers in measurements in physics?

When we measure something in physics we give a value plus or minus some uncertainty and assume that the measured value can take any value with the range with a certain confidence limit. I don't ...
7
votes
11answers
2k views

Is it possible for a physical object to have a irrational length?

Suppose I have a caliper that is infinitely precise. Also suppose that this caliper returns not a number, but rather whether the precise length is rational or irrational. If I were to use this ...
2
votes
1answer
72 views

How accurate is Newtonian mechanics? [closed]

How accurate is (theoretical, i.e. the mathematical formulation basing on "formalised" physical constants, such as material density) Newtonian mechanics? What does the accuracy depend on (material ...
6
votes
1answer
1k views

The uncertainty of a metre ruler?

I have been taught that the uncertainty in the measurement of a metre ruler is +-1 mm. However , I was also taught that the uncertainty is half of the smallest division in the measuring instrument. ...
1
vote
1answer
65 views

Deriving a POVM from a projective measurement

I understand how to show that every POVM is equivalent to a projective measurement on a larger Hilbert space, but I don't understand why the converse is true. The vast majority of explanations of ...
10
votes
3answers
12k views

Rough, easy DIY method of measuring magnetic field strength

How to easily, using standard DIY equipment measure the strength of magnetic field generated by a permanent magnet? Narrowing down the "loose language" of the above: strength of magnetic field: ...
0
votes
2answers
64 views

How are the SI units “generalised”?

How exactly are the SI units generalised from their definitions? E.g. the kilogram is a weight of an object of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of ...
1
vote
2answers
45 views

What is the relation between charge and polarization?

On one hand, in this "Measurement of polarisation" lab manual, polarization is surface charge density, $P=\frac{Q}{A}$, in other words, charge an polarization are essentially the same thing. On the ...
11
votes
3answers
370 views

What does multiplying two real-world values represent? [duplicate]

I totally get what division means in the real world. "dollars / hour", well, that's the number of dollars you will make in one hour. "kilometers / gallon" is the distance you can go with a gallon of ...
3
votes
2answers
120 views

How is dark matter detected?

What methods do we use to detect Dark Matter? If I understand correctly, due to lack of electromagnetic interaction it should be able to phase through normal matter nearly like through void - since ...
3
votes
2answers
269 views

How to use an accelerometer to correctly quantify the magnitude of vibrations

Consider the following setup: An accelerometer is placed inside a helicopter, measuring $g$ values along three axes $(x,y,z)$ every $10~\text{ms}$ (100 times a second), during a helicopter flight. ...
0
votes
1answer
35 views

Can conductivity (or resistivity) value be assigned to 2D materials?

2D materials, such as graphene monolayer or graphene nanoribbon, don't really have a cross section. By definition, conductivity is $\sigma = \frac{J}{E}$, and $J$, the current density, is calculated ...
0
votes
1answer
67 views

Calculating flow rate of a syringe

I wanted to know how would I go about calculating the flow rate of a syringe with a metal tip that is dispensing water using a pressurized air. I have had a look on internet about this and on this ...
1
vote
2answers
70 views

How to measure 8 μm surface bumps?

I want to measure surface bumps which are larger in size than $8\ \mathrm{\mu m}$ on a uncoated plastic mirror. So it can be any number of smaller bumps and that would not effect the application. I ...
4
votes
0answers
62 views

Measuring the Dirac field

If the Dirac field $\psi(x)$ is to the electron as the Electromagnetic field is to the photon, why is it that we can measure the Electromagnetic field, whereas the Dirac field we cannot?
1
vote
0answers
43 views

Measuring expectation value in quantum field theory and in quantum mechanics

There is a way of calculating the vacuum expectation value $\langle 0|\hat\phi|0\rangle$ theoretically in a quantum field theory like there is a rule to compute expectation value of any operator A ...
-1
votes
3answers
54 views

Why are measurements standardized the way they are?

Using meters as a base length, squaring or cubing lengths smaller than 0.67m makes the square term larger than the cubed term. This fact causes certain properties of physics (how rain needs to form?) ...
8
votes
5answers
776 views

How would I go about detecting monopoles?

A question needed for a "solid" sci-fi author: How to detect a strong magnetic monopole? (yes, I know no such thing is to be found on Earth). Think of basic construction details, principles of ...
0
votes
1answer
107 views

Wave Function State Reduction As a Result of Quantum Measurement

This is a question about indirect quantum measurement, involving an observable of an object of interest and a probe that is used to measure that observable. In this experiment, an observable of the ...
1
vote
0answers
77 views

Quantum indirect measurement with probes

I am wondering about the following hypothetical example regarding quantum measurements made by probes: An open, two-plate capacitor, in a LC circuit, has charge, q, that we want to measure. Each of ...
1
vote
3answers
26 views

Measuring Activity

The formula for Activity of a radioactive substance is $ \frac{dN}{dt}=A=λN $. If we have an initial number $N(0)$ of some Radionuclide, which has a halflife of, say, 12 hours, is there any ...
3
votes
0answers
77 views

How does one compute the state of a quantum system following imperfect measurement?

Suppose I have a quantum system $S$ ("system") with Hamiltonian $H_S$ and initial density matrix $\rho_S(0)$. I allow $S$ to interact with another system $P$ ("probe"), which has Hamiltonian $H_P$ and ...
4
votes
2answers
4k views

Measuring acceleration of earth due to its fall around the sun

Every orbiting of a satellite around a mass is nothing else but a constant fall - and therefore acceleration - towards this mass. In a way it is a "falling around" that mass. My question Is it ...
7
votes
2answers
3k views

Why does an infrared thermometer display very low temperature when being directed to the outer air?

I'm toying with an infrared thermometer - one which you point onto an object, press the button and it instantly measures the temperature by estimating the infrared radiation from the object. It shows ...
0
votes
0answers
22 views

Is the saturation magnetization of a thin film different if you apply the field at different directions?

I'm getting different values of the saturation magnetization when I measure them in-plane versus out of plane. The saturation magnetization for the out of plane measurement is greater than in the in ...