The result of sampling the property of a system

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1answer
81 views

Internal observers in a mathematical model

In this abstract the idea of a mathematical model having an internal observer is raised. A similar suggestion was made to me in a comment just recently. Is there a name for this methodology of ...
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1answer
103 views

Quantum physics and object location

If I understand correctly, according to quantum physics, subatomic particles can "leap" from one location to another (for example, from a location on earth to a location on the "other side" of the ...
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0answers
44 views

A Global Measure on a Gravitational System

I am a computer science engineer and I am trying to simulate the gravity on my computer; several objects with different masses randomly distributed in a Three-dimensional space. I would like to ...
8
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4answers
558 views

Doesn't the use of a thermometer alter the temperature of the system?

If I place a mercury thermometer in hot water, heat energy will transfer from the water to the mercury inside the thermometer. Will this continue until thermal equilibrium is reached and thus the ...
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9answers
3k views

How to measure speed of a ceiling fan?

I thought for fun I might measure how my ceiling fan slows down when turned off, and maybe find out what forces act on it when it slows down. My problem is that for that I need to measure its ...
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2answers
119 views

Why are all basic physical measurements ratios?

Why is it that "all basic physical measurements [...] are ratios"? This question comes from the book Mathematical Physics by Donald H. Menzel. Here is the explanation given: 1. The significance ...
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2answers
565 views

Measuring Cryogenic Temperatures

I need an inexpensive instrument to measure cryogenic temperatures (down to -200C). I can build a thermistor-based thermometer using an Arduino that is accurate to under 1 degree for 0 to 100C. ...
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0answers
235 views

How to measure the energy required to pedal an exercise bike

I have hacked a FitDesk exercise bike to output the RPM of the pedals and the position of the resistance control within its range of travel. I want to characterize the energy that is required to ...
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1answer
98 views

Obtaining a measurement very indirectly [closed]

While answering a question I came to reflect on the fact that in science experimental setups measure things indirectly. Example: the setup of the Pound-Rebka experiment. The amount of absorption was ...
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1answer
138 views

Measurement uncertainty basics

$$x = (\overline{x}-K)\pm \Delta x\tag{1}$$ $$\Delta x = s_{\overline{x}} = \sqrt{\frac t {\sqrt n} s_x} \tag{2}$$ $$s_x = \sqrt{\frac 1 {n-1} \sum^{n}_{i=1}(x_i-\overline{x})} \tag{3}$$ $$\frac ...
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3answers
836 views

How can I weigh liquid in a sealed container?

How do you measure the mass and volume of alcohol if it is in a sealed container? Are there clever ways of achieving this? The weight of the container is not known. I can't open the container, I ...
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1answer
137 views

Quantum mechanics and observation

I have read several times by different supposedly knowledgeable authors that conscious observation affects quantum experiments. I think these authors are confused, by which I mean they forget that ...
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2answers
132 views

How do we deal with irrationals in Physics? [duplicate]

This question is really very basic: how do we deal with irrationals in Physics? If for instance, in some meaningful calculation we get a length of $\pi$ meters, or a force of $\pi$ Newtons, how should ...
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2answers
283 views

How to measure declination of sun and moon?

I came by some graphs showing the declination for sun and moon (between angles of -30° and +30°). The graphs look very much like sine waves. I googled a bit and read about what the declination is, ...
2
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1answer
124 views

How is the energy of a lightning calculated?

I was wondering how is the energy of a lightning measured? Is it based on the luminous intensity and/or the generated sound? What about the generated heat and energy spent in the production of ozone? ...
4
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3answers
449 views

Are negativity of the Wigner function and quantum behaviour equivalent?

I've read the following question: Negative probabilities in quantum physics and I'm not sure I understand all the details about my actual question. I think mine is more direct. It is known that the ...
3
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0answers
199 views

Measure energy state of quantum harmonic oscillator

When discussing the quantum mechanical harmonic oscillator we are talking about energy eigenstates. How would one actually measure in which state an harmonic oscillator is in? Could you weigh it and ...
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2answers
156 views

Is there a fundamental limit to the temporal resolution of signals from space?

In Earth-based experiments, we can measure phenomenon very rapidly in an experiment given appropriate equipment. Clearly if something takes a long exposure to see (due to a weak signal), then the ...
2
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2answers
172 views

Uncertainty principle and multiple observers

My understanding is that an observer can measure the precise location of a particle so long as the corresponding uncertainty in momentum measurement is not an issue and vice-versa. Say there is ...
0
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1answer
218 views

Can an AC magnetic field be measured by a hall probe?

Many hall sensors on the market have a datasheet that explain the proportional relationship between the analog voltage that is outputted and the relative gauss/tesla reading (for example: 2.5mV = ...
0
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1answer
46 views

Where can I find a device or how can I make a device that measures only Alpha particles?

Are there special muller tubes that only measure alpha for example?
4
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1answer
175 views

Need to weigh a sack of kittens

Hypothetical brain teaser here, no real kittens involved! - For some bizarre reason I have a sack of kittens, I need to find the total weight of the kittens and the sack but I'm only allowed to use ...
1
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4answers
902 views

How does a dive computer calculate depth?

Depth is linked to pressure and water density. All depth gauges measure pressure. But is density also considered? And how? Specifically, I'd like to know: Is temperature measured? Is salinity ...
1
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1answer
3k views

Radius of curvature of lens: Newtons Ring method vs spherometer

I have to do an experiment to find out the radius of curvature of a lens using the Newtons ring method given that you know the wavelength of the monochromatic light used in the experiment. What the ...
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3answers
5k views

How is a second measured? And why is it measured that way?

The earth's rotation around the sun isn't exactly 24 hours. It off by some seconds which becomes somewhere around 6 hours per year and 1 day in 4 years(leap year), which brings the question why didn't ...
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1answer
198 views

How do we measure a quantum system properties? (length, mass and time) [closed]

How do we measure a quantum particle properties? (length, mass and time)
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3answers
723 views

Measuring speed of a vehicle

I'm very primitive with my thought. So please help if you can in layman terms with an answer to this question. Here we go- by using a street power pole as the source responder, is it possible to read ...
14
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1answer
256 views

GHz rate single photon counting

The fluorescent lifetimes of molecules used in biological applications tend to be in the sub-ns to a few ns timescale (let's say 0.8-4). The most direct methods to measure lifetimes typically involve ...
2
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1answer
225 views

How to measure the altitude of an object in the sky?

If I use a telescope to observe a plane in the sky, how can I find the altitude of the plane(altitude of the plane with respect to the ground)?
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2answers
425 views

Determination of mass density distribution of an object

This is a follow-up to a previous question How can you weigh your own head in an accurate way?. My purpose is not to restart the flurry of more or less humorous jokes (which are not such a bad thing ...
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25answers
13k views

How can you weigh your own head in an accurate way?

I read some methods but they're not accurate. They use the Archimedes principle and they assume uniform body density which of course is far from true. Others are silly like this one: Take a knife ...
2
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1answer
422 views

In terms of covariance matrices, are partial measurement and partial trace equivalent?

Partial measurement and partial trace There is a connection between a measurement of a part of a system and tracing this subsystem out. Say, we have a system composed of subsystems $A$ and $B$ in a ...
2
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1answer
396 views

Decibel equation, dyne or Watt

I know decibel is used as a ratio between two physical quantities with the same units on a logaritmic scale. Is also used for calculate the intensity of a sound, but I found it reported in two ...
2
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0answers
52 views

If the measurement of the proton size is correct and it is lower that it was expected than what are the theoretical implications?

In the press release of the recent measurement of the proton structure, they barely mention various theoretical explanations for the effect. What is the most likely (not assuming now that the ...
2
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1answer
251 views

How does an atom look like in momentum space?

I am reading an article in a popular science magazine about atoms in extremely low temperature (condensation?). It's interesting and I've got few questions regarding this phenomenon. 1) In the ...
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3answers
333 views

Does one second exist? [closed]

Let us assume .5 second has passed. Now similarly let .9 seconds also passed.. Then let's say that .99 second has passed...we're still not done because 1 second hasn't passed. Then follows .999 ...
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2answers
505 views

How do you come up with a POVM?

This is a made-up example, just to understand a concept. If changing the probability values aids your explanation, that's fine by me. Say you have a physical quantity $E$ that can take values 1, 2, 3 ...
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0answers
67 views

Physics (measurement) [closed]

In october,1707 four British warships ran aground because of error in position,sparking the effort to produce a marine clock that would be accurate to locate a position with in 30 miles after sailing ...
3
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1answer
117 views

Time of Measurement Vs Number of Measurements

Let's say that an experiment has to determine the number of cosmic muon at sea level. The appropriate equipment is ready to measure the counting rate. I can think of two ways Count for 10 minutes, ...
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1answer
95 views

How many measurements should be done? [closed]

I am measuring time of a computer operation. The operation should run roughly same time each time I measure it. How many times should I measure it to get good average and standard deviation?
3
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3answers
314 views

Precision of Coulomb's law

Up to which precision has the coulomb law proven to be true? I.e. if you have two electrons in a vacuum chamber, 5 meters appart, have the third order terms been ruled out? Are there any theoretical ...
6
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2answers
409 views

What is known about the Magnetic North Pole's location before 1800?

I was recently startled to find that the Earth's North Magnetic Pole is moving at upwards of 40 km per year, with an additional ~80 km daily elliptical drift about its mean position due to variations ...
1
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1answer
161 views

Measurement uncertainty of the quantity, that is function of two others quantities

I'm trying to compute uncertainty for the density of the ball. I measured its radius 6 times, so I was able to compute the stastistical uncertainty (we call it uncertainty type A, I don't know, if ...
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2answers
317 views

Is there any phenomenon in physics which is sensitive to irrational numbers?

We can measure only rational numbers by our scale. Here is an example where irrational numbers does makes sense. If so then this question may have some theoretical importance. Is irrational numbers ...
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2answers
282 views

How can we detect a black hole? [duplicate]

If black holes are phenomena of very high density (gravitational singularities) which don't emit radiation how can we detect them so far away from us where so much other radiation can hide the black ...
3
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0answers
542 views

projective measurement & POVM

Let us consider the following completely positive map $\mathcal{B}(\mathbb{C}^n)\ni\rho\mapsto L\rho L^\dagger$, where $L\in\mathcal{B}(\mathbb{C}^n)$ is any arbitrary operator (and can have rank ...
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2answers
132 views

Is there a naturally occuring sound pressure level reference?

There are lots of devices that purport to measure the absolute value of sound pressure levels. Here is an example, here's another, there's also this iPhone App. Putting such devices side-by-side in ...
13
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4answers
855 views

Can we measure an electromagnetic field?

As far as I can check, the Aharonov-Bohm effect is not -- contrary to what is claimed in the historical paper -- a demonstration that the vector potential $A$ has an intrinsic existence in quantum ...
7
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0answers
203 views

Why is the Planck length the shortest measurable length? [duplicate]

I quote from the Wikipedia article on Planck length: According to the generalized uncertainty principle, the Planck length is in principle, within a factor of order unity, the shortest ...
2
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2answers
433 views

Asymmetric uncertainties

Inspired by these two question on tex.SX Asymmetrical tolerancing Asymmetric uncertainties with siunitx package I'd like to ask for a nice explanation for these kind of uncertainties, like ...